19 August 2010 Accuracy of silicon versus thermopile radiometers for daily and monthly integrated total hemispherical solar radiation
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Abstract
Measurement stations for solar radiation resource assessment data are expensive and labor intensive. For this reason, long-term solar radiation measurements are not widely available. Growing interest in solar renewable energy systems has generated a great number of questions about the quality of data obtained from inexpensive silicon photodiode radiometers versus costly thermopile radiometers. We analyze a year of daily total and monthly mean global horizontal irradiance measurements derived from 1-minute averages of 3-second samples of pyranometer signals. The data were collected simultaneously from both types of radiometers at the Solar Radiation Research Laboratory (SRRL) operated by the National Renewable Energy Laboratory in Golden, Colorado. All broadband radiometers in service at SRRL are calibrated annually using an outdoor method with reference radiometers traceable to the World Radiometric Reference. We summarized the data by daily total and monthly mean daily total amounts of solar radiation. Our results show that systematic and random errors (identified in our outdoor calibration process) in each type of radiometer cancel out over periods of one day or more. Daily total and mean monthly daily total solar energy measured by the two pyranometer types compare within 1% to 2%. The individual daily variations among different models of thermopile radiometers may be up to twice as large, up to ±5%, being highest in the winter (higher average solar zenith angle conditions) and lowest in summer, consistent with the lower solar zenith angle conditions.
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Thomas L. Stoffel, Daryl R. Myers, "Accuracy of silicon versus thermopile radiometers for daily and monthly integrated total hemispherical solar radiation", Proc. SPIE 7773, Reliability of Photovoltaic Cells, Modules, Components, and Systems III, 777314 (19 August 2010); doi: 10.1117/12.859743; https://doi.org/10.1117/12.859743
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