On shadowband correction methods for diffuse irradiance measurements

Abstract

Diffuse irradiance, G,, is an important variable in solar resource assessment. The diffuse irradiante can be worked out from global, G, and direct, Gb. irradiance measurements, but this method involves the use of relatively expensive tracking mechanisms. Alternatively, a widely accepted technique uses a pyranometer with a shadowband. Because the shadowband screens the sensor from part of the diffuse radiation coming in from the sky, a correction must be made to the measurements. However, because of the anisotropy of diffuse radiation it is difficult to compute an exact theoretical correction. In this study we use two data sets registered in two locations in Spain. The first one consists in coincident hourly values of global, direct, and diffuse irradiance; the latter by means of shadowband. The other data set includes the same variables but as 5-minute values. Our goal is to study the necessary correction factor for diffuse irradiance measurements obtained by means of shadowband. After testing several well-known correction methods, we have developed two different correction models, using two-thirds of the hourly data set, while the remaining one-third and the whole S-minute data set have been used for validation purposes. The last validation test suggests that our anisotropic models provide reliable corrections for conditions different than the ones where they have been developed. The results obtained by the developed models show a negligible mean bias deviation. Approximately 55% of cases present deviations lower than 5% over the mean value of diffuse h-radiance.

Mapping a topographic global solar radiation model implemented in a GIS and refined with ground data

“…Antonic (1998) andBashahu (2003) propose correlating diffuse radiation with other meteorological data to improve its accuracy. Batlles et al (1995) also propose using a shadow-band correction to compute diffuse radiation, with acceptable results. Quereda (1982) obtains similar results between empirical global and diffuse measurements and theoretical computations, but Liepert (1997) shows us the importance of different cloud types when analysing the relationship between global and diffuse radiation.…”

Selection of cloudless sky conditions by applying solar global ultraviolet irradiance measurements

“…The second pyranometer was equipped with a 7.6 cm wide shadow-band with a radius of 31.70 cm for the diffuse irradiance. The diffuse irradiance measurements were corrected using Drummond's formula, as summarised in Iqbal (1983), although the use of this isotropic correction can lead to some error, see for example a study by Batlles et al (1995) made in another region of Spain. The pyranometers had a class 1 precision following the specifications of the World Meteorology Organization (WMO), and its cosine response is within ±2% from 0 to 70° zenith angle and ±5% for 70-80° zenith angles.…”

Determination of daily solar ultraviolet radiation using statistical models and artificial neural networks

“…Data were recorded at 20-s intervals and then averaged every 5 min. The TUVR values were temperature corrected following the calibration provided by the manufacturer, and diffuse radiation measurements were corrected following Batlles et al (1995), to take into account the sensor screening by the shadow band. Daily values for all the radiometric variables were obtained by integration during the day.…”