Estimation of the maximum power temperature coefficients of PV modules at different time scales

The maximum power temperature coefficients of a-Si modules are negative in indoor measurements, whereas the performance increases with module temperature in outdoor measurements. In order to resolve the discrepancy, we investigate the temperature coefficients of a-Si modules by two different methods corresponding to two different time scales. The results are that the temperature coefficient for a shorter time scale of several hours is negative, although the temperature coefficient for a longer time scale of seasons is positive. These results suggest that the discrepancy in temperature coefficient is partially caused by the effects of thermal annealing and light soaking. Therefore, the history of module temperature and received solar irradiance would be required for the correction of the performance of a-Si modules. In addition, we reveal that the effect of solar spectrum is also a contributory factor in the discrepancy.