Energy rating of photovoltaic systems - comparison between measurement and model

Cost of the energy generated by a Photo-Voltaic (PV) plant over the whole plant lifetime depends directly on the amount of energy the plant itself is able to generate under actual operating conditions (i.e. plant configuration and weather conditions). The so-called plant energy rating is a set of measures on arrays of PV modules or on complete plants over long periods, viz. months or years, carried out to measure actual module capability to generate current under different operating conditions. However, time required for these measures is generally incompatibles with industrial development, that need quick answer to address technological choices. Therefore, PV plant energy rating is estimated either by means of statistical-based assumptions, i.e. by coupling actual nominal module efficiency with standard derating factors, or by means of electrical and physical models simulating modules, strings, arrays and complete plants. The target of the present paper is the comparison between energy rating valuated with an yearly measurement on PV modules carried out at ENEL photovoltaic laboratory (Catania, Sicily) and the results obtained by an electrical model developed by Enel Research department in collaboration with Catania University. The technologies under testing were single-crystalline and multi-crystalline Silicon, CdTe, aSi/μcSi, heterojunction. The electrical model has been compared with several mathematical and physical models well known and currently used in scientific literature. Results show that while literature models fails under low irradiance conditions, good results have been obtained by the new model that was implemented, so that the new model guarantees much more precise energy rating capabilities. The model is now being used as the base for return on investment calculations of PV systems of different technologies and is being integrated into an on-line diagnostic systems for optimization of large-scale PV plant operation.