Encapsulation of phase change materials for heat storage

3.3 Experiments 40 3.3.1 Complex coacervation 40 3.3.2 Spray drying 42 3.4 Characteristics and performance of microcapsules 44 3.4.1 Energy storage and release capacities 44 3.4.2 Therm

ENCAPSULATION OF PHASE CHANGE MATERIALS (PCMS)

FOR HEAT STORAGE

MYA MYA KHIN (B.E., Yangon Technological University)

ƒ To my supervisors, Associate Professor M.S Uddin and Associate Professor M.N.A Hawlader for their constant guidance, kindness, forgiveness, care, concern shown throughout the project and time taken to read the manuscript

ƒ To all the technical and clerical staff in the Chemical & Environmental Engineering Department for their patience and help

ƒ To Dr Zhu Haijun for giving me some informations and literature for this project

ƒ To all my colleagues from E4A-07-07 especially Mr Peng Zanguo for their help

on different occasions, discussion and for their encouragement during my tenure at NUS

ƒ To my parents and family members and my best friend Miss Thin Thin Aye for their continuous love and encouragement throughout the study

Last but not least, my thanks to all who have contributed in one-way or another to make this thesis possible

1.2 Objective and scope of thesis 4

2.1 Thermal energy storage 6 2.2 Thermal energy storage techniques 7

2.3 Candidate heat storage materials 15

2.4 Factors affecting the energy storage capacity of PCM 20

2.5 Encapsulation of phase change materials 20

2.6 Methods of microencapsulation 25

2.7 Thermal cycling test for encapsulated PCM 30 2.8 Heat transfer of PCMs 31

2.9 Scope of the present work 34

3.1 Materials 36

3.3 Experiments 40

3.3.1 Complex coacervation 40 3.3.2 Spray drying 42 3.4 Characteristics and performance of microcapsules 44 3.4.1 Energy storage and release capacities 44 3.4.2 Thermogravimetric analysis 45

3.4.3 Surface morphology and characterization of inner 46 structure

3.4.4 Microencapsulation efficiency 47 3.4.5 Estimation of core to coating ratio 47 3.4.6 Chemical structure stability evaluation 48 3.5 Accelerated test process 48 3.6 Fluidized bed heat exchanger for microencapsulated PCM 51

4.1 Encapsulation efficiency 54

4.2 Estimation of core to coating ratio 63 4.3 Thermal performance 64 4.4 Surface morphology and inner structure characterization 68 4.4.1 Surface morphology 68 4.4.2 Inner structure 69 4.5 Thermogravimetric analysis 70

4.8 Thermal performance of microencapsulated PCM in 91

fluidized bed heat exchanger

4.8.1 Temperature profiles 91

4.8.2 Total heat storage and release 94

5.1 Conclusions 96

5.2 Recommendations 99

REFERENCES 100

APPENDICES 113

Microencapsulated PCMs are micron size phase change materials enclosed in a protective wrapping The microcapsule prevents the leakage of the material during its phase change

It also provides larger heat transfer area per unit volume of heat storage vessel It could

be used in solar energy storage, waste heat utilization, and space heating and cooling

This study investigated the use of complex coacervation and spray drying methods for microencapsulating paraffin wax by polymeric materials (gelatin and acacia) in an aqueous system Experiments on operational variables to select suitable conditions were carried out for complex coacervation Encapsulation efficiency was found to be higher when the products had lower core to coating ratios The optimum condition for various core to coating ratios was found to be 10 minutes homogenizing time and the amount of cross-linking agent 6~8 ml Non-linear regression was used to correlate the encapsulation efficiency and the parameters studied In spray-drying method, decrease in encapsulation efficiency with increase in the ratio of core to coating was observed The optimum core to coating ratio was found to be 1:2

In the studies on thermal performance analysis by Differential Scanning Calorimetery (DSC), the effect of core to coating ratio on energy storage/release capacities was investigated The higher paraffin wax content in the sample gave the higher energy storage/release capacities The energy storage/release capacities of the coacervated

prepared under different conditions

Further characterization for both coacervated and spray-dried samples focused on surface morphology and inner structure by using microtone and Scanning Electron Microscopy (SEM) SEM analysis showed that spray-dried samples were more regular and spherical

in shape compared to coacervated samples Both samples contained a few small globules Size of coacervated particles ranged from 3.3 to10.5 µm Spray-dried microcapsules had

a diameter ranging from 1.3-10.1 µm The inner structure characterization showed that both coacervated and spray-dried samples consisted of a polymeric matrix surrounding numerous globules

The thermal stability of both coacervated and spray-dried samples was estimated by using Thermogravimetry (TG) analysis Thermal decomposition temperatures of core and coating materials were determined from TG output curves The decomposition temperature of paraffin wax existed between 200 and 300°C, and the decomposition temperature of polymer network (gelatin and acacia) was observed between 300 and 400°C The TG output curve for spray-dried samples had two peaks between 300 and 400°C The second extra peak showed the decomposition temperature of unreacted coating materials

Thermal stability of microencapsulated PCMs was also checked through accelerated thermal (melt/freeze) cyclic tests Both samples were subjected to thermal cycle tests up

capacities, melting temperature and specific heat capacity after specific number of cycles Both the coacervated and spray-dried samples showed good thermal stability throughout cycling process Fourier Transform Infrared Spectophotometery (FTIR) analysis also confirmed distinct chemical stability of both samples throughout thermal cycling

Finally, the thermal performance of the PCM was carried out in as fluidized bed heat exchanger Heat transfer between the spray-dried encapsulated PCM and air was studied during heating and cooling process It was found that the time taken for charging and discharging the capsules was about 760 and 600 seconds, respectively Total energy and release amount were found to be 2953 and 2431 J Therefore, it was observed that it was efficient heat exchange system

Figure 2.1: Areas of research in thermal energy storage system 8

Figure 2.2: Thermal energy storage strategies 9 Figure 2.3: Various forms of capsules 21 Figure 3.1: Molecular structure of paraffinic hydrocarbons 37 Figure 3.2: Molecular structure for gelatin 38 Figure 3.3: Schematic representation of the coacervation process 41 Figure 3.4: Photograph of mini spray dryer used in microencapsulation of 43 paraffin wax

Figure 3.5: Schematic diagram of thermal cyclic system 50 Figure 3.6: Actual thermal cyclic system in laboratory 51 Figure 3.7: Schematic diagram of experimenatal set up for fluidized bed 53

Figure 4.1: Effect of homogenizing time and the amount of cross-linking 58 agent on encapsulation efficiency at 2:1 core to coating ratio (HCHO)

Figure 4.2: Effect of homogenizing time and the amount of cross-linking 58 agent on encapsulation efficiency at 1:1 core to coating ratio (HCHO)

Figure 4.3: Effect of homogenizing time and the amount of cross-linking 59 agent on encapsulation efficiency at 1:2 core to coating ratio (HCHO)

Figure 4.4: Effect of homogenizing time and the amount of cross-linking 59 agent on encapsulation efficiency at 2:1 core to coating ratio

Figure 4.8: The output curve of DSC for coacervated sample 2:1 67 Figure 4.9: SEM profile of the coacervated samples 68 Figure 4.10: SEM profile of the spray-dried samples 68 Figure 4.11: Inner structure of spray-dried samples (fresh) 71 Figure 4.12: Inner structure of spray-dried samples (cycled) 71 Figure 4.13: Inner structure of coacervated samples (fresh) 72 Figure 4.14: Inner structure of coacervated samples (cycled) 72 Figure 4.15: TG thermogram of coacervated microcapsules 74 Figure 4.16: TG thermogram of spray-dried microcapsules 74 Figure 4.17: DSC output curve for 1:1 coacervated sample after 500 cycles 79 Figure 4.18: DSC output curve for 1:1 coacervated sample after 2000 cycles 79 Figure 4.19: DSC output curve for 2:1 spray-dried sample at 0 cycle 80 Figure 4.20: DSC output curve for 2:1 spray-dried sample after 1500 cycles 80

Figure 4.21: DSC measurement of melting temperature of microencapsulated 84 paraffin wax (spray-dried 1:1) after 500 cycles

Figure 4.22: DSC measurement of melting temperature of microencapsulated 84 paraffin wax (spray-dried 1:1) after 2000 cycles

Figure 4.23: DSC measurement of melting temperature of microencapsulated 85 paraffin wax (coacervated 2:1) at 0 cycle

Figure 4.24: DSC measurement of melting temperature of microencapsulated 85 paraffin wax (coacervated 2:1) after 2000 cycles

Figure 4.25: DSC measurement of specific heat capacity of microencapsulated 86 paraffin wax (spray-dried 1:2) at 0 cycle

Figure 4.28: DSC measurement of specific heat capacity of microencapsulated 87 paraffin wax (coacervated 1:2) after 2000 cycles

Figure 4.29: FTIR output curve for spray-dried samples with 1:1 core to 90 coating ratio

Figure 4.30: FTIR output curve for coacervated samples with 1:1 core to 90 coating ratio

Figure 4.31: Temperature profiles during heat storage stage 93 Figure 4.32: temperature profiles during heat release stage 93 Figure 4.33: Heat storage with time during heat storage stage 95 Figure 4.34: Heat release with time during heat release stage 95

Table 2.1: Comparison of various heat storage media 13 Table 2.2: Physical properties of some PCMs 16 Table 2.3: Comparison of organic and inorganic materials for heat storage 19 Table 2.4: Important characteristics of energy storage materials 19 Table 2.5: List of published encapsulated PCM systems 24 Table 3.1: Materials used in the microencapsulating of paraffin wax 36 Table 3.2: Operating conditions for spray drying microencapsulation 44 Table 4.1: Encapsulation efficiency of coacervated capsules 55

Table 4.2: Encapsulation efficiency of coacervated capsules with 36% 55 formaldehyde

Table 4.3: Encapsulation efficiency of coacervated capsules with 50% 56 gluteraldehyde

Table 4.4: Non-linear regression analysis of encapsulation efficiency using 62 formaldehyde

Table 4.5: Non-linear regression analysis of encapsulation efficiency using 62 gluteraldehyde

Table 4.6: Encapsulation efficiency of spray-dried samples 63

Table 4.7: Comparison of experimentally measured core to coating ratio 64 with designed values

Table 4.8: Energy storage and release capacities for coacervated and spray-dried 66 microencapsulated paraffin wax

Table 4.9: Energy storage and release capacities for microencapsulated 77 paraffin wax (2:1 coacervated sample)

Table 4.10: Energy storage and release capacities for microencapsulated 77 paraffin wax (1:1 coacervated sample)

Table 4.13: Energy storage and release capacities for microencapsulated 78 paraffin wax (1:1 spray-dried sample)

Table 4.14: Energy storage and release capacities for microencapsulated 78 paraffin wax (1:2 spray-dried sample)

Table 4.15: Thermophysical properties of microencapsulated paraffin 82 wax (coacervated 2:1) with test cycles

Table 4.16: Thermophysical properties of microencapsulated paraffin 82 wax (coacervated 1:1) with test cycles

Table 4.17: Thermophysical properties of microencapsulated paraffin 82 wax (coacervated 1:2) with test cycles

Table 4.18: Thermophysical properties of microencapsulated paraffin 83 wax (spray-dried 2:1) with test cycles

Table 4.19: Thermophysical properties of microencapsulated paraffin 83 wax (spray-dried 1:1) with test cycles

Table 4.20: Thermophysical properties of microencapsulated paraffin 83 wax (spray-dried 1:2) with test cycles

CHAPTER 1 INTRODUCTION

1.1 General background

Renewable energy has been used over the last two decades to save the costs and adverse environmental pollution effects of fossil fuel (Klass, 2003) Today, use of the renewable energy provides electricity and it has been used to improve solar water heating and space application of an advanced power system over the past few years (Fath, 1995) However, the main problems for renewable energy are:

(1) Solar radiation is intermittent by its nature; its total available value is a factor of time, weather condition and latitude

(2) Energy sources and the demands, in general, do not match each other

Therefore, scientists investigated technically to solve these problems Finally, they found that energy storage is one of possible solutions for energy conservation and leveling of energy demand patterns Thermal energy storage (TES) is considered by many to be one

of the energy storage technologies (Dincer and Dost, 1996) TES contains a thermal storage mass, and can store heat or cool Basically, it can be classified as latent, sensible and thermo-chemical energy Among these energy storage types, the most attractive form

is latent heat storage in phase change material (PCM) because of the advantages of high storage capacity in a small volume and charging/discharging heat from the system at a

In a latent heat energy storage system, one of the main elements is the PCM and its selection criteria Most investigations were focused on salt hydrates, paraffin, non-paraffin organic acids, clathrates and eutectic organic and inorganic compounds (Lane, 1986) Among those materials, paraffin wax offers more desirable properties such as non-poisonous, chemically stable, self-nucleating, negligible supercooling, low vapor pressure

in the melt, no phase segregation and commercially available at reasonable cost (Abhat, 1978) Therefore, in this study, paraffin wax was emphasized

The application of conventional paraffin wax for heat storage has some limitations They are as follows:

(1) paraffin wax has low thermal conductivity approximately 0.18 W/m K (Abhat and Malatids, 1981) that leads to low heat transfer;

(2) energy withdrawn from paraffin wax during cooling is limited by the fact that the storage medium begins to solidify on the surface of heat exchangers, the layer of solid material can act as insulating material;

(3) large volume change during phase transition;

(4) if heat transport medium is air, oxidation of paraffin wax produces complex compounds, aldehydes, ketones, etc that can lead to toxic to our environment (Lane, 1986);

(5) conventional particles of paraffin waxes are slightly sticky and can stick together to form large lumps, clogging occurs in a heat storage system, resulting in failure to circulate heat transport fluid through the system (Winsters, 1991) These limitations can result in decreasing in energy storage capacity

In order to overcome these problems, Patel (1968), Patenkar (1980), Fouda et al (1984), Garg et al (1985) and Yanadori et al (1989) have identified heat transfer enhancement concepts such as the use of agitators, scrapers and slurries in heat exchangers The disadvantage of their heat exchanger development is increasing the cost and complexity

of thermal energy storage devices In order to solve these problems, both material investigation and heat exchanger development should be performed Therefore, the studies focused on both cases were investigated (Hawlader et al., 2000) They observed and reported that PCM should be bounded within a secondary supporting structure and the application of a packed/fluidized bed heat exchanger is a better way of heat transfer enhancement

Therefore, the progress in latent heat storage systems mainly depends on heat storage material investigations and on the development of heat exchangers that assure a high effective heat transfer rate to allow rapid charging and discharging The required heat transfer surfaces should be large to maintain a low temperature gradient during these processes (Banaszek et al., 1999)

Microencapsulation refers to a process where droplets of liquids, solids, or gases (core) are coated by thin films (coatings) that protect the core material (Sheu and Rosenburg, 1995) The National Cash Register for commercially applying in carbonless copy paper started encapsulation process at 1930 (Green and Schleicher, 1956) Recently, encapsulation processes have been developed in various fields such as pharmaceutical

exchanger are that it provides large heat transfer area per unit volume and provides higher heat transfer rate due to low thermal resistance between the heat transfer fluid and the PCM and high convective heat transfer by heat transfer fluid in a fluidized bed (Hawlader

et al., 2000)

1.2 Objective and scope of thesis

For many applications, encapsulated PCMs were produced by researchers Inaba et al.,

1997 prepared encapsulated paraffin wax by using interfacial polymerization and integrated the samples with building materials to reduce overheating in summer and to take effect storage discharge by ventilation Xiao et al., 2000 prepared matrix type microcapsules (paraffin wax) by using interfacial polymerization and used them as latent heat storage materials for thermal storage units Hawlader et al., 2002 prepared the microcapsules by using complex coacervation method and studied thermal performance in packed bed heat exchanger

All preliminary studies showed that encapsulated paraffin wax was prepared by interfacial polymerization and complex coacervation methods However, not much work was reported on the inner structure of the microcapsules, thermal cycles test on microcapsules

and the thermal performance of the encapsulated PCM in fluidized bed heat exchanger

The overall objective is to synthesize microencapsulated paraffin wax and to evaluate thermal performance of microencapsulated paraffin wax in fluidized bed heat exchanger The scope encompasses the following aspects of work:

(1) preparation of microencapsulated paraffin wax

(2) characteristic evaluation of encapsulated paraffin wax

(3) to study the effects of thermal cycling on the thermal properties of

microencapsulated paraffin wax

(4) to study heat transfer behavior in fluidized bed exchanger

This thesis is presented by organizing five chapters including introduction, Chapter 1 In chapter 2, literature review on the renewable energy and its application, thermal energy storage techniques and materials are presented Furthermore, literature review on encapsulation of PCMs, encapsulation techniques and heat transfer development for PCMs are also presented in this chapter Chapter 3 lists the materials used in this experiment, the experimental detail procedures, the evaluation techniques and the equipments used for characterization of microencapsulated paraffin wax Chapter 4 presents experimental results and discussion on characterization of microencapsulated paraffin wax, the effect of thermal cyclic test on thermal properties and heat transfer in fluidized bed heat exchanger Chapter 5 summarizes the conclusions of the present work and recommendation for future work

CHAPTER 2 LITERATURE REVIEW

In this chapter, literature review on the most challenging techniques of thermal energy storage and the advantages and disadvantages of each storage techniques are presented Moreover, the study of heat storage materials, encapsulation of PCMs and encapsulation techniques, laboratory test of freeze-melt behaviour of recent researches on heat transfer enhancement of PCMs has been included

2.1 Thermal energy storage

Renewable energy is an intermittent energy source For example, intermittence of solar energy is caused by day-night cycles, seasons and weather conditions Similar problems arise for waste heat recovery systems, where the waste heat availability and utilization periods are different Therefore, thermal energy storage (TES) is an essential technique for thermal energy utilization to solve the intermittence problems and levelling energy supply and demand A large volume of TES materials can store the entire daily and annual energy requirement The optimum size is mainly dependent upon meteorological conditions, storage temperature, storage heat losses, economic viability of storage medium, collector area and efficiency (Rosen, 1992)

Irrespective of their sizes, all TES system must satisfy certain characteristics The desired characteristics of TES are as follows:

ƒ compact, large storage capacity per unit mass and volume;

ƒ capability to charge and discharge with largest heat input/output rates but without large temperature gradients;

ƒ able to undergo large number of charging/discharging cycles without loss in performance and storage capacity;

ƒ small self-discharging rate i.e negligible heat losses due to surroundings;

2.2 Thermal energy storage techniques

For thermal energy storage, there are two alternatives:

ƒ sensible heat utilization;

ƒ latent heat utilization

An overview of major TES techniques is presented in Figure 2.2 The storage techniques, materials and their advantages and disadvantages are described in the following section

Thermal energy storage

Selection of materials in appropriate

Short term behavior

Long term behavior

Experimental research

Laboratory models

Prototypes

Pilot Plants

Incorporation into

heating/cooling systems

Field tests

Figure 2.1 Areas of research in thermal energy storage systems (Zalba et al., 2003)

Commercial product Thermal cycle

Figure 2.2 Thermal energy storage strategies (Zalba et al., 2003)

Thermal energy storage

Eutectics single temperature

Mixtures temperature interval

Paraffins (alkanes mixtures)

salts

Commercial grade

Analytical grade Thermo-

chemical

Sensible heat storage

Sensible heat storage medium is carried out by adding energy to a material to increase its temperature without changing its phase The amount of heat released or absorbed (Q), as the medium is cooled or heated between temperatures T1 and T2, can be mathematically illustrated by Equation 2.1

Q = the amount of heat released or absorbed (J)

m = the mass of heat storage/release material (g)

cp = specific heat capacity of heat storage/release material (J kg-1°K-1)

T1 and T2 = initial and final temperatures of heat storage medium (°C)

Sensible heat storage media can be classified on the basis of storage media as (1) liquid storage media (water, oil-based fluids, molten salts, etc.) and (2) solid media storage (rocks, metals and others) (Duffy and Beckman, 1989)

Water as storage material has the advantages of being inexpensive and readily available,

of having excellent heat transfer characteristics Hot water is required for washing, bathing, etc and it is commonly employed in radiators for space heating Water also can

be used as storage and as a transport medium of energy in a solar energy system Consequently, it is the most widely used storage medium today for solar based warm water and space heating applications However, its major drawbacks include difficulties:

insulation and pressure withstanding containment for high temperature applications and (3) large size and large temperature swing during the addition and extraction of energy (Wyman et al., 1980)

The most commonly proposed substitutes for water are petroleum based oils and molten salts The heat capacities are 25-40% of that of water on a weight basis However, these substitutes have lower vapor pressure than water and are capable of operating at high temperatures exceeding 300°C However, it can be limited due to stability and safety reasons and high cost In addition, it is highly corrosive, and there is a difficulty in containing it at high temperatures (Hasnain, 1998)

For a low as well as high temperature thermal energy storage, solid materials such as rocks, metals, concrete, sand and bricks etc can be used In this case, the energy can be stored at low or high temperatures, since these materials will not freeze or boil The difficulties of high vapor pressure of water and the limitations of other liquids can be avoided by storing thermal energy as sensible heat in solids Moreover, solids do not leak from the container

The pebble bed or rock pile consists of a bed of loosely packed rock material through which the heat transport fluid can flow The thermal energy is stored in the packed bed by forcing heated air into the bed and utilized again by recirculating ambient air into the heated bed The energy stored in a packed bed storage system depends, apart from the

Probably more important than rock size is uniformity of size If there is too much variation, the smaller stones will fill in the voids between the larger stones, thus increasing air blower power requirement When those types of rock tend to scale and flake, the resulting dust will be picked up by the heat transfer air and either clogs the furnace filters and, if the furnace is by-passed, dust is blown directly into the heating area (Hasnain, 1998)

Latent heat storage

The term “latent heat storage” can be generally described as the storage of heat in the form

of latent heat of fusion, vaporization and sublimation that can undergo phase separation at

a desired temperature level The heat storage process using such a phase-change medium can be represented mathematically, by the following Equation 2.2

T

mcpdT Hfusion

m mcpdT

Q (2.2)

Q = total amount of heat storage/release (J)

m = mass of heat storage material (g)

cp = specific heat capacity of heat storage material (J kg-1 °K-1)

Tm = melting temperature of heat storage material (°C)

T1 and T2 = initial and final temperatures of heat storage medium (°C)

∆H fusion = heat required to change from solid phase to liquid phase (J/g)

In a latent heat storage system, the sensible component of the heat storage is kept low

This enables the system to be operated at low temperature resulting in high efficiency of

the solar energy collection system in renewable energy application As shown in Table

2.1, latent heat storage media (PCMs) can store large quantity of heat in a smaller weight

and volume of material in comparison with sensible heat storage media Therefore, latent

heat storage media offers the following advantages: (1) it provides high-energy storage

capacity, (2) it can operate at narrow range of temperature and (3) heat store for phase

transition is significantly greater than sensible heat

Table 2.1 Comparison of various heat storage media (stored energy = 106 KJ,

Solid-solid, liquid-gas, and solid-liquid transformations can be found in PCM Heat can be

stored as the heat of crystallization, as the substance is transformed from one solid phase

to another solid phase in solid-solid PCM Relatively few solid-solid PCMs have been

identified that have heats of crystallization and transition temperatures suitable for thermal

considered for practical applications Therefore, solid-liquid transformation is commonly utilized and the energy stored could be discharged at a constant crystallization temperature

Basic technology for latent heat storage system design should be considered Any latent heat thermal energy storage system must possess at least the three following basic components:

(1) a heat storage substance that undergoes a solid-to-liquid phase transition in the required operating temperature range and where the bulk of heat added is stored as the latent heat of fusion;

(2) a container for holding the storage substance;

(3) a heat exchanging surface for transferring heat from the heat source to the PCM and from the later to heat sink

The type of the heat-exchanging surface plays an important role in the design of the system, as it strongly influences the temperature gradients for charging and discharging of the storage Therefore, the development of a latent heat thermal energy storage system involves two essentially diverse subjects:

(1) screening of heat storage materials (PCMs);

(2) heat exchangers for better heat transfer (Abhat, 1983)

A large number of organic and inorganic substances are known to melt with a high heat of fusion in any required temperature range, e.g 0-120°C However, for their employment as heat storage materials in latent heat thermal energy storage systems, these phase change

Moreover, economic considerations of cost and large-scale availability of the materials must be considered

2.3 Heat storage materials

Within operating temperature range of 0-120°C, candidates PCMs are grouped into two subfamilies: organics and inorganics Organic families include paraffin and non-paraffin organics Paraffins are substances having a waxy consistency at room temperature Paraffins contain in them one major component called alkanes, characterized by CnH2n+2; the n-alkane content in paraffin waxes usually exceeds 75% and may reach 100% The melting point of alkanes increases with the increasing number of carbon atoms; alkanes containing 14-40 carbon atoms possess melting points between 6 and 80°C and are generally termed as paraffins Commercial waxes, on the other hand, may have a range of about 8-15 carbon numbers Table 2.2 lists thermophysical data for some technical grade paraffin wax materials, which are paraffin mixtures and are not completely refined of oil, some fatty acids and salt hydrates Physical properties are also included in this table

Paraffins qualify as heat-of fusion storage materials due to their availability in a large temperature range and their reasonably high heat of fusion Furthermore, they are known

Table 2.2 Physical properties of some PCMs (Abhat, 1983)

(%)

Freez- ing point range (°C)

Thermal conduc- tivity (solid phase) (W/m K) C16-

-

- data not available, s-solid, l-liquid

to freeze without supercooling Due to cost considerations, however, only technical grade paraffins may be used as PCMs in the latent heat stores

Fatty acids are organic compounds characterized by CH3(CH2)2nCOOH with heat of fusion values comparable to that of paraffins Fatty acids are known to possess a reproducible melting and freezing behaviour and freeze with little or no supercooling They, hence, qualify as good PCMs Their major drawback, however, is their cost, which

is 2-2.5 times higher than that of paraffins (Lane and Glew, 1975)

Salt hydrates, characterized by M.nH2O, where M is an inorganic compound, form an important class of heat storage substances due to their high volumetric latent storage density In fact, their use as PCMs has been propagated as early as 1947 (Telkes, 1952)

The major problems in using salt hydrates as PCMs is the most of them melt incongruently, i.e they melt to a saturated aqueous phase and a solid phase which is generally a lower hydrate of the same salt Due to density differences, the solid phase settles out and collects at the bottom of the container, a phenomenon is called decomposition Unless special measures are taken, this phenomenon is irreversible, i.e during freezing, the solid phase does not combine with the saturated solution to form original salt hydrate

Another serious problem with salt hydrates is their poor nucleating properties resulting in

supercooling or reducing it to a minimum Typical methods suggested in the literature for this purpose are

(1) addition of nucleating agents that have a crystal structure similar to that of the parent substance (Telkes, 1952)

(2) using a “cold finger” in the PCM (Lorsch et al., 1975)

(3) promoting heterogeneous nucleation by using (rough) metallic heat exchanging surfaces in contact with salt hydrate (Abhat et al., 1981)

A comparison of the advantages and disadvantages of organic and inorganic materials is shown in Table 2.3 It can be concluded from the information complied that the main characteristics required of phase change materials are those indicated in Table 2.4

Organics have more advantages than inorganics, however, most of organics also present some problems, which limit their practical use Organic paraffin wax has low thermal conductivity, therefore, during the discharging process, as a material solidifies onto heat transfer surface, high thermal resistance is offered Hence, during heat exchanging, diffusion of heat transfer fluid is limited Large volume change during phase transition is another problem with some PCM such paraffin wax In addition, most conventional paraffin waxes are slightly sticky and can stick together to form large lumps, clogging occurs in heat storage system The above limitations can result in decreasing of energy

Table 2.3 Comparison of organic and inorganic materials for heat storage (Zalba, 2003)

Organics Inorganics Advantages

No corrosiveness

Low or none undercooling

Chemical and thermal stability

Disadvantages Lower phase change enthalpy

Lower thermal conductivity

Inflammability

Advantages Greater phase change enthalpy

Disadvantages Undercooling Corrosion Phase separation Phase segregation Lack of thermal stability

Table 2.4 Important characteristics of energy storage materials (Zalba, 2003)

Thermal properties Physical properties Chemical properties Economic properties

liquid and solid

phases (although not

always)

Low density variation

High density

Small or no undercooling

Stability

No phase separation Compatibility with container materials Non-toxic, non-flammable, non-polluting

Cheap and abundant

storage capacity Attempts have been made to address the problems by several researchers To overcome these problems, all preliminary studies show that paraffin wax should be bound within secondary supporting structure by means of microencapsulation (Hawlader et al., 2000) They reported that microencapsulated paraffin wax provides matrix type capsule in which paraffin wax is homogeneously distributed This could lead

to prevention in loss of heat storage capacity

2.4 Factors affecting the energy storage capacity of PCM

Oxidation of PCMs (e.g paraffin wax) results in formation of toxic compounds such as aldehyde, ketones, carboxylic acid, that can dissolve in pure PCM forming PCM solution Thus, oxidation of PCM leads to reduction in energy storage capacity, decrease in phase change temperature and broaden transition temperature range So it can be concluded that coating materials are required to prevent oxygen diffusion

Most inorganic salt hydrates and water-soluble organic PCMs are hygroscopic substances,

so they can absorb moisture easily forming higher hydrates resulting in decrease in energy storage capacity at its phase transition temperature Therefore, they need protective coating to reduce moisture gain Recently, encapsulation of PCMs has been developed to solve the above mentioned problems

2.5 Encapsulation of phase change materials

The encapsulation of the PCM has developed interest in several researchers (Zalba, 2003) Microencapsulated PCM means techniques in which the coating of the core PCM with

presented in the Figure 2.3 Encapsulation can be of many different forms such as a simple coating, a wall of spherical or irregular shaped, a multiwall structure with walls of the same or varying compositions or numerous cores agglomerate within the same walled

Figure 2.3 Various forms of capsules (Gibbs et al., 1999)

structure The matrix type of capsules contains matrix microparticles resembles that

of a peanut cluster where the core materials are buried into varying depths inside the wall material The matrix or capsule provides all the requirements of a container such as moisture barrier and physical containment (Johnson, 1984; Langer et al., 1969)

Coating materials used in the encapsulation of PCMs should meet the following characteristics:

ƒ Have high strenth, flexibility and thermal stability;

Simple

Irregular

Multi-wall

ƒ Capable of being used safely;

ƒ High thermal conductivity;

ƒ Not corrosive to container materials;

ƒ No migration of PCMs into coating materials;

ƒ No reaction between PCMs and coating materials

The advantages of heat storage with encapsulated PCM are the following:

ƒ Heat storage does not absorb heat energy directly, so the shape of storage is arbitrary, temperature gradient is more favorable, and longer PCM lifetime;

ƒ Bound PCM is always “dry” product and liquid handling is consequently eliminated as the phase change occurs within the coating material;

ƒ Air is directly taken into heat exchange surfaces, no additional media is needed;

ƒ Large heat transfer area is provided for effective heat transfer;

ƒ Large quantities of thermal energy can be stored and released at a relatively constant temperature without significant volume changes Sufficient free space exists within the supporting structure and size remains constant;

ƒ Meet the various needs for energy storage and suitable containment systems;

ƒ No expansion system is needed as the PCM propagate and contract directly in the microcapsules;

ƒ The microcapsules’ embedment in heat storage is simple;

ƒ PCM with different melting temperature could be used;

ƒ Avoid problems such as supercooling and phase separation

Successful utilization of encapsulated PCM heat storage media depends on developing means of containment Published encapsuletd PCM systems are summarized in Table 2.5 Most research efforts have centered on “microencapsulation”, the packaging of a mass of

a PCM in a sealed container, which itself serves as the heat exchange surface Different encapsulant materials were examined for suitability (Lane, 1977) The most promising film was a laminate consisting of inner film of heat-sealable polyethylene, a foil of aluminium, and outer film of PET polyester The plastic-aluminium foil laminate was not suitable for the organic PCMs tested Typically, the heat-sealed seam was attacked by the organic material It was also unsuitable for temperatures above about 70°C (Lane et al., 1978) For inorganic PCMs operating below this temperature, it seemed applicable Organic heat storage compositions were also rejected in the case of plastic as encapsulants Those tested migrated into plastic (Lane, 1980)

For many applications, PCMs are microencapsulated, and have been studied and developed by many researchers However, studies on encapsulated paraffin wax are extremely limited in scope, and the potential use of microencapsulated paraffin wax in various thermal control applications is not widely available Sanjay (1991) reported the existence of limitations in the formation of encapsulated PCMs due to leakage problem

Table 2.5 List of published encapsulated PCM systems

Mg(NO 3 ) 2 6H 2 O- MgCl 2 6H 2 O

polyester film

macrocapsules Preheating

domestic water

in a tank filled with encapsulated PCM Morikama et

al., 1985

Inorganic salt hydrate

Polyester Interfacial

polymerization

Matrix type microcapsules

Thermal control in the buildings Feldman et al.,

1989

Organic PCM 30% wt, gypsum, cement, sawdust, sand and water

Polyester resin Interfacial

polymerization

Floor, wall, ceiling tiles as composite materials

Storing peak electricity

Matrix type microcapsules

Integrating with building materials to reduce overheating in summer and to take effect storage discharge by ventilation Brown et al.,

1998

octadecane and paraffin

urea, cross-linked nylon, and gelatin

polymethylene-Interfacial polymerization

Microcapsules Gas-fluidized

bed Salyer, 1999 Eutectic PCM Polyester resin Interfacial

polymerization

Matrix type microcapsules

Insulation materials for using in clothing or bedding articles

Xiao et al.,

2000

Paraffin wax

Styrene-butadiene-styrene copolymer

Interfacial polymerization

Matrix type macrocapsules

Latent heat storage materials for thermal storage units Hawlader et

Packed bed heat exchanger

after 100 thermal cycle test runs (Bo et al., 1979; Hart and Thorton, 1982; McMahon, 1982; Colvin, 1986; Colvin 1989) Limited reports have been published on encapsulated paraffin wax

2.6 Methods of Microencapsulation

Various techniques are used for encapsulation (Dziezak, 1988) In general, three steps are involved: formation of the wall around the material, ensuring the leakage does not occur, and ensuring that the undesired materials are kept out The following encapsulation

methods will be discussed in the following sections: air suspension, coacervation-phase separation, solvent evaporation technique, spray-coating, interfacial polymerization and dip coating

Air Suspension

This method known as the Wurster process or fluidized bed coating involves dispersing solid particulate core materials in a supporting air stream and the spray coating of suspending materials The design of the chamber and its operating parameters effect recirculating flow of the particles through the coating zone of the chamber, where a coating material, usually a polymer solution, is sprayed onto the fluidized particles The cyclic process is repeated till the desired coating thickness is obtained The process is generally considered to be applicable to the encapsulation of solid core materials Extensive research has been carried out using this method of encapsulation (Zarn, 1995)

Coacervation-Phase Separation

Encapsulation by coacervation is one of the more popular methods commonly studied Coacervation is the separation of a macromolecular solution into two immiscible liquid phases: a dense coacervate phase, which is relatively concentrated in the macromolecules, and a dilute equilibrium phase The general outline of the process consists of three steps carried out under continuous agitation

Step 1 – The core material is dispersed in a solution of coating polymer

Step 2 – Deposition of the coating, accomplished by controlled, physical mixing of the coating material and the core material in aqueous phase

Step 3 – Rigidization of the coating by thermal, cross-linking or desolvation techniques, to form self-sustaining microcapsules

There are two types of coacervation: simple and complex Simple coacervation involves the use of only one colloid, e.g., gelatin in water, and involves removal of the associated water from the dispersed colloid by agents with a greater affinity for water, such as various alcohols and salts

Simple coacervation is induced by a change in conditions such as the addition of solvent, the addition of microions or a temperature change resulting in molecular dehydration of the macromolecules Complex coacervation is driven by electrostatic

non-Complex coacervation is a common method of microencapsulation Burgenberg de Jong and Kruyt (1929) showed that the solid particles could also be entrapped in coacervated systems On phase separation by complex coacervation, tiny coacervate droplets are formed which coalescence and sendiment to form a separate coacervate phase If a core material is present in a polyion system prior to complex coacervation, then the coacervate will deposit on and coalesce around those particles Agitation of the coacervate system by stirring or other means can prevent coalescence and sedimentation of the coacervate droplets The coacervate droplets can be crosslinked to form stable microcapsules by addition of a crosslinking agent, such as gluteraldehyde, or the use of heat

Complex coacervate formation is dependant on a number of factors such as pH, ionic strength, macromolecular weight, concentration and mixing ratio Charge is the most significant factor for complex coacervation Complex coacervation system includes gelatin-acacia, carbopol-gelatin, pectin-gelatin, gelatin-gelatin, sodium carboxymethylcellulose-gelatin

Although successful coacervate microencapsulation systems have been done at specific

pH, they require stabilization by the use of cross-linking agent or heat and the extent of crosslinking determines the retention of encapsulant Gluteraldehyde and formaldehyde are commonly used as crosslinking agents for protein-polysaccharide complex coacervate systems such as gelatin-acacia A condensation reaction occurs between the amino groups

of the protein and the aldehydes Since the core materials are microencapsulated while