Effects of module performance and long-term degradation on economics and energy payback: Case study of two different photovoltaic technologies

Both the economic viability and energy payback time of photovoltaic (PV) systems are inextricably tied to both the electrical performance and degradation rate of PV modules. Different module technologies exhibit different properties in response to varying environmental conditions over time. The purpose of this study is to quantify the effects of those differences on the life-cycle economical cost and energy payback time of two fielded PV systems; one system comprised of polycrystalline silicon (c-Si) modules and one featuring hydrogenated amorphous silicon (a-Si) modules. The DC operating current, DC operating voltage, AC power, and conversion efficiency of each system have been monitored for a period of over four years, along with plane-of-array (POA) irradiance, module temperature, and ambient temperature. Electrical performance is evaluated in terms of final PV system yield (Yf), reference yield (Yr), and performance ratio (PR), which are derived from the primary international standard used to evaluate PV system performance, IEC 61724. Degradation rates were evaluated over the four year period using regression analysis. The empirically determined trends in long-term performance were then used to approximate the energy produced by both system types under the same environmental conditions; most importantly, the same levels of solar irradiation. Based on this modeled energy production and economic conditions specific to the state of Florida, comparisons have been carried out for life-cycle costs and energy payback time.