Study Reveals Up To 15% Gap Between Expected And Actual Bifacial Gains
Researchers found that nominal bifaciality factors often overestimate performance in utility-scale solar plants due to temperature and irradiance variables.
A research team from the Universitat Politècnica de València and the engineering firm Grupotec has identified a significant discrepancy between the nominal bifaciality of solar modules and their actual performance in utility-scale power plants. The study, published in Renewable Energy, analyzed two years of operational data from a 50 MW bifacial plant in Spain, later validating the results against four additional sites in Spain and the United Kingdom.
The findings indicate that effective bifaciality is 13% to 15% lower than the nominal value of 0.70. At the reference plant, researchers calculated an average effective bifaciality of 0.610 on a time-weighted basis. This gap widens during periods of high irradiance, with effective bifaciality dropping to between 0.55 and 0.57 when irradiance levels exceed 1,000 W/m². The study attributes this decline to the interplay between rear-side irradiance, module temperature, and mounting configurations.
Data collected from the sites showed that while higher irradiance increases the contribution of rear-side power, the resulting rise in module temperatures reduces conversion efficiency. Specifically, researchers observed that as rear-side irradiance increased, string voltage fell by 8% to 9%, while module temperatures rose by as much as 3.5 C. The study also noted that mounting geometry, such as the use of single-axis trackers versus fixed-tilt systems, significantly influences how effective bifaciality responds to varying irradiance levels.
The researchers also addressed the regulatory bifacial factor of 1.15 established under Spain’s Royal Decree 997/2025. They argued that this figure functions as an administrative conversion coefficient rather than a realistic expectation for operational power gain. At the reference site, no observed bifacial power gain values approached the 1.15 threshold, with the mean value recorded at 0.495.
The authors concluded that relying on a constant nominal bifaciality factor in energy yield modeling can lead to an overestimation of performance. They recommended that designers incorporate the irradiance and temperature dependence of effective bifaciality into their models to better reflect actual operating conditions. Furthermore, the team suggested using field-data-based metrics to improve the accuracy of performance assessments across different climatic conditions and system designs.
