Assessment of Bifacial Photovoltaic Module Power Rating Methodologies -- Inside and Out

One-sun power ratings for bifacial modules are currently undefined. This is partly because there is no standard definition of rear irradiance given 1000 W·m^-2 on the front. Using field measurements and simulations, we evaluate multiple deployment scenarios for bifacial modules and provide details on the amount of irradiance that could be expected. A simplified case that represents a single module deployed under conditions consistent with existing one-sun irradiance standards lead to a bifacial reference condition of 1000 W·m^-2 G_front and 130-140 W·m^-2 G_rear. For fielded systems of bifacial modules, G_rear magnitude and spatial uniformity will be affected by self-shade from adjacent modules, varied ground cover, and ground-clearance height. A standard measurement procedure for bifacial modules is also currently undefined. A proposed international standard is under development, which provides the motivation for this paper. Here, we compare field measurements of bifacial modules under natural illumination with proposed indoor test methods, where irradiance is only applied to one side at a time. The indoor method has multiple advantages, including controlled and repeatable irradiance and thermal environment, along with allowing the use of conventional single-sided flash test equipment. The comparison results are promising, showing that indoor and outdoor methods agree within 1%-2% for multiple rear-irradiance conditions and bifacial module construction. A comparison with single-diode theory also shows good agreement to indoor measurements, within 1%-2% for power and other current-voltage curve parameters.