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Understanding PV module rating scheme

Posted: 06 Mar 2013     Print Version  Bookmark and Share

Keywords:photovoltaic  PSI Rating  Lifetime Energy Production 

Some PV modules perform better than others in off-peak conditions. Recognising the significance of this characteristic is important in selecting a superior PV module – one with higher Lifetime Energy Production. In order to calculate the impact on LEP, power must be measured at two different irradiance levels–typically 1,000 W/m² and 200 W/m². Thus, PSI has included the Power at low Irradiance / Power at High Irradiance Ratio characteristic in the PSI Rating: Plow/Pmax. Temperature and other test conditions are held constant during this test. Results from these measurements are used to determine a characteristic called the insolation response function of the PV module. The insolation response combined with the daily insolation is a key component of the LEP.

6. Annual power reduction
The power produced by PV modules may degrade over time for a variety of reasons, resulting in a characteristic PV module power reduction. Accelerated testing has been used to determine this degradation2, and it is of extreme significance to the manufacturers' warranty policies. Although the expected annual degradation data can be gathered in laboratory testing, the PSI Rating uses manufacturer's warranty values to calculate an annual power reduction value. A lower coefficient of degradation increases a PV module's PSI Rating.

7. Total area efficiency
PV modules are assembled from arrays of solar cells, and it is not always possible for the manufacturer to cover the entire surface of a module with solar cells. The amount of coverage combined with the solar cell's efficiency results in the Total Area Efficiency, and is calculated by dividing the module power by its total surface area. Since more concentrated wattage in a PV module improves design flexibility and efficient use of space—especially on rooftops—a higher surface power density increases the PSI Rating.

8. Failure rate
Perhaps one of the most difficult PV module characteristics to ascertain, Failure Rate, can impact Lifetime Energy Production through of loss of productivity while a failed module is offline and being replaced. Accelerated testing methods can provide some guidance to long term failure rates, but long-term field failure rate information is rarely available or disclosed. Because of the general lack of data, this characteristic is not presently included in the PSI Rating number.

The future looks bright
Until now, the absence of a consistent method for comparing solar power production systems left developers, designers and investors at the mercy of the manufacturer's sales and marketing departments to make substantial—and often critical—financial decisions. As in other mature industries, solar professionals need a systematic rating to compare PV modules—an unbiased rating designed to render performance information into quantifiable values.

The PSI Ratings are key criterion in PV module selection. Obviously, the solar developer should consider other factors as well, including: price, delivery costs and availability, manufacturer reputation and financial strength, warranty, and the module's effect on Balance of System (BOS) costs. The PSI Rating is a model-based comparison of LEP that specifically excludes these non-technical factors, which can change from deal-to-deal based upon circumstances and negotiation.

By creating this rating standard, PSI has removed one of the final barriers to the inevitable, mainstream adoption of solar power into the world energy mix. The PSI Rating will enable solar professionals to make intelligent, informed decisions for the selection of an optimum solar module, and help build the solar industry's solid foundation of credibility in the minds of energy leaders, investors and industries around the world.

1. Photovoltaic (PV) performance testing and energy rating.
2. History of accelerated and qualification testing of terrestrial photovoltaic modules: A literature review (2009), Osterwald, C. R., and T. J. McMahon. 2009. History of accelerated and qualification testing of terrestrial photovoltaic modules: A literature review. Progress in Photovoltaics: Research and Applications 17, no. 1: 11-33.

About the author
Matthew A. Thompson, Ph. D. is the Executive Director of Principal Solar Institute. He joined at its inception in 2012. Before that he worked at Motorola and Freescale Semiconductor for 23 years as a scientist in semiconductor process development, manufacturing and yield enhancement.

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