TECHNICAL EVALUATION OF A DUAL-JUNCTION SAME-BAND-GAP AMORPHOUS-SILICON PHOTOVOLTAIC SYSTEM

Monitoring of a 1.8-kW(ac) utility-interconnect photovoltaic (PV) syst em employing amorphous silicon (a-Si) modules was initiated on Decembe r 7, 1992, at our outdoor PV test site. This system was deployed to co nduct an in-situ technical evaluation of the PV array (in a high volta ge configuration), and system performance and reliability in a utility -interconnect application. The system is unique because construction-g rade insulation was added to the back of each PV module. Insulation wa s added in an attempt to levelize the annual array power output by ele vating the operating temperature of the modules (thus enhancing therma l annealing). Array and system performance data are presented and the effects of individual losses as well as seasonal changes on PV array a nd system performance are quantified. In agreement with past results w e 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 l ight-induced degradation are shown to be prominent influences on dual- junction a-Si PV array performance. We also show that fill factor is r elatively stable. This is attributed to the opposing influences of the rmal and spectral effects, causing a reduction in the seasonal variati on in fill factor.