Soiling loss characterization for Photovoltaics in buildings: A systematic analysis for the Madrid region

Dirt accumulation (“soiling”) on the surface of photovoltaic (PV) modules may cause significant energy output reductions (“soiling losses”). The adoption of suitable soiling mitigation strategies can make PV deployment more attractive, but it requires the assessment of the magnitude and the seasonal evolution of the soiling losses at local scale, as they are highly dependent on the regional climate, the meteorological conditions and the surrounding environment. For the purpose of estimating them for building integrated and building applied PV systems in Madrid city, Spain (i.e. under Mediterranean climate conditions), a systematic optical analysis was performed during a 15-month experimental campaign using a rooftop soiling test bench which allowed the exposure of multiple glass coupons at two tilt angles of interest (8° and 35°) in a semiurban environment. There was no cleaning schedule so that the exposed glass coupons were simply washed away by the rain. Broadband transmittance reductions (Tloss) indicated the occurrence of low soiling losses with yearly-averaged Tloss values of 1.49% at 8° and 1.04% at 35°. Regarding the soiling evolution, the local precipitation regime played a fundamental role. Soiling losses were negligible during frequent-rainfall periods, experienced a moderate increment in spring (March–April), and increased notably during the dry summer season, reaching annual maxima in July (5.13% at 8° and 3.89% at 35°). In order to minimize the local soiling losses, performing module cleaning operations at least during these two times of the year is encouraged. Concerning the soiling spectral effects, the optical loss increased towards shorter wavelengths, making wide-bandgap PV technologies more sensitive to local soiling. Finally, when applying PV performance prediction models which include soiling, the recommended local values for the daily soiling rate and the rain threshold are 0.11%/day and 5.3 mm at a tilt angle of 8° and 0.09%/day and 3.3 mm at a tilt angle of 35°.