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Photovoltaic Energy Ratings Methods Validation

The Photovoltaic (PV) Engineering group at NREL validates energy ratings methods by standards committees to establish an energy rating methodology. We are evaluating techniques to account for the impact on PV performance from variations in the spectral distribution of solar radiation.

Two types of methods were evaluated for correcting the short-circuit current of PV modules for variations in the solar spectrum under clear skies: (1) empirical relationships based on air mass, and (2) use of spectral irradiance models and PV module spectral response data. Methods of the first type were the Sandia National Laboratories absolute air-mass function, or f(AMa), and the CREST air-mass function, or f(AM). The second type used SEDES2 and SMARTS spectral irradiance models.

For predicting the short-circuit current under clear skies for a multicrystalline silicon PV module and an amorphous-silicon PV module, the methods using spectral irradiance models and PV module spectral response data performed better than the empirical air mass methods. This work continues to assess performance under both clear and cloudy skies.

Procedure for Correcting for Spectrum

Correcting the short-circuit current for variations in the solar spectrum is a key consideration for estimating the energy production. For some technologies it is important, for others it is less important. This work will evaluate different procedures that may be used to correct for spectrum (such as the Sandia air mass model), show their accuracy using experimental data measured at NREL and elsewhere, assess the sensitivity of various technologies to spectral variations, and make recommendations for implementation of the corrective methods considered into standard test methods such as energy ratings standards.

Energy Ratings for Concentrating PV

Compared to standards for flat-plate PV, energy ratings standards for concentrating PV are in the preliminary stage. NREL plans to identify all technical obstacles and work toward their solution using data collected to experimentally validate proposed and recommended procedures for PV concentrator energy ratings standards.