Single-Axis Tracking and Bifacial Gain on Sloping Terrain

This paper describes a mathematical model for dealing with large bifacial single-axis tracking photovoltaic (PV) plants over terrain of arbitrary orientation and slope. The only constraint is that the ground surface and the plane of the tracker axes must be parallel. This allows for two-dimensional (2D) modelling of the ground shading scene required for backtracking geometry and rear irradiance calculations. The model has been implemented in SISIFO, an open PV simulation tool developed by IES-UPM, which is slope aware since June 2022. In addition, a set of equations for upgrading tracker controllers, previously restricted to horizontal terrains, is also provided. As a representative case, a real 90 MW_p PV plant installed on an uneven terrain is analysed. The orography of the ground is described as a set of facets with different azimuth and slope angles. The plant is also described by the relative frequency distribution of the STC power over the different facets. A dedicated simulation exercise is then performed for each facet. Depending on the azimuth and slope values, the resulting final energy yield may be higher or lower than that associated with a horizontal terrain. The yield of the whole PV plant is calculated as a weighted average of the results, with the relative frequency being the averaging factor. If the plant is placed on favourable facets, the energy yield of the whole plant can be close to that calculated on horizontal terrain, at the cost of discarding a part of the available land. In this case, a 0.4% increase in yield is obtained at the cost of discarding about 40% of the available land.