Evaluating Solar Resource Data Obtained from Multiple Radiometers Deployed at the National Renewable Energy Laboratory

Solar radiation resource measurements from radiometers are used to predict and evaluate the performance of photovoltaic and concentrating solar power systems, validate satellite-based models for estimating solar resources, and advance research in solar forecasting and climate change. This study analyzes the performance of various commercially available radiometers used for measuring global horizontal irradiances (GHI) and direct normal irradiances (DNI). The radiometers in this study were deployed for one year (from April 1, 2011, through March 31, 2012) and compared to measurements from radiometers with the lowest values of estimated measurement uncertainties for producing reference GHI and DNI. The differences were calculated as a percent of reading for solar zenith angles ranging from 17.5 degrees to 85 degrees (the range of available solar zenith angles throughout the year at SRRL, excluding data near sunrise and sunset). Under clear-sky conditions when the solar zenith angle was less than 60 degrees, differences of less than 5% were observed for both GHI and DNI measurements when they were compared to the reference radiometers. These normalized differences increased during partly cloudy sky conditions and when the solar zenith angle was greater than 60 degrees. The intent of this paper is to present a general overview of each radiometer's performance. The National Renewable Energy Laboratory made no effort to ensure that the radiometers presented here were representative units; therefore, this paper does not guarantee the same results for all radiometers from the same manufacturer or model.