Master's thesis of Engineering: Performance enhancement of phase change materials as thermal energy storage for domestic solar water heating applications

Performance Enhancement of Phase Change Materials as Thermal Energy Storage for Domestic Solar Water Heating Applications

A thesis submitted in fulfillment of the requirements for the degree of Master of Engineering

Hossein Senobar

B.Sc. Mechanical Engineering, Azad University, Tehran, Iran

School of Engineering

College of Science, Technology, Engineering and Maths

RMIT University Melbourne, VICTORIA, Australia

November 2021

DECLARATION

I, Hossein Senobar, certify that except where due acknowledgement has been made, this

research is that of the author alone; the content of this research submission is the result

of work which has been carried out since the official commencement date of the approved

research program; any editorial work, paid or unpaid, carried out by a third party is

acknowledged; and, ethics procedures and guidelines have been followed.

In addition, I certify that this submission contains no material previously submitted for

award of any qualification at any other university or institution, unless approved for a

joint-award with another institution, and acknowledge that no part of this work will, in

the future, be used in a submission in my name, for any other qualification in any

university or other tertiary institution without the prior approval of the University, and

where applicable, any partner institution responsible for the joint-award of this degree.

I acknowledge that copyright of any published works contained within this thesis resides

with the copyright holder(s) of those works.

I give permission for the digital version of my research submission to be made available

on the web, via the University’s digital research repository, unless permission has been

granted by the University to restrict access for a period of time.

I acknowledge the support I have received for my research through the provision of an

Hossein Senobar

30 November 2021

Australian Government Research Training Program Scholarship

ACKNOWLEDGEMENTS

I would like to express my greatest gratitude to my supervisor Prof. Bahman Shabani for

the ongoing support, helpful guidance, and constructive advice during my study journey.

It was a great pleasure to have such an accompany in my time of study as a supervisor. I

would also gratefully thank my second supervisor Dr Biddyut Paul for his excellent inputs

during my research.

Special thanks to my dear wife, Maryam, who has always supported me by staying on my

side. In addition to the challenges of migrating to a new environment, she always

encouraged me in perfectionism, listened to all my complaints, endured my fatigue,

encouraged me to have good self-confidence, helped me understand the truth, and

celebrated good times with me. Maryam is a very patient and kind companion, and we

have progressed together during this time. She deserves endless appreciation because of

her honesty, kindness, and strength. I also thank my two dear daughters, Benita and

Emma, for helping me along the way by giving me energy and joy and cheering me up

with their childish words and passionate smiles with little complaints. I would like to

express my special thanks to my parents for their continuing support and care.

Finally, I would like to thank the workshop staff at RMIT School of Engineering for their

ongoing support. particularly during the COVID pandemic in 2020-2021. My, special

thanks go to Eliecer Bonilla Parra and Huw James that without their help setting up my

III

experimental rig was impossible.

TABLE OF CONTENTS DECLARATION .................................................................................................................................................. II

ACKNOWLEDGEMENTS ............................................................................................................................... III

TABLE OF CONTENTS ................................................................................................................................... IV

LIST OF FIGURES .......................................................................................................................................... VII

LIST OF TABLES .............................................................................................................................................. IX

EXECUTIVE SUMMARY .................................................................................................................................. 1

NOMENCLATURES and ABBREVIA ............................................................................................................ 7

1. INTRODUCTION ...................................................................................................................................... 8

1.1. Background........................................................................................................................................................... 8

1.2. Solar thermal collectors and thermal energy storage solutions.................................................. 11

1.3. Phase change materials ................................................................................................................................ 13

1.3.1. Overview ................................................................................................................................ 13

1.3.2. PCM properties and paraffin based PCMs ................................................................. 15

1.3.3. Thermal conductivity improvement of paraffin based PCMs ............................ 17

1.4. Research Gap..................................................................................................................................................... 19

1.5. Aim, Objectives, and Scope .......................................................................................................................... 19

1.5.1. Aim and objectives ............................................................................................................. 19

1.5.2. Research Scope .................................................................................................................... 20

1.6. Research Questions ........................................................................................................................................ 21

1.7. Research Outcomes ........................................................................................................................................ 21

1.8. Thesis Structure ............................................................................................................................................... 22

2. LITERATURE REVIEW ......................................................................................................................... 23

2.1. Overview ............................................................................................................................................................. 23

2.2. Phase Change Materials (PCMs) ............................................................................................................... 23

2.2.1. PCMs Classification ............................................................................................................ 23

2.2.2. Organic PCMs ....................................................................................................................... 26

2.2.3. Inorganic PCMs .................................................................................................................... 27

2.2.4. Eutectic mixtures ................................................................................................................ 28

2.3. Low Thermal Conductivity: A Major Challenge of PCMs ................................................................ 29

2.4. Enhancing the Thermal Conductivity of PCMs ................................................................................... 30

2.4.1. Overview of enhancement techniques studied to date ........................................ 30

2.4.2. Using metal foam to enhance the thermal performance of PCMs ..................... 34

2.4.3. Nanoscale materials to improve the effective thermal conductivity of PCMs

41

2.5. Integration of PCMs with Solar Thermal Collectors (STC)............................................................. 46

3. METHODS ................................................................................................................................................ 53

3.1. Overview .............................................................................................................................................................. 53

3.2. Literature Review............................................................................................................................................. 55

3.3 Material Selection and Application Requirements for the Experimental Study ................... 56

3.4 Experimental Study .......................................................................................................................................... 57

3.4.1 Thermal conductivity enhancement of PCM: comparing nanoparticles and metal foam .............................................................................................................................................. 57

3.4.2 Experiments with the solar thermal collectors .............................................................. 60

4. NANOPARTICLES AND METAL FOAMS FOR HEAT TRANSFER ENHANCEMENT OF PHASE CHANGE MATERIALS ..................................................................................................................... 63

5. EVACUATED TUBE SOLAR THERMAL COLLECTOR WITH ENHANCED PHASE CHANGE MATERIAL THERMAL STORAGE: AN EXPERIMENTAL STUDY ...................................................... 65

6. CONCLUSIONS AND RECOMMENDATIONS ................................................................................... 66

6.1 Overview ............................................................................................................................................................... 66

6.2 Responses to the research questions ........................................................................................................ 67

6.2.1 How much improvement in thermal performance of a selected PCM (i.e. suitable for use with solar thermal hot water systems) can be achieved when metal foams and nanoparticles are used separately and simultaneously? ................................ 67

6.2.2 How melting and freezing cycles can impact the stability and hence the effectiveness (i.e. in improving the thermal conductivity) of nanoparticles dispersed in PCMs, in terms of maintaining the improved thermal performance? ......................... 69

6.2.3 How do the thermal conductivity enhancement measures work in the context of a solar hot water system in terms of the improved experience of the end user? ......... 70

6.2.4 What are the challenges associated with using nanoparticles and metal foam in PCMs, and how these challenges can be addressed? .......................................................... 71

6.3 Recommendations for future work ....................................................................................................... 72

References ....................................................................................................................................................... 75

A. Appendix ................................................................................................................................................. 83

A.1. Publications From This Research Program ........................................................................................... 83

A.2. Solar Collector .................................................................................................................................................. 84

A.3. Paraffin (Phase Change Material).............................................................................................................. 85

A.4. Loading and Flowrate of hot water .......................................................................................................... 86

A.5. The PCM Thermal Storage Unit .................................................................................................................. 87

A.6. Setup experiment ............................................................................................................................................ 88

A.7. Copper Tube used in experiments ............................................................................................................ 89

A.8. Instrumentation ............................................................................................................................................... 90

A.9. Digi-Flow Rate meter...................................................................................................................................... 91

A.10. Material Safety Datasheets (Copper Oxide) ....................................................................................... 92

A.11. Safety Data Sheet of Copper Foam ......................................................................................................... 93

LIST OF FIGURES

Chapter 1 FIGURE 1- 1: CO2 EMISSION PER CAPITA IN 2017 OF IPCC (KEISUKE SADAMORI AUG 2021) ....................... 10 FIGURE 1- 2: AUSTRALIA'S PRIMARY ENERGY SUPPLY BY SHARE IN 2017. DATA: CALCULATED USING IEA (2019) ONLINE FREE VERSION. THIS DATASET IS THE ONLY AVAILABLE THAT SHOWS ALL ENERGY SOURCES. (ROSER 2019) ........................................................................................................................................ 11

FIGURE 1- 3: ENERGY VS. TEMPERATURE PLOT SHOWING THE ISOTHERMAL HEAT STORAGE EFFECT OF PCMS

(SOARES ET AL. 2013). ........................................................................................................................................... 14

CHAPTER 2 FIGURE 2- 1. PCMS TEMPERATURE VS. STORED HEAT FOR AN LHS SYSTEM (DOUVI ET AL. 2021) ................. 24 FIGURE 2- 2. CLASSIFICATION OF PCMS (LIN ET AL. 2018) ................................................................................... 25 FIGURE 2- 3. ORGANIC PCMS (PARAFFIN) (BAETENS ET AL. 2010) .................................................................... 27 FIGURE 2- 4. INORGANIC PCMS (SALTS) (BAETENS ET AL. 2010) ....................................................................... 28 FIGURE 2- 5 . EUTECTIC COMPOUNDS (THE BINARY EUTECTIC CHLORIDE/EG COMPOSITES %5 (A) AND %20

(B)) (TIAN ET AL. 2016) ........................................................................................................................................ 29 FIGURE 2- 6. SCHEMATIC OF DSC HEATING THERMOGRAM OF PCMS (FLEISCHER 2015A) ............................. 31 FIGURE 2- 7. SAMPLE OF NANOPARTICLES( COPPER OXIDE POWDER, 20 NM)(A), METAL FOAM (COPPER

FOAM, 10 PPI)(B), FIN ( COPPER FIN)(C), AND HEAT PIPE (HIGH-PERFORMANCE COPPER HEAT PIPE )(D) FOR ENHANCING THERMAL CONDUCTIVITY OF PCMS, (TIAN 2012; SOCACIU ET AL. 2016; LI ET AL. 2017) ......................................................................................................................................................................... 32

FIGURE 2- 8. THE FORMED SKELETON STRUCTURE OF THE METAL FOAM AND THE EXTRACTED

COMPUTATIONAL DOMAIN. COMPUTATIONAL DOMAIN (WANG ET AL. 2019B). .......................................... 34

FIGURE 2- 9. DIAGRAM OF TIME-TEMPERATURE WITH DIFFERENT PORE DENSITIES AT CONSTANT POROSITY (Ɛ = 0.90) FOR PCM COMPOSITES (A). DIAGRAM OF TEMPERATURE AND TIME WITH DIFFERENT POROSITIES IN FIXED PORE DENSITY OF 40 PPI, FOR PCM COMPOSITES (B)(W.Q. LI 2012) ................. 37

FIGURE 2- 10. THE EFFECT OF POROSITY OF METAL FOAM ON THE CENTER TEMPERATURE OF PCMS COMPOSITE DURING THE HEATING PROCESS (A) 13.65 W, AND (B) 16.66W. BASED ON THE CALCULATED RESULTS IN THIS STUDY, THE ENHANCED THERMAL CONDUCTIVITIES OF THE MF-PCMS ACHIEVE 27–134 TIMES (DEPENDING ON ITS POROSITY) OF THAT OF PARAFFIN RT55, WHICH SHOWS A TREMENDOUS ENHANCEMENT IN THERMAL CONDUCTION. (WANG ET AL. 2016) ....................................... 38 FIGURE 2- 11. EFFECT OF POROSITY OF FOAM METAL (WANG ET AL. 2019B) ..................................................... 39 FIGURE 2- 12. COMPARISON BETWEEN PURE PCM AND METAL-FOAM COMPOSITE PCMS, (HAN ET AL. 2013). ..................................................................................................................................................................................... 40

FIGURE 2- 13. DIFFERENT KINDS OF METAL FOAM (ZHAO ET AL. 2010; CHINTAKRINDA ET AL. 2011; EL IDI

ET AL. 2021) ............................................................................................................................................................. 40 FIGURE 2- 14. DIFFERENT KINDS OF NANOPARTICLES (HOSSEINI ET AL. 2013) ................................................. 45 FIGURE 2- 15. CONCLUDED THAT CU NANOPARTICLES HAVE THE BEST PERFORMANCE FOR HEAT TRANSFER

(KHODADADI 2015) ................................................................................................................................................ 46

FIGURE 2- 16. DIAGRAM OF PCM THERMAL STORAGE UNIT INTEGRATED WITH A SOLAR THERMAL

COLLECTOR ................................................................................................................................................................ 51

VII

CHAPTER 3 Figure 3- 1. Overview of research method........................................................................................................ 54 Appendix Figure A- 1. Schematic view of the collar collector GL 100-16 ----------------------------------------- 84 Figure A- 2. Data sheet of the paraffin RT44HC ---------------------------------------------------------- 85 Figure A- 3. The PCMS container design for experimental study ------------------------------------- 87 Figure A- 4. Photo of the equipment installed to perform the experiments ------------------------- 88 Figure A- 5. Digi-flow rate with technical details suitable for experiment -------------------------- 91

VIII

LIST OF TABLES

Chapter 1 TABLE 1- 1: AUSTRALIAN ENERGY CONSUMPTION - BY FUEL TYPE, AUSTRALIAN ENERGY STATISTICS (2020)

(DEPARTMENT OF INDUSTRY SEPTEMBER 2020) ................................................................................................ 9

TABLE 1- 2: SOLAR THERMAL TECHNOLOGIES & KEY CHARACTERISTICS (AUSTRALIAN RENEWABLE ENERGY)

(KEITH LOVEGROVE NOVEMBER 2019) .............................................................................................................. 12 TABLE 1- 3: THERMAL CONDUCTIVITY (TC) OF DIFFERENT MATERIALS (KASIRGA 2020)................................ 17 Chapter 2 TABLE 2- 1. COLLABORATION OF DIFFERENT TYPES OF PCMS (ZHOU ET AL. 2012) ......................................... 25 TABLE 2- 2. COMPARISON OF PROPERTIES OF SOME ORGANIC PARAFFINS (THOMAS BRAUN 2020) ............... 33 TABLE 2- 3. COMPARING THERMAL CONDUCTIVITY OF SOME METALS FOAM (CHINTAKRINDA ET AL. 2011;

EL IDI ET AL. 2021) ............................................................................................................................................... 40

TABLE 2- 4. COMPARISON OF THERMAL CONDUCTIVITY WITH DIFFERENT METAL MATERIAL ADDITIVES (LIN

ET AL. 2018) ............................................................................................................................................................. 41 TABLE 2- 5. EXPERIMENTAL STUDIES ON THERMO PHYSICAL PROPERTIES OF NANO-ENHANCED PARAFﬁN ... 44 TABLE 2- 6 . VARIOUS TYPES OF PCMS USED FOR THERMAL ENERGY STORAGE WITH VARIOUS SOLAR THERMAL COLLECTORS (DOUVI ET AL. 2021) ...................................................................................................................... 48

Chapter 3 TABLE 3- 1. COMPARISON OF PROPERTIES OF SOME ORGANIC PARAFFIN (THOMAS BRAUN 2020) ................. 56 TABLE 3- 2. PROPERTIES OF COPPER FOAM SELECTED FOR EXPERIMENTS (HANTER 2016) ............................ 57 APPENDIX TABLE A- 1. THERMAL ENERGY YIELD PER COLLECTOR MODULE {KWH/MODULE} AND HOT WATER DELIVERY [LITRES/MODULE] ................................................................................................................................................... 84 TABLE A- 2. FLOW RATES AND LOADING UNITES FOR HEATED WATER ................................................................... 86 TABLE A- 3. WATER AND GAS COPPER TUBES ACCORDING ASTM B88M- METRIC ............................................ 89 TABLE A- 4. THERMOCOPLE T-TYPE TABLE WITH WIDE RANGES OF VOLTAGE AND TEMPERATURE ................. 90

EXECUTIVE SUMMARY

Phase change materials (PCMs) for use in thermal energy storage applications can store

a large amount of thermal energy during melting through their latent heat and release

this heat back at an almost constant temperature during solidification. However, many

commonly used PCMs, such as paraffins, suffer from the major drawback of low thermal

conductivity that inhibits rapid heat transfer during their charging and discharging

modes (i.e. melting and solidification, respectively). This turns the enhancement of their

effective thermal conductivity to a research interest, with many studies reported on that

in the literature.

Amongst various enhancement methods introduced to date, the use of nanoparticles

and embedding metal foams (MFs) in paraffins have proven to be highly effective.

However, the literature review conducted as part of this research work indicated that

little has been reported on experimentally comparing or combining these two solutions,

particularly in the practical context of domestic water heaters. This gap shaped the

direction of the present study, its aim, and its objectives.

Focusing on this gap, two separate experimental studies were designed to conduct this

research. The first experiment was conducted to understand how above-mentioned

solutions are compared and whether if combining the two offers significant advantages

compared to using any of them separately. Hence, this investigation was done on small

scales about 65 g of PCMs at two constant temperature and constant heat flux

conditions.

With a constant temperature heat source, four samples, including pure paraffin (PP),

nanoparticles-enhanced paraffin (NP), metal foam enhanced paraffin (MFP), and

nanoparticles/metal foam enhanced paraffin (MFNP) were selected. In this experiment

hot water was used as the heat source (by considering the intended application of

domestic hot water supply). Temperature distributions, as well as melting and

solidification times, were measured for all samples under similar conditions to compare

and understand their performance enhancement effects. Initially, experiments were

performed in the pre-melting stage of PCM samples, and the heat transfer performance

of the samples from the best to the worst one was in the following order: MFNP, MFP,

NP, and PP. The experiments were then continued through the melting phase, and the

same order of performance was recorded. After the complete melting of all samples, the

experiments were continued until the samples reached 65-70 ͦ C, corresponding to what

is meant to be supplied by solar thermal collectors in domestic hot water applications.

Interestingly, at this stage, the PP sample showed the best heat transfer performance,

followed by MFNP and MFP samples. The reason for this behaviour involves the impact

of introducing nanoparticles and metal foam on suppressing the natural convection of

PCM in its liquid phase. The next step involved looking at the solidification behaviour of

the samples at a constant temperature. In reducing the temperature of the samples back

to the solidification point, the PP sample again showed the best performance, followed

by MFNP, MFP, and NP samples. However, during solidification, which was the core part

of the thermal energy discharging step, the samples were ranked as MFP, MFNP, and NP

in terms of their heat transfer performance. The details of the abovementioned rankings

and behaviour have been discussed in a paper published in Journal of Energy Storage,

as presented in chapter 4 of this thesis.

The experiments were also done on larger size samples about 400 g of PCMs using an

electric heating pad to supply a constant flux of heat. The samples were placed in a

cylindrical container. The heat generated by the thermal pad was supplied to the

samples from the bottom of the container, where the melting front started to propagate

throughout the samples, and the melting times were measured. The melting part of the

experiment ranked the samples (from the best to worst) as NP ، MFP, MFNP, and PP.

This is while the NP sample was not found to be able to offer a sustained good

performance since the particles settle as the PCM goes through multiple

charging/discharging cycles. On the other hand, the MFP samples showed a marginally

better melting performance than the MFNP sample, suggesting that using MF is a better

and more practical approach than introducing nanoparticles (with or without MF) for

improving the heat transfer performance of paraffin based PCMs (i.e. during melting

step). The experiments were continued by removing heat from the samples using flow

of water, to understand their solidification behaviour. Overall, the findings of the

experiments conducted on these four isolated samples (i.e. outside the context of the

intended application of solar water heating) can be summarised as follows:

The samples in which metal foam was used to boost the heat transfer mostly performed

the best by considering the long-term sustainability of the improved performance

achieved for them. Although in some cases MFNP sample showed good performance as

well, its superiority over the MFP samples was only marginal that could not justify the

additional cost and complexities associated with introducing nanoparticles to the MFP

sample. It is noteworthy that during the melting/solidification processes of PCMs, due

to their volumetric expansion, hollow spaces were created within the body of the PCMs

that caused discontinuity of the heat transfer routes and affected the functionally of the

PP sample. However, introducing metal foam and nanoparticles (particularly when they

were introduced together) helped significantly with addressing this challenge. By

considering the outcomes of this phase of the experimental study, the metal foam option

was selected as the most effective and practical approach for heat transfer enhancement

of the paraffin PCM energy storage unit used for this study. This option was then further

studied experimentally together with evacuated tube solar thermal collectors.

Solar thermal collectors can only deliver their function when they receive solar

radiation. However, an effective thermal energy storage arrangement is required to

extend the period of heat supply of these collectors. This study was scoped around this

application by using paraffin based PCM used for storing excess heat and releasing it

when the collectors are unable to meet the demand. The next phase of the experiment

has looked at the performance of PP as energy storage for solar thermal collectors and

quantify the benefit of using MFP based energy storage solution in this context (i.e.

compared to the base case of using PP thermal energy storage unit). It is important to

emphasise that most of the previous studies on thermally enhanced PCMs were on

isolated cases, and the use of PCMs in solar thermal systems (evacuated tubes in this

case) has been limitedly investigated in the literature. This major gap is identified and

addressed through a separate experimental study. In this experiment, a solar thermal

collector unit was used, and the heat supplied by the collectors was stored in a PCM unit

(during the day) and released when the supply from the collectors was insufficient to

meet the demand (e.g. during the night). Directed by the first phase of the experiment

on isolated samples, MFP based PCM thermal storage unit was selected for this phase to

be compared with the base case of pure paraffin. Obviously, in this phase of the

experiment, the hot water supplied by the collectors was the heat transfer agent. The

experiments were conducted at three different water flow rates of 1 l/min, 2 l/min and

3 l/min.

The results of the experiments suggested that introducing MF to paraffin helped reduce

the time required to take the paraffin to its melting point by around 50%, making the

system more responsive to deliver its function faster (by 2-3 times). It also helped

reduce the melting time by an average of 30%. By introducing metal foam to the sample,

more uniform temperature across the storage unit was achieved at all operating

conditions that the storage unit was tested at. It is noteworthy that in applications such

as domestic hot water supply, less fluctuations in hot water temperature are desired,

and the uniform temperature distribution achieved here serves towards this purpose.

The effect of introducing MF is particularly important during the solidification stage (i.e.

PCM thermal discharging). This part was done by simulating the worst-case scenario for

the storage system (that is zero supply of heat by the collectors during the night) by

completely covering the collectors after fully charging the thermal storage unit. The

results of this part of the experiments showed that pure PCM alone was not able to heat

the water flow effectively, and only lukewarm water at a low flow rate could be

maintained for a short period. However, this issue was addressed by introducing MF at

all flow rates. For example, at the flow rate of 1 l/min, the inlet water experienced up to

77% temperature increase, and this flow could be maintained for two hours. The only

negative impact observed for the metal foam paraffin PCM unit was an almost 10%

reduction in its storage capacity due to the fact that some paraffin content was removed

from the storage container to allow for accommodating the foam material, given that

the overall volume of the storage unit was kept the same.

The use of thermally enhanced PCM by introducing metal foam, showed promises to

replace the traditional bulky option of hot water based thermal energy option.

However, as for recommendations for future research of this topic, this solution can be

further studied by focusing on its optimisation for different applications. For example,

the type of MF and its characteristics or the shape and design of the thermal storage unit

can affect the performance of the unit in different application contexts. Moreover,

theoretical modelling of the system can facilitate a comprehensive parametric study and

help create new understandings of how different design parameters of both supply

(collectors) and storage (PCM) units interact to determine the overall performance of

the system. Moreover, the outcomes of such a model can be further studied and

validated experimentally.

NOMENCLATURES and ABBREVIA

Nomenclature

Specific heat capacity (kJ/kg.C) PCM liquid fraction Solar irradiance (kW/m) Heat transfer coefficient (kW/m2.C) Average enthalpy difference (kJ/kg) Specific heat (kJ/kg) Degree Centigrade Litter per minutes Solar fraction Time Temperature Average temperature difference (C)

Sensible heat storage Thermal Conductivity Solar thermal collector Solar Collector heat pipe Heat transfer Melting point Radiation Solid-state Outlet Solid Inside Flowrate Flowmeter

Copper Copper Oxide

Overall heat transfer coefficient (kW/m2.C) Abbreviations

Aluminium

Cp F G H Δh Q °C L/min SF T T ΔT SHS TC STC SC Hp HT Melt R S O Sd In FL Fm U

Evacuated tube solar collector

ETSC-PCMS Triplex tube heat exchanger

Nickle Latent Heat Energy Storage Valve Sensor of Vertical Sensor of Horizontal Sensor on wall Charge and Discharge

ETSC Cu CuO Al Ni

HTF LHS TES PCMS PP N MF MP NP LHES V SV SH SW C-D

Heat transfer fluid latent heat storage Thermal energy storage Phase Change Materials Pure paraffin Nanoparticles Metal Foam Metal foam & Paraffin Nanoparticle & Paraffin Metal foam &Nanoparticles & Paraffin

MFNP

1. INTRODUCTION

1.1. Background

In recent years, the rapid economic growth in the world and the increase in comfort

parameters have increased productivity and energy consumption. However, fossil fuels

still dominate the world energy market, with a share of about 80%, and their exploitation

entails relatively high environmental and economic costs that are now the biggest

concerns of society (da Cunha and de Aguiar 2020a). Hence two issues are being

considered: First, the use of fossil fuels leads to the release of harmful greenhouse gases

into the environment, which contributes to today's environmental concerns; second,

fossil fuel energy sources are limited, posing a threat to the future of energy supply.

Therefore, the optimal use of energy and the possibility of utilising renewable energy

sources are of great importance. (Kalnæs and Jelle 2015).

Based on the report of the Department of Industry, Science, Energy, and Resources

(Department of Industry September 2020), Australian Energy Statistics, Australians met

about 94% of their total energy needs by using fossil fuels (i.e. coal, oil, and gas) in 2018-

19. Australia's energy consumption has risen to 6,196 petajoules/year, showing

consistent increases in recent years. Oil has the largest share for energy in Australia at

39%, followed by coal with 29% and natural gas with 26% that leaves 6% to be supplied

through renewables (Table 1- 1). However, apparently following various renewable

energy targets at Federal, and States levels, the share of renewables is creeping up.

Table 1- 1: Australian energy consumption - by fuel type, Australian Energy Statistics (2020) (Department of Industry September 2020)

2018- 2019

Average annual growth

Source of Energy

Petajoules

2,402.10 1,801.60 1,592.70 399.60 6,196.00

Share (%) 38.8 29.1 25.7 6.4 100

2018-19 1.3 -2.5 2.2 4.6 0.6

10 years (%) 1.7 -2.5 2.7 3.9 0.7

Oil Coal Gas Renewables Total

Australia's coal-fired power plants account for 42% of Australia's greenhouse gas

emissions. Based on the Intergovernmental Panel on Climate Change (IPCC) reported in

2017, the result of the overuse of fossil fuels was that Australia alone produces about 400

million tonnes of CO2 per year (with a population of less than 25 million) (Keisuke

Sadamori Aug 2021). This amount of production in compared to the United Kingdom,

which is about 410 million tons of tons of CO2 with a population of 66 million, and Turkey

with a production of 398 million tons of CO2 suggests a high level of greenhouse gas with

(GHG) emission production per capita (i.e. 16 tonnes of greenhouse gas per year per

capita) (Figure 1- 1).

2017 Co2 emission per capita

a t i p a c r e p d e t t i

m E 2 o C f o s n o T

Austrailia

world

USA

China

European Union

Figure 1- 1: Co2 Emission per capita in 2017 of IPCC (Keisuke Sadamori Aug 2021)

The IPCC 2020 report indicated that the largest producer of GHG emissions in Australia

is the residential sector, with that used or water heating being a major contributor, i.e.

28% of home energy consumption. The Australian Government estimates that installing

a climate-friendly solar-powered hot water system can save a family up to $300-700 each

year.

There are different types of renewable energy sources in nature, such as solar energy,

wind energy, and geothermal energy, etc. Solar energy is one of the most stable and

cheapest forms of energy provided by nature that can supply a lot of thermal energy

during sunny days. However, the major challenge is how to make solar energy a reliable,

stable part of Australia's energy future, given the intermittency of that in supplying

energy. This essential question guided this study, as it is explained in the upcoming

sections of this chapter.

1.2. Solar thermal collectors and thermal energy storage

solutions

Solar heating is a cost-effective technology for providing renewable heat. This technology

has shown rapid growth globally in recent years (Tschopp et al. 2020). A general review

of the statistics of the amount of energy produced in the world in 2017 reported by IEA

(2019) is shown in Figure 1- 2, in which the solar thermal and electrical systems (i.e. by

solar thermal collectors photovoltaic panel, respectively) produce less than 2% of the

total energy in the world (Roser 2019). Although this ratio in recent years has reached

Figure 1- 2: Australia's primary energy supply by share in 2017. Data: Calculated using IEA (2019) online free version. This dataset is the only available that shows all energy sources. (Roser 2019)

about 4%, the full potential of this technology is yet to be unleashed (Bhusal et al. 2020).

A solar thermal collector receives solar radiation and converts it to heat, which in turn

can be used directly, stored, or used to produce fuels or electricity. Solar thermal

technology solutions must be optimised for required temperature ranges.

The range of collector technology options available and heat transfer fluids (HTF) that

can be used are summarised in Table 1- 2 (Agency 2019). A suitable collector is selected

by considering the required temperature and volume of hot water to be supplied.

Evacuated tube solar collector is a kind of solar thermal technology that received

increasing attention in recent years due to covering a wide range of temperatures,

relatively low cost, and high efficiency that make them suitable options for use in a variety

Table 1- 2: Solar thermal technologies & key characteristics (Australian Renewable Energy) (Keith Lovegrove November 2019)

Collector Type

Temperature

Note

Unglazed flat plate

20 – 40 ͦ C

Fabricated from rubber or PVC tubing. HTF: water, air

Glazed flat plate

30 – 85 ͦ C

HTF: water, air, glycol

Evacuated tube

50 – 150 ͦ C

HTF: water, glycol

Parabolic trough

100 – 450 ͦ C

HTF: water, steam, synthetic oil (molten salt)

Linear Fresnel

100- 450 ͦ C

HTF: water/steam, synthetic oil (molten salt

of applications (Aramesh and Shabani 2020b).

Overall, Australia is among the countries with the highest solar irradiation in the world.

Higher irradiation directly translates to higher penetration and hence lower cost of solar

thermal technologies (Panchal et al. 2019). However, the radiation can be interrupted

due to the presence of clouds and moisture in the air, and of course at making the

collectors unable to produce hot water that highlighted the importance of developing

reliable thermal energy storage solutions to support solar thermal collectors (Panchal et

al. 2019). The storage solution captures the excess thermal energy generated during

sunny periods and stores it to be released during periods of low supply by the collectors

(e.g. evenings). Thermal energy storage solutions play an important role in making solar

thermal solutions more practical for the end-users and hence their widespread adoption

(Khan et al. 2018).

While there are many approaches to thermal storage under development worldwide,

there are only a few commercially available. Researchers have widely considered phase

change materials (PCMs) as a promising solution to store thermal energy in the form of

latent heat that supports the continuity of supply in solar thermal systems (e.g. those used

for hot water supply). This is further discussed in the following section.

1.3. Phase change materials

1.3.1. Overview

Phase Change Materials (PCMs), which release or absorb thermal energy during melting

and solidification processes, offer an excellent capability to store a massive amount of

heat during their phase change processes (Lin et al. 2018). The thermal storage

mechanism of PCMs is shown in Figure 1- 3 (Soares et al. 2013). Latent heat storage has

a much higher energy density than sensible heat storage, resulting in less required

material mass and/or smaller storage tank volumes. Latent heat storage systems are also

easier to work with other types of thermal storage solutions (e.g. thermochemical

storage). The latent heat storage method with a more negligible temperature difference

between storing and releasing heat, provides a much higher storage density, unlike the

Figure 1- 3: Energy vs. Temperature plot showing the isothermal heat storage effect of PCMs (Soares et al. 2013).

sensible heat storage method. (Kuznik et al. 2011).

Storing and releasing heat energy at the same temperature by changing the phase, i.e.

isothermal energy storage, is a feature that makes PCMs an attractive option for a wide

range of applications (El Idi and Karkri 2020). This isothermal heat transfer enables PCMs

to reduce system outlet temperature fluctuations in systems that use PCM thermal energy

(Mavrigiannaki and Ampatzi 2016). Overall, latent heat-based energy storage solutions

(i.e. using PCMs) offer various advantages such as high energy storage for a given volume,

uniform energy storage/supply, flexibility to be defined for specific temperature ranges,

isothermal supply of heat, etc. that make them very attractive.

It must be considered that paraffins have weaknesses such as low thermal conductivity

and high-volume expansion that limit their use in various applications. (Titin Trisnadewi,

2020) Titin et all and Many studies in recent years have been dedicated to investigating

the low thermal conductivity challenge of PCMs such as paraffins ( El Idi and Karkri 2020;

Kasirga 2020). On the other hand, various methods are discussed in the literature to

overcome this challenge (Huang et al. 2019), that will be further discussed in this chapter.

There are also some other challenges associated with PCMs, such as foamability and

significant volume variation during phase change (Sevault et al. 2017). The latter is

briefly covered by this study.

1.3.2. PCM properties and paraffin based PCMs

An ideal PCM must have desirable thermophysical properties (Mohamad et al.) (da Cunha

• long-term thermal stability;

• high melting time;

• effective heat-transfer during the phase change to store maximum energy;

• high latent heat of phase change to increase its energy storage capacity;

• high thermal conductivity in both solid and liquid phase;

• transmission temperature in the range of the desired operating temperature;

• no separation during phase change;

• high density; and

• minimum volume change during phase change, to avoid disconnection in heat

and de Aguiar 2020a), such as:

transfer pathways.

• non-corrosive;

• non-flammable;

• non-toxic; and

• environmentally friendly.

They are also preferred to be: (Sevault et al. 2017):

In fact, pure PCMs may not have all these features. In this study, the selected PCM is for

use in residential hot water applications, i.e. together with solar energy systems. An

option with as many as the features mentioned above is preferred suggesting paraffin to

be an attractive option (Cabeza et al. 2011; Dhaidan 2021). Paraffin, in particular, is a low-

cost option that offers a high heat storage capacity, great chemical stability, and absorbs

and releases heat at relatively constant temperatures (Wang et al. 2019a). However, with

phase change, it loses its structural integrity that impact its heat transfer performance.

Paraffins also suffer from poor thermal conductivity (Wei et al. 2018; Aramesh and

Shabani 2020b). The latter is one of the most significant disadvantages of paraffin, which

hinders the widespread adoption of this option. Therefore, many studies have been

dedicated to date to improving the effective thermal conductivity of paraffin based PCMs

(Wei et al. 2018; Aramesh and Shabani 2020b) is an important consideration.

Table 1- 3, shows the thermal conductivity of different materials, indicating how low the

thermal conductivity of paraffin is, i.e. almost the same as some insulating materials

(Chen et al. 2014). Moreover, phase change in paraffin causes them to lose their structural

Table 1- 3: Thermal Conductivity (TC) of different materials (Kasirga 2020)

integrity, which further reduces their effective thermal conductivity.

Metals

PCMs

Substance

TC (k) (W/m°k)

TC (k) (W/m˚k)

TC k (W/m°k)

1 Aluminium Polyglycol E600 0.190 205 Brick 0.71

2 Copper Capric Acid 0.153 400 Glass 0.80

406 Silver Paraffin C13-C124 0.210 Water 0.60 3

50.2 Steel/SS Paraffin RT27 0.200 Nylon 0.25 4

109 Brass Paraffin wax 0.221 PVC 0.19 5

Iron 80 Paraffin Rt100 0.200 Epoxy 0.17 6

0.15 Diamond 1000 CaCl2,6H2O 0.56 Wood 7

Gold 314 NaSO4,10H2O 0.544 Asbestos 0.08 8

Mercury 8.3 Mg (NO3)2,6H2O 0.490 Fiberglass 0.04 9

1.3.3. Thermal conductivity improvement of paraffin based PCMs

It was discussed that the enhancement of the thermal conductivity of paraffin-based

PCMs, especially when they are in solid phase (Huang et al. 2019), is one of their most

urgent challenges to be addressed in order to pave the way for wider employment of

these materials in thermal energy storage applications (Fleischer 2015b; Zarei et al.

2020a).

Recent studies have demonstrated that the integration of paraffin based PCMs with some

other materials can help improve their thermal performance by improving their overall

effective thermal conductivity. These studies mostly suggested five ways to increase the

thermal conductivity of PCMs:

• Introducing nanoparticles to PCMs;

• Using metal foam and mesh materials;

• Embedding large surfaces in PCMs (e.g. fins);

• Using heat pipes; and

• Using shaped containers.

The extensive literature review conducted as part of this study showed that the first two

solutions have been very popular and received significantly more attention than others

(Wu et al. 2020). This is mainly because of their effectiveness in enhancing the heat

transfer performance of PCMs and the ease of their implementation. However, while

many research studies have been conducted on these options for various different cases,

only a limited number of studies were conducted systematically to compare these two

solutions on similar cases and/or in similar applications, e.g. solar water heating (He et

al. 2019). Moreover, few studies can be found in the literature looking at simultaneous

use of these two options (Yang et al. 2019b). It is noteworthy that the literature suggests

that enhancing the thermal conductivity of PCMs in an important consideration when

they are used to support solar water heating system as this help the PCMs with absorbing

and releasing heat at the required rate. In other words, the excess heat from the collectors

can be absorbed by the PCM unit as it becomes available for storage. Later, the heat is

released quick enough from the PCM unit as demanded by the end user (i.e. supplying hot

water at the right rates).

1.4. Research Gap

Based on the literature review conducted to support this research, the following major

gaps are identified:

• The existing literature did not detail the effect cycling on the performance of

PCMs (e.g. paraffin) that are thermally enhanced using nanoparticles.

• There is only limited understanding of how metal foams and nanoparticles can

• Most studies have been conducted to date are on various isolated samples and

work together to improve the effective thermal conductivity of PCMs.

no experimental studies looked at paraffin based PCMs with enhanced thermal

properties in the applied context of solar thermal systems.

1.5. Aim, Objectives, and Scope

1.5.1. Aim and objectives

This project aims to experimentally study the thermal conductivity enhancement of

paraffin based PCMs by using metal foam materials and nanoparticles, alone and

together, as well as the advantages offered by these options when used in the context of

solar thermal applications (i.e. for domestic hot water supply). Following this aim, the key

objectives of the project are thus to:

• understand the state of development of enhanced PCMs (with special focus on

paraffin) in which nanoparticles and metal foams are used to improve their thermal

conductivity;

• experimentally quantify the long-term performance of PCMs in which nanoparticles

are used for enhancing their thermal conductivity (i.e. the effect of cycling);

• experimentally study the effect of adding metal foams on effective thermal

conductivity of PCMs that have already been improved by dispersing nanoparticles;

• quantify how using PCMs with enhanced thermal conductivity, when used as thermal

• provide recommendations on the future study of using thermally enhanced PCMs for

energy storage, can enhance the performance of a solar thermal system; and

solar thermal applications

1.5.2. Research Scope

The following boundaries are considered in pursuing the proposed research study:

• This study is conducted on an experimental basis.

• Domestic hot water supply using evacuated tube solar thermal collectors has

been selected as the applied context for this study.

• Paraffin has been chosen to be used as a PCM for this study due to its attractive

techno-economic properties for use with solar thermal systems in domestic hot

water applications.

• The effect of scale factor is not studied.

• The effect of the shape of the PCM container is not investigated in this study.

1.6. Research Questions

The main research questions that are addressed by this research include:

1) How much improvement in thermal conductivity of a selected PCM (i.e. suitable

for use with solar thermal hot water systems) can be achieved, when metal foams

and nanoparticles are used separately and simultaneously?

How melting and solidification cycles can impact the stability and hence the 2)

effectiveness (i.e. in improving the thermal conductivity) of using nanoparticles

dispersed in PCMs, in terms of maintaining the improved thermal performance?

How do the thermal conductivity enhancement measures work in the context 3)

of a solar thermal hot water system in terms of the improved experience of the end

user?

4) What are the challenges associated with using nanoparticles and metal foam in

PCMs, and how these challenges can be addressed?

1.7. Research Outcomes

The major outputs of this study include:

• Experimental data on comparing nanoparticles and metal foam solutions for

thermal conductivity enhancement of a selected PCM.

• Experimental data on using metal foam based enhanced PCMs as thermal energy

storage employed to support a solar thermal hot water system.

• A thesis reporting the details of this study focusing on above-mentioned objectives

and research questions.

• Two journal papers published in Q1 Journal of Energy Storage, reporting on the

findings of this experimental work.

1.8. Thesis Structure

The thesis has been arranged into six chapters. The present chapter provides an overview

of thermal energy storage applications and their challenges to support renewable

thermal energy systems. This chapter though, highlights the research gap in a systematic

manner to sets the aim and objectives of this work to address the identified research gaps.

The research questions to be answered are presented accordingly, and the scope and

expected outcomes of this study are stipulated in this chapter. This is followed by a

detailed targeted literature review in chapter 2 that elaborates on the state of the article

and the research gaps introduced in chapter 1. Chapter 3 reviews the research method

and provides details on how this research is planned to address the aim, objectives, and

research questions outlined in chapter 1. The thesis will then be supported by two journal

papers in chapters 4 and 5 in which the core objectives and research questions of this

study are dealt with. The first paper to be presented in chapter 4 reports the details of

studying the use of metal foam and nanoparticles to enhance the heat transfer of phase

change materials using small scale isolated samples. This experimental study helps

narrow down the options to be used in the context of the solar thermal system that is the

next phase of the study to be reported in chapter 5. Chapter 5 then provides the details

of another experimental study in which metal foam was used to enhance the performance

of a PCM unit used in a solar thermal hot water system. The thesis is then concluded in

chapter 6, where the research questions posed in chapter 1 are exclusively addressed,

and direction and recommendations on future research on this topic are provided.

2. LITERATURE REVIEW

2.1. Overview

This chapter systematically reviews the literature to provide an overview of the state of

PCM, its advantages and disadvantages, the drawback of PCM can make using that

challenging. The low thermal conductivity challenge of PCMs, its high cost, flammability

and low melting point are the limitations of using pure paraffin, but research to date has

shown that there are ways to overcome these challenges. With respect to low thermal

conductivity, the main methods presented to improve the thermal conductivity of PCM to

date include I) adding nanoparticles in PCM, II) embedding metal foam in PCM, III) using

heat pipes in PCM, and IV) inserting fins inside the paraffin. These methods are briefly

discussed in this chapter. Moreover, preparation of thermally enhanced PCMs is

reviewed, and finally, an overview of solar thermal collectors (STCs), which is the applied

context focused by this thesis, is provided. This literature review further elaborates on

the research gap stipulated in chapter 1 of the thesis.

2.2. Phase Change Materials (PCMs)

2.2.1. PCMs Classification

Materials used as thermal energy storage media that use latent heat as the main

mechanism for storing heat are known as phase change materials (PCMs). PCMs may have

transitioned from a solid phase to a liquid phase, from a solid phase to a gaseous phase,

or from the solid to gaseous phase by evaporation (Chieruzzi et al. 2013). While solid to

gas and liquid to gas transitions have phase changes with a high enthalpy of evaporation

and high energy transfer, challenges such as large volume changes during phase change

hinder their practicality for use in energy storage applications (Elias and Stathopoulos

2019). Such challenges are less pronounced on solid-liquid PCMs (Liu and Ma 2002;

Chieruzzi et al. 2013), while they release or absorb a large amount of thermal energy

during the melting and freezing processes (Aramesh et al. 2019; Afshan et al. 2020a; da

Cunha and de Aguiar 2020a; Sun et al. 2020; Dhaidan 2021).

Figure 2- 1 shows how solid-liquid PCMs store/release heat as their temperature varies.

In particular, the functionality of PCMs is driven by their capacity for storing thermal

Figure 2- 1. PCMs temperature vs. stored heat for an LHS system (Douvi et al. 2021)

energy through latent heat (LH) (Huang et al. 2017).

A wide range of PCMs, including organic, inorganic, and eutectic compounds, are made

from organic and/or inorganic materials (Mosaffa et al. 2013). Organic, inorganic, or

eutectic are often used in waste heat recovery systems and buildings, while inorganic or

eutectic can be used in solar power plants and other high temperature applications

(Chandel and Agarwal 2017). Organic PCMs, include mainly paraffin and other non-

paraffin substances such as esters, fatty acids, alcohols, and glycols. The category of

inorganic PCMs consists of salt hydrates and metallics, and finally, eutectics can be

mixtures of organic-organic, inorganic-inorganic, and inorganic-organic PCMs (Javadi et

Figure 2- 2. Classification of PCMs (Lin et al. 2018)

al. 2020). Figure 2- 2, summarises the classification of PCMs.

As presented in Table 2- 1, each of these PCMs offers advantages and disadvantages

Table 2- 1. Collaboration of different types of PCMs (Zhou et al. 2012)

Classification

Advantages

Disadvantages

Organic PCMs

1. Low thermal conductivity (around 0.2 w/m ͦk). 2. Flammability 3. Relatively significant volume change

1. High heat of fusion 2. Availability in a large temperature range 3. No supercooling 4. Good compatibility with other materials 5. Chemically stable and recyclable

that will be further discussed in the following sections.

1. Supercooling 2. Corrosion

Inorganic PCMs

1. High heat of fusion 2. High thermal conductivity (around 0.5 W/m˚K). 3. Low volume change 4. Availability in low cost

Eutectics

Lack of currently available test data of thermophysical properties

1. Sharp Melting temperature 2. High volumetric thermal storage density

PCMs are selected by considering the intended applications as well as the following

points (Alva et al. 2017; Huang et al. 2017; Lin et al. 2018):

• Phase transition temperature (i.e. to meet the needs of the intended application)

• latent heat storage capacity (i.e. to satisfy the application requirement)

• Thermal conductivity (be enhanced if needed)

• The stability of chemical and thermal properties

• They must be non–toxic, non–corrosive, and harmless to the environment

• Cost and availability

• Volume change (to be minimum)

• Supercooling (to be avoided)

Organic PCMs

2.2.2.

Organic PCMs can be described as paraffin and non-paraffin substances. Organic

materials are characterised as chemically stable (Figure 2- 3). They are made of chains

of carbon and hydrogen atoms. Pure paraffins generally contains 14- 40 carbon atoms,

and their melting points depend on the number of carbon atoms that make the chain (da

Cunha and de Aguiar 2020a).

Paraffin-type organic PCMs are cheap and have high heat of fusion (usually more than

170 Kj/kg). No supercooling is associated with them; they are non-corrosive, chemically

inert, and show low volumetric changes. They were also reported to be stable at

temperatures below 500 ° C (Kalnæs and Jelle 2015). All these properties make them safe

Figure 2- 3. Organic PCMs (Paraffin) (Baetens et al. 2010)

and reliable to be used in a wide range of thermal applications.

Inorganic PCMs

2.2.3.

Inorganic compounds are classified as hydrated and metal salts. Hydrated salts have been

extensively studied in thermal energy storage (TES) systems (Figure 2- 4). They have

high phase-change temperatures, are non-flammable, and their thermal conductivity is

higher than organic PCMs (~0.5 W/m ͦ K). Moreover, they exhibit small volume changes

during phase change. However, they suffer from disadvantages such as over-cooling,

corrosion, high cost, phase separation, thermal instability, and high weight that impact

their popularity for use in a wide range of TES applications. This is while some of their

advantages, such as small volume changes, high heat of fusion, and high thermal

Figure 2- 4. Inorganic PCMs (salts) (Baetens et al. 2010)

conductivity, are favourable (Baetens et al. 2010).

2.2.4. Eutectic mixtures

Eutectic compounds have some superior properties; for example, the process of

preparing eutectic mixtures is very simple. They only need to mix two different

compounds mechanically and don`t require any environment. They are relatively

environmentally safe. However, eutectic compounds are relatively rare. Generally

eutectic compounds have high thermal conductivity (i.e. good heat transfer), wide

melting point range, high volumetric heat storage density, and symmetrical phase change

compared to organic and inorganic paraffins. Compared to water in the melting range

(phase change time), eutectic compounds can store an enormous amount of thermal

energy in the form of latent heat. On the other hand, these compounds have

disadvantages such as low specific heat, high cost and pungent odor. (Zhou et al. 2012).

Little experimental data are available in the literature on thermal-physical properties of

eutectic compounds. Some eutectic compounds are shown in Figure 2- 5.

Figure 2- 5 . Eutectic compounds (The binary eutectic chloride/EG composites %5 (a) and %20 (b)) (Tian et al. 2016)

2.3. Low Thermal Conductivity: A Major Challenge of PCMs

From the point of view of the environmental properties, PCMs must have low energy

consumption associated with their production, ease of separation from other materials,

high recyclability, and low environmental impact. Finally, regarding the economic

viewpoint, ideal PCMs must be available and abundant at low costs in order to offer

competitiveness for use in thermal applications (da Cunha and de Aguiar 2020a). Paraffin

can meet many of these requirements; however, their low thermal conductivity remains

to be their major challenge (Table 1-3). The thermal conductivity of paraffin is low

compared to many other materials, sometimes close to materials used for thermal

insulation, i.e. ~0.2 W/m K (Singh et al. 2018c).

This low thermal conductivity makes the heat transfer rate in PCMs very low, especially

when they are in the solid phase (Kok 2020b; Zarei et al. 2020a). This shows that the

process of storing energy, i.e. charging PCMs, involves moving from the solid phase to the

liquid phase. In addition, as the stored thermal energy is released, solidification of PCMs

in areas close to the heat reservoir reduces the rate of heat transfer from other areas to

the heat reservoir. In many cases, because of the low thermal conductivity, PCMs are not

able to release the stored thermal energy at the right rate required by the demand side

(Kalnæs and Jelle 2015; Zhang et al. 2020a).

2.4. Enhancing the Thermal Conductivity of PCMs

2.4.1. Overview of enhancement techniques studied to date

Increasing the thermal conductivity of PCMs is one of the topics that has been the subject

of scientific research in recent years. It is widely accepted that the low thermal

conductivity of materials with high energy storage capabilities is one of the significant

barriers to the wider implementation of thermal energy storage applications (Fleischer

2015b). The high amount of heat absorbed by the paraffin in the melting process is

released into the surrounding area in a cooling process that starts at the PCM’s

crystallization temperature (Mondal 2008). A typical differential scanning calorimetric

(DSC) heating thermogram for PCMs melting is schematically shown in Figure 2- 6.

Figure 2- 6. Schematic of DSC Heating Thermogram of PCMs (Fleischer 2015a)

Low thermal conductivity and large volume variation during phase change are the main

disadvantages of the PCMs that affect their performance in TES systems (Wei et al. 2018;

Yang et al. 2019b). In particular, paraffins, which are commonly used organic PCMs, come

with significantly low thermal conductivity. Hence, and increasing their thermal

conductivity is one of the most critical issues in PCM for energy storage applications. As

indicated in the literature, there are five popular methods to increase the effective

thermal conductivity of PCMs, (Figure 2- 7) (Tian 2012). This includes:

1) Adding macroscale metallic structures such as meshes or foam (metal foam).

2) Introducing nanoscale materials to create colloidal PCMs suspensions with

improved thermal properties such as carbon particles, Aluminium particles,

Copper particles, etc.

3) Optimizing the design of the PCM container by considering different shapes such

as rectangular cavities, spherical, etc.

4) Embedding extended surfaces in PCMs such as fins; and

Figure 2- 7. Sample of Nanoparticles( Copper oxide powder, 20 nm)(a), Metal foam (copper foam, 10 PPI)(b), Fin ( Copper fin)(c), and Heat pipe (High-Performance Copper Heat Pipe )(d) for enhancing thermal conductivity of PCMs, (Tian 2012; Socaciu et al. 2016; Li et al. 2017)

5) Using heat pipes.

The first two in the list, in particular, received significant attention by the literature

(Socaciu et al. 2016) and reported to be relatively more effective or easier to implement

than other methods, especially in reducing the melting time and freezing times of PCMs.

Although many studies were reported on these two methods in the literature,

comparative experimental studies (i.e. by applying the two methods on the same case)

and the effect of simultaneous application of these methods have not been reported in

the literature. In particular, no experimental data has been reported in the literature on

the effectiveness of these methods in the applied context of domestic solar water heaters.

It is noteworthy that these solutions also come with challenges that need attention. For

example, although nanoparticles within PCM lead to better thermal properties, it is hard

to keep nanoparticles-PCM composites stable in terms of thermodynamic properties, as

the composite goes through multiple thermal charging/discharging cycles (Alva et al.

2017). The addition of metal foams also considerably enhances the heat transfer

properties of PCMs, specifically by increasing the overall thermal conductivity of the

PCMs unit. However, according to the literature, depending on the PCMs thermal and

geometrical specifications, the addition of metal foam can impact the capacity of the

composite in storing heat (by keeping the volume constant); moreover, the presence of

metal foam can impact the natural convection of the PCM in is the liquid phase that will

be discussed further. Overall, there must be an optimum metal foam porosity, the lower

or higher levels of which would be less effective in improving the heat transfer process

(Lin et al. 2018). Such system modelling and optimisation remain out of the scope of this

study while can be pursued in future studies.

As mentioned earlier, paraffin was selected from among other phase change materials for

this study. Based on the purpose of this study, to provide domestic hot water, RT44HC

paraffin was selected from hundreds of other types of paraffin according to its thermal

conductivity, melting time, and heat transfer properties. Because this paraffin has

proximity properties to the objectives required for this project (Table 2- 2. Comparison

Table 2- 2. Comparison of properties of some organic paraffins (Thomas Braun 2020)

Paraffins

RT42

RT47

RT44 HC

RT50

RT54HC

TR55

RT62HC

Melting Area (ͦC)

38-43

41-48

41-45

45-51

53-54

51-57

62-63

165

160

250

160

200

170

230

Heat Storage capacity (KJ/Kg ͦ k) Latent Heat (W/m ͦ k) Specific Heat capacity (KJ/Kg ͦ k)

of properties of some organic paraffins (Thomas Braun 2020).

2.4.2. Using metal foam to enhance the thermal performance of PCMs

A very common and effective method that has been practiced to date to enhance the

thermal performance of PCM is the addition of metal foam (Wu et al. 2020). Metal foams

are especially effective solutions for improving the thermal conductivity of PCMs due to

their high surface / mass ratio (Figure 2- 8). When metallic foams are used, heat flows

along a metallic path, in this case, the ligaments of the foam, and then into smaller

separated masses of PCMs. In other words, heat transfer is distributed through a smaller

portion of PCM rather than its entire body, which increases the heat transfer rate,

Figure 2- 8. The formed skeleton structure of the metal foam and the extracted computational domain. Computational domain (Wang et al. 2019b).

particularly when the PCM is in the solid phase.

One of the earliest studies reported on using metal foam (copper) with porosities of 85%

and 95% and pore densities of 10 PPI and 30 PPI, for enhancing the thermal performance

of paraffin (RT58) is the experimental one conducted by Tian and Zhao (2011a), in which

a constant heat flux source was used. They found that the addition of metal foams can

considerably enhance the PCM heat transfer rate by up to 10 times.

In a similar experiment conducted by Yang et al. (2016a), similar results were reported.

They used paraffin wax as a PCM in their experiments, and copper foam with 0.92

porosity and a pour density of 20 PPI was embedded in that. The paraffin composite and

metal foam were placed in a rectangular container covered by coated with plexiglass

sheets (k = 0.042 W / m °C) to minimise heat loss. Water at high temperature was used

to create as a heat transfer fluid passed through the pipes within the PCM sample.

Temperature changes were recorded in different parts of the PCM sample. Experimental

results showed that PCM composite with metal foam took more than 1/3 less time to melt

completely than the pure PCM. They also reported a more uniform temperature within

the PCM after the metal foam was introduced.

Through another research, Li et al. (2017) conducted a study to increase the thermal

conductivity of PCMs by placing metal foam in them. In this study, copper foams with two

porosities of 0.9 and 0.95and the pore size of 10 PPI were used as the porous material.

This experimental system was designed to evaluate the thermal performance of PCM

metal foam composite, which consists of three parts: the D.C. power supply, the test

section, and the data collection system, in which an electric heater pad (model: 50 V / 50

W, electric heater) was attached to the outer surface of the PCM tank to generate thermal

energy. The container containing PCM was insulated with plexiglas polyurethane

material (k = 0.02 W/m ͦK) with a thickness of 40 mm. The surface temperature of pure

paraffin first increased significantly from 20 ° C to 60 ° C in less than 12 min, then further

increased to 87 °C almost linearly over 30 min. This dramatic increase in surface

temperature can be attributed to the combined effects of low thermal conductivity of

pure PCM, low heat transfer efficiency, and thermal resistance between PCM particles.

This particularly led to a significant increase in surface temperature and a large internal

temperature difference near the wall. However, in the PCM composite with metal foam,

the temperatures inside the sample at different locations increased almost

simultaneously with time. This phenomenon is due to the integration of the internal

temperature of the paraffin foam composite, which is caused by the increased heat

transfer suggested by the presence of metal foam. By using metal foam with less porosity

of the thermal performance improved more than that suggested by the foam sample of

higher porosity. By using the metal foam sample with 90% porosity, the surface

temperature of the PCM composite decreased by about 47%. It also helped the internal

temperature difference of paraffin (reported by different thermocouples) by more than

four times before complete melting. The use of foam encouraged a uniform heat transfer

across the PCM that led to the complete melting of paraffin in a short time.

W.Q. Li (2012) has specifically studied the effect of foam porosity and pore density on the

thermal performance enhancement of metal foam embedded PCMs. They performed

experiments by using seven samples of copper foam with porosities of 90%, 95%, 98%,

and pore densities of 10 PPI, 20 PPI, and 40 PPI. Paraffin was used as the phase change

material (PCM) in their experimental study. A Plexiglas cavity is designed to cage

paraffin-saturated metal foam based on the size of the metal foam. All samples of copper

foam used in this study had the same dimensions (45 × 100 × 100 mm3) and the PCM

container was thermally insulated using urethane foam. In this experiment, copper foams

were tested once with different porosity percentages with equal to pore density, and once

with different pore densities with the same porosity. The internal temperature

distributions of PCM samples were measured during the phase change of saturated

paraffin and in metal foams during the melting process. Electric heaters with constant

heat flux were used during the. Figure 2- 9 (a) shows that the heat transfer of PCMs

improves with decreasing the pore density, and Figure 2- 9 (b) shows that the heat

transfer of PCMs improves by decreasing the porosity of metal foam. In general, this

study suggested that the uniformity of temperature distribution in saturated PCMs is

Figure 2- 9. Diagram of time-temperature with different pore densities at constant porosity (ɛ = 0.90) for PCM composites (a). Diagram of temperature and time with different porosities in fixed pore density of 40 PPI, for PCM composites (b)(W.Q. Li 2012)

increased either by reducing the metal foam's porosity or pore density.

In a theoretical study by Xiao et al. (2014), copper foams with a pore size of 25 PPI and a

porosity of 96.95%, 92.31%, and 88.89% were used to improve the thermal performance

of a paraffin-based PCM sample. In this experiment, they kept the volume of the paraffin

constant; hence by introducing metal foam, the overall volume of the composite increased

slightly (i.e. compared to the original paraffin samples). They showed that copper foam

(with the abovementioned porosities) increased the thermal conductivity of PCMs

composition by about 13, 31, 44 times more than pure PCMs, respectively.

Similarly, Wang et al. (2016) conducted studies on the effect of metal foam with different

porosity on PCMs. In this study, RT55 paraffin was used as phase change material and

copper foam as metal foam. Three thermal sinks were used for MF-PCM composites with

three different porosities of 95%, 85%, 75%. In the melting process, similar to other

studies reported here, it was observed that decreasing the metal foam porosity further

improved the heat transfer performance of the MF-PCM composites. After complete

melting, it was observed that the temperature difference between the three porosities

increases with increasing input power, which indicates that less porosity material has

better performance at complete melting of MF-PCM composite. Figure 2- 10, shows the

Figure 2- 10. The effect of porosity of metal foam on the center temperature of PCMs composite during the heating process (a) 13.65 W, and (b) 16.66W. Based on the calculated results in this study, the enhanced thermal conductivities of the MF-PCMs achieve 27–134 times (depending on its porosity) of that of paraffin RT55, which shows a tremendous enhancement in thermal conduction. (Wang et al. 2016)

complete process before and after melting.

Through another theoretical study by Wang et al. (2019b), paraffin RT44 was selected as

PCM, and the effect of introducing aluminium foam with porosities of 90.97%, 88.78%,

and 86.42% were investigated in the presence of a constant heat source. The graph

obtained from their simulation (Figure 2- 11) shows that the melting rate increases by

Figure 2- 11. Effect of porosity of foam metal (Wang et al. 2019b)

decreasing the porosity.

Tian and Zhao (2009) also reported that metal foams with less porosity are more effective

than those with higher porosities in improving the heat transfer of PCMs. A similar

conclusion was made by Han et al. (2013), who studied PCM samples and metal foams

with 85% and 95% porosities (Figure 2- 12).

Figure 2- 12. Comparison between pure PCM and metal-foam composite PCMs, (Han et al. 2013).

Another factor to consider when choosing metal foam to enhance the heat transfer of PCM is

the material that the metal foam is made of (Table 2- 3. Comparing Thermal Conductivity

of some Metals foam (Chintakrinda et al. 2011; EL IDI et al. 2021) diamond has a high

thermal conductivity compared to other alternatives listed in the able; however, the

drawback of this material option is its high cost. Hence, the studies conducted to date

mostly used aluminum foam (Chintakrinda et al. 2011), copper foam (Zhao et al. 2010),

Table 2- 3. Comparing Thermal Conductivity of some Metals foam (Chintakrinda et al. 2011; EL IDI et al. 2021)

1 Aluminium

2 Copper

3 Silver

4 Steel/SS

5 Brass

6 Iron

7 Diamond

8 Gold

9 Nickel

and Nickel foams (EL IDI et al. 2021) to enhance the heat transfer of PCMs (Figure 2- 13).

R Metals

1000

400

406

Thermal Conductivity (W/m°k)

Aluminium foam

Copper foam

Nickel foam

Figure 2- 13. different kinds of metal foam (Zhao et al. 2010; Chintakrinda et al. 2011; EL IDI et al. 2021)

314 97.5 109 205

Previous studies have investigated the thermal conductivity of paraffin and composite

metal of PCMs and showed PCMs and nickel foam have lower thermal conductivity than

copper. Nevertheless, it is still 3-5 times larger than pure paraffin. Thapa et al. (2014) also

obtained an equivalent conductivity of 3.7 W / m ͦ K in the experiment by adding copper

foam as a PCMs compound. Lin et al. (2018) also conducted a study on the effect of adding

metal foam by using different materials and picked copper as the best option for

improving the overall effective thermal conductivity of the PCM (Table 2- 4). That

explains why over a third of studies on MF for thermal performance enhancement of

Table 2- 4. Comparison of thermal conductivity with different metal material additives (Lin et al. 2018)

PCMs were conducted by using copper (Ali et al. 2019).

PCMs Metal Fraction

Paraffin Kf (W/mK) 0.354 Kp (W/mK) 398 Copper foam Keff (W/mK) 16.01 Increase times 45.23

Paraffin 0.305 Nickel foam 91.4 2.33 6.58

Eryhritol (solid) 0.733 Nickel foam 90.3 4 5.46 88.89% (porosities) 90.61% (porosities) 35 vol wt%

Paraffin 53 0.305 Nickel Foam 91.4 1,2 3.93

Paraffin 0.305 Copper foam 398 4.9 16.06 Porosities >95% Porosities >95%

2.4.3. Nanoscale materials to improve the effective thermal conductivity of

PCMs

One of the most popular methods to enhance the thermal performance of PCMs is

introducing nanoparticles. Due to their high thermal conductivity, low density, high

usability, and stable chemical nature have been considered. Increased heat transfer has

been reported in the PCMs phase change systems by dispersion of nanoparticles.

Wu et al. (2010)created a new type of nanoparticles enhanced PCM by dispersing small

amounts of nanoparticles, with a weight fraction of 1%, in melted paraffin and studied it

numerically. Cu, Al, and C / Cu nanoparticles were used in this study to increase the heat

transfer rate of paraffin. They found that copper nanoparticles have the best performance

for heat transfer enhancement of the PCM compound they studied compared to Al and C

/ Cu nanoparticles. They further stated that copper nanoparticles could reduce the

melting and freezing time of nanofluid paraffin by 30.3% and 28.2% compared to pure

paraffin, respectively.

In another study, Shi et al. (2013) added graphite nanoparticles (xGnP) to a paraffin

sample to improve its effective thermal conductivity. They performed their experimental

study by loading 10% by weight of xGnP into paraffin. They found that the thermal

conductivity of composite nanoparticles (graphene) increased compared to pure PCMs

and that graphite improved the effective thermal conductivity of PCM composites by

more than 10 times.

In another experimental investigation, graphene exfoliated graphite was used (Xia Liu

2017). In this experiment, the mass fraction of nanoparticles varied between 0 and 2.0%

for graphene or exfoliated graphite used; hot water at 75 ° C was used as a constant

temperature source of heat. Graphene powder with a mass fraction of 0.2 wt% was able

to increase the thermal conductivity of paraffin/graphene composite from 0.28 to 0.33

(W/m.°K) (13.8 %). On the other hand, graphite particles with the same mass fraction

could improve the thermal conductivity of graphite PCM composite up to 0.31 (W/m.°K),

which was very close to that achieved using graphene nanoparticles.

A numerical study on the effect of the mass fraction of nanoparticles (used for thermal

performance enhancement of PCMs) was performed by Ling et al. (2015). In this study,

RT44HC paraffin and expanded graphite (EG) have been selected as phase change

materials and nanoparticles. Paraffin / E.G. composites were prepared by dispersing 25 -

wi 35 wt% of E.G. powder in paraffin at 60 °C and melting and freezing processes were

investigated. The conditions of the composite were monitored by collecting data every

minute at temperatures of 35 °C, 40 °C, 41 °C, 42 °C, 43 °C, 44 °C, 45 °C, 50 °C, and 60 °C.

The latent heat of fusion of PCM composites with mass fractions of 25% and 35% of EG

were improved by 152.5 kJ/kg and 168.1 kJ/kg, respectively. On the other hand, the

results showed that by increasing the mass fraction of EG, the thermal conductivity of the

composite increased and with the mass fraction of 35%, the enhancement of thermal

conductivity was 25% more than that achieved by using30 wt% of the mass fraction.

A summary of the experimental investigations accomplished for thermal conductivity

and respective latent heat capacity of enhanced nano-paraffin is given in Table 2- 5 (Bose

and Amirtham 2016).

Table 2- 5. Experimental studies on thermo physical properties of Nano-enhanced parafﬁn

References

Remarks

Comment

Concentrations PW

K (W/mͦK)

LH (kJ/kg)

Nano particle added SWCNT

0.1 vol%

N/A

168.2

0.4 vol%

170.6

Shaikh et al (2008)

0.7 vol%

174.6

1.0 vol%

176.6

Latent heat capacity was increased by 12.98%, with 1.0 vol%

MWCNT

0.1 vol%

165.2

0.4 vol%

168

0.7vol%

170.9

Numerically and experimentally studied and found that the intermolecular attraction between the molecules of nanoparticles and paraffin wax was considered the possible reason for enhancing latent heat values.

1.0 vol%

172.1

MWCNT

0.2 wt%

0.39

165.4

0.5 wt%

0.35

165.1

Wang et al. (2009)

1.0 wt%

0.37

164.9

2.0 wt%

0.28

163.8

Concluded that PW/MWCNT composites have enhanced thermal conductivities compared to the pure P.W., with increasing the mass fraction of MWCNTs

Al2O3

5 wt %

225.6

Ho & Gao (2013)

0.128 55℃

10 wt %

212.3

Thermal conductivity values were increased by 35% in solid and 45% in liquid with 2 wt% Thermal conductivity values were increased by 6%, with 10 wt%

0.142 55℃

CuO

N/A

Jesumathy et al (2012)

Thermal conductivity values were enhanced

1 wt % 2 wt % 5 wt % 10 wt %

0.398 0.411 0.439 0.45

2.8 vol %

252.9

GO sheet

Mehrali et al (2013)

5.8 vol %

251.3

8.5 vol %

250.6

0.24 20 ℃ 0.28 20℃ 0.32 20℃

Found that a non-linear increase of the measured thermal conductivity and dynamic viscosity for the nanoparticle-in-parafﬁn emulsions formulated with the mass fraction of the nanoparticles compared with that for the pure parafﬁn, depending on the temperature Found that charging time is decreased by 13, 21%, and 35%, and discharging time is also decreased by 13, 19%, and 34% for various concentrations. Studied and found that, the thermal conductivity of the composite PCM was highly improved from 0.305 to 0.985 (W/mk) and suggested that good option for TES because of its acceptable thermal properties, good thermal reliability, chemical stability, and thermal conductivities.

1 wt %

0.253

193

2 wt %

0.261

186.1

Şahan et al (2014)

3 wt %

0.261

169.6

The composite PCM had good thermal reliability and chemical stability after 2500 melting/ freezing cycles Thermal conductivity values were increased by

Showed that nano magnetite parafﬁn is a thermally efﬁcient and cost-effective material that can be used in many

4 wt %

0.266

161.1

5 wt %

0.274

142.3

34%, with 1 wt%

thermal energy storage applications.

10 wt %

0.309

113.6

0.1 wt%

136.1

Wang et al (2010)

0.5 wt%

136.3

1.0 wt%

134.1

Thermal conductivity values were increased by 26% with, 5 wt%

0.27 15 ℃ 0.26 15 ℃ 0.28 15 ℃

Showed that, melting time is reduced by 22.9% and 33.6% and that for solidiﬁcation by 21.5%and 24.8% with 0.5% and 1% wt concentration of nanoparticles

Previous studies have shown that popular nanoparticles for testing include aluminium,

Figure 2- 14. Different kinds of Nanoparticles (Hosseini et al. 2013)

copper oxide, aluminium oxide, copper, and titanium oxide (Figure 2- 14).

Khodadadi (2015) presented the benefit of using a variety of nanoparticles including Cu,

CuO, Al2O3and TiO2 dispersed in melting paraffin. They obtained that copper

nanoparticles showed the best heat transfer enhancement effect for PCMs than other

types of nanoparticles (Figure 2- 15).

Figure 2- 15. Concluded that Cu nanoparticles have the best performance for heat transfer (Khodadadi 2015)

In a study by Arifutzzaman (2020), MXene nanomaterials were used with paraffin PW70

as a phase change material (PCM) to study these nanoparticles' thermal conductivity

enhancement effect. They prepared MXene nanoparticle PCM composites with different

concentrations. They found that MXene nanoparticles with a weight percentage of 0.3%

increased the effective thermal conductivity of the composite by 16% in the molten state

compared to pure paraffin wax, while the percentage of this improvement was 6.1%,

12.4%, when 0.1 wt%, 0.2 wt% were used, respectively.

2.5. Integration of PCMs with Solar Thermal Collectors (STC)

Solar thermal collectors are used to convert solar radiation into heat for applications such

as water heating, air heating, industrial processes, etc. Several solar thermal technologies

are commercially available and used for various applications. The selection of a suitable

technology for an application is made by considering temperature requirements, flow

rates (e.g. for hot water), and the environmental condition in which the collector is

utilised (Zheng 2017; Keith Lovegrove November 2019). Conventional collectors options

include evacuated tube solar thermal collectors, concentrated solar collectors, flat panel

solar collector, solar ponds, unimaginable concentrators, paraboloid solar concentrators,

Fresnel lens reflective concentrator, Transmission-type line-focus Fresnel solar

concentrators solar tower concentrators, and multi surface or multielement compound

solar concentrators. The main function of a solar thermal system is to collect the solar

energy and convert that to thermal energy that is usually collected by a fluid (Zheng

2017).

Among the above-mentioned solar collectors, evacuated tube solar collectors (ETSC)

have gained popularity in recent years as they can be operated over a wide range of

temperatures suitable for various domestic and industrial applications. (Chopra et al.

2018; Aramesh and Shabani 2020b). Their tubular shape helps them collect the solar

radiation from different directions without the need for any sun tracking arrangement

(i.e. the longest solar radiation absorption time during the day compared to other types

of collectors). Moreover, the vacuum inside the pipes makes it a good insulator against

heat loss, and hence most of the solar energy that is converted to heat is passed to the

user (Greenland 2018; Bhusal et al. 2020). The thermal efficiency of ETSCs is about 70%

that is generally better than other types of collectors with comparable operating

temperature ranges (Douvi et al. 2021).

ETSCs, like other types of collectors, have a major drawback in that they cannot provide

continuous thermal energy and have limited energy production during cloudy days and

nights (Aramesh and Shabani 2020b). This is while their supply when the sun is shining

can be above that demanded by the end user, which means a suitable thermal energy

solution can store this surplus and release it back to the system when the supply of heat

falls short due to a low level of solar radiation hitting the collectors (Zhou et al. 2015;

Tong et al. 2019). As discussed earlier in this thesis PCMs are generally used for this

purpose. Table 2- 6 summarises various studies reported in the literature in which PCMs

Table 2- 6 . Various types of PCMs used for thermal energy storage with various solar thermal collectors (Douvi et al. 2021)

The melting point of PCMs

Investigation Method

Solar Collector

Type of PCMs

Form of PCMS

Reference

Paraffin wax

54-57 °C

PCM around the U- tubes

Experimental

M. Essa (2018)

Evacuated Tube

Senthil (2020)

Parabolic dish

Experimental

PCMs are filled in the receiver as thin layers

117.7 °C 166 °C 210 - 220 °C

Abuska

Flat plate air

Sugar alcohols Erythritol D-Mannitol Eutectic mixture 60% KNO3 40% NaNO3 RT54HC

53-54 °C

PCM-honeycomb core

Experimental

M. (2019)

Flat plate

Experimental Dengjia Wang

(2019)

70 °C 15 °C

2 PCMs, not specified

An s-shaped pipe is located below the absorber plate, and two kinds of PCMs wrapped with Aluminium foil are placed in the space area under the absorber plate

Flat plate

5 °C

Numerical

F. Zhou (2019)

Mixed organic PCMS

Under the absorber Plate

were used with different types of solar thermal collectors.

Li et al. (2018)

119 °C

Experimental + numerical

Evacuated Tube

Erythritol Composite PCM (3%wt E.G. and 97%wt erythritol)

An Aluminium pipe with the diameter of 42mm is inserted next to the evacuated pipe, which is used in the vicinity the composite PCMS

217 °C

Experimental + numerical

D. Guerraiche (2020)

Salt hydrates 60% NaNO3+ 40% NO3

Concentric absorber tube with phase change material

Parabolic shape solar concentrator

210-220 °C

Experimental

Senthil (2020)

Parabolic dish

eutectic mixture 60% KNO3 + 40% NaNO3

Stearic acid

57-60 °C

Experimental Chopra et al.

(2019)

Evacuated Tube

SA-67

67.10 °C

Experimental Chopra et al.

(2020a)

Evacuated Tube

Rectangular PCM filled enclosures of 30mm width in the solar receiver Array of ten heat pipe equipped evacuated tubes connected to a manifold filled with PCMS Integration of PCM with heat pipe equipped ETC

Table 2-6, indicates that only a few studies with limited scopes have been performed on

the integration of thermally enhanced PCMs with solar thermal collectors in general and

with ETSCs in particular. In one of these rare studies on the latter, conducted by Li et al.

(2018), they reported that by adding expanded graphite nanoparticles to 17 kg paraffin,

the effective conductivity of the PCM increased by 2.4 times. As the result of this

improvement, the daily energy storage efficiency has increased by an average of about

40% after introducing graphite, and almost a uniform temperature distribution across

the PCMs was achieved on sunny days. The positive impact of this approach was reported

to be even more pronounced in cloudy periods when the role of thermal energy storage

is presumably more critical.

The study by Kumar and Mylsamy (2019) is another one of this kind in which SiO2

nanoparticles were used to improve the thermal conductivity of a paraffin based PCM.

They reported an increase from ~70% to ~75% in the overall thermal efficiency of the

ETSC studied. However, the experiments did not attempt to compare the system with the

one in which pure PCM is used rather than a thermally enhanced PCM composite.

Several other studies reported on using PCM with evacuated tube solar thermal collectors

tried to accommodate the PCM inside the tubes. Researchers placed the PCMs inside the

evacuated tube's solar collection tubes. The experiment was performed on a typical day;

the results showed that the overall thermal efficiency of the system increased by about

13% (Abokersh et al. 2017). In 2019, an experimental study was performed using pure

PCMs inside a solar collection manifold on cold winter days (Kumar and Mylsamy 2019).

they do so, the water temperature supplied by the system was increased from 33.1 ͦ C to

37 ͦ C and the overall thermal energy efficiency so it can be that energy efficiency

increased nearly 11%.

Chopra et al. (2019) studied a solar collector with 26 kg PCMs stearic acid used inside the

solar collector manifold. Data were collected at both middles of the day and at the end of

the day. The thermal energy efficiency improved from 55% to 72% and from 52% to 62%

for half-a-day and full-day cases, respectively.

Another experimental study was performed on a heat pipe collector with 2.25 kg/tube

PCM inside the tubes (Chopra et al. 2020a). The experiments were conducted with

different rates of water flow inside the solar thermal collector. The difference in water

inlet and outlet temperature was 43 ° C when the PCMs were in the collector and 30 ° C

when the solar collector was used without the PCM. The daily efficiency of the PCM

collector system was 32- 37% higher than the collector without PCM.

As indicated by this literature review, previous studies mainly focused on either

integrating the PCM thermal storage unit with the ETSC tubes or placing the PCM in the

manifold of these collectors. However, the use of a thermally enhanced PCM as a separate

independent unit has not been reported in the literature (Figure 2- 16). This offers the

flexibility of sizing the storage unit independently to match the demand and applying

enhancement measures without being restricted by the collector's size and geometry. It

must be noted that separating the storage unit from the collectors will open the

possibility of losing heat through the pipelines that must be minimised by properly

Figure 2- 16. Diagram of PCM thermal storage unit integrated with a solar thermal collector

insulating the system.

Hence the present study is an attempt to make a contribution to addressing the above

mention gap through a systematic experimental study. The data collected here can be

used to further understand the advantages and challenges of using thermally enhanced

PCMs with ETSCs and create a platform for further research and development around this

idea in the future.

3. METHODS

3.1. Overview

This study starts with reviewing the literature, which is always considered an essential

part of shaping research. This research was designed to be done experimentally to

address the gap identified and presented in chapter 2, and the present chapter provides

an overview of the experimental method used to conduct this study.

Guided by the literature, the target materials to conduct this experimental study were

identified. That included paraffin RT44HC as the phase change material, copper oxide

particles as nanoparticles, and copper foam for experiments. Detailed technical

specifications of these materials will be covered in chapter 4. Four experimental samples

were prepared to be studied, including pure paraffin (PP), paraffin with nanoparticles

(NP), metal foam embedded paraffin (MFP), and paraffin sample with both metal foam

and nanoparticles (MFNP). Initially, the heat transfer performance of the samples was

investigated at both the constant temperature and constant heat flux. This phase of the

experimental study helped with identifying the best option to be further studied for use

in a thermal storage unit together with an evacuated tube solar thermal collector unit.

Figure 3- 1. provides an overview of the method, which will be further discussed in the

following sections.

Figure 3- 1. Overview of research method

3.2. Literature Review

Focussing on PCMs and their challenges for use in solar thermal systems, over 50 articles

relevant articles were reviewed to understand the behavioural characteristics of PCMs,

their strengths, and challenges. While their ability to store a large amount of makes PCMs

an attractive option for use in thermal energy storage applications, the literature

highlighted low thermal conductivity as one of the major challenges of commonly used

PCMs in such applications (e.g. paraffin) (Singh et al. 2018c). Hence, many studies have

been performed on various techniques for enhancing the thermal conductivity of PCMs,

as presented in the literature (Tian 2012). Several methods were proposed in the

literature to increase the thermal conductivity of PCMS, such as using heat pipes,

introducing extended surfaces (i.e. fins), introducing nanoparticles, and embedding metal

foams or mesh materials in the PCM.

Amongst these methods, the two latter techniques attracted a significant level of

attention. While many studies focused on these two methods, the literature review

indicated a gap related to comparing these two options and understanding the effect of

their simultaneous use (Wu et al. 2020). No experimental data were found focusing on

the performance of such thermally enhanced PCMs in the context of solar thermal

systems for domestic hot water supply.

3.3 Material Selection and Application Requirements for the

Experimental Study

The ideal domestic hot water temperature for the end users is about 40 °C, while the

storage temperature if traditional water based thermal storage is used, must be above 60

°C (i.e. to prevent bacteria from growing in the tank). This is a significant limitation

restricting the use of this thermal storage option with solar thermal systems since the

system may produce hot water at temperatures well below 50 °C during periods of low

solar radiation. This situation makes PCMs even more attractive for storing heat in

domestic solar hot water systems. Therefore, Paraffin (RT44HC) was selected for this

experiment due to the melting point of 41-45 ͦ C, which is around the temperature that

can be used directly by the end user. The melting temperature is in a suitable range for

domestic hot water, while a high heat storage capacity is offered by paraffin- RT44HC.

Moreover, paraffin with the high latent heat of fusion can store thermal energy in this

Table 3- 1. Comparison of properties of some organic paraffin (Thomas Braun 2020)

Paraffin Melting Area

Heat Storage capacity (KJ/Kg ͦ k)

Latent Heat (W/m ͦ k)

Specific Heat capacity (KJ/Kg ͦ k)

250

( ͦC) 41- 45

RT44HC

temperature range and then release them in the form of latent heat (Table 3- 1).

Copper was selected as the material for the metal foam (to be introduced to the paraffin)

due to its compatibility with paraffin while offering a network of material with high

thermal conductivity (400 W/m ͦk). The focus of this study was not optimising the pore

size, porosity, etc., of the metal foam for conductivity enhancement of paraffin. Hence, the

research was guided by the previous studies reported in the literature to select these

specifications (see chapter 4 for details). It is noteworthy that the paraffin is supposed to

move throughout the foam materials; hence, open-pore metal foams were used for this

study. The copper foam selected for this study came with the specifications provided in

Table 3- 2. Properties of copper foam selected for experiments (Hanter 2016)

Porosity

Pore size

Kind of cell

Pores per inches (PPI)

Density

Thermal conductivity

Metal Foam

90 %

5-7 mm

Open Cell

10 PPI

400 (w/m ͦ k)

Table 3- 2.

Copper Foam

8.954 g/cm3

Various nanoparticles were used in different studies to increase the effective thermal

conductivity of PCMs. Nanoparticles enhance the heat transfer performance of paraffin,

especially when it is in the liquid phase (i.e. potentially through encouraging convection).

Hence, less emphasis was put on the thermal conductivity of the core material used for

making nanoparticles, and the existing 20 nm copper oxide powder that existed in our

labs, also recommended by the literature, was used in this experimental study. In terms

of concentration, the literature mostly agrees on 0.4 wt% as a good value which was also

used for this experimental study. More details can be found in chapters 2 and 4 of this

thesis as well as the appendix.

3.4 Experimental Study

3.4.1 Thermal conductivity enhancement of PCM: comparing nanoparticles and

metal foam

The first part of the experiment looked at the impact of introducing nanoparticles and

metal foams, individually and together, on the thermal conductivity improvement of a

selected paraffin sample (RT44HC). As indicated by the literature review, comparative

studies of this type on similar samples were limitedly reported. This part of the study was

then used as a guide to narrow down the thermal conductivity enhancement options in

the second part of the experiment, in the presence of the collectors (i.e. the practical

application context of this study).

The following four options were used to understand the melting and solidification

process of the PCM sample by adding nanoparticles and metal foam to pure paraffin:

• Pure paraffin (PP)

• Nanoparticles are added to paraffin (NP)

• Metal foam and paraffin (MFP)

• Paraffin and nanoparticles on metal foam (MFNP)

To prepare the samples with nanoparticles, first the pure paraffin was heated in a

magnetic stirrer and after complete melting of the PCM, 0.4% (by mass fraction) of

nanoparticles (CuO-Nano) was added to the PCM while still being in the stirrer. The PCMS

mixture was then stirred for another 5 minutes and then refrigerated for rapid

solidification. 0.4 wt% value was selected based on earlier finding in the literature about

the optimum mass fraction of nanoparticles (CuO) in the PCM for thermal performance

enhancement. (Khodadadi (2015)).

The experiments were designed with two types of heat sources: constant temperature

and constant heat flux. In the first experiment, hot water was at a controlled temperature

of about 65-70 ͦ C to create a constant temperature heat transfer scenario, although using

PCM creates the possibility of storing heat at a lower temperature. All samples were filled

into a small cylindrical container by using 65 g of paraffin.

In the second set of experiments, an electric plate heater was used as the heat source. The

samples were placed in a cylindrical container with a height of 110 mm. By generating

heat at the bottom of the container, the samples could absorb thermal energy at a

constant heat flux . The sample was cooled from the top using a cold-water stream

(passing through pipes) to create a heat flow throughout the samples. The mass of

paraffin in the samples was 400 g, which is the same for all PCM samples, and the mass of

CuO-particle was 1.6 g in the samples of MFNP and NP, equivalent to 0.4 wt%. The MFP

and MFNP samples had 64 g of Cu foam with 80-90% porosity and 10 PPI.

The melting and solidification times were obtained, and the samples were fully

monitored using seven temperature sensors located 1.5 cm apart in the samples. Seven

thermocouples (T-Type) were connected to the data logger (Pico TC-08) for data

recording. The error of these thermocouples in the range of 250-1300 ͦC is 0.025 ͦC. In

addition to each compound's melting and solidification times, the thermal performance

inside them was investigated, which is one of the other objectives of this study. The

uniformity and non-uniformity of heat distribution in each compound indicated the

quality of its heat transfer that will be thoroughly discussed in chapter 4.

In order to compare the thermal conductivity of PP, NP ،MFP, and MFNP samples and to

evaluate the heat transfer rate of PCMs, all samples were tested under equal conditions.

By measuring the melting and solidification times of each sample, the heat transfer rates

of the samples were compared, and the percentage of improvement in the amount of heat

transfer rate in each sample was discussed.

In each stage of these experiments, the melting and solidification process was performed

more than 30 times. The test results for each of the compounds were evaluated to ensure

that the results were reproducible and proper uncertain analysis could be conducted.

There were also challenges identified during these experiments, for example, creating

hollow spaces between two layers during melting and solidification cycling or challenges

associated with keeping the samples containing nanoparticles stable.

These two experiments helped narrow down the four options to one: i.e. metal foam,

which was used in the applied context of solar thermal heating in the next part of this

experimental study. The details of the experiments leading to these narrowed down

solutions are provided in chapter 4 of this thesis.

3.4.2 Experiments with the solar thermal collectors

The thermal conductivity enhancement option (i.e. metal foam) selected using the first

part of this experimental study was then applied in the practical context of a solar thermal

hot water system. This is to understand how this option improves the performance of the

thermal storage unit compared to pure PCM in this context. Evacuated tube solar

collectors (ETSCs) have a relatively wide temperature range between 50 ° C and 200 ° C

and high thermal efficiency of more than 70%. Cylindrical collecting pipes are less

dependent on sunlight due to their geometric shape because they can receive sunlight

from different directions (Bhusal et al. 2020). The vacuum layer inside the collecting

pipes helps reduce heat loss. It can be said that ETSCs have the highest thermal efficiency

compared to other solar heat collectors that making them a popular option for solar hot

water applications (Aramesh and Shabani 2020b).

A water efficient showerhead uses ~7-9 l/min; however, the thermal storage unit used in

this study was only based on a fraction of this need, as it still could serve as a tool for

conducting a comparative experimental study. Having this in mind, the total mass of the

PCM used in the PCM unit was 5.8 kg. By keeping volume as a constraint, this mass was

decreased to 5.2 kg after introducing metal foam.

The heat storage unit (PCMs Box) has a cuboid shape with a total volume of 8.31 litter, i.e.

25.0 cm in length, 12.5 cm in width, and 26.6 cm in height, and it is thermally insulated.

Part of this volume is occupied with 10 pipes carrying the water throughout the storage

box. More details about this are provided in Chapter 5. It is noteworthy that the

experimental setup comprised an ETSC, a PCMs heat storage unit, a pump and several

valves for flow control, and a flowmeter. All pipes external to the storage unit were fully

insulated to prevent heat loss from the pipelines. No water-based thermal storage tank is

used in the system.

One of the other objectives of this experiment was to understand how the thermally

enhanced PCM unit can support the continuity of hot water supply at higher flow rates.

Hence, the system was tested at three different water flow rates of 1 l/min, 2 l/min, and

3 l/min. The water stream from the solar thermal system absorbs solar energy in the form

of heat and transfers it to the PCM unit. Melting time was measured and recorded while

the condition of different sections of the unit was monitored by 13 temperature sensors

located inside the PCM init. Temperature uniformity across the storage unit and

temperature fluctuation of the hot water supplied by the unit were measured for pure

and metal foam enhanced PCM cases.

4. NANOPARTICLES AND METAL FOAMS FOR HEAT TRANSFER ENHANCEMENT OF PHASE CHANGE MATERIALS

The details of the effect of using nanoparticles and metal foams on the performance of

phase change materials (PCMs) are described in this chapter, in the form of a paper

published in Energy storage. This chapter includes a comparison of the effectiveness of

using copper oxide nanoparticles and copper metal foams, together and separately, for

improving the performance of a selected PCM during melting and solidification processes

and for different heat transfer configurations. Two types of experiments were conducted,

at constant temperature and using a constant heat flux heat source. This study also shed

light on some other challenges associated with using PCM and the role and the practicality

of applying the above-mentioned thermal techniques in addressing them. This chapter

provides answers to research questions 1, 2, and 4 and contributes to objectives 1, 2, and

3 as detailed in chapter 1.

https://doi.org/10.1016/j.est.2020.101911

5. EVACUATED TUBE SOLAR THERMAL

COLLECTOR WITH ENHANCED PHASE CHANGE MATERIAL THERMAL STORAGE: AN EXPERIMENTAL STUDY

A case study of experimental investigation of the integration of PCMs and the solar

thermal collector system is presented in this chapter, in the form of a paper submitted to

the Journal of Energy storage. Based on the results of the study obtained in Chapter 4,

introducing metal foam was selected for thermal performance enhancement of the PCM

used with the collectors and the combination was studied experimentally. This chapter

contributes to answering research questions 1 and 3 and helps achieve objective 4, as

outlined in chapter 1.

https://doi.org/10.1016/j.est.2021.103838

6. CONCLUSIONS AND RECOMMENDATIONS

6.1 Overview

This study was conducted experimentally to investigate melting and solidification

performance characteristics of PCM compounds in which nanoparticles and metal foam

were used for enhancing their thermal performance. The first phase of this study was

designed to select the best thermal enhancement solution (from above-mentioned

options) to be further studied in the real context of a solar thermal system used for

domestic hot water supply.

As per the first objective of this work, a thorough literature review was conducted to

understand the state of development of enhanced PCMs (with special focus on paraffin)

in which nanoparticles and metal foams are used to improve their thermal conductivity.

This review helped with identifying the gaps of knowledge to shape the present study.

Inspired by this literature, in order to enhance the transfer performance of a paraffin

based PCM sample (suitable for domestic hot water supply applications) the effect of

introducing nanoparticles and metal foam were investigated. These two methods were

applied separately and simultaneously on the selected PCM sample. The PCM composites

studied then included, pure PCM (PP), PCM nanoparticle (NP), PCM-foam metal (MFP),

and PCM-nanoparticle-metal foam (MFNP). Their thermal performance (i.e. melting and

solidification) were studied by focusing on melting and freezing performance of PCMs

(pure and composite samples) by exposing them to constant temperature and constant

heat flux heat transfer conditions. These experiments helped with narrowing the options

down to using metal foam only that then was employed and further studied in the context

of a solar thermal system designed for domestic hot water supply. Although the use of

metal foam for performance enhancement of PCMs has been reported in the literature,

no experimental studies were reported on applying this solution in the real context of a

solar thermal domestic hot water arrangement. This study though is an attempt to

address this gap. With this introduction, here the research questions presented in chapter

1 of this thesis are exclusively reviewed and answered as follows.

6.2 Responses to the research questions

6.2.1 How much improvement in thermal performance of a selected PCM (i.e.

suitable for use with solar thermal hot water systems) can be achieved when

metal foams and nanoparticles are used separately and simultaneously?

This research question is mainly linked to the second and third planned objectives of the

work. Answering to this research question provided direction on the best option to be

used in the second phase of the experiments together with a solar thermal collector in

real world operation. By doping the experiments using a constant temperature heat

source (i.e. hot water at a controlled temperature), the following key results related to

this question were found:

For MFNP, MFP, and NP composites, it took about 32%, 19%, and 4% less time compared

to the PP sample to go from the initial temperature of 18 °C to the starting melting (41°C).

Overall, the time needed to move from 18 °C to the end of melting for MFNP, MFP, and NP

samples reduced by 24%, 17%, and 13% compared to that recorded for the PP sample.

The heating of the samples continued after the melting region (41-44 °C) until reaching

52 °C. In the same above-mentioned order, in comparison with the PP sample, about 10%,

4%, and 0% less time were required to raise the temperature after melting to reach 52

°C.

After this, the experiments with a constant temperature heat source continued to

investigate and compare the solidification behaviour of the samples. The PMC composites

of MFNP, MFP, and NP showed 65%, 26%, and 14% faster sonification than PP samples,

respectively, and with the same order, in the cooling process down to 30 °C (below the

solidification point), the time was reduced by about 20%, 24%, and 12% compared to the

PP sample.

Using a different setup, the experiments with the above-mentioned samples were

repeated in the presence of a constant heat flux source of heat. This experiment was

performed by heating the samples from the initial temperature of 18 °C until the samples

were completely melted at 44 °C. The results showed that the MFNP, NP, and MFP

samples were 12, 11, 7 times faster than the PP samples in reaching this final state (i.e.

completely melted at 44 °C).

Both nanoparticles and metal foam were effective in improving the thermal performance

of the pure PCM sample. However, adding nanoparticles to metal foam (i.e. combing the

two solutions) offered only a marginally better performance than using either of these

solutions separately. This marginal improvement though may not justify the additional

costs and challenges associated with using nanoparticles, particularly the challenges

suggested by the settlement of the particles that will be further discussed. Hence, for the

third part of this experimental study, in which an ETSC arrangement was used as the

source of heat, the investigation was performed using an MFP sample only.

6.2.2 How melting and freezing cycles can impact the stability and hence the

effectiveness (i.e. in improving the thermal conductivity) of nanoparticles

dispersed in PCMs, in terms of maintaining the improved thermal

performance?

This question is designed to particularly support the second objective of this research

and generate more data on practicality of using nanoparticles for enhancing thermal

performance of PCMs. The melting time in NP in the first cycle of the melting and freezing

performances were close to those recorded for the MFNP sample; however, they started

to show signs of deterioration in subsequent cycles pushing it closer to the performance

PP sample while still remaining marginally better than that. The gradual settlement of the

particles explains this. It was observed that after the complete melting of PCM, the

nanoparticles settled down inside both NP and MFNP samples. This phenomenon has

further happened after several melting/freezing cycles, and a considerable amount of the

nanoparticles dispersed in the paraffin, settled at the bottom of the samples. For this

reason, the heat transfer rate at the bottom of the container was very high, while the top

part showed a behaviour close to the PP sample (i.e. in the absence of particles). Figure

6 in chapter 4 shows this phenomenon in a molten NP sample obtained for five

consecutive melting/freezing cycles.

6.2.3 How do the thermal conductivity enhancement measures work in the

context of a solar hot water system in terms of the improved experience of

the end user?

The initial experiments narrowed down the thermal performance improvement options

to only introducing a metal foam structure to the PCM used in this study. This option was

then used to prepare a relatively small-scale thermal storage unit to go with an evacuated

tube solar hot water system. This particularly serves towards achieving the forth

objective of the thesis as outlined in chapter 1. The results showed that by introducing

metal foam, the PCM thermal storage system could play its storage role much more

effectively. This improved performance was evident by the significantly reduced

temperature fluctuation of the hot water supplied by the storage unit to the end user at

different flow rates (up to 3 l/min). The excellency of the PCM-MF composite over pure

PCM in this application can be further explained by the following observations: The

composite of MFP reduced the melting time at flow rates of 1 l/min, 2 l/min, and 3 l/min

(compared to the PP sample) by about 33%, 27%, and 21%, respectively. Slimier

improvement effects were also observed during thermal discharging mode (i.e.

solidification). At flow rates of 1 l/min, 2 l/min, and 3 l/min, the MFP composite reduced

the solidification time compared to the PP sample by about 40%, 32%, and 30%,

respectively.

A small pump was added to the system to change the direction of water flowing through

the system. This can help with investigating the effect of natural convection and further

understanding the melting and solidification behaviours of the storage unit that is

recommended to be the topic of future studies.

6.2.4 What are the challenges associated with using nanoparticles and metal

foam in PCMs, and how these challenges can be addressed?

The PCM selected in this study has a 12.5% volumetric expansion coefficient. This

expansion coefficient is more or less present in all PCMs. When the molten PCM gets solid,

heat escapes from the sides of the body of the container of PCM, and the PCM begins to

solidify. But in the central parts of the PCM, empty spaces and cavities are created due to

the contraction of the volume. The creation of hollow spaces after the melted sample has

solidified a challenge associated with using PCMs. These holes act as insulators

interrupting the heat transfer path throughout the PCM. Introducing nanoparticles were

effective in eliminating these hollow spaces, but however, this approach did not offer a

perfect solution to fully eliminate this negative effect. This is while introducing metal

foam was found to be generally more effective in addressing this issue.

As discussed earlier, the obvious challenge associated with using nanoparticles was their

settlement after several melting/freezing cycles. After the PCM was fully melted, the

nanoparticles inside the NP and MFNP samples started to settle down. This issue was

pronounced even more after a few melting/solidification cycles, and a considerable

portion of the nanoparticles (almost 80% or more) diffused in the PCM and settled down

at the bottom part of the PCM container. This challenge was also observed in the presence

of metal foam (i.e. MFNP sample). The metal foam was open cell type; hence, the

nanoparticles could move inside the porous medium. This study though flagged the

sedimentation of nanoparticles as a major drawback of this solution.

The critical factor of heat transfer in pure PCM is its natural convection. For this reason,

a high temperature difference is created between the location of the heat source and the

next points in the heat transfer path. In the case of metal foam composite (PCM-MF), the

metal foam structure can significantly suppress the natural convection of the pure PCM

in its liquid phase. When using PCM-MF composites, the conduction heat transfer is

supported, and the temperature difference between the points in the heat transfer path

is much smaller (when the PCM is in the solid phase). However, in the liquid phase, this

positive effect can be offset by the negative impact of using MF on the natural convection

heat transfer through the PCM.

Generally, the use of metal foam was found to be effective for improving the thermal

properties of the PCM used in this study. However, it must be noted that the PCM

composition is affected when such metal structured are introduced. Introducing metal

foams with a density of 90% means phasing out 10% of the PCM if the volume of the PCM

container is kept the same. This literally means that the thermal storage capacity of the

PCM unit is impacted as the metal materials replacing this 10% of PCM do not offer the

same thermal storage capacity as that offered by the PCM. However, this reduction in

storage capacity can still be justified by the significant thermal performance

improvement offered by introducing metal foams.

6.3 Recommendations for future work

As outlined by the last objective of this study presented in chapter 1, this research was also

designed to identify areas for further research and development. In line with this objective,

the recommendations for future work can be summarised as below:

• Placing the thermally enhanced PCM unit in series in the system, i.e. between the

collector and end user, can delay the immediate access of the end user to hot water.

In this case, the addition of a small water tank can offer a solution while it also

further buffers the supply of hot water. Studying the dynamic effect of this

arrangement and understanding how the two storage systems can complement each

other can be done in the future.

• The experimental study with an evacuated tube solar thermal collector was

conducted using a basic small-scale configuration. However, the effect of different

parameters such as demand pattern, collector size, storage size and configuration,

operating temperature, flow rate, etc., are recommended to be studied in detail. A

comprehensive simulation validated through this study can then be used in the

future for detailed parametric study of the system and understanding the effect of

different parameters such as those mentioned here. The model can then help with

identifying ways to further enhance the performance of this arrangement. This

modelling exercise can also facilitate a full exergy analysis of the system to further

understand opportunities for performance improvement.

• The solar thermal hot water system used in this study was investigated over selected

and limited periods. The study can be designed to be conducted over an extended

seasonal period by using some arrangements for the automation of the system and

data acquisition.

• The PCM unit used in this case study solely relied on the collectors for melting and

storing heat. The system, though, can be combined with renewable based electric

heaters (i.e. PVs), and the performance of such a hybrid system can be further

investigated. This is particularly of interest by considering the rapid decrease in the

cost of PVs in recent years.

• A comprehensive life cycle analysis (LCA) to compare this system with water based

thermal storage systems is recommended.

• The present study was conducted in the context of a domestic hot water supply

application; however, it can be expanded to industrial applications with different

supply temperature and flow rate needs.

References Rubitherm Phase Change Material, RT44HC. Abokersh MH, El-Morsi M, Sharaf O, Abdelrahman W. 2017. On-demand operation of a compact solar water heater based on U-pipe evacuated tube solar collector combined with phase change material. Solar Energy 155: 1130-1147. Afshan ME, Selvakumar A, Velraj R, Rajaraman R. 2020a. Effect of aspect ratio and dispersed PCM balls on the charging performance of a latent heat thermal storage unit for solar thermal applications. Renewable Energy 148: 876-888. Afshan ME, Selvakumar AS, Velraj R, Rajaraman R. 2020b. Effect of aspect ratio and dispersed PCM balls on the charging performance of a latent heat thermal storage unit for solar thermal applications. Renewable Energy 148: 876-888. Al-Jethelah M, Ebadi S, Venkateshwar K, Tasnim SH, Mahmud S, Dutta A. 2019. Charging nanoparticle enhanced bio-based PCM in open cell metallic foams: An experimental investigation. Applied Thermal Engineering 148: 1029-1042. Ali HM, Janjua MM, Sajjad U, Yan W-M. 2019. A critical review on heat transfer augmentation of phase change materials embedded with porous materials/foams. International Journal of Heat and Mass Transfer 135: 649-673. Alsayed MF. 2021. Solar water heater sizing and economic evaluation for local hospital. in 2021 12th International Renewable Engineering Conference (IREC), pp. 1-6. Alva G, Liu L, Huang X, Fang G. 2017. Thermal energy storage materials and systems for solar energy applications. Renewable and Sustainable Energy Reviews 68: 693-706. Aramesh M, Ghalebani M, Kasaeian A, Zamani H, Lorenzini G, Mahian O, Wongwises S. 2019. A review of recent advances in solar cooking technology. Renewable Energy 140: 419-435. Aramesh M, Shabani B. 2020a. On the integration of phase change materials with evacuated tube solar thermal collectors. Renewable and Sustainable Energy Reviews 132. Aramesh M, Shabani B. 2020b. On the integration of phase change materials with evacuated tube solar thermal collectors. Renewable and Sustainable Energy Reviews 132: 110135. Arıcı M, Bilgin F, Nižetić S, Karabay H. 2020. PCM integrated to external building walls: An optimization study on maximum activation of latent heat. Applied Thermal Engineering 165. Arifutzzaman NARSA. 2020. Experimental investigation of energy storage properties and thermal conductivity of a novel organic phase change material/MXene as A new class of nanocomposites. Journal of Energy Storage 27. B K, Pandey AK, Shahabuddin S, Samykano M, M T, Saidur R. 2020. Phase change materials integrated solar thermal energy systems: Global trends and current practices in experimental approaches. Journal of Energy Storage 27. Baetens R, Jelle BP, Gustavsen A. 2010. Phase change materials for building applications: A state- of-the-art review. Energy and buildings 42: 1361-1368. Bhusal Y, Hassanzadeh A, Jiang L, Winston R. 2020. Technical and economic analysis of a novel low-cost concentrated medium-temperature solar collector. Renewable Energy 146: 968-985. Bose P, Amirtham VA. 2016. A review on thermal conductivity enhancement of paraffinwax as latent heat energy storage material. Renewable and Sustainable Energy Reviews 65: 81-100. Buonomo B, Pasqua Ad, Ercole D, Manca O. 2018. Numerical Study of Latent Heat Thermal Energy Storage Enhancement by Nano-PCM in Aluminum Foam. Inventions 3. Cabaleiro D, Agresti F, Barison S, Marcos MA, Prado JI, Rossi S, Bobbo S, Fedele L. 2019. Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low-temperature applications. Applied Thermal Engineering 159. Cabeza LF, Castell A, Barreneche Cd, De Gracia A, Fernández A. 2011. Materials used as PCM in thermal energy storage in buildings: A review. Renewable and Sustainable Energy Reviews 15: 1675-1695. Chandel S, Agarwal T. 2017. Review of current state of research on energy storage, toxicity, health hazards and commercialization of phase changing materials. Renewable and Sustainable Energy Reviews 67: 581-596.

Chen Z, Gao D, Shi J. 2014. Experimental and numerical study on melting of phase change materials in metal foams at pore scale. International Journal of Heat and Mass Transfer 72: 646- 655. Cheralathan M, Velraj R, Renganarayanan S. 2007. Performance analysis on industrial refrigeration system integrated with encapsulated PCM-based cool thermal energy storage system. International Journal of Energy Research 31: 1398-1413. Chieruzzi M, Cerritelli GF, Miliozzi A, Kenny JM. 2013. Effect of nanoparticles on heat capacity of nanofluids based on molten salts as PCM for thermal energy storage. Nanoscale research letters 8: 1-9. Chintakrinda K, Weinstein RD, Fleischer AS. 2011. A direct comparison of three different material enhancement methods on the transient thermal response of paraffin phase change material exposed to high heat fluxes. International Journal of Thermal Sciences 50: 1639-1647. Chopra K, Pathak AK, Tyagi V, Pandey A, Anand S, Sari A. 2020a. Thermal performance of phase change material integrated heat pipe evacuated tube solar collector system: an experimental assessment. Energy Conversion and Management 203: 112205. Chopra K, Pathak AK, Tyagi VV, Pandey AK, Anand S, Sari A. 2020b. Thermal performance of phase change material integrated heat pipe evacuated tube solar collector system: An experimental assessment. Energy Conversion and Management 203. Chopra K, Tyagi V, Pandey A, Sari A. 2018. Global advancement on experimental and thermal analysis of evacuated tube collector with and without heat pipe systems and possible applications. Applied energy 228: 351-389. Chopra K, Tyagi V, Pathak AK, Pandey A, Sari A. 2019. Experimental performance evaluation of a novel designed phase change material integrated manifold heat pipe evacuated tube solar collector system. Energy Conversion and Management 198: 111896. Coccia G, Aquilanti A, Tomassetti S, Comodi G, Di Nicola G. 2020. Design, realization, and tests of a portable solar box cooker coupled with an erythritol-based PCM thermal energy storage. Solar Energy 201: 530-540. D. Guerraiche CB, K. Guerraiche, L. Valenzuela, Z. Driss. 2020. Experimental and numerical study of a solar collector using phase change material as heat storage. Energy Storage, 27. da Cunha SRL, de Aguiar JLB. 2020a. Phase change materials and energy efficiency of buildings: A review of knowledge. Journal of Energy Storage 27: 101083. -. 2020b. Phase change materials and energy efficiency of buildings: A review of knowledge. Journal of Energy Storage 27. Dengjia Wang HL, Yanfeng Liu. 2019. Frost and High-temperature resistance performance of a novel dual-phase change material flat plate solar collector. Solar Energy Materials and Solar Cells 201. Department of Industry S, Energy and Resources. September 2020. Australian Energy Update 2020 Australian Energy Statistics, Canberra. . Dhaidan NS. 2020. Thermal Performance of Constrained Melting of PCM Inside an Elliptical Capsule of Two Orientations. Iranian Journal of Science and Technology, Transactions of Mechanical Engineering. Dhaidan NS. 2021. Thermal performance of constrained melting of PCM inside an elliptical capsule of two orientations. Iranian Journal of Science and Technology, Transactions of Mechanical Engineering 45: 515-521. Douvi E, Pagkalos C, Dogkas G, Koukou MK, Stathopoulos VN, Caouris Υ, Vrachopoulos MG. 2021. Phase Change Materials in Solar Domestic Hot Water Systems: A review. International Journal of Thermofluids: 100075. El Idi MM, Karkri M. 2020. Heating and cooling conditions effects on the kinetic of phase change of PCM embedded in metal foam. Case Studies in Thermal Engineering 21: 100716. EL IDI MM, KARKRI M, KRAIEM M. 2021. Preparation and effective thermal conductivity of a Paraffin/Metal Foam composite. Journal of Energy Storage 33: 102077. Elias CN, Stathopoulos VN. 2019. A comprehensive review of recent advances in materials aspects of phase change materials in thermal energy storage. Energy Procedia 161: 385-394.

F. Zhou JJ, W. Yuan, X. Zhao, S. Huang. 2019. Study on the PCM flat-plate solar collector system with antifreeze characteristics. International Journal of Heat and Mass Transfer 129: 9. Fan L-W, Wu Y-Y, Xiao Y-Q, Zeng Y, Zhang Y-L, Yu Z-T. 2016. Transient performance of a thermal energy storage-based heat sink using a liquid metal as the phase change material. Applied Thermal Engineering 109: 746-750. Fleischer AS. 2015a. Thermal Energy Storage Using Phase Change Materials. Springer briefs in thermal engineering and Applied science, Springer Cham Heidelberg New York Dordrecht London Fleischer AS. 2015b. Thermal energy storage using phase change materials: fundamentals and applications. Springer. Gil A, Oró E, Miró L, Peiró G, Ruiz Á, Salmerón JM, Cabeza LF. 2014. Experimental analysis of hydroquinone used as phase change material (PCM) to be applied in solar cooling refrigeration. International Journal of Refrigeration 39: 95-103. Greenland. 2018. Solar Thermal Energy. Website by SOLAR THERMAL, Australia. Hajizadeh MR, Selimefendigil F, Muhammad T, Ramzan M, Babazadeh H, Li Z. 2020. Solidification of PCM with nano powders inside a heat exchanger. Journal of Molecular Liquids 306. Han X, Tian Y, Zhao C. 2013. An effectiveness study of enhanced heat transfer in phase change materials (PCMs). International Journal of Heat and Mass Transfer 60: 459-468. Hanter. 2016. Safety Data Sheet - Hunter Industrials. infosafe He J, Yang X, Zhang G. 2019. A phase change material with enhanced thermal conductivity and secondary heat dissipation capability by introducing a binary thermal conductive skeleton for battery thermal management. Applied Thermal Engineering 148: 984-991. Heyhat MM, Mousavi S, Siavashi M. 2020. Battery thermal management with thermal energy storage composites of PCM, metal foam, fin and nanoparticle. Journal of Energy Storage 28. Hossain R, Mahmud S, Dutta A, Pop I. 2015. Energy storage system based on nanoparticle- enhanced phase change material inside porous medium. International Journal of Thermal Sciences 91: 49-58. Hosseini SMJ, Ranjbar AA, Sedighi K, Rahimi M. 2013. Melting of nanoprticle-enhanced phase change material inside shell and tube heat exchanger. Journal of Engineering 2013. Huang R, Li Z, Hong W, Wu Q, Yu X. 2019. Experimental and numerical study of PCM thermophysical parameters on lithium-ion battery thermal management. Energy Reports. Huang X, Alva G, Jia Y, Fang G. 2017. Morphological characterization and applications of phase change materials in thermal energy storage: A review. Renewable and Sustainable Energy Reviews 72: 128-145. Iasiello M, Mameli M, Filippeschi S, Bianco N. 2021. Metal foam/PCM melting evolution analysis: Orientation and morphology effects. Applied Thermal Engineering 187. Javadi F, Metselaar H, Ganesan P. 2020. Performance improvement of solar thermal systems integrated with phase change materials (PCM), a review. Solar Energy 206: 330-352. Kabeel AE, Abdelgaied M. 2017. Observational study of modified solar still coupled with oil serpentine loop from cylindrical parabolic concentrator and phase changing material under basin. Solar Energy 144: 71-78. Kalnæs SE, Jelle BP. 2015. Phase change materials and products for building applications: A state- of-the-art review and future research opportunities. Energy and Buildings 94: 150-176. Kalogirou SA. 2004. Solar thermal collectors and applications. Progress in Energy and Combustion Science 30: 231-295. Kasirga TS. 2020. Thermal Conductivity Measurements in Atomically Thin Materials and Devices. Springer Nature. Keisuke Sadamori TG. Aug 2021. The intergovernmental Panel on Climate Change IPCC (ed. FB Mary Burce Warlick). © 2021 Intergovernmental Panel on Climate Change (IPCC). Keith Lovegrove DA, Roman Bader, Stephen Edwards, Michael Lord, Ahmad Mojiri,. November 2019. Renewable energy option for industrial process heat ITP Thermal Pty Limited Austrailia. Khan MMA, Ibrahim NI, Mahbubul I, Ali HM, Saidur R, Al-Sulaiman FA. 2018. Evaluation of solar collector designs with integrated latent heat thermal energy storage: a review. Solar Energy 166: 334-350.

Khan RJ, Bhuiyan MZH, Ahmed DH. 2020. Investigation of heat transfer of a building wall in the presence of phase change material (PCM). Energy and Built Environment 1: 199-206. Khodadadi JM. 2015. Nanoparticle-enhanced phase change materials (NEPCM) with improved thermal energy storage. Google Patents. Kok B. 2020a. Examining effects of special heat transfer fins designed for the melting process of PCM and Nano-PCM. Applied Thermal Engineering 170. -. 2020b. Examining effects of special heat transfer fins designed for the melting process of PCM and Nano-PCM. Applied Thermal Engineering 170: 114989. Kumar PM, Mylsamy K. 2019. Experimental investigation of solar water heater integrated with a nanocomposite phase change material. Journal of Thermal Analysis and Calorimetry 136: 121- 132. Kumaresan V, Chandrasekaran P, Nanda M, Maini AK, Velraj R. 2013. Role of PCM based nanofluids for energy efficient cool thermal storage system. International Journal of Refrigeration 36: 1641-1647. Kuznik F, David D, Johannes K, Roux J-J. 2011. A review on phase change materials integrated in building walls. Renewable and Sustainable Energy Reviews 15: 379-391. Li B, Zhai X. 2017. Experimental investigation and theoretical analysis on a mid-temperature solar collector/storage system with composite PCM. Applied Thermal Engineering 124: 34-43. Li B, Zhai X, Cheng X. 2018. Experimental and numerical investigation of a solar collector/storage system with composite phase change materials. Solar Energy 164: 65-76. Li C, Zhang B, Xie B, Zhao X, Chen J, Chen Z, Long Y. 2019. Stearic acid/expanded graphite as a composite phase change thermal energy storage material for tankless solar water heater. Sustainable Cities and Society 44: 458-464. Li MY, Li B, Liu C, Su S, Xiao H, Zhu C. 2020a. Design and experimental investigation of a phase change energy storage air-type solar heat pump heating system. Applied Thermal Engineering 179. Li W, Wan H, Lou H, Fu Y, Qin F, He G. 2017. Enhanced thermal management with microencapsulated phase change material particles infiltrated in cellular metal foam. Energy 127: 671-679. Li WQ, Qu ZG, He YL, Tao WQ. 2012. Experimental and numerical studies on melting phase change heat transfer in open-cell metallic foams filled with paraffin. Applied Thermal Engineering 37: 1- 9. Li Z, Shahsavar A, Al-Rashed AAAA, Talebizadehsardari P. 2020b. Effect of porous medium and nanoparticles presences in a counter-current triple-tube composite porous/nano-PCM system. Applied Thermal Engineering 167. Lin Y, Jia Y, Alva G, Fang G. 2018. Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage. Renewable and sustainable energy reviews 82: 2730-2742. Ling Z, Chen J, Xu T, Fang X, Gao X, Zhang Z. 2015. Thermal conductivity of an organic phase change material/expanded graphite composite across the phase change temperature range and a novel thermal conductivity model. Energy Conversion and Management 102: 202-208. Liu Y, Duan J, He X, Wang Y. 2018. Experimental investigation on the heat transfer enhancement in a novel latent heat thermal storage equipment. Applied Thermal Engineering 142: 361-370. Liu Z, Ma C. 2002. Numerical analysis of melting with constant heat flux heating in a thermal energy storage system. Energy Conversion and Management 43: 2521-2538. Lv Y, Liu G, Zhang G, Yang X. 2020. A novel thermal management structure using serpentine phase change material coupled with forced air convection for cylindrical battery modules. Journal of Power Sources 468. M. Abuska SS, A. Kayapunar. 2019. Experimental analysis of solar air collector with PCM- honeycomb combination under the natural convection. Sol Energy Mater Sol Cells, 195: 9. M. Essa NM, M. Ibrahim. 2018. An experimental investigation of the phase change process effects on the system performance for the evacuated tube solar collectors integrated with PCMs. Energy Conversion and Management 177: 10.

Mahdi JM, Mohammed HI, Hashim ET, Talebizadehsardari P, Nsofor EC. 2020. Solidification enhancement with multiple PCMs, cascaded metal foam and nanoparticles in the shell-and-tube energy storage system. Applied Energy 257. Mahdi JM, Nsofor EC. 2017a. Melting enhancement in triplex-tube latent heat energy storage system using nanoparticles-metal foam combination. Applied Energy 191: 22-34. -. 2017b. Solidification enhancement in a triplex-tube latent heat energy storage system using nanoparticles-metal foam combination. Energy 126: 501-512. Manoj Kumar P, Mylsamy K. 2018. Experimental investigation of solar water heater integrated with a nanocomposite phase change material. Journal of Thermal Analysis and Calorimetry 136: 121-132. Mavrigiannaki A, Ampatzi E. 2016. Latent heat storage in building elements: A systematic review on properties and contextual performance factors. Renewable and Sustainable Energy Reviews 60: 852-866. Mazraeh AE, Babayan M, Yari M, Sefidan AM, Saha SC. 2018. Theoretical study on the performance of a solar still system integrated with PCM-PV module for sustainable water and power generation. Desalination 443: 184-197. Mehrali M, ten Elshof JE, Shahi M, Mahmoudi A. 2021. Simultaneous solar-thermal energy harvesting and storage via shape stabilized salt hydrate phase change material. Chemical Engineering Journal 405. Mehryan SAM, Heidarshenas MH, Hajjar A, Ghalambaz M. 2020. Numerical study of melting- process of a non-Newtonian fluid inside a metal foam. Alexandria Engineering Journal 59: 191- 207. Meng X, Meng L, Zou J, He F. 2021. Influence of the Copper Foam Fin (CFF) shapes on thermal performance of Phase-Change Material (PCM) in an enclosed cavity. Case Studies in Thermal Engineering 23. Mohamad AT, Sidik NAC, Abdulwahab MR. The Significant Effect of Graphene on Inorganic Salt Hydrated Phase Change Material Thermal Physical Enhancement. Journal of Advanced Research Design 59: 1-10. Mondal S. 2008. Phase change materials for smart textiles–An overview. Applied thermal engineering 28: 1536-1550. Mosaffa A, Ferreira CI, Talati F, Rosen M. 2013. Thermal performance of a multiple PCM thermal storage unit for free cooling. Energy Conversion and Management 67: 1-7. Nagappan B, Alagu K, Devarajan Y, Munuswamy DB. 2018. Energy and Exergy Analysis of Multi- Temperature PCMs Employed in a Latent Heat Storage System and Parabolic Trough Collector. Journal of Non-Equilibrium Thermodynamics 43: 211-220. Nguyen HQ, Shabani B. 2020a. Metal hydride thermal management using phase change material in the context of a standalone solar-hydrogen system. Energy Conversion and Management 224: 113352. -. 2020b. Proton exchange membrane fuel cells heat recovery opportunities for combined heating/cooling and power applications. Energy Conversion and Management 204: 112328. -. 2021. Review of metal hydride hydrogen storage thermal management for use in the fuel cell systems. International Journal of Hydrogen Energy 46: 31699-31726. Nie C, Liu J, Deng S. 2021. Effect of geometric parameter and nanoparticles on PCM melting in a vertical shell-tube system. Applied Thermal Engineering 184. Nourani M, Hamdami N, Keramat J. 2017. Preparation and evaluation of a stable nanoalumina- paraffin composite: Melting rate investigation using image analysis. Journal of Dispersion Science and Technology 39: 1385-1393. Panchal H, Patel J, Chaudhary S. 2019. A comprehensive review of solar cooker with sensible and latent heat storage materials. International Journal of Ambient Energy 40: 329-334. Parida A, Bhattacharya A, Rath P. 2020. Effect of convection on melting characteristics of phase change material-metal foam composite thermal energy storage system. Journal of Energy Storage 32. Qu S, Ma F, Ji R, Wang D, Yang L. 2015. System design and energy performance of a solar heat pump heating system with dual-tank latent heat storage. Energy and Buildings 105: 294-301.

Ren Q, Meng F, Guo P. 2018. A comparative study of PCM melting process in a heat pipe-assisted LHTES unit enhanced with nanoparticles and metal foams by immersed boundary-lattice Boltzmann method at pore-scale. International Journal of Heat and Mass Transfer 121: 1214- 1228. Roser HRaM. 2019. Energy Mix, global primary energy consumption by source OXFORD UNI, England and Wales. Sarafraz MM, Safaei MR. 2019. Diurnal thermal evaluation of an evacuated tube solar collector (ETSC) charged with graphene nanoplatelets-methanol nano-suspension. Renewable Energy 142: 364-372. Saydam V, Duan X. 2018. Dispersing different nanoparticles in paraffin wax as enhanced phase change materials. Journal of Thermal Analysis and Calorimetry 135: 1135-1144. Senobar H, Aramesh M, Shabani B. 2020. Nanoparticles and metal foams for heat transfer enhancement of phase change materials: A comparative experimental study. Journal of Energy Storage 32: 101911. Senthil R. 2020. Effect of charging of phase change material in vertical and horizontal rectangular enclosures in a concentrated solar receiver. Case Studies in Thermal Engineering 21. Sevault A, Kauko H, Bugge M, Banasiak K, Haugen NEL, Skreiberg Ø. 2017. Phase change materials for thermal energy storage in low-and high-temperature applications: a state-of-the-art. Shabani B, Biju M. 2015. Theoretical Modelling Methods for Thermal Management of Batteries. Energies 8: 10153-10177. Shi J-N, Ger M-D, Liu Y-M, Fan Y-C, Wen N-T, Lin C-K, Pu N-W. 2013. Improving the thermal conductivity and shape-stabilization of phase change materials using nanographite additives. Carbon (New York) 51: 365-372. Singh R, Sadeghi S, Shabani B. 2018a. Thermal Conductivity Enhancement of Phase Change Materials for Low-Temperature Thermal Energy Storage Applications. Energies 12. -. 2018b. Thermal Conductivity Enhancement of Phase Change Materials for Low-Temperature Thermal Energy Storage Applications. Energies 12: 75. -. 2018c. Thermal conductivity enhancement of phase change materials for low-temperature thermal energy storage applications. Energies 12: 1-20. Soares N, Costa JJ, Gaspar AR, Santos P. 2013. Review of passive PCM latent heat thermal energy storage systems towards buildings’ energy efficiency. Energy and buildings 59: 82-103. Socaciu L, Giurgiu O, Banyai D, Simion M. 2016. PCM selection using AHP method to maintain thermal comfort of the vehicle occupants. Energy Procedia 85: 489-497. Sun W, Huang R, Ling Z, Fang X, Zhang Z. 2020. Numerical simulation on the thermal performance of a PCM-containing ventilation system with a continuous change in inlet air temperature. Renewable Energy 145: 1608-1619. Thapa S, Chukwu S, Khaliq A, Weiss L. 2014. Fabrication and analysis of small-scale thermal energy storage with conductivity enhancement. Energy conversion and management 79: 161-170. Thomas Braun LK. 2020. Rubitherm Technologies GmbH Imhoffweg 6. (ed. Rubitherm), p. 1, Imhoffweg 6 12307 Berlin. Tian H, Wang W, Ding J, Wei X, Huang C. 2016. Preparation of binary eutectic chloride/expanded graphite as high-temperature thermal energy storage materials. Solar Energy Materials and Solar Cells 149: 187-194. Tian Y. 2012. Heat transfer enhancement in phase change materials (PCMs) by metal foams and cascaded thermal energy storage. University of Warwick. Tian Y, Zhao C-Y. 2009. Heat transfer analysis for phase change materials (PCMs). -. 2011a. A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals. Energy 36: 5539-5546. Tian Y, Zhao CY. 2011b. A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals. Energy 36: 5539-5546. Tong X, Li N, Zeng M, Wang Q. 2019. Organic phase change materials confined in carbon-based materials for thermal properties enhancement: Recent advancement and challenges. Renewable and Sustainable Energy Reviews 108: 398-422.

Tschopp D, Tian Z, Berberich M, Fan J, Perers B, Furbo S. 2020. Large-scale solar thermal systems in leading countries: A review and comparative study of Denmark, China, Germany and Austria. Applied Energy 270: 114997. Venkateshwar K, Joshy N, Simha H, Mahmud S. 2019. Quantifying the nanoparticles concentration in nano-PCM. Journal of Nanoparticle Research 21. W.Q. Li ZGQ, Y.L. He, W.Q. Tao. 2012. Experimental and numerical studies on melting phase change heat transfer

in open-cell metallic foams filled with paraffin. Applied Thermal Engineering 37: 9. Wan X, Chen C, Tian S, Guo B. 2020. Thermal characterization of net-like and form-stable ML/SiO2 composite as novel PCM for cold energy storage. Journal of Energy Storage 28. Wang F, Lin W, Ling Z, Fang X. 2019a. A comprehensive review on phase change material emulsions: Fabrication, characteristics, and heat transfer performance. Solar Energy Materials and Solar Cells 191: 218-234. Wang G, Wei G, Xu C, Ju X, Yang Y, Du X. 2019b. Numerical simulation of effective thermal conductivity and pore-scale melting process of PCMs in foam metals. Applied Thermal Engineering 147: 464-472. Wang H, Wang F, Li Z, Tang Y, Yu B, Yuan W. 2016. Experimental investigation on the thermal performance of a heat sink filled with porous metal fiber sintered felt/paraffin composite phase change material. Applied Energy 176: 221-232. Wang Z, Yang L, Hu J, Gu Z. 2017. Analysis on Cascade Heat Utilization of Solar Energy in Building Heating Based on Phase Change Thermal Storage. Procedia Engineering 205: 2266-2272. Wei G, Wang G, Xu C, Ju X, Xing L, Du X, Yang Y. 2018. Selection principles and thermophysical properties of high temperature phase change materials for thermal energy storage: A review. Renewable and Sustainable Energy Reviews 81: 1771-1786. Weiss W, Spörk-Dür M. 2020. Solar Heat Worldwide 2020. International Energy Agency Solar Heating & Cooling Programme. Wu J, Tremeac B, Terrier M-F, Charni M, Gagnière E, Couenne F, Hamroun B, Jallut C. 2016. Experimental investigation of the dynamic behavior of a large-scale refrigeration – PCM energy storage system. Validation of a complete model. Energy 116: 32-42. Wu S, Yan T, Kuai Z, Pan W. 2020. Thermal conductivity enhancement on phase change materials for thermal energy storage: A review. Energy Storage Materials 25: 251-295. Wu S, Zhu D, Zhang X, Huang J. 2010. Preparation and melting/freezing characteristics of Cu/paraffin nanofluid as phase-change material (PCM). Energy & fuels 24: 1894-1898. Xia Liu ZR. 2017. Experimental study on the thermal performance of graphene and exfoliated graphite sheet for thermal energy storage phase change material. Thermochimica Acta 647: 6. Xiao C, Zhang G, Li Z, Yang X. 2020. Custom design of solid–solid phase change material with ultra- high thermal stability for battery thermal management. Journal of Materials Chemistry A 8: 14624- 14633. Xiao X, Zhang P, Li M. 2014. Effective thermal conductivity of open-cell metal foams impregnated with pure paraffin for latent heat storage. International Journal of Thermal Sciences 81: 94-105. Yang J, Yang L, Xu C, Du X. 2016a. Experimental study on enhancement of thermal energy storage with phase-change material. Applied Energy 169: 164-176. Yang X-H, Tan S-C, Ding Y-J, Wang L, Liu J, Zhou Y-X. 2017a. Experimental and numerical investigation of low melting point metal based PCM heat sink with internal fins. International Communications in Heat and Mass Transfer 87: 118-124. Yang X-H, Tan S-C, Liu J. 2016b. Numerical investigation of the phase change process of low melting point metal. International Journal of Heat and Mass Transfer 100: 899-907. Yang X, Feng S, Zhang Q, Chai Y, Jin L, Lu TJ. 2017b. The role of porous metal foam on the unidirectional solidification of saturating fluid for cold storage. Applied Energy 194: 508-521. Yang X, Guo Z, Liu Y, Jin L, He Y-L. 2019a. Effect of inclination on the thermal response of composite phase change materials for thermal energy storage. Applied Energy 238: 22-33.

Yang X, Niu Z, Bai Q, Li H, Cui X, He Y-L. 2019b. Experimental study on the solidification process of fluid saturated in fin-foam composites for cold storage. Applied Thermal Engineering 161: 114163. -. 2019c. Experimental study on the solidification process of fluid saturated in fin-foam composites for cold storage. Applied Thermal Engineering 161. Yang X, Wei P, Cui X, Jin L, He Y-L. 2019d. Thermal response of annuli filled with metal foam for thermal energy storage: An experimental study. Applied Energy 250: 1457-1467. Yang X, Yu J, Xiao T, Hu Z, He Y-L. 2020. Design and operating evaluation of a finned shell-and- tube thermal energy storage unit filled with metal foam. Applied Energy 261. Zambolin E, Del Col D. 2010. Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions. Solar Energy 84: 1382-1396. Zarei MJ, Bazai H, Sharifpur M, Mahian O, Shabani B. 2020a. The effects of fin parameters on the solidification of PCMs in a fin-enhanced thermal energy storage system. Energies 13: 198. -. 2020b. The Effects of Fin Parameters on the Solidification of PCMs in a Fin-Enhanced Thermal Energy Storage System. Energies 13. Zhang C, Yu M, Fan Y, Zhang X, Zhao Y, Qiu L. 2020a. Numerical study on heat transfer enhancement of PCM using three combined methods based on heat pipe. Energy 195: 116809. -. 2020b. Numerical study on heat transfer enhancement of PCM using three combined methods based on heat pipe. Energy 195. Zhang P, Xiao X, Meng ZN, Li M. 2015. Heat transfer characteristics of a molten-salt thermal energy storage unit with and without heat transfer enhancement. Applied Energy 137: 758-772. Zhao C, Lu W, Tian Y. 2010. Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs). Solar energy 84: 1402-1412. Zheng H. 2017. Chapter 2—Solar Energy Utilization and Its Collection Devices. Solar Energy Desalination Technology: 47-171. Zheng L, Zhang W, Xie L, Wang W, Tian H, Chen M. 2019. Experimental study on the thermal performance of solar air conditioning system with MEPCM cooling storage. International Journal of Low-Carbon Technologies 14: 83-88. Zhou D, Zhao C-Y, Tian Y. 2012. Review on thermal energy storage with phase change materials (PCMs) in building applications. Applied energy 92: 593-605. Zhou Z, Liu J, Wang C, Huang X, Gao F, Zhang S, Yu B. 2018. Research on the application of phase- change heat storage in centralized solar hot water system. Journal of Cleaner Production 198: 1262-1275. Zhou Z, Zhang Z, Zuo J, Huang K, Zhang L. 2015. Phase change materials for solar thermal energy storage in residential buildings in cold climate. Renewable and Sustainable Energy Reviews 48: 692-703.

A. Appendix

A.1. Publications From This Research Program

Journal Papers

Senobar H, Aramesh M, Shabani B. 2020. Nanoparticles and metal foams for heat transfer enhancement of phase change materials: A comparative experimental study. Journal of Energy Storage 32: 101911.

https://doi.org/10.1016/j.est.2020.101911

Senobar H., Aramesh M., Shabani B. 2022, Evacuated tube solar thermal collector with enhanced phase change material thermal storage: an experimental study. Journal of Energy Storage 46, 103838.

https://doi.org/10.1016/j.est.2021.103838

A.2. Solar Collector

Figure A- 1. Schematic view of the collar collector GL 100-16

Table A- 1. Thermal energy yield per collector module {kWh/module} and hot water delivery [Litres/module]

The specifications of the items used in constructing the solar thermal hot water system are presented in this section.

A.3. Paraffin (Phase Change Material)

Figure A- 2. Data sheet of the paraffin RT44HC

The specifications of the paraffin used in the PCM composite in the experiment.

A.4. Loading and Flowrate of hot water

Table A- 2. Flow rates and loading unites for heated water

The table below shows the peak spa consumption for a residential unit used for calculations and design. This information is taken from Australian standards. The flow rates to fixtures, appliances, taps and valves shall be not less than the flow rates specified in Error! Reference source not found..