Abstract
Accurate solar radiation data are essential for the development of solar energy application systems. The limited availability of solar radiation data, and especially diffuse irradiance values, makes it vital to develop models to estimate these data. The development of estimation models has been the objective of many studies. This paper presents an extended review of the diffuse ratio (k) vs. clearness index (kt) annual, monthly, daily, and hourly frequency regression models. It is however interesting to note that there is a dearth of such knowledge for diffuse ratio–clearness index regressions that are based on averaged data. Monthly-averaged daily global irradiation data are now easily available from the NASA website for any global location. Using existing models, it is possible to decompose the daily to averaged-hourly global irradiation values. The missing link so far has been hourly averaged diffuse irradiation. This article presents regression equations which could be used to estimate that information. For this purpose, hourly global and diffuse irradiation data was pooled from 19 different locations to obtain three latitude-dependent regression models relating the monthly-averaged hourly diffuse ratio ( k ¯ ) to the clearness index ( k t ¯ ). The results show a high relationship between both variables. These regression equations could be used to estimate the averaged diffuse irradiation values from averaged global irradiation values, which are more easily available.
Highlights
Alternative or renewable energy sources are able to provide the global current and future energy, with zero or almost zero greenhouse gas and pollutant emissions [1]
The amount of energy generated from solar energy depends directly on the quantity of solar irradiance that is received in the earth and the efficiency of the devices used in the conversion process [2]
Stanhill [16] used three years of radiation values records in the south of Israel to develop a correlation between the annual diffuse to extraterrestrial ratio, and compared these relationships with those obtained in other locations to verify their applicability in the region
Summary
Alternative or renewable energy sources are able to provide the global current and future energy, with zero or almost zero greenhouse gas and pollutant emissions [1]. Even though global irradiation at a monthly, daily, or hourly basis is the most commonly recorded parameter, it is only available in a limited number of meteorological stations. The contribution of the diffuse sky component in the irradiance received in the earth’s surface is high enough to consider the diffuse irradiation an essential parameter for projects related to solar energy. The limited availability of solar radiation data records makes it essential to estimate irradiation values for sloped surfaces given values for horizontal surface. To introduce an extended review of the existing regression models relating the diffuse ratio (k) and the clearness index (kt ) at an annual (Section 2), monthly (Section 3), daily (Section 4), and hourly (Section 5) frequency. This article intends to provide regression models to estimate the average-hourly diffuse irradiation using the freely accessible NASA website data
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