Review of Solar Radiation Utilizability

Abstract

The utilizability concept was originally developed for estimating the performance of flat-plate solar collectors over long time periods. In this application, low critical radiation levels are desirable in that they result in high utilizability values and thus efficient collection of solar energy. For this reason, early correlations of the utilizability function were designed to provide accurate estimates of the utilizability at low to intermediate critical levels. It is now apparent that the utilizability concept is useful for other applications such as passive and photovoltaic system analysis. In these latter applications, high critical levels and thus low utilizability (unutilizability) values are desirable in that they imply a better match of collectable solar energy to the load. Utilizability correlations that retain acceptable accuracy at high critical levels are needed for these applications. A complicating factor is that as the critical level increases, utilizability becomes more sensitive to the distribution of solar radiation. At high critical levels, utilizability may be sensitive to variations in the solar radiation at time intervals shorter than one hour. These factors place a limit on the accuracy achievable with a location-independent correlation. To improve accuracy, it may be necessary to introduce one or more additional parameters (in additionmore » to the clearness index) to characterize the local solar climate. The utilizability function also contains information about operating times through its derivative with respect to critical level. None of the existing correlations were developed with the concern to accurately represent both the utilizability function and its derivative and so the estimation of operating time is perhaps not as accurate as it could be.« less