Civil Engineering Reference
In-Depth Information
e
th
¼
g
c
Q
u
¼
g
c
g
th
I
¼
n
th
I
ð
14
Þ
where g
c
is the ideal Carnot efficiency (Bosanac and Sørensen
2003
):
293 K
293 K
þ
T
wm
T
a
g
c
¼
1
ð
15
Þ
ð
Þ
where T
wm
is the final temperature of the work medium.
The electrical exergy is written as follows:
e
e
¼
g
e
I
¼
n
e
I
ð
16
Þ
The overall exergy efficiency could be written as follows:
n
o
¼
g
c
g
th
þ
g
e
ð
17
Þ
The exergy efficiency has considered the energy grade difference between heat
and electricity and therefore is a more rational index to evaluate performance of
the PV/T systems.
Primary-energy-saving efficiency
Huang et al. (
2001
), Huang (
1993
) proposed another performance evaluation
method to recognise the energy grade difference between heat and electricity,
namely the primary-energy-saving efficiency (E
f
), which is given by
E
f
¼
g
e
g
power
þ
g
th
ð
18
Þ
where g
power
is the electrical power generation efficiency for a conventional power
plant which is considered 0.38. For simplicity, the efficiency of conventional
heating systems is considered 100 % which is achievable if a condensing boiler is
used. Huang et al. (
2001
) suggested that primary-energy-saving efficiency of a PV/
T system should be higher than 0.50, in order to compete a pure solar hot water
system.
Solar Fraction
From the primary energy saving point of view, solar fraction (f) can also be used to
evaluate the performance of PV/T system. It is defined as the fractional ratio of
primary energy saving that a PV/T system can obtain to the overall energy demand
and could be written as follows:
f
¼
1
Q
load
;
t
Q
aux
;
t
Q
load
;
t
þ
Q
load
;
e
Q
aux
;
e
Q
load
;
e
2
ð
19
Þ
where Q
load,t
and Q
aux,t
are the overall thermal load and auxiliary heat required;
Q
load,e
and Q
aux,e
is the total electrical load and auxiliary electricity needed.
Kalogirou (
2001
) indicated that the solar fraction is lower in the winter months
and higher in the summer months reaching an annual value of 0.49 for a hot water
supply system.
