Irradiance Monitoring for Bifacial PV Systems' Performance and Capacity Testing

Three standards for photovoltaic (PV) performance and capacity testing are evaluated for bifacial PV system reporting: performance ratio, ASTM E2848, and a new draft of IEC 61724-2. In this context, challenges and recommendations for rear irradiance instrumentation are described for three types of bifacial irradiance sensors-horizontal albedometer, backward-facing reference cells (or pyranometer), and bifacial reference module. A year of operating field data for single-axis tracked bifacial and monofacial systems was collected, including periods of high surface albedo due to snow ground cover. If snowy conditions are included, we found that all three methods performed comparably to the monofacial baseline case, but only if rear-measured irradiance is incorporated into the expected energy calculation. The lowest RMS error was obtained by following the draft IEC 61724-2 standard and using a calibrated bifacial reference module for bifacial irradiance resource. If measured rear irradiance is unavailable, field conditions either need to be filtered to avoid variable (snowy) albedo or an albedometer measurement can be used in conjunction with modeled rear irradiance along with the draft IEC procedure. Additional practical factors are described, including the proper placement of rear irradiance sensors and the proper interpretation of IEC 61724-1 bifacial performance ratio calculations.