DNI Measurements in the South of Portugal: Long Term Results through Direct Comparison with Global and Diffuse Radiation Measurements and Existing Time Series

Abstract

The present work describes the measurement effort for DNI resource evaluation in the sunny south of Portugal, with a network of eight radiation measurement stations in several locations (including Évora) providing good coverage of the region. This new initiative for DNI measurement will still need many years (typically 10 or more) to produce a time series which can claim having long term statistical value. This problem can, however, be temporarily solved by measuring DNI at the same time as GHI and DHI, in a place where long term series dating back, already exist for those two. Correlations can then be established and a long term statistical value attributed to the short and recent DNI series. It so happens that a long term series (20 years) of global and diffuse solar irradiation exists for the location Évora. The paper describes the setup of the measuring stations and presents the preliminary measurements obtained. It further presents the first correlations of hourly and monthly averages between normal beam (DNI), global and diffuse radiation. It then uses these correlations, admittedly without acceptable statistical significance (short series of less than one year of measured data), to exemplify how to get a prediction of long term DNI for Évora. The value obtained is compared to that predicted by the commercial algorithm Meteonorm.