New method to measure energy yield of bifacial PV systems

Researchers from the Sunlab laboratory at the University of Ottawa have devised a new way to measure the energy yield of bifacial PV systems. The innovation is intended to complement the International Electrotechnical Commission (IEC) measuring standard introduced in 2019.

The new scaled rear irradiance (SRI) method purportedly improves IEC measurements by accommodating the effects of different ground covers’ spectral albedo in its calculation of the bifacial system’s rear-side irradiance.

To prove the concept, the scientists modeled the performance of a bifacial silicon heterojunction (HJT) solar module under the IEC bifacial illumination technique and the SRI method. They mounted the system on a horizontal single-axis tracker 1.22 meters from the ground in Boulder, Colorado, United States. The ground covers considered were soil, red brick, green grass, roof shingle, concrete, sand, dry grass, and snow.

Results show that for a rear irradiance of 200 W/square meters, the IEC measurement does not distinguish between the different ground covers, overestimating bifacial gain by between 14% and 1.4% for all ground covers except snow. The SRI method, on the other hand, shows a clear differentiation between the range of operating conditions.

“Implementation of this method into international standards for such panels can enable predictions of outdoor bifacial panel performance to within 2% absolute,” said researcher Erin Tonita.

The new method can also predict energy yield gains in locations other than Boulder with an error of less than 7%, according to the academics.

The innovation could be used to compare different bifacial technologies and enhance system performance via ground cover-specific design optimization, the authors said. They shared their findings in “A general illumination method to predict bifacial photovoltaic system performance,” which was recently published in Joule.