Potential of Decimeter-Resolution Ground Albedo Data for Bifacial Photovoltaics

Abstract

Performance assessments of bifacial photovoltaic (PV) plants can exhibit notable deviations in particular due to often inaccurate rear side irradiance information. Such inaccuracies can result from the spatial and temporal variability of ground albedo which is often used to derive rear side irradiance. In this contribution we investigate the potential of high-resolution (15-30 cm) satellite images to obtain highly accurate and spatially resolved ground albedo information within the area of existing bifacial PV plants. We perform a case study with exemplary PV plants in Spain and Germany and evaluate the benefit of such data for a more accurate performance assessment and fault detection for bifacial PV plants. The resolution of proposed satellite images is high enough to distinguish PV modules, areas shaded by PV modules and unshaded ground. This allows us to determine a ground albedo map only using suitable, unshaded pixels. From this highly resolved albedo map we calculate 1) the overall representative ground albedo for each PV plant, 2) local albedo data mimicking albedo measurements in the PV plant and 3) 500 m x 500 m-resolution albedo averages, thereby mimicking state-of-the-art satellite albedo data such as MODIS [1]. For this 500-m resolution albedo we include also pixels featuring PV and shadows. Finally, we model the different annual productions of the PV plants detected in the satellite images applying ground albedo from the three different sources and compare the results. The mimicked ‘MODIS’ albedo deviates from the representative one by -0.1...0.04. These deviations are explained by different vegetation types within and outside the PV plants and also by disturbances introduced by dark PV modules and their shadows. The mimicked albedo measurements deviate by up to -0.08…+0.09 from the representative albedo, depending on the evaluated site and the simulated albedometer’s location. The predicted PV production can deviate notably (-1.6…+2.1%) from the reference case with a representative ground albedo if only a single albedometer is used. Between 40-60% of the measurement points in the studied PV plants lead to absolute deviations of the modeled PV production of ≥1%. With the ‘MODIS’ albedo, the predicted PV production deviates by -1.9…+1% from the reference case. In summary, our work shows that high-resolution satellite images can address the shortcomings of the state-of-the-art solutions to assess ground albedo. We also discuss how important seasonal effects such as snow and vegetation change could be included. Further, approaches to combine high-resolution albedo information with ground-based albedo measurements and state-of-the-art satellite data to combine the strengths will be presented.