Quoted from: Veeran P K ,  Kumar S . Analysis of monthly average daily global radiation and monthly average sunshine duration at two tropical locations[J]. Renewable Energy, 1993, 3(8):935-939.

https://www.sci-hub.ren/10.1016/0960-1481(93)90054-k

The monthly average daily totals of global solar radiation on a horizontal surface at Madras (13°0′N, 80°11′E) and Kodaikanal (10°14′N, 77°28′E) during the period 1983–1987 have been processed, analysed and presented. The regression coefficients, a and b , of ngstrom linear type correlations for the monthly mean daily global solar radiation have been determined for these two locations. A comparison of average global radiation and sunshine duration obtained here (5 years) with long term data (more than 15 years) shows that a minimum of 5 years is sufficient to obtain average values for radiation, whereas the same does not hold true for the duration of sunshine.

INTRODUCTION

The monthly average of the daily global solar radiation at a location is an important parameter for the design and prediction of solar system performance, thermal or photovoltaic. Information on solar radiation at a location is obtained by measuring sunshine duration : hourly and daily values of global, direct and diffuse radiation. Sunshine duration is the most commonly measured parameter and, therefore, scientists have used various methods to correlate global radiation with sunshine duration. Recent data on sunshine duration and hourly global radiation were made available  for Madras and Kodaikanal in India, and this paper presents the results of the analyses conducted based on these data.

The aim of this paper is to present the regression coefficients a and b for Madras and Kodaikanal, and secondly, to compare the 5-years data (present study) with those of Mani and Rangarajan (21 years for Madras and 16 years for Kodaikanal) in order to see whether 5-years data is sufficient to predict the average values of daily global radiation and sunshine duration. This study is important since it is not always clear how many years data is necessary to estimate average values, though for solar radiation, a minimum of 4 years is considered necessary. Earlier studies on solar radiation in India are discussed briefly as follows.

 Veeran and Kumar  proposed the following linear equation for two tropical locations in India:

\( 𝐻/𝐻_0 = 0.34 + 0.32 𝑆/𝑆_0 , \)

\( 𝐻/𝐻_0 = 0.27 + 0.65 𝑆/𝑆_0 . \)

CONCLUSION

The regression analysis was carried out to correlate the monthly average daily global solar radiation on a horizontal surface in Madras and Kodaikanal with the sunshine hours recorded for the period 1983 1987. The regression constants a and b have been determined and compared with the values reported by different authors for these two locations. A study on the number of years of data necessary for long term calculation of radiation and sunshine duration has shown that 5-years data will be sufficient for radiation analysis whereas the same could not be said for sunshine duration.