2. Literature review
Iziomon and Mayer evaluated global solar radiation for lowland and a mountain site with data from 1991 to 1994. Tested models were broadly categorised as cloud-based (Kasten) and sunshine-based (Ångström–Prescott, Garg and Garg, Sivkov). Adjustable parameters in the models were determined. Observed monthly mean values of solar radiation G and those estimated using Kasten model agreed within 2.5% for the lowland site and 13% for the mountain site. Root mean square errors of estimated hourly values of G using Kasten model appreciated significantly with fractional cloud cover N (particularly for N>4 octals). Monthly mean values of G estimated using Ångström–Prescott model agreed with observation within 2.5% for the lowland site and 3.4% for the mountain site. The effect of air mass, latitude and water vapour terms on the Ångström–Prescott relation has also been investigated. In general, Ångström–Prescott as well as Garg and Garg models yielded the least RMSE (less than 0.047) for the study sites and are thus recommended for estimating G for an arbitrary location .
Ali Rahoma analyzed global index and diffuse fraction for clear-sky conditions using direct, global, and diffuse solar radiations (cloudless) taken over a one year period at Helwan. The results highlight the need for routine instantaneous surface meteorological data to compute global and diffuse radiations on a horizontal surface in the absence of any other radiation measurements. The spectral composition of the global solar-radiation was found to be 4.3% UV band, 32.5% band range 250–630 nm, 13.74% red band, 52.75% infrared band and 29.7% diffuse solar-radiation The spectral distribution of direct solarradiation ratio of the extraterrestrial solar radiation was found to be: 0.69% green and blue band, 47.5% yellow and orange band 45% red band, and 52.7% infra-red band .
Shafiqur Rehman, et al adopted a geostatistical technique consisting of (i) data collection, (ii) univariate analysis, (iii) experimental variogram calculations and model fitting, (iv) estimation using kriging, and (v) plotting contour maps for the estimation of solar radiation in Saudi Arabia. Variogram models are fitted to measured variograms for each month of the year. Estimates obtained for 1500 grid points (30 × 50) with resolution of 55 × 33 km, were used to plot monthwise solar radiation contours. The error analysis showed that the mean percent errors to vary between 0.5% and 1.7%. This technique could be used for the spatial estimation of solar radiation on regional and continental scales .
Ahmet Aksakal and Shafiqur Rehman, presents the actual global solar radiation on a horizontal surface in the Arabian Gulf Coast near the city of Dhahran, based on monthwise high resolution, real time solar radiation and meteorological data for a year. Hourly, daily, and monthly statistics of solar radiation was made from the one-minute averaged recorded values. The highest measured daily, and monthly mean solar radiation was found to be 351 and 328 W/m-2, respectively. The highest one-minute averaged solar radiation values up to 1183 W/m-2 were observed in the summer season, from May–September. The highest hourly solar radiation value was recorded as 1053 W/m-2 in the middle of June .
Ramachandra and Subramanian., estimated the solar radiation for Uttara Kannada district based on the solar radiation and climatological data got from IMD, Pune, for Karwar, Honnavar, Shirali, Mangalore, and Goa. The computed and measured values of global solar radiation agrees with the range of 2 –5 % for most months the computed and the estimated values are with in the range of ± 5%. Karwar has a global solar radiation range of 5.5 – 6.5 kWh/sq.m for January–May and 4 - 5 kWh/sq.m during July–September, while in Honnavar it ranges from 5.47–6.5 kWh/sq.m for January–May and minimum during July–September. The study demonstrates that good solar energy potential is available in this region during most months of the year. The amount of solar energy that could be harnessed by utilizing 5% of the wasteland as collector area is found to be of the order of 95.72 million units (MkWh) annually .