Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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2.2.2 IV-translation applied to thin film technologies (WP2.4)
Within the diploma work of Stephanie Malik presented in [REF4] ‘three IV translation procedures have
been investigated. This was performed with respect to the current discussion of a new version of the
IEC Standard 60891 ‘Procedures for temperature and irradiance corrections of current-voltage characteristics’.
The methods analyzed are a modified version of the Blaesser method, recently introduced by
W.Hermann from TUV and the linear interpolation approach, published by Hishikawa from AIST. The
original IEC approach is used as a reference.
This study consists in the following of the diploma work of Stephanie Malik where the same procedure
as for c-Si modules has been applied to Thin Film.
Approach
This study focus on the three translation methods:
� The existing IEC 60891
� The Modified Blaesser-Method (TÜV-Rheinland)
� The Linear Interpolation (Hishikawa)
Details of these methods are presented in [REF4]. Three thin film modules, which are one FirstSolar
(FS60), one Kaneka (K60) and one UniSolar (ES-62T) are considered within this assessment. The
base data consisted of IV-measurements measured with a class A solar simulator at 5 irradiance levels
(200, 400, 600, 800 and 1000W/m²) and 8 temperatures, ranging from 25-60 °C (at 1000 W/m²
only at 25 °C). To start with, some base I-V curves were defined for each method. The base curves
were then extrapolated and/or interpolated to the remaining environmental conditions and compared
to the measured ones.
Results
Table 2 summarises the results for Pmax errors obtained with the three IV translation methods.
� G=200 W/m² G=400-1000 W/m²
� T=25-65°C T=25-65°C
avg. error St.Dev avg. error St.Dev
IEC60891
4.80% 4.39% -0.03% 1.41%
Modified Blaesser -1.95% 1.94% 0.56% 0.86%
Linear interpolation 1.00% 0.60% 0.55% 1.26%
Table 1: Average over a defined range of irradiances and temperatures of the avg. Pmax error of three
a-Si modules obtained by the three IV-translation methods. Note: standard deviation in italic.
The original standard (ed. 1987) recommends that the target irradiance should be within ±30% of the
base irradiance. This is why we distinguish here between the error at 200W/m² (outside of 800W/m²
± 30% range) and the other irradiance levels. As represented in Table 2 the highest errors are obtained
at low irradiances. In general, the less accurate method resulted to be the original IEC60891
procedure. The trend on the results on thin film modules is the same as obtained with c-Si modules,
but with significant higher errors. For instance, in some cases IV translation methods led to errors in
Isc translation of up to 20%.This can be explained by the not applied spectral mismatch correction at
the different irradiance levels.
In order to improve the results an approach based on the self reference method is currently investigated
at ISAAC. The latter consists in the use of module short circuit current (Isc) at STC for determination
of irradiance instead of reference cell value.
Results will be available beginning of 2009.
PERFORMANCE, G. Friesen, SUPSI, ISAAC-TISO
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