Effects on photovoltaic solar module performance in the UK climate

Abstract

SYNOPSIS Presently, PV devices are rated on the basis of a standard test condition (STC) efficiency and not on their energy production, which would be a much better parameter for their cost per generated kwh. There is a lack of knowledge of factors affecting performance of thin film PV devices, especially operating in maritime climates such as the UK's. Modules studied in this investigation include crystalline, polycrystalline and amorphous silicon, cadmium telluride and copper indium diselenide. The different factors affecting the performance of PV modules operated in the UK climate are investigated. The main effects identified are changes in the spectrum, operating temperature and irradiance, each affecting device performance according to the type of cell material. The spectral effect is predictably more pronounced for the wider band gap materials (amorphous silicon and cadmium telluride) and includes two separate identifiable effects, termed primary and secondary. The primary spectral effect depends on the availability of spectrally useful irradiance and is more pronounced for the amorphous silicon (a-Si) single junctions, where it varies by + 5% to—9% with respect to the annual average. There is also evidence of a secondary effect in the a-Si multi-junctions, due to mismatch of current between the series connected sub-cells, the magnitude of which is much less than the primary spectral effect, but is noticeable nevertheless. The materials with narrower band gaps (polycrystalline, crystalline and copper indium diselenide) suffer more significantly from thermal effects and less from spectral effects than do devices with a wider band gap. The effect of radiation intensity is device-specific; some devices (crystalline and polycrystalline) benefit from increased irradiance, although some thin films show a deterioration of efficiency with increasing irradiance. The importance of shunt and series resistance is discussed in this context.