A detailed analysis of gains and losses of a fully-integrated flat roof amorphous silicon photovoltaic plant

In 2003 a fully-integrated photovoltaic (PV) plant composed by amorphous silicon PV modules was installed on top of a flat roof in Lugano (Southern Switzerland) – a site representative for most of continental Europe – and continuously monitored since. This work follows a previous study which analyzed the first 2 years of operation of the plant, ascribing most of the noticeable winter losses to reflection losses due the lower position of the sun in the sky. Other loss mechanisms were discussed only from a qualitative point of view. The energy production of this particular PV installation is in fact influenced by several combined phenomena such as Staebler–Wronski, spectral variations, temperature and optical losses effects.The present work aims to widen the analysis by discerning between these partly competitive effects and attempts to give a quantitative description of the influence which each single phenomenon has on the energy performance of the PV plant. For this purpose, single PV modules similar to those of the plant (triple-junction a-Si) were subjected to several indoor and outdoor tests.By means of indoor characterization we found that reflection losses become significant for angles of incidence larger than 50°. Repeated indoor and outdoor degradation–recovery cycles underlined the influence of annealing time and temperature on the recovery of the PV modules. In particular outdoor degradation tests showed that at our latitudes (46°N) the influence of the Staebler–Wronski effect on the output power of these devices is around 10% (±5% around an annual average value).The influence of the spectral effects on the current of amorphous silicon modules was assessed by means of outdoor IV characterization: the short circuit current decreases linearly with AM value at a slope between 4% and 8% per AM-unit depending on the technology under investigation.Combining these three effects with the effect of temperature the authors are able to perform a simulation of gains and losses of the a-Si modules which well approximates the energy performance of the CPT-Solar plant over a whole year.

► We monitor a triple-junction a-Si PV plant integrated into a flat roof.
► We analyse four effects influencing the energy behaviour of the plant.
► Reflection losses, Staebler–Wronski, spectral variations and temperature effects.
► We develop a theoretical model describing the real energy performance of the plant.