Assessment of Solar Radiation Data Used in Analyses of Solar Energy
Systems
University of Wisconsin-Madison, (2004)Abstract
Solar radiation data is essential to conducting performance analyses
of solar energy systems. In the US, the National Solar Radiation
data base has archived 30 years of hourly data for 239 US cities.
Since the computational effort required to simulate systems for 30
years is excessive, it is more convenient to use typical meteorological
year (TMY) data in performance analyses. TMY data provide hourly
solar radiation and meteorological data representative of one âtypicalâ
year for the same 239 locations in the database. Simulation studies
have generally used these hourly values, although solar radiation
can exhibit wide variations during an hour. Variations in solar radiation
during an hour could result in inaccurate performance estimates for
some types of solar systems, such as photovoltaic systems, that respond
quickly and non-linearly to solar radiation. Short-term radiation
data is not as readily available as hourly data. One year of 1- minute
data for one US location and one year of 3-minute data for eight
US locations were made available for this research. The impact of
using short-term radiation data in performance analyses instead of
hourly data and the accuracy of using statistically formulated TMY
data instead of the actual long-term data was investigated. In addition,
the behaviors of both data sets were studied in terms of diffuse
fraction and frequency distributions. Correlations and distribution
curves have been previously developed and comparisons of real data
to these correlations were made to determine their accuracy. To quantify
impacts, calculation of radiation on tilted surfaces using known
models were made for both TMY data and short-term radiation data
sets, and utilizability analyses were made to quantify the impact
on performance analyses in a system-independent manner.