solar thermal storage system

8/1/2019 Solar Thermal Storage System

1/63

``

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


2/63

``

i

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 .


3/63

``

1

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.


4/63

``

2

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.


5/63

``

3

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


6/63

``

materials and the properties of various sensible and latent heat storage

materials are given in this section.

4

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


7/63

``

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

5

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


8/63

``

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

6

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.


9/63

``

7

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:


10/63

``

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.

8

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.


11/63

``

Fig. 1.1 Classification of Phase Change Materials

9

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.


12/63

``

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

10

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


13/63

``

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.

11

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


14/63

``

12

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.


15/63

``

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).

13

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.


16/63

``

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.

14

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.


17/63

``

15

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.


18/63

``

16

Fig.3.1 Schematic Diagram of Experimental Setup


19/63

``

17

Fig.3.2 Photographic View of the Experimental Setup


20/63

``

18

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.


21/63

``

19

Fig.3.3 Photographic View of Thermal Storage Tank

Fig.3.4 Photographic View of Thermal Storage Tank Showing

Paraffin Filled Containers


22/63

``

20

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


23/63

``

21

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.


24/63

``

22

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.


25/63

``

23

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.


26/63

``

24

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.


paraffin during discharging.


paraffin during discharging.


27/63

``

25

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


28/63

``

26

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



29/63

``

27

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



30/63

``

28


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.



31/63

``

29


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




32/63

``

30


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




33/63

``

31

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.


34/63

``

32

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


35/63

``

33

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


Discharging of Paraffin for Tank 1


36/63

``

34

5.1.2 Water temperature variation

The variations in water temperature in the storage tank are noted

and plotted with respect to time..




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



37/63

``

35

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

`


38/63

``

36

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


Charging of Water for Tank 1


39/63

``

37

50

55

60

65

70

75

80

85

90

0 50 100 150 200 250 300

TIME (min)

TEMPERATURE('C)

TW


Discharging of Water for Tank 1


40/63

``

38

5.2 STORAGE TANK 2 WITH PARAFFIN QUANTITY = 15 kg

5.2.1 Paraffin temperature variation:


are noted and plotted against time.




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.




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



41/63

``

39

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


Charging of Paraffin for Tank 2 with 15 kg Paraffin


42/63

``

40

TIME (min)


Discharging of Paraffin for Tank 2 with 15 kg Paraffin


43/63

``

41

5.2.2 Water temperature variation:

The variation in water temperature in the storage tank is noted and

plotted with respect to time.




minutes. And after reaching maximum of 80oC the temperature is almost


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



44/63

``

42

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


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


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)



45/63

``

43

Fig.5.7. Variation of Temperature Vs Time forCharging of Water for Tank 2 with 15 kg Paraffin


46/63

``

44

50

55

60

65

70

75

80

85

0 50 100 150 200 250 300 350 400

TIME (min)

TEMPER

ATURE('C)

TW


Discharging of Water for Tank 2 with 15 kg Paraffin


47/63

``

45

5.3 STORAGE TANK2 WITH PARAFFIN QUANTITY = 18 kg

5.3.1 Paraffin temperature variation


are noted and plotted against time.




minutes. After reaching maximum of 76oC the temperature is almost


standby loss.




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



48/63

``

46

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


Charging of Paraffin for Tank 2 with 18 kg Paraffin


49/63

``

47

50

55

60

65

70

75

80

0 50 100 150 200 250 300

TIME (min)

TEMPERATURE('C)

T1

T2

T3

T4

T5

T6


Discharging of Paraffin for Tank 2 with 18 kg Paraffin


50/63

``

48

5.3.2 Water temperature variation

The variation in water temperature in the storage tank is noted and

plotted with respect to time.




minutes. And after reaching maximum of 76oC the temperature is almost


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.


51/63

``

49

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


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


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)



52/63

``

50

0

10

20

30

40

50

60

70

80

0 50 100 150 200 250 300 350 400

TIME (min)

TEMPERATURE('C)

Tw


Charging of Water for Tank 2 with 18 kg Paraffin


53/63

``

51

50

55

60

65

70

75

80

85

0 50 100 150 200 250 300

TIME (min)

TEMPERATURE('C)

Tw


Discharging of Water for Tank 2 with 18 kg Paraffin


54/63

``

52

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.


55/63

``

53

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.


56/63

``

54

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


57/63

``

55

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



58/63

``

56

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


P=18kg

Fig.5.15. Variation of Temperature Vs Time for Discharging of

Paraffin



59/63

``

57

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


P=15kg


P=18kg

Fig.5.16. Variation of Temperature Vs Time for Discharging of

Water



60/63

``

58

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.


61/63

``

59

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.


62/63

``

60

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.


63/63

``

61

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.

solar thermal storage system

Documents