Validation of a cloud detection algorithm with an All-Sky Imager

Abstract

With the increase in renewables in the future power mix, fluctuations in the power production become larger due to the intermittency of these energy sources. Accurate forecasting of solar energy production can help to improve unit-commitment decisions and reduce ancillary costs. Various methods can be used for solar forecasting. Most methods, e.g. satellite techniques, often overlook small and thin clouds. The high temporal as well as spatial resolution of All Sky Imagers enables the opportunity to take these small and thin clouds in to account, even at fast changing cloud conditions. Recent efforts at EKO Instruments let to the development of a new cloud detecting algorithm called TRINITY. This study aims at comparing the performance of the new TRINITY algorithm in combination with an All Sky Imager to the existing BRBG and CDOC cloud detecting algorithms. The new algorithm is validated using two approaches. The first method uses shortwave irradiance by determining the clearness index and diffuse fraction as proxies for Cloud Cover Fraction. The other method calculated the Cloud Cover Fraction by using downward longwave irradiation. Data is provided by two cases studies, where data is collected in Utrecht (NL) and Denver (US). Results of the shortwave irradiance method show that lowest errors where achieved by using the diffuse fraction as a proxy. Overall, the mean absolute error of the new TRINITY algorithm was 12%, whereas the BRBG and CDOC algorithms had errors of 17% and 14%, respectively. When differentiating for different sky conditions the TRINITY algorithm outperforms BRBG and CDOC at clear sky conditions, whereas in overcast conditions it outperforms the BRBG algorithm. Furthermore, the unreliable sunrise and sunset periods affect the accuracy of the algorithms and radiation measurements. Excluding the sunrise and sunset improves the accuracies with 11%, 15% and 2% for the BRBG, CDOC and TRINITY, respectively. Testing the effect of the solar position on the performance of the algorithms showed that the BRBG algorithm is most sensitive to low elevation angles, leading to higher errors. The TRINITY algorithm achieved similar performance for all elevation angles and is more stable than the other algorithms. For elevation angles of 35° and higher, all algorithms perform similarly. Preliminary results for using longwave downward radiation show that the accuracies of all algorithms are comparable (53%, 54% and 56% for BRBG, CDOC and TRINITY, respectively) with lowest errors for the BRBG algorithm. Overall, TRINITY is found to perform best followed by the CDOC and BRBG algorithm. Accurate cloud detection by All Sky Imagers will improve the accuracy of short-term solar forecasting.