Encapsulation of phase change materials for heat storage
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ENCAPSULATION OF PHASE CHANGE MATERIALS (PCMS)
FOR HEAT STORAGE
MYA MYA KHIN
(B.E., Yangon Technological University)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF CHEMICAL & ENVIRONMENTAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2003
Acknowledgement
ACKNOWLEDGEMENT
I wish to express my thanks to the following institution and persons, without whose
assistance and guidance this thesis would not have been possible.
To the National University of Singapore, for the postgraduate research
scholarship, without which I would not be able to continue my higher degree
studies.
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.
To all the technical and clerical staff in the Chemical & Environmental
Engineering Department for their patience and help.
To Dr Zhu Haijun for giving me some informations and literature for this project.
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.
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.
i
TABLE OF CONTENTS
ACKNOWLEDGEMENT
i
TABLE OF CONTENTS
ii
SUMMARY
v
LIST OF FIGURES
viii
LIST OF TABLES
xi
CHAPTER 1 INTRODUCTION
1
1.1 General background
1
1.2 Objective and scope of thesis
4
CHAPTER 2 LITERATURE REVIEW
6
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
CHAPTER 3 MATERIALS AND EXPERIMENTAL
METHODS
3.1 Materials
34
36
36
ii
3.2 Characteristics of core and coating materials
37
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
CHAPTER 4 RESULTS AND DISCUSSION
54
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
4.6 Thermal cyclic test
75
iii
4.7 Structural stability
88
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
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
96
5.1 Conclusions
96
5.2 Recommendations
99
REFERENCES
100
APPENDICES
113
iv
SUMMARY
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
microcapsules were higher than those of the spray-dried samples. Energy storage/release
v
capacities were found to be in the range of 91-239 J/g for microencapsulated PCMs
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
vi
to 2000 cycles. DSC analysis was carried out to measure energy storage/release
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.
vii
LIST OF FIGURES
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
paraffin wax
43
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
agent on encapsulation efficiency at 2:1 core to coating ratio
(CH3CHO)
59
Figure 4.5: Effect of homogenizing time and the amount of cross-linking
agent on encapsulation efficiency at 1:1 core to coating ratio
(CH3CHO)
60
Figure 4.6: Effect of homogenizing time and the amount of cross-linking
agent on encapsulation efficiency at 1:2 core to coating ratio
(CH3CHO)
60
viii
Figure 4.7: The output curve of DSC for spray-dried sample 2:1
67
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
paraffin wax (spray-dried 1:1) after 500 cycles
84
Figure 4.22: DSC measurement of melting temperature of microencapsulated
paraffin wax (spray-dried 1:1) after 2000 cycles
84
Figure 4.23: DSC measurement of melting temperature of microencapsulated
paraffin wax (coacervated 2:1) at 0 cycle
85
Figure 4.24: DSC measurement of melting temperature of microencapsulated
paraffin wax (coacervated 2:1) after 2000 cycles
85
Figure 4.25: DSC measurement of specific heat capacity of microencapsulated
paraffin wax (spray-dried 1:2) at 0 cycle
86
Figure 4.26: DSC measurement of specific heat capacity of microencapsulated
paraffin wax (spray-dried 1:2) after 1000 cycles
86
ix
Figure 4.27: DSC measurement of specific heat capacity of microencapsulated
paraffin wax (coacervated 1:2) after 500 cycles
87
Figure 4.28: DSC measurement of specific heat capacity of microencapsulated
paraffin wax (coacervated 1:2) after 2000 cycles
87
Figure 4.29: FTIR output curve for spray-dried samples with 1:1 core to
coating ratio
90
Figure 4.30: FTIR output curve for coacervated samples with 1:1 core to
coating ratio
90
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
x
LIST OF TABLES
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%
formaldehyde
55
Table 4.3: Encapsulation efficiency of coacervated capsules with 50%
gluteraldehyde
56
Table 4.4: Non-linear regression analysis of encapsulation efficiency using
formaldehyde
62
Table 4.5: Non-linear regression analysis of encapsulation efficiency using
gluteraldehyde
62
Table 4.6: Encapsulation efficiency of spray-dried samples
63
Table 4.7: Comparison of experimentally measured core to coating ratio
with designed values
64
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
paraffin wax (2:1 coacervated sample)
77
Table 4.10: Energy storage and release capacities for microencapsulated
paraffin wax (1:1 coacervated sample)
77
Table 4.11: Energy storage and release capacities for microencapsulated
paraffin wax (1:2 coacervated sample)
77
xi
Table 4.12: Energy storage and release capacities for microencapsulated
paraffin wax (2:1 spray-dried sample)
78
Table 4.13: Energy storage and release capacities for microencapsulated
paraffin wax (1:1 spray-dried sample)
78
Table 4.14: Energy storage and release capacities for microencapsulated
paraffin wax (1:2 spray-dried sample)
78
Table 4.15: Thermophysical properties of microencapsulated paraffin
wax (coacervated 2:1) with test cycles
82
Table 4.16: Thermophysical properties of microencapsulated paraffin
wax (coacervated 1:1) with test cycles
82
Table 4.17: Thermophysical properties of microencapsulated paraffin
wax (coacervated 1:2) with test cycles
82
Table 4.18: Thermophysical properties of microencapsulated paraffin
wax (spray-dried 2:1) with test cycles
83
Table 4.19: Thermophysical properties of microencapsulated paraffin
wax (spray-dried 1:1) with test cycles
83
Table 4.20: Thermophysical properties of microencapsulated paraffin
wax (spray-dried 1:2) with test cycles
83
xii
Chapter 1
Introduction
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
nearly constant temperature (Abhat, 1983).
1
Chapter 1
Introduction
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, nonparaffin organic acids, clathrates and eutectic organic and inorganic compounds (Lane,
1986). Among those materials, paraffin wax offers more desirable properties such as nonpoisonous, 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.
2
Chapter 1
Introduction
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
industry, food industry, biomedical field, coating of PCMs for better heat storage system
and so on. The advantages of using microencapsulated paraffin wax in fluidized bed heat
3
Chapter 1
Introduction
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:
4
Chapter 1
Introduction
(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.
5
Chapter 2
Literature Review
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:
compact, large storage capacity per unit mass and volume;
heat storage medium with suitable properties in the operating temperature range;
6
Chapter 2
Literature Review
capability to charge and discharge with largest heat input/output rates but without
large temperature gradients;
able to undergo large number of charging/discharging cycles without loss in
performance and storage capacity;
small self-discharging rate i.e. negligible heat losses due to surroundings;
long life;
inexpensive.
The research areas for TES systems are shown in the Figure 2.1. New TES concepts and
improvements required in the performance of TES system, the design of compact TES
systems, and the use of TES in practical energy applications are outlined. Research on
TES has been broad based and productive, and directed towards the resolution of specific
TES issues and new TES material.
2.2 Thermal energy storage techniques
For thermal energy storage, there are two alternatives:
sensible heat utilization;
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.
7
Chapter 2
Literature Review
Thermal energy storage
Materials research
Heat exchanger development
Selection of materials in appropriate
range
Selection of type of exchanger and
parameters
Thermal analysis
Thermal storage material
Thermophysical
property data
Melting-solidifying
characteristics DSC
(Differential Scanning
Calorimetry)
TA (Thermal
Analysis)
Construction of material
Compatibility of
materials
Simulation
Experimental
research
Laboratory models
Prototypes
Short term behavior
Pilot Plants
Long term behavior
Thermal cycle
Useful life
Incorporation
into
heating/cooling
systems
Field tests
Final cost analysis
Commercial product
Figure 2.1 Areas of research in thermal energy storage systems (Zalba et al., 2003)
8
Chapter 2
Literature Review
Thermal energy
storage
Thermochemical
Sensible heat
Latent heat
Gas-liquid
Solid-gas
Solid-liquid
Solid-solid
Organics
Eutectics single
temperature
Inorganics
Mixtures
temperature
interval
Paraffins
(alkanes
mixtures)
Commercial
grade
Eutectics single
temperature
Fatty acids
Mixtures
temperature
interval
Hydrated
salts
Analytical
grade
Figure 2.2 Thermal energy storage strategies (Zalba et al., 2003)
9
Chapter 2
Literature Review
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.
T2
Q = ∫ mcpdT
(2.1)
T1
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:
(1) system corrosion and leakage, (2) due to its high vapor pressure, it requires costly
10
Chapter 2
Literature Review
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
thermophysical properties of the material, on several parameters, including rock size and
shape, packing density, heat transfer fluid etc.
11
Chapter 2
Literature Review
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.
Tm
Q=
∫
T1
T2
mcpdT + m∆Hfusion + ∫ mcpdT
(2.2)
Tm
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)
12
