26 November 2019

The Bifacial Year

Evaluations over a 12-month period at BiTEC confirm the trend that the SF7 Bifacial solar trackers in 2P obtain significantly more Bifacial Gain than 1P, 2.1%. Albedo and tracker geometry are critical for Bifacial Gain.

During the wet season, rainwater puddles caused reflections on the surface, with an unusual effect on bifacial modules: higher rear irradiation due to short-lived albedo increasement. However, such effect did not result in significant power generation gains. In fact, albedo decreased during the following days due to the dark color of wet soil.

Figure 9. Example of reflection effects caused by puddles in January 2019. Source: BiTEC

Ground surface conditions were subject to continuous changes impacting their albedo. As an example, the experience at BiTEC supports the hypothesis that albedo, and in turn irradiation reflected on the rear side of modules, decreases in humid environments [5].

Measurements for the different types of surfaces analyzed at BiTEC confirm that soil changes in time have a direct impact on the albedo value. The figure 10 shows measurement variability compared to the range of typical values established by NREL for each type of surface.

It is important to mention that bifacial gain estimates for long-term analyses requires the use of representative albedo values throughout the period being analyzed.

Figure 10. Albedo measured at BiTEC for each type of soil versus the range of NREL values. Source: 8

1. Annual Results at BiTEC: Albedo Bifacial Gain

Figure 11 shows full-year results for different albedo combinations.

As seen in the table:

  • For high albedo (55.6%), that is, white sand or snow-covered ground, the bifacial gain of a 2P SF7 bifacial tracker is 15.7%
  • For a medium albedo (29.5%), such as sand, the bifacial gain is around 9.6%.
  • For cultivation areas with an albedo of 19.9%, the bifacial gain of a 2P SF7 bifacial tracker is 7.3%.

BiTEC analyses revealed a linear dependency between bifacial gain and albedo. Besides, the influence of the remaining parameters under analysis, such as height, GCR and temperature, is always affected by the albedo. However, its influence can be assessed separately.

Figure 11. Dependency of yearly bifacial gain in relation to albedo. Yearly Values Measured ar BiTEC. Source: Soltec
Type of groundAlbedoBifacial Gain


Javier Guerrero-Perez holds a Ph.D. in Renewable Energy. His professional activity spans over ten years in the solar industry within multinational EPC operations. He has published several papers on modelling electrical behavior of both PV modules and inverters, oriented to large scale simulation. Since 2015, when he was part of the team of La Silla PV plant (2015), current research lines are focus on modelling the bifacial PV modules behavior while he is managing Soltec’s Bifacial Tracker Evaluation Center in Livermore, California.
Irene Muñoz Benavente holds an International Ph.D. with special honors in Renewable Energy from the Technical University of Cartagena, working in the renewable energy sector since 2016, specifically in Photovoltaic. Focused in optimizing tracking algorythms for solar trackers.
José Navarro Berbel holds a Master of Science degree in Sustainable Energy of the Technical University of Denmark. He has worked as a Termotechnology Product Manager and right now he is a simulation specialist at bifacial solar plants in Soltec.