Effects of Sky Conditions Measured by the Clearness Index on the Estimation of Solar Radiation Using a Digital Elevation Model

Abstract

This study evaluated the effects of sky conditions (measured by the clearness index, K T) on the estimation of solar radiation and its components. Solar radiation was calculated by a digital elevation model derived from the Shuttle Radar Topography Mission (SRTM). The calculated radiation was parameterized and validated with measured solar radiation from two stations inside the urban perimeter of the city of Cuiabá, Brazil, during 2006 to 2008. The measured solar radiation varied seasonally, with the highest values in December-March and the lowest in June-September. Comparisons between calculated and measured values for two sites in Cuiabá demonstrate that the model is accurate for daily Rg estimates under clear sky conditions based on Root Mean Square Error, Mean Bias Error and Willmott's index. However, under partially cloudy and cloudy sky conditions the model was not able to provide robust estimates. Spatially, the highest values of incident Rg occurred on strands with North, Northeast and Northwest orientations and were lowest on those oriented to the South, Southeast and Southwest.

Performance Analysis of Clear Sky Global Horizontal Irradiance Models: Simple Models Adapted for Local Conditions

“…Very low values of the daily clearness index are an indication of low global irradiance which may mainly be due to the presence of clouds in the sky that obstruct the sun's disc without significantly increasing the diffuse component of the incoming solar irradiance. Different authors have specified varying thresholds of K t that signify a clear sky, for example, Iqbal [39] defined the range of K t for clear sky as 0.7 ≤ K t < 0.9, Alves et al [42] considered the range 0.65 < K t < 1 for clear sky, Li and Lam [43] considered K t > 0.7, while Jung et al [44] used K t > 0.63 for clear skies.…”

Decomposition and transposition model-matching technique in the absence of plane-of-array measurements and the evaluation of tilted solar collectors and their harvested solar resource

“…Figure 1(b) shows the monthly average daily clearness index, K T , where 0:41 K T 0:50 with a minimum of 0.41 in August, a maximum of 0.5 in February, and average values of 0.48 and 0.43 for the dry (January-May) and wet (June-December) seasons, respectively. The sky conditions for the typical days of the months during a year can thus be classified as intermediate or partly cloudy [22][23][24] on which basis one can reasonably deduce that a transposition model that assumes pure or near-isotropic diffuse irradiation conditions (consistent with overcast or nearovercast conditions) would not be accurate for Trinidad as it does not reflect the inherent meteorological conditions of typical days. Figure 2 shows the variation of the monthly optimum tilt angles for the six transposition models and the beam irradiation only-the former plotted as lines and the latter as point plots for the sake of clarity.…”

Optimized indoor daylight for tropical dense urban environments

“…As stated by Alves et al (2013), the sky cover condition in the urban area of Cuiabá is influenced by diverse factors, such as cloudiness and atmospheric combustion products. Another phenomenon is a typical characteristic of the central-western Brazilian territory: outbreaks of fire in the surrounding native vegetation.…”