Measuring dirt on photovoltaic modules. Part II

The accumulation of surface dirt on photovoltaic (PV) modules installed at MIT Lincoln Laboratory test sites can cause degraded electrical performance until it is removed either by washing or by rain. Modules with only RTV as a solar-cell encapsulant are more prone to dirt retention than modules covered with glass. Two techniques for measuring and quantifying surface dirt and its effects on module output are described. One technique, using a standard portable glossmeter, measures the scattering of specular light by surface-dirt particles. This measured value can then be correlated with the peak power performance before and after cleaning. A second technique is also described which enables an investigator, in the field, to take dirt samples or replicas of the accumulated surface dirt from the PV module. Photomicrographs of urban, suburban, and rural dirt particles are shown. Measurements of module peak power before and after cleaning indicate that dirt particles in urban environments are more degrading to PV performance than dirt particles found in rural areas. The significance of the measurements is not obvious, however, after examination of the photomicrographs of the dirt particles. It was found that the optical transmission of the particles has a greater effect on peak power performance than the quantity of these particles.