Energy and Power Engineering, 2012, 4, 415-421 http://dx.doi.org/10.4236/epe.2012.46055 Published Online November 2012 (http://www.SciRP.org/journal/epe) Estimation of Global Solar Radiation Using Clearness Index and Cloud Transmittance Factor at Trans-Himalayan Region in Nepal Khem N. Poudyal 1 , Binod K. Bhattarai 1 , Balkrishna Sapkota 1 , Berit Kjeldstad 2 1 Institute of Engineering, Tribhuvan University, Kathmandu, Nepal 2 Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway Email: khem@ioe.edu.np Received August 12, 2012; revised September 20, 2012; accepted September 30, 2012 ABSTRACT This paper presents the global solar radiation (GSR) and cloud transmittance factor (cf) measured at the horizontal sur- face over a period of one year from 2009 to 2010 using CMP6 Pyranometer and NILUUV Irradiance Meter at Lukla (Latitude 26.69˚N, Longitude 86.73˚E and Altitude 2850 m) in the foothills of the Mt Everest (8850 m high). Monthly and seasonal variations of global solar radiation as well as correlation between clearness index and cloud transmittance factor at Lukla are presented. The annual average daily global solar radiation is about 3.83 kWh/sq·m/day which is suf- ficient to promote solar active and passive energy technology at high mountain terrain where there is no other viable alternative sources of energy. The maximum and minimum global solar radiation of 5.33 and 2.08 kWh/sq·m/day is recorded in April and September 2010 respectively. The seasonal variation of solar energy is about 2.87 kWh/sq·m/day and 4.83 kWh/sq·m/day in summer and spring respectively which is not in line with the general trend. The coefficient of determination (R 2 ) between cloud transmittance factor (cf) and clearness factor (K) is found to be 0.97. This novel result can be utilized to estimate the global solar radiation at the horizontal surface where K and cf are available. Keywords: Global Solar Radiation; Seasonal Variation; Cloud Transmittance Factor; Clearness Index; Coefficient of Determination; High Himalaya Region 1. Introduction Nepal is located in favorable latitude (26˚N to 29˚N), and receives ample solar radiation throughout the country. The average global solar radiation varies from 3.6 - 6.2 kWh/sq·m/day, and the sun shines for about 300 days a year. The national average sunshine hours and solar insolation are 6.8/day and 4.7 kWh/sq·m/day respec- tively [1]. However the energy scenario of Nepal is alarming situation. Currently the electricity users in the country are facing 4 to 16 hours of load shedding in sum- mer (rainy season) and dry season (spring) respectively. The load shedding causes huge losses to commercial, industrial, residential customers, and the office buildings including academic and government institutions, health service providers and NGOs and so on. Some losses can be directly estimated whereas some others require in- direct methods to estimate the losses. The geophysical structure of Nepal is unique. It expands from low plain area from 60 m to the world’s highest peak Mt Everest. There is vast climatic variation in every 100 to 200 m change of altitude. There are series of zigzag shape of small and high mountains so even the satellite cannot measure the exact value of solar radiation in such com- plex terrain of Himalaya. Therefore, it is very important to validate the ground base measurement data with em- pirical model for the confirmation of global solar radi- ation at different parts of the country. The study of solar radiation potential will support the promotion of solar photovoltaic technology which is one of the most viable options of renewable energy resources for rural electrification in Nepal [2]. There is no detailed study about the global solar radiation and UV radiation in the Himalaya Region. Only few scattered and short term measured data of solar radiation and ultraviolet radiation is available. However, these data will be helpful to ex- plore the trend of global solar radiation and ultraviolet radiation in the Himalaya Region of Nepal [3]. Alternative Energy Promotion Centre (AEPC) under the Government of Nepal has conducted a project—Solar and Wind Energy Resource Assessment (SWERA) under United Nations Environment Program (UNEP)/Global Environment Fund (GEF) from March 2003 to 2006. The study recommended the annual average solar insolation of about 4.7 kWh/sq·m/day in Nepal. SWERA report Copyright © 2012 SciRes. EPE