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Chapter 6: Where can you obtain solar radiation data?

Chapter 7

How accurate do the data need to be?

The required accuracy of the solar radiation data for a site depends on the application. When the cost of the solar conversion device is low compared with the overall system cost, we can account for uncertainties in the solar radiation data by using "engineering judgment" to increase the size of the solar collectors. However, as the solar energy conversion system increases in size and cost, this becomes less acceptable, and we need more accurate solar radiation data to optimize the design and project the cost.
"Utility engineers need solar radiation data accurate to within plus or minus 5% to assess the resource, estimate the output of a solar system, and determine whether the system can reliably and economically meet daytime demand and energy requirements. Because there are few sites with data of this accuracy, we need monitoring stations to collect the data at proposed PV sites."

--J.E. Bigger, Electric Power Research Institute

For large-scale applications of solar energy conversion technologies, most experts agree that solar radiation data should be accurate to within 5% so they can make reasonable assumptions concerning energy output to evaluate the performance and economics. Unfortunately, not much available solar radiation data are accurate to within 5%. This is due to the measurement uncertainties of the instruments used and the limited number of measurement sites. Consequently, designers today have to apply these data more conservatively than is ultimately desirable. The SOLMET/ERSATZ data are the most widely used solar radiation data. On an average for all sites, they are accurate to within about 10% for average daily values on an annual basis. But for average daily values on a monthly basis for an individual site, they can be in error by 20% or more. For interpolating data for sites between SOLMET/ERSATZ stations, microclimate differences due to terrain and local weather conditions can also increase the uncertainty of the data.

Chapter 8: How will we meet our solar radiation data needs?

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