Effect of using global horizontal or plane of array irradiance for monitoring sun tracking solar photovoltaic plants performance

Abstract

The performance ratio (PR) is a metric commonly used for assessing the performance of a photovoltaic (PV) plants worldwide. The standard PR calculation method (PRPoA) used in solar PV industry as per the International Electro-technical Commission (IEC) standard uses the solar irradiance in the Plane of Array (PoA) as reference. The CSIR PR calculation method (PRGHI) used Global Horizontal Irradiance (GHI) as a reference for the first phase of PV plants realized as part of the Energy Autonomous Campus program. The PRGHI holds the EPC contractor liable for any underperformance due to poor layout or tracker operations. The PRGHI is variable in the short term and highly variable across the seasons compared with the PRPoA. The temperature corrected PRGHI and PRPoA are calculated to minimize any biases due to temperature which may arise from different weather conditions. The PRPoA is less variable in the short term and across the seasons, so any loss or gain in performance is more easily identified for both the single axis and dual axis tracker systems. The seasonal effect on PRGHI is larger for the dual axis tracker system compared to single axis tracker PV system. The increasing PRGHI between summer and winter solstice for the dual axis tracker system is due to a decrease in the reference irradiance caused by the higher angle of incidence of the sun on the GHI reference sensor and is not due to an increase in PV electrical output. The predicted and actual PRGHI and PRPoA are compared for the single and dual axis tracker systems for a period of one year without temperature correction. The absolute delta between the predicted and actual PR for the single axis tracker was -10% for PRGHI and -4.5% for PRPoA. The absolute delta for the dual axis tracker was -4.5% for PRGHI and +0.4% for PRPoA. The large absolute PRGHI delta between the actual and predicted PRs for the single axis tracker is investigated and this paper focuses only on the tracker performance. The actual tracker tilt angle performed optimally during the summer solstice period but suboptimally during the early morning and late afternoon between the summer and winter equinoxes. Future work will characterize the effect of the sub-optimal tracker performance in terms of PoA irradiance and energy production.