Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

3.2.2 Verification of the delivered power (Pa)
The difference between Power at Purchase (Pa) and Nominal Power (Pn) of the modules shouldn’t be
greater than Production Tolerance (tp).
In test cycle 10 the power at the purchase of a c-Si module was on average 1.5% lower with respect to
Pn but ranging from 0.2% and -3.8% (see Table 1). In 3 of 11 cases power at purchase was outside
production tolerance.
The initial power Pa of the FS-60 module (CdTe) is not available, due to problems with the measurements.
In thin-film a-Si modules the given power is that already stabilised. Stabilisation of the performance of
a module normally takes a few months and verification is only possible when power is stabilized. Nevertheless,
the initial power of the K60 modules is 39.9% higher and at the planning stage the system
has to work at higher power, current and voltage for a number of months
3.2.3 Verification of the stabilized power (P15)
After one year of exposure, stabilization has occurred. PV module lifespan is long and generally is
considered as being over 25 years. The great reliability and stability of PV modules justifies this warranty
of more than 20 years and the stabilized value P15 can be considered the reference value of
real power.
Warranty limits are normally defined as a percentage with respect to the nominal value and sometimes
with respect to the minimal tolerance value {(1 – tp)*Pn}.
The stabilized power of a c-Si module after 15 months is on average 3.6% lower with respect to Pn
and ranges from -0.7% to -8.2%. All type of modules are within the warranty limits.
For three types of module the warranty limits were not clearly defined.
Both the power of the a-Si K60 modules and that of the CdTe FS-60 modules respected the warranty
limits. The stabilized power of ES-62T modules after 15 months is on average 12.3% lower with respect
to Pn. In the datasheets the warranty limits were not clearly defined.
The Sunpower modules have high efficiency A-300 type cells with EWT (Emitter Wrap-through) back
contacts. The high efficiency of SunPower’s A-300 solar cell is obtained in part by covering its front
surface with a proprietary coating which prevents the loss of the charge carriers generated by sunlight.
However, this creates a problem called ‘surface polarization’ which risks limiting output power.
The BP7180 modules have Saturn 7BP Solar cells. Internal communications of BP say that these cells
do not have an Anti Reflective Coating (ARC). In spite of the change in cell type the name of the module
is still the same. As a consequence, modules exist on the market with the same name but with
different performances.
3.2.4 Power degradation after exposure
Almost all standard c-Si PV modules tested in cycle 5 to 10 showed a degradation in performance
when exposed for the first time to sunlight. Such power degradation occurs during the first hours of
exposure (H=2.5 kWh/m 2 ) and ranges normally between 0 and 5 %. In order to avoid initial degradation
effects influencing the determination of module energy yield, the test procedure of the ISAAC-
TISO include, for c-Si modules, a period of light soaking of 20kWh/m 2 to 40 kWh/m 2 followed by
3 months of stabilization.
Prior sunlight exposure and storage time and subsequent pre-degradation of the modules purchased
is not known, so the initial power Pa measured at ISAAC-TISO could correspond to the power of already
degraded c-Si modules.
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Centrale di test ISAAC-TISO, D. Chianese, ISAAC-TISO
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