A new performance index for PV system analysis

Abstract

This paper describes a term called the Performance Index (P.I.) for analyzing photovoltaic (PV) system performance. The P.I. is the dimensionless ratio of actual to expected generation over any time interval.PV system performance is often described by applying terms widely used for conventional power plants. However, there are drawbacks to using some of the most common quantities, such as capacity factor, efficiency, and availability. For PV systems, these quantities exhibit wider seasonal variations and smaller magnitudes than for conventional power plants. Consequently, PV systems are often judged to be operating poorly, even in the absence of failures.At PVUSA, the P.I. is now being used for both real-time power plant monitoring and for longer-term analyses. A description of the calculation procedure and sample results are included. I NTRO D U CTlO NAnswering a simple question regarding PV system performance such as "How well is it working?" proves to be more difficult than it appears. The capacity factor is probably the most useful statistic to answer such a question for many types of power plants, but not for PV. Because of the time-dependent nature of the fuel (solar) resource, normal daily and seasonal variations in solar radiation result in low capacity factors and can mask losses due to component failures or soiling.Other common performance measures can be calculated, such as energy, average and peak power as a percentage of rated power, efficiency, availability, and equivalent forced outage rate, but with similar drawbacks. Taken together, the additional measures can yield fairly Rated power under a set of reference test conditions. Manufacturers' standard test conditions (STC) of 1,000 W/m2 plane of array irradiance, air mass 1.5, and 25 or 28 C cell temperature have been widely used. As these conditions rarely occur outdoors, PVUSA has developed a more representative set of test conditions (PTC), defined as 1,000 W/m2 plane of array irrad. (850 W/m2 direct normal irrad. for concentrators), 20 C air temperature, and 1 m/s wind speed at 10 m height. PTC ratings tend to be about 10% lower than STC ratings. comprehensive assessments, but they also make for cumbersome and potentially misleading interpretations of PV system performance.For example, a PV system with a wintertime peak equal to 60% of its rated output, operating at 8% efficiency, 15% capacity factor, and 35% availability may have nothing wrong with it, while similar values in the summer would suggest problems exist. Those familiar with conventional plants might mistakenly conclude the PV system had been operating poorly during the winter and might also correctly conclude the summertime performance was poor, but possibly for the wrong reasons.This confusion prompts another extreme to occur -a tendency to report oversimplified on/off status answers that say little or nothing about how well the system is operating. Both the detailed and oversimplified approaches are inadequate, suggesting the need for a capacity factor-like statistic that doesn't...

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