Determination of Circumsolar Radiation and its Effect on Concentrating Solar Power

Kurzfassung

Depending on the atmospheric conditions, a considerable fraction of solar radiation is scattered towards the circumsolar region. Circumsolar radiation is only partially used by concentrating collectors and has to be considered for performance evaluation of CSP plants. No information on circumsolar radiation is available for some sites that are currently of interest for CSP projects. Hence, sunshape measurement systems are developed and applied in this work. The measurements from two sites are evaluated in order to characterize circumsolar radiation and its effect on CSP plants. One of the developed systems, the SFERA system, consists of the SAM instrument, a sun photometer and post-processing software. The overall uncertainty of the SFERA system is a signigcant improvement compared to previous measurement systems, e.g. the former DLR sunshape camera. One and a half years of measurements with the SFERA system from PSA (Plataforma Solar de Almeria) and approximately one year from Masdar are evaluated and compared to previous measurements. The SFERA average sunshape for the exemplary data from PSA is similar to the standard solar scan (SSS). The average sunshape for the exemplary dataset from Masdar has a circumsolar ratio (CSR) that is about twice as high as that of the SSS and the SFERA result for PSA. The correct processing of time series of circumsolar radiation data is discussed and implemented for the raytracing tool SPRAY. EuroTrough plants and tower plants are modeled at PSA and Masdar. Signifcant errors occur when calculating with so-called standard sunshapes instead of measurements for the instantaneous received power and also for the daily and the annual heat that is provided to the power block. Raytracing with a disk sunshape leads to overestimation of the annual yield. Overestimation of 1.6 % is obtained for the tower at PSA and 3.9 % for Masdar. Even for the troughs the overestimation is noticeable when calculating with the disk sunshapes. Approximately 0.5 % overestimation occurs for PSA and 1.1 % for Masdar. A remarkable overestimation is also found for the tower plant at Masdar when calculating with existing average sunshapes as the SSS or the sunshape \DLRMean". The overestimation is 1.7 % for DLRMean and 2.1 % for the SSS. For the tower at PSA, calculating with the average sunshapes results in relatively small overestimation (0.7 % for the SSS, 0.5 % for DLRMean). The deviations of the annual yield for the trough in Masdar are smaller than for the tower. The SSS results nearly in the same annual yield as the sunshape measurements and DLRMean causes overestimation of about 0.4 %. For PSA, average sunshapes lead to underestimation of up to -0.4 %. Based on the raytracing result, parameters for the description of the efect of circumsolar radiation on the exemplary plants are studied. The CSR and also the circumsolar contribution (e.g. from the region between 0.64° and 3.2° distance from the sun's center) are promising parameters for simple models for the effect of circumsolar radiation on the effciency of CSP plants. Circumsolar radiation measurements should be included in solar resource assessment, plant design, yield assessment, plant operation and CSP performance tests. In the best case, the circumsolar radiation measurements are represented by sunshape measurements. However, also the less demanding measurement of the CSR or the circumsolar contribution can be sufficient. Hence, alternative measurement systems are investigated in this thesis. One of the tested systems is the Black Photon Instruments CSR460 sensor. It uses two pyrheliometers with different penumbra functions. The relative deviations between the two pyrheliometers agree well with the relative deviations expected from the SFERA measurements. Also a method to derive the CSR from these deviations is tested. The accuracy of this CSR determination is lower than for the SFERA system as the involved Buie sunshapes are steeper than most of the measured sunshapes. Furthermore, an algorithm is developed that allows the determination of circumsolar contributions with rotating shadowband irradiometers. The now available sunshape measurement systems and the correct processing method of circumsolar radiation data in raytracing tools can further improve the accuracy of CSP plant yield assessment.