Bifacial Photovoltaic Modules: Measurement Challenges

The photovoltaic market is currently competing for high efficiency cell technologies. Several of these technologies are inherently bifacial. For large commercial systems, the expected annual bifacial gain is significant, from 5 to over 15% [1]. But the lack of standardization [2] and feedback on large systems seems to limit the proliferation of bifacial modules. There are no Standard Test Conditions defined for their measurement, and no available commercial simulator that can predict their energy production. As a result, investors are still reluctant to choose bifacial technologies as an alternative to the standard monofacial ones. In this paper, we analyse three different approaches for bifacial module performance measurements. The first approach consists in measuring both sides independently with a standard solar simulator and build an equation to extrapolate the contribution of back side illumination to the front-side power value. The second approach consists in illuminating both sides simultaneously with a specific double illumination characterization setup [3] that verifies the results of the first approach. Finally, the third approach compares outdoor and indoor behaviour of the modules. The results presented in this paper show how the different approaches are complementary to help building up Standard Test Conditions and outdoor simulation tools for bifacial modules.