Estimating the horizontal diffuse solar radiation over the Central Anatolia Region of Turkey Haydar Aras a , Ozgur Balli b , Arif Hepbasli c, * a Department of Mechanical Engineering, Engineering and Architecture Faculty, Osmangazi University, Eskisehir, Turkey b First Air Supply and Maintenance Center, TUAF, Eskisehir, Turkey c Department of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey Received 26 June 2005; accepted 28 November 2005 Available online 18 January 2006 Abstract The main objective of the present study is to develop new hybrid models to predict the monthly average daily diffuse solar radiation on a horizontal surface over Turkey’s Central Anatolia Region (CAR), which covers the 12 provinces (Afyon, Ankara, Cankiri, Corum, Eskisehir, Kayseri, Kirsehir, Konya, Nevsehir, Nigde, Sivas and Yozgat), as an exam- ple. The models proposed by many investigators to estimate the diffuse solar radiation were reviewed. Although the global solar radiation and sunshine duration have been measured by the Turkish State Meteorological Service (DMI) over all the country since 1964, the diffuse solar radiation has not been measured. The twelve new hybrid models for estimating the monthly average daily diffuse solar radiation on a horizontal surface in the CAR were validated, and thus, the most accu- rate model was selected for guiding future projects. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Diffuse solar radiation; Empirical correlation model; Hybrid correlation model; Solar energy; Central Anatolia Region; Turkey 1. Introduction Sustainable development and environmental protection need renewable and clean energy resources such as solar energy [1]. Turkey has a better chance than other countries in terms of solar energy potential because of its geographical situation in the northern hemisphere with latitudes of 36–42°N and longitudes 26–45°E [2–4]. The yearly average total solar radiation varies from a low of 1120 kW h/m 2 per year in the Black Sea Region with an annual sunshine duration of 1971 h to a high of 1460 kW h/m 2 per year in the Southeast Anatolia Region with an annual sunshine duration of 2993 h. The annual average total insolation duration is 2640 h (7.2 h/day), while the average annual solar radiation is 1311 kW h/m 2 /year (3.6 kW h/m 2 /day) in Turkey 0196-8904/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2005.11.024 * Corresponding author. Tel.: +90 232 3884000x1918; fax: +90 232 3888562. E-mail addresses: haras@ogu.edu.tr (H. Aras), hepbasli@bornova.ege.edu.tr, hepbasli@egenet.com.tr (A. Hepbasli). Energy Conversion and Management 47 (2006) 2240–2249 www.elsevier.com/locate/enconman