Effects of solar irradiation conditions on the outdoor performance of photovoltaic modules

Energetic losses, relative to the standard conditions of testing, in photovoltaic modules in outdoor operation, were analyzed and the role of the optical effects is discussed. The following four loss effects were estimated: (a) reflection of unpolarized light, (b) spectrum, (c) intensity of the light and (d) temperature of the module. Four independent models were used to describe these four losses. The models were validated by the experimental data of an outdoor measurement campaign performed on 0 degrees tilted modules at 41 degrees N latitude in South Italy. Disagreement reaching 5% under clear sky conditions was found between theoretical predictions and experimental data for the instantaneous total loss. As a result of a critical analysis of the literature data on this subject, it could be explained by invoking the presence of a fifth loss mechanism: the polarization of the incident light. Final relative losses, due to the particular state of the incident sunlight, amount to about 7-8% of a total of 14-15%. Of these, 3% is due to the low irradiation level, 1-2% to the polarization of the skylight and 3% to the reflection of the incident light on the front cover of the module. The spectral effects are negligible (less than 1%). The remaining 7% loss is due to temperature effects on the module. All the loss data are reported as a function of the air mass (AM). The maximum operating efficiency is reached at AM approximate to 1.5. (C) 1998 Elsevier Science B.V. All rights reserved.