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5. Methodology

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Stations where measurements of global solar radiation were available, data was used directly and for locations where the data was not available indirect methods were used. They are as follows,

1. From extra terrestrial radiation, allowing for its depletion by absorption and scattering by atmospheric gases, dusts, aerosols and clouds. This is theoretically based and requires some approximation of the absorbing and the scattering property of the atmosphere (given in Eq. 6).
2. From other meteorological elements, such as duration of sunshine and cloudiness using regression technique (Eqs. 9, 10 and 20). This method is empirical based, and the form usually used involves actual and potential hours of sunshine, which gives the regression constants for global and diffused solar radiation at the particular location or site          

For practical purpose it is convenient to divide the entire radiation regime within the earth’s atmosphere into two parts, the solar or the short-wave radiation and the terrestrial or long-wave radiation. To derive the detailed solar radiation climatology of a region and to estimate its solar energy potential, it is necessary to collect extensive radiation data such as daily global solar radiation, daily diffused solar radiation, sunshine hours, and maximum sunshine hour etc. of highest accuracy at a larger number of stations covering all the climatic zones of the region. In the absence of actual measured data, radiation data can be computed from other meteorological parameters to provide necessary data coverage for the region [5].

India Meteorological Department (IMD) collects climatological data in all districts of Karnataka while solar radiation data is collected in stations of Bangalore, Mangalore, Bellary, and Raichur. Fig. 2 shows the meteorological stations in Karnataka. These are ordinary stations equipped for continuous recording of global solar radiation and sunshine. Since not all the districts have solar radiation monitoring station, the values for such districts were estimated using the data collected from the neighboring station (with solar radiation data). The climatological parameters such as relative humidity, sunshine hours, and mean temperature for calculating global solar radiation was collected from IMD, Pune for meteorological stations in Karnataka located at Bangalore (ordinary radiation station), Mangalore (down ward radiation measured), Bellary (under crop weather observation), and Raichur (co-operation ordinary radiation station).

Regression relations between radiation with sunshine duration and climatological parameters was found out for Bangalore, Mangalore, Raichur, Bellary districts (in Karnataka) and Goa based on 25 years of data collected from Goa, 20 years data from Mangalore, 12 years data from Bellary and 7 years data from Raichur. It is found that computed values of solar radiation agree within 5-10% of the observed values.

With the knowledge of relationship between global radiation with sunshine duration and climatological parameters at Mangalore, Bangalore, Bellary, Raichur, and Goa interpolation was done to determine global solar radiation in other districts of the State, where solar radiation data is not available. Climatological mean monthly data of temperature, hours of sunshine, relative humidity etc., for all the districts were analysed and used to estimate solar radiation for all districts in Karnataka.

Thus, solar radiation for Kolar, Tumkur, Mysore, Mandya and Chamrajnagar were estimated based on the relation derived from Bangalore station, as Bangalore is closer to these districts. Similarly values for Udupi, Shimoga, Chikmagalur, Hassan and Kodagu were estimated based on the relation derived from Mangalore station. Relation derived from Bellary station was used in Haveri, Gadag, Davangere, Koppal, and Chitradurga and that from Raichur station were used in Bidar, Gulbarga, Bijapur, and Bagalkote. Since Uttara Kannada, Dharwad, and Belgaum were closer to Goa, global solar radiation was estimated based on the regression relationship derived for G’ and climatological parameters of Goa. The station in Goa is a principal radiation station, where there is continuous recording of global, diffused solar radiation and sunshine.

The solar energy potential is truly enormous, and the amount that can be readily accessed with existing technology greatly exceeds the world’s primary energy consumption. In determining the realistic potential of solar energy conversion systems, it is quite evident that we have to consider the availability of solar energy associated with time such as daily variation due to day and night cycles, seasonal changes due to the earth’s motion around the sun, and variation due to local weather condition. With the detailed investigation of solar energy availability and changes at various places in Karnataka, the potential that could be harnessed to meet the community energy needs in the immediate future can be known.

Due to the dilute nature of solar energy flux at the earth’s surface large collecting surfaces are required in many applications. This makes it necessary to look at the land use pattern of the region and also at the land availability for harnessing solar energy.

The average monthly global solar radiation is calculated using Eq. 17. Based on the R2 value and the least value of standard error of the y estimate, empirical formula consisting of specific humidity and mean temperature is the best relationship compared to others.

where, Tm is mean temperature and SH is specific humidity (given by Eq. 19).

The resulting global solar radiation is further classified based on seasons as:

– Global solar radiation during summer (February–May)
– Global solar radiation during monsoon (June–September)
– Global solar radiation during winter (October–January)