Energetica India Magazine: september 2020

SOLAR POWER How to obtain the extra energy gain promised by bifacial technology? For the solar recycling industry to grow sustainably, it will ultimately need supportive policies and regulations. The EU model of having producers finance the take-back and recycling of solar panels might be a good one to emu- late. But before that is going to happen, we need to recognize that the problem exists and is going to get bigger. 26 energetica INDIA- September_2020 Javier Tamayo Design Engineer, STI Norland Introduction Over the last few years, there has been an increasing trend in the use of bifacial solar panels, a technology already assessed a few decades ago, but which due to its high cost failed to ex- pand. It has been recently that optimized manufacturing pro- cesses have allowed bifacial panels to compete with monofa- cial panels. The solar generation gain in the change from monofacial to bi- facial is currently around 6-8%, a percentage known as bifacial gain , which is conditioned by multiple factors. More specifically, there are three base parameters directly de - termining photovoltaic project bifaciality, namely the following (in order of relevance): - Amount of irradiance : Obviously, the more light received, the more electricity can be generated from solar resources. It is important to mention that most captured irradiance comes in the form of direct irradiance, which is reflected on terrain areas not affected by the solar panel own-shading on the ground. - Albedo : It refers to the proportion of irradiance actually re- flected by the ground. It depends on the type of terrain used in each project and varies throughout the year. As a reference, common sandy terrain has an average albedo value of 0.25. - View factor : It is directly linked to the geometry and posi- tion of the irradiance capturing surface (panels), in relation to the irradiance-emitting surface (ground). Thus, analyzing the tracker choice is paramount to ensure bifaciality adequacy. In other words, it is essential to thoroughly assess the amplitude with which tracker panels capture ground-reflected irradiance. As above mentioned, out of these three key factors, the first two are specifically defined for each project: - Irradiance is determined by geographical location. - As for albedo and despite the availability of techniques to improve this parameter, it is not so clear to date to which extent investing and optimizing albedo will establish itself as a com- mon application of bifacial projects. However, the geometric form factor does vary within a project for the same irradiance and albedo, depending on the solar tracker used. Analytical approach to bifaciality Normalized height – aspect ratio According to an analytical approach, irradiance reflected and captured by a collector is not related to collector size. As long as the collector is installed at a distance proportional to the surface that reflects solar irradiance, it should in return reflect the same irradiance. The width of the emitting surface grows proportionally to the increase in the collector height, so it should capture the same irradiance in return.