Evaluation of the Heliosat-II method using daily irradiation data for four stations in Iran Isaac Moradi a, * , Richard Mueller b , Bohloul Alijani c , Gholam Ali Kamali d a Satellite Atmospheric Science (SAT) Group, Institutionen fo ¨ r Rymdvetenskap (Department of Space Science) (IRV), Lulea ˚ University of Technology (LTU), Kiruna, Sweden b German National Meteorological Service (DWD), Climate Monitoring Satellite Application Facility (CM-SAF), Offenbach, Germany c Department of Geography, Tarbiat Moalem University, Tehran, Iran d Atmospheric Sciences and Meteorological Research Center, Tehran, Iran Received 24 November 2006; received in revised form 23 October 2007; accepted 10 July 2008 Available online 9 August 2008 Communicated by: Associate Editor David Renne Abstract Efficient use of solar radiation needs detailed knowledge of its spatial and temporal variations. Such information can be achieved using interpolating measured irradiance by ground stations. But more reliable results can be obtained by processing geostationary satel- lite images. Heliosat is an algorithm which has been developed to estimate global horizontal irradiance at ground level from images taken in the visible band by the Meteosat satellites. The aim of this study was to evaluate the Heliosat-II model by using daily global solar irradiation data measured at the four radio- metric stations in Iran as well as Meteosat-5 images which are recorded by a spacecraft over 63°E. Mean RMSD% and MBD% for all stations were 11.7% and 1.9%, respectively. The mean values of intercept, slope and correlation coefficient were 0.82 (kWh m 2 ), 1.05 and 0.93, respectively. Seasonally, the maximum RMSD occurs in autumn (22.1%) and the minimum is experienced in spring (8.4%). This accuracy is a great achievement for producing a high quality solar radiation atlas in a country such as Iran with very sparse radiometric network and frequently unreliable measured irradiation data. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Heliosat; Meteosat; Solar radiation atlas; Geostationary satellite; Iran 1. Introduction The estimation of fluxes of incident shortwave radiation at the earth’s surface is important for a number of applica- tions including climate monitoring, regional solar energy availability assessment for heating and electrical power generation purposes and for the evaluation of the cloud and radiation parameterization used in weather and cli- mate models (Diak et al., 1996). The availability of solar radiation data is limited by the sparsity of the existing networks. Zelenka et al. (1999) and Perez et al. (1997) state that estimating solar radiation data from satellite images is more accurate than interpolating data measured by a modern radiometric network. In daily basis, the error resulted from estimating incoming solar radiation by use of satellite images is equal to the error related to interpolating measured data in a radiometric net- work with 50 km distance between stations (Fig. 1). The error of interpolation increases with the distance between stations; for a radiometric network such as the one existing in Iran with average distance more than 500 km this error 0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.07.010 * Corresponding author. Address: Satellite Atmospheric Science (SAT) Group, Institutionen fo ¨r Rymdvetenskap (Department of Space Science) (IRV), Lulea ˚ University of Technology, Box 812, Kiruna 98128, Sweden. Tel.: +46 980 7 9177, fax: +46 980 7 9050. E-mail address: imoradi@www.sat.ltu.se (I. Moradi). www.elsevier.com/locate/solener Available online at www.sciencedirect.com Solar Energy 83 (2009) 150–156