Karachi University Journal of Science, 2008, 36, 19-22 19 0250-5363/08 © 2008 University of Karachi Recital of Some Existing Sunshine-Based Models of Global Solar Radiation for Selected Stations of Punjab, Pakistan: A Comparative Study Intikhab Ulfat 1,2,* , Firoz Ahmed 2 , Farrukh Javed 3 , Asif Usman 4 and Faiza Kanwal 2 1 Department of Applied Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden; 2 Energy & Environment Research Group, Department of Physics, University of Karachi, Karachi- 75270, Pakistan; 3 School of Economics and Management , Department of Statistics, Lund University, SE-220 07 Lund; Sweden; 4 Pakistan Metrological Department, Karachi-75270, Pakistan Abstract: An essential requirement of design of a solar energy conversion system is the precise knowledge of the availability of global solar radiation and its components at the location of interest. In developing countries like Pakistan such data is not available for all locations owing to which there is need of employing different models for the estimation of global solar radiation that use climatological parameters of the location under study. In the present paper the monthly average daily global solar radiation at two stations (Lahore and Multan) in the Punjab province of Pakistan has been predicted using empirical models of Angstrom, Rietveld and Glover Mc Culloch. The performances of these models have been assessed employing different statistical tests. Punjab is the largest province of Pakistan by population and the global solar radiation data for a long period of time is available only for the stations mentioned above. This study will help solar energy research workers in the region to use the empirical models for the places with similar conditions and having no facilities of recording the radiation data. Key Words: Global Solar Radiation, Empirical Models. 1. INTRODUCTION Information concerning the accessibility of global solar radiation and its components at a specific site is vital for practical utilization of solar energy. In developing countries like Pakistan the lack of the facilities for recording solar radiation is an obstruction for most of the ventures. Due to this verity it becomes essential to imply empirical models based on various climatological factors. The most significant factor in this regard is the sunshine hours whose recording is quite often. In Pakistan the daily sunshine hours is recorded at more than 40 stations whereas the daily global solar radiation is recorded only at five stations. This paper presents a comparison to evaluate the prognostic ability of some existing empirical models based on sunshine hours called sunshine-based models for two stations of the region of Punjab, Pakistan. Being largest by population and the second largest by area, Punjab is a very important province of Pakistan. It is situated at the eastern periphery of the Iranian Plateau and the northwestern border of the geologic Indian plate in South Asia [1]. Punjab is the only province of Pakistan that touches all the other provinces of the country and can thus be regarded as central region of the country. This significant geographical situation and a huge number of inhabitants greatly affects Punjab's stance on issues of state echelon. The selected stations from the province i.e., Lahore and Multan are two large and the only cities of this province where the global solar radiation data is measured. *Address correspondence to this author at the Department of Applied Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden; E-mail: intikhab@chalmers.se 2. MODELS USED 2.1. Angström–Prescott–Page Model The most commonly used sunshine based model used for the estimation of monthly average daily global solar radiation (H) measured in MJm 2 is of Angstrom 2 developed later by Page 3 is given as H=H 0 a+b (n/N) where H 0 is the monthly average daily extraterrestrial radiation, n is the day length, N is the maximum possible sunshine duration, and a and b are empirical coefficients signifying the measure of the overall atmospheric transmission under the total cloudy condition (n/N = 0) and the rate of increase of H/H 0 with n/N respectively. These are also referred as Angstrom coefficients and there value has been reported for a number of stations 4. We have previously reported their values for Karachi, Pakistan 5,6. In the present paper we report the values of these constants for Lahore and Multan for the first time considering a long- range data of 25 years, which has not been done so far. The monthly average daily extraterrestrial solar radiation on a horizontal surface i.e., H 0 is one of the basic input parameter for estimation and can be calculated with fair amount of accuracy from the geographical information of a particular place in MJm 2 7. 2.2. Rietveld Model Rietveld examined several published values of the a and b coefficients 8 and pointed out that a is related linearly