Spectrally selective sensors for PV system performance monitoring

The main purpose of PV system performance monitoring is to determine whether the system is operating as expected. This requires measuring the actual output of the system as well as the conditions under which it is operating. Solar radiation intensity in the plane of the array (POA) is by far the most important operating condition and is the basis for calculating performance ratios (PR). However, differences in spectral and directional response between pyranometers and PV modules lead to intraday as well as seasonal fluctuations in the performance ratio, even though the system may be operating without faults or degradation. For c-Si PV systems, a reference cell can be used instead of (or in addition to) a pyranometer to measure POA irradiance. Since reference cells have spectral and directional responses that are similar to PV modules, their output signals correlate better with system output. As a result, it is becomes easier to identify abnormal system operation. In this paper, we present a practical alternative to using a spectrally matched reference cell for measuring POA irradiance. The method uses dual sensors with different spectral responses whose outputs are combined to produce a composite signal. That composite signal can effectively match the spectral response of any PV module type over a wide range of irradiance conditions. As a result, this method makes possible the rapid and accurate detection of abnormal system operation for thin-film systems.