Civil Engineering Reference
In-Depth Information
2.3 Performance Evaluation Standards
Several national/regional standards are currently available for evaluating the per-
formance of solely arranged solar thermal and PV devices. For solar thermal, the
available standards include EN 12975 (
2006a
,
b
), EN 12976 (
2006a
,
b
), EN 12977
(
2008
,
2010a
,
b
,
c
and
d
), Solar Keymark (
2010
), ISO 9806 (
1994
,
1995a
,
b
), MCS
004 (
2008
) and other national solar thermal themes; for PV, standards, including
IEC 61215 (
2005
), IEC 61646 (
2008
), IEC 61730 (
2004a
,
b
), UL 1703 (
2002
), UL
1741(
2010
) and UL 4703 (
2005
), IEEE 1262 (
1995
) and IEEE 929 (
2000
),
Mark (
2010
) and other national electric codes, are in place. No published legal
standards were found to address the performance issues of the PV/T. Instead, the
methods for evaluating the PV/T were suggested in several academic papers. To
summarise, the technical performance of the PV/T systems is usually evaluated
using several indicative parameters including overall energy efficiency, overall
exergy efficiency, primary-energy-saving efficiency and solar fraction. The eco-
nomic performance of the PV/T systems is measured with life cycle cost (LCC) and
cost payback time (CPT), and the environmental benefit of the system is justified
using the energy payback time (EPBT) and greenhouse gas payback time (GPBT).
These parameters are briefed as below.
2.3.1 Technical Performance Evaluation Parameters
Overall Energy Efficiency
Overall energy efficiency is the ratio of collected electrical and heat energy to
incident solar radiation striking on the PV/T absorber. It is yielded from the first
law of thermodynamics and indicates the percentage of the energy converted from
the solar radiation. In a PV/T module, the electrical efficiency is much lower than
the thermal efficiency, and therefore, the overall energy efficiency will largely rely
on the thermal energy conversation of the system. It should be pointed out that the
overall energy efficiency ignores the difference between heat and electrical energy
in terms of the energy grade (quality) and therefore is inadequate to fully justify
the energy performance of the PV/T systems.
Overall Exergy Efficiency
Overall exergy efficiency takes into account difference in energy grades between
heat and electricity and involves a conversion of low-grade thermal energy into the
equivalent high-grade electrical energy using the theory of Carnot cycle. The
overall exergy (e
o
) of the PV/T could be written as follows:
e
o
¼
e
th
þ
e
e
¼
n
th
þ
n
e
ð
Þ
I
¼
n
o
I
ð
13
Þ
where e
th
and e
e
are the thermal and electrical exergy, respectively; n
th
and n
e
are
the thermal and electrical exergy efficiency; n
0
is the overall exergy efficiency. The
thermal exergy could be further written as follows:
