Physikalische Weiterentwicklung einer Wolkenkamera-basierten Messmethode für Diffusstrahlung

Abstract

Ground-based measurements of direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI) are of interest for applications in solar energy. However, measurement systems established in the market either have high acquisition costs, require intensive maintenance, or are prone to increased deviations. For these reasons, the measurement of solar radiation using a combined measurement system, consisting of an all-sky camera (ASI) and a pyranometer, henceforth referred to as PyranoCam, is currently under investigation at the German Aerospace Center (DLR). The PyranoCam measurement method analyzes the global horizontal irradiance (GHI) measured by the pyranometer in combination with cloud images captured by the ASI and derives DHI and DNI from it. Yet, the measurement method still shows potential for improvement in terms of accuracy and transferability between sites. In the present diploma thesis, an enhanced physical model of a cloud camera-based measurement method for the quantification of DHI and DNI is introduced. The core of the study is the development of a dynamic diffuse broadband correction factor. This factor is designed to address the errors arising from the spectrally limited sensitivity of the cloud camera sensor. At present, an empirical static diffuse broadband correction factor is applied to the pixel intensity values of captured all-sky images to estimate the broadband DHI, thereby improving the accuracy of DHI measurement. The novel model calculates the diffuse broadband correction factor dynamically based on the ratio of the red to blue color channels. For this purpose, an all-sky image dataset is fused with spectroradiometer measurements to correlate the measured broadband factor with the color channel ratio. A corresponding regression model is presented, and the results are showcased for two sites: Tabernas in southern Spain and Oldenburg in the northwest of Germany. The validation at these sites indicates that the new correction leads to a higher measurement accuracy while being physically more adequate.