Page 260 - Advances in Renewable Energies and Power Technologies

2. Performance Parameters of Photovoltaic Systems 233

Four performance parameters established by the International Energy Agency
(IEA) Photovoltaic Power System Program and described in the IEC standard
61724 define the overall system performance with respect to the energy production
[4], solar resources, rated power, and overall effect of system losses. These perfor-
mance parameters are, namely, the reference yield (Y r ), array yield (Y a ), final yield
(Y f ), and finally the performance ratio (PR). These parameters are given by the
following equations:
Dt
R
0 GðtÞdt
Y r ¼ (7.1)
G ref
2
where G ref is the irradiance at standard test conditions (STCs), 1000 W/m spectrum
Air mass 1.5 (AM1.5), and G is the measured irradiance.
R Dt
0 E dc ðtÞdt
Y a ¼ (7.2)
P o
R Dt
0 E ac ðtÞdt
Y f ¼ (7.3)
P o
where E dc and E ac are the generated energy at the DC and AC sides, respectively, of
the PV system and P o is the nominal power of the PV array.
The PR is the ratio of the useful energy that would be generated by an ideal PV
system working at STC temperature, 25 C, and real irradiance conditions of work.

The PR can be evaluated as follows from Eqs. (7.1) and (7.3):

Y r
PR ¼ (7.4)
Y f
On the other hand, there are several sources of losses originated by different kind
of faults that can be present in a GCPVS. Table 7.1 presents a list of possible origins
of faults that may appear in a GCPVS. These faults are divided in terms of their ef-
fects in the PV system behavior. Faults can generate permanent energy losses or tem-
porary energy losses. The normalized total inherent losses in the PV system, L, can
be calculated by the following expression:
H i E dc
L ¼ Y r ðG; T C Þ Y a ðG; T C Þ¼ ðG; TcÞ ðG; TcÞ (7.5)
G ref P o
where Yr(G,Tc) and Ya(G,Tc) are the reference and array yields, respectively, at real
working irradiance, G, and real module temperature, Tc, and H i is the total plane of
2
array irradiation in Wh/m .
These losses can be divided into two groups, the first one associated with thermal
effects, L T , and the second one comprising the rest of possible effects that cause loss
of power, miscellaneous losses, L M , thus:

(7.6)
L ¼ L T þ L M