Optical losses of photovoltaic modules due to mineral dust deposition: Experimental measurements and theoretical modeling

Deposition of particles on photovoltaic (PV) modules has the potential to increase costs of solar energy production and maintenance and to affect grid-connected energy forecasting. Particles deposited on PV modules can reduce the optical transmission to the PV semiconductor significantly (>50%) due to absorption and scattering. Although there are many previous studies of PV module efficiency losses with respect to exposure time, angle tilt of the PV module, and other environmental factors, there has been little study of PV module efficiency losses with respect to the optical characteristics of the deposited particles (e.g., refractive index, optical depth). Here, we deposited two types of dust onto glass slides, optically absorbing dust and optically non-absorbing dust. We systematically increased the mass per unit area deposited onto the glass slides and measured the optical depth and total transmission (i.e., direct plus diffuse light) using a spectrophotometer with an integrating sphere detector system. Our experimental measurements were compared with a two-stream radiative transfer model, and with Monte Carlo radiative transfer calculations, yielding good agreement for both absorbing and non-absorbing dust. Our results indicate that total transmission decreases approximately linearly as a function of dust mass deposited per unit area, with the slope being highly sensitive to the absorptivity of the dust. The obtained results and models used in this study can be used in conjunction with deposition models to predict the degradation of the optical transmission of PV modules with respect to mass per unit area dust loading.