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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]

The solar field of a parabolic trough power plant is a complex thermal and hydraulic system. It is used to bring the heat transfer fluid (HTF) up to the desired temperature. The mass flow for the entire solar field is usually controlled from the plant control room. In the ideal case, the controller will adjust the mass flow so that the desired temperature is reached and all collector in the solar field are in full focus. Under transient conditions like a cloud passing through, different temperatures of the HTF can occur in the solar field. For example, the area not covered by a cloud can become hotter whereas temperatures in the area with cloud cover drop. In case the mass flow is adapted to the average irradiance level in the field, this scenario can lead to significant defocusing of those collector rows seeing high DNI but suffering from reduced mass flow. State of the art solar field controllers can use information on current DNI derived from one or a few pyrheliometers distributed over the solar field. Information on DNI distribution or an appropriate average DNI are thus not available. Recent developments in all sky imager (ASI) systems provide such spatially resolved DNI information and open the way for new control approaches. In this study, an irradiance forecasting system is used based on all sky imagers (ASI) to provide spatially resolved DNI maps of the whole solar field. With the ASI system, the controller can access information about the spatial DNI distribution and thus has knowledge about individual clouds in the solar field. In this paper, a model based approach to improve the feed forward control of the mass flow is investigated The detailed model used by the controller represents the current state of the solar field using most recent sensor data from the real solar field. With the forecasts of the ASI system as input, several simulations are performed to determine a suitable mass flow for the upcoming minutes. Under cloudy conditions, the new system shows advantages compared to a reference controller using only the DNI information from the elapsed time step. The investigation are carried out with ideal forecast and forecasts including uncertainties. For a test set of 27 days, the new control concept is able to reduce the number of emergency defocusing events by 9% under ideal forecasts and 5.7% when considering uncertainties. The average focus state of the field is increased by about 0,25%. [ABSTRACT FROM AUTHOR]

Solar fields of parabolic troughs are extensive complex thermal hydraulic facilities. Intra-hour and intra-minute variabilities of the DNI, mainly caused by passing clouds, pose an operational challenge for parabolic trough power plants. Under perfect circumstances a solar field controller would adjusts the mass flow in such a way, that the design temperature is always maintained constant with a maximized focus rate. However, heterogeneous irradiance conditions or flow distribution may cause some solar field sections to temporarily overheat while others may not reach the set point temperature, which in turn leads to an economic loss. State of the art solar field controllers have only access to incomplete information on spatial DNI variability, from DNI measurements of few pyrheliometers. Solar field controllers could be optimized with access to highly resolved DNI informations both in space and time. Such DNI information can be provided by all sky imager (ASI) based irradiance monitoring systems. In a previous study we developed and benchmarked new solar field controllers with access to spatial DNI information from an ASI system for a 50 MWe plant close to Córdoba (Spain). Significant improvements in revenue were observed. Yet, this previous study was limited to 22 days only. In this study, we estimate the potential benefit of these new solar field controllers over a 2 year period on the basis of the simulation results over 22 days. The upscaling method makes use of DNI variability classes. Using the ASI data we obtain a significant improvement in revenue up to 2% for the 2 year period. [ABSTRACT FROM AUTHOR]

The yield of solar thermal power plants is strongly affected by disturbances in the irradiation condition through passing clouds. This effect is even more prominent when the clouds shade the solar field only partially as the irradiance situation cannot be fully measured by few measurement points in the field. In this paper, the benefit of using nowcasting systems to produce highly resolved irradiance maps on the field during start-up is studied. A simulation tool is used to dynamically model all the loops and piping in a reference commercial solar field. Automatic solar field controllers allow the model to operate during different situations. The performance of the field under a reference state-of-the-art controller is compared against a controller using the irradiance maps from nowcasting systems. The benefit during start-up processes is found to be significantly lower than the benefit the system makes during normal solar field operation. However, there is more potential shown for more advanced controllers, like model-predictive control. [ABSTRACT FROM AUTHOR]

Controlling parabolic trough solar thermal power plants during transient events, such as passing clouds, is challenging for the power plant operators. Solar field controllers try to operate the field with the suitable fluid mass flow to reduce any waste of solar irradiation under very different weather and operational conditions. However, finding the optimal operation trajectory is not possible or guaranteed due to the lack of information about many influencing parameters in the solar field including, for example, effect of spatial variation of irradiance and flow maldistribution in the field. In this paper, the in-house transient simulation tool, the Virtual Solar Field (VSF) is used to test full-sized solar power plants under realistic weather conditions. Automatic field controllers have been developed with the help of our industrial partners to act as reference for further developments in field control. Thus, the VSF can estimate the potential benefit of advanced controllers using, for instance, nowcasting systems provided by cloud cameras, as well as loop control valves to manipulate the flow control in individual loops. It is shown how the VSF can act as a platform to test different control strategies and provide assessments of the detailed performance of the controller with different control components. The implemented automatic field controllers proved to be robust for different irradiance types and, hence, can be used as basis for further analysis. In addition, the controller can be instructed to dump some of the incident solar energy through defocusing in cases when the available solar energy is higher than the energy consumption by the power block and storage. In summary, VSF allows the developers of solar field controllers to test their implementations under different weather conditions and provide accurate assessment without disturbing power plant operation and before investing in new technologies, for instance nowcasting systems. [ABSTRACT FROM AUTHOR]