Measurement of direct, diffuse, and total solar radiation with silicon photovoltaic cells

Abstract

Abstract The paper presents the results of tests carried out during the spring of 1962 at the Yellott Solar Energy Laboratory to determine the suitability of commercially available silicon photovoltaic cells for use in solar radiometers. Since the short-circuit current of the silicon cell has been shown to exhibit linear variation with varying intensity of incident radiation, this quantity was measured for six cells, selected at random, under widely varying temperatures, under both tungsten illumination and solar radiation. Over the range of temperatures likely to be encountered in solar radiometry (40 to 160 degrees F), the short-circuit current varied from 10 to 15 percent, under tungsten illumination. Under natural sunlight, the maximum variation was 6.3 percent. By measuring the millivolt drop across a small resistance, instead of determining the short-circuit current directly with a milliammeter, temperature compensation is shown to be adequately accomplished by the use of a thermistor and a Manganin wire shunting resistance, which are maintained at the temperature of the cell. The effect upon the cell coefficient of solar altitude, in terms of deviation from the cosine response and air mass effect, was then investigated, using a silicon cell radiometer which could serve both as a pyrheliometer and as a pyranometer. It was found that, for solar altitudes above 60 degrees, no correction for cosine deviation or air mass effect is necessary. For lower solar altitudes, these factors must be taken into consideration by determining the coefficient at varying solar altitudes, through calibration against a standard thermopile-type instrument. A day-long comparison of the output of a silicon cell pyranometer with the output of an Eppley 180-degree horizontal pyrheliometer showed an agreement in total radiation within 1 percent. It is concluded that, with proper calibration and adequate attention to the effects caused by solar altitude, a temperature-compensated silicon-cell pyrheliometer can be used with confidence to measure direct, total, and diffuse solar radiation with an accuracy of at least ±3 percent.