TRANSACTION ON CONTROL AND MECHANICAL SYSTEMS, VOL.1, NO.2, PP. 87-92, JUN., 2012.

RECEIVED: 26, APR., 2012; REVISED: 14, MAY, 2012; ACCEPTED: 29, JUN., 2012; PUBLISHED: 30, JUL., 2012.

Abstract: The objective of the present study is to investigate

the characteristic of thermal energy storage in solar system

using phase change materials (paraffin wax), here the

experiments were performed with phase change materials in

which PCM storage tank was designed and developed to

enhance the heat energy in the storage system. The fabricated

PCM storage unit consists of a number of copper tubes; each

tube is filled with PCM materials. During charging process heat

energy can be stored in copper tube as latent heat, the same

heat can be recovered during discharging process by applying

cold water .The PCM storage unit kept in well insulated storage

tank. It carries minimum of 45 liters capacity of water with

glass wool insulation. The fabricated PCM storage tank

received hot water from solar tank, solar tank integrated with

evacuated glass tubes collector. Solar energy is absorbed and

stored in PCM storage unit as latent heat. Large quantity of

solar energy can be stored in a day time and same heat can be

retrieved for later use. The tank was instrumented to measure

inlet and outlet water temperature. Flow meters were used to

vary the mass flow rate.

Keywords: PCM, Solar, Thermal Energy Storage.

1. INTRODUCTION

Energy storage leads to saving premium fuels and makes

the system more cost effective by reducing the wastage of

energy. In most systems, there is a mismatch between the

energy supply and consumption of energy. The energy

storage systems can overcome the imbalanced system and

thereby helps in saving capital costs. It is desirable for more

effective and environmentally benign energy use. A variety

of thermal energy storage techniques have been developed

over the past four to five decades. Increasing energy

demands, shortages of fossil fuels, and concerns over

environmental impact have provided impetus to the

development of renewable energy sources. Solar energy is a

major renewable energy resource, the intermittent nature

and its effective utilization is in part dependent on efficient

and effective energy storage systems. The present work is to

analyze the application of the PCM in thermal energy

storage systems, and the enhancement of the heat transfer

from the solar tank to the PCM storage tank. PCM materials

and their performance of charging and discharging of a

storage tank were tested experimentally.

1 B. Kanimozhi is with Research Scholar, Department of Mechanical &

Production Engineering, Sathyabama University, Chennai-600 119, India. (Email: kanihwre@yahoo.com) 2 B. R. Ramesh Bapu is with Principal, RVS Padhmavathy College of

Engineering and Technology, Thiruvallur District– 517588, India. (Email: brrbapu.aero@gmail.com)

2. LITERATURE REVIEW

Heat transfer enhancement techniques are required for

many latent heat thermal energy storage systems; various

methods are proposed to enhance the heat transfer in a latent

heat thermal energy storage system, such as Metallic fillers,

metal matrix structures, and finned tubes were used to

improve the thermal conductivity of the phase change

materials. Efterkhar et al. [1]

have experimentally studied a

different heat transfer enhancement method for melting of

paraffin by constructing a model that consists of vertically

arranged fins between two isothermal planes which not only

provides additional conduction paths but also promotes

natural convection with the molten PCM.

Ananthanarayanan et al. [2]

developed a computer model for

the estimation of temperature profiles of the solid and the

fluid along the length of the packed bed of self-encapsulated

Al-Si PCM shots as functions of distance along the bed and

time during a series of heat storage and utilization cycles.

Air was used as HTF in their study.

Chen and Yue [3]

developed an ID porous medium model

to determine the thermal characteristics of ice-water cool

storage in packed capsules for air conditioning.

Comparisons of this theory with experimental data of

temperature profiles of PCM (water) and coolant (alcohol)

for various porosities flow rates and different inlet coolant

temperatures showed good agreements. Lacroix has

presented a theoretical model for predicting the transient

behavior of a shell and tube storage test in which annular

fins externally fixed in the inner tube with the PCM on the

shell side and the HTF flowing inside the tube. The

numerical results have also been validated with

experimental data for various parameters like shell radius,

mass flow rate inlet temperature of the HTF. L.C. Chow et

al [4]

studied about two thermal conductivity enhancement

techniques are investigated the first technique focuses on

placing encapsulated PCM of various shapes in a liquid

metal medium.

The second technique involves a metal / PCM composite.

Velraj et al. [5]

have presented the theoretical and

experimental work for a thermal storage unit consisting a

cylindrical vertical tube with internal longitudinal fins and it

was concluded that this configuration which forms a

v-shaped enclosure for the phase change material gives

maximum benefit to the fin arrangement. I. M. Bugaje [6]

studied thermal response of paraffin wax contained in

plastic tubes and used in latent heat storage systems was

enhanced by the use of metal matrices R. Velraj et al. [7]

, In

Experimental Study of Thermal Energy Storage in Solar

System Using PCM

B. Kanimozhi1, and B. R. Ramesh Bapu

2

TRANSACTION ON CONTROL AND MECHANICAL SYSTEMS, VOL. 1, NO. 2, PP. 87-92, JUN., 2012.

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this paper a detailed investigation of the different heat

transfer enhancement methods for the latent heat thermal

storage system has been carried out, Enhancement has been

done with fin configuration and by Lessing rings are used to

increase the storage capacity. Ismail et al. [8]

represented a

model for simulation of the process of heat transfer

(charging and discharging) of a latent heat storage system of

packed bed of spherical capsules filled with PCM was

developed and solved numerically by using a finite

difference approach and moving grid technique.

The numerical grid was optimized and the predicted

results were compared with experimental measurements to

establish the validity of the model V. Arun prasad Raja et al [9]

developed numerical simulation method, he has enhanced

heat transfer rate for water and air, computational fluid

dynamics software (FLUENT) used. In which numerical

solution were obtained for constant properties, forced

convection heat transfer in fully developed flow were

compared with thermally developing laminar flow, it was

found that heat transfer coefficients were higher due to

small channel spacing and developing laminar flow. The

author E. Assis et,al. [10]

presented the process of melting of

a phase-change material (PCM) in spherical geometry has

been explored experimentally and numerically. Melting

temperature of the PCM was incorporated in the simulations

along with its other properties, Nallusamy et al. [11]

, deals

with the experimental evaluation of thermal performance of

a packed bed latent heat TES unit integrated with solar flat

plate collector.

The TES unit contains paraffin as phase change material

(PCM) filled in spherical capsules, which are packed in an

insulated cylindrical storage tank. The water used as heat

transfer fluid (HTF) to transfer heat from the solar collector

to the storage tank also acts as sensible heat storage material,

the same author did the work of thermal energy storage

system for the use of hot water at an average temperature of

45ºC for domestic applications using combined sensible and

latent heat storage concept. The objective of the present

work is to enhance the heat transfer performance in thermal

energy storage system using PCM filled copper tubes, the

storage unit integrated with solar collector water heating

system. The storage tank contained encapsulated copper

tubes surrounded by hot water. Parametric studies were

carried out and the effectiveness of the storage unit

performed.

3. PCM STORAGE TANK AND COPPER

TUBE SPECIFICATION

The PCM storage tank contains PCM storage unit, the

storage unit having 50 numbers of copper tube, each tube

carries minimum of 400 gm PCM, totally 20 kg of PCM

were used, and the entire setup kept in well insulated

stainless steel tank with45 litres capacity of water. The heat

transfer fluid flows by convection through the copper tubes.

The storage tank accurately considered as adiabatic no heat

transfer. The experimental test unit consisted of two

concentric cylinder with75cm lengths. The inside cylinder

with inner diameter of 30 cm and outer diameter of 37 cm

was made of stainless steel (304L), inside the tank the

number of copper tubes has kept with dimension of length

60cm, 1.5cm diameter and thickness of 1.5 mm. In order to

reduce the heat transfer to the environment the storage tank

was thermally well insulated as glass wool. Water was used

as the HTF and it circulated throughout the tank between

the copper tubes.

The tube was filled with the PCM are shown in Figures

1 and 2.

Fig. 1. Copper tube arrangements

Fig. 2. Fabricated PCM storage tank

Copper possess very good thermal conductivity and is

also available easily, so copper materials can be used

effectively in thermal storage systems. A bundle of 50

copper tubes were fabricated for the purpose of installing

PCM materials. Insulation is an important factor in thermal

storage systems. Hence proper capping in each of the

copper tube was done after installing the PCM material, for

effective insulation work. Proper insulation and high

thermal conductivity of these copper tubes provide secure

thermal storage system which retains heat for long time and

reduce energy wastage.

KANIMOZHI et al.: EXPERIMENTAL STUDY OF THERMAL ENERGY STORAGE IN SOLAR SYSTEM USING PCM.

TRANSACTION SERIES ON ENGINEERING SCIENCES AND TECHNOLOGIES (TSEST) ©

89

4. SPECIFICATION OF SOLAR

COLLECTOR AND SOLAR TANK

Storage capacity of solar tank has 100 liters, Solar

collector having 15 numbers of evacuated glass tube,

Length of the collector tube 138cm, Breadth of the solar

collector100cm, Height of the solar tank from the base of

the collector 65cm, Gap between the tubes 4 cm, The

maximum permissible working pressure of solar water

heater with evacuated tube collector is 0.4kg/cm2.Solar

collector having the evacuated glass tube. the evacuated

tube solar collector are the key component of solar water

heater, two concentric borosilicate glass tubes configure

each of them, the outside surface of inner glass tube is

coated with special solar selective coating, which absorbs

and converts the maximum amount of solar radiation into

heat.

Fig. 3. Experimental setup

The space between outer and inner glass tubes is

evacuated and permanently sealed off, the vacuum acts as

an excellent insulator. The schematic diagram of

experimental view as shown in Fig. 3.

5. EXPERIMENTAL PROCEDURE

The experimental set-up has three storage tanks; the first

one is a cold water tank with a capacity of 500 liters; the

second one is solar tank with a capacity of 100 liters, and

the third one is the PCM storage tank with a capacity of 45

litres; the tank was instrumented to measure the inlet and

outlet water temperatures with a thermocouple arrangement.

Flow meters were fit across the pipe line to vary the flow

rates.

Here, water is used as the Heat transfer fluid and

paraffin and honey waxes are used as phase change

materials. To analyses the temperature variation of the HTF

and PCM in the copper tube, the length of the copper tube is

divided into four divisions or segments (levels) from the

base, each division of which in and out of the copper tubes

is connected to thermocouples with sensors. Inside the

copper tube the PCM is presented, and outside the copper

tube the HTF is circulated. Hence, we can read the

temperature for both the HTF and the PCM at all levels. The

line diagram of the experimental setup as shown in the Fig.

4.

Fig. 4. Line Diagram of the Experimental setup

The flow starts from cold water storage tank, then it

goes to evacuated solar tube collector area with 1.5 m2 and

is saved as hot water in the solar tank, then it passes to PCM

storage tank while adjusting the valve position. For the

charging process: during the active phase of the sun light

the heat energy can be stored in the solar tank through the

evacuated glass tube collector. Then the Heat Transfer Fluid

(HTF) from the solar tank is allowed at 76C with a flow

rate of 6kg/min to the PCM storage tank. Then the PCM and

HTF temperature are noted every 10 minutes at four levels;

the experiment is carried for nearly 160 minutes for the

charging process. For the discharging process, the cold

water is allowed to the PCM tank with a flow rate of 6

kg/min at 32C. then the HTF and PCM temperature are

noted every 10 minutes at four levels; the cold water

absorbs heat from the PCM, and releases hot water

continuously. The experiment is completed when the PCM

and water temperatures are the same. The same procedure is

repeated for different mass flow rates, and the heat transfer

rate and heat absorption are calculated.

6. RESULTS AND DISCUSSION

A. Charging Process Of Paraffin Wax

The charging process was carried by applying HTF at

76C with a flow rate of 6 kg/min. Due to variation of sun

radiation the additional heat sources are provided to

maintain the temperature of HTF at 76 C. The HTF is

flowing through the storage tank then the temperature of

HTF and PCM have been noted for every 10 minutes once

at four different levels. The temperature profiles for HTF

and PCM have been drawn for each level as shown in

Figures 5 and 6. It is observed from the figures that the

temperature variation between each levels (x=0.25, x=0.5,

x=0.75, x=1) having differences among them due to the

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flow direction, time consumption and surface contact of

HTF around the copper tubes.

Fig. 5. Temperature of HTF at 6 kg/min (Charging process

of paraffin wax)

Fig. 6. Temperature of PCM at 6 kg/min (Charging process

of paraffin wax)

Figure 7 shows the heat absorption curve for the

charging process of the paraffin wax. The graph has been

plotted between time verses heat absorption. It is observed

from the graph that the heat absorption is increases with

respect to the time. The effect of temperature variations

between the levels are clearly shown in the graphs during

the absorption of heat in the charging process. As the

melting process proceeds, the temperature increases at

higher axial positions becomes comparatively faster. It is

due to the direction of HTF and increased natural

convection in the paraffin wax, which results from the fact

that the solid is down and the melted paraffin wax is in the

top because of density difference. A visible variation is

observed in the profile with different height of the levels

inside the storage tank.

Figure 8 shows the graph plotted between temperature

verses heat absorption; it is observed from the graph that the

heat absorbed by paraffin wax in the storage tank has a

combined storage of sensible heat and latent heat of the

systems. Paraffin starts with sensible heat until the paraffin

reaches its melting temperature, and then the energy storage

is achieved by melting the paraffin at a constant temperature.

Then the energy is stored as sensible heat in liquid paraffin.

Fig. 7. Time Vs Heat absorption

Fig. 8. Temperature Vs Heat absorption

Calculation of heat absorption during the Charging

process of paraffin wax as given in the following Equation

from (1) to (3)

When, m = 6kg/min. X = 0.25

Heat absorbed by PCM(Qp)

= Sensible Heat of PCM (Heat of solid medium)+ Latent Heat of PCM+ Sensible Heat of PCM(Heat of liquid medium) (1)

Qp = ∫ mCpdT + mam∆hm + ∫ mCpdTTf

Tm

Tm

Ti

(2)

Qp = m[Csp(Tm − Ti) + am∆hm

+ Clp(Tf − Tm)] (3)

where, Csp = specific heat of solid medium

Clp

= specific heat of liquid medium am = melting fraction of PCM ∆hm = Latent heat of fusion

B. Discharging Process of paraffin wax

The discharging process has been carried out by

applying cold water at 32 oC to the storage tank. The

temperature variation of the HTF and PCM of discharging

process was noted for every 10 minutes in all four levels,

and graphs were plotted for both the HTF and the PCM

(paraffin wax).

The rate of heat recovery is large at the beginning of the

30

40

50

60

70

80

0 100 200

HTF

Te

mp

era

ture

in

cels

ius

Time in min

X=0.25

X=0.5

X=0.75

30

40

50

60

70

80

0 50 100 150 200

PC

M T

em

pe

ratu

re in

ce

lsiu

s

Time in min

x=0.25x=0.5x=0.75x=1

0

1000

2000

3000

4000

5000

6000

7000

0 100 200

He

at a

bso

rbe

d b

y p

araf

fin

w

ax in

KJ

Time in min

X=0.25X=0.5X=0.75X=1

0

1000

2000

3000

4000

5000

6000

7000

40 50 60 70 80

He

at a

bso

rbe

d b

y P

araf

fin

w

axin

kJ

Temperature in celsius

x=0.25x=0.5x=0.75x=1

KANIMOZHI et al.: EXPERIMENTAL STUDY OF THERMAL ENERGY STORAGE IN SOLAR SYSTEM USING PCM.

TRANSACTION SERIES ON ENGINEERING SCIENCES AND TECHNOLOGIES (TSEST) ©

91

discharging process and decreases with time because of the

change in the thermal resistance of the solidified layer of the

paraffin and decrease in temperature difference between the

solidified paraffin wax (PCM) and HTF. In the case of

discharging process the HTF outlet temperature decreases

continuously with time.

At the beginning of the discharging process the

temperature drop is large until the paraffin reaches its phase

transition temperature. After that the temperature drop in the

paraffin wax is negligible for a long duration, as the paraffin

wax (PCM) releases its latent heat at constant temperature

for a period of nearly 110 minutes. After the complete

solidification of the paraffin wax (PCM), its temperature

starts decreasing; however, the rate of the temperature drop

is not as high as in the beginning of the discharging process.

This is due to the low temperature difference between the

paraffin wax (PCM), and the cold water inlet temperature.

Figures 9 and 10 show the temperature profile of HTF

and PCM between the levels inside the storage tank during

the discharging process of paraffin wax with a flow rate of

6kg/min.

Fig. 9. Temperature of HTF

Fig. 10. Temperature of PCM

Figure 11 shows the heat rejection of paraffin wax with

respect to time at different levels inside the storage tank.

The paraffin temperature decreases to its melting

temperature when the cold water is applied to the storage

tank, paraffin begins to release the stored energy, it takes a

long time to completely discharge the latent heat.

Fig. 11. Time Vs Heat rejected

Fig. 12. Temperature Vs Heat rejected

Figure 12 shows the heat rejection curve for the paraffin

wax with the flow rate of 6kg/min of cold water supply to

the storage tank during the discharging process.

C. The Effect of mass flow rate

The effect of mass flow rate of HTF can be obtained by

varying the flow rate as 2kg/min, 4kg/min, and 6kg/min

during the charging and discharging processes. Figure 13

show that an increase in mass flow rate has a significant

temperature difference between them. The flow rate

increases the time required for the complete charging which

becomes smaller. This is because of an increase in surface

heat transfer coefficient between the HTF and PCM tubes.

Hence mass flow rate has significant effect on the charging

and discharging processes in the storage tank.

7. CONCLUSION

The thermal energy storage system is the combination of

sensible heat and latent heat storage system. In the sensible

heat storage, the temperature of the storage material

increases as the energy is stored without change the phase

of the materials, while the latent heat storage makes use of

the energy stored when a substance changes from one phase

to another. A thermal energy storage system has been

developed for the use of hot water at an average temperature

of 60oC, for domestic applications using combined sensible

and latent heat storage concept. Mass flow rate has a

30

40

50

60

70

80

0 100 200 300HTF

Te

mp

era

ture

in c

els

ius

Time in min

x=0.25x=0.5

x=0.75x=1

30

40

50

60

70

80

0 100 200 300

PC

M T

em

pe

ratu

re in

ce

lsiu

s

Time in min

x=0.25x=0.5x=0.75x=1

0

1000

2000

3000

4000

5000

6000

7000

0 100 200 300He

at r

eje

cte

d b

y p

araf

fin

w

ax in

kJ

Time in min

x=0.25

x=0.5

x=0.75

x=1

0

1000

2000

3000

4000

5000

6000

7000

40 60 80He

at r

eje

cte

d b

y p

araf

fin

wax

in

kJ

Temperature in º C

x=0.25

x=0.5

x=0.75

x=1

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significant effect of temperature difference between them.

Fig. 13. Effect of mass flow rate during charging process

It is concluded from the experimental results that the

enhancement technique were implemented through the

numbers of copper tube in the fabricated storage tank, the

smaller diameter of copper tubes could effectively enhance

the heat from the HTF to the PCM as well as from the PCM

to the HTF during charging and discharging processes.

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[8] Ismail and R. Henr_ıquez “Numerical and

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[9] Arun Prasad Raja et al. “heat transfer and fluid flow in

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30

40

50

60

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0 100 200 300Tem

pe

ratu

re in

ce

lsiu

s

Time in min

m=6 kg/min

m=4 kg/min

m=2 kg/min

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Cite this work as: B. Kanimozhi, and B. R. Ramesh Bapu, “Experimental

Study of Thermal Energy Storage in Solar System

Using PCM” TSEST Transaction on Control and Mechanical Systems, Vol. 1, No. 2, PP. 87-92, Jun.,

2012.