A new look at the correlation of Kd and Kt ratios and at global solar radiation tilt models using one-minute measurements

• Published in: Solar energy 1984, Vol.32 (1), p.99-107
• Main Authors: Smietana, P.J., Flocchini, R.G., Kennedy, R.L., Hatfield, J.L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Energy; Exact sciences and technology; Natural energy; Solar energy
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/0038-092X(84)90053-7

One-minute measurements of horizontal diffuse, D h , and global, G h , solar radiation components were used to calculate K d and these ratios were compared with values determined from previously published functions of the independent parameter K t . The spectrum of the 9906 ratios, used in the comparison, was taken from representative days with percent sunshine ranging from 10 to greater than 90 per cent and from different seasons. The results of this comparison show that K d is not accurately determined by these functions of K t alone and from functions fitted to our data, and suggest that other parameters might be needed. The calculation of global radiation on a tilted surface from models which depend on D h were also analyzed by comparisons with global tilt radiation measured at 45° and 90° tilt angles at each of the four cardinal directions. The poor determination of K d causes large scatter in calculated global tilt radiation values because of the corresponding poor value of D h . However, if a good value of D h is used then the global tilt models replicate measured global tilt radiation values to within 10 per cent.