Albedo measurements and energy yield estimation uncertainty for bifacial photovoltaic systems

Abstract

The deployment of bifacial photovoltaic (PV) modules for utility-scale PV plants has quickly proliferated in recent years. As they can harness the irradiance received at the rear side of bifacial generators—which depends on the albedo of the reflector ground—bifacial modules provide higher productivity than monofacial modules, in exchange for a low increase in manufacturing costs. However, the reduction of PV performance estimations uncertainty still represents a main concern for both PV and financial actors, which seek to reduce financial risks of bifacial PV systems. The aim of this work is to provide a better understanding of the impact that albedo measurements have on PV yield estimations uncertainty. We analysed the albedo sensitivity coefficient relating albedo measurement and subsequent energy yield estimations' uncertainties through a representative yield simulation exercise performed for three different sites (Chile, Spain and Sweden) and two types of PV plants: static and single-axis tracker. A formula considering the impact of Ground Coverage Ratio and Inverter Loading Ratio is proposed for this coefficient. We performed an experimental campaign to evaluate the uncertainty associated to different albedo measurement conditions. Surface reflectance of five different ground surfaces has been measured using four sensors, which allowed to obtain three useful types of albedos that we defined and described. A series of recommendations for short-term ground-based albedo measurements that entail a maximum of 1.5% yield uncertainty contribution in the frame of commercial PV plants are then derived. A representative case of actual albedo measurement practices at the current PV market scene is also presented in the form of a commercial albedo measurement project for a future 50 MW bifacial PV plant. Despite being a local characteristic, the most relevant uncertainty source found is associated to the non-homogeneity of the future PV plant's surface. An important consideration is that changes in the use of land caused by the construction of a PV plant lead to an albedo uncertainty that predominates over all the uncertainty sources associated to the albedo measurement conditions. This can also be the case for long-term seasonal variations, which can become the dominant uncertainty source for high latitude locations.