This paper proposes a method to obtain spectral sky radiances, at three wavelengths (464, 534 and 626 nm), from hemispherical sky images. Images are registered with the All-Sky Imager installed at the Andalusian Center for Environmental Research (CEAMA) in Granada (Spain). The methodology followed in this work for the absolute calibration in radiance of this instrument is based on the comparison of its output measurements with modelled sky radiances derived from the LibRadtran/UVSPEC radiative transfer code under cloud-free conditions. Previously, in order to check the goodness of the simulated radiances, these are compared with experimental values recorded by a CIMEL sunphotometer. In general, modelled radiances are in agreement with experimental data, showing mean differences lower than 20% except for the pixels located next to the Sun position that show larger errors. The relationship between the output signal of the All-Sky Imager and the modelled sky radiances provides a calibration matrix for each image. The variability of the matrix coefficients is analyzed, showing no significant changes along a period of 5 months. Therefore, a unique calibration matrix per channel is obtained for all selected images (a total of 705 images per channel). Camera radiances are compared with CIMEL radiances, finding mean absolute differences between 2% and 15% except for pixels near to the Sun and high scattering angles. We apply these calibration matrices to three images in order to study the sky radiance distributions for three different sky conditions: cloudless, overcast and partially cloudy. Horizon brightening under cloudless conditions has been observed together with the enhancement effect of individual clouds on sky radiance.

Different meteorological data series called multiyear data, long-term average measured data series, or test reference years (TRYs) are required for solar energy system simulation. It is known that the use of the multiyear data approach requires a great effort in time and computation, long-term average measured data do not have the extreme values of weather data given along the year, and TRYs represent typical references rather than extreme conditions and facilitate the comparison in the performance of energy systems. In this paper, TRYs have been generated, using three different methodologies, from hourly meteorological data measured in two cities, Madrid and Valladolid (Spain). In order to evaluate them, the performance simulation of three solar energy systems (thermal, passive, and photovoltaic) with long-term measured meteorological data has been compared with estimated performance simulations with TRYs. Root-mean-square and mean bias errors and relative differences have been used as estimator... Abstract Different meteorological data series called multiyear data, long-term average measured data series, or test reference years (TRYs) are required for solar energy system simulation. It is known that the use of the multiyear data approach requires a great effort in time and computation, long-term average measured data do not have the extreme values of weather data given along the year, and TRYs represent typical references rather than extreme conditions and facilitate the comparison in the performance of energy systems. In this paper, TRYs have been generated, using three different methodologies, from hourly meteorological data measured in two cities, Madrid and Valladolid (Spain). In order to evaluate them, the performance simulation of three solar energy systems (thermal, passive, and photovoltaic) with long-term measured meteorological data has been compared with estimated performance simulations with TRYs. Root-mean-square and mean bias errors and relative differences have been used as estimator...

Daylight downward longwave irradiance data recorded over a flat place for the period between April 2001 and December 2004 in Valladolid, Spain, have been compared with estimates generated using four different schemes. The parameterization schemes of Brutsaert, Swinbank, Idso, and Brunt have been considered and calibrated for the comparison. Root-mean-square errors (rmse), mean bias errors, and linear regression correlations have been used to compare measured and estimated values. The results of this comparison show that, for clear-sky conditions, rmse values range between 19.57 and 8.85 W m−2 for calibrated schemes and between 39.78 and 11.13 W m−2 for original ones. The Idso and Brunt schemes give the best results with calibrated coefficients, and the Brunt scheme performs the best with original coefficients. A new scheme for estimating daylight downward longwave irradiance under “all-sky” conditions has been developed based on clear-sky schemes and solar global shortwave irradiance, and, after comparing measured and estimated values by calibrated schemes, it has been found that the Idso and Brunt schemes give the best results.