User's Manual for TMY2s


Table of Contents

Acknowledgments


SECTION 1

Overview

This user's manual describes typical meteorological year (TMY) data sets derived from the 1961-1990 National Solar Radiation Data Base (NSRDB). Based on more recent and accurate data, these data sets are recommended for use in place of earlier TMY data sets (NCDC 1981) that were derived from the 1952-1975 SOLMET/ERSATZ data base (SOLMET - Vol. 1 1978 and SOLMET - Vol. 2 1979). To distinguish between the two TMY data sets, the new TMY data sets are referred to as TMY2s.

TMY and TMY2 data sets cannot be used interchangeably because of differences in time (solar versus local), formats, elements, and units. Unless they are revised, programs designed for TMY data will not work with TMY2 data.

Section 1 of the manual provides general information about the TMY2s and how they were developed; Section 2 lists the stations and provides station identifying information and classification; Section 3 details the contents of the TMY2 files and provides the location in the hourly records of data values and their source and uncertainty flags; Section 4 compares the TMY2s with 30-year data sets; Appendix A provides a description of the procedures used to develop the TMY2s; Appendix B provides a key for present weather elements; and Appendix C contains a table of unit conversion factors for converting SI data to other units.

Typical Meteorological Year - A Description

A TMY is a data set of hourly values of solar radiation and meteorological elements for a 1-year period. It consists of months selected from individual years and concatenated to form a complete year. The intended use is for computer simulations of solar energy conversion systems and building systems. Because of the selection criteria, TMYs are not appropriate for simulations of wind energy conversion systems.

A TMY provides a standard for hourly data for solar radiation and other meteorological elements that permit performance comparisons of system types and configurations for one or more locations. A TMY in not necessarily a good indicator of conditions over the next year, or even the next 5 years. Rather, it represents conditions judged to be typical over a long period of time, such as 30 years. Because they represent typical rather than extreme conditions, they are not suited for designing systems and their components to meet the worst-case conditions occurring at a location.

NSRDB - Source of Data for the TMY2s

The TMY2s were derived from the NSRDB, Version 1.1, which was completed in March 1994 by the National Renewable Energy Laboratory (NREL). The NSRDB contains hourly values of measured or modeled solar radiation and meteorological data for 239 stations for the 30-year period from 1961-1990. A complete description of the NSRDB and how it was produced is presented in its user's manual (NSRDB - Vol. 1 1992) and the final technical report (NSRDB - Vol. 2 1995). The original version of the NSRDB, Version 1.0, was completed in August 1992. Version 1.1 corrects two types of minor errors in Version 1.0 that affected about 10% of the stations (Rymes 1994).

There are two types of stations in the NSRDB: primary (denoted by asterisks in the station map in Figure 1-1) and secondary (denoted by dots in the station map in Figure 1-1). The 56 primary stations measured solar radiation for a part (from 1 to 27 years) of the 30-year period. The remaining 183 stations, designated as secondary stations, made no solar radiation measurements and therefore use modeled solar radiation data that are derived from meteorological data, such as cloud cover. Both primary and secondary stations are National Weather Service stations that collected meteorological data for the period 1961-1990.

Succeeding the older 1952-1975 SOLMET/ERSATZ data base, the NSRDB accounts for any recent climate changes and provides more accurate values of solar radiation for several reasons:

A comparison of the old and new data bases provided an incentive for developing the TMY2s. On an annual basis, 40% of the NSRDB and SOLMET/ERSATZ stations are in disagreement for global horizontal radiation by more than 5%, with some stations showing disagreement of up to 18% (Marion and Myers 1992). For direct normal radiation, 60% of the NSRDB and SOLMET/ERSATZ stations are in disagreement by more than 5%, with some stations showing disagreement of up to 33%. Disagreement between the two data bases is even greater when compared on a monthly basis.

An analysis of cloud cover data indicated little or no change for the two periods; consequently, most of the disagreement for NSRDB and SOLMET/ERSATZ data is attributed to differences in reconstructing the instrument calibrations and differences in the solar radiation models (NSRDB - Vol. 2 1995).

Because of differences in the data bases from which they were derived, the old TMYs and the new TMY2s will differ. For some stations, the differences may be minor, but other stations will have large differences.

Methodology

Except for a few changes to the weighting criteria, which accounts for the relative importance of the solar radiation and meteorological elements, the TMY2s were created using similar procedures that were developed by Sandia National Laboratories (Hall et al. 1978) to create the original TMYs from the 1952-1975 SOLMET/ERSATZ data. Studies by Freeman (1979), Siurna, D'Andrea, and Hollands (1984), and Menicucci and Fernandez (1988) have shown that this procedure gives reasonable results. Sandia's procedure has also been adopted by Siurna, D'Andrea, and Hollands (1984) for developing TMYs for Canada.

The Sandia method is an empirical approach that selects individual months from different years from the period of record. For example, in the case of the NSRDB that contains 30 years of data, all 30 Januarys are examined, and the one judged most typical is selected to be included in the TMY. The other months of the year are treated in a like manner, and then the 12 selected typical months are concatenated to form a complete year.

The 12 selected typical months for each station were chosen from statistics determined by using five elements: global horizontal radiation, direct normal radiation, dry bulb temperature, dew point temperature, and wind speed. These elements are considered the most important for simulation of solar energy conversion systems and building systems.

For other elements in the TMY2s, the selected months may or may not be typical. Cloud cover, which correlates well with solar radiation, is probably reasonably typical. Other elements, such as snow depth, are not related to the elements used for selection; consequently, their values may not be typical. Even though wind speed was used in the selection of the typical months, its relatively low weighting with respect to the other weighted elements prevents it from being sufficiently typical for simulation of wind energy conversion systems.

Appendix A contains a more detailed description of the procedures used to develop the TMY2s.

TMY2 Station Classification

The TMY2 station classification pertains to the amount of measured meteorological data available for a station to select typical months to form the typical meteorological year. Of a possible 30 candidate months, Class A stations had a minimum of 15 candidate months, without more than 2 consecutive hours of missing data, from which a typical month was selected. For Class B stations to achieve a minimum of 15 candidate months, data filling for periods of up to 47 hours were required. For some elements not required for the selection of the typical meteorological months, the data are unfilled in the TMY2 data files. The elements horizontal visibility, ceiling height, and present weather may be missing for up to 2 consecutive hours for Class A stations and for up to 47 hours for Class B stations. No data are missing for more than 47 hours, except for snow depth and days since last snowfall for Colorado Springs, Colorado.

Data Elements

Table 1-1 shows the data elements in the TMY2 data files. These are the same elements as for the 30-year NSRDB, except that illuminance and luminance elements were added to support building energy analysis. The table includes information by element and station classification to alert the user to the possibility of missing data. Definitions of the elements and their units are provided in Table 3-2 of Section 3.

Where to Order

TMY2 data sets are available over Internet from NREL's Renewable Resource Data Center (RReDC). The Universal Resource Locator (URL) address of the RReDC is http://www.nrel.gov/rredc. Users should have World Wide Web (WWW) browsing software, such as Firefox, MS Internet Explorer or Netscape, to access the RReDC.

References

Freeman, T. L. (1979). Evaluation of the "Typical Meteorological Years" for Solar Heating and Cooling System Studies. SERI/TR-8150-1. Golden, CO: Solar Energy Research Institute.

Hall, I.; Prairie, R.; Anderson, H.; Boes, E. (1978). Generation of Typical Meteorological Years for 26 SOLMET Stations. SAND78-1601. Albuquerque, NM: Sandia National Laboratories.

Marion, W.; Myers, D. (1992). A Comparison of Data from SOLMET/ERSATZ and the National Solar Radiation Data Base. NREL/TP-463-5118. Golden, CO: National Renewable Energy Laboratory.

Menicucci, D.; Fernandez, J. (1988). A Comparison of Typical Year Solar Radiation Information with the SOLMET Data Base. SAND87-2379. Albuquerque, NM: Sandia National Laboratories.

NCDC (1981). Typical Meteorological Year User's Manual. Asheville, NC: National Climatic Data Center.

NSRDB - Vol. 1 (1992). User's Manual - National Solar Radiation Data Base (1961-1990). Version 1.0. Golden, CO: National Renewable Energy Laboratory and Asheville, NC: National Climatic Data Center.

NSRDB - Vol. 2 (1995). Final Technical Report: National Solar Radiation Data Base (1961-1990). NREL/TP-463-5784. Golden, CO: National Renewable Energy Laboratory.

Rymes, M. (1994). "Beyond the NSRDB Version 1.0." Proceedings of the 1994 Annual American Solar Energy Society Conference; June 25-30, 1994, San Jose, CA; pp. 445-450.

Siurna, D.L.; D'Andrea, L.J.; Hollands, K.G.T. (1984). "A Canadian Representative Meteorological Year for Solar System Simulation." Proceedings of the 10th Annual National Conference of the Solar Energy Society of Canada; August 2-6, 1984, Calgary, Alberta; pp. 85-88.

SOLMET, Vol. 1 (1978). User's Manual - Hourly Solar Radiation-Surface Meteorological Observations. TD-9724. Asheville, NC: National Climatic Data Center.

SOLMET, Vol. 2 (1979). Final Report - Hourly Solar Radiation-Surface Meteorological Observations. TD-9724. Asheville, NC: National Climatic Data Center.


Section 2   Stations

Table of Contents


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