Comparison of techniques for the estimation of daily global irradiation and a new technique for the estimation of hourly global irradiation

Global irradiation and sunshine duration data recorded at Trieste (CNR, Istituto Talassografico di Trieste) during the 11-year period 1972–1982 are analyzed using the classical Angstrom equation H = H0 (a + bS/S0) and the equation H′ = H0 (a + bS/S′0) suggested by Hay [7] for incorporating the effects of (i) multiple reflections, and (ii) not burning of the sunshine recorder chart for small elevation of the sun. The values of the regression constants and the correlation coefficients are calculated using each yearly data set separately. Correlation coefficients of 0·89 or more are obtained for the 11 years. Substantial unsystematic scatter is obtained in the values of a as well as b for different years. The use of the equation H′ = H0 (a + bS/S′0) is not found to either decrease this scatter or to give better values of the correlation coefficients. Hourly global irradiation data is also analyzed. Eleven-year mean values of the ratio hourly/daily are plotted against the solar time for each of the 12 months of the year. The normal distribution curve P(t) = 12πrexp−(t−12)22σ is found to fit the data closely. The mean of the normal distribution is taken at the solar noon and the σ values are obtained for each month by matching the experimental and the theoretical values at the solar noon. The σ values so obtained are found to bear an excellent linear correlation (r = 0·996) with S0, viz.σ = 0·461 + 0·192 S0. This provides a simple and elegant technique for estimating hourly irradiation from the daily values and may be of universal applicability. The technique enables the estimation of global irradiation for any smaller interval of time as well.

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