politecnico di milano · of “laurea magistrale” my love is divided for three cities i.e....

POLITECNICO DI MILANO

Dipartimento di Scienze e Tecnologie Aerospaziali

Corso di Laurea Magistrale

Ingegneria Spaziale

Phase Change Material as a Heat Sink Device

for Small Satellites

Relatore: Dr.Francesco Topputo

Co - relatore: ACTIVE SPACE TECHNOLOGIES GmbH

Tesi di Laurea Magistrale di:

Siddharth Tiwari

Matr. 780101

Anno Accademico 2013 - 2014

Abstract

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Abstract

The thesis primarily investigates the possibility of adapting the high heat

capacity displayed by some materials near their melting point, from henceforth

called Phase Change Materials (PCM), for their use as a heat sink device aboard

small satellites. Theoretically, under their optimum performing conditions not

only do PCMs display better behaviour than conventional heat sinks, but they

also lower the mass budget required for the thermal control of the spacecraft

and its components, while simplifying the whole process of thermal control (by

making it passive).

However, PCM based heat sinks do have some shortcomings of which low

thermal conductivity and high volumetric expansion (during phase change) are

particularly significant and which make them difficult to be exploited for space

based applications. While the thesis will tackle the problem of low thermal

conductivity by the insertion of fins made up of high conductive materials, the

problem of volumetric expansion of the PCM will be accounted by leaving a

clearance volume in the PCM box (for this study), so as to avoid its failure.

Finally to sum it up, the primary goal of the thesis is to validate the concept of

PCM as an efficient heat sink device for its use aboard small satellites and to

obtain an acceptable coherence between the simulation and experimental results

with the intention of possessing the ability to simulate large variations of PCM

boxes and to be able optimise one for a given application or mission in the

future.

Key words: Phase Change Material, Fins, Satellites, Heat Sink

Sommario

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Sommario

‘Il presente lavoro di tesi si concentra sull’analisi di materiali a cambiamento di

fase (Phase Change Materials – PCM) per applicazioni aerospaziali,

proponendo una innovativa soluzione tecnologica per il controllo termico di

satelliti di piccole dimensioni. In condizioni teoriche ottimali i PCM, infatti,

dimostrano non solo di essere molto piú performanti di un radiatore

convenzionale ma anche vantaggiosi in termini di massa, consentendo una

significativa riduzione del mass budget per controllo termico e quindi

permettendo aumentazione della massa a disposizone per il payload.

La bassa conducibilitá termica e l’espansione del materiale subita in seguito a

fusione – elementi che appaiono proibitivi per un effettivo utilizzo dei PCM in

ambito aerospaziale – vengono risolti ricorrendo all’espediente di fins metallici,

opportunamente inseriti nella scatola contenente i PCM e lasciando un po di

spazio vuoto nella scatola (per questo studio) rispettivamente.

L’obiettivo dell’analisi condotta é dunque la validazione della suddetta tecnica

adoperata, mostrando una buona corrispondenza tra i risultati numerici e

sperimentali ottenuti testando diverse configurazioni di contenitori per i PCM

allo scopo di pervenire ad un design ottimale che possa essere implementato in

future missioni spaziali

Key words: Materiali a Cambiamento di Fase, Fins, Satellite, Radiatore.

Acknowledgements

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Acknowledgements

For all the interesting and stimulating technical notes that have been composed

in this document, this section is probably my most preferred one because it gives

me the opportunity to be grateful to all the beautiful people in my life, who

made this journey possible. It’s fair to say, that at the end of this little pilgrimage

of “Laurea Magistrale” my love is divided for three cities i.e. Bombay, Milan &

Berlin and its people.

Bombay. How could I have ever managed anything without the help and

support of my loving parents and my family. They stood by me through the

thick and thin and always supported my decisions. I am thankful to my Sister,

who with her work ethic and determined attitude has had a profound effect on

me all throughout my life. My Logistics Guru, My Uncle Sanjay, has always

made sure that my relocation to a new city has been planned to perfection,

constantly pointing out the things that I must be careful about. The chiding and

scolding of Mrs.Chopra, my childhood mentor, still reverberates every time I am

about to go wrong. I am highly indebted to Rishikesh for introducing me to the

world of Aerospace through the very many gliders we built together.

Milan. Saying that the journey through Polimi was a cakewalk would make me

the biggest liar in the world. But during the testing times (and not so testing

ones), I had a bunch of awesome friends from Campus Certosa who ensured for

a fun filled time outside the university. We cooked together, played together,

laughed together and learned to share our joys and problems. Visiting them in

Teramo and Rocarasso on the occasion of Christmas and New Years formed

quite the highlight of my stay in Italy. Life would have been much different

(read worse) without them. Not to forget my classmates at Polimi, who had to

bear the burden of my stupid questions and jokes in class, and who guided me

through the magistrale years. Special thanks to My Dearest Uncle Vico, who

was always calling me up to check if everything was alright, and who along with

Aunt Ester, entertained me with their adventurous life stories on some special

Sunday afternoons at their home. Last but not the least, I am very grateful to

Prof.Bernelli and Prof.Topputo, who made all this happen. Who would have

thought that a hopeful mail in 2010 for an application for internship would lead

to all this? Thank you so much!

Berlin. It surely is the city that never sleeps and I have been lucky to have

learned so much from my experience here. Firstly, a big thanks to Riccardo for

having faith in me and giving me the opportunity to perform my thesis at Active

Space Technologies (AST). Thanks to all the guys of the thermal department i.e.

Acknowledgements

5

Asli, Gosia & Luca for coming up with suggestions for improvements from time

to time. Special thanks to (Senor) Matthew for helping me with the 3-Omega

measurements for the PCM and for being my “Scientific Guide” during my stay

at AST. Thanks to my Active Space Mother, Fariba, for taking care of me

during my time at AST. A noteworthy mention also to Sasol GmbH for being

highly interested in the project and for readily providing us with samples of

PCM. I am also grateful to all my friends in Berlin who made certain that I

always had diversions after work for recreation.

Lastly, I see this as a culmination of the efforts of all the teachers I have ever

had and to whom I will always be grateful to.

-Sid

Index

1 LITERATURE SURVEY 12

1.1 INTRODUCTION TO PCM 13 1.2 CLASSIFICATION OF PCM 15 1.3 CHALLENGES IN THE USE PCM 17

2 CHARACTERISATION OF PCM 21

2.1 SELECTION OF PCM 21 2.2 DETERMINATION OF MELTING POINT 22 2.3 DETERMINATION OF DENSITY 28 2.4 DETERMINATION OF THERMAL CONDUCTIVITY 29 2.5 DETERMINATION OF LATENT HEAT OF MELTING 32 2.6 SUMMARY OF THE THERMOPHYSICAL PROPERTIES 33

3 TEST CONFIGURATION AND SETUP 34

3.1 MATERIALS 34 3.2 TEST SETUP 40 3.3 CONTROL PARAMETERS FOR THE TEST SETUP 45

4 TEST RESULTS 47

4.1 PCM BOX WITHOUT FINS 47 4.2 PCM BOX WITH FINS 52

5 MODELLING & SIMULATION OF PCM IN ESATAN-TMS 55

5.1 APPROACHES TO PCM MODELLING 55 5.2 THERMAL MATHEMATICAL MODELLING 58 5.3 DISCREPANCY BETWEEN TEST SETUP AND SIMULATION MODEL 63 5.4 IDENTIFICATION OF OPTIMAL FIT WITH PARAMETRIC ANALYSIS 67 5.5 SIMULATION FOR VARIOUS NUMBER OF FINS 75 5.6 COMPARISON OF PCM BOX IN VARIOUS CONFIGURATIONS 76

6 CONCLUSION 78

6.1 GOALS ACHIEVED 78 6.2 SCOPE FOR IMPROVEMENT AND FUTURE WORK 80 6.3 FINAL REMARKS 84

APPENDIX 85

A. SIZING OF THE ELECTRONIC BOX 85 B. PERFORMANCE OF PCM WITH FILLERS 87 C. RADIATOR SIZED FOR PEAK DISSIPATION V/S PCM SYSTEM 91

REFERENCES: 95

Index of Figures

8

Index of Figures

Figure 1-1: PCM Heat Sink and its basic setup .................................................. 13 Figure 1-2-(a-d): Various stages of operation of a PCM box ............................. 14 Figure 1-3: Classification of PCM ...................................................................... 16 Figure 1-4: Subcooling of PCM .......................................................................... 18

Figure 2-1: Specific heat capacity for NE12 ....................................................... 22 Figure 2-2: Newton’s cooling curve ................................................................... 23 Figure 2-3: Representation of the experimental setup for Newton’s cooling ..... 23

Figure 2-4: Cooling curve data from all sensors ................................................. 24 Figure 2-5: Cooling curve data from middle sensor ........................................... 25 Figure 2-6: Algorithm to obtain melting point .................................................... 26 Figure 2-7: Graphical result after post processing .............................................. 27

Figure 2-8: Hardware for the 3-Omega method .................................................. 29 Figure 2-9: Sample of Aerogel for 3-Omega characterisation ............................ 30

Figure 2-10:NE12 sample for 3-Omega characterisation ................................... 30 Figure 2-11: Trend of thermal conductivity with temperature ............................ 31

Figure 2-12: Trend of diffusivity and specific heat capacity .............................. 32

Figure 3-1: Universal housing used for PCM ..................................................... 35

Figure 3-2: Detailed view of the universal housing ............................................ 35 Figure 3-3: Aluminium plate (dummy E-box) with heater and sensors .............. 37

Figure 3-4: Assembly of Aluminium plate (E-box) and PCM box ..................... 37 Figure 3-5: RTD inserted inside the PCM box ................................................... 38 Figure 3-6: PCM box with fins ........................................................................... 39

Figure 3-7: The assembly of the Cooling Machine, TVC & Data Logger ......... 40 Figure 3-8: External view of PCM box completely sealed ................................. 41

Figure 3-9: Graphical representation of Orientation 1 ........................................ 42 Figure 3-10: Graphical representation of Orientation 2 ...................................... 43 Figure 3-11: Graphical representation of Orientation 3 ...................................... 44

Figure 3-12: Serial communication setup using RS232 for pressure data .......... 46 Figure 4-1: Tests result for PCM box without fins-various powers.................... 48 Figure 4-2: Tests result for PCM box without fins -10 watt ............................... 48

Figure 4-3: Sectional front & side view of the PCM box ................................... 49 Figure 4-4: Data from sensor on the E-box & inside the PCM box .................... 50 Figure 4-5: Tests result for PCM box without fins w/ melting line .................... 51 Figure 4-6: Test results for PCM box with fins .................................................. 52 Figure 4-7: Comparison of test result for PCM box w/ & w/o fins .................... 53

Figure 5-1: Finite difference modelling of PCM ................................................ 55 Figure 5-2: Graphical representation of variable heat capacity modelling ......... 56 Figure 5-3: Enthalpy V/s Temperature near phase change temperature ............. 57

Figure 5-4: Physical representation of the model (front view) ........................... 58

Index of Figures

9

Figure 5-5: Sectional front view of the PCM box ............................................... 60 Figure 5-6:Non-Sectional (Normal) front view of PCM box .............................. 60 Figure 5-7: Sectional top view of the PCM box ................................................. 61 Figure 5-8: Sectional top view of the PCM box with fins .................................. 61

Figure 5-9: Representation of Thermal Mathematical Model ............................. 63 Figure 5-10: Comparison of exp. & sim. result w/o correction .......................... 64 Figure 5-11: Sectional view of unmelted PCM ................................................... 65 Figure 5-12: Sectional view of partially melted PCM in Space .......................... 65

Figure 5-13: Sectional view of partially melted PCM on Ground ...................... 66 Figure 5-14: Sensitivity analysis for Gravity correction factor .......................... 66 Figure 5-15: Comparison of exp & sim. results for PCM box w/o fins-10 watt. 69 Figure 5-16: Simulation with variable number of PCM elements ...................... 70

Figure 5-17: Comp. of exp. & sim. results for PCM box w/o fins-var. powers .. 71 Figure 5-18: Comp. of exp. and sim. results for PCM box w/ fins-10 watt ........ 73 Figure 5-19: Comp. of exp. & sim. results for PCM box w/ fins-var. powers .... 74 Figure 5-20: Simulation of PCM box with variable number of fins ................... 75

Figure 5-21: E-box temperature trend in various orientations ............................ 76 Figure 6-1: Experimental setup for PCM box to reduce the effect of gravity .... 83

Figure A-1: Graphical representation of PCM at the end of melting cycle ........ 88

Figure A-2: PCM performance with variation in filler ....................................... 89

Figure A-3: Duty cycle of cyclically dissipating component .............................. 91 Figure A-4: Mass of radiator system sized for Max.dissip. V/s PCM sys. ......... 93

Index of Tables

10

Index of Tables

Table 0-1: List of Acronyms ............................................................................... 11 Table 1-1: Technology Readiness Level in the ESA [Reference [19]] ............... 12 Table 2-1: Properties of NACOL Ether 12 ......................................................... 21 Table 2-2: Results after post processing data ...................................................... 27

Table 2-3: Error in temperature measurement of RTD ....................................... 28 Table 2-4: Density values for NE12 .................................................................... 28 Table 2-5 : Errors /Uncertainties in 3-Omega characterisation method ............. 31

Table 2-6: Average values for the thermophysical properties ............................ 32 Table 2-7: Latent heat values for NE12 .............................................................. 32 Table 2-8: Summary of Thermophyiscal properties of NE12 ............................. 33 Table 3-1: Geometric dimensions of the universal housing................................ 36

Table 3-2: Location of RTDs on PCM box ......................................................... 39 Table 3-3: Performance capability of TVC ......................................................... 40

Table 3-4: Summary of possible test orientations ............................................... 44 Table 4-1: Summary of tests performed.............................................................. 47

Table 5-1: Colour representation of elements ..................................................... 59

Table 5-2: Colour representation of elements in Figure 5-7 ............................... 61

Table 5-3: Colour representation of elements in Figure 5-8 ............................... 62 Table 5-4: Colour code for graphical representation .......................................... 65

Table 5-5:Cont. Resistance & Gravity correction factor (10 W w/o fins) .......... 69 Table 5-6: Parameters for evaluation of fit (10 W w/o fins) ............................... 69 Table 5-7: Cont. Resistance & Gravity correction factor (w/o fins) ................... 71

Table 5-8: Parameters for evaluation of fit (w/o fins) ......................................... 72 Table 5-9: Cont. Resistance & Gravity correction factor (10 W w/ fins) ........... 73

Table 5-10: Parameters for evaluation of fit (10 W w/ fins) ............................... 73 Table 5-11: Cont. Resistance & Gravity correction factor (w/ fins) ................... 74 Table 5-12: Parameters for evaluation of fit (w/ fins .......................................... 74

Table 6-1:Summary of tests and simulations ...................................................... 78 Table 6-2: Evaluation of TRL ............................................................................. 80 Table A-1: Description of a generic E-box for small satellites ........................... 85

Table A-2: Thermophysical properties of various components of E-box ........... 85 Table A-3: Thermophysical property of Aluminium alloy ................................. 86 Table A-4: Input values for system comparison ................................................. 93

Acronym

11

Acronym

AST Active Space Technologies

PCM Phase Change Material

ESATAN-TMS European Space Agency Thermal Analysis

Network-Thermal Modelling Suite

RTD -Resistance Temperature Detector

PT100 Platinum type RTD with 100 ohm at 0ºC

N/A Not Applicable

TVC Thermal Vacuum Chamber

NE12 NACOL Ether 12

TRL Technology Readiness Level

ESA European Space Agency

E-box Electronic box

PSA Pressure Sensitive Adhesive

S/C Spacecraft

Table 0-1: List of Acronyms

Literature Survey

12

1 Literature Survey

Prior to the commencement of the literature survey phase of the thesis, specific

aim and goals of the thesis were well identified in order to be able to channelize

the efforts and concentrate only on the relevant literature, out of the vast

resource which is available on the subject of Phase Change Materials (PCM).

With the primary aim of the thesis being adoption of PCM as a heat sink for

electronic boxes aboard small satellites, efforts were directed towards review of

scientific papers which talked about attempts to do the same. References [1], [9],

[14], [18], [21] & [22] discuss the aforementioned issue.

Low thermal conductivity being one of the major shortcomings of PCM,

references which talked about possible methods of augmentation of the heat

transfer within PCM were surveyed. This has been examined in deep in

References [1], [6], [10] & [12].

Volumetric expansion of the PCM on melting is a major issue which needs to be

tackled before its use aboard a satellite and with that in mind References [9],

[22], [23], [24] & [25] were reviewed. However, considering the already sheer

volume of work required to be done (minus the volumetric expansion on

melting) to validate the concept of PCM for thermal control, it was decided that

a small gap would be left inside the PCM box (equivalent to the volumetric

expansion) to tackle this problem temporarily.

A Technology Readiness Level (TRL) of 4 for the PCM Heat Sink box or PCM

based thermal control was aimed to be reached at the end of the thesis.

Technology

Readiness Level

Description

TRL1 Basic principles observed and reported

TRL2 Technology concept and/or application formulated

TRL3 Analytical & experimental critical function and/or

characteristic proof-of-concept

TRL4 Component and/or breadboard validation in laboratory

environment

TRL5 Component and/or breadboard validation in relevant

environment

TRL6 System/subsystem model or prototype demonstration in a

relevant environment (ground or space)

TRL7 System prototype demonstration in a space environment

TRL8 Actual system completed and "Flight qualified" through test

and demonstration (ground or space)

TRL9 Actual system "Flight proven" through successful mission

operations

Table 1-1: Technology Readiness Level in the ESA [Reference [19]]

Literature Survey

13

1.1 Introduction to PCM

It is an inherent property of most materials in nature to exhibit a large heat

capacity (latent heat capacity), when undergoing phase transition. This

behaviour is due to the large energy required to overcome the molecular forces

of attraction when there is a change of state from a denser to sparser medium,

making the process endothermic during melting or vaporisation. A similar effect

takes place when there is freezing or condensation of the material, but the

process is exothermic.

In Reference [1], it was noted that such phenomenon could be useful for cyclic

processes i.e. storing heat energy when there is excess of it (endothermic) and

releasing when there is lack of it (exothermic). To elaborate, a spacecraft

experiences cyclic variation in ambient conditions i.e. when facing the sun, the

temperature of the spacecraft rises and, on the contrary, the period of eclipse is

marked by large heat loses from the spacecraft resulting in excess cooling of the

spacecraft. With the help of PCM, the excess of heat could be absorbed from the

spacecraft and stored in the form of melting or vaporisation of the PCM and

then during the cold cycles this heat would be released causing freezing or

condensation of the PCM with the effect of heating the spacecraft.

The same concept is also valid for components which undergo cyclic heat

dissipations during operation and is expatiated below.

A classical PCM based heat sink designed for a component has been shown

below:

Figure 1-1: PCM Heat Sink and its basic setup

Reference [1]

Literature Survey

14

The component is mounted atop a PCM box which in turn is mounted on a

radiator which helps in rejecting the heat absorbed by the PCM during

dissipation cycle and making it operational again. The PCM box itself acts as

heat accumulator , but the system of the PCM box along with a small radiator

(to reject the heat to deep space) constitutes the heat sink system.

When the dissipation phase of the component starts, the PCM is completely in

solid state (figure1.2.a).

As heat is absorbed by it, the process of melting is initiated (figure 1.2.b).

The PCM box has to be sized in such a manner that as soon as the dissipation

cycle of the component ends, all the PCM in the PCM box is in liquid state

(figure 1.2.c) i.e. the right most layer has just changed its state from solid to

liquid when the dissipation ends.

This is then followed by period of radiative cooling where the PCM freezes

again (figure 1.2.d) and as soon as it completely turns to solid (figure 1.2.a), the

dissipation of the components starts again and the whole cycle repeats itself.

1.2.a: PCM in solid state 1.2.b: Partially melted PCM

1.2.c- Fully melted PCM 1.2.d-Partially frozen PCM

Figure 1-2-(a-d): Various stages of operation of a PCM box

Reference [1]

Literature Survey

15

Such materials by nature would passively control the temperature of the

spacecraft around its phase change temperature, eliminating the need for any

sort of active control, and thus, simplifying the whole process of thermal control

of the component or the S/C.

Reference [1] has also elaborated the advantages of a PCM based thermal

control system over a radiator based thermal control system, stating that

although a radiator sized for a given component’s dissipation cycle maintains

the temperature below the highest allowable value during the dissipation cycle,

it continues to lose heat to the deep space when the component is not functional

and this causes excess cooling of the component, which necessitates the

deployment of heaters. This results in higher power requirements by the S/C

along with the necessity for a continuous active control.

However, there are a few challenges and obstacles which need to be tackled

before this technology can be used for space applications. These primarily

include low thermal conductivity exhibited by phase change materials and

volumetric expansion upon phase change.

Low thermal conductivity results in non-uniform distribution of heat within the

PCM box, which results in reduced efficiency of the system. Volumetric

expansion upon phase change (only melting/vaporisation) causes increased

stresses on the PCM vessel. This makes the design of the vessel highly

challenging. These issues will be discussed in deep at a later stage of this

chapter.

Note: Although a detailed discussion about the chemistry and classification of

PCM is not the main aim of the thesis, a brief description in reference to this

topic has been given below.

1.2 Classification of PCM

Having described the salient features of a phase change system, one can then

infer about the thermophysical properties required by such system. This will

help in the elimination of a large category of materials and we will be left with a

select few, which are most suitable for the design of a PCM based heat sink and

related applications.

Reference [4] states that materials to be used for phase change thermal energy

storage must have a large latent heat and high thermal conductivity. They should

have a melting temperature lying in the practical range of operation, melt

Literature Survey

16

congruently with minimum sub cooling and be chemically stable, low in cost,

non-toxic and non-corrosive.

For space based applications, where mass of the system is a major issue, it is

safe to say that they should have low volumetric expansion upon phase change

(so that the vessel which houses the PCM is not bulky). This criterion eliminates

all the liquid-gas transformations, because the change in volume that occurs in a

liquid-gas transformation is much higher than that of solid-liquid

transformation.

It is fair to say that there is no “one” material that satisfies all these criterions at

the same time, but there are families of materials which do satisfy few of the

required criterions (Reference [5]) and they have been listed below:

Figure 1-3: Classification of PCM

Literature Survey

17

1.2.1 Organic

Organic PCM mainly include paraffins and non-paraffins, both of which

showcase high latent heat without much degradation of performance with

repeated cycles.

1.2.2 Inorganic

Inorganic PCM are further classified into Salt Hydrates and Metallics.

The solid–liquid transformation of salt hydrates is actually a dehydration of the

salt, although this process resembles melting or freezing thermodynamically.

The major problem of salt hydrates as PCM is sub cooling and phase

segregation, which occurs on phase change and this results in the degradation of

thermal performance.

Metallics include the low melting metals and metal eutectics. These metallics

have not yet been seriously considered for PCM technology because of weight

penalties.

1.2.3 Eutectic

A eutectic is a minimum-melting composition of two or more components, each

of which melts and freeze congruently forming a mixture of the component

crystals during crystallisation.

1.3 Challenges in the use PCM

As mentioned earlier, there are a few issues which need to be resolved prior to

harvesting PCM technology for space based or other terrestrial applications.

These have been listed below:

1.3.1 Low thermal conductivity

Most PCMs that have been mentioned in the classification earlier exhibit a low

value of thermal conductivity (mean of around 0.1-0.3 W/m-K Reference [5]).

This low value of thermal conductivity creates a hindrance for the transfer of

heat through the PCM vessel, and thus, the effective efficiency is low.

In order to overcome this issue, substitution of some of the PCM with high

thermal conductivity fillers (like metals) has been suggested in References [1],

[3] and [6].

In Reference [1], an analytical analysis of the acceptable amount of filler to

PCM ratio that must be present in the system so as to improve its performance,

and still have a pre dominantly PCM system with some metal filler rather than

vice versa, has been discussed. For further details check Appendix B. (From the

Literature Survey

18

discussion made in the Appendix B one can also infer how PCM is better than a

metallic sink in overall performance).

Reference [3] & [10] have extensively discussed the use of fins along the depth

of the PCM vessel to efficiently transport heat to the internal layers and to

promote the simultaneous melting of a larger amount of PCM, thus improving

the performance of the system. The effects of fins on the system have been

discussed in a profounder manner at a later stage in the thesis.

1.3.2 Sub cooling

Sub cooling is a phenomenon which results in the delay of the crystallisation of

the PCM. Generally crystallisation starts at a point in the liquid solution and

then propagates throughout the volume of the PCM. However, owing to certain

reasons like lack of impurities, very rapid cooling etc. there is a delay in the

crystallisation and the crystallisation fails to initiate at any point in the solution

at its standard freezing temperature and the material continues to exist in liquid

state. This temporary reduction in heat capacity of PCM delay could result in the

temperature of the component (for which the PCM is acting as heat sink) to be

cooled below its allowable limit, and thus, resulting in its failure.

Once crystallisation takes place at any point after the delay, the PCM once

again exhibits normal behaviour.

This phenomenon can be observed in the figure below:

Figure 1-4: Subcooling of PCM

[Reference [7]]

Literature Survey

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1.3.3 Degradation in thermal behaviour

Some families of PCM show degradation in their thermophysical properties

arising from numerous reasons like chemical decomposition, phase segregation,

incompatibility with materials of construction etc. (Reference [5]).

Salt hydrates are especially susceptible to this owing to phase segregation.

When they melt, salt hydrates separate in two discrete phases i.e. salt and the

hydration medium. Salts settle at the bottom (owing to their larger density) and

fail to mix uniformly with the hydrating medium upon freezing. This

degradation increases as the number of cycles increase and after the passage of a

certain amount of time there is significant difference in the latent heats of the

material at the start and the end of large number of cycles.

This phenomenon can be overcome with the help of mechanical stirring or

mixing of the solution before freezing.

It is interesting to note, however, that the main reason for the phase segregation

is gravity and in the absence of it (like in space conditions), this phenomenon is

not going to have significant effect on the material’s thermophysical properties.

For a small satellite which is commissioned in LEO orbit, the PCM should be

capable of surviving around 30,000 melting /freezing cycle without significant

degradation.

1.3.4 Mechanical Design of the Vessel

The next two points mainly deal with mechanical design of the PCM box or

vessel which houses the PCM.

On melting, most PCMs undergo a 10 % increase in volume (Reference [18] )

due to change in density from the solid to liquid state and this puts two rigid

constraints on the design of the vessel:

1.3.4.1 Liquid tight vessel

On melting, there will be tendency of the low density liquid PCM to flow

outside the box. The tendency is further augmented due to the fact that there is

vacuum outside the box.

1.3.4.2 Failure Proof PCM Vessel

The low density liquid also causes creation of large stresses on the walls of the

vessel and if not designed properly, can result in the mechanical failure of the

vessel. At the same time, care has to be taken to avoid creation of a very bulky

vessel because then the whole concept of PCM as heat sink would lose its

meaning, which is -efficient thermal control at low cost and low mass budget.

Literature Survey

20

It would seem that a quick fix for this problem would be leaving a small void

equivalent to the expansion of the PCM in the liquid state. However, besides the

fact that this gap leads to inefficient heat transfer from the component to the

PCM and reduced capacity of the PCM box, the location of such a gap cannot be

controlled in space environment owing to the lack of gravity, which makes it

difficult to simulate the behaviour of the PCM system which will change each

time the location of this void changes.

In the chapters ahead, a more detailed discussion of each step required for the

fulfilment of the aim of the thesis has been done.

Characterisation of PCM

21

2 Characterisation of PCM

The aim of this phase of the project was to be able to precisely determine the

thermophysical properties of a given PCM, and hereby establish standard

procedures for similar characterization tests to be performed in the future for a

wide range of PCM. This included determining the melting point, density,

thermal heat capacity, latent heat of melting etc.

2.1 Selection of PCM

For the initial tests, the PCM was provided by SASOL GmbH from Hamburg.

The preferable temperature of melting was around 30-35 ˚C. This temperature

range was chosen even though theoretically we require a melting temperature

around 20 ˚C for most space applications. This is because for practical purpose

it was easier to handle solid PCM at room temperature, rather than liquid PCM.

Based on available PCMs at Sasol, NACOL Ether 12 (NE12) was found most

suitable for our application. Another reason for which NACOL Ether 12 was

selected was that it showcased a sharp melting point between 30-32ºC, which

suits the sub-routine developed to simulate phase change materials

accompanied with a high value of latent heat. The physical and chemical

properties provided by the supplier for NE12 are given below:

Name NACOL Ether 12

Appearance Slightly yellow after melting

Contents

Dilaurylether 85 wt%

Water 0.1 wt%

Ester Number 5 mgKOH/g

Acid Number 1 mgKOH/g

Thermophysical

Properties

Melting Point/Range 30˚-32˚C

Density 0.789-0.804 g/ml

Flash Point 194˚C

Latent Heat 201 kJ/kg

Table 2-1: Properties of NACOL Ether 12

provided by supplier


22

Given below is the three layer calorimeter analysis data provided by the

manufacturer for NE12.

Figure 2-1: Specific heat capacity for NE12

2.2 Determination of Melting Point

2.2.1 Newton’s Cooling Curve

The process of phase transition is governed by Newton's law of cooling, which

states that the rate of change of temperature of an object is proportional to the

difference between its own temperature and the temperature of its surroundings,

which in equation form can be presented as [16]

Where T and To are the body and surrounding temperature respectively and k a

constant which depends on thermal and physical properties of the body and its

ambient conditions.

It is expected that during phase change (solidification), the Newton’s curve

would show the following trend:


23

Figure 2-2: Newton’s cooling curve

[Reference [16]]

Near its melting point it will show a plateau like region (considering that the

ambient temperature remains constant) and this could be used to obtain the

melting point.

2.2.2 Experiment for Newton’s Cooling Curve

A melted sample of PCM was taken in a metal can and was cooled very slowly

from the top and sides of the can through natural convection. Three Resistance

Temperature Detectors (RTD) were inserted inside the can as follows:

Figure 2-3: Representation of the experimental setup for Newton’s cooling


24

There was one on the top surface, one in the middle and lastly one right at the

bottom of the can. The idea behind putting more than one sensor inside the can

arises from the following reasoning:

1. The top and bottom surfaces are cooled relatively quickly and this makes the

prediction of the melting point of the PCM difficult (As was experienced in

some previous attempts).

2. More cooling curves will ensure more than one experimental value for

melting point, and thus, providing us with an opportunity to tally their respective

values and being able to judge the accuracy of the post-processing of the

experimental data as well.

The RTDs which were used for the experiment were all of 2 wire PT100 type.

In order to be decently far away from the melting point to obtain a good curve

for Newton’s cooling, the sample of PCM was heated roughly 20 ºC more than

the melting point (mean for the three sensors), since we already had a rough idea

about its melting point (via the datasheet), and cooled 7 ºC below melting point

(mean for the three sensors). The cooling time for the PCM was almost eight

and a half hours. The graphical result of the experiment is as follows for the

three sensors:

Figure 2-4: Cooling curve data from all sensors


25

The middle sensor shows the lowest cooling rate, and thus, is the best case for

our analysis.

The similarity between the Figure 2-2 and the cooling curve for the middle

sensor can be noticed below:

Figure 2-5: Cooling curve data from middle sensor

In order to obtain the melting point of the PCM sample, a linear regression

algorithm was applied and it has been described below.

2.2.3 Method of Linear Regression

The method involves fitting the experimental data points to obtain a line parallel

to X-axis (constant temperature line) in the temperature v/s time curve for the

cooling curve, and eliminating all those which do not do so. Since only the data

points which represent melting will fulfil this criterion, one can then obtain all

the data points which represent melting, and thus, the melting point.

The process of fitting is as follows:

Once a line (parallel to X-axis) has been obtained which is the median of the

data values, the data on either side of the line which are far away from it are


26

eliminated and the data size is reduced until one obtains only those values which

lie very close to the melting point line (they would represent points which are

undergoing phase change). The algorithm used is as follows:

Figure 2-6: Algorithm to obtain melting point

from the method of Linear Regression

A fit is considered to be good, if the reduced chi square value is close to 1 (that

means error in fitting is only due to sensor noise) and cumulative probability

distribution of the curve greater than 95 % (meaning that if we were to repeat

the experiment a number of times, 95% of the time we would obtain the same

value for the melting point –Reference [17]). The R.M.S value for sensor noise

was taken as 0.02 ºC. This was obtained by taking the mean of a series of sensor

values in quick succession when it was in steady state.

The Analysis revealed that the melting point for the sample (from the data

obtained from the sensor in the middle which had the slowest cooling rate and


27

thus possibly the best specimen for curve fitting) is 33.12ºC. The complete result

of post processing for all the sensors are as follows:

Sensor Location Bottom Middle Top

Melting Point

obtained from

algorithm (ºC)

33.07 33.12 33.41

Reduced chi square 1.204 1.072 1.166

1-cumulative

prob.distribution

0.149 0.06 0.0523

Table 2-2: Results after post processing data

The slight difference in the predicted value of melting point of bottom and top

sensor is mainly due to the following:

1. The sensor wire length not being of the same length.

2. Sensor noise.

3. A better fit was obtained for the sensor in the middle than that at the bottom

and top, as the sensor in the middle cooled slower, and so, the melting line was

flatter. And hence, the melting point predicted by the middle sensor is much

more accurate. Give in figure below is the result before and after fitting.

Figure 2-7: Graphical result after post processing


28

In the Figure 2-7, the upper curve is a representation of all the points of the

cooling curve and the lower represents all the points remaining after the

trimming away of data points, which were far away from the fitted line was

performed. In other words, the first and the last point of this line would

represent the beginning and end of the melting process. From such an analysis

one can also say that the freezing time for our sample was just slightly more

than 2.5 hours.

Note: The determination of a melting point rather than a melting range can be

justified by the fact, that melting behaviour was predominantly showcased at

this point and, in any case, the narrow range of melting can easily be

compensated for in the simulations.

2.2.4 Correction for length of the wire of RTD

The use of two wire RTD inherently introduces in the system extra resistance of

the wire and increases the resistance measured by the data logger for the sensor,

and thus, the temperature measured. To account for this error, the RTD along

with wires was placed in an ice bath and the resistance of the system at 0 ºC was

measured. Knowing that the RTD100 must have a resistance of 100 Ω at 0 ºC

along with the knowledge of the Ω/ºC for the RTD, one can estimate the error in

temperature measurement.

Resistance at 0ºC 100.5 Ω

Resistance of length of wire 0.5 Ω

Ω/ºC for the RTD 0.3908 Ω/ºC

Error in Temperature 1.3 º C

Melting Point before correction 33.12

Melting Point after correction 33.12-1.3=31.82 ºC

Table 2-3: Error in temperature measurement of RTD

2.3 Determination of Density

The density was determined simply by measuring accurately the mass (using an

accurate and sensitive weighing scale) and the volume (using a measuring

cylinder) of the sample and mathematically dividing them.

State of PCM Solid Liquid

Density(kg/m3) 830 775

Table 2-4: Density values for NE12


29

The net result of the analysis is that the volumetric expansion which takes place

in PCM due to phase change is about 7.1 %.

2.4 Determination of Thermal Conductivity

The 3-Omega method [Reference [15]], which has been developed at AST for

determination of thermal conductivities of aerogels was used for this purpose.

The method relies on measuring the output signal (voltage) obtained from a

heater wire placed inside the sample given an input signal (current). The output,

which is a function of the temperature of the wire, varies depending on the

thermal conductivity of the sample and by using the method of curve fitting the

output is correlated to the analytical closed form solution of Fourier’s heat

equations with internal heat generation to obtain the thermal conductivity. The

method also gives the thermal diffusivity and combined with density it provides

also the thermal heat capacity.

The figure given below shows the Experimental Setup of the 3-Omega method:

Figure 2-8: Hardware for the 3-Omega method


30

Figure 2-9: Sample of Aerogel for 3-Omega characterisation

Figure 2-10:NE12 sample for 3-Omega characterisation

The sample consists of the PCM inserted in an aluminium vessel (Figure 2-10)

and a very thin copper wire inserted in it. An RTD is also inserted in the sample

so as to be able to track the temperature of the sample. Through this wire, very

small currents are passed through the PCM (in the order of micro amperes) and

depending on the thermal conductivity of the substance one sees a different

behaviour in the electrical resistance of the wire.

The salient features of the method are as follows:


31

1. It has a thermal conductivity measurement range of 0.001 to 1 W/m-K

2. Measurable temperature range of -273ºC to 403ºC.

3. Since thermal wave due to conduction travels faster than that of convection,

the method can effectively measure the thermal conductivity of liquids without

introducing error due to convection.

4. The measurement accuracy is as follows:

Conductivity 3-10 %

Diffusivity 7-15 %

Heat Capacity 10-20 %

Table 2-5 : Errors /Uncertainties in 3-Omega characterisation method

5. The minimum sample size required is very small i.2. 10 x 1 x 1 cm or 300 ml

for liquid or powders.

6. It is much quicker than other methods like the hot plate method.

The results of the measurements performed on NACOL Ether 12 are as follows:

Figure 2-11: Trend of thermal conductivity with temperature

for NE12


32

Figure 2-12: Trend of diffusivity and specific heat capacity

for NE12

Once the temperature trend was obtained for each of the thermophysical

properties, the average value was taken for each state.

The result was follows:

Solid State Liquid State

Conductivity(W/m-K) 0.259 0.175

Diffusivity(mm2/s) 0.2 0.08

Heat Capacity(kJ/kg-K) 1522.2 2656.5

Table 2-6: Average values for the thermophysical properties

of NE12

2.5 Determination of Latent Heat of Melting

For the latent heat, the data provided by the manufacturer was used.

For specific heat v/s temperature trend refer to Figure 2-1.

It is visible that the trend in specific heat capacity with temperatures is slightly

different depending on whether the process is melting or freezing. The latent

heat values between the range 30-32 ºC can be summarised as follows:

Heating Cooling

Latent Heat (kJ/kg-K) 220 186

Table 2-7: Latent heat values for NE12


33

2.6 Summary of the thermophysical properties

Thus the results for the thermophysical properties for the given sample of NE12

can be summarised as follows:

Physical Properties Solid

State

Liquid State State

Independent

Melting Point (ºC) - - 30-32

Thermal Conductivity (W/m-K) 0.259 0.175 -

Thermal Diffusivity (mm2/s) 0.21 0.08 -

Density (kg/m3) 830 775 -

Volumetric Expansion (%) - - 7.1

Thermal Heat Capacity (J/kg-K) 1522.2 2656.5 -

Table 2-8: Summary of Thermophyiscal properties of NE12

Test Configuration and Setup

34

3 Test Configuration and Setup

In this chapter, a brief description of the test setup and the various elements

which constitute the tests have been discussed. Justifications for the

experimental arrangement have been given along with factors or control

parameters affecting the tests.

3.1 Materials

Under this heading, various constituents of the test along with their suppliers

have been discussed.

3.1.1 PCM

The PCM was intended to be purchased by one of the companies from Germany

to facilitate reduction in handling and transportation charges, apart from the fact

that it would also be more suitable from a temporal standpoint.

For the tests, the PCM was provided by SASOL GmbH from Hamburg and

discussion about this has already been done in the previous chapter

(Characterisation of PCM).

3.1.2 PCM Box

The PCM box houses the PCM and the box itself is mounted on the Electronic

box (Figure 1-1). In order to optimise cost and time, it was decided that a

universal housing like the one shown below (Figure 3-1) by Hammond

Electronics would be used for the PCM box. It perfectly suits the application, as

it is liquid tight, and the base has flanges which makes its mounting on a

secondary surface easier (if mounting is needed).

It is made of aluminium, which quite suits our application because of its high

thermal conductivity and low density (compared to other metals).


35

Figure 3-1: Universal housing used for PCM

Detailed view of the universal housing looks as follows:

Figure 3-2: Detailed view of the universal housing

Since it was an off-the-shelf product, customised dimensioned box could not be

selected and the box with dimensions closest to that being suitable for an

electronic box for a small satellite was selected.

The dimension of this PCM box is given below:


36

Length 121 cm

Breadth 66 cm

Height 35.5 cm

Thickness of Lid 0.2 cm

Thickness of walls 0.4 cm

Table 3-1: Geometric dimensions of the universal housing

3.1.3 Electronic Box

The experimental electronic box consists of the following components:

3.1.3.1 Plate for Electronic Box

A thin aluminium plate of the same cross section of the PCM box was used as a

dummy electronic box in order to be able to simulate its behaviour. The Material

for the box was chosen to be Aluminium, as owing to its high thermal

conductivity it would have uniform temperature and could possibly be modelled

as a single node in the simulations (also because no specific design of the

electronic box was available). Thickness of the Aluminium plate, which is the

variable parameter in this case, was taken to be 1 cm, so that it would have

roughly the same thermal capacitance (product of density, heat capacity and

volume-it is a measure of the heat required to change the temperature by 1

degree) as a standard electronic box. This was done by surveying the standard

dimensions and components of an electronic box and obtaining the thermal

capacitance of a standard electronic box for a small satellite, and then replicating

the same thermal capacitance for the aluminium alloy plate (Al-Mg-Si 0.5),

knowing its thermophysical property. A detailed discussion about this has been

done in the Appendix A.

The Electronic box was manufactured by W & P Geat GmbH.

3.1.3.2 Heater

The heaters help in the simulation of heat generation inside the electronic box.

The kind of heater which was chosen for this purpose was Kapton insulated

heater of dimension 5 x 5 cm with pressure sensitive adhesive (PSA). The

heater has a maximum heat generation density of 10 W/ in2 (1.55 W/m2 ) at

28V, which amounts to a maximum heat generation of about 40 W. The heaters

were purchased from Omega Engineering.

The heater was placed on top of the plate to simulate the internal heat generation

inside the electronic box as show in the figure below:


37

Figure 3-3: Aluminium plate (dummy E-box) with heater and sensors

It was expected that this system should replicate the behaviour of an electronic

box (at least thermally), since it has the same thermal capacitance as the

electronic box.

The Electronic box was mounted rigidly on top of the PCM box using bolts and

the complete assembly looked as follows:

Figure 3-4: Assembly of Aluminium plate (E-box) and PCM box


38

3.1.4 Temperature Sensors

The temperature sensors that were used for the tests were 2 wire Pt-100

Resistance Temperature Detector (RTD) type. “Pt” indicating that the elements

of the sensor are made up of platinum, and 100 indicating the resistance of the

sensor at 0 .

RTD sensors are chosen specifically because they have a much more stable

behaviour and linear change in resistance over wide range of temperatures, as

compared to other temperature sensors.

The RTD’s were located in the following positions:

1. Two RTD’s were located on the top of the aluminium plate, which helped in

simulating the electronic box as shown in Figure 3-3. These were two of the

most important sensors because one of the most important parameters that we

are tracking during the tests is the temperature of the electronic box.

2. Two Sensors were placed inside the PCM box.

It was very tricky to prepare these sensors, as regular insulated wires would not

permit the proper closing and sealing of the box if they were inserted inside the

PCM box. So for this purpose, very thin copper enamelled wires of 0.1 mm

diameter were used. The figure below illustrates one such sensor:

Figure 3-5: RTD inserted inside the PCM box


39

The thick insulation part provides reinforcement, while the thin copper wires

permit the closing of the box without causing leakage.

Note: The presence of enamel insulation also prevents the short circuit of the

temperature measurement circuit.

3. Two RTD’s were located at the bottom of the PCM box, which were used to

monitor the transfer of heat from the electronic box through the PCM box.

4. One RTD was used just to track the ambient temperature inside the TVC.

Number of RTD Location

2

Aluminium plate

(that simulates the electronic box)

2 Inside the PCM

2 Sides of the PCM box close to the bottom

1 Ambient Temperature inside the TVC

Table 3-2: Location of RTDs on PCM box

The RTD’S were purchase from Omega Engineering.

3.1.5 Fins

Fins, as mentioned previously in Chapter 1: Literature Survey, help in increasing

the heat flow through the PCM. Fins were manufactured and were glued to the

border of the vessels using high conductivity glue. They had a thickness of

around 2 mm and were made of Al-Mg-Si 0.5 Alloy.

Figure 3-6: PCM box with fins

(in liquid and solid state of PCM)

Just like the aluminium plate, the fins were manufactured at W & P Geat GmbH.


40

3.2 Test Setup

The tests were performed in a thermal vacuum chamber (TVC) in AST facilities,

to eliminate the heat loss due to external convection from the system. The

performance capabilities of the TVC have been summarised below:

Leak Rate <0.01 mbar/hr

Vacuum Level - mbar

Working Temperature -80˚C/100˚C

Table 3-3: Performance capability of TVC

at AST

Note: The lowest possible pressure of 0.001mbar can be reached under the ideal

conditions i.e. absence of slowly outgassing elements in the chamber and all the

existing bolts of the chamber door well tightened.

For the given setup and its elements, and the current TVC performing

conditions, a pressure of 2 mbar was reached.

Figure 3-7: The assembly of the Cooling Machine, TVC & Data Logger

(From L to R) at AST

The cold plate, inside the chamber was decided not to be kept operational for

the following reasons:

1. The operation of the cold plate would introduce other extra parameters that

would be needed to be modelled in the simulations thus increasing complexity

of the model. (In the eventually selected configuration, the PCM is insulated

from the environment and only accumulates heat from the E-box acting more

like a heat accumulator).


41

2. In spite of the fact that there was a small gap left to account for the expansion

of the PCM upon melting, very slight amount of leakage was anyways observed

(due to the pressure gradient between the internal and the external of the box)

and the introduction of any fluid inside the vacuum chamber is highly

undesirable as it results in degradation of performance (lowest achievable

pressure).

Note: This leakage into the chamber did not happen in the eventual

configuration as the PCM box was kept inside a small cardboard box (which

was internally lined with aluminium foil to reduce radiation losses) and then

inside the TVC as shown below:

Figure 3-8: External view of PCM box completely sealed

inside the cardboard box (graphical representation)

The above figure, represents the internal view inside the chamber for the test

configuration where the green plate is the cold plate and the brown cardboard

box houses the PCM box.

3. In any case, the main aim of the thesis was to be able to obtain an acceptable

compliance between the test and simulation results, which could also be

achieved without keeping the cold plate active.

Keeping all these points in consideration, only the heating cycles for the E-box

were performed.

With regards to the possible orientations of the PCM box inside the TVC, there

were three possibilities and they are listed below:


42

3.2.1 Orientation 1 - Electronic Box on the lower side (Figure 3-9)

In this orientation, the Electronic box would be facing downwards and in contact

with the base of the cardboard box. However, this would result in a lot of heat

loss at the contact interface between the electronic box and the cardboard box

besides the fact that placement of sensors for E-box would be difficult.

Such a configuration would also result in very strong vertical convection

currents setup inside the box.

Figure 3-9: Graphical representation of Orientation 1

(without cardboard box top)

3.2.2 Orientation 2 - Electronic Box on the Upper side (Figure 3-10)

In this orientation, the Electronic box would be facing upwards and would not

be in contact with the base of the cardboard box.

In this configuration, the convection currents, although present, would be much

weaker than the ones in orientation 1.

Since the box is not designed to withstand the expansion of the PCM when it

melts (like a pressure vessel), a pre-calculated gap is left inside the box to

account for expansion as mentioned earlier. If the box were to be kept in this

orientation, then due to this gap there would be no direct heat transfer between


43

the lid and the top layer of PCM (thus in a way contact between the electronic

box and the top layer of the PCM will be disrupted). This would result in

efficient transfer to the PCM from the electronic box not taking place, which is

one of the main goals of the project.



3.2.3 Orientation 3 - Electronic Box facing sideways (Figure 3-11)

In this orientation, the Electronic box would be facing sideways and only its

longer edge would be in contact with the cardboard box. Although convection

currents would be present in this orientation (still less than that in orientation 1),

a good contact between (the upper layer of) the PCM and the electronic box will

also be achieved (owing to gravity), resulting in efficient heat transfer to the

PCM.

Considering the merits and demerits of the different orientations, this one was

considered to be more feasible than the others. And thus, orientation 3 was

selected for the test setup.

Also, when the PCM was frozen in ambient conditions (outside the TVC) in

between tests, it was kept in the same orientation to maintain the contact

between the solid PCM and the lid.


44



Given below is a summary of all the orientations:

Orientation Description Comments Decision

Orientation 1

E-box facing downward

Large heat loss at contact

interface of E-box with base

Orientation 2

E-box facing upward

No contact between upper layer

of PCM & Lid (due to gap)

Orientation 3

E-box facing sideward

Good contact between upper

layer of PCM & Lid

Table 3-4: Summary of possible test orientations

Note: In all of the aforementioned configurations, gravity still plays a significant

role and is something that significantly alters the behaviour of PCM box on

terrestrial environment, as compared to its behaviour in space. A discussion on

this topic has been made in Chapter 5: Modelling & Simulation of PCM in

ESATAN-TMS.

Also, the absence of a heat sink to reject the accumulated heat from the

electronic box makes the PCM system in the tests behave more like a heat

accumulator rather than a heat sink.


45

3.3 Control parameters for the test setup

3.3.1 Heater

Different power curves for the PCM box were achieved by varying the power

dissipation of the heater.

The heater power was controlled by a Programmable power supply (HAMEG

HMP2030), which was controlled by the computer using a RS-232 interface.

The values of voltage and current were recorded by the computer and a track of

the power dissipated with time was kept.

The powers selected for evaluating the behaviour of the PCM box were 5 W, 7.5

W & 10 W. These were selected based on the power dissipation trend of

electronic boxes. Powers below this range do not demand the need of a heat sink

owing to sufficient thermal inertia (or capacitance) and powers beyond this

range resulted in high leakage from the box owing to higher temperatures

reached (discussed later). Hence, this range was found to most suitable for the

conduction of the experiments.

3.3.2 Number of Fins

Depending upon the number of fins inserted inside the PCM, the performance of

the system varies. Generally, with the increase in number of fins the heat

transfer to the internal layers of the PCM increases and improves the

performance of the PCM box.

Four fins were inserted inside the PCM box, which accounted for around 10 %

of the cross sectional area. Simply inserting large number of fins inside the box

would have resulted in a lower total heat capacity of the box owing to lesser

PCM. It would have also made the comparison of the performance of the cases

in which the PCM box has fins and in which it does not have fins difficult.

However, the insertion of just four fins results in not a large amount of PCM

being substituted by fins, thus making the comparisons between different cases

(PCM box with fins and without fins ) still possible and also, as suggested in

Appendix B, results in significant improvement in the performance of the PCM

box.

3.3.3 Pressure Level in TVC

The amount of heat loss due to convection is dependent on the pressure level

inside the vacuum chamber and to eliminate it, an attempt was made to evacuate

the chamber to the lowest possible pressure. The vacuum level in the vacuum

chamber was kept track of by interfacing the vacuum gauge with the help of a

RS-232 interface to the computer. The subsequent tracking of the pressure levels

with time gave a good indication of the possible errors/heat losses that were

introduced in the system due to convection.


46

Figure 3-12: Serial communication setup using RS232 for pressure data

Test Results

47

4 Test Results

With the Test setup well defined, the tests were carried out inside the TVC at

AST. The tests were performed for two configurations of the PCM box i.e. PCM

box without fins and with fins at various powers. They have been summarised

below:

PCM Box Configuration Power Values

PCM Box w/o fins 5 W

7.5 W

10 W

PCM Box with fins (4 fins) 7.5 W

10 W

Table 4-1: Summary of tests performed

The selection of these powers has already been justified in the Chapter 3: Test

Configuration and Setup.

All the above tests were conducted with the TVC evacuated to a pressure of 2

mbar.

The results of the tests have been discussed below:

4.1 PCM Box without fins

For this case, the tests were performed at three power points, beginning at 5 W

until 10 W with a step of 2.5 W i.e. 5, 7.5, 10 watt.

The tests were limited to relatively low power levels mainly because of the large

volume expansion of the PCM, which takes place at high temperatures

(associated with high power levels) resulting in leakage of PCM from the PCM

box.

The graphical result for the different power values are given below. The plot is

for the Electronic box temperature with time. The results have been shown for

each case until a point slightly beyond all the PCM inside the box has melted. In

the due course of the tests, it was realised that 60 degrees was a good threshold

value to avoid large leakage of PCM from the PCM box i.e. for none of the

power cases temperature of the electronic box was allowed to exceed 60

degrees.

Test Results

48

Figure 4-1: Tests result for PCM box without fins-various powers

A more profound analysis of the melting process can be done by isolating one of

the power cases and studying the various phases involved along with the trend in

the electronic box temperature. The case selected for this purpose was the 10

watt power one.

Figure 4-2: Tests result for PCM box without fins -10 watt

Test Results

49

One can identify three phases in the temperature v/s time curve for the electronic

box.

Phase 1

This is the phase where the temperature of the electronic box increases linearly

with time. The PCM box is not very effective in this range because the melting

of the PCM has not yet initiated, and so the sensible heat capacity of the PCM is

being used to store heat (which is not very efficient). This takes place almost

until a temperature slightly above 30 degrees is reached.

Phase 2

Once the electronic box temperature reaches slightly above 30ºC, the melting of

the PCM begins and the electronic box starts utilizing the large reservoir of

latent available in the PCM to store the heat. This cause decay in

of the

Electronic box. This is the regime for which the PCM box displays optimal

behaviour (large heat capacity) and it lasts until all the PCM has melted.

Phase 3

This phase begins as soon as all the PCM in the box has melted and the

electronic box starts using once again the sensible heat capacity of the PCM to

store heat. Since the electronic box is utilizing the sensible heat capacity of the

PCM, this phase (like the phase 1) is not very efficient and shows an almost

linear trend in temperature with time.

In order to further verify that the identification of the phases and the cause of it

(for example linear increase in temperature is taking place because all the PCM

has melted), the data of an RTD sensor placed inside the PCM box close to one

of the longer side was analysed. Its position has been shown below:

Figure 4-3: Sectional front & side view of the PCM box

with internal sensor

As the melting proceeds, the PCM melts from the sides where it is in contact

with the metal box and the block of PCM sinks staying always in contact with

the side on which it is standing due to gravity. Thus, a sensor placed in contact

Test Results

50

with this longer side (on which the PCM always has contact) will give a good

indication of when all the PCM has melted.

The results for this sensor were as follows:

Figure 4-4: Data from sensor on the E-box & inside the PCM box

for PCM box without fins-10 watt

The blue curve represents the temperature of sensor place inside the PCM box

and the red curve for the electronic box. One can easily identify all the phases

(discussed earlier) from the temperature of the PCM sensor (blue curve) as well.

Phase 1 The range where the temperature of the PCM increases linearly (sensible heat

capacity), the temperature of the Electronic box also increases linearly

Phase 2

In between the range of temperature 30ºC-32ºC, the PCM undergoes phase

change and exhibits high heat capacity (latent heat capacity) accompanied by an

almost constant temperature.

This is accompanied by reduction in the rate of increase of temperature of the

Electronic box.

Test Results

51

Phase 3 Once all the PCM has been melted, strong convection currents are (possibly)

setup in the PCM box and this causes an almost instantaneous increase in the

temperature of the PCM element and the temperature of the RTD in the PCM

sensor increases instantaneously. This can be seen by the vertical line around

100 minutes. Simultaneously, the temperature of the Electronic box goes from

displaying a stable increase in temperature to rapidly increasing and linear trend

in temperature.

Thus, one can conclude that the initial conclusions drawn from the trend in

temperature of the Electronic box were precise.

The small dip in temperature of the Electronic box, which takes place in and

around the complete melting of the PCM, is also caused due to convection

currents (possibly). This in a way helps in the identification of the complete

melting of the PCM inside the box.

Note: Although theoretically melting in the box should happen in the way

mentioned earlier, there is still a certain amount of uncertainty associated to it as

there is no visual verification of the melting process, and so, the causes for the

phenomenon at the end of phase 2 (sudden rise in temperature of the sensor and

a dip in the electronic box temperature) can only be stated with some

uncertainty.

Given below is the graph for all the power values again with a vertical line, so as

to indicate when the melting process finishes.

Figure 4-5: Tests result for PCM box without fins w/ melting line

-various powers

Test Results

52

4.2 PCM Box with fins

Following the tests for PCM box without fins, the ones with fins were

performed. As mentioned earlier, a total of 4 equidistant fins were place inside

the box to augment the heat transfer to the PCM in the box.

The test was performed only for two power values and the results are given

below:

Figure 4-6: Test results for PCM box with fins

- various powers

Not much of a different conclusion can be drawn from this graph, besides the

identification of the general three phases discussed earlier with a few

differences, which have been mentioned below :

1. Like the case for without fins, the end of melting phase cannot be identified

very definitely in the case with fins. No RTD data was available, since none

were inserted inside because of inconvenience. It would be interesting, however,

to compare the results of the case with fins to their respective counterparts

without fins.

2. When comparing the melting curve of the PCM box with fins and without

fins, one can notice that the phase 2 of the box without fins is much more stable

(less fluctuating or noisy) than the phase 2 of the box with fins. One possible

reason for this could be diffusion of PCM from one PCM fin compartment to

Test Results

53

another due to the small gaps in the contact between the fins and the border of

the vessel.

A comparison of the two cases (with and without fins) can be seen below:

Figure 4-7: Comparison of test result for PCM box w/ & w/o fins

-various powers

The above graph represents a very interesting result.

One can see that in the case of fins, the electronic box shows a much flatter

behaviour (almost a plateau like trend, which is highly desirable) than its

counterpart without fins. This is mainly because there is larger amount of

simultaneous melting of the PCM taking place inside the case with fins. Fins

thus effectively transport the heat from the sides of the box to the middle/upper

layers of the PCM, as expected during theoretical studies.

The presence of fins also causes a faster melting of the PCM and the electronic

box (with fins) enters the phase 3 slightly earlier, than the case without fins

(besides the fact there is lesser PCM).

Another observation that can be made comparing the two cases of 7.5 W and 10

W, is that the difference in temperature of electronic box between the case with

fins and without fins is higher for higher power. This is mainly because as the

Test Results

54

amount of power increases, the ratio of the ability to transport heat by the PCM

(due to low thermal conductivity) to the heat coming in becomes lower, so more

of the sensible heat capacity of the PCM gets utilised. However with fins, this

problem is overcome as the fins transport the heat directly to the internal PCM

layers through metal contact and the problem of low thermal conductivity is

overcome. This is one of the reasons why the test for PCM box at 5W was not

performed, as in this case the Electronic box already displays an almost plateau

trend in temperature with time(without fins) and the difference between the case

with and without fins would not have been very large

Modelling & Simulation of PCM in ESATAN-TMS

55

5 Modelling & Simulation of PCM in

ESATAN-TMS

5.1 Approaches to PCM Modelling

One of the biggest challenges for the simulation of PCM with thermal software

lies in the analysis of the PCM not in the sensible heat domain, but in the latent

heat domain. Most thermal softwares available commercially don’t deal with

problems in the latent heat domain. Hence, it is required by the user to develop a

subroutine/model for this purpose and include the subroutine/model to the main

model. Of all the available choices, ESATAN-TMS (European Space Agency

Thermal Analysis Network) was chosen for this purpose primarily because a

subroutine for phase change element was already developed at AST, as a part of

an internal research project, and one has to simply include this sub model in the

main model to simulate an element as PCM. In general, there are two ways of

approaching the problem of modelling in the latent heat domain. They are as

follows [Reference [7]]:

Explicit Method: Melting in a range of temperature with variable heat

capacity

Implicit Method: Melting at a fixed temperature (in the enthalpy

regime)

Figure 5-1: Finite difference modelling of PCM

[Reference [7]]


56

5.1.1 Explicit Method

This approach involves melting not at a given temperature, but a range of

temperatures. This algorithm does not actually enter the enthalpy regime directly

when running the problem, but the adjustments for latent heat are pre-calculated.

In the range of temperature in which melting is supposed to take place, the

specific heat is not constant, but it is linear. The specific heat varies linearly

from a virtual solid state value to a virtual liquid state value (or vice versa), with

the slope calculated in such a way that the integration of the over the

temperature range (and hence the heat absorbed) is equal to the latent heat of the

substance (plus the sensible heat in that range). It is important to note that both

the virtual state specific values are not equal to the real state specific heat

values.

Figure 5-2: Graphical representation of variable heat capacity modelling

The graph displayed above illustrates the fact explained earlier. The temperature

range generally taken is 2 , where is the temperature offset on either side

of the mid-point of the melting temperature ( an algorithm input variable).

2 effectively becomes the range in which the anomalous behaviour of

specific heat takes place to account for the latent heat.

The graph on the left represents the amount of sensible heat absorbed during the

temperature change from - to in reality. The light grey shaded

area represents the sensible heat (it does not include the latent heat).

The graph on the right represents the case for linearly varying virtual , which

takes into account the latent heat. The sensible heat absorbed in this case is

represented by a similar coloured area (as in the previous case). The heat


57

capacity values in this case are different than that in the real case, because of the

presence of the latent heat (represented by the dark grey area on the graph on the

right) and the (virtual) linear trend in heat capacities.

One of the biggest drawbacks of such an algorithm is that the time step for

integration has to be very small, so that the temperature changes are

infinitesimal and the correct trend for temperature change due to heat absorbed

can be replicated, and this inherently makes the algorithm slow.

5.1.2 Implicit Method

In this case melting takes place at a fixed temperature.

This approach basically transfers the problem in the enthalpy regime, where in

when the melting point is reached, the PCM node acts like a boundary node

(incapable of changing temperature) and changes to a diffusion node(capable of

changing temperature) only when heat equal to latent heat for that node has been

absorbed. The approach can be summarized by the following graph:

Figure 5-3: Enthalpy V/s Temperature near phase change temperature

[Reference [7]]

The model is linear, in the sense, that the percentage of solid fraction or liquid

fraction is calculated linearly depending on percentage of heat absorbed or lost

by a given element.

For example for a melting process, the percentage of liquid is calculated as

follows:

Liquid fraction=

This liquid fraction or solid fraction can then be used to estimate the thermal

conductivity, heat capacities and density of the resultant partially melted mixture

by linear interpolation. With this data, one can then calculate the thermal path


58

conductance (or GL’s in ESATAN-TMS), which are generally needed by the

software for thermal calculations.

The advantage of this approach is that there are no restrictions on the time step

for integration.

This approach was thought to be much more robust and simple, and thus, it was

chosen by AST for developing the sub routine to simulate PCM.

5.2 Thermal Mathematical Modelling

The various system elements which need to be considered while creating the

model for a PCM Heat Sink/Accumulator are as follows:

PCM box with PCM

Electronic box

Contact Resistances

The figure below shows the physical representation of the model:

Figure 5-4: Physical representation of the model (front view)


59

5.2.1 Electronic Box

The choice for the electronic box was a 1 cm thick aluminium plate with same

cross section as PCM box. Such a metal plate would maintain a uniform

temperature at all its points owing to its not so large dimensions and high

thermal conductivity of aluminium and it was modelled as a single node with

lumped thermophysical properties.

5.2.2 PCM box

The PCM box was modelled without performing any geometrical

transformation. A 3D thermo-mathematical model was created for it. It consists

of a vessel of finite thickness with lid and PCM inserted inside it. A detailed

description has been given below:

5.2.2.1 Meshing of the PCM Box

For a PCM box (without any fins), there are two kinds of elements i.e. vessel

elements (made of high conductive materials like metals) and the PCM. One has

to create the nodes in such a manner that these two are always discrete.

The axis co-ordinates are chosen as shown in the Figure 5-4.

An parametric input file was created such that the user just specifies the number

of elements of PCM along X, Y and Z. The Borders of the vessel (made of

metal) on the right and left just have one element along the X- direction and the

borders of the vessel on the top and bottom have just one element along the Y

direction (Figure 5-5). Thus, if one chooses 3 x 3 (elements along X and Y) for

PCM, a section of the vessel will be effectively divided into 5 x 5. Refer to the

Figure 5-5 below, which is the sectional front view for Figure 5-4. The elements

numbered in red belong to the vessel and the elements numbered in black belong

to the PCM.

Element Colour

PCM Black

Vessel (metal) Red

Table 5-1: Colour representation of elements

in Figure 5-5 & Figure 5-6


60

Figure 5-5: Sectional front view of the PCM box

Similarly, having a look at the front view (without any sections been taken), the

nomenclature would be as shown below. All are in red because all the nodes

belong to the vessel.

Figure 5-6:Non-Sectional (Normal) front view of PCM box


61

The view from the top for the box (Y-axis) is as follows:

There are three layers of PCM (which is the user input for number of elements

along Z direction) in white background and the extra two for the borders in blue

background. To have a unique number for each node of different layers, each

layer has been assigned a value in thousands shown below. To get the global

number of a node, the layer number is added to the local number. So the node 1

in the Figure 5-6 (belonging to the metal layer or border) above which has a

local number 1 will have global number of 1001.

5000

4000

3000

2000

1000

Figure 5-7: Sectional top view of the PCM box

Element Background

Colour

PCM White

Vessel

(metal)

Blue

Table 5-2: Colour representation of elements in Figure 5-7

Similar modelling approach is made for PCM boxes with fins.

The top view representation in the case of fins is represented below.

Figure 5-8: Sectional top view of the PCM box with fins


62

Element Background

Colour

PCM White

Vessel

(metal)

Blue

Fins Green

Table 5-3: Colour representation of elements in Figure 5-8

In this case, the blue background represents vessel, the white background

represents PCM layers and the green background represents the fins. One can

see from two cases represented, with three fins and two fins, that each fin layer

is surrounded by two layers of PCM on each side. The choice of each fin being

surrounded by two layers of PCM on either side is a modelling preference and

not a rigid modelling parameter.

Note: It is important to note that a PCM layer or fin layer does not mean that

each node in the layer is made up of PCM or fin respectively. It just means that

the layers except the borders are made up of PCM or fin. The borders will

always be made up of vessel material like in the sectional front view of the PCM

box shown before, because of the design of the box.

5.2.3 Contact Resistances

Once the Electronic and the PCM box have been modelled, one has to then

consider the contact resistances between the various interfaces.

For the PCM box without fins the following contact resistances were

considered:

Contact Resistance between the lid and Electronic box

Contact Resistance between the lid and the rest of the PCM Vessel.

For the case with fins, there are two more contact resistances in addition to the

ones mentioned before. They are:

The contact resistance between the top of the fin and the lid of the PCM

box (the gap between the top of the fin and lid has been modelled as a

contact resistance).

The contact resistance between the sides edges of the fin and vessel

(where the fins make contact with the rest of the vessel).

The whole model has been summarised below:


63

Figure 5-9: Representation of Thermal Mathematical Model

5.3 Discrepancy between Test Setup and Simulation Model

One of the biggest challenges in getting the simulation model verified with the

test results was to account for gravity and convection. They have been discussed

below:

5.3.1 Effect of Gravity

As the heater is switched on, the surface of the PCM in contact with the sides of

the vessel melts and the block of the PCM sinks and falls in contact with the

side on which it is standing, due to gravity. Although this effect is favourable in

keeping the temperature of the electronic box under control (as it is augmenting

the heat transfer to the PCM and partially overcoming the drawback of low

thermal conductivity of the PCM), it diverges from the simulation model, which

has been created for space conditions (which have an absence of gravity).

5.3.2 Effect of Convection

Convection, just like gravity, cannot be eliminated in a terrestrial environment.

There will always be some convection currents setup in any given configuration.

The effective result of both these effects is that the heat transfer to the PCM is

augmented and it melts faster (which although beneficial is not desired).

This can be seen from the graph below:


64

Figure 5-10: Comparison of exp. & sim. result w/o correction

for PCM box without fins-10 watt

The above simulation has been carried out with appropriate value of contact

resistances (found and discussed later) and without any correction for gravity

and convection. One can see the large error in such a scenario.

However, ways and means to account for them were thought of. They have been

discussed below:

5.3.3 Approaches to overcome the discrepancies:

5.3.3.1 Approach 1

This would be more of a traditional approach, which would include the

predicting the mode of convection and modelling convection in the simulation

model. This approach would include neglecting the effect of gravity. The

difficulty would lie in predicting the nature of convection, which in this case is

not so well defined.

The drawback of this approach is that even if a complex convection model

would be implemented there would still be an error between the test and

simulation result due to the non-consideration of gravity.


65

5.3.3.2 Approach 2

This would be more of a non-standard approach which would basically involve

increasing the thermal conductivity once a small element in the simulation

model has melted. This would then increase the heat transfer to the internal

layers which have not yet melted and, effectively, simulate simultaneously the

effect of gravity and convection. Approach 2 has been described with the help of

some graphics below:

The following colour scheme is valid for the figures

Yellow Solid PCM

(Before Melting)

Red Liquid PCM

(After Melting)

White Vessel Border

Table 5-4: Colour code for graphical representation

given below

Figure 5-11: Sectional view of unmelted PCM

In the figure above (sectional view of PCM box without fins), before the

beginning of the melting cycle all the PCM is in solid state (hence yellow)

Figure 5-12: Sectional view of partially melted PCM in Space


66

In Space conditions, the melting would take place from the borders and the

yellow elements (unmelted PCM) would remain in the centre owing to the lack

of gravity.

Figure 5-13: Sectional view of partially melted PCM on Ground

However, in terrestrial environments the melting pattern would take place as

shown in the figure above i.e. the elements would melt and fall to make contact

with the sides of the vessel, owing to gravity.

Since the simulation model has been done for the melting pattern to take place

as shown in Figure 5-12, one way of simulating the corresponding behaviour of

Figure 5-13 is by augmenting the thermal conductivity of the melted elements

(in red) around the unmelted elements (in yellow).

The result of such a correction is that it tends to bring the red curve shown in

Figure 5-10 closer to the test data. Shown below are various values of multiples

of liquid thermal conductivity taken and their result on the simulation:

Figure 5-14: Sensitivity analysis for Gravity correction factor


67

In the cases above, the multiple indicates the multiple of the liquid thermal

conductivity of the PCM which has been taken (from henceforth also called as

the gravity correction factor). So in this case, a gravity correction factor of 1

indicates no correction has been applied.

To find the optimal value for a given power case a large parametric analysis was

done, which included taking multiples of the thermal conductivity of PCM in

liquid state (larger than its original value), and picking the value which matches

the test data the best.

Keeping these points in mind, Approach 2 was thought to be most suitable for

our applications from the point of simplicity and effectiveness.

5.4 Identification of optimal fit with parametric analysis

In order to find a good conformity between the test and simulation results, a

parametric analysis of large number of cases with a variation in the free

parameters was carried out, as mentioned before, and the best fit was be found.

The free parameters in this case are:

1. The multiple of the liquid thermal conductivity of PCM.

2. The contact resistance between the lid of the box and the rest of the box.

3. The contact resistance between the electronic box and the PCM box

Accordingly, hundreds of cases were run in ESATAN-TMS using the

parametric manager, selecting a logical range for the free parameters.

5.4.1 Approach

In order to select the case which best fits the test data, the following logic was

used:

Step 1

This involved selecting the number of “power” cases for which the optimal

solution is desired i.e. finding the best fit for just one power case (e.g. 10 W) or

best for three power cases (e.g. 5, 7.5,10 W) i.e. a single set of value for the free

parameters, which suits only one power case or a single set, which suits all the

power cases.


68

Note: A single set of parameter which satisfies best one power case may not

satisfy the other power cases. Also, as the number of power cases to be

considered increase, the probability of finding a set of single free parameters,

which satisfies all the power cases becomes lower.

Step 2

A maximum allowable difference of temperature for a single data point is

selected. This is the maximum allowable difference between an experimental

and simulation data point. This is a good first criterion to filter out cases. All the

cases which did not respect this criterion were discarded. An initial estimate of

5ºC was good value for the maximum allowable difference.

To elaborate further, this means that while comparing the test data and

simulation data for a given single set of parameter, even if there is one point (at

the same time instant) in the test and simulation data with temperature

difference between them greater than the assigned value (for e.g. 5 ºC), then that

parametric case is discarded.

Step 3

This involved finding the difference between every experimental data point and

simulation data point (at the same time) from the non-discarded cases and

finding the one which has least sum of absolute error for all the data points.

The results have been discussed from the next page onwards:


69

5.4.2 Fit for PCM box without fins

5.4.2.1 Fit with one power case

An initial fit considering only the 10 watt case was performed and the results

were as follows:

Figure 5-15: Comparison of exp & sim. results for PCM box w/o fins-10 watt

Contact Resistance between the lid and the

vessel for PCM box

(

0.002

Contact Resistance between the lid and the

electronic box

(

0.0025

Multiple for the heat conductivity of PCM in

liquid state (gravity correction factor)

7.5

Table 5-5:Cont. Resistance & Gravity correction factor (10 W w/o fins)

for Best fit

Max Temperature difference between a

data point (ºC)

3.6976

Average Temperature difference

between a data point (ºC)

0.82611

Table 5-6: Parameters for evaluation of fit (10 W w/o fins)


70

An important observation that can be made from the simulation results is the

non-linear nature of the simulation trend for the electronic box temperature.

This can be owed to the following reasons:

1. The discontinuity in the initial part is due to the fact that in and around the

melting point, the PCM (in reality and not in simulations) exhibits not a sensible

heat capacity value, but a value which is higher, like a melting range, as can be

seen in Figure 2-1). This results in a very smooth transition from phase 1 to

phase 2, and not abruptly like in the simulation.

2. The current analysis has been done by considering 5x5x5 PCM elements.

When the PCM melts, it acts like a boundary node which does not experience a

change in the temperature, and when it absorbs heat equal to latent heat it

changes to diffusion node which is capable of changing its temperature. This

accompanied with fact that the ratio between the sensible heat capacity to latent

heat is very low, causes a sudden change in the temperature as a layer of PCM

melts, and hence, there is a slight discontinuity as there is a temporary change in

the heat capacity of the PCM box. This stabilises as another layer adjacent to

this layer starts melting.

This discontinuity can be overcome by creation of larger number of elements in

ESATAN-TMS as shown below:

Figure 5-16: Simulation with variable number of PCM elements

for PCM box w/o fins-10 watt


71

One can see that the result for the simulation of 7x7x7 is much smoother than

3x3x3.

However, the complier coupled with way the source code was written did not

permit the number of elements to be higher than a certain value (discussed in

Chapter 6: Conclusion).

5.4.2.2 Fit with three power cases

The fit analysis was then extended to three power cases i.e. 5, 7.5,10 watt cases

and the best fit was attempted to be found.

The result is as following:

Figure 5-17: Comp. of exp. & sim. results for PCM box w/o fins-var. powers

Contact Resistance between the lid and

the vessel for PCM box

(

0.00143

Contact Resistance between the lid and

the electronic box

(

0.002

Multiple for the heat conductivity in


6.25

Table 5-7: Cont. Resistance & Gravity correction factor (w/o fins)

for best fit


72

Power Case 5 W 7.5 W 10 W

Average Temperature

Difference (ºC)

0.796 0.992 2.953

Maximum Temperature

Difference (ºC)

2.700 4.200 3.217

Table 5-8: Parameters for evaluation of fit (w/o fins)

It is interesting to note that the contact resistance value and gravity correction

factor change slightly for the best for three power cases, but they are still close

to the value for fit for only one power i.e. 10 W.

5.4.3 Fit for PCM box with fins

A similar approach for the fitting was followed for the case with fins.

The contact resistance values which were obtained in the previous fitting were

used and the new free parameters were:

1. The contact resistance between the fin and sides of the vessel

2. The contact resistance between the fin and the lid.

Note: The top of the fin does not make contact with the lid and this gap (actually

filled by PCM due to sideways orientation of the box during tests) has been

modelled as a contact resistance

3. The value for the liquid thermal conductivity of PCM (this was also a free

parameter in the previous case).

Note: This correction parameter is not a physical constant and is a way of

approximating for convection and gravity and thus it changes with change in

orientation.

The results are as follows:


73

5.4.3.1 Fit with one power case

Considering only the 10 watt power case:

Figure 5-18: Comp. of exp. and sim. results for PCM box w/ fins-10 watt

Contact Resistance between the fins and

side of the vessel

(

0.00025

Contact Resistance between the fin and the

lid of the vessel

(

0.002

Multiple for the heat conductivity in liquid

state (gravity correction factor)

10

Table 5-9: Cont. Resistance & Gravity correction factor (10 W w/ fins)

for best fit

Power Case 10 W

Average Temperature

Difference (ºC)

0.7484

Maximum Temperature

Difference (ºC)

4.1688

Table 5-10: Parameters for evaluation of fit (10 W w/ fins)


74

5.4.3.2 Fit with two power cases

Considering the 10 watt and 7.5 watt power case:

Figure 5-19: Comp. of exp. & sim. results for PCM box w/ fins-var. powers

Contact Resistance between the fins and

side of the vessel

(

0.0002857

Contact Resistance between the fin and

the lid of the vessel

(

0.002

Multiple for the heat conductivity in


10

Table 5-11: Cont. Resistance & Gravity correction factor (w/ fins)

for best fit

Power Case 7.5 W 10 W

Average Temperature

Difference (ºC)

1.12227 0.74928

Maximum Temperature

Difference (ºC)

5.5828 4.1698

Table 5-12: Parameters for evaluation of fit (w/ fins)


75

5.5 Simulation for various number of fins

Once the fit was done and the parameters for various contact resistances and

correction for gravity were obtained, simulation for various number of fins were

carried out and their results were analysed. One such graph has been given

below:

Figure 5-20: Simulation of PCM box with variable number of fins

-10 watt

As discussed earlier, the insertion of fins causes better heat transfer to the

internal layers of the PCM and improves the capacity of the PCM box to absorb

power dissipated by the electronic box. This results in a flatter curve for the

temperature of the electronic box with time when the PCM is melting, which

improves with the number of fins, and is something that can be observed in the

graph above.


76

5.6 Comparison of PCM box in various configurations

In this section, an attempt has been made to justify the use of PCM box for the

current application and it has been shown how it is effective in temperature

control of the electronic box and thus its efficient functioning.

The cases which have been compared are as follows:

1. Electronic Box with no PCM Box

In this case, the external of the Electronic box has been painted black and it

loses heat via radiation to the internal sides of the satellite, which is considered

to be within a fixed temperature range.

2. Electronic Box with PCM Box without fins for thermal control

This includes a PCM box with NE12 as PCM and the box has no fins.

3. Electronic Box with PCM Box with fins for thermal control

This includes a PCM box with NE12 as PCM and the box has variable number

of fins. Different cases with different number of fins have been simulated.

The results have been shown below:

Figure 5-21: E-box temperature trend in various orientations


77

It can be seen that in the absence of the PCM box, the temperature of the

electronic box shoots to a very high value very fast. While in the presence of

the, PCM box the temperature of the electronic box is kept within a safe range

and the performance for withstanding high powers improves with the insertion

of fins.

Conclusion

78

6 Conclusion

To summarise the thesis, it can be stated that an off-the-shelf universal housing

with dimensions suitable for a small satellite was selected for the insertion of

PCM and an aluminium metal plate (with same thermal capacity as a generic

electronic box) with a heater mounted on it was used to simulate the behaviour

of an electronic box. The tests were carried out in different configurations of the

box and power dissipation inside a TVC at 2 mbar pressure and a short summary

has been given below:

PCM Box Configuration Power Values

for tests

Average temp difference

between the experimental

and simulation results for

E-box

PCM Box w/o fins 5 W

7.5 W

10 W

0.89 ºC

0.99 ºC

0.80 ºC

PCM Box with fins (4

fins)

7.5 W

10 W

0.75ºC

1.12 ºC

Table 6-1:Summary of tests and simulations

6.1 Goals Achieved

With the completion of the aforementioned tasks the, the following goals have

been achieved:

1. The primary mission of validation of concept for PCM based system for

thermal management of components has been achieved. The ability of PCM

Heat Sinks to effectively keep the temperature of a component (for which it has

been sized for) below its maximum operating temperature, as predicted

theoretically, has been demonstrated.

2. Although a direct comparison between PCM and other thermal control

techniques has not been made experimentally, an insight into the potential of

PCM Heat Sink technology has been realized, not just for space based

applications but also for other terrestrial applications.

3. A coherence between the experimental results and the simulation in

ESATAN-TMS has resulted in the ability to simulate the behaviour of PCM

Conclusion

79

Heat Sinks (during melting) with an acceptable degree of accuracy. This

accompanied with a parametric input file that contains a modest library housing

the thermophysical properties of all relevant PCM, vessel and fin materials

renders the ability to simulate the behaviour of any PCM system and evaluate its

feasibility for a given application. e.g. By changing the inputs in a parametric

file, one can evaluate the trend of the temperature for the electronic box for a

PCM box with wax as PCM inserted in an aluminium vessel with 10 copper fins

of 1mm thickness, and conclude if its suitable for a given application.

With reference to point 2, which talked about the applications of PCM

technology for space as well as terrestrial applications, it is fair to say that since

the simulations did take into account the effect of gravity and convection (which

itself is due to gravity), the simulation tools can also be used to simulate a PCM

based system for terrestrial applications.

4. An ability to effectively characterize any given sample of PCM in a given

state (solid or liquid) with an acceptable range of uncertainty (or error) has been

successfully accomplished. This is an important step in obtaining good

coherence between the experiments and the simulations, as the simulation

results depend on the accuracy of the thermophysical properties obtained via

characterization.

Besides this, it also eliminates the dependence on data sheets from PCM

manufacturers, which it has been observed have a low level of accuracy. For

example error of 0.1 W/m-K has been commonly observed in PCM data sheet,

which if the thermal conductivity is 0.2-0.3 W/m-K, accounts for 30-50 % error

in thermal conductivity with respect to its original value.

5. Critical issues with regards to the design of the PCM vessel have been

recognized. This includes making the vessel leak proof or liquid tight, along

with it being able to withstand the stresses on melting. A discussion about this

will be made with much more detail at a later stage in this chapter.

6.A TRL of 4 for the demonstration of thermal control of a component with a

PCM based system has been achieved i.e. the tasks of observation of basic

principles (TRL1), formulation of technology concept (TRL 2), experimental

proof-of-concept (TRL3) and component validation in laboratory

environment(TRL4) have been completed. The table given below summarizes

the situation:

Conclusion

80

TRL Description Task

Completed

1 Basic principles observed and reported

2 Technology concept and/or application formulated

3 Analytical & experimental critical function and/or characteristic

proof-of-concept

4 Component and/or breadboard validation in laboratory environment

5 Component and/or breadboard validation in relevant environment

6 System/subsystem model or prototype demonstration in a relevant

environment (ground or space)

7 System prototype demonstration in a space environment

8 Actual system completed and "Flight qualified" through test and

demonstration (ground or space)

9 Actual system "Flight proven" through successful mission operations

Table 6-2: Evaluation of TRL

6.2 Scope for improvement and Future Work

In spite of the completion of the majority of the goals which were sought to be

accomplished at the end of this project, certain insufficiencies were also

identified along the way. These arguments need to be attended to before a fully

functional PCM based Heat Sink can be adopted for space applications and a

brief discussion on each of them has been done below:

6.2.1 Simulation Issues

6.2.1.1 Simulation of a Range of Melting Point

The current sub-routine that has been used in ESATAN-TMS has ability to

simulate melting of PCM only at a given melting point i.e. all the latent heat of

the PCM is concentrated at a given a temperature. While true for an ideal case, it

is far from the reality as most of the PCMs exhibit not a sharp melting point, but

a range of meting point. While some PCMs have a small range, some others do

have a large range, in which case it becomes difficult to compensate for this

range and results in large discrepancies between reality and simulation.

So, in order to be able to simulate a larger group of PCMs, this subroutine has to

be improved in order to be able to take as input not just a single melting point

temperature, but a range of melting point temperatures.

Conclusion

81

6.2.1.2 Simulation code for Complex Fin and Filler Orientation

The algorithm used for the generation for the nodes and conductance has been

generated manually in ESATAN-TMS and not using the GUI. While this

approach has its advantages, it also makes the task of code developing highly

tedious. For this reason, the code has been restricted to simulate only a simple

PCM box without fins and PCM box with vertical fins (with variable number of

fins and their thickness).

While this is good enough until the concept validation phase of the project, in

order to simulate a box optimized for a given space mission with complex

arrangement of fins and fillers, extensive work has to be done to improve the

algorithm for the code generation.

6.2.1.3 Linearity in the Solution

The non-linearity of the simulation results had been discussed earlier and it had

already been stated that the reason for this behaviour was the scarcity (in terms

of computing standards) of number of PCM elements or nodes.

The abrupt change of a PCM element from a boundary node (a node incapable

of changing its temperature) to a diffusion node (a node capable of changing its

temperature) upon complete melting, results in a localized (in time) change in

the behaviour of the PCM system, which eventually stabilizes quickly, but does

make the solution slightly non-linear.

This drawback can be overcome by increasing the number of nodes. As trivial as

it sounds, the shortcoming in this case is not due to the incompetence in

programming but due to the limitation of the ESATAN-TMS complier and

needs a change in approach in the algorithm. And hence, it was not dealt with in

this thesis.

6.2.2 Cooling Cycles

In this thesis, emphasis has been given only to the heating cycle of the

component, making the PCM box act more like a heat accumulator, partly

because heating cycles play more part in the design of the box (like the amount

of PCM needed, number of fins required, max temperature of the PCM achieved

and thus the mechanical design etc.) and thus the majority of the constraints are

due to it. Thus, keeping this in mind and the time constraints for the thesis,

emphasis was placed only on the heating cycle. (There were other secondary

Conclusion

82

reasons as well like – issues with cooling machine, cooling cycle would mean

the box would be in contact with the cold plate of the TVC and in the case of

leakage of the box there would be PCM inside the TVC which is not advisable

etc.).

However, with a view to design an efficient PCM system, the analysis of the

cooling cycle becomes crucial primarily for two reasons:

1. Design of a heat sink system for the PCM System itself to freeze the PCM

back to solid state

2. PCMs generally do not exhibit exactly the same behaviour in freezing as they

do in melting (there is a slight difference in the melting range and the latent heat

as well), and hence, the freezing cycle itself requires close examination so that

this variation in behaviour can be taken into account for design and simulation.

6.2.3 Issues with gravity and convection

As discussed earlier, results for the behaviour of a PCM system on Earth and

Space showcase a significant difference in performance, which is primarily due

to the presence of gravity (which also introduces convection).

In general the effect of gravity is that it causes the PCM to fall and always

maintain contact with one of the metal borders (depending upon the orientation

of the box), thus causing faster melting of the PCM (In the absence of gravity,

the block of PCM inside the box would remain in the centre, not falling down to

maintain contact with the borders and taking larger time to melt completely

owing to the low thermal conductivity of the PCM).

While a method for the compensation for these effects has been discussed in the

section talking about Simulations, there is no straightforward method to

eliminate gravity in order to evaluate the behaviour and capacity of the system

in space conditions. However, there are ways and means by which gravity can

be partially compensated for and they have been discussed below:

1. The box can be placed horizontally as shown below with the electronic box

on its upper side and the lower end in contact with the cold plate (cooled by

cooling machine).

Conclusion

83

Figure 6-1: Experimental setup for PCM box to reduce the effect of gravity

The cold plate is cooled to a low temperature so as to effectively simulate a heat

sink (or a radiator) for the PCM box. This would cause the melting to take place

primarily from the top, thus eliminating two factors at the same time -gravity

and convection. However, in order for that to happen, the PCM box has to be

full of PCM until the brim (in solid state) without any gap, so as to efficiently

transfer heat from the electronic box to the top layer of the PCM (Something

which was not possible at this stage of the project).

2. The more the simultaneous melting of PCM, the lesser will be the difference

in the absence of gravity. More simultaneous melting of the PCM can be

achieved with the help of insertion of large number of high conductive fins and

fillers. This happens because the fins and the fillers perform the task of

efficiently carrying heat from the vessel to the internal PCM layers and melting

of the internal PCM layers happens because of the heat carried predominantly

by these fins/fillers, and not because of the PCM-metal contact.

Introduction of large number of mesh-like structures made of high conducting

material could be one way of implementing the concept mentioned above. This

would increase the surface in contact with the PCM without the introduction of

excess weight into the system as would be the case with large number of fins.

6.2.4 Mechanical Issues

6.2.4.1 Leak Proof

As mentioned earlier, making the PCM box 100% leak proof or liquid tight has

to be one of the priorities of any future work. This is one of the most crucial

Conclusion

84

aspects of the project, and failing this there would be failure not only of the

PCM system and in the prediction of its behaviour but also possibly of the

component for which it has been designed for.

6.2.4.2 Failure Proof

Making the box capable of withstanding large stresses is complementary to the

point mentioned earlier. Such a box has to be designed with the constraint that it

cannot be very bulky, which would result in PCM box losing its merits when

compared to an orthodox thermal control technique.

One way of reducing the work required on structures, is to select a PCM which

has low volumetric expansion on melting. This would partially reduce the

constraints on the work to be done in making the box rigid enough to take care

of expansion but most probably at the expense of reduced thermal heat capacity.

A right choice has to be made on this matter depending on the functional

requirements of the system.

6.3 Final Remarks

A proof-of-concept has been achieved via this thesis for the operation of PCM

for thermal management of components aboard satellites, which has a wide

avenue of applications from civil to space industry. Besides the fact that the

PCMs are mass and cost effective, they have a much larger capacity per mass to

store and release heat (in the form of their latent heat), and could not only be

used to automate and reduce the power requirements for a satellite, but also for a

number of domestic and other terrestrial applications.

While the technology is ready to be harvested for terrestrial purposes, it is fair to

say that it still needs some effort and research before it can be used extensively

for space-based applications. But once tamed, this technology can be used not

only for individual electronic boxes, but also for the whole spacecraft, rovers,

cooling satellite antennas etc. The concept of heat switches is based on the same

principle as PCM and mastering of this technology could open up also the

possibilities for other heat switch based applications. It can be aptly stated that

the two are very much complementary.

Appendix

85

Appendix

A. Sizing of the Electronic Box

The procedure that was followed for the selection of the dimensions of the

aluminium plate was as follows:

The trends in the dimensions of the electronic boxes for small satellites were

identified along with its various components as listed below:

Dimensions 80 x 85 x 25 mm

Components Battery, Processor, Sensors, Different Voltage buses,

Communication units etc.

Table A-1: Description of a generic E-box for small satellites

A mean or resultant of thermophysical properties for the electronic box was

obtained knowing the thermophysical properties of the individual components

and having a rough idea about their geometrical dimensions inside the box. The

result of such an analysis has been given below.

Property PCB Aluminium Battery Weighted Mean on basis of

geometric

distribution

Thermal Conductivity

(W/m-K)

39.358 237 0 56.85

Specific Heat Capacity

(J/kg-K)

578.49 897 1140 1254.52

Density (kg/m) 2514 2700 198.5 956.64

Table A-2: Thermophysical properties of various components of E-box

Resultant Thermal Capacitance = = 200 J/K

Where -density

-specific heat capacity

-volume

Since the cross section of the aluminium plate depends on the selection of the

PCM box (which was not custom made but was selected from a select available

products), only the height could be altered to obtain the same thermal capacity

as the electronic box.

The material that was chosen for the electronic box was an alloy of Aluminium

(composition: Al Mg1% Si 0.5%) with the following thermophysical properties:

Appendix

86

Property Value

Thermal Conductivity (W/m-K) 220

Specific Heat Capacity (J/kg-K) 960

Density (kg/m) 2630

Table A-3: Thermophysical property of Aluminium alloy

used for E-box

The result of this was that an Aluminium plate of 1 cm thickness would produce

the same capacitance as that of the ideal electronic box.

Appendix

87

B. Performance of PCM with fillers

Reference [1] discusses a simple exercise for analysing the behaviour of PCM

with the variation in the amount of fillers. This analysis can be carried out with

the four equations given below.

1. Resultant thermal conductivity of the system with filler

Where

The fillers are assumed to be mixed homogenously with the PCM and the

resultant thermal conductivity of the mixture is calculated by a highly optimistic

linear formula. This assumption will result in the discrepancy between theory

and actual results. But the main motive behind this exercise is to have an idea of

the amount of fillers needed and the resultant behaviour of the PCM System.

2. Energy absorbed by system

+ [

Appendix

88

3. Temperature constraint

The conditions of the PCM at the end of melting are as follows:

Figure A-1: Graphical representation of PCM at the end of melting cycle

At the end of the cycle of PCM, there is a temperature gradient within the PCM

and this condition has been represented as follows:

Where

4. Mass of the System

At the end of sizing of the PCM box, it is desirable to know the mass of such a

system to evaluate its overall performance and compare it with other available

heat sinks or thermal management techniques. For this purpose, an empirical

formula has been suggested

( √ )

Where

-Total mass of the PCM system including PCM, container, fillers etc.

–Density of the container material

-Length of the container (for all practical purposes can be taken as the length

of the PCM)

Appendix

89

The four equations have been solved for the PCM NE-12 with L, &

being the unknowns for a component with 10W dissipation for 60 mins

considering a melting point of 32ºC for the PCM (it can also be considered as

the equilibrium or optimum performance temperature of the component),

considering an aluminium filler and an aluminium vessel, and the result of such

an analysis is given below:

Figure A-2: PCM performance with variation in filler

In this graph, abscissa value of 0 means that the PCM system is completely

made up of PCM without any fillers and the abscissa value of 1 implies that the

system is completely made up of filler (completely metallic heat sink).

The Results suggests that the optimum behaviour of the PCM in terms of

temperature occurs in and around 50 % of PCM to filler ratio.That is at

this point the temperature of the component is minimum.

This system is not only much more efficient than a metallic heat sink in the

thermal management of the component, but is also lighter.

Appendix

90

Considering the fact that the slope of the curve for diminishes as

it approaches 50 % filler material, the mass of the system can be further reduced

by using only 30 % filler, not the optimal point but in the process saving

significant mass with a small compensation in thermal performance.

Appendix

91

C. Radiator Sized for Peak dissipation V/s PCM System

In this part of the Appendix, it has been theoretically proved that under certain

conditions or specific applications a PCM box with an associated averaged sized

radiator is much more effective, than a radiator sized for maximum radiation

dissipation in terms of mass, something which had been directly stated earlier.

The two systems in comparison are:

1. A Radiator sized for Peak Load Dissipation

This solely includes a radiator system sized for peak dissipation, which

discharges all the heat to space when the component is dissipating and then

overcools the component when it is not in dissipation mode, which is one of its

disadvantages.

2. A PCM system with an associated averaged sized radiator This includes a PCM with an associated average sized radiator that dissipates

the energy absorbed by the PCM to deep space, helping it in freezing again and

making it ready once again for component dissipation.

Given below is a graph of a periodically operating component:

Figure A-3: Duty cycle of cyclically dissipating component

[Reference [1]]

According to Reference [20], there are certain values of radiation of duty cycle

below which a PCM system is more effective mass wise than a radiator based

system. This has been proved below.

Mass for Radiator sized for maximum dissipation of the component is as

follows:

Appendix

92

Mass for a PCM system sized for the same purpose is given as:

Mass for average sized Mass for PCM

Radiator

Where

-Mass of Radiator sized for peak dissipation.

-Power Dissipation of Component

-Mass of Raditaor per sq.meter

-Power Dissipation Capacity of Radiator System per unit area

-Mass of PCM system

-duty cycle

-orbit time

-Latent Heat Capacity of PCM

Note: It is important to note that the mass of radiator systems refers to mass of

thermal finishes, heat pipes system etc.

Equating these two equations to each other, one can find the value for below

which a PCM based system is lighter than a solely radiative system. This

optimum value of i.e. is given by:

The analyses were carried out for the following input:

Appendix

93

1 kW

90 mins

300 W/

35 W-hr/kg

5 kg/

Table A-4: Input values for system comparison

And the result is as follows:

Figure A-4: Mass of radiator system sized for Max.dissip. V/s PCM sys.

with average sized radiator

One can see that below a duty cycle of around 35% the PCM system is lighter

than a radiator sized for peak load. In fact, for very low values of β it is not

advisable to such a system because the mass of the equipment or component

being cooled may be large enough to directly absorb (thermal capacitance) the

Appendix

94

heat generated without appreciable temperature rise and no PCM is needed.

Thus there is a very narrow range in which PCM is highly effective and where

its use is theoretically justified.

On the other hand, with higher the value of β, the PCM +radiator system

becomes ineffective as can be seen in the above graph.

References:

95

References:

[1] David G.Gilmore, Thermal Control Handbook, Second Edition, The

Aerospace Press.

[2] Wiley J.Larson and James R.Wertz, Space Mission Analysis and

Design, Second Edition, Microcosm Inc .ECSS Thermal Control

Handbook-European Space Agency (ESA)

[3] S.Suresh et al, Experimental Configuration of PCM based heat sink with

different configuration of internal fins, International Conference on

Advance Research in Mechanical, Aeronautical And Civil, Pattaya

[4] M.M.Farid et al, A Review on phase change energy storage material and

application, Volume 45, Issue 9, Elsevier Publications

[5] Atul Sharma et al, Review on thermal energy storage with phase change

materials and applications, Volume 13, Issue2, Elsevier Publications

[6] P.Zhang, L.Xia, R.Z.Wang, The Thermal Response of Heat Storage

System with Paraffin and Paraffin/Expanded Graphite Composite for

Hot Water Supply, World Renewable Energy Congress 2011, Sweden

[7] Jacques Bony, Stephane Citherlet, The Numerical Model and

experimental validation of heat storage with Phase Change Material,

Volume 39, Issue 10, Elsevier Publications

[8] Amelia Carolina Sparavigna et al, Behaviour of Thermodynamic Models

with Phase Change Materials under Periodic Conditions, Volume 3,

Issue 2, Scientific Research Publications

[9] Dr.Timothy Knowles, PCM thermal control of Nickel-Hydrogen

Batteries, Energy Science Laboratories Inc

[10] Al-Dadah et al, Experimental Investigation of the effect of fin

configuration on the thermal performance of various PCM based heat

sinks, 4th International Conference on Applied Energy, 2012, China,

[11] S.A.Khot et al, Experimental Investigation of Phase Change

Phenomenon of Paraffin wax inside a capsule, International Journal of

Engineering Trends and Technology- Volume2 Issue2- 2011.

[12] Omar Sanusi et al, Energy Storage and solidification of paraffin phase

change material embedded with graphite nanofibres, Volume 54, ISSUE

19, Elsevier Publications

[13] A.F.Mahrous, Thermal Performance of PCM based heat sinks,

International Journal of Mechanical Engineering, Volume 2 Issue 4

References:

96

[14] J.Kӧhln et al, Design of an experimental PCM Solar Tank- Integrated

Communication and Thermal Managment Microsystem for advanced

S/C, Presentation from The Angstrom Space Technology Centre.

[15] David De Koninck-Thermal Conductivity Measurement using the 3-

Omega Technique

[16] http://amrita.vlab.co.in/ -Note on Newton’s Law of Cooling

[17] http://maxwell.ucsc.edu/-Chi-square:Testing for goodness of fit

[18] Timothy R.Knowles, Phase Change Composites for Thermal

Management, IECEC 2011, San Diego CA

[19] European Space Agency, Technology Readiness Levels Handbook for

Space Applications

[20] M. S. Busby and S. J. Mertesdorf, The Benefit of Phase Change Thermal

Storage for Spacecraft Thermal Management, AIAA-87-1482 (1987)

[21] Dr.Timothy Knowles, Phase Change Composites Phase Change

Composites, for Thermal Management, IECEC, San Diego CA, 2011

[22] Gregory Hickey, Ram Manvi & Timothy Knowles, Phase Change

Materials for Advanced Mars Thermal Control, Jet Propulsion

Laboratory Document, 1996

[23] Klett et al, Pitch-Based Carbon Foam Heat Sink with Phase Change

Material, United States Patent, 2000

[24] Hamilton Sundstrand Space Systems International Inc, Phase Change

Material Heat Sink, European Patent, 2013

[25] Knowles et al, PCM/Aligned Fibre Composite Thermal Interface,

United States Patent, 2004

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