Investigation into the Enhancement of the Thermal Conductivity of Phase

Change Materials Project Report

Project Group O

4/21/2015

http://strathclydephasechange.squarespace.com

Project Group O consists of Calum Ronaldson, Conor Foley, Jack Wright, Peter Hastie and Takuma Kitagawa. Group O are the service provider to Dr. Nicolas Kelly of the Mechanical and Aerospace Engineering Department of Strathclyde University (the client).

i

Contents Figures ............................................................................................................................................................. iv

Tables ................................................................................................................................................................ v

Equations .......................................................................................................................................................... v

Acknowledgements ............................................................................................................................................. 1

1 Introduction ................................................................................................................................................. 2

2 Project Outline ............................................................................................................................................. 3

2.1 Aim and Objectives .............................................................................................................................. 3

2.2 Project Organisation ............................................................................................................................ 3

2.3 Project Management ........................................................................................................................... 3

3 Experimental Design .................................................................................................................................... 5

3.1 Location ............................................................................................................................................... 5

3.2 Design Specifications ........................................................................................................................... 5

3.2.1 Purpose ........................................................................................................................................ 5

3.2.2 Key Measurements ...................................................................................................................... 5

3.2.3 Constraints ................................................................................................................................... 5

3.3 Initial Concepts .................................................................................................................................... 5

3.3.1 Concept 1 - Heater and Pump ..................................................................................................... 5

3.3.2 Concept 2 - Internal Heating Element ......................................................................................... 5

3.3.3 Concept 3 - Modified Tea Urn ..................................................................................................... 6

3.3.4 Concept 4 - Circulating Water Bath ............................................................................................. 6

3.4 Selection Matrix ................................................................................................................................... 6

3.5 Final Concept ....................................................................................................................................... 7

3.5.1 Water Bath .................................................................................................................................. 7

3.5.2 PCM and Heat Exchanger Outer Casing....................................................................................... 7

3.5.3 Heat Exchanger ............................................................................................................................ 7

3.5.4 Temperature Measurement ........................................................................................................ 7

3.5.5 Experimental Set up .................................................................................................................... 7

3.6 Detailed Design .................................................................................................................................... 8

3.6.1 List of components and sizes ....................................................................................................... 9

4 Thermal Analysis ........................................................................................................................................ 10

4.1 Effective Thermal Conductivity ......................................................................................................... 10

4.2 Power Equations ................................................................................................................................ 12

5 Sourcing Materials ..................................................................................................................................... 16

ii

5.1 Phase Change Material ...................................................................................................................... 16

5.1.1 Paraffins ..................................................................................................................................... 16

5.1.2 Fatty acids .................................................................................................................................. 16

5.1.3 Hydrated Salts............................................................................................................................ 17

5.1.4 Final selection of PCM ............................................................................................................... 17

5.2 Container and construction materials ............................................................................................... 17

5.3 Cost Analysis ...................................................................................................................................... 17

6 Manufacturing ........................................................................................................................................... 19

6.1 Hardware Manufacturing .................................................................................................................. 19

6.1.1 Assembling the Perspex Box ...................................................................................................... 19

6.1.2 Piping ......................................................................................................................................... 20

6.1.3 Thermocouple Installation ........................................................................................................ 20

6.2 Software Manufacturing ................................................................................................................... 22

7 Experimental Set-Up .................................................................................................................................. 25

8 Experimental Procedure ............................................................................................................................ 28

8.1 Heating .............................................................................................................................................. 28

8.2 Cooling ............................................................................................................................................... 28

8.3 Set-up Variations ............................................................................................................................... 28

9 Experimental Results ................................................................................................................................. 29

9.1 Heating .............................................................................................................................................. 29

9.1.1 No Fins ....................................................................................................................................... 29

9.1.2 Fins ............................................................................................................................................. 30

9.1.3 Mesh .......................................................................................................................................... 31

9.2 Cooling ............................................................................................................................................... 32

9.2.1 No Fins ....................................................................................................................................... 32

9.2.2 Fins ............................................................................................................................................. 33

9.2.3 Mesh .......................................................................................................................................... 34

9.3 Summary of Results ........................................................................................................................... 35

9.4 Analysis .............................................................................................................................................. 35

9.5 Experimental Issues and Setbacks ..................................................................................................... 37

10 Computational Analysis ......................................................................................................................... 38

10.1 Introduction ....................................................................................................................................... 38

10.2 Theory ................................................................................................................................................ 38

10.2.1 Convection ................................................................................................................................. 39

iii

10.2.2 Mushy Region ............................................................................................................................ 39

10.3 2D CFD Analysis ................................................................................................................................. 40

10.3.1 CFD Geometry ........................................................................................................................... 40

10.3.2 Material properties.................................................................................................................... 41

10.3.3 Mesh .......................................................................................................................................... 41

10.3.4 Mushy Region ............................................................................................................................ 42

10.3.5 Boundary Conditions ................................................................................................................. 42

10.3.6 Initial Conditions ........................................................................................................................ 42

10.3.7 Time step ................................................................................................................................... 42

10.3.8 Results ....................................................................................................................................... 43

10.3.9 Discussion .................................................................................................................................. 45

10.4 3D CFD Analysis ................................................................................................................................. 46

10.4.1 CFD Geometry ........................................................................................................................... 46

10.4.2 Material Properties .................................................................................................................... 46

10.4.3 Mesh .......................................................................................................................................... 46

10.4.4 Mushy Region ............................................................................................................................ 47

10.4.5 Boundary Conditions ................................................................................................................. 47

10.4.6 Initial Conditions ........................................................................................................................ 47

10.4.7 Time Step ................................................................................................................................... 47

10.4.8 Results ....................................................................................................................................... 47

11 Group Reflection.................................................................................................................................... 48

11.1 Project Management ......................................................................................................................... 48

11.1.1 Calum Ronaldson (13 Oct 16 Nov) .......................................................................................... 49

11.1.2 Takuma Kitagawa (17 Nov - 21 Dec) .......................................................................................... 49

11.1.3 Conor Foley (22 Dec 22 Feb) ................................................................................................... 49

11.1.4 Peter Hastie (23 Feb - 29 Mar) .................................................................................................. 49

11.1.5 Jack Wright (30 Mar 29 Apr) ................................................................................................... 50

11.2 Group Performance ........................................................................................................................... 50

11.2.1 Familiarisation ........................................................................................................................... 50

11.2.2 Planning and Preparation .......................................................................................................... 50

11.2.3 Time Management .................................................................................................................... 51

11.3 Reflection ........................................................................................................................................... 51

12 Conclusion ............................................................................................................................................. 53

13 Recommendations ................................................................................................................................. 53

iv

14 Bibliography ........................................................................................................................................... 54

15 Appendices ............................................................................................................................................... I

15.1 Appendix A ........................................................................................................................................... I

15.2 Appendix B ........................................................................................................................................... II

15.2.1 Heating Results ............................................................................................................................ II

15.2.2 Cooling Results ........................................................................................................................... VI

15.3 Appendix C .......................................................................................................................................... IX

15.4 Appendix D ........................................................................................................................................ XII

15.5 Appendix E ........................................................................................................................................ XIV

15.6 Appendix F ......................................................................................................................................... XV

Figures Figure 1: Schematic of the experimental set up with thermocouples shown as black dots in the heat

exchanger circuit ................................................................................................................................................. 8

Figure 2: CAD image of final design ..................................................................................................................... 8

Figure 3: Actual Configuration ........................................................................................................................... 11

Figure 4: Energy equation representation ........................................................................................................ 13

Figure 5: Enthalpy change of PCM versus temperature .................................................................................... 13

Figure 6 : laser cutting profiles .......................................................................................................................... 19

Figure 7: Rig case before use ............................................................................................................................. 20

Figure 8: rig design ............................................................................................................................................ 21

Figure 9: Electronic breadboard and data acquisition logger ........................................................................... 22

Figure 10: Rig ready for experimentation ......................................................................................................... 22

Figure 11: LabVIEW block diagram .................................................................................................................... 23

Figure 12: lab view user interface ..................................................................................................................... 24

Figure 13: Rig (pipe no fins) before use ............................................................................................................. 25

Figure 14: Two views of the LM35s in position in the test rig ........................................................................... 25

Figure 15: Heat exchanger with fins at different stages of heating .................................................................. 26

Figure 16: Stainless steel mesh before and after being positioned in the test rig ............................................ 26

Figure 17: Rig with mesh in place ...................................................................................................................... 27

Figure 18: No fins heating ................................................................................................................................. 29

Figure 19: Fins no additive heating ................................................................................................................... 30

Figure 20: Fins mesh no additive heating .......................................................................................................... 31

Figure 21: No fins, no additives cooling ............................................................................................................ 32

Figure 22: Fins No additives cooling .................................................................................................................. 33

Figure 23: Fins, mesh, no additives cooling run ................................................................................................ 34

Figure 24: The mesh allows heat to penetrate through the solid region of PCM. ............................................ 36

Figure 25: CFD Geometry .................................................................................................................................. 40

Figure 26: Liquid faction heating timeline ......................................................................................................... 43

Figure 27: Liquid faction cooling time line ........................................................................................................ 44

Figure 28: 3D CFD analysis geometry ................................................................................................................ 46

v

Figure 29 .............................................................................................................................................................. X

Figure 30: Temperature cooling timeline ........................................................................................................... XI

Tables Table 1: Drop dead dates ................................................................................................................................. 3

Table 2: Project Management ............................................................................................................................. 3

Table 3: Design matrix ......................................................................................................................................... 6

Table 4: Cost analysis......................................................................................................................................... 18

Table 5: Experimental set up ............................................................................................................................. 28

Table 6: Summary of Results ............................................................................................................................. 35

Table 7: Material properties of copper ............................................................................................................. 41

Table 8: Material properties of PMMA [11] ...................................................................................................... 41

Table 9: Material properties of Lauric acid [12] ................................................................................................ 41

Table 10: Edge sizes ........................................................................................................................................... 42

Equations Equation 4:1 ...................................................................................................................................................... 11

Equation 4:2 ...................................................................................................................................................... 12

Equation 4:3 ...................................................................................................................................................... 12

Equation 4:4 ...................................................................................................................................................... 12

Equation 4:5 ...................................................................................................................................................... 13

Equation 4:6 ...................................................................................................................................................... 13

Equation 4:7 ...................................................................................................................................................... 14

Equation 4:8 ...................................................................................................................................................... 14

Equation 4:9 ...................................................................................................................................................... 14

Equation 4:10 [6] ............................................................................................................................................... 14

Equation 4:11 .................................................................................................................................................... 14

Equation 4:12 .................................................................................................................................................... 15

Equation 4:13 .................................................................................................................................................... 15

Equation 4:14 .................................................................................................................................................... 15

Equation 4:15 [7] ............................................................................................................................................... 15

Equation 10:1 [9] ............................................................................................................................................... 38

Equation 10:2 .................................................................................................................................................... 38

Equation 10:3 [9] ............................................................................................................................................... 38

Equation 10:4 .................................................................................................................................................... 39

Equation 10:5 .................................................................................................................................................... 39

Equation 10:6 .................................................................................................................................................... 39

Equation 10:7 [8] ............................................................................................................................................... 40

1

Acknowledgements The authors of this report would like to acknowledge the following people for their invaluable contributions

to this paper: John Redgate, James Doherty, Chris Cameron, Dr. Thomas Scanlon, Drew Irvine, Duncan

Lindsay, and Steven Black.

We would also like to thank GrafTech International Ltd for supplying free samples of expanded graphite for

use in our experiments.

The group would like to give special thanks to Dr. Nicolas Kelly for his support and guidance throughout this

project.

2

1 Introduction Rapid economic development and population growth has led to huge energy demand across the globe. This

has resulted in diminishing fossil fuel levels and high levels of greenhouse gas (GHG) emissions in the

atmosphere, which may have contributed significantly to global warming. In 1997, the Kyoto Protocol was

adopted by most developed countries. It has placed a heavier burden on developed nations to assume

responsibility for the current high levels of GHG emissions resulting from more than 150 years of industrial

activity. Hence, developed nations have been trying to find new, renewable energy sources to reduce

dependency on fossil fuels as well as methods to conserve energy consumption.

To maximise energy efficiency and to reduce greenhouse gas emissions, thermal energy storage

technologies have been the subject of extensive research because of their potential use within a sustainable

energy infrastructure. Thermal energy storage technologies make it possible to reduce peaks in power

consumption and to shift part of the load from periods of maximum demand. As a result of this, storage of

thermal energy has become an important aspect in engineering applications, especially in energy

conservation in buildings.

One of the various thermal energy storage methods to have attracted attention is latent heat storage. In a

latent heat storage system (LHSS), thermal energy is stored during a phase transition, such as melting or

solidification of a Phase Change Material (PCM). The system is capable of absorbing and releasing thermal

energy with minimal temperature variation. Furthermore, compared to sensible heat storage, phase change

materials allow large amounts of energy to be stored in relatively small volumes due to their high storage

density, resulting in some of the lowest storage media costs of any storage concepts [1].

Energy storage using PCMs in combination with solar collectors has been studied as thermal energy storage

technology has the potential to solve a critical obstacle of solar energy use, namely the time mismatch

between solar energy supply and electricity demand. In addition, solar thermal energy for domestic hot

water heating is one of the most cost effective and efficient areas of alternative energy exploitation [2].

Thus, it has been considered to be the most advantageous means that the heat energy obtained from the

solar receiver is conveyed into PCM via water. When solar radiation is available, the heat energy can be

stored via hot water in the PCM by changing the phase of the PCM from solid to liquid. Later on, when there

is higher electricity demand or during cloudy periods, the stored heat can be recovered and used for hot

water generation.

However, most PCMs are not usable for direct application in LHSS because of their disadvantages. Take fatty

acid as an example, the most serious problem with this solid-liquid PCM is the relatively low thermal

conductivity which restricts its heat transfer rate. Previous works indicate that certain additives are able to

enhance the thermal conductivity of the PCMs by compounding with the PCMs [3] [4]. However, there is

plenty of research yet to be done into using additives in a practical way due to the lack of knowledge on how

to uniformly distribute the additive throughout the PCM. This project looks into methods of increasing the

thermal conductivity of PCMs using additives and heat exchanger configurations.

3

2 Project Outline

2.1 Aim and Objectives The aim of this project was to design and build a thermal energy storage test rig using PCMs to investigate

the efficiency of PCMs and to verify their performance as a latent heat storage system. Furthermore, the

project set out to investigate methods of improving the thermal conductivity of PCMs during heating and

cooling cycles. The original contract signed by the supervisor can found in Appendix E.

To achieve these aims there were a number of objectives and sub objectives, all of which are identified in

the project Gantt chart in the Appendix D.

Some of these objectives were subject to time limitations and all the major objectives were assigned latest

possible end dates (drop dead dates) after which the objective would not be attempted. The latest end dates

for all the major objectives can be found below in table 1.

Table 1: Drop dead dates

Objective Latest end Date

Manufacturing of Test Thermal Store 27 Feb 15 Execution of Experiment and review of results 6 Apr 15 Construction of Computational Simulation 6 Apr 15

All the deadlines discussed in table 1 were met.

2.2 Project Organisation The project was executed by Group O. Group O was allocated 500 to assist in the completion of this task.

Group O had the facilities of a lab space and technical support from the University of Strathclyde. Technical

support included consultation academic support for Dr Nicolas Kelly and technical construction support for

the universitys technicians. Group O reserved the right to revise external funding sources. The University of

Strathclyde made licences for ANSYS (both APDL and WorkBench), LabVIEW and MATLAB available to group

O.

Group O consisted of 5 members: Conor Foley, Peter Hastie, Takuma Kitagawa, Calum Ronaldson, and Jack

Wright, from 13th of October 2014 to the 19th of January 2015. From the 19th January 2015 to the end of the

project group O consisted of 4 members: Conor Foley, Peter Hastie, Calum Ronaldson, and Jack Wright.

2.3 Project Management The role of project manager was rotated by the members of Group O over the course of the project in a 5

week rotation. The project management changed at midnight on the Sunday at the end of the last week of

the managers term (table 2).

Table 2: Project Management

Month Project Manager

13 Oct 16 Nov Calum Ronaldson 17 Nov 21 Dec Takuma Kitagawa 22 Dec 22 Feb Conor Foley 23 Feb 29 Mar Peter Hastie 30 Mar 26 Apr Jack Wright

4

The Project manager was responsible for:

Overseeing the progress of the group

Ensuring the group was able to meet deadlines and complete objectives

Decision making and documentation of the decision making process

All official communications between the group members

5

3 Experimental Design

3.1 Location The experiments were conducted in M5 Upper. It was agreed with the Laboratory Manager Chris Cameron

that the sink should not be used as a drain during the experiments to avoid blockages. The size of the

experimental set up was also restricted by the lab space given to the group.

3.2 Design Specifications

3.2.1 Purpose

The test rig needed to be capable of melting and re-solidifying a PCM in a controlled manner while allowing

data to be collected from the experiment. The test rig had to allow for different additives to be added to the

PCM so that comparisons of the additives could be made from the data collected. The rig had to be designed

in such a way that the PCM could be readily emptied without having to perform a complete dismantling of

the testing facilities. The effective thermal conductivity of the test rig could be improved both by using

additives and by altering the heat exchanger design.

3.2.2 Key Measurements

There were key measurements that had to be provided by the experiment in order to provide the

information required to successfully analyse the thermal conductivity of the PCM:

Temperature of the PCM measured radially in at least 3 different positions.

Mass flow rate of water through the heat exchanger.

Time taken for a given change in temperature of the PCM.

3.2.3 Constraints

The constraints on the experiment mainly arose from the lab conditions. In the lab space the group were

given for the experiment, the hot tap could not be used as a heat source for the PCM because it was an

unreliable source of hot water. Time constraints were also placed on the experiment as the analysis should

ideally have a relatively fast running time so that the PCM did not have to be left for days to melt and re-

freeze. Therefore the heat exchanger set up had to be capable of running both hot and cold water through

the rig on demand.

3.3 Initial Concepts

3.3.1 Concept 1 - Heater and Pump

The first concept for the experimental design was the use of an external heating unit (similar to a shower

heating unit) to heat water in a closed loop system along with a pump to circulate the water through a heat

exchanger surrounded by PCM. The pump and the heater would provide circulated hot water through the

heat exchanger in a closed loop system during heating; however, the closed loop system would then need to

be opened to allow cold water to run passed the PCM meaning that a drain is needed. This system would

potentially take a long time to heat up as the water is being circulated and the temperature would be

difficult to accurately control.

3.3.2 Concept 2 - Internal Heating Element

The second concept included a heating element inside the heat exchanger tube running through the PCM so

that the required water could be heated up quickly and the energy from the heating element could be

6

pushed through the PCM efficiently. The PCM could then be cooled by flushing the system with cold water.

This concept could work with both open and closed loop systems and in an open loop system it would not

require a pump. However, it would be difficult to manufacture a custom made heating element to fit inside

the heat exchanger.

3.3.3 Concept 3 - Modified Tea Urn

A third initial concept was to make use of a water reservoir (in the form of a tea urn) which could be heated

and the water could then be pumped in a closed loop system through the heat exchanger. The large

reservoir of hot water would ensure that the circulated water remained at a constant temperature. This

concept included the use of a modified tea urn to provide the heating along with a separate water pump for

circulation. The system would then still need to be flushed with cold water in an open loop system.

3.3.4 Concept 4 - Circulating Water Bath

The fourth initial concept was a simpler (but more expensive) variation of the third concept. Instead of a

modified tea urn to use as a hot water reservoir, the forth concept makes use of a circulating water bath

which would provide the experiment with a constant temperature hot water reservoir and an integrated

pump, therefore eliminating the need for modifications and a drastic reduction in build time. The water bath

concept has the added advantage of remaining a closed loop system during both heating and cooling as the

temperature can be electronically altered via an interface.

3.4 Selection Matrix A selection matrix was used to determine the best method of providing heating and cooling to the heat PCM.

Table 3: Design matrix

Selection Criteria Heater and Pump

(no reservoir) Internal Heating

Element Modified tea urn

and pump Circulating water

bath

Construction time

s -- - ++

Experimental running time

s ++ + ++

Reliance on drain s + s +

Construction complexity

s -- + +

Cost s + s --

Ease of use s s s +

Total - 0 4 1 2

Total + 0 4 2 7

Total Score 0 0 1 5

The circulating water bath concept came out best in the selection matrix as it satisfied all of the design

constraints and greatly simplified the experimental set-up, it was therefore taken to be the final concept.

The main drawback to this concept was the large cost of a circulating water bath.

7

3.5 Final Concept

3.5.1 Water Bath

The water bath (Grants TC120-P12 stirred water bath) removed the need for separate heaters and pumps in

the system. The water bath had an incorporated pump with a volume flow rate of 16L/min and a

temperature range of 10-120C. The pump provided a sufficient flow rate of water to ensure that the water

flowing through the heat exchanger was fully turbulent, therefore increasing the heat transfer from the

water to the PCM.

3.5.2 PCM and Heat Exchanger Outer Casing

The PCM and heat exchanger were held in a Perspex outer casing which allowed the experiment to be

viewed from all angles when in operation. The Perspex casing also provided insulation to the PCM to reduce

the heat loss from the PCM to the environment. The Perspex casing included an accessible top section which

could be lifted open to access the heat exchanger fins and PCM. The PCM could be poured into the Perspex

casing in liquid form and additives or alternative PCMs would also be added to the test rig through the

hinged top section of the casing.

3.5.3 Heat Exchanger

It was decided that the heat exchanger aspect of the experiment should be allowed to vary in order to test

its effects on the rigs effective thermal conductivity. Therefore a degree of flexibility was incorporated into

to the heat exchanger design. The heat exchanger was a thin walled copper tube running through the centre

of the PCM. The experiment can be conducted with the copper tube alone, however the design allowed for

optional fins to be added to the copper pipe. The effect of adding fins to the heat exchanger can then be

investigated. The copper fins were rectangular and filled the entire cross section of the Perspex test rig to

distribute the heat throughout the full test specimen.

3.5.4 Temperature Measurement

The temperature of the PCM at different locations in the PCM was measured using a number of LM35

temperature sensors. The LM35s were placed through the Perspex walls and held in position by the wires

that connect them to the National Instruments (NI) board (see figure 2). The position of the LM35s (as

shown by the black dots in figure 2) was designed to measure the temperature gradient both radially from

the heat exchanger and between the fins.

3.5.5 Experimental Set up

The experimental set up is shown in figure 1.

8

Figure 1: Schematic of the experimental set up with thermocouples shown as black dots in the heat exchanger circuit

The data from the LM35s was processed by the National Instruments board before being analysed on the

laptop.

3.6 Detailed Design To keep the experimental running time short, the dimensions of the PCM and heat exchanger have been

kept small, with the Perspex inner casing having dimensions of 100mmx100mmx100mm. A full detailed

drawing of the design can be found in appendix A.

Figure 2: CAD image of final design

National

Instruments

Board

Laptop Water Flow

Variable Temperature

Water Bath

PCM

Integrated

Pump

Closed-loop Heat exchanger circuit

Data Analysis circuit Heat Exchanger Fins Perspex

Casing

Power

source

9

3.6.1 List of components and sizes

The following components were needed for the experimental set up:

Copper piping (15mm outer diameter)

Copper sheeting for the heat exchanger fins (1mm thickness)

Pipe expander and reducer (from copper pipe to water bath pipe)

Water bath with integrated pump (Grants TC-120 P12)

Water bath circulating pipes (6mm at inlet, 11m at outlet)

Perspex (5mm thick)

National Instruments Board (NI USB 6210 data logger)

Laptop plus connecting cables (provided by a group member)

PCM (5kg)

Thermocouples

Flexible tubing

Adhesive and sealant

The set up was then modelled in PTC Creo to check the dimensions and ensure that the design was realistic.

10

4 Thermal Analysis

4.1 Effective Thermal Conductivity An accurate analytical model of the system is vital in the determination of the thermal conduction properties

from the measurements taken during the experiment. The experiment begins with the PCM solely in one

phase, either liquid or solid, and will then experience a phase change during the experiment. At the end of

the experiment, the PCM is once again in the form of only one phase.

To work out the thermal conductivity of the PCM (both solid and liquid states), it is much simpler to consider

a steady-state problem. To achieve the steady-state conditions, the experiment would keep running after

the phase change is complete, until all temperature sensors indicate constant temperature. In the steady-

state problem, the PCM is either 100% solid or liquid, depending on which phase change was completed, so

in order to find the thermal conductivity values for the solid and liquid phases both problems will be

analysed analytically.

When the PCM is in its solid phase, and undergoing steady-state heat transfer, the following processes take

place throughout the experimental set-up:

1 Convection from the water to the inside of the copper pipe

2 Conduction through the thickness of the pipe

3 Conduction through the PCM (+convection for liquid phase)

4 Conduction through the Perspex casing

5 Convection from the Perspex surface to the surrounding air (radiation considered negligible)

While convection is a major contributor to the heat transfer through the liquid state of the phase change, it

was decided to work out an effective heat transfer coefficient while using the conduction equations. To

determine the effective heat transfer coefficient, the heat transfer per unit length from position TC1 (Figure

3) to the surrounding air was equated to the heat transfer per unit length from TC1 to TC3.

For the heat conduction equations, a method which uses a conduction shape factor is used [5]. This shape

factor method can be used to bridge the gap for a circular to square cross section as well as conduction

through all four walls of the Perspex. A simple convection correlation is used to describe the convection from

the Perspex box, assuming the bottom surface is well enough insulated to be considered negligible.

11

Figure 3: Actual Configuration

The equation used for the heat transfer per unit length from TC1 to the surroundings is shown below, with

the shape factors taking an impact on the thermal resistance through the system.

Equation 4:1

1 = (1 )

= ln (1.08

21

)

2+ 0.785ln (

)

2+

1

Where a, b and P represent the external side length of the Perspex box, the internal side length of the

Perspex box, and the exposed (for convection) perimeter of the Perspex box (3*a). ka is the thermal

conductivity of Perpex and kpcm is the effective thermal conductivity through the PCM. All other symbols

have their usual meanings.

Equation 2 shows the heat transfer from TC1 to TC3. As the heat transfer travelling through from TC1 to TC3

has not yet reached the extremities of the box, it is assumed that the conduction can be modelled using the

concentric cylinders conduction equation, where all the symbols have their usual meaning.

12

Equation 4:2

= 2 (1 3)

ln (31

)

For a steady-state problem, the heat flux from TC1 to the surroundings and TC1 to TC3 must be equal. An

equation for kpcm can be derived, as shown in equation 4:3.

Equation 4:3

=

(1 ) ln (1.08

21

) (1 3)

ln (31

)

1 3

ln (31

) (

0.785ln ()

+2

)

It is important to remember that this kpcm value calculated is an effective k value and doesnt necessarily give

an accurate description of the thermal conductivity of the material, and is only used as a comparison method

for the different experimental variations. This thermal conductivity value also includes the effects of

convection, additives, and heat exchanger design. Therefore this does not represent the actual thermal

conductivity of the PCM.

4.2 Power Equations Assuming that there is negligible radiation heat loss the energy equation of the PCM in the test rig can be

written as:

Equation 4:4

= +

13

Where Hpcm is the enthalpy change of the PCM.

The enthalpy change of the PCM can be found by using sensible and latent heat effects as shown in the

figure below.

Stages 1 and 3 are sensible heating and the enthalpy change can be found using the sensible heating

equation.

Equation 4:5

=

2

1

The enthalpy change over the phase change stage (2) is the latent heat of fusion.

Equation 4:6

=

Qpipe

Qloss

Temperature, T (K)

Enthalpy, H (J)

1 2 3

Figure 4: Energy equation representation

Figure 5: Enthalpy change of PCM versus temperature

14

The total enthalpy required to take the PCM of mass m from the start temperature (T1) to the end

temperature (T2) can be found as:

Equation 4:7

= 1 + 2 + 3

= ( 1) + +(2 )

= (( 1) + + (2 ))

The heat loss is transient and can be approximated using a discrete method where the total heat loss is the

sum of the heat loss of all the time steps.

Equation 4:8

=

=1

The heat loss by the PCM over each time step can be found for both the solid and the liquid phase as the

sum of the heat losses in all directions assuming there are no end effects.

Equation 4:9

= + +

The assumption is made that the PCM is universally at the same temperature and that the free stream

temperature of the air is a constant 200C (293K). The rig is assumed to be on a table that is 0.05m thick and

heat is lost via convection on the bottom surface. As there is no heat convected though the PCM while it is in

solid phase it is assumed that the temperature on the inside wall of the rig is the temperature of the PCM.

Hence the Q out can be calculated as:

Equation 4:10 [6]

=

{

+

1

+

+1

+

+

< 42 ()

1

+

+1

+

1

+

+

1

+

1

+

+

> 42 ()

The heat transfer coefficients, h can be found using the Nusselt Number, Nu:

Equation 4:11

=

=

15

Where L is the length factor and k is the thermal conductivity. The Nusselt number can be found using

Reynolds number. Reynolds number can be found as using the Grashof number (Gr) and the Prandtl number

(Pr). In this case all convection is natural convection.

Equation 4:12

= =3

2

Equation 4:13

=

Equation 4:14

=3

Here, g is the acceleration due to gravity, is the coefficient of thermal expansion, v is the dynamic viscosity

and is the thermal diffusivity. For the maximum temperature difference possible the Reynolds number is

less than 109, hence the Nusselt number can be found as:

Equation 4:15 [7]

=

{

0.541/4

0.68 +0.671/4

(1 + (0.492 )

9/16

)

4/9

0.271/4

Where =0.8 kg/m3K and thermal diffusivity is 1x10-7 m/s. The values of h can be found at each instant of

time during the experiment. The power into the system can them be found by dividing Qin by the time taken

for the run.

16

5 Sourcing Materials

5.1 Phase Change Material Selection of a suitable phase change material for the purposes of investigation was of key importance to the

project. In order to make a worthy selection, the material had to fit certain criteria. Since the

experimentation involved a thermodynamic study of the material, the thermodynamic criterion for the

material was extremely important. For a phase change material, the ideal thermodynamic criterion is as

follows:

Melting point in the desired operating temperature range

High latent heat of fusion per unit mass, so that a lesser amount of material stores a given amount

of energy

High density, so that a smaller container volume holds the material

High specific heat to provide for additional significant sensible heat storage effects

High thermal conductivity, so that the temperature gradients required for charging and discharging

the storage material are small

Congruent melting: the material should melt completely so that the liquid and solid phases are

identical in composition. Otherwise, the difference in densities between solid and liquid cause

segregation resulting in changes in the chemical composition of the material

As well as matching the thermodynamic criteria as best possible, the material also had to adhere to other

levels of criteria. The material should exhibit little to no levels of supercooling in order to maximise the

efficiency of its energy storage capabilities. In terms of chemical properties, the material should exhibit

chemical stability, non-corrosiveness to construction materials, no chemical decomposition (so that a high

latent thermal energy storage system life is assured) and must be non-flammable, non-poisonous and non-

explosive. As for the economic criteria, the material should be available to purchase in large quantities and

at a reasonable price.

Once a criterion was established a study was performed to examine the properties of the three main groups

of commercially available PCMs to determine whether they were suitable for the scope of this project.

5.1.1 Paraffins

Paraffins are a straight-chain hydrocarbon group with good thermal storage capacities. They have proven to

undergo solid-liquid phase change without super cooling and generally they have a high latent heat of

fusion. They possess the chemical stability required for the material to undergo many heating and freezing

cycles, and will not corrode any construction materials. The main problem with paraffins however is the

pricing. Due to the required costs of separation from crude oil, getting a pure enough product makes most

paraffins commercially unviable. Additionally, the melting points within the human comfort range for

paraffins are few and far between, and to achieve a certain temperature will often require a blend of two or

more paraffins, which creates a difference in melting/freezing point which has an adverse effect on the

latent heat capacity of the material. Paraffins are also slightly flammable but certain simple precautions can

be taken towards negating any potential safety threat.

5.1.2 Fatty acids

The emergence of vegetable-based fatty acids as a PCM has led to breakthroughs in development due to its

superior environmental and economic benefits over paraffins. They also possess the longevity required for

17

many cycles, and are far less flammable than their naturally-occurring competitors. They possess equal

performance characteristics at a fraction of the price, but their conductivity levels are very low in the solid

state.

5.1.3 Hydrated Salts

The inorganic hydrated salt group are typically the most inexpensive PCMs and have been the subject of

extensive research as latent heat storage materials. They possess excellent heat storage capabilities and are

capable of conducting heat at a much higher rate than both paraffins and fatty acids. There are certain

impracticalities which are associated with the inorganic compounds, namely the large degree in change of

volume between the solid and liquid phase, as well as the tendency to supercool without the necessary

nucleating agent and low cycle life.

5.1.4 Final selection of PCM

After studying the commercially available phase change materials and examining each materials eligibility

for the project, the number of available materials was quickly narrowed down. Paraffin waxes were

eliminated from the selection since it would not be possible to acquire enough material for experimentation

without exceeding the budget constraint. The impracticalities of changing volume associated with salt

hydrates did not justify the complications that would arise in designing a suitable container, and were

therefore eliminated. This left the fatty acid group, of which several candidates were suitable. In the end

lauric acid was chosen for its favourable melting temperature, high latent heat of fusion and relatively low

price compared to other fatty acids in bulk.

5.2 Container and construction materials PMMA (Perspex) was the material chosen for the container. Its transparency allows the visible changes

within the container during phase change to be seen, and its reputable thermal insulation properties help to

mitigate the heat loss from the phase change material to the atmosphere. For the inner piping and fins,

copper is the preferable choice as it is inexpensive in pipe and sheet form, and has a very high thermal

conductivity value which is paramount to this study. Rubber tubing will connect the copper piping to the

water bath, as the rubber acts as a thermal insulator ensuring that the temperature of the water reaching

the rig has little variation from the water bath temperature.

5.3 Cost Analysis Like all projects, one of the main drivers was the operational budget. The role of an engineer is to achieve a

solution to a problem using as little resources as possible, as this minimises expenditure on a project which

therefore benefits shareholder wealth. It also engages creative thinking in the design process, knowing that

there will be limited resources available. While a university project has no shareholders as such, it is good

practice to operate within the assigned budget, as every engineer will face the same constraints throughout

their career. At the beginning of the project the group was assigned a budget of 100 per group member.

Since there were initially five members of the group the operating budget for the project was 500. A

breakdown of the groups expenditure over the course of the project is depicted below.

18

Table 4: Cost analysis

Component Quantity Price/unit Price

Lauric Acid 5kg 65.00 65.00

LM35 6 2.00 12.00

copper Pipes 3m 0.00 0.00

flexible tubing 1m 0.00 0.00

Jubilee clips 4 0.00 0.00

NI Board 1 0.00 0.00

Water bath 1 Subsidised by University 211.98

Perspex 0.15m3 8.67 8.67

Copper-Zinc nanoparticles 5g 46.56 46.56

Expanded Graphite 2 0.00 0.00

Electronic breadboard 1 6.84 6.84

Ball valve 1 6.09 6.09

Website Domain (Monthly Instalment) 2 6.73 13.46

Heat shrink 3m 7.26 7.26

Total 375.86

Budget 500.00

Remaining 126.14

Many of the components had a price of zero. In most cases these components were provided by the MAE

store, and the expanded graphite samples were kindly donated to us by GrafTech International for research

purposes. Initially, the water bath had a price tag of 723.96, which was beyond the groups budget. The

group felt that this particular water bath was perfect for the research purposes of the project, and argued

that this piece of equipment could be of great use to the department in the years to follow, so the

department and Dr Kelly agreed to subsidise the cost of the water bath, thus freeing up the groups flexibility

of working capital for the rest of the projects requirements.

19

6 Manufacturing The manufacturing stage of the project began on the 26th of January and lasted until the 15th of February. In

conjunction with the manufacturing of the test rig, the creation of a Virtual Instrument (VI) on LabVIEW had

to be completed at the same stage in order to ensure a smooth transition into the testing stage.

6.1 Hardware Manufacturing

6.1.1 Assembling the Perspex Box

The first duty was to cut the Perspex casing into shape. It was a top priority to ensure that all the separate

pieces of the rig fitted together with minimal spacing between areas where two sides met. To achieve this

precision the group opted to have the Perspex laser-cut into shape.

Drawings for each part were created in CREO. Each side had a specified dimension, and 4 mm holes were

placed strategically across the rig to make the rig easier to screw together. Two 15.5 mm holes were created

at two opposing sides to allow the copper pipe to run through the rig.

Figure 6 : laser cutting profiles

After the pieces were cut to shape, 3.5 mm holes were drilled at the designated points and screws were then

installed to ensure rigidity throughout the rig.

Base Front & Back

Sides Top

20

Figure 7: Rig case before use

Even though this rigidity gave the rig its structural soundness, screws were not enough to prevent the

leakage of fluid from the inside of the rig. A waterproof silicone sealant was chosen due to its high

temperature sustainability and wide market availability. The inner edges excluding the top of the rig were all

sealed, and the copper pipe was then passed through the centre of the rig and sealed at the inner and outer

junctions at both faces.

6.1.2 Piping

Once the sealant had been allowed to set, the copper pipe which runs through the centre of the rig was

connected to a hose at the output and held in place with a jubilee clip. As the water baths intrinsic pumping

system became active as soon as the bath was turned on, it was necessary to have a mechanism to prevent

the water flow through the test rig before it reached the desired temperature. A ball valve was installed

between the copper pipe and another hose to facilitate this requirement. The unconnected ends of the

hoses were then attached to the appropriate connections of the pumping system. The fins of the heat

exchanger were brazed onto the pipe to ensure there was good contact to allow for thermal conductivity.

The rig was then filled with water and 80C water was pumped through the piping system. The water was

then allowed to heat inside the rig. This was to test for any leaks in the Perspex, a number of which were

found. After consulting a technician, the sealing method was changed which yielded a more robust and leak-

free rig which was now ready to hold the phase change material.

6.1.3 Thermocouple Installation

Six thermocouples were to be placed inside the rig, in such a position that the thermodynamic

characteristics of the fluid could be properly analysed, as depicted in figure 8. A seventh thermocouple was

also created to monitor the water bath temperature.

21

Figure 8: rig design

The thermocouples used for measurement were LM35s. The LM35 was chosen as its output voltage was

directly proportional to the Celsius temperature and required minimal calibration. Each had to be wired and

protected from fluid damage. Each LM35 was soldered to three different-coloured wires, in order to

distinguish between the power supply pin, ground pin and temperature sensor output. After each LM35 had

been soldered to the wires, shrink wrap was used to make each node watertight, as any fluid coming into

contact with the wires would disrupt the function of the LM35.

Before the LM35s were placed into the rig, they were calibrated using the VI created in LabVIEW to ensure a

level of overall consistency in the readings. A standard deviation of 0.4C was achieved over the six LM35s

inside the rig, which was a satisfactory level of consistency for the purposes of the investigation. Once

satisfied, they were tested in water to ensure each LM35 had been sealed properly.

The LM35s were then carefully placed within the rig. Great care was taken to ensure each LM35 was

accurately placed in its correct position. The wires were then fixed into place using sealant on the top of the

rig. Each wire at the other end was connected to an electronic breadboard (figure 9) powered by an op-amp

to boost the LM35 voltage signal. The breadboard was connected to an NI Data Acquisition Logger which

connected directly to a computer for logging the data, the software for which is spoken about in the next

section. Figure 10 depicts the rigs hardware ready for commissioning.

22

Figure 9: Electronic breadboard and data acquisition logger

Figure 10: Rig ready for experimentation

6.2 Software Manufacturing In order to capture, compute, visualise and make good use of the data required to make further calculations,

data acquisition software was required. LabVIEW had the required facilities but none of the group members

had any experience with the software. Jack and Conor familiarised themselves with the fundamentals of

LabVIEWs operating processes and attended a training session to help with the learning process. Eventually

we possessed the skillset required to create a block diagram fit for our purposes in LabVIEW

23

Figure 11: LabVIEW block diagram

In the block diagram shown above, it can be seen that the DAQ Assistant block is the raw data coming in

from the data acquisition board. Within the DAQ assistant block, the data can be scaled to ensure accuracy

of the devises and this was done for each LM35. From the raw data, it is then seen that the multiple data

feeds are split into separate flow lines, in which they are all averaged over their last 10 readings (this was

done to smooth over the readings and avoid random peaks and troughs from the sensors). The averaged

values are then sent to thermometers to aid the user in seeing the individual temperatures at each point.

Finally the data is grouped back together and sent to a graph block, in which all data lines are displayed on

the same plot, and a write to measurement file block which is a way of saving the data. A save button was

also introduced so the program could be run with or without saving data. The block in the above diagram

which features a watch was to cause a 1 second delay after each reading, this was to ensure there was not

an overload of data.

The block diagram produced a VI which allows the user to visualise what is going on inside the rig, and lets

them have control over the starting and stopping of data acquisition.

24

Figure 12: lab view user interface

The above figure is a snapshot of a heating run, with each data line representing the changes in temperature

over time for each LM35. The higher temperature data lines indicate areas where the material is melting at a

faster rate.

25

7 Experimental Set-Up The set-up of the heat exchanger circuit is shown in figure 10. The heat exchanger part of the circuit was

made up of the water bath connected to the test rig by garden hose.

Figure 13: Rig (pipe no fins) before use

Figure 13 shows the ball valve connected to the inflow side of the test rig so that water could be heated or

cooled in the water bath while the valve was closed while not having an effect on the PCM in the test rig.

The heat exchanger pipe was initially used without fins, as shown in figure 14.

Figure 14: Two views of the LM35s in position in the test rig

Once the test rig was sealed, the PCM was put in place and the heat exchanger side of the experiment was

complete. The LM35s (shown in figure 14) taking the temperature readings were then connected to the data

analysis side of the circuit.

The wires from the six LM35s were connected into a breadboard, before the output signal wires were

connected to the NI USB 6210 board. The LM35s were powered using a 1.5A 9V regulated DC supply. The

data from the NI board was then analysed in a computer using LABVIEW. Each of the LM35s were calibrated

from 0 to 80 degrees and a scale was placed on them in LABVIEW.

26

Figure 15: Heat exchanger with fins at different stages of heating

When the experiments with the heat exchanger without fins had been completed, the finned heat exchanger

was substituted into the test rig as shown in figure 15.

Figure 16: Stainless steel mesh before and after being positioned in the test rig

Figure 16 shows the stainless mesh that was manufactured and placed inside the test rig. Figure 17 shows

the final set up of the experiment with the stainless steel mesh inside the test rig along with the finned heat

exchanger.

27

Figure 17: Rig with mesh in place

28

8 Experimental Procedure The NI board readings were interpreted and recorded in LabVIEW before being analysed in MS Excel. In

LabVIEW, data from the LM35s was recorded every second and averaged over 10 readings to smooth the

readings out. Before starting any experiment, the LABVIEW data recording was turned on so that no data

was missed.

The experiments were carried out in two phases: heating and cooling.

8.1 Heating During the heating phase the water bath was heated to 80C with the circulating valve closed. When the

water bath reached the desired temperature of 80C the circulating valve was opened allowing the heated

water to come in contact with the water. At this point the LabVIEW data started recording the temperature

from the six LM35s placed inside the test rig the heating phase of the experiment would then run until the

test rig had reached steady state conditions.

8.2 Cooling After the heating phase was complete and steady state conditions had been reached, the cooling phase of

the experiment could commence. The circulating valve was closed to stop the hot water from circulating

before emptying out the water bath. The water bath was then filled with water that was heated up to 20C

and the circulating valve was once again opened. The LabVIEW data was recorded from when the circulating

valve was reopened allowing the cooling water to pass through the PCM. The cooling run continued until the

PCM reached a steady state condition as indicated by the LM35s.

8.3 Set-up Variations The test rig conditions were altered to test the overall heat transfer rate. The configurations shown in table 5

were used to compare the effectiveness of each additive and heat exchanger configuration and determine

the best configuration. Each experiment was repeated at least three times for experimental accuracy.

Table 5: Experimental set up

Experiment No. Heat Storage Heat Exchanger Type Additive

1 Lauric Acid None None

2 Water Fins None

3 Lauric Acid Fins None

4 Lauric Acid Fins Expanded graphite TG679 2g/L

5 Lauric Acid Fins Expanded graphite TG679 15g/L

6 Lauric Acid Fins Copper-zinc nanoparticles 5g/L

7 Lauric Acid Fins Stainless steel mesh

29

9 Experimental Results The results from the experiments were plotted in Microsoft Excel. The temperature readings from the LM35s

were plotted against the time to produce the graphs shown in this section. The complete experimental

results can be found in Appendix C.

Each graph gives a general overview of the changes in temperature throughout the rig. Certain observations

can be made from an initial overview of the graphs, for example, the evident reduction in cooling time the

mesh provides (figure 23) in comparison with the same rig with only fins used as a heat exchanger alteration.

In order to gain more insight into the results, the equations developed in section 4 were applied to the data

acquired by the LabVIEW software.

9.1 Heating

9.1.1 No Fins

Figure 18: No fins heating

Temperatures converge at around 8,000 seconds in Figure 18.

0

10

20

30

40

50

60

70

80

90

0 2000 4000 6000 8000 10000 12000

Tem

pe

ratu

re, T

(C

o)

Time, t (s)

No Fins, No Additive: Heating Run 1

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

Water Bath

30

9.1.2 Fins

Figure 19: Fins no additive heating

Temperatures converge at around 3,500 seconds in Figure 19.

0

10

20

30

40

50

60

70

80

90

0 1000 2000 3000 4000 5000 6000 7000 8000

Tem

pe

ratu

re, T

(C

)

Time, t (s)

Fins, No Additive: Heating Run 2

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

WaterBath

31

9.1.3 Mesh

Figure 20: Fins mesh no additive heating

Temperatures converge at around 3,400 seconds in Figure 20. This indicates a marginal improvement from

the configuration before the mesh was added.

0

10

20

30

40

50

60

70

80

90

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Tem

pe

ratu

re, T

(C

)

Time, t (s)

Fins, Mesh, No Additives: Heating Run 2

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

Water Bath

32

9.2 Cooling

9.2.1 No Fins

Figure 21: No fins, no additives cooling

Water bath temperature equals PCM temperature at around 20,000 seconds in Figure 21.

0

10

20

30

40

50

60

0 5000 10000 15000 20000 25000 30000 35000

Tem

pe

ratu

re, T

(oC

)

Time, t (s)

No Fins, No Additives: Cooling Run 3

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

33

9.2.2 Fins

Figure 22: Fins No additives cooling

Water bath temperature equals PCM temperature at around 16,000 seconds in Figure 22.

0

10

20

30

40

50

60

70

80

0 5000 10000 15000 20000 25000 30000

Tem

pe

ratu

re, T

(C

)

Time, t (s)

Fins, No Additive: Cooling Run 4

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

WaterBath

34

9.2.3 Mesh

Figure 23: Fins, mesh, no additives cooling run

Water bath temperature equals PCM temperature at around 13,500 seconds in Figure 23.

0

10

20

30

40

50

60

70

80

0 5000 10000 15000 20000 25000 30000

Tem

pe

ratu

re, T

(C

)

Time, t (s)

Fins, Mesh, No additives: Cooling Run 2

Length 1

Length 2

Length 3

Radius 1

Radius 2

Radius 3

WaterBath

35

9.3 Summary of Results Table 6 shows the average power from the heating and cooling cycles, the average steady state

temperature, and the effective thermal conductivity (heating and cooling) for each experiment variant. The

power was calculated using the method described in the section 4. Refer to table 5 for a description of the

experiment number.

Table 6: Summary of Results

Experiment No.

Description Heating Power (W)

Cooling Power (W)

Average Steady State Temperature (C) Heating/Cooling

Average Effective Thermal Conductivity

(W/m2K) Liquid/Solid

1 No fins, no additives

27.4 14.2 63.02/20.98 14.57/0.09*

2 Fins, water 53.7 47.8 73.16/26.5 56.70/3.15 3 Fins, No

additives 45.7 16.5 70.57/21.65 19.52/0.37

4 Fins, 2g/L expanded graphite

46.0 14.8 70.77/22.4 28.94/0.36

5 Fins, 15g/L expanded graphite

44.4 13.3 70.99/21.84 25.97/0.35

6 Fins, 5g/L copper-zinc

nanoparticles

49.3 15.1 70.8/22.94 47.74/-

7 Fins and mesh, no additives

47.3 15.0 71.26/22.96 25.10/1.72

*These are not the true liquid and solid effective thermal conductivities since the PCM did not fully

melt/freeze

9.4 Analysis An effective thermal conductivity could not be obtained for experiment 1 during the heating cycle due to the

fact that the PCM never reached a fully liquid state. The results show that the addition of fins to the pipe

greatly reduced the experiment running time and increased the heat transfer through the PCM, and this also

allowed the PCM to fully melt with in an appropriate time scale.

As can be seen in Table 6, the average heating power suggests that the addition of the 2 g/L of expanded

graphite slightly improved the heating characteristics of the PCM solution, while the cooling power suggests

that the graphite hindered the cooling process of the PCM. The average effective thermal conductivity from

table 6 is consistent with the results that the expanded graphite (2 g/L) did improve the PCM during the

heating cycle, but lowered the performance (negligible) in the cooling run. The cooling run is an important

feature of the cycle as this is what will return the heat back to the user, because of this, it is not a desirable

impact for the cooling run to be hindered.

When more expanded graphite was added into the mixture (15 g/L), the power calculation suggests that the

performances in both the heating and cooling cycles were diminished. When the average effective heat

36

transfer coefficients are examined, this suggests that the effective heat transfer in the liquid stage was

improved, but again declined in the solid stage. This means while the power calculation suggests the

performance in the heating cycle has declined, the heat transfer coefficient suggests that the performance

through the liquid phase improved. This difference can be explained as the power calculation takes place

throughout the whole phase change process, in which the solid state will still exist and have an effect,

whereas the heat transfer coefficient is only averaged over the steady-state section of the cycle, where the

PCM is solely liquid.

When the zinc-copper nanoparticles were introduced as an additive, some temperature sensors became

temperamental and required to be monitored at all times. This resulted in not being able to calculate

average effective heat transfer coefficient during the solid phase, and the liquid phase should only be used

for rough indication. The power calculations suggest that the heating cycle improved, again with the cooling

cycle performance lower than that with no additives. The heat transfer coefficient for the heating cycle is

consistent in that the performance improved, but again should only be used as indication.

The stainless steel mesh was the final experiment variation to be run. The power calculations show similar

results to other experiments with additives in the solution, that the heating cycle improved in performance

but once again, the cooling cycle performance slightly decreased compared to the no additive configuration.

When looking at the effective heat transfer coefficients, these suggest that the coefficients have increased

from the no additive situation in both the solid and liquid states of the solution. This contradicts the power

calculation, which may be because the power is averaged over the whole process and not all of the

experiments ran for same amount of time, this was down to different durations of phase change and access

to the experimental set up.

The results indicate that the mesh and fin arrangement made the greatest improvement to the PCM solution

overall, however the mesh had a minimal effect when compared to the impact of the fins. The improvement

may be furthered by the use of a better constructed mesh with better contact points for the pipe and better

contact through the mesh structure itself. The effects of the mesh when good contact was made with the

fins can be seen in figure 24.

Figure 24: The mesh allows heat to penetrate through the solid region of PCM.

37

As it can be observed that the biggest improvement was when the heat exchanger fins were implemented,

further research should be carried out into how more complex and efficient heat exchangers can be

implemented into certain geometries.

9.5 Experimental Issues and Setbacks The LM35s had to be made completely waterproof which was achieved by covering them in heat

shrink and then using silicon sealant to seal any remaining gaps. The PCM would still eventually work

into the heat shrink covering and short circuiting remained an issue throughout the project.

LM35s should be calibrated upon purchase but since they were covered in heat shrink and sealant,

calibration was carried out on the LM35s using the water bath as a calibrator. Any discrepancies in

the temperature readings across the LM35s were corrected using a scaling factor so that all LM35s

gave the same reading at the same thermometer. However, this scaling means that the originally

stated error of 0.6C was slightly distorted.

The test rig was held together by screws and sealed using silicon sealant which should operate up to

200C. However, it was found during testing that the liquid lauric acid was degrading the sealant and

slowly causing leaks in the rig. A leak in the test rig would mean that experiments had to be stopped

while the test rig was resealed, often leading to a day or more of lost time deconstructing the rig and

waiting for the newly applied sealant to dry.

38

10 Computational Analysis

10.1 Introduction The experimental testing of thermal phase change stores provided accurate and realistic information on the

use of phase change thermal stores. However the experimental results could only show the local conditions

around the LM35 thermal probes - they could not directly show the internal dynamics of the system. Due to

the complicated geometry of the phase change store and the changing material properties of the PCM, the

use of algebraic models to understand the internal dynamics from the experimental results is extremely

complex. Due to this complexity, computerised simulations of models with similar conditions to that of the

experimental model were created to understand the internal dynamics of the phase change store.

These simulations provided information that became useful in the design process and aided the

understanding of the behaviour of the PCM during phase change as well as providing topics for future study.

10.2 Theory As discussed in the previous section of this report, the phase change of a material absorbs or releases energy

without an increase or decrease in temperature- the latent heat effect. This latent heat was modelled in the

simulation as a change in enthalpy at the phase change temperature. This change in phase was also coupled

with a change in properties.

The CFD simulation is based on the finite volume method and uses the Navier-Stokes equations to evaluate

the flow characteristics in each volume. For the phase change solver the liquid faction of the each cell is

evaluated using the equation below [8]:

Equation 10:1 [9]

=

{

0 <

< <

1 >

Where Tsoildus and Tliquidus are the solidus and liquidus temperatures respectively. The enthalpy change due to

the latent heat effect can be found for each cell dependent on the liquid faction.

Equation 10:2

=

Where L is the latent heat. The sum of the latent heat and the sensible enthalpy are then used to find the

overall enthalpy change in material. In the melting and solidification problem for CFD models the energy

equation can be written as:

Equation 10:3 [9]

() + . () = . () +

Here v is the velocity of the fluid, is the density, k is the thermal conductivity and S is the source term.

39

10.2.1 Convection

Convection was found to be the primary source of heat transfer in PCM in liquid phase. The convection in

the test rig is natural convection caused by a difference in density. The amount of heat transferred via

natural convection Q over an area A can be found using the heat transfer equation.

Equation 10:4

= ()

Where T is temperature and h is the heat transfer coefficient. The heat transfer equation can be found using

the Nusselt number, Nu [10, p. 105].

Equation 10:5

=

Where D is diameter. Nusselt number can be found as a function of the Reynolds, Re, and Prandtl, Pr,

numbers. The Nusselt number is proportional to the Reynolds number. The Reynolds number can be

calculated as:

Equation 10:6

=3

= 3

Where g is the acceleration due to gravity, is the coefficient thermal expansion, L is the length, is the

thermal diffusivity and is the dynamic viscosity of the medium.

The initial models were FEA model using ANSYS with the assumption the viscosity of the liquid phase PCM

was high and the density difference was low in liquid PCM at low temperatures, hence, Reynolds and Nusselt

numbers were low and the majority of heat transfer in the liquid phase was carried out by conduction. The

FEA models the change of phase using an increase in enthalpy at the phase change temperature without a

change in viscosity.

Both the experimental results and the further research found that the FEA assumptions were not accurate

and so a model was created using CFD including the effect of convection. Fluent uses an enthalpy-porosity

method [8]to model natural convention during the phase change.

10.2.2 Mushy Region

When the temperature of the PCM is between the liquidus temperature and the solidus the PCM material is

said to be in the mushy region where there is a mixtu