Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
280 heaters with paraffin as thermal energy storage material.
The first system had tank in tank type storage and the second had integrated type of storage using a reflector.
Both systems were able to deliver hot water during the night and in morning on a 24 h cycle basis the two
systems were found to be 45 and 60 efficient respectively. Canbazoglou et al [12] presented some
results of investigations on solar energy storage performance using sodium thiosulfate pentahydrate in a
conventional solar water heating system. In this study, authors used an experimental, open-loop conventional
passive solar energy system with a natural circulation to provide domestic hot water. The system consisted of
solar collectors, hot and cold water tanks and was equipped with sensors to take measurements. Al-Hinti et
al [6] made an experimental study of the performance of water-phase change material PCM storage for use with
conventional solar water heating systems. Paraffin wax contained in small cylindrical aluminum containers was
used as the PCM. The containers were packed in a commercially available, cylindrical hot water storage
tank on two levels. The PCM storage advantage is demonstrated with controlled energy input experiments
with the aid of an electrical heater on an isolated storage tank, with and without the PCM containers. It was found
that the use of the proposed configuration can provide a 13–14
o
C advantage in the stored hot water temperature for extended periods of time.
In this work, typical results from a solar installation developed at Technological Educational Institution of
Halkida to test PCM effect on solar energy storage under Greek climate conditions are presented and discussed.
Results show clearly enhancement of solar thermal energy storage performance compared to a conventional
system. The suggested storage configuration is simple and can be easily used with existing conventional
systems without major or expensive modifications.
II. Sensible and Latent Storage Heat
he basic methods for storing thermal energy are sensible heat storage and latent heat storage. In sensible
heat storage heating a liquid or a solid takes place without changing phase. Sensible heat storage systems
utilize the heat capacity and the change in temperature of the material during the process of charging and
discharging. In sensible heat storage, the temperature of the medium changes during charging or discharging of
the system. The amount of energy stored depends on the temperature change of the material and can be expressed
by:
2 1
T p
T
Q m c dT
=
∫
1 where m is the mass and c
p
the specific heat at constant pressure. T
1
and T
2
represent the lower and upper temperature levels between the storage operation.
In latent heat storage during heating, the medium temperature remains more or less constant since it
undergoes a phase transformation. Latent heat storage systems offer high storage capacity as compared to
sensible heat storage and also involve low heat losses. The amount of energy stored Q in this case depends
upon the mass m and latent heat of fusion of the material. The storage operates isothermally at the melting
point of the material. If isothermal operation at the phase change temperature is difficult, the system operates over
a range of temperatures T
i
to T
f
that includes the melting point. The storage capacity of a latent heat storage
system with a PCM medium is given by:
f m
m L,PCM
p p
i m
m L,PCM
sp m
i lp
f m
Q m
Q mC dT
mC dT m
Q C
T T
C T
T
α α
= +
+ =
⎡ ⎤
= +
− +
− ⎣
⎦
∫ ∫
2 where,
α
m
is the PCM fraction melted, Q
L,PCM
is heat of fusion Jkg, m, is mass of heat storage medium kg, T
f
is final temperature °C, T
i
is initial temperature °C, T
m
is melting temperature °C, C
p
is specific heat Jkg K, C
lp
average specific heat between T
m
and T
f
Jkg K, C
sp
is average specific heat between T
i
and T
m
Jkg K. The selection of the storage method should consider
among others the temperature range, over which the storage has to operate; the rate of charging and
discharging; the effect of storage capacity on the operation of the rest of the system; a smaller storage unit
operates at a higher mean temperature; cost of the storage unit.
III. Experimental Facility
A schematic diagram of the developed experimental setup is shown in Fig. 1.
Fig. 1 View of the experimental facility 1: solar collector without PCM, 2: solar collector with PCM
T
boiler
, temperature inside boiler, T
iwoc
, temperature at open circuit inlet T
owoc
, temperature at open circuit outlet, T
iwcc
, temperature at closed circuit inlet, T
owcc
, temperature at closed circuit outlet
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
281 The installation comprised two thermal solar
collectors constructed of aluminum profile, with single cover glass and black-painted absorber plate, each of
area 2,2 m². Solar collectors were oriented to south having tilt angle of 30
o
with horizontal plane. They were also connected to a pump and a water storage tank of
1000 lt as shown in Fig. 1. They were also connected to boilers each of 132lt.
Cold water supply to the system was achieved from the water storage tank through plastic tubes under forced
circulation conditions open circuit. The return of hot water from the system to the same water storage tank
was done under natural circulation conditions through well insulated plastic tubes. Anti-freeze solution was
used in the closed circuit consisting of water and antifreeze chemical.
30.6l of PCM were placed inside the boiler of one of the solar collectors in 17 metal tubes of 0.95m length and
0.05m diameter with complement of 85 which were sealed to avoid possible leakages. The PCM used was
paraffin. The thermo physical properties of the paraffin wax are given in Tables I, II. The insulation material of
the boiler was glass wool.
TABLE I
P
ROPERTIES
O
F
M
ATERIALS
F
OR
B
OTH
B
OILERS
BOILER WITH PCM no
Material Volume
Density Mass
Cp Heat fusion
Heat capacity lt
kg kJkgK kJkg
kJK 1 Water 96,19 0,996 95,81
4,186 401,05
2 PCM 30,60 0,850
26,01 2,130
214 5621,54
3 Steel 5,21 7,850 40,87
0,480 19,62
Total 132,00 162,69 6042,21
Theoretical capacity for energy storage m x c +m x
∆Η x ∆ 3600 kWh 59,89
Experimental capacity for energy storage kWh 26,32
PCM fraction melted 39,15
26,32 PCM thermal diffusivity
k ρ c
1,10E-07 BOILER WITHOUT PCM
no Material
Volume Density
Mass Specific heat
Heat of fusion Heat capacity
lt kg kJkgK JK
1 water 132 0,996 131,47
4,186 550,34
Total 132 131,47
550,34 Theoretical capacity for energy storage
m x c x ∆ 3600 5,22
Experimental capacity for energy storage kWh 5,22
TABLE II
PCM T
HERMOPHYSICAL
P
ROPERTIES
Melting Temperature
o
C 58 Heat capacity kJkgK
2,13 Thermal conductivity WmK
0,2 Latent Heat kJkg
214 Solid density kgm
3
850 Liquid density kgm
3
775
Temperature was recorded at the open circuit inlet and outlet, closed circuit inlet and outlet, and inside the boiler
as depicted in Fig. 1. The measurement system included T type
thermocouples, two ADAM 4018 modules connected to a PC to enable the continuous recording of the
temperature readings. A meteorological station was placed 5m away from the solar collectors measuring
environmental temperature, relative humidity, direct and diffuse radiation, wind velocity and direction, total
sunshine duration. Data recording has been scheduled every ten minutes. Pump’s flow rate of the open circuit
was calculated 0,246m
3
h. The experiments were conducted during June at the
campus. This site is located at 38°3440.42N and 23°3834.52E with an altitude of around 42 m.
Table III shows meteorological data of the campus area. The climate at the campus area is typically
Mediterranean and is characterized by around 2851h of sunshine per year.
TABLE III
M
ETEOROLOGICAL
D
ATA
O
F
T
HE
C
AMPUS
A
REA
M ea
n da il
y m
in im
u m
a ir
te m
pe ra
ture
o
C M
ea n da
il y
m ax
im u
m a
ir
te m
pe ra
ture
o
C
S uns
hi ne
dur at
ion h
m ont
h S
uns hi
ne dur
at ion
h d
ay Re
la ti
ve hum
idi ty
Irra d
ia ti
o n
o f
gl oba
l r adi
at ion
hor iz
ont al
kW h
m
2
Jan 3,6 11,9 135 4,4 79 63
Feb 3,3 13,6 142 5,1 75 70
Mar 5,1 16,0 186 6,0 69 107
Apr 8,1 19,8 236 7,9 64 154
May 12,9 25,3 299 9,7 58 193 Jun 16,4 30,1 339
11,3 52 205 Jul 19,1 32,6 371
12,0 50 215
Aug 18,9 32,1 352 11,3 52 197
Sep 15,2 28,2 281 9,4 59 154 Oct 11,6 22,6 216 7,0 69 98
Nov 8,0 17,5 160 5,3 77 62
Dec 5,4 13,5 133 4,3 79 48
Year 1561
IV. Experimental Procedure and
