Estimation of Global Solar Radiation Using Clearness Index and Cloud Transmittance Factor at Trans-Himalayan Region in Nepal ()
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 respectively [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 summer (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 indirect 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 complex terrain of Himalaya. Therefore, it is very important to validate the ground base measurement data with empirical model for the confirmation of global solar radiation 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 explore 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 showed that there is lower solar radiation potential at low altitude plain region than at high altitude mountains and north western part of the country. The Solar Resource Map developed by DLR Germany satellite estimates 3.5 - 4 kWh/sq·m/day of energy at the central mid hill region and higher energy of 5 - 5.5 kWh/sq·m/day is found in the north western region. Similarly, the solar map report of National Renewable Energy Laboratory (NREL), USA shows that there is almost equal amount –4.5 to 5 kWh/sq·m/day of solar radiation found throughout the country. But in the North-Western Region of Nepal the solar insolation is found to be 6 - 6.5 kWh/sq·m/day based on the results of the measurement carried out by SWERA. These Solar Resource Satellite derived DLR and NREL solar resource maps are compared with the ground level measured data of 3 sites. The relative biasness of the model data with respect to the ground level measured data is analyzed considering point to point as well as region to region monthly and annual variation. It shows that within a particular point of location, DLR satellite data has higher Relative Biasness in comparison to NREL data [4].
It is necessary to develop the authentic database for modeling and also investigate the solar active and passive energy application in Nepal for the industrial, tourism trade, communication, education, modern agriculture and public health. Further, it will be used to study the impacts on human health, solar energy budget, climate change, global warming and impacts on agriculture production in the long run [5].
It is known that the higher the altitude greater the total solar irradiance under the clear and intermediate sky conditions, but under the overcast days the solar irradiance is very low in comparison with sunny days [6]. The solar radiation increases with increase in altitude mainly due to decreasing amounts of air molecules, ozone, aerosols and clouds in high altitude atmosphere. Actually, mapping the solar radiant energy on the earth’s surface is a requirement not only in the studies of climate change, effects on ecosystems, environmental pollution but also in agriculture, hydrology, food industry and non conventional energy development programs [7].
The utilization of solar energy, like any other natural resource, requires detailed information on availability. Since solar radiation reaching the Earth’s surface depends on many factors which are not of global character, a study of solar radiation under local weather conditions is also essential.
The present study is expected to provide more information about the total solar radiation potential which will be crucial for designing and predicting the performance of solar energy equipment and solar energy potential [8].
The global solar radiation is affected by clouds, water vapor, aerosols, ozone, and other gases. The cloud is the key affecting factor for solar radiation. The water vapor varies over a day and season. There is more water vapor in the air during wet season compared to the dry season. These are not the only parameters affecting the amount of solar radiation arriving at the earth surface, but the incident angle of solar rays also affects the of solar radiation. This angle varies with time, season and location. The geographical distribution of solar radiation over a region is normally different from other regions due to the position and atmospheric constituents of the local weather condition [9]. Clouds is the major cause of fluctuation in the solar radiation and sunshine hours on the ground surface. The variation, however, is not due to the angle of incidence of the sun’s rays with ground surface and its azimuth [10].
The cloud transmittance factor (cf) is the ratio of measured radiation to the calculated clear sky radiation without aerosols and zero surface albedo. It is measured in percentage.
The main aim of this study is to estimate the global solar radiation using the cloud transmittance factor (cf) and clearness factor (K). This paper presents the trend of monthly and seasonal variation of global solar radiation at High Mountain with complex terrain where there is no viable alternative option of energy supply for the tourism business and the residential customers of electricity.
Measuring Site
Lukla lies in the Khumbu Valley just south of the Mt. Everest. It is located in the central part of Himalayan Range and partially includes the area of Sagarmatha National Park. This area is characterized by poor annual mean precipitation mostly concentrated during summer monsoon season. Winter synoptic circulation is dominated by western streams bringing events which lead to snowfalls in the western and central Himalayan Range and the Tibetan Plateau. In summer, southern monsoon streams dominate carrying damp ocean air toward interior of the continent. The measuring site is situated at 2850 meters above from the mean sea level. The surrounding terrain, thin air, highly changeable weather and fast changing visibility are the characteristics of the sensitive weather region from the aviation perspective. The climate is sub-tropical with wet summers and chilly, dry winters mainly affected by its altitude and the summer monsoon season. The temperature ranges from 4˚C to 27˚C and –15˚C to 6˚C in summer and winter respectively [11].
2. Instruments and Methods
The global solar radiation on a horizontal surface was measured using Kipp and Zonen CMP6 Pyranometer and NILU-UV Irradiance Meter measures the cloud transmittance factor at Lukla. These instruments are installed at Lukla (Lat. 26.69˚N, Long. 86.73˚E, Alt. 2850 m). The figures of CMP6 Pyranometer and NILU-UV Irradiance Meter is shown in Figures 1(a) and (b).
The CMP6 pyranometer has wide spectral range of instrument from 310 nm to 2800 nm. The operating temperature is from –40˚C to 80˚C. The sensitivity of instrument and field of view are 5 to 15 µV/W/sq·m and 180˚ respectively. All the measuring data is recorded by LOGBOX SD data logger within a minute resolution for 24 hours. Its special features are low noise, high resolution and low power consumption. It can be used in all weather conditions. It collects the data at real time for the needs of meteorology and slow signal analysis. For data logging 128 KB of memory is available. We can insert the SD memory card for long term data storage. For the communication LOGBOX uses either RS232 or RS485 communication port [12].
(a)
(b)
Figure 1. (a) CMP6 pyranometer; (b) NILU-UV irradiance meter.
The cloud transmittance factor (cf) is measured by sophisticated NILU-UV irradiance meter. This device is a six-channel radiometer designed to measure hemispherical irradiances as well as cloud transmittance factor on a plane surface. The different effects of cloud on global solar radiation and UV radiation are estimated on the basis of cloud transmittance factor. The measured cf is utilized to explain the availability of solar energy in different locations. It is the ratio of measured radiation to the calculated clear sky radiation without aerosols and zero surface albedo. It is measured in percentage. The range of cloud transmittance factor value ranges from 0 to 1 [13].
The global solar radiation (Hg) is measured using CMP6 Pyranometer on the horizontal surface at Lukla. Similarly the cloud transmittance factor cf is measured by NILU-UV Irradiance Meter.
However the extraterrestrial global solar radiation H0 is in Joules per square meter and Isc is in W/m2. H0 is calculated using equation (1.10.3) [14].
(1)
where φ is the latitude (rad) and δ is the solar declination angle (rad). ω is sunset hour angle for typical day and n is mean day of each months where, n is the day of the year. January first n = 1 to 365 days.
(2)
The relation of day length is,
(3)
(4)
where ω is the sunset hour angle.
In this paper we compare the monthly and seasonal variation of global solar radiation on the basis of cf and K. The use of these two coefficient factors, we can found the coefficient of determination. On the basis of R2, the Hg/H0 can be found using the linear equation and after applying the Equation (1) for the extraterrestrial global solar radiation is found. At the end Hg can be calculated which will be novel work in our complex terrain of Himalaya region where all types of instruments cannot installed.
3. Results and Discussions
3.1. Diurnal Variation of Global Solar Radiation
Figure 2 shows the measured and estimated value of
Figure 2. Diurnal Variation of global solar radiation in clear sky day on 26 January, 2010 at Lukla.
diurnal variation of global solar radiation at Lukla on 26th January 2010. However, maximum solar radiation of 868.61 W/sq·m is recorded in the noon. The measured value is 38.12 percent less than estimated value of global solar radiation. The statistical analysis of the data gives the correlation coefficient of 0.96 between measured value and calculated value of solar radiation. Similarly, the standard deviation and P-value are 30.29 and 0.0001 are found respectively. Hence statistical analysis indicates that the measured data are relevant as well as sufficient to explore the solar energy even in January, i.e. during winter season.
3.2. Relationship between Clearness Index (K), Cloud Transmittance Factor (cf) and Measured Global Solar Radiation at Lulka
The global solar radiation and cloud transmittance factors have are measured at the above mentioned site continuously by Pyranometer and NILU-UV irradiance meter. The measurements are recorded at intervals of 1 minute. Based on these 1 minute data for the whole year, monthly mean values of global solar radiation, clearness index and cloud transmittance factor have been computed and presented in Table 1.
The clearness index is defined as the ratio of measured global solar radiation (Hg) to the extraterrestrial global solar radiation (H0). Similarly the cloud transmittance factor is the ratio of measured radiation to the calculated clear sky radiation without aerosols and zero surface albedo. It is measured in percentage. Figure 3 shows that the coefficient of determination of 0.97 is obtained from clearness factor (K) and cloud transmittance factor (cf). It is observed that sufficient amount of energy is available for extraction even during the winter season.
Figure 4 shows the relationship between global solar radiations (GSR), clearness index (K) and cloud transmittance factor (cf). Figure 4 shows that the increasing and decreasing trend of GSR, cf and K are tentatively similar it means that there is strong correlation in between GSR, cf and K. Hence if cf values are available, the GSR can be predicted which will be the new way in coming days. The given figure clearly indicates that maximum amount of solar energy can be harvested in spring and minimum in summer which is quite different trend compared to Kathmandu, Pokhara and Biratnagar [10].