Comparison of diffuse/global ratios calculated from one-minute, hourly and daily solar radiation data

• Published in: Solar energy 1984, Vol.32 (3), p.417-423
• 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)90286-X

One-minute values of direct, diffuse and global radiation have been continuously collected at Davis, California (38.5°N, 121.1°W) since 1 January, 1979. These datasets are quality controlled to insure the most accurate and reliable data possible. Analysis of one-minute data has provided an opportunity to evaluate some of the bias that may be inherent in statistical representation of solar radiation data. A simple mean and standard deviation do not adequately describe the variation in the data and we show that a more representative treatment includes the box and whisker analysis. In this the mean, median, first and third quartiles, and the maximum and minimum ranges are presented. It is possible to compute the variability between days more completely with this technique while the means may be very close. This has application to evaluation of solar collectors as a better method of evaluating theire efficiency. This is applied to diffuse/global ratios which show a seasonal dependence although some clear winter days have ratios close to clear summer values; however, the first and third quartile and median distinctly separate these days. Analysis of solar radiation data should be conducted with caution as shown by these results. A simple model is proposed to compute hourly global values from the integrated daily total. Comparisons of calculated with measured hourly values indicated less than a 10 per cent error between 0700 to 1600 with the maximum value being slightly underestimated. This procedure allows one to evaluate solar collectors with only daily values and presents a method for thoroughly evaluating our solar resources.