2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 226 -
Thailand. It was found that the cost of energy (COE) was
24.67 Baht/kWh.
TABLE I
I
NVESTMENT AND ELECTRICITY GENERATION COSTS FOR
CSP
TECHNOLOGIES
[1]
System
Investment cost
(
Baht/kW)
Electricity
generation cost
(
Baht/kWh)
The parabolic trough system
140,000
–
160,000
6.0
–
7.5
The tower system
200,000
–
225,000
7.5
–
10.0
The dish/Stirling engine
system
500,000
–
600,000
10.0
–
12.5
In Thailand, the CSP technology is unsuitable because
the direct normal solar radiation is relatively low. A
solution for this problem is the use of flat-plate or
evacuated-tube solar collectors that take the total solar
radiation for a heat source and work with organic
Rankine cycle (ORC) that could generate power from
different low temperature heat source such as waste heat
[3], solar thermal [4, 5], biomass [6], geothermal [7], etc.
There are some reports on the use of low temperature
ORC for power generation. Wei et al. [8] studied
performance analysis and optimization of an ORC system
using R245fa as working fluid. Wang et al. [9] studied
performance evaluation of a low-temperature solar
Rankine cycle system utilizing R245fa.
The objective of this work was to investigate the
values of levelized electricity costs (LEC) of the solar
organic Rankine cycle (SORC) with flat-plate and
evacuated-tube solar collectors as heat sources. The ORC
is operating with R245fa and the weather conditions of
Chiang Mai and Ubon Ratchathani are taken as the input
data of the calculations.
III. M
ATERIAL AND
M
ETHOD
A. Solar Organic Rankine Cycle (SORC)
A schematic diagram of a SORC is shown in Fig. 1.
The SORC system is a solar water heater combined with
an ORC. The unit consists of a set of solar collectors with
a thermal energy storage and an ORC having internal heat
exchanger. There is a water loop to extract heat from the
solar system and transfer to the ORC cycle. Fig. 2 shows
a T-s diagram of the ORC cycle. It could be noted that the
refrigerant is a dry-type therefore the state of the fluid
during expansion is superheated.
For simplicity, the pressure drops in the components
other than the pump and the turbine, such as the solar
collectors, the heat exchangers, the evaporator, condenser
and the piping system, are ignored. The energy equations
obtained are summarized below.
- Pump
1 2 1
_
2 1
ORC
PUMP ORC
ORC a
P
m v P P
W
m h h
K
.
(1)
- Evaporator
_
3 2
EV ORC ORC
IHE
Q
m h h
_
_
EV ORC WEV ORC p TL FL
Q
m C T T
.
(2)
- Turbine
_
3 4
TUR ORC ORC
T
W
m h h
K
.
(3)
- Condenser
_
4
1
CO ORC ORC IHE
Q
m h h
.
(4)
- Internal heat exchanger
_
4 4
4
IHE ORC ORC p a a
IHE
Q
m C T T
_
2 2
2
IHE ORC ORC p a IHE a
Q
m C T T
_
4
2
min
IHE ORC ORC p
a
a
Q
mC T T
H
.
(5)
- Thermal efficiency
_
_
_
_
TUR ORC PUMP ORC
th ORC
EV ORC
W W
Q
K
.
(6)
The heat exchange process between the hot water and
the working fluid in the evaporator of ORC is shown in
Fig. 3. The energy balances could be expressed as
3
_
_
_
ORC
PP ORC WEV ORC TL PP W
m h h
m
h h
(7)
_
_
_
2
ORC PP ORC IHE WEV ORC PP W FL
m h
h
m
h
h
.
(8)
The model for evaluating the temperature of water in
the thermal energy storage is applied from a lump model
by considering the storage be unstratified. With the finite
difference method, the temperature of water in the
thermal energy storage can be calculated from
_
[ ( )
(
)]
(
)
(
)
c R T
L S a
t t
t
S
S
WEV ORC p TL FL
S p
S a
A F I
U T T
t
T T
m C T T
M C
UA T T
WD
'
½
°
°
'
®
¾
°
°
¯
¿
(9)
where
t t
S
T
'
is the temperature of water at time
t
t
'
and
t
S
T
is the temperature of water at time
t
.
B. Conditions for Analysis
The flat-plate solar collectors with
R
F
WD
= 0.682,
R L
F U
= 4.5392 W/m
2
K and the evacuated-tube solar
collectors with
R
F
WD
= 0.5,
R L
F U
= 1.6 W/m
2
K were used
for generating heat water. The area of the solar collectors
was between 300 and 800 m
2
and the ORC was operating
with R245fa of which the evaporating temperature of the
ORC was between 75 and 85
o
C. The weather conditions
of Chiang Mai and Ubon Ratchathani were taken as the
input data of the calculation. The overall coefficient of
heat loss (
UA
) from the thermal energy storage was 5
W/K and the pressure of the thermal energy storage was 5
bar. The conditions for the ORC analysis were:
1. Condensing temperature was at 35
o
C.
