Spectral Influences on Performance: A South African PV Plant after 4 years of operation

The performance ratio of a South African photo-voltaic (PV) plant, over 4 operational years, is discussed by assessing the normal and weather-corrected performance ratio calculation under well-known spectral influences, being airmass and precipitable water. Performance analysis indicates that the normal performance ratio is overestimated during the Southern hemisphere winter months and underestimated during the summer months. The performance ratio is determined for different intervals of airmass and precipitable water to identify scenarios in which the normal and weather-corrected performance ratio shows significant deviations. In addition, the spectral irradiance of polycrystalline PV modules is simulated for different airmass and precipitable water values to identify the wavelengths where the PV module underperforms. Both the normal and weather-corrected performance ratio is decreased when airmass levels are higher than 3, precipitable water is greater than 0.8 cm, and calculated cell temperatures below 30° C. The corrected performance ratio is more stable for ideal operating conditions, such as higher irradiance, moderately higher cell temperatures, lower airmass, and clearer skies where the precipitable water is lower. The performance ratio shows significant deviations under nonideal operational circumstances and the results indicate that the spectrum has considerable influence on the performance of the photovoltaic module. The spectrum is affected in the 0.9 to 1.0 µm wavelengths for changes from standard testing conditions in both the airmass and precipitable water. Changes in airmass have a greater effect on spectral irradiance for wavelengths from 0.7 to 0.8 µm. The PV module's spectral response is higher in the 0.7 to 1.0µm wavelengths, which results in a more pronounced effect on the PV module performance.