Planned Field Test of Soiling and Irradiance Measurement Uncertainties in Bifacial PV Systems Using In-Situ Reference Modules

We present a study plan to investigate uncertainties in soiling and irradiance measurement in a bifacial photovoltaic (PV) power plant using in-situ I-V measurements of module power. Soiling ratio is the ratio of actual module power output to expected power under clean conditions. However, precise determination of expected module power output for clean conditions is challenging for bifacial systems because of the need to account not only for front-side but also for rear-side irradiance contributions which have greater sources of variability. We present plans for a recently initiated field test to assess two methods of determining soiling ratio. In both methods we will use in-situ module I-V measurement to directly measure power output of a soiled reference module within the plant, yielding the numerator of soiling ratio. In method one, for the denominator of soiling ratio, the expected module power, we will use power measured from another reference module that is routinely cleaned. In method two, for the denominator we will use module power estimated from front and rear-side cleaned reference cells. Our aim is to determine the precision of soiling ratio measurements in a bifacial system given the challenge of normalizing for rear-side irradiance. For both methods we plan to quantify drift in the soiling ratio baseline corresponding to zero soiling loss and use this as a measure of soiling ratio precision.