Photovoltaic Efficiency Measurements
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Photovoltaic Efficiency Measurements Keith Emery Introduction A variety of performance indicators have been employed by the photovoltaic (PV) community to rate the performance of PV cells, modules, and arrays. The PV power conversion efficiency under a set of standard reporting conditions serves as a basis for meaningful comparisons of PV performance, within and among PV technologies. PV efficiency is defined as 100 times the maximum PV electrical power density produced divided by the incident light power at a reference PV temperature, total irradiance, and spectral irradiance. Standard reporting conditions commonly used by the PV community are summarized in Table I. The direct and global reference spectra were generated by a comprehensive Monte-Carlo computer model and actually integrate to 768 W m"2 and 964 W m~2. The terrestrial PV community has arbitrarily assigned a 1-sun total irradiance of 1,000 W nT2 for the global and direct reference spectra. The term "direct" in Table I refers to the direct-normal (5° field of view about the sun) component of the global spectral irradiance distribution. The term "global" in Table I refers to the spectral irradiance distribution on a 37-tilted southfacing surface with a solar zenith angle of 48.2° (AM1.5). The terms AMI or AM1.5 are often used to refer to standard spectra, but the relative optical air mass (AM) is a geometrical quantity and can be obtained by taking the secant of the angle between the sun and the zenith. The AMO total irradiance used in calculating the efficiency has varied from 1,353 W m"2 to 1,372 W m~2.5 Many groups still rely on the less accurate value of 1,353 W m"2 total AMO irradiance.1 The best estimates are 1,367 W ITT2 recommended by
the World Radiation Center or 1,372 W rrT2 used by NASA Lewis Research Center.3"5 Both values were obtained from long-term monitoring of the solar irradiance with an
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active-cavity radiometer on the Solar Max, Nimbus 7, and other satellites. Fortunately, the 1,353 W m~2 total AMO irradiance used by many groups does not enter into the AMO PV power measurements used in sizing arrays for spacecraft because AMO reference cells are not calibrated at a given total irradiance but at whatever irradiance exists at the time of calibration at distance of 1 astronomical unit from the sun. Efficiency measurements related to standard reporting conditions do not distinguish between cells and modules. Definitions for cells and modules have been proposed.6 A module consists of several electrically interconnected cells. The area of a cell is taken to be the total area of the space-charge region including grids and contacts. The standard area definitions for a cell replace the term "space-charge region" with "frontal area" but this term does not adequately account for more than one cell on a single substrate or superstrate. The PV efficiency is proportional to the area definition used. In fact, differences in the area definition often account for the greatest differences ,in efficiency among various groups and values published in
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