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
Calculations
The experimental procedure lasts 24h and is divided in two stages, charging and discharging. The charging
stage starts when both boilers have been filled with cold water from the water storage tank. This corresponds to
about 09:00am in the morning when solar radiation
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
282 significantly increases and lasts until the sunset. As solar
radiation levels increase during the day, water temperature inside the boiler increases. Simultaneously,
paraffin wax temperature increases. Paraffin begins to melt at 55-58
o
C where latent heat storage begins. The latter heat will be emitted again to the water when
paraffin will be solidified. As tubes material is of high thermal conductivity, it has been assumed that paraffin
wax temperature does not deviate from that of water inside boiler.
Discharging begins when solar radiation levels are low sunset, circuit valves open and hot water returns
back to the water storage tank. It lasts until the end of the 24h cycle. During discharging stage water flows at the
open circuit and temperature is recorded at its outlet. It is expected that the boiler with PCM will require more time
to reduce water temperature in comparison with that of water at the boiler without PCM outlet as in the first case
the PCM latent heat will be emitted again to the water when PCM will be solidified.
For the evaluation of the system and the energy storage during charging and discharging stages the
following calculations were applied:
Charging
w ,c ,b w ,o ,b
st PCM ,S
w,c,b P
w,o,b P
s ,ch ch
st p
PCM p
sensible heat - PCM at solid state PCM
L ,PCM latent heat absorption from PCM during its melting
w,c,b
m C
m C
T Q
m C m
C t
m Q
t m
C +
+ ⎧
⎫ ∆ ⎪
⎪ =
+ ⎨
⎬ +
+ ∆
⎪ ⎪
⎩ ⎭
+ ∆
+ +
w,c ,b w ,o ,b
st PCM ,L
P w,o,b
P l ,ch
st p
PCM p
sensible heat - PCM at liquid state
m C
T m C
m C
t +
+ ⎧
⎫ ∆ ⎪
⎪ ⎨
⎬ +
+ ∆
⎪ ⎪
⎩ ⎭
3
where
w,c,b
m : is anti-freeze solution from closed circuit
mass inside boiler kg,
w,c ,b
P
C : is specific heat capacity
of anti-freeze solution inside boiler JkgK,
w,o,b
m : is
water mass from open circuit inside boiler kg,
w,o ,b
P
C :
is specific heat capacity of water mass from open circuit inside boiler JkgK,
st
m : is mass of steel tubes
that contain PCM kg,
st
P
C
: is specific heat capacity of steel tubes that contain PCM JkgK,
PCM
m : is PCM
mass kg,
PCM ,S
P
C : is specific heat capacity of PCM at
solid state JkgK,
PCM ,L
P
C : is specific heat capacity of
PCM at liquid state JkgK,
L,PCM
Q : is PCM heat of
fusion Jkg, ∆T
s,ch
: is temperature difference during charging until PCM melting K,
∆T
l,ch
: is temperature difference after melting during charging where PCM is
at liquid state K. Closed circuit:
Heat inlet from closed circuit is calculated by:
ch,c c
p ,c c
ch
Q m C
T t
= ∆ ∆
4 •
c
m , anti-freeze solution feed rate from closed circuit
kgs •
p ,c
C
, anti-freeze solution specific heat JkgK •
∆ Τ
c
, temperature change of anti-freeze solution boiler inlet-outlet – closed circuit
Κ •
ch
t
∆ , charging time
Open circuit:
ch,o
Q
=
Discharging
w,c ,b w,o ,b
st PCM ,L
w,c,b P
w,o,b P
l ,dis st
p PCM
p sensible heat - PCM at liquid state
PCM L,PCM
latent heat emmision from PCM during its solidification
m C
m C
T Q
m C m
C t
m Q
t +
+ ⎧
⎫ ∆ ⎪
⎪ =
+ ⎨
⎬ +
+ ∆
⎪ ⎪
⎩ ⎭
− ∆
+ +
w ,c ,b w ,o ,b
st PCM ,S
w,c,b P
w,o,b P
s ,dis st
p PCM
p sensible heat - PCM at solid state
m C
m C
T m C
m C
t +
+ ⎧
⎫ ∆ ⎪
⎪ ⎨
⎬ +
+ ∆
⎪ ⎪
⎩ ⎭
5
where
w,c,b
m : is anti-freeze solution mass inside boiler
kg,
w ,c ,b
P
C : is specific heat capacity of anti-freeze
solution inside boiler JkgK,
w,o,b
m : is water mass
from open circuit inside boiler kg,
w ,o ,b
P
C : is specific
heat capacity of water mass from open circuit inside boiler JkgK,
st
m : is mass of steel tubes that contain
PCM kg,
st
P
C
: is specific heat capacity of steel tubes that contain PCM JkgK,
PCM
m : is PCM mass kg,
PCM ,S
P
C : is specific heat capacity of PCM at solid state
JkgK,
PCM ,L
P
C : is specific heat capacity of PCM at
liquid state JkgK,
L,PCM
Q : is PCM heat of fusion
Jkg, ∆T
s,dis
: temperature difference for a certain time period
∆t during discharging after PCM solidification K,
∆T
l,dis
: temperature difference after melting during discharging where PCM is at liquid state K
Open circuit:
dis,o o
p ,o o
dis
Q m C
T t
= ∆
∆
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
283 •
o
m , water feed rate discharging at the open circuit
kgs •
p ,o
C , water specific heat at the open circuit JkgK
• ∆
Τ
ο
, water temperature change at the open circuit boiler inlet-outlet- open circuit
Κ •
dis
t ∆
, discharging time Closed circuit:
dis ,c
Q =
V. Results and Discussion
