Quoted from:[1] Jain S ,  Jain P C . A comparison of the Angstrom-type correlations and the estimation of monthly average daily global irradiation[J]. Solar Energy, 1988, 40(2):93-98.

https://www.sci-hub.ren/10.1016/0038-092x(88)90076-x

Abstract

Linear regression analysis of the monthly average daily global irradiation and the sunshine duration data of eight Zambian locations has been performed using the least squares technique. Good correlation (r ≥95) is obtained in all the cases showing that the Angstrom equation is valid for Zambian locations. The values of the correlation parameters thus obtained show substantial unsystematic scatter. The analysis was repeated after incorporating the effects of (1) multiple reflections of radiation between the ground and the atmosphere and (2) not burning the sunshine recorder chart. The surface albedo measurements at Lusaka were used. The scatter in the correlation parameters was investigated by graphical representation, by regression analysis of the data of the individual stations as well as the combined data of the eight stations. The results show that the incorporation of the two effects does not reduce the scatter significantly.

A single linear equation obtained from the regression analysis of the combined data of the eight stations is found to be appropriate for estimating the global irradiation over Zambian locations with an average accuracy of 4 to 10%. 

INTRODUCTION

A reasonably accurate knowledge of the availabilityof the solar resource at any place is required bysolar engineers，architects, agriculturists，and hy-drologists in many applications of solar energy. Themost important parameter that is often needed isthe long-term average daily global irradiation. Un-fortunely, the measurements of this parameter aredone only at a few places. For places where no mea-sured values are available, a common practice hasbeen to assess this parameler using appropriate cor-relations that are empirically established by usingthe measured data at some selected places.Thesecorrelations estimate the values of H from the morereadily available meteorological，climatological,and geographical parameters such as the sunshineduration,temperature, and latitude.The first correlation proposed for estimating themonthly average daily global irradiation is due to Angstrom. He used the sunshine duration datafor estimating the global irradiation.