Numerical modelling of the coupling of thermal and photoelectric effects for the photovoltaic modules under low concentration

The 'Aleph' project (Amelioration de l'Efficacite Photovoltaique) was inspired by the design constraints of conventional horizontal-plane photovoltaic (PV) installations, such as rooftop installations and large-scale solar farms installed in the field. In a typical installation, the ground coverage ratio (the ratio between the PV cell and ground surfaces) is limited by the need to minimize shading periods which occur in the morning and evening periods. The resulting system is of a relatively low angle of inclination of the PV modules (typically 10 deg. less than the latitude) and an inter-row spacing equal to 2-5 times the module height. This arrangement results in the inter-row space being illuminated during the periods of highest solar irradiation (around solar noon) without contributing to electricity production. The Aleph project aims to explore the benefit of adding interrow planar reflectors to increase the system yield, and define clear rules for optimal settings of such systems in a given location and under a given climate. Existing literature shows that the use of planar reflectors is an already established technique for increasing the solar flux incident on a collector. Various studies have been performed for estimating the increase in irradiation from a given collector-reflector system geometry, most frequently focusing on solar thermal systems. However, very few extensive studies have been carried out on planar reflector-equipped PV systems. Existing studies rarely involve both numerical and experimental parts or test more than one PV module technology in parallel. This work combines experiments on PV module behavior in an outdoor environment on the SIRTA (Site Instrumental de Recherche par Teledetection Atmospherique) meteorology platform (Palaiseau, France, 48.71 deg. N, 2.21 deg. E) and a multiphysics numerical model used to simulate the system behaviour considering all the important physical phenomena (notably the optical, photoelectric, and thermal effects). Two prominent PV technologies are tested for performance with this type of system: hydrogenated amorphous silicon (a- Si:H) and polycrystalline silicon (p-Si). The experimental data show significant gains in produced energy brought by the reflectors, and highlight the advantage of a-Si:H modules compared to p-Si modules. The model is built in a modular way, allowing for testing of several modelling approaches of a given physical phenomenon (e.g. analytical vs. stochastic optical model). Some of the highlights of the modelling work include a Monte-Carlo ray-tracing optical model and a cell-level photo-electric model. The proposed integrated model is calibrated with outdoor measurements using an evolutionary algorithm. Once calibrated, the model demonstrates good performance in predicting the module power output as a function of atmospheric and irradiance data. Several model applications are demonstrated through case studies, such as designing for a specific reflector performance, evaluating achievable gains under different climates, and evaluating the impact of string mismatch. (author)