Measurement of Soiling Loss on Bifacial PV Modules

Bifacial photovoltaic (PV) modules can absorb light from both front and rear sides of the module, resulting in an additional gain of ~ 10-15% in output. The additional gain depends on the amount of reflected light (albedo) reaching the rear side. This is highly depended on the local site layout, ground cover, distance between modules and the ground, distance between modules in a row and the distance between the rows. There is lot of uncertainty in energy forecast due to the variabilities of albedo due to all these factors. These uncertainties are a hindrance in adoption of bifacial modules in the industry and hence have attracted lot of research interest recently. Measurement of soiling loss in bifacial modules is another area that is very important and suffers from uncertainties in the albedo at a site. The problem is compounded due to a slight spectral mismatch between the front and rear side, and anticipated differences in the rate of soiling on two sides. In this paper we report on experiments using two identical half cut cell bifacial PV modules in Logan, UT for about 6 months during June-December 2020. The short circuit current of the two modules has been measured along with the supporting measurements of incident and reflected light intensity underneath each module. A method is proposed to separate the contributions to module output from front and rear sides. The soiling loss on both sides can also be calculated separately, using this method. The soiling loss is dominated by soiling on the front side as expected. The soiling loss on the rear side is minimal and may be affected by the spatial variations in the albedo across the two modules under comparison.