Uncertainty Calculations in Pyranometer Measurements and Application

The uncertainty of pyranometer measurements should be traced back to the World Radiation Reference (WRR), a standard that is specified by the mean sensitivity of the World Standard Group (WSG). The WSG is build up by 7 primary standard pyrheliometers, operated at Davos, Switzerland. Analyzing the complete calibration chain for an individual field pyranometer, usually the uncertainty of its calibration constant is extracted as a unique figure depending on the calibration method. The common representation of the expanded uncertainty, specified by the manufacturer is global information on the accuracy of daily averages. For the use of pyranometers for e.g. test of solar energy components as solar collectors, this information on the global daily accuracy of the pyranometer is not sufficient. As the response of a solar collector to the irradiance shows nonlinearities, a more detailed analysis of the pyranometer uncertainties is necessary. This will be demonstrated for the analysis of the uncertainties of the test results — i.e. the collector coefficients and their uncertainties — and the resulting predictions of the energy gain by these devices. Ancillary information by the manufacturers will be used to discuss the uncertainty of individual measurements depending on e.g. ranges of those parameters that originate the uncertainties, depending on the geometry (incidence angle), ambient temperature and the sky conditions. Based on this information the inter-comparability of test performed at different times or with different instruments will be discussed.