Applicability of Equivalent Diode Models to Modeling Various Thin-Film Photovoltaic (PV) Modules in a Wide Range of Temperature and Irrandiance Conditions
Abstract
In the paper results of fitting of current-voltage (I-V) curves acquired in a wide range of irradiances and temperatures with use of equivalent either single (SEM) or double (DEM) diode model as applied to several commercial thin-film photovoltaic (PV) modules are presented. It is shown that like in case of crystalline silicon PV modules also for CIGS (CuInGaSe2) as well as CdTe thin-film modules both models may be reliably applied whereas in case a-Si (amorphous silicon), whether it is single or multi-junction structure, obtained results can not be accepted as being credible.
Introduction
Equivalent electrical models, either single or double diode, of photovoltaic (PV) cells and modules can be very useful tools when predicting performance of PV generator to be operated in natural, changeable conditions. On the other hand, such models may be also very helpful even when analyzing basic physical processes occurring in the devices, including recombination/diffusion processes. Applicability of both models has been commonly accepted when applied to crystalline silicon PV cells and modules yet there is a very little work analyzing usefulness of these models for thin-film PV devices existing on the market. Here we would like to present a unique approach of applying both models to various commercial thinfilm PV modules, like CIGS. CdTe or even amorphous Si. using large number of I-V curves acquired in natural outdoor conditions during long term monitoring [1]. For that purpose special software allowing numerical fit of measured I-V curves to either of the two models was developed [2]. It automatically imports 7-Kdata stored in SolarLab's database enabling fitting of thousands curves within reasonably short time. After completing each fitting process, which typically takes few seconds only, obtained results are immediately processed showing on the screen dependences of such parameters as components of the dark saturation currents as well as series and parallel parasitic resistances on module temperature and irradiance.
To perform experiments data collected for four commercial modules were analysed. These were Cu(GaIn)Se2 (CIGS) thin-film module ST40 from Shell Solar. CdTe FS50D from First Solar, single junction amorphous silicon (a-Si) A13P-1X from Free Energy Europe and finally triple junction a-Si from Unisolar. As for I-V curves of the last of listed modules practically no satisfactory fitting with neither of the models was obtained hence no data for this module will be presented in further parts of tliis work.
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