Predicting kWh/kWp performance for amorphous silicon thin film modules

The BP Solar factory in Toano, VA is producing tandem amorphous silicon PV modules called Millennia. With this product commercially available it is important to be able to predict the performance and energy production obtainable from these modules anywhere in the world. The formulas developed to predict crystalline silicon module kWh/kWp performance do not work because for tandem junction a-Si: (1) the temperature coefficients, particularly the one for peak power, are functions of temperature and irradiance. Therefore a single coefficient cannot be used to calculate expected power under different operating conditions; and (2) the output power depends on the history of the module. During operation the module is continually undergoing light induced degradation and thermal annealing.