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Parabolic trough power plants are complex hydraulic and thermal systems that use parabolic mirrors to heat a fluid for energy generation. The solar field is divided into several loops through which the fluid flows and is heated. A loop can be more than 600 m long [3] and consists of parabolic mirrors that focus the sun's radiation onto an absorber tube. To achieve the desired outlet temperature, the amount of fluid brought into the solar field can be controlled. The fluid is distributed in the solar field in such a way that approximately the same amount flows through each loop. In case of homogeneously distributed irradiation, e.g. under clear sky conditions, the control with the mass flow works well. Problems may occur in transient conditions such as cloud passing. In this case, shaded loops can receive too much mass flow and the fluid cools down, or loops that are not shaded by a cloud can receive insufficient mass flow and overheat in the process. As an emergency function, the parabolic mirrors are moved out of focus to prevent damage in the event of high temperatures. In order to be able to react better in such situations, a system was developed that provides a better overview of the irradiation in the solar field. A camera system with an all sky imager (ASI) takes pictures of the sky and the software creates spatially resolved irradiation maps. This information can be used in different ways. This paper presents different implementation depths for parabolic trough power plants. The implementation levels range from the ASI system as stand-alone system up to a fully integrated automatic control of the solar field. For each implementation level, the benefits of the provided information are shown and an overview on the implementation for the operator effort is provided. [ABSTRACT FROM AUTHOR]

Because of forward scattered radiation in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in direct normal irradiance (DNI) measurements at the usual 2.5° pyrheliometer opening half angle, but only partially intercepted by the receivers of focusing solar collectors. Therefore, circumsolar radiation measurements are recommended to be included in solar resource assessment. Circumsolar radiation can be characterized using the radial angular distribution of the radiance around the center of the sun - the so-called sunshape. Several sunshape measurement methods have been developed recently. Most approaches use cameras or pyrheliometers with different apertures which require daily maintenance. The Rotating Shadowband Irradiometer (RSI) based method discussed here uses a conventional RSI without any hardware modification to enable sunshape measurements without affecting the RSI's fundamental function as a DNI measurement device. Thus, it allows to measure circumsolar radiation without any additional hardware and with significantly lower maintenance requirements. The presented RSI-based sunshape measurement algorithm has been validated with four RSIs and more than two years of data. After a short description of the method to derive the circumsolar contribution, the validation results are shown. Then the required calibration method is discussed, followed by the results from the measurement campaigns at four sites in Spain, India and Morocco. It was found that no individual calibration is required per instrument and the algorithm can be used for automatic data processing so that common RSI stations can measure the sunshape with comparably low extra effort. Furthermore, we explain how to derive sunshapes from the RSI measurements. [ABSTRACT FROM AUTHOR]

Concentrating solar power projects require accurate direct normal irradiance (DNI) data including uncertainty specifications for plant layout and cost calculations. Ground measured data are necessary to obtain the required level of accuracy and are often obtained with Rotating Shadowband Irradiometers (RSI) that use photodiode pyranometers and correction functions to account for systematic effects. The uncertainty of Si-pyranometers has been investigated, but so far basically empirical studies were published or decisive uncertainty influences had to be estimated based on experience in analytical studies. One of the most crucial estimated influences is the spectral irradiance error because Si-photodiodepyranometers only detect visible and color infrared radiation and have a spectral response that varies strongly within this wavelength interval. Furthermore, analytic studies did not discuss the role of correction functions and the uncertainty introduced by imperfect shading. In order to further improve the bankability of RSI and Si-pyranometer data, a detailed uncertainty analysis following the Guide to the Expression of Uncertainty in Measurement (GUM) has been carried out. The study defines a method for the derivation of the spectral error and spectral uncertainties and presents quantitative values of the spectral and overall uncertainties. Data from the PSA station in southern Spain was selected for the analysis. Average standard uncertainties for corrected 10 min data of 2% for global horizontal irradiance (GHI), and 2.9% for DNI (for GHI and DNI over 300 W/m?) were found for the 2012 yearly dataset when separate GHI and DHI calibration constants were used. Also the uncertainty in 1 min resolution was analyzed. The effect of correction functions is significant. The uncertainties found in this study are consistent with results of previous empirical studies. [ABSTRACT FROM AUTHOR]