National Solar Radiation Data Base User's Manual (1961-1990)

Table of Contents

How to Use this Manual

Part 1: How to Use and Interpret Data Base Products

1.0 Background and Overview: The National Solar Radiation Data Base (Version 1.0)

Part 1 of this manual describes ways of acquiring and reading the information contained in the National Solar Radiation Data Base (NSRDB), Version 1.0. The foundation for the NSRDB is the hourly measured solar radiation data collected by the National Weather Service (NWS) over the past several decades. Although measured solar radiation data constitute less than 7% of the NSRDB, they provided the benchmark for model estimates of solar radiation. The METSTAT model used for producing approximately 93% of the solar radiation data in the NSRDB was developed by the National Renewable Energy Laboratory (NREL), using the relatively good quality solar radiation data collected by the NWS from 1977 through 19801. These measured and modeled solar radiation data were combined with meteorological data (used by the solar energy industry to evaluate the performance of its systems) to form the NSRDB.

1.1 General Description

The NSRDB is a serially complete collection of hourly values of the three most common measurements of solar radiation (global horizontal, direct normal, and diffuse horizontal) over a period of time adequate to establish means and extremes, and at a sufficient number of locations to represent regional solar radiation climates. The solar radiation and meteorological elements contained in the data base are listed in Table 1-1.

National and international meteorological practices (WMO 1967) call for the use of a 30-year period of record to establish normals, means, and extremes for meteorological variables. Because the National Oceanic and Atmospheric Administration (NOAA) updates the normals, means, and extremes for the United States each decade, the period from January 1961 through December 1990 was used for the NSRDB.

Standard International (SI) units are used for all elements in the data base except atmospheric pressure. Atmospheric pressure is reported in millibars because these units are commonly used in computer models to estimate solar radiation and are consistent with standard NWS reporting practices. Appendix A contains factors to convert all elements to other commonly used units.

All data are referenced to local standard time. The solar radiation elements are the radiant energy integrated over the hour preceding the designated time. Meteorological elements are the values observed at the designated time.

When a station contains only modeled solar radiation data, it is referred to as a Secondary station. Primary stations contain measured solar radiation data for at least a portion of the 30-year record. The NSRDB contains a total of 56 Primary and 183 Secondary stations. Their locations are shown in Figure 1-1. Primary stations are listed in Table 1-2 and Secondary stations are in Table 1-3. The Weather Bureau Army Navy (WBAN) numbers used to identify the stations are given in these tables. Appendix B provides more comprehensive information about each of the stations incorporated into the data base.

1.2 Rationale for a New Data Base

The NSRDB replaces the SOLMET/ERSATZ data base (SOLMET Vol. 1 1978 and Vol. 2 1979), which we will refer to as the SOLMET data base. A number of investigators have examined data from the SOLMET data base as well as the models used in its preparation. Randall and Bird (1989) present a good summary of the results of these investigations. They found a great deal of variation in the apparent quality of the SOLMET data. For example, differences in the values for mean monthly global horizontal radiation from the SOLMET data base, which covers the period from 1952 to 1975, and monthly means from measurements taken from 1977 to 1980 were as great as 20%.

Differences between SOLMET and 1977 to 1980 values for mean monthly direct normal radiation were as great as 50%. This was probably due in part to the fact that the SOLMET data base contained only modeled estimates of direct normal radiation.

The large differences found between monthly mean values in the SOLMET data base and monthly mean values of data collected by NWS stations from 1977 to 1980 provided the primary motivation for upgrading the data base. The need to update the data base was apparent because the last data in the SOLMET data base were collected in 1975. Thus, the U.S. Department of Energy (DOE), in cooperation with the NCDC, undertook this effort to update the solar radiation data for the United States and to develop statistics consistent with standard climatic practices.

1Because of the importance of these and other measured data, a brief history of solar radiation measurements in the United States is presented in Part 2.

Table 1-1. Solar Radiation and Meteorological Elements in the NSRDB

Table 1-2. Primary Stations (with measured solar radiation for at least one year)

Table 1-3.. Secondary Stations (without measured solar radiation data)

2.0 Data Base Product Options

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