NREL’s performance ratio method applied to thousands of PV systems

Researchers have analyzed the monthly performance ratio of 8000 PV installations distributed across the United Kingdom by using the calculation model of the U.S. National Renewable Energy Laboratory. They measured, in particular, thermal impact on the performance ratio (PR) and found that Spring and Summer seasons have higher monthly PR compared to Autumn and Winter.

A research group at the UK University of Huddersfield has assessed the performance ratio (PR) of 8,000 PV power generators spread across England, by applying the monthly PR calculation method developed by the U.S.Department of Energy’s National Renewable Energy Laboratory (NREL).

The PR is a parameter that defines the relationship between the actual and theoretical power production of a PV system and is largely unrelated to an installation’s location and orientation. This value is used to understand how efficiently the PV system is operating. “PR is a popular metric used to indicate the total generation of the output power of PV installations; wherein theory, 100% shows no losses associated with the PV installation,” the researchers said. “In existing PV installations, as a worldwide point of view, the PR is usually ranging from as low as 10–20% up to 95%.”

The NREL model was applied to residential PV systems located in northern England, the Midlands, and southern England, and relying on conventional crystalline silicon modules, over a period of four years from 2015 to 2019.

Through their analysis, the group found that the installations located in the Midlands have the largest “scale” monthly PR of 88.12%, while southern and northern England were found to have a monthly PR of 85.12% and 83.98%, respectively.

According to them, the Midlands has the lowest seasonal fluctuations in solar radiation and temperature over the year, which means a lower thermal impact on the performance of a PV system. “In contrast, Northern England has the highest rates of thermal instabilities,” they stated.

The measurements also showed that the highest monthly PR of 91.62% was registered in the spring season, while the lowest of 84.04% was reported for the winter season. The lower PR in the winter was mainly due to increasing shading conditions and the existence of hotspots.

The results of the analysis were presented in the paper Thermal impact on the performance ratio of photovoltaic systems: A case study of 8000 photovoltaic installations, published in Case Studies in Thermal Engineering.

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