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Chapter 5: How do we use solar radiation data?

Chapter 6

Where can you obtain solar radiation data?

The National Weather Service of the National Oceanic and Atmospheric Administration (NOAA) operates monitoring stations in the United States to collect and disseminate information about solar radiation. This information is available on computer-readable magnetic tape from NOAA's National Climatic Data Center, Federal Building, Asheville, NC 28801, (704) 259-0682.

Most of NOAA's solar radiation data sets are from 26 SOLMET stations and 222 ERSATZ stations and consist of hourly values of solar radiation and meteorological data from 1952 to 1975. For the SOLMET stations, instruments measured the global horizontal solar radiation and researchers modeled the direct beam solar radiation data. For the ERSATZ stations, although no solar radiation measurements were made, researchers modeled global horizontal radiation based on observed meteorological data such as cloudiness and minutes of sunshine; the ERSATZ data do not include direct beam radiation. Because the ERSATZ data are modeled, these data are less accurate than the SOLMET data.

NOAA also has available more recent data for the periods 1977 to 1980 and 1988 to the present. The data include hourly values of measured global horizontal solar radiation for 38 stations, measured direct beam solar radiation for 32 stations, and measured diffuse horizontal radiation for 9 stations.

Two of NOAA's data sets are of particular interest to designers and engineers: the typical meteorological year (TMY) data set and the weather year for energy calculations (WYEC) data set. For these, researchers extracted information from SOLMET/ERSATZ data to make data sets of hourly values spanning 1 year. For the ERSATZ TMY data, researchers included values of direct beam radiation with modeled values of global horizontal radiation. These data sets represent typical values occurring from 1952 to 1975, and not the minimum or maximum values. For example, a cloudy year in this period may have had an annual solar radiation value 10% below the TMY values, and a very cloudy month in this period may have had a solar radiation value 40% percent below its TMY value. A difference between TMY and WYEC data is that the TMY data are weighted toward solar radiation values and their hourly distribution, whereas the WYEC data are weighted toward average monthly values of temperatures and solar radiation. Researchers recently revised the WYEC data to include estimates of direct beam and diffuse solar radiation and estimates of illuminance for lighting applications. Illuminance refers to solar radiation in the visible region of the solar spectrum to which the human eye responds.

Solar radiation data derived from the SOLMET/ERSATZ data sets are also published in tabular form by the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161. Two of these tabular data sets are listed below.

Maps are available that depict long-term average solar radiation data for each month. This is a convenient way to show variations in the amount of solar radiation and for interpolating data between stations. For the United States, these maps were made using solar radiation data from the SOLMET/ERSATZ data base. The Solar Radiation Energy Resource Atlas of the United States was published by the Superintendent of Documents, but is out of print. This atlas is available at some university and city libraries.

The University of Lowell compiled an international solar radiation data base for locations outside the Unites States. This data base presents average daily values by month and year for global horizontal solar radiation. It is available from the University of Lowell Photovoltaic Program, 1 University Avenue, Lowell, MA 01854, (508) 934-3377.

Solar radiation data recorded for l-minute intervals are available for four locations: Albany, New York; Atlanta, Georgia; Davis, California; and San Antonio, Texas. The data were recorded over periods of 1 year or more by university meteorological research and training stations. Because of the time scale used, these data are primarily of interest to researchers studying transient responses in solar energy technology systems. These data are available from the National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401.

A spectral solar radiation data base representing a range of atmospheric and climatic conditions is also available from NREL. This data base includes more than 3000 spectra measured over a wavelength range from 300 to 1100 nanometers at 2-nanometer increments (1 nanometer is one-billionth of a meter) and is the result of a cooperative effort between NREL, the Electric Power Research Institute, the Florida Solar Energy Center, and the Pacific Gas and Electric Company. Spectral solar radiation was measured at three sites: Cape Canaveral, Florida; San Ramon, California; and Denver, Colorado. This data base can help determine whether spectrally selective technologies (such as photovoltaics and biomass) are optimized for a particular location and climate.

Additionally, other sources of solar radiation data are state and local governments, utilities, and universities. Examples include the Pacific Gas and Electric Solar Insolation Monitoring Program, the University of Oregon/Pacific Northwest Solar Radiation Data Network, and the Historically Black Colleges and Universities Solar Radiation Monitoring Network.

Chapter 7: How accurate do the data need to be?

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