Technical evaluation of a dual-junction same-band-gap amorphous silicon photovoltaic system

Monitoring of a 1.8-kW(ac) utility-interconnect photovoltaic (PV) system employing amorphous silicon (a-Si) modules was initiated on December 7, 1992, at our outdoor PV test site. This system was deployed to conduct an in-situ technical evaluation of the PV array (in a high voltage configuration), and system performance and reliability in a utility-interconnect application. The system is unique because construction-grade insulation was added to the back of each PV module. Insulation was added in an attempt to levelize the annual array power output by elevating the operating temperature of the modules (thus enhancing thermal annealing). Array and system performance data are presented and the effects of individual losses as well as seasonal changes on PV array and system performance are quantified. In agreement with past results we show that the seasonal variations in array peak power largely result from changes in current that are well-correlated with temperature and inversely correlated with air mass. Thermally-induced annealing and light-induced degradation are shown to be prominent influences on dual-junction a-Si PV array performance. We also show that fill factor is relatively stable. This is attributed to the opposing influences of thermal and spectral effects, causing a reduction in the seasonal variation in fill factor.