The clearness index for hourly and daily radiation is an important parameter in describing solar radiation. Liu and Jordan demonstrated that the monthly average daily clearness index could be used to predict the long-term distribution of daily solar radiation in a month. This paper reviews recent literature on the prediction of hourly and daily frequency distributions and cumulative frequency distributions of clearness indices. Ten years of measured weather data for six cities in the U.S. are used to investigate the nature of the hourly and daily frequency distributions. A second set of ten years of data for six cities is used to verify the predictions. A bi-exponential probability density function is proposed that fits the observed bimodal nature of the data better than existing models. A case is made for the function being universal.
Frequency Distributions for Hourly and Daily Clearness Indices
Contributed by the Solar Energy Division of the American Society of Mechanical Engineers for publication in the ASME JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received by the ASME Solar Energy Division November 2000; final revision July 2001. Associate Editor: A. Walker.
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Iban˜ez, M., Beckman, W. A., and Klein, S. A. (July 1, 2001). "Frequency Distributions for Hourly and Daily Clearness Indices ." ASME. J. Sol. Energy Eng. February 2002; 124(1): 28–33. https://doi.org/10.1115/1.1445443
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