DEVELOPMENT OF A THERMAL ENERGY STORAGE SYSTEM FOR …

DEVELOPMENT OF A THERMAL ENERGY

STORAGE SYSTEM FOR COLD CHAIN

APPLICATION

Author

Md. Monir Hossain

DEPARTMENT OF MECHANICAL ENGINEERING

DHAKA UNIVERSITY OF ENGINEERING & TECHNOLOGY, GAZIPUR

DECEMBER 2019



APPLICATION

A Thesis

by

Md. Monir Hossain



DECEMBER 2019



APPLICATION

A Thesis

by

Md. Monir Hossain

Student No.: 142301 (P), Session: 2014-2015

Supervisor

Dr. Hasan Mohammad Mostofa Afroz

Professor, Department of Mechanical Engineering

Director, Institute of Energy Engineering

Dhaka University of Engineering & Technology, Gazipur

Submitted to the



In partial fulfillment of the requirements for the award of the degree

MASTER OF SCIENCE IN MECHANICAL ENGINEERING

DECEMBER 2019

The Thesis entitled “Development of a Thermal Energy Storage System for Cold Chain

Application” submitted by Md. Monir Hossain, Student No.: 142301-P has been accepted as

satisfactory in partial fulfillment of the requirement for the Degree of Master of Science in

Mechanical Engineering on 17th December, 2019.

BOARD OF EXAMINERS

1. (Dr. Hasan Mohammad Mostofa Afroz)

Professor

Department of Mechanical Engineering


Gazipur-1707, Bangladesh.

Chairman

(Supervisor)

2.

(Dr. Mohammad Zoynal Abedin)

Professor and Head




Member

(Ex-officio)

3.

(Dr. Md. Anowar Hossain)

Professor




Member

4.

(Dr. Md. Mostaqur Rahman)

Associate Professor




Member

5.

(Dr. Mohammad Ariful Islam)

Professor


Khulna University of Engineering & Technology

Khulna, Bangladesh.

Member

(External)

DECLERATION

I confirm that the work showed in this thesis or any portion of thesis has not been submitted

previously, to qualify for the award of any other academic degree. The data and information

are derived from the published and unpublished work of others has been acknowledged in the

text and a list of references is given.

Signature

.......................................

(Md. Monir Hossain)

Signature of Thesis Supervisor

...................................................................

(Dr. Hasan Mohammad Mostofa Afroz)

Professor, Department of Mechanical Engineering

Director, Institute of Energy Engineering



Nomenclature

Abbreviation Full Meaning

COP : Coefficient of Performance

LMTD : Log Mean Temperature Difference

PCM : Phase Change Material

PV : Photovoltaics

SHS : Sensible Heat Storage

STC : Standard Test Condition

TES : Thermal Energy Storage

VCR : Vapour Compression System

LHS : Latent Heat Storage

i

Content Page No.

Acknowledgement _________________________________________________________ vi

Abstract __________________________________________________________________vii

Chapter 1: Introduction _______________________________________________________ 1

1.1 Background ___________________________________________________________ 1

1.2 Literature Review ______________________________________________________ 2

1.3 Scope of Work ________________________________________________________ 5

1.4 Objectives of the Current Research Work ___________________________________ 5

Chapter 2: Phase Change Material (PCM) as Thermal Energy Storage __________________ 6

2.1 Introduction to Phase Change Material (PCM) _______________________________ 6

2.2 Categorization of PCMs _________________________________________________ 6

2.2.1 Organic Type PCM _________________________________________________ 7

2.2.2 Inorganic Type PCM ________________________________________________ 7

2.2.3 Eutectic Type PCM _________________________________________________ 8

2.3 Implementation of PCMs as Thermal Energy Storage __________________________ 8

2.3.1 Solar Photovoltaic (PV) System _______________________________________ 8

2.3.2 Air Conditioning applications _________________________________________ 9

2.3.3 Solar Water Heating Application _______________________________________ 9

2.3.4 Biomedical applications ______________________________________________ 9

2.3.5 Thermal Energy Storage in Buildings __________________________________ 10

2.3.6 Nuclear Thermal Energy Storage Systems ______________________________ 11

2.3.7 Storage and Transport of Thermal Sensitive Products _____________________ 11

2.3.8 Other Applications _________________________________________________ 12

Chapter 3: Experimental Setup & Methodology __________________________________ 13

3.1 Experimental Facility __________________________________________________ 13

3.2 PCM Heat Exchanger __________________________________________________ 14

3.2.1 Casing or Shell ____________________________________________________ 15

Table of Contents

ii

3.2.2 PCM Plate _______________________________________________________ 16

3.2.3 Heat Exchanger Coil _______________________________________________ 17

3.2.4 Fin _____________________________________________________________ 18

3.2.5 Refrigerant Distributor ______________________________________________ 18

3.2.6 Condensing Unit __________________________________________________ 19

3.3 Fabrication of the Experimental Setup _____________________________________ 20

3.4 Design of the PCM Heat Exchanger _______________________________________ 21

3.5 Equipment used in the Experiment ________________________________________ 22

3.6 Experimental Procedure ________________________________________________ 26

Chapter-4: Results & Discussions _____________________________________________ 28

4.1 Solidification and Melting Characteristics of PCM ___________________________ 28

4.1.1 Temperature Distribution during Solidification (Basic Model)_______________ 29

4.1.2 Temperature Distribution during Melting (Basic Model) ___________________ 32

4.1.3 Temperature Distribution during Solidification (Fins Incorporated Model) _____ 34

4.1.4 Temperature Distribution during Melting (Fins Incorporated Model) _________ 36

4.1.5 Temperature Distribution during Solidification (Refrigerant Distributor Model) _ 38

4.1.6 Temperature Distribution during Melting (Refrigerant Distributor Model) _____ 40

4.1.7 Comparisons of Solidification Phenomena for all Models __________________ 41

4.1.8 Melting Phenomena Comparison for all Models __________________________ 43

4.2 Performance Analysis of VCR System _____________________________________ 45

4.3 Performance Analysis of Air Side of PCM Heat Exchanger ____________________ 46

Chapter-5: Conclusion and Further Research _____________________________________ 49

5.1 Conclusion __________________________________________________________ 49

5.2 Future Research Work _________________________________________________ 50

References ________________________________________________________________ 51

iii

Caption Page No.

Figure 2.1: Classification of PCM (Solid-Liquid) __________________________________ 6

Figure 3.1: Schematic Diagram of the Experimental Setup __________________________ 13

Figure 3.2: Photographic View of the Experimental Setup __________________________ 14

Figure 3.3: PCM Heat Exchanger at different Manufacturing Level ___________________ 15

Figure 3.4: PCM Plates with PCM and Heat Exchanger Coil ________________________ 16

Figure 3.5: PCM Plates with Heat Exchanger Coil ________________________________ 16

Figure 3.6: PCM Heat Exchanger Coils outside and within PCM Plates ________________ 17

Figure 3.7: Helical Coil Tube with its Dimensions ________________________________ 17

Figure 3.8: Heat Exchanger Coils with Incorporated Fins ___________________________ 18

Figure 3.9: Positions and locations of fins within PCM plates ________________________ 18

Figure 3.10: Refrigerant Distributor Attached to the Experimental Setup _______________ 19

Figure 3.11: Photographic view of Condensing Unit with its Components ______________ 19

Figure 3.12: Positions of Thermocouples within PCM Plates ________________________ 20

Figure 4.1: PCM Heat Exchanger Models (a) Basic Model (b) Fins Incorporated Model (c)

Refrigerant Distributor Model ________________________________________________ 28

Figure 4.2: Plate and Coil Numbering of the PCM Heat Exchanger ___________________ 29

Figure 4.3: Temperature Distribution during Solidification (all plates) without Fin and

Distributor ________________________________________________________________ 29

Figure 4.4: Temperature Distribution during Solidification (a) Plate-1 (b) Plate-2 (c) Plate-3

without Fin and Distributor ___________________________________________________ 30

Figure 4.5: PCM Heat Exchanger during Solidification (Basic Design) without Fin and

Distributor ________________________________________________________________ 31

Figure 4.6: Temperature Distribution during Melting (all plates) without Fin and Distributor

_________________________________________________________________________ 32

Figure 4.7: Temperature Distribution during Melting (a) Plate-1 (b) Plate-2 without Fin and

Distributor ________________________________________________________________ 32

Figure 4.8: Temperature Distribution during Melting (a) Plate-3 without Fin and Distributor

_________________________________________________________________________ 33

Figure 4.9: Temperature Distribution during Solidification (all plates) with fin __________ 34

List of Figures

iv

Figure 4.10: Temperature Distribution during Solidification (with fin) (a) Plate-1 (b) Plate-2

(c) Plate-3 with fin _________________________________________________________ 35

Figure 4.11: Temperature Distribution during Melting (all plates) with fin ______________ 36

Figure 4.12: Temperature Distribution during Solidification (with fin) (a) Plate-1 (b) Plate-2

(c) Plate-3 with fin _________________________________________________________ 37

Figure 4.13: Temperature Distribution during Solidification (all plates) with Fin and

Distributor ________________________________________________________________ 38

Figure 4.14: Temperature Distribution during Solidification with Distributor and Fin (a) Plate-

1 (b) Plate-2 (c) Plate-3 with Fin and Distributor __________________________________ 39

Figure 4.15: Image Taken during Solidification with Fin and Distributor _______________ 39

Figure 4.16: Temperature Distribution during Melting (all plates) with Fin and Distributor _

_________________________________________________________________________ 40

Figure 4.17: Temperature Distribution during Melting with Distributor and Fin (a) Plate-1 (b)

Plate-2 with Fin and Distributor _______________________________________________ 40

Figure 4.18: Temperature Distribution during Melting with Distributor and Fin at Plate-3 with

Fin and Distributor _________________________________________________________ 41

Figure 4.19: Temperature Distribution during Solidification at plate-1 for different Design

Conditions ________________________________________________________________ 41


Conditions ________________________________________________________________ 42


Conditions ________________________________________________________________ 42

Figure 4.22: Temperature Distribution during Melting at plate-1 for different Design

Conditions ________________________________________________________________ 43


Conditions ________________________________________________________________ 44


Conditions ________________________________________________________________ 44

Figure 4.25: R-22 P-h Chart for Existing Refrigeration System ______________________ 45

Figure 4.26: Bypass Factor of PCM Heat Exchanger at Different Velocities ____________ 47

Figure 4.27: Average Inlet Air Temperature, Average Outlet Air Temperature and Average

Plate Surface Temperature ___________________________________________________ 47

Figure 4.28: Sensible Heat Transfer Rate by Air at Different Time ____________________ 48

v

Caption Page No.

Table 3.1: Specification of PCM Heat Exchanger _________________________________ 15

Table 3.2: Specification of PCM Heat Exchanger Casing ___________________________ 15

Table 3.3: PCM Plates Specification ___________________________________________ 17

Table 4.1: Pressure and Temperatures at different Positions of the Refrigeration System __ 46

Table 4.2: Bypass Factor of PCM Heat Exchanger at Different Air Velocities ___________ 46

List of Tables

vi

Acknowledgement

Foremost, I would like to offer this endeavor to our almighty Allah who showered his utmost

blessings upon me during the challenging moments of this journey. He always provided me the

strength, good health and peace of mind for doing the research.

It is a pleasure to express my heartiest gratitude to my thesis supervisor, Prof. Dr. Hasan

Mohammad Mostofa Afroz, Director, Institute of Energy Engineering. His motivation

stimulated me in all the time of study and writing of the thesis. I could not have imagined

having a better mentor than him.

My sincere thanks also goes to Prof. Dr. Mohammad Zoynal Abedin, Head, Department of

Mechanical engineering for his encouragement and insightful comments.

Also, I convey my humble gratefulness to Prof. Dr. Md. Ariful Islam, Prof. Dr. Md. Anowar

Hossain and Dr. Md. Mostaqur Rahman, the committee members of my defense, for their

willingness to manage time even at hardship and also for advices with constructive suggestions.

Also, I would like to thank Md. Abdul Aziz, Lecturer, Institute of Energy Engineering for his

valuable support during the experiment. Finally, I must express my very profound gratitude to

my family members for providing unfailing support and continuous encouragement throughout

the years of study and through the process of researching and writing this thesis. This

accomplishment would not have been possible without them. Thanks all.

vii

Abstract

Post-harvest losses having a great impact in the economy of developing countries like

Bangladesh. To obstruct post-harvest losses and ensure better quality of foods, suitable cold

chain is necessary from harvesting to market selling. Storing thermal energy within phase

change material (PCM) in cold chain application is more advantageous than conventional

refrigeration system. In this aspect proper design of PCM energy storage devices and their

better performance should be assured to preserve cold chain.

In this thesis, three PCM heat exchanger models are developed to investigate the performance

analysis of the heat exchanger models and to compare their research output. In the first basic

model the refrigerant is distributed through a header mechanism to the evaporator coil and in

the second model fins are incorporated at the evaporator coil side and the header mechanism

at the inlet side of the evaporator is replaced by an efficient refrigerant distributor in the third

model. In the first model during charging period it is found that the nearest coil region solidified

within 2 hours and furthermost regions remain in liquid state for very longer time.

Incorporating fins in the evaporator coil side give improvement in solidification characteristics

in the second model but all the PCM regions are not completely solidified although it takes

more than 6 hours for partial solidification. In the third model where, refrigerant distributor is

used the solidification is completed in all the regions within 3.0 hours. During discharging

(melting) period the last model ensures almost 4.0 hours melting period.

In the refrigerant distributor model experiment shows that the heat exchange rate between air

and PCM is higher at the initiation of melting period and reduced with increasing time of

melting. The temperature difference between inlet and outlet air is higher at the initial

discharging period and becomes lower with increasing time. The sensible heat transfer rate

decreases with increasing time period and it varies from 2200 to 1300 W within 4 hours time

period. The PCM plate temperature is also increased with increasing duration of melting.

Temperature difference varies from 16 to 9C within 4 hours time period. The air bypass factor

of the PCM heat exchanger is increased by increasing air velocity. Bypass factor increases 6%

from the air velocity 1.2 m/s to 2m/s.

The performance of refrigerant distributor model is quite satisfactory and some other

modification can improve its heat transfer rate further. This type of heat exchanger is best suited

for cold chain application like refrigerated vehicle or cold storage.

1

Chapter 1: Introduction

1.1 Background

Thermal energy storage (TES) is now-a-days a major concern due to higher cost of energy and

inadequacy of energy resources. TES can be utilized either latent heat (changing phase of

storage material) or sensible heat (changing the storage material temperature). In sensible heat

storage (SHS) system the physical state of the storage material remains unchanged such as

solid to solid, liquid to liquid or gas to gas. But in latent heat storage (LHS) system the physical

state of the storage material is changed to another state like solid to liquid, liquid to gas or gas

to solid. The most suitable phase transition is solid-liquid transition due to greater heat of

fusion. These solid-liquid latent heat storage materials are also known as phase change material

(PCM). The latent heat storage system is more convenient to store thermal energy (hot or cold)

due to its compactness, higher heat storing capacity, changing phase at a constant temperature.

The PCM latent storage method suitable for food cold chain application, air-conditioning,

electrical load shifting and so on. Food cold chain is most sensitive part for human health

security. Different foods like fish, vegetables, meat have to store at every stage (from harvest

to consumption) at different standard temperature to minimize the metabolic and microbial

deterioration. In developing countries like Bangladesh, the use of refrigeration system to store

foods increasing rapidly during last decade to reduce the post-harvest losses. The additional

load for the refrigeration system drives to load variation in electricity demand and supply. In

this circumstances PCM storage system can give better solution by shifting load from peak to

off-peak hour. During night period when the electricity demand is less, the energy can be stored

to PCM and at the day-time this stored energy from PCM can be used by simple solidification-

melting process.

Transportation of foods from one place to another by using vapour compression driven

refrigerated vehicle is another problem due to use of large amount of fuel, increasing CO2

emission. This problem can be resolved by using PCM based refrigerated vehicle where PCM

plate is placed on the wall of vehicle refrigerated space and heat is exchanged within foods and

PCM. In all the stages of food cold chain PCM can be a better alternative than existing systems.

A very important issue is to select suitable PCM, since different products have to store different

standard temperature and humidity. Different PCMs change their phases at different

temperature. So, it is very inefficient to use different PCM in a single cold storage or many

cold storages from which each cold storage is based on a single PCM. A major drawback of

PCM is the lower thermal conductivity at solid phase that reduces heat transfer from PCM to

foods.

2

Developing a PCM based heat exchanger could be a better alternative which will be placed

outside the cold storage or refrigerated vehicle and supply cold to the chamber. The

experimental analysis of energy storage using PCM heat exchangers is a burning topic,

primarily due to the high latent heat of PCMs upon freezing and melting nearly at constant and

workable desired temperatures. However, the technology of the PCM based heat exchanger, its

heat transfer characteristics, performance, sizes should be analyzed before its

commercialization. That’s why the performance, air side heat transfer characteristics,

solidification and melting is analyzed throughout the research work.

1.2 Literature Review

Refrigeration makes reduction of the rate at which food changes is occurred. These changes

mainly microbiological (microorganisms’ growth), physiological (e.g. respiration, ripening),

biochemical (e.g. pigment degradation, browning reactions and lipid oxidation) and moisture

loss. An effective cold-chain can prevent or slowing these changes efficiently.

Temperature effect in storage conditions already analyzed for varieties foods such as

vegetables [1], frozen dough [2], ice cream [3-5], and meat [6-7]. Duun et al. [6] investigated

the drip loss from pork roast under various storing conditions. Greater drip loss was occurred

from meat subjected to temperature abuse than storing meat at a fixed temperature. Yanniotis

et al. [7] investigated the bovine muscle drip loss (Musculus semimembranosus) stored in a

freezing chamber running with defrosting cycles. The drip loss with defrosting was higher

comparing without defrosting cycle in the meat samples. Ice cream is also under investigations

to check the impact of storage conditions. Flores and Goff [5] analyzed that ice crystals size

was larger under shifting conditions comparing constant storage temperature. Donhowe and

Hartel [3, 4] investigated the same result as size increasing of crystal.

In reality a small amount of foods (almost 10%) are usually refrigerated and post-harvest losses

tends to 30% of total production [8]. So, providing safe, high quality food products the cold-

chain from the initial freezing, storage, transport, to retail display in every aspect must be paid

attention. But due to cost increasing and lack of energy resources many researchers focused on

thermal energy storage. It is currently using in household water heating; its use is essential for

solar heating; off-peak air conditioning and load factor of electric utilities can be improved [9].

In off-peak air conditioning [9] 'coolness' is produced during off-peak time when the generation

of electricity exceeds demand, stored in a TES material and used during peak hour of electricity

demand. TES can be achieved either through sensible heat or through latent heat (phase

change). The significance of PCM for energy storage is well established, primarily due to their

higher thermal energy storage density.

The performance of a latent heat storage module consists of cylindrical shaped capsules packed

with PCM and heat transfer fluid passing along the capsules, this heat exchange is already

3

investigated for different PCMs [10-12]. Green and Vliet [13] developed a shell and tube type

heat exchanger having baffles where water is using on the shell side as the heat transfer fluid.

The PCM was encapsulated in the horizontal tubes bundle. The rectangular encapsulation was

also analyzed where the PCM was accommodated alike to plate-type heat exchanger

arrangement. Abhat [14] and Humpries et al. [15] have abridged about the materials that can

be used as latent heat storage material. Commercially available paraffin waxes are less costly,

having intermediate thermal storage densities, and a broad span of melting temperatures. But

most of these having lower thermal conductivities that bound their used. To ensure fair heat

transfer rate at solidification or melting periods of the PCM, a notable temperature difference

must be maintained between the temperature of heat transfer fluid and the temperature of

melting of PCM. So that the PCM should be chosen based on their melting temperatures and

higher latent heats of fusion. Admitting this situation, Farid [16] suggested to use PCM's having

different melting temperatures to enhance heat transfer rate in a rectangular encapsulation

storage unit. The concept was enlarged further by Farid et al. [17] for PCM's encapsulation in

multirows of upright cylinders with heat exchange medium as air flowing covering them. The

simulation result of that unit represents notable development in heat charging and discharging

in which three PCM's of dissimilar melting temperatures were utilized. The waxes were

presumed to be melted at certain temperatures in this simulation that can be valid only for pure

compounds.

Due to availability PCMs at different temperatures allow wide range of application for storing

thermal energy and load shifting purposes. In spite of the great advantages of PCM that can

dispense, many systems are quiet using alternative methods and giving the idea of PCM heat

exchangers for storing thermal energy. Heat extraction from pool-type nuclear reactor is an

example. Pool water temperature is increased by 2 °F per 1 h when in operation, so the cooling

rate is less than 2 °F per 48 h after shutdown. To retrieve this, Castano et al. [18] theoretically

designed a sensible heat storage system using chilled water/glycol tank to exchange heat in

looping with the main cooling system of reactor and cool pool water of 30,000 gallon from 88

°F to 68 °F in 1.5 h. For the extended period of reactor at full power operation and power

upgrade in future, a higher cooling mechanism is required than the finite sensible heat of water.

Using PCM in TES systems can solve these restrictions. PCM has higher storage density in a

compact size and having the ability to discharge at constant temperatures compared to water

with sensible heating systems.

Thermal energy storage using PCM can also be used in data centers like Apple, Google,

Facebook and so on where the temperature rises due to heat generated by electronic circuits. In

2008, ASHRAE’s standards and thermal guidelines 2008 [19, 20] amplified the maximum

permissible operating temperature range of A1 class servers and data centers to 32 °C and

recommended 27 °C for all classes. So, the thermal load of data center can be encountered by

the chilled water supply of 22–24 °C temperature. It is feasible to produce 16–17 °C or lower

4

water temperature for data centers and store “cold” energy in PCM at off-peak periods at night

from cooling tower in absence of mechanical chiller systems.

PCM thermal energy storage system gives a simpler and workable solution. PCMs are specified

by their less thermal conductance that can reduce the heat transfer rate. In order to recompose

for the less thermal conductivities, very efficient design must be employed. Coil-in-PCM

arrangement heat exchanger analyzed by researchers [21–23], their major problem is the less

heat transfer area to volume [24]. Many researchers investigated disseminate particles of high

thermal conductivity and micro encapsulation. The investigation performed by Martin et al.

[25] in a porous graphite matrix utilizing encapsulated PCM of paraffin and 50% performance

was enhanced for storing energy in response time. However, 20% less energy storage density

was detected. Zhao et al. [26] analyzed implantation of metal foam as a metallic matrix

structure and result showed 3 to 10 times improvement of the overall heat transfer (U) rate.

Medrano et al. [24] investigated experimentally five distinct categories PCM heat exchangers,

counting plate type heat exchanger design and shell-and-tube design also. During testing under

the similar environment, the highest average thermal power found at compact plate type design

and shell and tube design having better heat transfer area to heat transfer volume. Lin et al. [27]

analyzed plastic slabs matrix which is packed by PCM. The result shows that the compactness

and slabs thickness play crucial role to achieve highest heat transfer.

The use of well conductive fins provides a feasible solution to enhance the performance of

thermal energy storage unit heat transfer [28]. In addition, no difficulties during fabrication,

lower cost, and fins gain maintenance is a prospective way comparing with other heat transfer

enhancement approaches [29, 30]. Using fin to raise heat transfer, corrosion potential and

thermal conductivity are two main criteria for selecting fin materials. In addition, the cost of

fin materials different thermophysical properties, and machinability also important parameters

[31]. Shaon et al. [32] investigated solidification and melting characteristics of PCM

numerically and validated with experimental data to examine the performance of an innovative

latent heat based thermal energy storage system. In melting period higher heat flux, energy

storage and faster solidification is obtained for PCM pack of 6.5 cm thickness with maximum

number of fins. For the design of PCM based thermal energy storage the review by the

researcher is available in [33, 34].

In brief analyzing many researches work it is found that the performance of thermal energy

storage devices solely depends on the surface area between PCM and heat transfer fluid. PCM

thermal conductivity can be increased by macro encapsulations but this application is limited

for weight, costings and energy storage density draining due to the presence of additives. The

incorporating fins also give some improvement. The uniform distribution of heat throughout

the PCM is very crucial.

5

1.3 Scope of Work

This research work represents the suitable design of a PCM latent thermal energy storage

system by analyzing different characteristics like solidification-melting phenomena, heat

transfer rate, air side temperature variations, bypass factor and system COP. In this aspect

initially a PCM heat exchanger is designed, fabricated and analyzed. Based on the result the

design is modified and again analyzed to reach the optimum design approach. Water is used as

a PCM that having phase transition temperature at 0C. Using water as a PCM positive

temperature can be maintained within cold storage or refrigerated vehicle and wide range of

fresh fruits and vegetables can be stored by controlling the cold flow to the cold storage.

1.4 Objectives of the Current Research Work

The following objectives will be considered in the present study:

a. To develop a PCM based energy storage unit with heat exchanger.

b. To check the performance of the heat exchanger such as solidification, melting

characteristics of PCM, air side bypass factor, air temperature variations at inlet and

outlet of the heat exchanger.

c. To select suitable design of PCM heat exchanger from different modifications.

d. To calculate system capacity needed for the designed PCM heat exchanger.

6

Chapter 2: Phase Change Material (PCM) as Thermal Energy Storage

The storing system of thermal energy can be sensible or latent. Latent heat storage system has

some advantages over sensible heat storage system like higher heat of fusion/vaporization,

energy density etc. Within latent heat storage system liquid to solid phase change has greater

flexibility than liquid-gas phase change. Liquid to solid phase change latent storage material is

also known as phase change material (PCM).

2.1 Introduction to Phase Change Material (PCM)

Phase change materials (PCM) are the substances which have high latent heat of fusion,

solidification and melting is occurred at a fixed temperature, stores and releases large amount

latent heat energy.

2.2 Categorization of PCMs

An extensive range of PCMs with varying working temperature and thermal properties are

available. The classification of PCM (solid-liquid families) shown in figure 2.1. These PCMs

broadly categorized as eutectic, inorganic and organic [34].

Figure 2.1: Classification of PCM (Solid-Liquid)

7

2.2.1 Organic Type PCM

Paraffin and Non-Paraffin are the major category of organic PCMs. Paraffin is composed of

saturated hydrocarbon chain (CnH2n+2). The length of chain dependent to the latent heat of

fusion and it maintains a proportional relationship. Paraffin have the special properties such as

non-toxic, having better stability upon cycling, non-corrosive, ability to crystallize with little

amount of subcooling or no subcooling [35]. They are quite flammable, lower thermal

conductivity and changing the volume is high during phase transition. The modern class of

PCMs for TES is non-paraffin materials [34]. Although there are different types of non-paraffin

but, fatty acids having preferable properties like thermophysical and kinetic. It has the high

research of interest now-a-day. The general construction chain of fatty acids is

CH3(CH2)2nCOOH [36]. Fatty acids having the properties such as greater latent heat, changes

of volume are small during phase change, fixed temperature of melting and show reproducible

behavior during melting and solidification compared with paraffin. Since they are composed

of single component that ensures ideal stability on cycling [37]. Fatty acids have some similar

properties as like paraffin. These are less thermal conductivity, flammable at higher

temperature and small or no supercooling [33]. The latest organics PCMs, sugar alcohols

having distinguish formula HOCH2 [CH(OH)]nCH2OH [37]. Since these are in the initial stage

of research phase so slight information is found. The have the properties such as higher heat of

fusion and higher density compared with other different organic PCMs. However, they have

some amount of subcooling compared with other organic PCMs. Fatty acid combining with

alcohol can produce esters which is also known as fatty acid esters. They have the properties

of non-toxicity and exhibit no supercooling. They are commercially available for various

application [38].

2.2.2 Inorganic Type PCM

Metallic alloys and salt hydrates are main two categories of inorganic PCMs. Normally

inorganic PCMs having greater heat of fusion than organic PCMs except sugar alcohols [37].

Salt hydrates comprised of salt and water and after solidification these make crystal matrix

[39]. Their melting temperature vary from 15 to 117C with high latent heat [40]. Molten salts

melting temperature vary from 250 to 1680C [41]. Salt hydrates PCM groups have the great

significance in research and chosen by many researchers due to their wide range of applications

in the TES system. Due to their lower cost and availability of salt hydrates, they are using many

commercial applications for TES. The example of extensively available salt hydrates for

commercial application at low cost are CaCl2.6H2O and Na2SO4.10H2O. These two salt

hydrates PCMs have comparatively high thermal conductivity with others and show small

volume change during solidification/melting. The thermal conductivity varies from 0.4 to 1

W/m-K [39, 40, 42]. One of the major problems of salt hydrates PCMs is the instability on

8

cycling and salt-water separation which leads to reduction in latent heat of energy. Adding

thickened compound could be a solution of this problem. Supercooling and causing corrosion

to metals are another problem of salt hydrate. Metallic alloys are another inorganic PCM having

high weight. Their thermal conductivity is comparatively high and greater latent heat of fusion.

But they are not selected as commercial PCMs for their bulkiness.

2.2.3 Eutectic Type PCM

Eutectic mixtures of inorganic or organic compound makes new PCMs with better properties

and various melting temperatures. Two or more inorganic or organic PCM components consist

mixture which melt and solidify concurrently [39]. Mixing of PCMs is used get new eutectic

PCMs which change their phase at the same temperature but new eutectic PCM have better

melting performance [34]. Eutectic mixtures are usually analyzed by diverse experiments to

find the distinct behavior like fixed melting temperature and greater enthalpy. If some

components give good results then again several experiments are conducted for optimization

such as optimized composition. This requires a lot of research work and time.

2.3 Implementation of PCMs as Thermal Energy Storage

There is vast commercial application of PCMs in TES system. In this part this application will

be discussed. The cost, capacity of thermal energy storage can be reduced by using PCMs in

various applications. It is systematic and environment friendly way of usage of energy. By

using PCM for TES the greenhouse gas emission can be reduced by 40% [43]. PCMs play a

major role for TES application. However, in special cases using PCMs can save 50% energy

can be saved TES systems [35].

2.3.1 Solar Photovoltaic (PV) System

Generation of energy from PV panels solely rely on the solar radiation intensity, panels quality

and panels temperature [44]. PV panel generation efficiency reduces with increasing panels

temperature. From the solar radiation only 10 to 20% is found as electrical energy and rest

amount is used to increase temperature of the PV panels and reflected [43]. The power

efficiency of PV panels is decreased by 0.5% per kelvin with the rising of temperature. The PV

module temperature increase to 80C or higher that leads to 28% loss in electrical energy

conversion from solar energy with reference to the standard test condition (STC). Different

research works have been performed such as use of fans to supply air, pumps to circulate water

or other energy consumable devices to improve PV panels cooling and efficiency. However, to

avoid overheating of PV panels, PCMs integration in the panels is an efficient solution. This

type of PV-PCMs integration system ensure longer life and desired temperature with better

efficiency. Surprisingly PV-PCMs integration system have higher potentiality comparing with

9

other natural ventilation or cooling since PV-PCMs integration is free from the effect of wind

direction. To increase conversion coefficient and reduce power loss due to temperature Hasan

et al. [45] established a PV-PCM system. The PV panel peak temperature is maintained less

than 45C at 950 W/m2 solar irradiation. In another research Atkin and Farid [46] established

a new PV-PCM system. In this system external fins are used in PCM infused graphite to

upgrade the heat transfer at the back side of the panel. By this thermal system 13% efficiency

can be raised. PV-PCM system is viable economically if the excess heat is used to supply

building heating system by introducing solar thermal system with PV-PCM system [47].

2.3.2 Air Conditioning applications

Many researchers investigated PCMs based air conditioning systems. The energy consumption

in air conditioning system based on PCM is lower than existing traditional air conditioning

systems. PCMs extract heat during phase change that reduces the entering temperature of air

to the evaporator coil. This system can increase thermal performance of the air conditioner. A

PCM based air conditioner is analyzed by Chaiyat et al. [48] that ensures the improvement of

thermal efficiency and reducing electricity consumption efficiently. Another researcher

Vakilaltojjar [49] developed a PCM based air conditioner where cold air is passed into several

thin flat PCM containers. The investigation determines the PCM slab thickness and air gaps

effects. Spherical shaped packed capsule PCM bed used in air conditioning system was

analyzed that gives better performance [50].

2.3.3 Solar Water Heating Application

Integration of PCMs into solar heating system can improve efficiency of the solar heating

system. Prakash et al. [51] investigated a PCM based solar heating system (water) which have

PCM layer filled at the bottom. Water is heated first by the sun during day time and then the

heat is transferred into PCM in the latent heat form at the bottom layer. When sun is not

available the heat is transferred from PCM into water to increase its temperature. This system

shows a low efficiency due to imperfect heat transfer between PCM and water. The improved

design of this storage system was analyzed by Ghoneim [52]. A cylindrical shaped tank that is

used as storage of PCM and multiple tubes attached with solar plate is a system of such type.

This system enhances the heat transfer between PCM and water and ensure longer time heat

reservation.

2.3.4 Biomedical applications

In biomedical applications PCMs give important solutions for various biomedical system

which needs thermal protection. Comfort skin temperatures can be maintained by using PCM

based textiles and special bandages that is used for heating or cooling therapy such as burning

10

therapies [53]. At a recent time, hot and cold PCM pads is used to treat several pains and is

available at the doorstep. A special thermal protection device is proposed by Mondieig et al.

[54], which is made of PCM with double walled packaging and very useful for blood storage

and transportation. An innovative concept was developed by Lv et al. [55] in the thermal

protection during cryosurgery by using PCMs. From thermal injury healthy tissues located near

the cancerous tumor can be protected located near the cancerous tumor. A different

investigation performed by Wang et al. [56] for highly effective and sensitive thrombin

detection by using RNA thermal probes with PCM nanoparticles. The thermal probes with

PCM extract heat and exhibit thermal signal at the time of temperature scan. The area and

position of the peak determines the thrombin amount and existence.

2.3.5 Thermal Energy Storage in Buildings

PCMs are used to make comfort range temperature and reduce power consumption in buildings

by storing and releasing heat. In the daytime when sunshine available the energy is stored in

PCMs and at night this energy can be released. The energy is possible to store in different ways

such as using solar cells, PCM within concrete walls, tiles, wallboards [41, 42]. Due to

availability, simplicity and easy installing ceiling tiles and wallboard are commercially very

popular. PCM within the building structure is an efficient management system. It is used as an

alternative of conventional mechanical heating or cooling system since it maintains comfort

temperature in the room. It also helps to reduce the electrical energy consumption by

temperature shifting to the off-peak hour. It also reduces the peak power demand notably. The

ventilated PCM ceiling tiles or wallboards also act as a discharge system at night and can

improve the efficiency of PCM thermal system of the building.

When outside temperature is raised at daytime and it closes to the PCM melting temperature,

all the PCM melts and store thermal energy in the form of latent heat energy. At night the

temperature is low and PCM again solidified by discharging its heat to the environment. The

ventilation system ensures full discharge of PCM and after that new day cycle begins. PCM is

selected based on its phase change temperature to adjust the environment. Pure PCM is not

found for all melting temperatures. In these case eutectic mixtures give the best solution to find

new PCM mixture at desired melting temperatures. Many researchers studied about the PCMs

for building TES system. It is found that the PCM technique in building application can save

14% to 87% thermal energy based on specified environment and parameters [56]. PCM used

as a building TES system can be compared with other building sensible equipment and due to

latent heat storage PCM ensures greater benefit. One of building PCM is Lauryl Alcohol which

melting point is at 23.83C and heat of fusion is 216.5 kJ/kg. On the other hand, the water and

concrete rock heat capacity expect to be 0.750 kJ/(Kg-K) and 4.182 kJ/(kg-K), respectively

[41].

11

2.3.6 Nuclear Thermal Energy Storage Systems

Many researchers already proposed different methods to store excess heat generated at lower

electricity demand by nuclear reactor [57-60]. Nuclear power plants should operate at the

constant base load power for its operational reliability and economic purpose. Due to variable

demand of electricity nuclear reactor must operate at uneven load. Economic penalties can be

occurred due to uneven power demand and capacity factor can be affected badly [60]. In this

type of odd situation, the integrated thermal energy storage system helps the reactors to meet

the demand variations. When demand is low the excess heat can be stored PCM based heat

storage tank by changing PCM phase at an appropriate temperature. Lee et al. [58] suggested

to use underground rocks in the replacement of storage tank with PCM. This excess heat can

be used to produce electricity by conventional thermal power plant when peak demand is

available. Although the availability of various TES system for nuclear power plant, but PCM

is most popular due to its unique advantages such as suitable phase change temperature and

greater storage capacity. Saeed et el. [60] proposed an advanced model where Lithium chloride

is used as a PCM in the storage tank. Nuclear reactor thermal power was retained at fixed level

since the demand is gathered from PCM based TES tank. The research work also considered

the natural safety at unplanned shutdown. Natural circulation is used to transfer decay heat to

the TES tank. The reactor attained a state of shutdown less than half an hour and normal

temperatures is maintained within the safety range during the transient. High melting

temperature PCMs like molten salts or their mixture is selected to design this type of heat

storage tank. The melting temperatures of these PCMs range between 250 and 1680 °C, the

heat of melting range between 68 and 1041 J/g [38].

2.3.7 Storage and Transport of Thermal Sensitive Products

Storage and transport of temperature sensitive products are regular commercial application of

PCMs. In this fields the common products are fresh vegetables, fish, medical commodities,

beverages etc. The performance of a modified refrigerator with PCM was analyzed by Azzouz

et al. [61, 62]. The PCM based refrigerator operates within 5 to 9 hours backup without power

supply based on load condition. The test was performed for various PCM thickness, outside

temperature and thermal loads. The final result showed that the performance is 10 to 30%

higher compared with conventional systems. Gin et al. [63] analyzed PCMs effectiveness

against heat load in a freezer. The common heat loads are electrical power failure, defrosting,

openings of freezer door. The analysis concluded that if PCM is used then the temperature does

not rise remarkably that leads to less energy consumption. It is also demonstrated by

Subramanian et al. [64] that domestic refrigerators with PCM incorporated can enhance the

food quality by reducing hysteresis cycle rate. The transportation of thermal sensitive products

within PCM based containers is cost effective and maintain great quality. PCM containers are

comparatively cheap and it can control the temperature and heat throughout the transportation

12

period. The heat transfer across the wall of transportation trucks can be reduced by using PCMs

[65]. The investigation showed that 29.1% heat loss can be pull back from the refrigerated

trucks. This massive energy savings leads to lower size refrigeration system capacity. This

also increase the operational life for refrigerated trucks and diesel emission is reduced

significantly if it is diesel driven. Using special container for transportation of thermal sensitive

product is an important concern. This can be achieved by using special PCM equipped rigid or

soft containers and having PCM with lower melting temperature [35]. Due to high latent heat

of fusion the PCM is able to maintain the food or commodities at desired temperature level for

a long time period without external source.

2.3.8 Other Applications

The PCMs application in various sector is under development. The possible technique to use

PCMs as TES system are boundless. The use of PCM for special body cooling is proposed for

athletes which ensures comfort by regulating body temperature, reduce fatigue, dehydration

[66]. Another important application is PCM integrated walls. The PCM integrated wall is very

efficient to capture and release heat and very effective in peak load shifting [67]. A thin was

integrated with PCM can act as a high thermal energy reservoir. PCM is also can be used in

the transparent part of the buildings such as special glass window filled with PCM [68]. The

window filled with PCM is more efficient than air filled window and having the capacity to

filter thermal radiation. PCMs can be used in Lithium-ion batteries for cooling purpose to

increase better lifespan by avoiding overheating [69]. PCM can also be used for cooling the

engines either electric or combustion engine [70]. PCMs can also be used for thermal control

in spacecraft, greenhouses and solar cooking system [71, 72, 73].

13

Chapter 3: Experimental Setup & Methodology

3.1 Experimental Facility

In the experimental setup it has three major components such as condensing unit, PCM heat

exchanger and data logging-acquisition system. The schematic diagram of the experimental

setup is shown in figure 3.1. A condensing unit supplies cool refrigerant to the PCM heat

exchanger’s evaporator coil and receives hot refrigerant after exchanging heat to the PCM. In

this case PCM heat exchanger acts as an evaporator. The PCM heat exchanger assembly

consists of a stainless-steel vessel to contain the three PCM plates, baffles and twelve helical

tubes set. PCM plates hold PCM within the plates and copper tubes set (evaporator coils)

submerged within the water. The refrigerant flows through the helical submerged copper tubes

that act as wall between water and refrigerant. Heat is transferred between refrigerant and water

through copper tubes. Water was chosen as PCM for its availability and wide range of

application. During charging cycle the condensing unit is turned on and cool refrigerant passes

through the evaporator coil and PCM becomes solidified. During discharging cycle, the

1. Compressor 6. Mass flow filter 10. Helical coil tube 15. Data to computer

2. Condenser 7. Mass flow meter 11. Thermocouple 16. Computer

3. Receiver 8. Expansion valve 12. PCM plate

4. Filter-Dryer 9. Pressure & temperature 13. Thermocouple line

5. Sight glass sensor (to data logger) 14. Data logger

1

2 3

4 5 6 7 8

9

10

11

12

13

14 16 15

Figure 3.1: Schematic Diagram of the Experimental Setup.

14

condensing unit is turned off and air is sucked from the lower portion of the PCM heat

exchanger and after exchanging heat between PCM and air separated by steel plate, the cold

air then moves away through the upper portion of the PCM heat exchanger and melting of PCM

is occurred. The experimental data is logged with the help of a data logger with data acquisition

system (laptop). In the figure 3.1 the yellow lines indicate refrigeration cycle between

condensing unit and PCM heat exchanger coil and the green lines indicate data receiving and

acquisition from different sensors and thermocouples. The image of the experimental setup

with different components is shown in figure 3.2. The details of each of components of the

experimental setup will be described in the following sections.

3.2 PCM Heat Exchanger

The PCM heat exchanger shown in figure 3.3 is a stainless-steel cased box having three PCM

plates. These plates are small boxes also made from stainless steel and placed within the PCM

box that contain PCM (water). The PCM heat exchanger box is insulated by 1.5-inch (38.1mm)

cork sheet to prevent the heat loss between outside air and PCM heat exchanger. The PCM

plates are separated by three stainless steel baffles which is placed inside the PCM box and

around the plates. The lower and the upper portion of the PCM heat exchanger have opening

to suck and deliver air. The flowing air is directed through PCM plates’ gaps with the help of

baffles like shell and tube heat exchanger. The flow of air through the PCM heat exchanger is

maintained by a runner.

PCM Heat Exchanger

Data Acquisition Laptop

Pressure Sensor

Filter and Drier

Flow Meter

Data Logger

High & Low Pressure Lines

Condensing Unit

Liquid Receiver

Wire Thermocouples

Experiment Table

Figure 3.2: Photographic View of the Experimental Setup

15

The specifications of the PCM heat exchanger are also given in Table 3.1.

Table 3.1: Specification of PCM Heat Exchanger

Parameter Description

Box Size (L × B × H) 0.45 m × 0.40 m × 0.46 m

Box Sheet Thickness 1.5 mm

Plate Size (L × B × H) 0.41 m × 0.10 m × 0.31 m

Amount of PCM (Water) 32.33 kg

Melting or Solidification Point of PCM (Water/Ice) 0 C

Latent of heat of PCM (Water/Ice) 334 kJ/kg

Total Stored Energy 10800 kJ

Thermal Conductivity of PCM (Water) 0.59 W/m.K

Thermal Conductivity of PCM (Ice) 2.20 W/m.K

3.2.1 Casing or Shell

The steel vessel act as a shell which encompasses all the components of the PCM heat

exchanger and ensures the reduction of heat loss since the outer surface of the casing is properly

insulated. Full heat exchanger covered with white PVC sheet of 3 mm thick. The casing holds

the runner which sucks air from the lower portion opening of the heat exchanger and deliveries

it to the upper portion opening (shown in figure 3.3). The specification of the casing given in

the table 3.2 as following.

Table 3.2: Specification of PCM Heat Exchanger Casing.


Casing Material Stainless Steel

Casing Material Thermal Conductivity 45.3 W/m.K at 20C

Insulation Material Cork Board

Insulation Thickness 38.1 mm

PVC Sheet Thickness 3 mm

Insulation Thermal Conductivity 0.04 W/m.K

Figure 3.3: PCM Heat Exchanger at different Manufacturing Level

16


Size (L × B × H) without insulation 0.45 m × 0.40 m × 0.46 m

Size (L × B × H) with insulation 0.53 m × 0.48 m × 0.54 m

3.2.2 PCM Plate

The PCM plates are small boxes that contain PCM (water) and evaporator coils submerged into

water. Three PCM plates placed within the PCM box and baffles around these to maintain

uniform gap between plates. The size of the PCM plates were designed by considering the

amount of water, volume expansion of water, volume required for evaporator coil. The

designed size of each of PCM plate is 0.41 m × 0.10 m × 0.31 m (L × B × H). The external

surfaces of the plate come into contact with air to transfer heat from solid ice during discharge

cycle. The photographic view of the PCM plates shown in figure 3.4 and 3.5. The specification

of the PCM plates given in table 3.3.

PCM Plates

Casing

Runner

Figure 3.4: PCM Plates with PCM and Heat Exchanger Coil

Plate-3

Plate-2

Plate-1

Coil no. 1

Coil no. 2

Coil no. 3

Coil no. 4

Figure 3.5: PCM Plates with Heat Exchanger Coil

17

Table 3.3: PCM Plates Specification


Plate Size (L × B × H) 0.41 m × 0.10 m × 0.31 m

Plate Material Stainless Steel

Thermal Conductivity 45.3 W/m.K at 20C

Plate Material Thickness 1.5 mm

3.2.3 Heat Exchanger Coil

The helical shape of copper tubes (coil) incorporated in the heat exchanger plate and this shape

of the tubes ensure high density of tubes i.e. compact structure. The helical coil shaped tubes

ensure high heat transfer area compared with straight tube. The PCM heat exchanger has 12

sets of helical coils where each of the plate contains 4 sets of tube. At each plate four coil sets

are connected by supply and return distribution header (shown in figure 3.6). The distribution

headers are connected by inlet and outlet header. The specification of each helical coil is shown

in figure 3.7.


Tube Material Copper

Tube outer diameter 9.525 mm

Tube inner diameter 8.001 mm

Tube Wall thickness 0.762 mm

Helix diameter 76.2 mm

Pitch 25.4 mm

Height 330.2 mm

Coil revolution 13

Figure 3.7: Helical Coil Tube with its Dimensions

Heat Exchanger Coil

Coil Distribution Header

Figure 3.6: PCM Heat Exchanger Coils outside and within PCM Plates

Inlet Header

Outlet Header

18

3.2.4 Fin

In the second modification of the PCM heat exchanger copper fins are attached within the heat

exchanger helical tubes to expand the heat transfer surface area during charging period of PCM.

Two different fin sizes are used to transfer heat flow in both longitudinal and traverse direction

of PCM plates. The incorporated fins within the coils shown in figure 3.8 with fin specification

and the locations of fins within PCM plates shown in figure 3.9.

3.2.5 Refrigerant Distributor

In the third modification of PCM heat exchanger a refrigerant distributor is included in the

supply refrigerant line to distribute refrigerant uniformly through all the helical evaporator

coils. The refrigerant distributor is attached to the inlet side of the PCM heat exchanger and

twelve coils are connected individually with the outlet side of the refrigerant distributor. The


Fin Material Copper

Fin thickness 0.89 mm

Long fin size (L × B) 375 mm × 35 mm

Short fin size (L × B) 100 mm × 35 mm

No. of long fin per plate 5

No. of short fin per plate 20

Figure 3.8: Heat Exchanger Coils with Incorporated Fins

Figure 3.9: Positions and locations of fins within PCM plates

19

refrigerant distributor is properly insulated to avoid the heat loss through it. The image of

refrigerant distributor connected to the PCM heat exchanger is shown in figure 3.10.

3.2.6 Condensing Unit

The condensing unit is used to supply cool refrigerant to the PCM heat exchanger and solidify

the PCM. The capacity of the condensing unit was 2.0 tons of refrigeration (7.03 kW) that is

capable to continuous supply of refrigerant through the cycle. The refrigerant used in this

condensing unit was R-22.

Refrigerant

Distributor

Figure 3.10: Refrigerant Distributor Attached to the Experimental Setup

Dryer

Liquid Receiver

Condenser

Condenser Fan

Compressor

Accumulator

Expansion Valve

Figure 3.11: Photographic view of Condensing Unit with its Components.

20

The main parts of the condensing unit are as following and also shown in figure 3.11:

a. Compressor

b. Condenser

c. Evaporator

d. Expansion Valve

e. Receiver

f. Filter-Dryer

g. Accumulator

3.3 Fabrication of the Experimental Setup

After manufacturing the PCM heat exchanger the experimental setup was equipped with

several sensors at different positions. Pressure sensors and thermocouples at four positions to

check the system coefficient of performance (COP), two anemometers at the inlet and outlet

air flow positions to check the air velocities variations. Pressure and temperature of the

refrigerant at four different positions such as before and after condenser, before and after the

PCM heat exchanger (evaporator) were obtained by using digital pressure-temperature

combined sensors to examine the system performance. A refrigerant mass flow meter also

attached in the refrigeration line after the condenser to inspect the mass flow rate of the

refrigerant. Twelve thermocouples were submerged into the PCM to check solidification and

melting temperatures variations. Four thermocouples were placed within each PCM plate. The

positions of thermocouples within the plates are shown in figure 3.12. The schematic diagram

full fabricated experimental setup is shown in the figure 3.1.

Figure 3.12: Positions of Thermocouples within PCM Plates

21

3.4 Design of the PCM Heat Exchanger

The heat transfer coefficient at refrigerant side of the evaporator is calculated according to

shah’s [74] method for two phase flow and the heat transfer coefficient at PCM (water) side is

calculated by considering natural convection over horizontal tube or cylinder. Material

properties and different data after calculation is given as following.

PCM Properties Refrigerant (R-22) Properties

Phase Transition Temperature : 0C Saturation Temperature : -6C

Initial/Ambient Temperature : 25C Mass Flow Rate : 145 kg/h

Average/Film Temperature : 12.5C Saturation Pressure : 407.69 kPa

Latent Heat of Solidification : 334 kJ/kg Latent Heat of Vaporization : 209.76 kJ/kg

Density of PCM : 998.9

kg/m3

Density (Liquid Phase) : 1301.6 kg/m3

Viscosity : 1.215 Pa-s Density (Vapor Phase) : 17.504 kg/m3

Thermal Conductivity (Liquid

Phase)

: 0.59

W/mK

Specific Heat (Liquid

Phase)

: 1.154 kJ/kgK

Thermal Conductivity (Solid

Phase)

: 2.2 W/mK Thermal Conductivity

(Liquid Phase)

: 0.0975 W/mK

Prandtl Number : 8.64 Surface Tension : 0.0126 N/m

Viscosity (Liquid Phase) : 0.000233 Pa-s

Viscosity (Vapor Phase) : 0.00001124 Pa-s

• Size of each PCM plate = 0.4046 m × 0.1016 m × 0.3048 m (16 in × 4 in × 12 in)

• No. of PCM plate = 03

• Total Volume of PCM plates = 0.0376 m3

• Total amount of PCM (water) = 32.34 kg (10.78 kg in each plate)

• Approximate time required for solidification = 2.75 hours

• Volume/space required for PCM (water) in PCM plate = 0.0324 m3

• Volume/space required for tube = 0.0017 m3

• Volume/space required for PCM solidification = 0.0030 m3

• Total volume/space required for PCM = 0.0371 m3

• Space available (0.0376 m3 - 0.0371 m3) = 0.0005 m3

• Energy stored in PCM during solidification = 10800 kJ

The calculated heat transfer rate and the size of tube is as following.

Parameter Value

Heat transfer coefficient at refrigerant side, htp : 2677.64 W/m2K

Heat transfer coefficient at PCM side, hT : 70.46 W/m2K

Overall heat transfer coefficient, U : 68.65 W/m2K

22

Parameter Value

Log mean temperature difference, LMTD : 15.22 K

Heat flux, q : 1045.12 W/m2

PCM charging rate, q : 1090.91 W

Tube diameter, d : 0.009525 m

Total length of tube required, L : 34.88 m

Length of tube required per plate : 11.63 m

No. of coil per plate : 04

Length of tube required per coil per plate : 2.91 m

The tubes turned into helix form in the PCM heat exchanger and each plate contains four helix

tubes. So length of per helix is 2.91 m. So helix size will be calculated as following:

Helix size Calculation:

Helix Length, L (m)

𝐿 = 𝑛√(𝐶2 + 𝑃2)

Pitch, P (m) Helix

Diameter, D

(m)

Helix

Circumference, C

(m)

No. of Turns,

n

2.91 m 0.0254 0.0762 0.2394 12.07 13

3.5 Equipment used in the Experiment

The equipment used in this research are listed as following with their specifications.

Equipment Name with Photo Specifications

Refrigerant Mass Flow Meter

Brand: Max Machinery, Inc.

Model: P214 Piston Flow Meter (Frequency)

Flow Range: 5 cc/min to 10,000 cc/min

Accuracy (at 3 cP): ± 0.2% of reading over a 200:1

range

Maximum Operating Pressure: 210 bar (3000 psi)

Recommended Filtration: 10 micron

Fluids: Most non-aqueous, organic liquids

23


Fine Wire Duplex Insulated

Thermocouple Wire

Brand: OMEGA

Model: GG-K-36

Type: K

Maximum Temperature: 482C

Nominal Size: 0.8 mm × 1.1 mm

Pressure Transducer

Brand: Firstrate

Model: FST800-217

Pressure range: up to 600 bar

Operating temperature range: (-40°C to 125°C)

Supply Voltage: 9~30 VDC/ 15~30 VDC

Signal Output: 4~20 mA/ 0~5VDC

Accuracy: ±0.5%FS

Refrigerant Recovery System

Brand: Yellow Jacket

Model: RecoverXLT2-AP™

Capabilities: Vapor, liquid, push/pull

Refrigerants: R-32, R-1234yf, R-1234ze as well as the

most common CFC, HFC and HFO

refrigerants, including R-22, R-404A,

R-407, R-410A, R-448A and R-449A

Compressor: ½ hp, twin cylinder, reciprocating, oil-less

Power Source: 230V - 50/60 Hz

Amps: 4.0

Operating Temperature: 40° to 122° F (4° to 50° C)

Refrigerant Leak Detector

Brand: D-Tek Select

Model: 712-202-G1

Minimum sensitivity: 0.10 oz. / yr. (3 g/a)

Power source: NiMH batteries (rechargeable)

Probe: Rubber-coated flexible metal,

approx. 15 in. (38 cm) long

24


Digital Psychrometer


Model: 69008

Temperature range: -4° to 122°F (-20° to 50°C)

Resolution: 0.1°F/ºC, 0.1% RH

Operating temperature range: 4° to 122°F

(-20° to 50°C) 0-100% RH

Accuracy: Relative humidity functions

(± 3% RH at 25°C)

Temperature ± 1°F (0.6°C)

Sensors: Temperature: thermistor bead

Humidity: polymer film (capacitance)

Update rate: 2-6 seconds depending on function

Recovery Burnout Filter for Dirty

Systems


Model: 95014

High capacity inline filter

Removes Moisture and Burnt Oil as well as Acid

Refrigerant Charging Scale

Brand: Inficon

Model: Wey-TEK HD Wireless

Accuracy: ±0.06% of reading, ±0.25 oz. (10 g)

Resolution: 0.25 oz. (10 g)

Wireless range 30 ft. (10 m) v

Vacuum Pump Systems


Model: 93560

Free air displacement: 6.0 cfm

Motor: 1/2 hp - 1725 rpm

Weighing capacity: 250 lb. (115 kg)

Display: Full-color graphical display

Wireless range: 30 ft. (10 m)

25


Wireless Vacuum Gauge and

vacuume pressure & Temperature

gauge


Model: ManTooth-PTV (67023)

Pressure Sensing Resolution: 0.1 psi, 0.1 bar, 1 kPa,

0.001 MPa, 0.01 kg/cm^2

Pressure Sensing Accuracy: 0.5% of full scale at 25°C;

1% of full scale 55°F to

130°F; 2% of full scale -40°F to 248°F

Working Pressure: 0 – 700 psia (48.3 bar)

Temperature Sensor Range (Instrument) Sensing

element: -40°F to 266°F (-40°C

to 130°C)

Sight glass


Model: 41145

Connection: 1/4" Flare x 1/4" Female Flare

Item Weight: 5.4 ounces

Humidity and temperature display

and data logger

Brand: Tektronix

Model: KEITHLY 3706A

Measuring: Temperature, pressure

Up to 576 two-wire or 720 one-wire multiplexers

channels in one mainframe

Expansion Valves

Brand: Danfoss

Model: 068Z3209

Type: Thermostatic

Maximum working pressure: 34 bar

Minimum Temperature: –60°C

Refrigerant: R-22, R-134a, R-407C, R-404A

26


Annemometer

Brand: HTC Instrument

Model: AVM – 07

Air Velocity Range: 0.40 ~ 45.0 m/s

Resolution: 0.1 m/s

Accuracy: ± (2% +0.1m/s)

3.6 Experimental Procedure

During charging cycle the water converted into ice by rejecting heat into the refrigerant which

flows through the heat exchanger coil. After solidification, the ice is used to supply cold to the

room or cold storage chamber. The air comes into the heat exchanger from the lower portion

of the heat exchanger and it touches the PCM plates and becomes cold by exchanging heat

from air to PCM plate that contained ice. This cold air is supplied to the required medium. The

air side heat transfer rate can be enlarged by incorporating fin at the PCM plates.

The PCM heat exchanger was initially designed by considering two phase flow at refrigerant

side and PCM side. After that, manufacturing of the heat exchanger was completed by using

welding, joining, bending, flaring, insulating etc. process. Then the experiment was begun as

following.

I. The full setup was fabricated with twelve thermocouples within PCM plates, four

thermocouples and two anemometers at air inlet and outlet side of PCM heat exchanger,

pressure and temperature sensors at four positions of the refrigeration circuit to take the

readings.

II. Then the condensing unit was in operational mode for a certain amount of time to

examine the solidification characteristics like temperature distribution, solidification

time.

III. After solidification of water, the melting characteristics like temperature distribution,

melting time also observed by using several thermocouples with the help of data

acquisition system. This solidification and melting phenomena were observed 5 times.

IV. Based on the experimental results the design of the PCM heat exchanger was slightly

modified i.e. fins were incorporated with heat exchanger coils. Then again solidification

and melting characteristics were observed 5 times.

27

V. Although solidification and melting characteristics were improved but refrigerant

distribution was not uniform. For that reason, the design of PCM heat exchanger is

again modified by installing refrigerant distributor in replacement of inlet header

mechanism.

VI. During melting period, the inlet and outlet air temperatures and velocities variations

also observed with the help of data logger.

VII. Then these 5 sets of experimental data were analyzed to make a conclusion about the

design of PCM heat exchanger.

28

Chapter-4: Results & Discussions

4.1 Solidification and Melting Characteristics of PCM

Solidification and melting characteristics were observed for three different models. In the first

model which is termed as basic model, header mechanism is used to distribute refrigerant

through evaporator coils. In the second model fins were attached to enhance heat transfer at the

PCM side of the PCM heat exchanger. The header mechanism is still present in this model and

termed as fins incorporated model. In the last model header mechanism was replaced by

refrigerant distributor to distribute refrigerant through evaporator coils. In this model the fins

are also present and name of this model is refrigerant distributor model. These three PCM heat

exchanger models is shown in figure 4.1.

To check the temperature distribution within the plates four thermocouples were placed at each

of the plate. So total twelve thermocouples positioned within three plates. The positions of

twelve thermocouples within plates is shown in figure 3.12. Each of plates having four

evaporator helical coils, one thermocouple is positioned at each of the coils. So, every plate

contains four thermocouples. The numbering of the thermocouple is as following.

𝑇𝑚𝑛 = 𝑇ℎ𝑒𝑟𝑚𝑜𝑐𝑜𝑢𝑝𝑙𝑒 𝑎𝑡 𝑝𝑙𝑎𝑡𝑒 𝑛𝑜. ′𝑚′ 𝑎𝑛𝑑 𝑐𝑜𝑖𝑙 𝑛𝑜. ′𝑛′

Plate number starts from the right side, viewing from the front side of the PCM heat exchanger

and coil number starts from far most region to the closest region of the inlet header. The plate

and coil numbering are shown in figure 4.2.

(a) (b) (c)

Figure 4.1: PCM Heat Exchanger Models (a) Basic Model (b) Fins Incorporated Model (c) Refrigerant Distributor Model

29

During solidification and melting the temperatures were recorded at 30 seconds interval. After

incorporating fins, the temperatures were also recorded and finally installing a refrigerant

distributor (replacement of header) at the inlet side of the PCM heat exchanger the temperature

readings were also observed. The temperature distributions during solidification and melting

are described at the following sections.

4.1.1 Temperature Distribution during Solidification (Basic Model)

The temperature readings at twelve different positions were obtained during solidification at

30 seconds interval in the basic PCM heat exchanger model where header mechanism was used

to distribute refrigerant. Temperature readings of all thermocouples is plotted against duration

(time needed to solidify) shown in figure 4.3 and temperature distribution for each of plates

Figure 4.3: Temperature Distribution during Solidification (all plates) without Fin and Distributor

Figure 4.2: Plate and Coil Numbering of the PCM Heat Exchanger

30

shown in figure 4.4. It is observed that, at plate-1 thermocouple T1-4 & T1-3 show negative

temperature but T1-2 & T1-1 are positive and close to zero. So, the regions at thermocouples T1-

4 & T1-3 are solidified first and other two regions were not completely solidified. So, at plate-1

coil-3 & 4 regions solidified less than one hour, coil-2 region solidified within 2.5 hours and

coil-1 region solidified more than 3.5 hours. At plate-2 & plate-3 thermocouple T2-4 & T3-4 are

negative respectively and all the other thermocouples show positive temperature close to zero.

At plate-2 coil-4 region (T2-4) takes almost 1 hour for solidification, coil-2 and 3 regions (T2-2,

T2-3) solidified within 3.5 hours and coil-1 region (T2-1) takes more than 3.5 hours for complete

solidification. At plate-3 coil-4 region (T3-4) takes less than 1 hour for solidification and coil-

1, 2 and 3 (T3-1, T3-2 and T3-3) regions were not completely solidified. It is observed that the

amount of solidification is higher at plate-1 than other two plates and each coil-4 region PCM

at all plates solidified first. Amount of solidification at plate-1 is higher because huge amount

of refrigerant passes through plate-1 evaporator coils compared with other plates coils. So, the

(a) (b)

(c)

Figure 4.4: Temperature Distribution during Solidification (a) Plate-1 (b) Plate-2 (c) Plate-3 without Fin and Distributor

31

refrigerant distribution among plates are not uniform. The position of each coil no. 4 of all

plates close to the inlet header (shown in figure 4.1) so vast number of refrigerant passes

through all coil no. 4 and these become solidified first and after solidification these go to

negative temperature. It takes only a few hours to solidify completely at coil no. 4 region. After

solidification of all coil no.4 regions the heat is transferred from nearest coil (coil no. 4) to far

most coil (coil no. 1) in a slow manner since the thermal conductivity of ice very low. Through

other coils except coil no. 4 pass a small amount of refrigerant so it takes much amount of time

to solidify and show a positive temperature close to zero. So, it is clear that in the basic design

of PCM heat exchanger the refrigerant distribution is not uniform and most of the refrigerant

passes through all nearby coils. The image taken after solidification is shown in figure 4.5.

Intel Header

Outlet Header

Plate-1

Plate-2

Plate-3

Figure 4.5: PCM Heat Exchanger during Solidification (Basic Design) without Fin and Distributor

32

4.1.2 Temperature Distribution during Melting (Basic Model)

At melting (discharging) stage the ambient air comes into contact with heat exchanger plate

and goes out to the atmosphere. In the basic design, during solidification the PCM near the

header region is solidified first and maximum solidification is occurred at plate-1. The melting

temperature distribution at all plates shown in figure 4.6 and individual plates separately in

figure 4.7 and 4.8.

Figure 4.6: Temperature Distribution during Melting (all plates) without Fin and Distributor

(a) (b)

Figure 4.7: Temperature Distribution during Melting (a) Plate-1 (b) Plate-2 without Fin and Distributor

33

During melting all the thermocouples of plate-1 (T1-1, T1-2, T1-3 and T1-4) start with a negative

temperature and become positive after a certain time changing their phase. At plate-1 all the

PCM regions take 6 to 8 hours for complete melting since plate-1 was completely solidified.

At plate-2 coil-4 region (T2-4) takes almost 6 hours for melting, coil-2 and 3 regions (T2-2 and

T2-3) melt within 2.5 hours and coil-1 region (T2-1) shows sensible temperature variations. In

plate-3 only thermocouple coil no. 4 (T3-4) shows both sensible and latent heat transfer and this

region PCM takes almost 5 hours for complete melting and other thermocouples (T3-1, T3-2, and

T3-4) indicate only sensible heat transfer. The melting duration is fully depending on amount of

solidification. In this case only plate-1 was completely solidified so it shows longer melting

time but other plates were not completely solidified and show uneven melting duration.

So, in the basic header mechanism design plate-1 is more efficient than all other plates. Overall

the temperature distribution is not uniform in both solidification and melting and heat is

concentrated to the nearest header region.

So that in the second model fins were incorporated to distribute heat from the nearest header

region PCM to far region PCM.

Figure 4.8: Temperature Distribution during Melting (a) Plate-3 without Fin and Distributor

34

4.1.3 Temperature Distribution during Solidification (Fins Incorporated Model)

Attaching fin to the evaporator coils ensures better heat transfer during solidification. The heat

is transferred from the nearest header coil region (coil no. 4 region) to far most coil region (coil

no. 1 region). In this case the heat transfer from nearest region to furthermost region is little bit

higher since fins are incorporated. The solidification with fins incorporated for all plates shown

in figure 4.9 & 4.10.

At plate-1 all the regions are solidified completely and take less amount of time than other

plates. In this plate coil-3 and 4 regions (T1-3 and T1-4) PCM solidified within 4 to 5 hours and

coil-1 & 2 regions (T1-1 and T1-2) take almost 7 to 8 hours for complete solidification. At plate-

2 coil-3 & 4 regions (T2-3 and T2-4) PCM solidified within 4 hours, coil-2 region (T2-2) solidified

within 8 hours and coil-1 region (T2-1) was not completely solidified. The solidification

behavior is fairly improved at plate-2 than without fins design (basic design). At plate-3 two

thermocouples (T3-4 & T3-3) indicate better solidification takes almost 6 hours for solidification

but other two thermocouples show positive constant temperature which clarify that those

regions are not completely solidified.

By incorporating fin, the solidification characteristics is improved than without fin and

solidification phenomenon is quite satisfactory at plate-1 and plate-2. Although heat is

transferred from header nearest region to far most region but still far most regions are not fully

solidified as like nearest regions. In generally heat was more or less distributed through all the

PCM regions but it takes longer time for solidification and some of the regions are still not

solidified.

Figure 4.9: Temperature Distribution during Solidification (all plates) with fin

35

(a) (b)

(c)

Figure 4.10: Temperature Distribution during Solidification (with fin) (a) Plate-1 (b) Plate-2 (c) Plate-3 with fin

36

4.1.4 Temperature Distribution during Melting (Fins Incorporated Model)

Attaching fins to the existing PCM heat exchanger design show some improvement during

solidification in some of the region but some of the PCM regions still not solidified. So the

melting of the PCM did not show significant improvement in all regions during melting.

In figure 4.12 at plate-1, it is observed that three thermocouples (T1-4, T1-3 & T1-2) show normal

melting and takes almost 4 to 6 hours for discharging but another thermocouple (T1-1) indicates

less than 1 hour discharging time. At plate-1 coil-2, 3 & 4 regions take 4 to 6 hours for complete

melting and coil-1 region takes less than 1 hour for melting. At plate-2 all the PCM regions

take 2.5 to 6 hours for complete melting. At plate-3 coil-4 region takes 3 hours, coil-3 region

takes 1.5 hours and coil-1 & 2 take less than 1 hour for melting. The melting temperature

distribution is solely depending on amount of solidification and these melting characteristics

indicates non uniform temperature distribution. So, in the third model (refrigerant distributor

model) a refrigerant distributor was installed to distribute refrigerant uniformly for uniform

solidification.

Figure 4.11: Temperature Distribution during Melting (all plates) with fin

37

(a) (b)

(c)

Figure 4.12: Temperature Distribution during Solidification (with fin) (a) Plate-1 (b) Plate-2 (c) Plate-3 with fin

38

4.1.5 Temperature Distribution during Solidification (Refrigerant Distributor

Model)

Installing a refrigerant distributor at the inlet line by replacing header ensures uniform

distribution of the refrigerant through all the evaporator coil. The temperature distribution

during solidification for all plates shown in figure 4.13 and individual plates in figure 4.14.

From the figure it is shown that all the plates are solidified uniformly and within a shorter

period of time than with or without fin. Each and every thermocouple starts with positive

temperatures and changing its phase at 0℃ and then going to negative temperatures. These

thermocouples show normal phase change behavior. The variation of temperature from one

thermocouple to another within a plate is not significant. The image during solidification is

shown in figure 4.15. It takes almost 3 hours for complete solidification of all PCM plates. So,

the optimized solidification phenomenon is found in distributor based PCM heat exchanger

with fin.

Figure 4.13: Temperature Distribution during Solidification (all plates) with Fin and Distributor

39

Wire Thermocouples

Figure 4.15: Image Taken during Solidification with Fin and Distributor

(a) (b)

(c)

Figure 4.14: Temperature Distribution during Solidification with Distributor and Fin (a) Plate-1 (b) Plate-2 (c) Plate-3 with

Fin and Distributor

40

4.1.6 Temperature Distribution during Melting (Refrigerant Distributor Model)

All the plates were fully solidified during charging period. In discharging period all of the

thermocouple starts with a negative temperature then change their phase at 0℃, after go to

positive temperature. All the plates grant more than 4 hours during discharging period. The

melting temperature distribution is shown in figure 4.16, 4.17 and 4.18. All the plates show

almost same characteristics and melting period.

Figure 4.16: Temperature Distribution during Melting (all plates) with Fin and Distributor

(a) (b)

Figure 4.17: Temperature Distribution during Melting with Distributor and Fin (a) Plate-1 (b) Plate-2 with Fin and Distributor

41

4.1.7 Comparisons of Solidification Phenomena for all Models

The temperature distributions during solidification at different plates for different models are

shown in figure 4.19, 4.20 & 4.21. The comparisons considered for 3.5 hours solidification

period.

In the basic model (without fin and distributor) at plate-1 two evaporator coils regions solidified

within half an hour and other two thermocouples take more than 3.5 hours for solidification. It

indicates that the refrigerant passes through those two coils. At the same plate after

incorporating fins, all the evaporator coils regions solidified uniformly but takes more than 3.5

Figure 4.18: Temperature Distribution during Melting with Distributor and Fin at Plate-3 with Fin and Distributor

Figure 4.19: Temperature Distribution during Solidification at plate-1 for different Design Conditions

42

hours for complete solidification. A significant improvement is occurred when refrigerant

distributor is used. In this case all the evaporator coils regions completely solidified within 2.5

hours.

At plate-2 without fin and distributor design only one evaporator coil solidified within one hour

and other evaporator coils regions need more than 3.5 hours for solidification. But in case of

fins incorporated design it exhibits same characteristics as like plate-1. For with fin and



43

distributor design one evaporator coil region takes almost 1.3 hours for solidification and other

evaporator coils regions need 2.7 hours for complete solidification.

At plate-3 in basic design one evaporator coil region takes less than half an hour to solidify and

other thermocouple regions maintain a positive constant temperature and need more than 3.5

hours for solidification. At the same plate the incorporated fins design all the evaporator coils

regions exhibit constant phase change temperature and need more than 3.5 hours for

solidification. But with fin and distributor design shows better solidification than other designs.

In this case only 2 hours is needed for complete solidification.

Overall the basic model shows uneven solidification through all the regions, fins incorporated

model shows some improvement in temperature distribution but it takes longer time for

solidification. The refrigerant distributor model shows uniform temperature distribution for all

the regions of the PCM plates. So, it is concluded that refrigerant distribution through header

does not ensure uniform distribution

4.1.8 Melting Phenomena Comparison for all Models

The melting temperature distributions for different plates at different design conditions are

shown in figure 4.22, 4.23 and 4.24. The melting comparison considered for 6 hours time

period. In the basic model plate-1 PCM takes more than 6 hours during melting. But after

incorporating fin, melting time maintains more than 5 hours but one thermocouple indicates

Figure 4.22: Temperature Distribution during Melting at plate-1 for different Design Conditions

44

less than one hour. But at the same plate the backup time reduces to 4 hours but all the region

show simultaneous melting in the refrigerant distributor model.



45

At plate-2 the basic design condition ensures more than 5 hours melting time for one region of

PCM but other PCM regions shows less than 2.5 hours which is unusual. But at incorporated

fins design two PCM regions show better melting than other two regions. At the fin and

distributor design the melting behavior is uniform with more than 3.5 hours backup. At plate-

3 only refrigerant distributor model has uniform melting and significant melting time but other

two models show non-uniform melting distribution with uneven melting time from region to

region.

Finally, it is clear that the melting phenomena is better in the refrigerant distributor model than

other two models. Although plate-1 shows better performance in all models.

4.2 Performance Analysis of VCR System

To analyze the performance the Vapor Compression Refrigeration (VCR) system the pressure

and temperature readings at four different position were observed at different periods. The

average pressure and temperature are shown at table 4.1. Using these data and with the help of

P-h chart the Coefficient of Performance (COP) of the system is calculated using the following

formulas.

𝐶𝑂𝑃 (𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑃𝑒𝑟𝑓𝑜𝑟𝑚𝑎𝑛𝑐𝑒) =𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡

𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑊𝑜𝑟𝑘

𝑜𝑟, 𝐶𝑂𝑃 =𝐸𝑛𝑡ℎ𝑎𝑙𝑦 𝑎𝑡 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑜𝑟 𝑂𝑢𝑡𝑙𝑒𝑡 − 𝐸𝑛𝑡ℎ𝑎𝑙𝑦 𝑎𝑡 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑜𝑟 𝐼𝑛𝑙𝑒𝑡

𝐸𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑎𝑡 𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑂𝑢𝑡𝑙𝑒𝑡 − 𝐸𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑎𝑡 𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝐼𝑛𝑙𝑒𝑡=

ℎ1 − ℎ4

ℎ2 − ℎ3

𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 𝑀𝑎𝑠𝑠 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 × 𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡 = �̇� × (ℎ1 − ℎ4)

Figure 4.25: R-22 P-h Chart for Existing Refrigeration System

46

Table 4.1: Pressure and Temperatures at different Positions of the Refrigeration System

Measuring Point Pressure, psi [MPa] Temperature [℃] Enthalpy (kJ/kg)

After Evaporator (PCM Heat Exchanger), h1 37.56 [0.259] -10.2 402.6

After Compressor, h2 293.85 [2.026] 91.8 459.1

After Condenser, h3 291.96 [2.013] 49.5 261.2

Before Evaporator (PCM Heat Exchanger), h4 42.79 [0.295] -15.9 261.2

Mass flow rate of refrigerant = 40.22×10-3 kg/s (1808.80 cc/min)

So, 𝐶𝑂𝑃 = 402.6−261.2

459.1−402.6≈ 2.503

𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 40.22 × 10−3 (402.6 − 261.2) = 5.6871 𝑘𝑊 = 1.6249 𝑇𝑅

The capacity of the system falls to 1.63 TR from 2.0 TR. It is because the system was modified

to maintain low evaporator temperature.

4.3 Performance Analysis of Air Side of PCM Heat Exchanger

Two thermocouples were placed at the air inlet, two thermocouples were placed at the air outlet

and three thermocouples were placed on the plates of the PCM heat exchanger. The temperature

readings at inlet and outlet at different velocities shown in table 4.2. Using the values, the

bypass factor of the PCM heat exchanger is calculated with the help of following formula.

𝐵𝑦𝑝𝑎𝑠𝑠 𝐹𝑎𝑐𝑡𝑜𝑟 = 𝑈𝑛𝑡𝑜𝑢𝑐ℎ𝑒𝑑 𝐴𝑖𝑟

𝑇𝑜𝑡𝑎𝑙 𝐴𝑖𝑟=

𝑂𝑢𝑡𝑙𝑒𝑡 𝐴𝑖𝑟 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 − 𝑃𝑙𝑎𝑡𝑒 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

𝐼𝑛𝑙𝑒𝑡 𝐴𝑖𝑟 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 − 𝑃𝑙𝑎𝑡𝑒 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒

The variation of bypass factor with average velocity is shown in figure 4.26. The bypass factor

is increased with increasing air velocity. From the air velocity 1.2 m/s to 2 m/s the bypass factor

is increased almost 6%. The variation of average inlet air, outlet air and plate surface

temperatures at different times are shown in figure 4.27.

Table 4.2: Bypass Factor of PCM Heat Exchanger at Different Air Velocities

Air Velocity (m/s) Inlet Air Temp. (℃) Outlet Air Temp. (℃) PCM Plate Temp. (℃) Bypass Factor

1.2 28.16 12.22 0.2894 0.4281

1.3 28.81 12.90 0.3599 0.4408

1.5 28.91 12.98 0.3549 0.4421

1.6 28.68 13.25 0.4163 0.4540

1.9 28.65 13.88 0.3966 0.4772

2.0 28.49 13.94 0.4096 0.4818

The average inlet and outlet air temperature difference is high at the initial stage of melting and

becomes low after certain period of backup. The inlet air temperature is nearly average

throughout the melting period and average plate surface temperature increases with increasing

duration of melting. The average plate surface temperature varies from 0C to 5C within the

47

melting period. Temperature difference between inlet and outlet varies from 16 to 9C within

4 hours time period.

The sensible heat transfer rate from the heat exchanger PCM by the air throughout the melting

period is shown in figure 4.28. The sensible heat transfer rate is high at the initial stage of

melting since the temperature difference was high. The sensible heat transfer rate decreases

with increasing time period and it varies from 2200 to 1300 W within 4 hours time period.

Figure 4.26: Bypass Factor of PCM Heat Exchanger at Different Velocities

Figure 4.27: Average Inlet Air Temperature, Average Outlet Air Temperature and Average Plate Surface Temperature

48

Figure 4.28: Sensible Heat Transfer Rate by Air at Different Time

49

Chapter-5: Conclusion and Further Research

5.1 Conclusion

In the present work a PCM heat exchanger is developed to investigate the performance analysis

of the heat exchanger such as solidification-melting behavior, bypass factor phenomenon, heat

transfer rate variation and average temperature variation for cold chain application like

refrigerated vehicle or cold storage. Three models of the PCM heat exchanger were developed

to compare the performance variations and to select the best one. In the first basic model the

evaporator (PCM heat exchanger coil) refrigerant is distributed through a header mechanism

and in the second model fins are incorporated at the evaporator coil side to enhance the heat

transfer rate during solidification & melting. The header mechanism at the inlet side of the

evaporator is replaced by an efficient refrigerant distributor in the third model.

The solidification and melting behavior are analyzed for all the models of the PCM heat

exchanger and compared. The nearest header region PCM is solidified first and other regions

take a huge amount of time to solidify in the basic PCM heat exchanger model. It is occurred

due to non-uniform distribution of refrigerant and lower heat conductivity of PCM after

solidification. In order to minimize the heat transfer problem from solidified PCM to liquid

PCM fins are incorporated in the second model. After attaching fins, the heat transfer rate is

improved than first model but it takes longer time to completely solidify all the PCM. So that

in the third model a refrigerant distributor is introduced by replacing inlet header of the

evaporator. Due to refrigerant distributor and incorporated fins (third model) the solidification

characteristics is improved than other two models. In the refrigerant distributor model, it takes

3.0 hours to completely solidify all the PCM.

During melting period in the first two models show that different PCM regions have different

melting time since complete solidification was not ensured. But in the refrigerant distributor-

based model all the PCM regions indicate almost same amount of melting time during

discharging process. So, the third model is most efficient during solidification and melting.

The air side performance of PCM heat exchanger is analyzed for the third model due to its

appropriateness. Experiment shows that at the initial stage of melting period the sensible heat

transfer rate is higher and becomes lower within the duration. The sensible heat transfer rate

degradation shows polynomial behavior during discharging period. The sensible heat transfer

rate decreases with increasing time period and it varies from 2200 to 1300 W within 4 hours

time period. The temperature difference between inlet and outlet air higher at the initial melting

period and becomes lower with increasing time. Temperature difference varies from 16 to 9C

within 4 hours time period. The PCM plate temperature is also increased with increasing

50

duration of melting. The air bypass factor of the PCM heat exchanger is increased by increasing

air velocity. Bypass factor increases 6% from the air velocity 1.2 m/s to 2m/s.

5.2 Future Research Work

The PCM heat exchanger models developed in this research work is a good initiative to design

air to PCM heat exchanger. But some other important challenges can also be investigated in

future. The important analysis should be considered in the future are:

Water is considered as only PCM in the research work. Some other PCM such as

ethylene glycol, water-salt solution can be considered for further investigation.

Using nanoparticle within the PCM can increase heat transfer rate within PCM. This

research can also be investigated further.

To improve the air side heat transfer fins can be attached in the air side and the effect

of air side fins can be explored.

The temperature distribution in the air side of the PCM heat exchanger can be observed

in the future to investigate air side performance.

The copper was used as fin material in this research works. The effect of other fin

materials can be analyzed.

The number of PCM plates and their orientation is an important factor for heat transfer.

Changing the number of PCM plates with its orientation can be analyzed in future.

The condensing unit capacity in this research work is minimal. Higher capacity

condensing unit can reduce solidification time and can change the scenario of the

temperature distribution.

51

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[2] Phimolsiripol, Y., Siripatrawan, U., Tulyathan, V., Cleland, D.J., 2008. Effects of

freezing and temperature fluctuations during frozen storage on frozen dough and bread

quality. Journal of Food Engineering 84, 48–56.

[3] Donhowe, D.P., Hartel, R.W., 1996a. Recrystallization of ice during bulk storage of ice

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