solar thermal storage system
TRANSCRIPT
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SOLAR THERMAL STORAGE SYSTEM
A PROJECT REPORT
Submitted by
MD. SADIQ. B
MOHAMMED FAIZAN. A
In partial fulfillment for the award of the degree
Of
BACHELOR OF ENGINEERING
In
MECHANICAL ENGINEERING
PRIYADARSHINI ENGINEERING COLLEGE, VANIYAMBADI
ANNA UNIVERSITY :: CHENNAI 600 025
MAY 2006
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ABSTRACT
Increasing energy consumption, shrinking resources and rising
energy costs will have significant impact on our standard of living for
future generations. In this situation, the development of alternative, cost
effective sources of energy has to be a priority. Designing energy
efficient and affordable dwellings located in harsh climate regions
present significant challenges.This project presents the advanced
technology and some of the unique features of a novel solar system that
utilizes solar energy for space heating and water heating purpose in
residential housing and commercial buildings.
The improvements in solar technology offers a significant cost
reduction, to a level where the solar system can compete with the energy
costs from existing sources.The main goal of the project is to investigate
new or advanced solutions for storing heat in systems providing heating.
which can be achieved using phase change material(PCM).A phase
change material with a melting/solidification temperature of 50C to
60C is used for solar heat storage. When the PCM undergoes the phase
change, it can absorb or release a large amount of energy as latent heat.
This heat can be used for further applications like water heating and space
heating purposes. Thus solar thermal energy is widely used for space
heating and domestic water heating .
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CHAPTER 1
INTRODUCTION
1.1 UTILISATION OF RENEWABLE ENERGY
The aim of the project is to utilise the renewable energy wasted in
large amount. One of the major renewable energy resources is the solar
energy which sun emits to the earth. The solar energy can be utilised to a
higher extent in areas having harsh climatic conditions like India.
The sun emits solar radiation as much as 1395 W/m2. To utilise
this energy solar collectors are used. The most economic and efficient
solar collector is the flat plate collector which absorbs solar radiation and
the heat is transferred to the water inside the tubes of the collector.
Further, to store the energy and utilise later, phase change material(PCM) is used which absorbs large amount of heat from water inside the
tank and releases its latent heat when the temperature of PCM reaches its
melting point.
1.2 PRINCIPLE OF HEAT RECOVERY FROM SOLAR ENERGY
Solar energy is recovered using a collector with a tank connected to
it through hoses which forms a closed circuit. The principle behind the
Solar Thermal Storage System (STSS) is the Gravity Convection which is
the natural movement of heat that occurs when a warm fluid (water) rises
and a cooler fluid sinks under the influence of gravity. This is also called
Thermosyphoning.
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1.3 NECESSITY OF THERMAL STORAGE SYSTEM
Thermal storage units have received greater attention in solar and
waste heat recovery thermal applications because of its large heat storage
capacity and their isothermal behaviour duringcharging and discharging
process. The major technical constraint, which prevents successful
implementation of heat recovery system, is intermittent and time
mismatched demand and availability. In order to overcome the above
constraint, thermal storage unit can be implemented. Thermal energy
storage provides one practical means of storing energy during the
availability and use this energy when need arises.
1.4 TYPES OF THERMAL STORAGE SYSTEM
Thermal energy storage can be achieved in the form of sensible
heat of a solid or liquid medium, latent heat of a phase change material or
by a chemical reaction. The choice of storage media depends on the
amount of energy to be stored in unit volume or weight of the medium
and temperature range at which it is required for a given application.
1.4.1 Sensible Heat Storage System
The commonly used material in the sensible heat storage system
are water, pebble beds, packed solid beds, refractory materials,
hydrocarbon oils, organic and metallic salts. The main advantage of the
sensible heat storage system is easy recovery of energy, as the surface
convective heat transfer coefficient is very high. However the sensible
heat storage materials have very low heat capacity per unit volume.
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1.4.2 Latent Heat Thermal Storage System
Latent Heat Thermal Storage System (LHTS) unit is particularly
attractive due to its high energy storage capacity and its isothermal
behavior during charging and discharging processes.
1.4.3 Combined Storage System
Sensible heat storage does not exhibit isothermal charging and
discharging and is of low heat capacity. Although these drawbacks are
overcome in a latent heat unit, they are not commercially used. The main
reason is that during phase change, the solid-liquid interface moves away
from the convective heat transfer surface due to which the thermal
resistance of the growing layer of solidified medium increases, thereby
resulting in poor heat transfer rate. Following are the advantages of using
combined storage system.
Isothermal charging and discharging
Higher heat capacity
Less reduction in heat transfer rate due to poor thermal
conductivity of the solid medium.
Compact size
Economy of operation
1.5 STORAGE MATERIALS
One major area in the field of thermal storage is the material
investigation. The various criteria that govern the selection of storage
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materials and the properties of various sensible and latent heat storage
materials are given in this section.
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1.5.1 Sensible Heat Storage Materials
Desirable characteristics of the sensible storage material include the
following:
High thermal heat capacity
High thermal diffusivity
High density
Reversible heating and cooling
Chemical and geometrical stability
Non combustible, non corrosive and non toxic
Low vapour pressure to reduce the cost of containment
Low cost of material and storage unit fabrication
The properties of some sensible heat storage material are given in
Table 1.1
Table 1.1 Properties of Sensible Heat Storage Materials
Material Density
kg/m3
Specific heat
kJ/kgK
Volumetric specific heat
MJ/m3K
Water 1000 4.20 4.20Scrap Iron 7800 0.46 3.60
Scrap Aluminium 2700 0.92 2.50
Rock 2000 0.90 1.80
Brick 2000 0.90 1.80
Feolite 3900 0.92 2.61
1.5.2 Latent Heat Storage Materials
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The term latent heat storage as we generally understand it today,
applies to the storage of heat as the latent heat of fusion in suitable
substances that undergo melting and freezing at a temperature level.
Consequently it is also often called the Heat-of-fusion storage. Typical
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Heat-of-fusion storage substances well known to all of us are ice, paraffin
or Glaubers salt.
The term latent heat storage may also be applied to include
taerythritol, wherein heat is stored as the heat of crystallization, as the
substance is transformed from one solid phase to another. The stored heat
is recovered in alike wise manner as the original solid phase is regained.
Excluded in the present definition of latent heat storage is, however, the
heat stored in materials that undergo a liquid-to-vapour phase transition,
e.g. water-to-steam. Although the later phase transitions are associated
with a phase transition that is almost an order-of-magnitude higher than
that for solid-to-liquid or solid-to-solid phase change, the practicalproblems of storing a gaseous phase and the necessity of pressurized
containers for this purpose rule out their potential utility.
The review article relates to the discussion of heat-of-fusion
storage, a technique range of 0-120 degree Celsius to cover variety of low
temperature applications, such as domestic hot water production, direct or
heat pump assisted space heating, green house heating, solar cooling, etc,.is considered. It should however, be emphasized here, that although heat
storage in solid-solid phase transition is much less understood today, it
does hold out future promises.
1.5.3 Principle of Latent Heat Energy Storage
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Latent heat is the quantity of energy which needs to be absorbed or
released when a material changes its phase from solid to liquid termed as
fusion (melting) or from liquid to solid state termed as crystallization
(freezing). These phase changes take place at constant temperature and
for certain materials the process of melting and freezing can be repeated
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For an unlimited number of cycles with no change in their physical and
chemical properties.
The following are the necessary criteria for selection of PCM:
The material should have large heat of fusion
The material should have a congruent melting point
The material should not super cool, i.e., during the cooling of
liquid phase of the material, the melt should solidify at the
thermodynamic melting point.
The material should be stable
The material should not interact with the container
The material should not be dangerous
The material should be cheap and readily available
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.
0oC 120
oC. However, for the employment as heat storage materials in
LHTS systems, phase change materials must exhibit certain desirable
thermodynamic, kinetic and chemical properties. Moreover, economic
considerations of cost and large-scale availability of the materials must be
considered.
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1.6 VARIOUS CRITERIA
The various criteria that govern the selection of phase heat storage
are summarized below.
1.6.1 Thermodynamic criteria
The phase change materials must possess:
a) A melting point in the desired operating temperature range.
b) High latent heat of fusion per unit mass, so that a lesser amount of
material stores a given amount of energy.
c) High density, so that a smaller container volume holds the material.
d) High specific heat that provides additional sensible heat storage
effect and also avoid sub cooling.
e) High thermal conductivity so that the temperature gradient required
for charging the storage material is small.
f) Congruent melting: The material should melt completely so thatthe liquid and solid phases are identical in composition. Otherwise
the difference in densities between solid and liquid cause
segregation, resulting in changes in the chemical composition of
the material.
1.6.2 Kinetic criteria:
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It should exhibit little or no super cooling during freezing. The
melted material should crystallize at its freezing point. This is achieved
through a high rate of nucleation and growth rate of the crystals.
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1.6.3 Chemical criteria:
The phase change material should show
a) Change stability
b) No chemical decomposition, so that the LHTS system life is
assured.
c) No corrosiveness to construction material.
d) The material should be non-poisonous, non-flammable and non-
explosive.
It is quite apparent that no single material can fully satisfy the long list
of criteria mentioned above. Trade off is hence made in selection of heat
storage materials. Within the operating temperature range PCMs are
grouped into the families of organic and inorganic compounds and their
eutectics as shown in Fig1.1 subfamilies of organic compound include
paraffin and non-paraffin.
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Fig. 1.1 Classification of Phase Change Materials
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Paraffin wax consists of primarily straight chain hydrocarbons.
Paraffin contains in them one major component called alkanes, CnH2n+2.
Pure paraffin contains only alkanes. For example paraffin ocradecane
(C18H38). Alkanes containing 14 to 40 carbon atoms possess melting point
between 6o
to 80oC and are generally termed as paraffin. Commercially
waxes may have a range of about 8-15 carbon atoms.
Paraffins qualify as heat of fusion storage material due to their
large availability in a large temperature range and their reasonably high
heat of fusion. The heat of fusion and recrystallization of paraffin sum
upto about 210 to 252 kJ/kg and the temperature range of fusion point
vary from 20o
to 99oC.
For moderate temperature region salt hydrates are most suitable.
During their melting process, high latent of fusion is absorbed. Few
examples are CaC12.6H20, Na2SO4.10H2O (Glaubers salt) etc.
The latent heat storage system has the advantage that, phase changematerials are available for practically all temperature range of operation.
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The compounds with different composition can also achieve required
fusion temperature. When using eutectic systems various transition
temperatures may be achieved with the same group of substances. The
eutectic substances always produce lower fusion points than the pure
components.
1.6.4 Paraffin
Paraffins consisting of a waxy consistency at room temperature is
grouped in the organic family. The substances are made up of straight
chain hydrocarbons with 2-methyl branching groups near the end of
chain. The paraffins are classified into two main groups, even chained
(n-paraffin) and odd chained (iso-paraffin). Whether paraffin falls into an
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Even chain or odd chain group is dependent upon the content of alkanes
within the substance (ranging from 75-100%).
The melting point of paraffins is directly related to the number of
carbon atoms within the material structure with alkanes containing 14-40
carbon atom possessing melting points between 6 and 80 degrees
centigrade. These are termed pure paraffins and should not be confused
with paraffin waxes. Paraffin waxes contain only 8-15 carbon numbers
with lower melting points than pure paraffins at 2-45 degree celsius.
When paraffins reach their melting points an allotropic
modification takes place with the material being soft and plastic with
individual crystals being needle shaped. Additionally a second allotropic
modification occurs below melting point forming a brittle hard structure
(similar to that of a section of unit candle) with disc shaped crystals.
Paraffins form an ideal PCM candidate for residential heating
applications due to there large temperature range and there various forms
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of structure allowing specific paraffins to be selected for a certain
temperature range.
The material being of an organic compound is cheap and in huge
quantity. The storage capacity is relatively compared with other
compounds, plus the materials are proven to freeze without super cooling
(the entire material content change phase resulting in maximum thermal
capacity without any segregation over longevity).
The thermo physical properties of paraffin are given in Table 1.3.
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Table 1.2 Thermo Physical Properties of Paraffin
S.NO. PROPERTY VALUE
1 Latent heat of fusion 214 kJ/kg
2 Specific heat capacity 2.9 kJ/kgK
3 Thermal conductivity 0.2 W/Mk
4 Density
Solid
Liquid
850 kg/m3
750 kg/m3
5 Phase transformation temperature
range ( solid-liquid ) 50-60oC
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CHAPTER 2
LITERATURE REVIEW
In this part an attempt has been made to review the literature for
analysis of this study. The present energy crisis has focused international
attention upon the different use of available supplies of energy.
A lot of research in the field of thermal energy storage from the
renewable energy source has being done. They mainly concentrate on the
solar energy which is wasted in huge amount.
1. Velraj .R. (1998) have recommended a combined sensible and
latent heat storage system for thermal energy storage. He has designed a
storage system for solar hot water application. He found that the one
fourth of the days requirement is stored by the water in the tank as
sensible heat, and the three fourth of the requirement is stored in the
PCM. This reduces the size of the storage system.
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2. Conventionally, sensible heat storage systems are commonly
used, where as latent heat storage units have not proved successful on a
large scale. A lot of research in the field of phase change heat storage,
especially on salt hydrates, has been done by Lane (1983). His book gives
a detailed account of the development of phase change materials (PCM),
criteria for selection of PCM and the chemical aspects of phase change
phenomena. A detailed review of low temperature phase change material
has been done by Abhat (1983).
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3. The use of paraffin as phase change material has been
investigated by Fieback and Gutberlet (1998) for the development of
compact storage units, which provide a high thermal energy storage
capacity for many thermal storage applications.
4. Annathanarayan et al., (1987), Beasley D.E. and Ramanarayanan
.C (1989) has studied about fixed bed/packed bed type of heat storage
units utilizing phase change materials.
5. Rosen M.A. (1992), has studied in details the thermodynamic
performance of thermal energy storage system. He discussed several
definitions of energy and energy efficiency for closed system for thermal
energy storage.
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6. Telkes . M., (1955), has given brief idea about most commonly
used solar energy storage techniques and how to utilize the amount of
solar energy incident on collector efficiently.
7. Fender, D.A. and J.R. Dunn (1978), has given and explained the
various steps for analyzing and testing the solar panels. They have also
described the methodology of testing the solar panels.
8. Jrinak. J.J and S.I. Abdul-Khalik, (1979), have presented paper
on the performance study of solar system utilizing phase change energy
storage and have described various factors incurred in it.
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9. Kauffman. K. and I. Gruntfeest, (1973), have given a joint report. Thisreport gives detailed study of how to use congruently melting material for
thermal energy storage.
10. Beasley D.E. and Ramanarayanan C. (1989), have jointly proposed a
journal on thermal response of a packed bed of spheres containing a
phase-change material which gives wide idea for how to use different
types of phase change materials.
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CHAPTER 3
EXPERIMENTAL SETUP
The experimental setup consists of a flat plate collector, thermal
storage system, thermocouples and temperature indicator. The schematic
diagram of the experimental setup is shown in Fig.3.1and photographic is
shown in Fig. 3.2.
3.1 SOLAR COLLECTOR
The flat plate collector has a heating capacity of 100 litres per
day. It consists of frame, absorber plate with copper tubes and cover
plate.
3.1.1 Frame
A hollow rectangular box shaped frame is used into which all the
other elements of the collector are fitted. It is made up of aluminium.
3.1.2 Absorber plates with copper tubes
The absorber plate is the main part of the collector as it absorbs
solar radiation and converts it into heat energy. It is made of aluminium
(and painted black) welded to the copper tubes. The water circulates
through the copper tubes. Both the absorber plate and the copper tubes
are painted black.
3.1.3 Cover plate
A plain glass plate is fitted above the absorber plate to transmit the
solar radiation to the blackened surface.
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Fig.3.1 Schematic Diagram of Experimental Setup
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Fig.3.2 Photographic View of the Experimental Setup
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3.2 THERMAL STORAGE TANK
The storage tank is a stainless steel vessel containing water as
sensible heat storage material and paraffin as the latent heat storage
material. Hence it is called combined sensible and latent heat storage
system. Paraffin filled containers, made of tin (coke tin) are placed inside
the tank. Each container contains 240 grams of paraffin wax. The tank is
well insulated by using fiber coir to prevent heat radiation to the
surroundings. The photographic view of storage tank is shown in Fig.3.3.
The solar thermal storage system was experimented with two
different capacities of storage tank to compare the amount of energy
stored in each tank for the same collector. The details of storage tanks 1
and 2 are given in tables 3.1 and 3.2 respectively.
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Fig.3.3 Photographic View of Thermal Storage Tank
Fig.3.4 Photographic View of Thermal Storage Tank Showing
Paraffin Filled Containers
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3.2.1 Thermal storage tank 1
The specification of storage tank 1 is given in Table 3.1.
Table 3.1 Specification of Thermal Storage Tank 1
S. No. Particulars Symbol Value
1 Height of storage tank hs1 34 cm
2 Diameter of the tank Ds1 30 cm
3 Volume of the tank Vs1 24033 cm3
4 Mass of water in the tank mw1 15 kg
5 Mass of paraffin in the tank mp1 8 kg
6 Thickness of insulation tins 0.5 cm
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3.2.2 Thermal storage tank 2
The specification of storage tank 2 is given in Table 3.2.
Table 3.2 Specification of Thermal Storage Tank 2
S. No. Particulars Symbol Value
1 Height of storage tank hs2 41 cm
2 Diameter of the tank Ds2 43 cm
3 Volume of the tank Vs2 59540 cm3
4 Mass of water in the tank mw2 30 kg
5 Mass of paraffin in the tank mp2 15 & 18 kg
6 Thickness of insulation tins 0.5 cm
3.3 LOCATION OF THERMOCOUPLE
J-type thermocouple is used to measure temperature at different
locations in the tank. All the thermocouples are connected to a
temperature indicator which provides instantaneous digital outputs. Six
thermocouples are placed in three different horizontal planes in paraffin
containers inside the storage tank. In each plane two thermocouples are
placed radially and uniformly spaced.
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The location of thermocouples in the storage tank is shown in Fig
3.4.
Fig. 3.4 Location of thermocouples in the storage tank
Where
T1, T2 - Thermocouples to measure temperature of paraffin at the
bottom of the tank.
T3, T4 - Thermocouples to measure temperature of paraffin at the middle
of the tank.
T5, T6 - Thermocouples to measure temperature of paraffin at the top
portion of tank.
Tw - Thermocouples to measure temperature of water inside the tank.
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CHAPTER 4
EXPERIMENTAL PROCEDURE
Storage tank filled with water is connected to the flat plate
collector through the high temperature resistant polymer hoses. Water
from the storage tank enters into the copper tubes of the collector and gets
heated up. No circulating pump is necessary as the mass flow is very low.
The temperature variations at different locations in the tank are
taken with respect to time.
4.1 CHARGING OF STORAGE TANK
The water in the copper tube gets heated up and water circulates by
natural circulation between the storage tank and the collector. The
principle which lies behind the system is gravity convection.
Gravity convection or thermosyphoning is a process that makes watercirculates automatically between a warm collector and a cooler storage
tank. There is a continuous heat transfer taking place between the
collector and the tank and between water and paraffin. Hence water in the
storage tank gets heated up to a high temperature. Temperature variations
for every 30 minutes time interval are taken for each tank.
The readings for charging of different storage tank are taken and
tabulated.
Table 4.1 shows the observed readings for storage tank1 during charging
of paraffin and water.
Table 4.3 shows the observed readings for storage tank 2 with 15 kg of
paraffin during charging.
Table 4.5 shows the observed readings for storage tank 2 with 18 kg ofparaffin during charging.
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4.2 DISCHARGING OF HEAT
After charging the storage tank is disconnected from the collector
and is left undisturbed with complete insulation. At regular intervals of
time the temperatures inside the storage tank are taken for a specific
period. It is seen that the temperature of water and paraffin reaches a
maximum of 95oC in storage tank 1 and 80
oC in case of storage tank 2.
The readings for discharging of different storage tanks are taken and
tabulated.
Table 4.2 shows the observed readings for storage tank1 during
discharging.
Table 4.4 shows the observed readings for storage tank 2 with 15 kg of
paraffin during discharging.
Table 4.6 shows the observed readings for storage tank 2 with 18 kg of
paraffin during discharging.
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OBSERVED READINGS IN TEMPERATURE INDICATOR:
Table 4.1 Temperature at various locations in the storage tank-1 for
different time intervals during charging
TIME
IN Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 30 30 30 30 30 30 30
30 32 32 32 33 33 33 35
60 35 35 34 37 37 36 40
90 42 41 41 45 45 44 47
120 43 43 47 46 49 47 57
150 45 45 50 50 53 52 63
180 48 48 50 53 54 53 66
210 70 71 74 75 71 71 75
240 83 85 87 87 81 79 85
270 90 91 96 93 90 89 92
300 98 99 99 98 97 95 97
330 96 97 98 97 95 94 96
360 94 95 96 95 94 92 95
390 92 92 93 95 92 91 93
Mass of Paraffin = 8 kg
Mass of water = 15 kg
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Table 4.2 Temperature at various locations in the storage tank-1 for
different time intervals during discharging
TIME
IN Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 88 87 90 89 88 87 88
20 86 86 88 87 87 86 86
40 83 83 86 85 85 84 84
60 83 82 85 84 84 84 83
80 81 81 83 83 83 82 82
100 80 80 81 81 81 80 81
120 78 78 80 80 79 78 79
140 77 76 79 79 77 77 77
160 75 75 78 78 76 76 76
180 73 73 76 74 74 74 74
200 72 71 74 73 73 72 73
220 70 70 72 71 71 70 71
240 68 68 70 69 69 69 69
Mass of Paraffin = 8 kg
Mass of water = 15 kg
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Table 4.3 Temperature at various locations in the storage tank-2 with
15 kg of paraffin for different time intervals during charging
TIME
IN Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 30 30 30 30 30 30 30
30 32 33 32 33 33 33 32
60 38 39 39 38 39 38 39
90 47 48 48 47 48 48 48
120 52 52 55 52 52 51 55
150 53 54 56 53 55 55 56
180 61 62 63 61 62 61 63
210 62 63 63 62 63 63 63
240 69 69 71 70 69 69 71
270 74 74 75 73 73 72 75
300 75 74 76 75 74 75 79
330 77 78 79 78 79 78 79
360 78 79 80 78 78 79 80
Mass of paraffin = 15 kg
Mass of water = 30 kg
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Table 4.4 Temperature at various locations in the storage tank-2 with
15 kg of paraffin for different time intervals during discharging
TIME
IN Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 78 79 80 78 78 79 80
30 77 78 79 78 79 78 79
60 76 77 78 78 77 78 78
90 75 75 76 76 75 75 76
120 74 74 74 73 74 74 74
150 72 72 73 72 71 72 73
180 70 70 71 71 70 71 71
210 70 69 70 70 70 70 70
240 69 68 69 69 69 68 69
270 68 67 67 67 68 67 67
300 67 66 66 66 67 66 65
330 65 65 65 65 66 65 64
360 63 63 63 64 64 63 63
Mass of paraffin = 15 kg.
Mass of water = 30 kg
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Table 4.5 Temperature at various locations in the storage tank-2 with
18 kg of paraffin for different time intervals during charging
TIME
IN Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 30 30 30 30 30 30 30
30 31 30 32 31 31 31 32
60 33 32 34 32 32 32 34
90 36 35 38 37 36 38 38
120 41 40 42 42 41 42 42
150 44 43 45 44 43 43 45
180 49 48 50 50 49 49 50
210 51 50 53 53 52 50 53
240 55 54 57 56 55 57 57
270 62 61 63 62 62 62 63
300 69 68 71 70 69 69 71
330 72 71 74 73 72 72 74
360 75 74 76 75 74 74 76
Mass of paraffin = 18 kg
Mass of water = 30 kg
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Table 4.6 Temperature at various locations in the storage tank-2 with
18 kg of paraffin for different time intervals during discharging
TIME
IN
Min
T1
oC
T2
oC
T3oC
T4
oC
T5
oC
T6
oC
TwoC
0 75 76 76 75 76 76 76
30 72 72 73 73 72 73 73
60 70 70 71 72 71 70 72
90 69 68 69 70 69 68 70
120 67 66 68 69 67 67 69
150 66 65 66 67 66 66 67
180 64 63 64 65 65 64 66
210 63 63 63 64 63 63 65
240 61 62 62 63 62 62 63
Mass of paraffin = 18 kg
Mass of water = 30 kg
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CHAPTER 5
RESULTS AND DISCUSSIONS
In the present chapter, the experimental results are enumerated in
the form of various graphs of temperature variation. Variation of water
and paraffin temperature in the storage tank at different positions and
other performance parameters are studied. Based on the graphs,
inferences are given for various observations made on two storage tanks.
5.1 STORAGE TANK 1
5.1.1 Paraffin temperature variation
The variations in temperature for the different positions of paraffin
are noted and the variation is plotted with respect to time.
The time required to reach the maximum temperature of about
95oC is around 360 minutes. It is seen from the Fig 5.1 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes and on reaching the melting point there is a rapid increase in
temperature about 15-20oC for same interval, after which the increase in
temperature is linear about 9-10oC . After reaching maximum of 95
oC the
temperature is almost constant and then it decreases. This is due to
radiation and is termed as standby loss.
The tank is kept undisturbed and the discharging temperature
variations with respect to time are noted. The Fig 5.2 indicates that there
is steady temperature fall i.e. linear throughout the discharge period. The
temperature drop is only about 5oC per hour. The fall in temperature after
360 minutes is about 26oC i.e., the temperature of paraffin after 360
minutes is about 64oC.
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0
20
40
60
80
100
120
0 50 100 150 200 250 300 350 400 450TIME (min)
TEMPERATURE('C)
T1
T2
T3
T4
T5
T6
Fig.5.1. Variation of Temperature Vs Time for
Charging of Paraffin for Tank 1
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50
55
60
65
70
75
80
85
90
95
0 50 100 150 200 250 300
TIME (min)
TEMPERATURE(
'C)
T1
T2
T3
T4
T5
T6
Fig.5.2. Variation of Temperature Vs Time for
Discharging of Paraffin for Tank 1
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5.1.2 Water temperature variation
The variations in water temperature in the storage tank are noted
and plotted with respect to time..
The time required to reach the maximum temperature of about
94oC is around 360 minutes. It is seen from the Fig 5.3 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes. It is also seen that there is no rapid increase in temperature at the
phase change point. And after reaching maximum of 95 oC the
temperature is almost constant and then it decreases. This is due to
radiation and is termed as standby loss.
Temperature discharging rate for the water is taken and graph is
plotted. The Fig 5.4 clearly shows that there is steady temperature fall i.e.
linear throughout the discharge period. The temperature drop is only
about 5 oC per hour. The fall in temperature after 360 minutes is about
26oC i.e., the temperature of water inside the storage tank after 360
minutes is about 64oC.
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5.1.3 Charging rate
It is defined as the average rate at which heat is supplied to the
storage tank. This is calculated by ratio of heat stored in water and
paraffin to the time consumed for charging.
mwCpw (95-30)+ mpL + mpCpp(95-57)
Charging rate =
Time consumed
15 x 4.18 x (95-30) + 8 x 214 + 8 x 2.9 (95-57)
=
6
= 1111.51 kJ/hr
5.1.4 Energy saved
It is the amount of energy saved for the period of charging and is
calculated as
Energy saved = mwCpw (95-30)+ mpL + mpCpp (95-57)
=15 x 4.18 x (95-30) + 8 x 214 + 8 x 2.9 x (95-57)
= 6669.1 kJ in six hours
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0
10
20
30
40
50
60
70
80
90
100
110
0 50 100 150 200 250 300 350 400 450
TIME (min)
TE
MPERATURE('C)
Tw
Fig.5.3. Variation of Temperature Vs Time for
Charging of Water for Tank 1
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50
55
60
65
70
75
80
85
90
0 50 100 150 200 250 300
TIME (min)
TEMPERATURE('C)
TW
Fig.5.4. Variation of Temperature Vs Time for
Discharging of Water for Tank 1
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5.2 STORAGE TANK 2 WITH PARAFFIN QUANTITY = 15 kg
5.2.1 Paraffin temperature variation:
The variations in temperature for the different positions of paraffin
are noted and plotted against time.
The time required to reach the maximum temperature of about
81oC is around 360 minutes. It is seen from the Fig 5.5 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes. After reaching maximum of 81oC the temperature is almost
constant and then it decreases. This is due to radiation and is termed as
standby loss.
The tank is kept undisturbed and the discharging temperature
variations with respect to time are noted. The Fig 5.6 indicates that there
is steady temperature fall i.e. linear throughout the discharge period. The
temperature drop is only about 5 oC per hour. The fall in temperature after
360 minutes is about 25oC i.e. the temperature of paraffin after 360
minutes is about 56oC.
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0
10
20
30
40
50
60
70
80
90
0 50 100 150 200 250 300 350 400 450
TIME ( min )
TE
MPERATURE('C)
T1
T2
T3T4
T5
T6
Fig.5.5. Variation of Temperature Vs Time for
Charging of Paraffin for Tank 2 with 15 kg Paraffin
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TIME (min)
Fig.5.6. Variation of Temperature Vs Time for
Discharging of Paraffin for Tank 2 with 15 kg Paraffin
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5.2.2 Water temperature variation:
The variation in water temperature in the storage tank is noted and
plotted with respect to time.
The time required to reach the maximum temperature of about
80oC is around 360 minutes. It is seen from the Fig 5.7 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes. And after reaching maximum of 80oC the temperature is almost
constant and then it decreases. This is due to radiation and is termed as
standby loss.
The temperature discharging rate for the water is taken and graph is
plotted. The Fig 5.8 clearly shows that there is steady temperature fall i.e.
linear throughout the discharge period. The temperature drop is only
about 5oC per hour. The fall in temperature after 360 minutes is about
25oC i.e. the temperature of water inside the storage tank after 360
minutes is about 56oC.
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5.2.3 Charging rate
It is defined as the average rate at which heat is supplied to
the storage tank. This is calculated by ratio of heat stored in water and
paraffin to the time consumed for charging.
mwCpw (80-30)+ mpL + mpCpp(80-57)
Charging rate =
Time consumed
30 x 4.18 (80-30) + 15 x 214 + 15 x 2.9 (80-57)
=
6
= 1746.75 kJ/hr
5.2.4 Energy saved
It is the amount of energy saved for the period of charging and is
calculated as
Energy saved = mwCpw (80-30) + mpL + mpCpp(80-57)
= 30 x 4.18 (80-30) + 15 x 214 + 15 x 2.9 (80-57)
= 10480.5 kJ in six hours
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Fig.5.7. Variation of Temperature Vs Time forCharging of Water for Tank 2 with 15 kg Paraffin
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50
55
60
65
70
75
80
85
0 50 100 150 200 250 300 350 400
TIME (min)
TEMPER
ATURE('C)
TW
Fig.5.8. Variation of Temperature Vs Time for
Discharging of Water for Tank 2 with 15 kg Paraffin
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5.3 STORAGE TANK2 WITH PARAFFIN QUANTITY = 18 kg
5.3.1 Paraffin temperature variation
The variations in temperature for the different positions of paraffin
are noted and plotted against time.
The time required to reach the maximum temperature of about
76oC is around 360 minutes. It is seen from the Fig 5.9 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes. After reaching maximum of 76oC the temperature is almost
constant and then it decreases. This is due to radiation and is termed as
standby loss.
The tank is kept undisturbed and the discharging temperature
variations with respect to time are noted. The Fig 5.10 indicates that there
is steady temperature fall i.e. linear throughout the discharge period. The
temperature drop is only about 5 oC per hour. The fall in temperature after
240 minutes is about 14oC i.e., the temperature of paraffin after 240
minutes is about 61oC.
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0
10
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
TIME (min)
TEMPERATURE('C)
T1T2
T3
T4
T5
T6
Fig.5.9. Variation of Temperature Vs Time for
Charging of Paraffin for Tank 2 with 18 kg Paraffin
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50
55
60
65
70
75
80
0 50 100 150 200 250 300
TIME (min)
TEMPERATURE('C)
T1
T2
T3
T4
T5
T6
Fig.5.10. Variation of Temperature Vs Time for
Discharging of Paraffin for Tank 2 with 18 kg Paraffin
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5.3.2 Water temperature variation
The variation in water temperature in the storage tank is noted and
plotted with respect to time.
The time required to reach the maximum temperature of about
76oC is around 360 minutes. It is seen from the Fig 5.11 that temperature
rise in the beginning of charging period is about 3-5oC for every 30
minutes. And after reaching maximum of 76oC the temperature is almost
constant and then it decreases. This is due to radiation and is termed as
standby loss.
The temperature discharging rate for the water is taken and the
graph is plotted. The Fig 5.12 clearly shows that there is steady
temperature fall i.e. linear throughout the discharge period. The
temperature drop is only about 5oC per hour. The fall in temperature after
240 minutes is about 13oC i.e., the temperature of water inside the storage
tank after 240 minutes is about 63oC.
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5.3.3 Charging rate
It is defined as the average rate at which heat is supplied to
the storage tank. This is calculated by ratio of heat stored in water and
paraffin to the time consumed for charging.
mwCpw (76-30) + mpL + mpCpp (76-57)
Charging rate =
Time consumed
30 x 4.18 (76-30) + 18 x 214 + 18 x 2.9 (76-57)
Charging rate =
6
= 1768.7 kJ/hr
5.3.4 Energy saved
It is the amount of energy saved for the period of charging and is
calculated as
Energy saved = mwCpw (76-30) + mpL + mpCpp (76-57)
= 30 x 4.18 (76-30)+ 18 x 214 + 18 x 2.9 (76-57)
= 10612.2 kJ in six hours
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50
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
TIME (min)
TEMPERATURE('C)
Tw
Fig.5.11. Variation of Temperature Vs Time for
Charging of Water for Tank 2 with 18 kg Paraffin
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50
55
60
65
70
75
80
85
0 50 100 150 200 250 300
TIME (min)
TEMPERATURE('C)
Tw
Fig.5.12. Variation of Temperature Vs Time for
Discharging of Water for Tank 2 with 18 kg Paraffin
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5.4 COMPARATIVE STUDY OF RESULTS
From the results of storage tank with different capacity and paraffin
quantity following comparative results are made.
The charging rate for tank1 is more when compared with storage
tank2 with paraffin quantity 15kg and storage tank2 with 18kg paraffin
this is because, the quantity of water and paraffin used in tank1 is less and
the amount of heat energy required to charge this capacity is also less
hence while charging this amount of water and paraffin a maximum
temperature of about 950C is obtained. But the amount of energy stored in
tank1 is less when compared to storage tank2 with 15kg paraffin and
storage tank2 with 18kg paraffin.
Similarly for storage tank2 with 15kg paraffin when compared
with storage tank2 with 18kg paraffin charging rate is more but the
amount of energy stored is less. Thus it is seen that when the quantity of
paraffin is increased for same capacity of water the charging rate
decreases but the energy stored is increased considerably.
Hence this comparison is clearly shown using graphs. The
variation of temperature with respect to time for charging of paraffin in
different storage tank is shown in Fig. 5.13. The graph clearly shows that
charging rate is initially more for tank1 when compared to storage tank2
with 15kg paraffin and storage tank2 with 18kg paraffin. It is also seen
that the charging rate for storage tank2 with 15kg paraffin is more when
compared with storage tank2 with 18kg paraffin.
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Fig.5.14 shows the variation of water temperature with respect to
time for charging period of water in different storage tanks. Here also the
same characteristic can be observed i.e. similar to the charging of
paraffin.
Fig.5.15 shows the variation of paraffin temperature with respect to
time for discharging period in different storage tanks. It is seen from the
graph that the standby heat loss is more in storage tank1 when compared
to storage tank2 with 15kg paraffin and storage tank2 with 18kg paraffin.It is also seen that the standby loss in storage tank2 with 15kg paraffin is
more when compared to standby loss that occurred in storage tank2 with
18kg paraffin.
Fig.5.16 shows the variation of water temperature with respect to
time for discharging period in different storage tanks. Here also the samecharacteristic can be observed i.e. similar to the discharging of paraffin.
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0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
TIME (min)
TEMPERATUR
E('C)
Charging curve for Tank1
Charging curve for Tank2 with P=1
Charging curve for Tank2 with P=1
Fig.5.13. Variation of Temperature Vs Time for Charging of Paraffin
In different Storage tanks
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0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300 350 400 450TIME (min)
TEMPERATURE('C)
Charging curve for tank1
Charging curve for Tank2 with P=15kg
Charging curve for Tank2 with P=18kg
Fig.5.14. Variation of Temperature Vs Time for Charging of Water
In different Storage tanks
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50
55
60
65
70
75
80
85
90
95
0 50 100 150 200 250 300 350 400TIME (min)
TEMPERA
TURE('C)
Disharging curve for Tank1
Disharging curve for Tank2 with
P=15kg
Disharging curve for Tank2 with
P=18kg
Fig.5.15. Variation of Temperature Vs Time for Discharging of
Paraffin
In different Storage tanks
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50
55
60
65
70
75
80
85
90
95
0 50 100 150 200 250 300 350 400
TIME (min)
TEMPERATURE('C)
Disharging curve for Tank1
Disharging curve for Tank2 with
P=15kg
Disharging curve for Tank2 with
P=18kg
Fig.5.16. Variation of Temperature Vs Time for Discharging of
Water
In different Storage tanks
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CONCLUSION
The experimental setup and details of the thermal storage systems
are outlined in this project work.
The time variations of the temperature in the storage tank are
experimentally studied to know the characteristics of the system.
The performance parameters like charging rate and the amount of
energy saved are analyzed to merits and demerits of the STSS.
Based on the results the following conclusions are drawn:
1. The thermal energy stored in the STSS is retained for as long
as 15 hours with some standby loss.
2. The combined storage system has greater storage efficiency
as compared to conventional systems.
3. The combined storage system overcomes the main drawback
of the sensible storage system by exhibiting isothermal
behavior.
4. The combined storage system also reduces the size of the
STSS considerably.
5. The charging efficiency and the percentage energy saved of
the STSS can be increased by applying advanced insulation
techniques.
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6. Increasing the capacity of the storage tank to 50 litres, i.e.,
storage tank 2 and keeping the mass of paraffin to mass of water
ratio unchanged, the maximum temperature attained in the tank
was reduced to 80oC. This is due to the increase in capacity and
surface area of the tank which enhances the standby loss while
charging itself.
7. For the same storage tank 2, without changing the mass of
water, the mass of paraffin is increased from 15 kg to 18 kg. In
this case the maximum temperature reached is only 76oC. It
shows that increase in mass of paraffin affects the heat storage
capacity of the STSS.
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REFERENCES:
1. Abhat. A (1983) Low temperature latent heat, thermal energy
storage: Heat storage material, Solar Energy, Vol.30, pp.313 - 332
2. Ananthanarayan. V et al., (1987) Modeling of fixed bed heat
storage units utilizing phase change materials, Metallurgical
Transactions B, Vol. 18B, pp.339346.
3. Beasley D.E. et al., (1989) Thermal response of a packed bed of
spheres containing a phase-change material, International Journal of
Energy Research, Vol. 13, p.253265.
4. Fieback. K et al., (1986) The use of paraffin waxes in thermal
energy storage applications, Proc. 1st
IEA workshop on Phase Change
Materials and Chemical Reaction for Thermal Energy Storage
5. Lane G.A (1983) solar heat storage: Latent heat materials,
Lane G.A. (Editor), CRC Press, Inc., Boca Raton, Florida.
6. Rosen M.A. (1992) Appropriate thermodynamic performance
measures for closed system for thermal energy storage, Journal of Solar
Energy Engineering, Vol. 114, pp.100105.
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7. Fender, D.A et al., (1978), Theoretical Analysis of solar
collector/ solarpanels ASME winter annual meeting, 78WA/sol-11.
8. Jrinak, J.J et al., (1979), Paper on the performance of solar
heating system utilizing phase change energy storage
9. Kauffman .K et al., (1973), Congruently melting material for
thermal energy storage Report NCEMP-20 University of Pennsylvania.