User's Manual for TMY2s

SECTION 4

Comparison with Long-Term Data Sets

The TMY2 data were compared with 30-year data sets to show differences between TMY2 data and long-term data for the same stations. Comparisons were made on a monthly and annual basis for global horizontal, direct normal, and south-facing latitude tilt radiation; and for heating and cooling degree days. These comparisons give general insight into how well, with respect to long-term conditions, the TMY2s portray the solar resource and the dry bulb temperature environment for simulations of solar energy conversion systems and building systems. On an annual basis, the TMY2s compare closely to the 30-year data sets. The monthly comparisons are less favorable than the annual comparisons.

Solar Radiation Comparisons

Monthly and annual solar radiation for the TMY2 data sets were compared with previously determined (Marion and Wilcox 1994) monthly and annual averages for the 1961-1990 NSRDB, from which the TMY2 data sets were derived. These comparisons were made for global horizontal, direct normal, and a fixed surface facing south with a tilt angle from horizontal equal to the station's latitude.

Results of these comparisons are shown in Figures 4-1 through 4-6. TMY2 values for all stations are plotted against their respective 30-year average from the 1961 - 1990 NSRDB. As indicated by the scatter of the data and the statistical information at the top of the figures, agreement is better on an annual basis than on a monthly basis. This is a consequence of cancellation of some of the monthly differences when the monthly values are summed for the annual value. The statistical information presented is the mean difference between the TMY2 value and the 1961-1990 average and the standard deviation of the differences.

Table 4-1 provides 95% confidence intervals, determined as twice the standard deviation of the differences between TMY2 and NSRDB values, for TMY2 monthly and annual solar radiation. The confidence intervals are given in units of kWh/m²/day. Differences between TMY2 and NSRDB 30-year values should be within the confidence interval 95% of the time.

Heating and Cooling Degree Day Comparisons

Degree days are the difference between the average temperature for the day and a base temperature. If the average for the day (calculated by averaging the maximum and minimum temperature for the day) is less than the base value, then the difference is designated as heating degree days. If the average for the day is greater than the base value, the difference is designated as cooling degree days.

Monthly and annual heating and cooling degree days (base 18.3°C) calculated from the TMY2 data sets were compared with those for the same stations from NCDC's data tape, "1961-1990 Monthly Station Normals All Elements." This data tape includes temperature and degree day normals for about 4775 stations in the United States and its territories. The normals are averages computed by NCDC for the period 1961-1990.

Results of these comparisons are shown in Figures 4-7 through 4-10. TMY2 values for all stations are plotted against their respective 30-year average from NCDC's data tape. As seen for solar radiation, agreement is better on an annual basis than on a monthly basis.

Table 4-2 provides 95% confidence intervals, determined as twice the standard deviation of the differences between TMY2 and NCDC values, for TMY2 monthly and annual heating and cooling degree days. The confidence intervals are given in units of degree days. Differences between TMY2 and NCDC 30-year values should be within the confidence interval 95% of the time.

References

Marion, W.; Wilcox, S. (1994). Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors. NREL/TP-463-5607. Golden, CO: National Renewable Energy Laboratory.

Appendix A Procedures for Developing TMY2s

Table of Contents

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