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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]

Operators of a solar-thermal power plant bear a high responsibility as they are in charge of the plant integrity, the safety of staff and optimal performance. At the same time, the complexity of such plants is very high. Thus, skilled operators are needed to understand what they do and, especially, how to react in non-standard plant conditions. Training the operators for these situations is very important for trouble-free operation. Those items become particularly crucial for testing of parabolic trough plants with molten salt as working fluid, as they inherit the danger of HTF freezing at normal ambient conditions. DLR developed a training simulation software as a platform to train the operators in dealing with various scenarios and test automation concepts for plant control for the test facility in the HPS2 project. The simulator software features a detailed model for transient simulation of the solar field based on DLR’s Virtual Solar Field (VSF), a simulator management system and a human-machine interface (HMI). VSF communicates via a standard TCP/IP interface with the simulator management. The plant model features detailed modelling of the hydraulic network and storage system, alarm functions and control logics derived from the functionality of the test plant. The HMI is designed according to the real process control software. Simulations with measured weather data validated the correct behavior of the physical models. To understand the behavior of the plant, the plant model can be completely manually operated. Various weather scenarios can be loaded to show difficulties of transient situations, like a sudden direct normal irradiation (DNI) rise or drop. This enables the trainer to show real operation scenarios of the molten salt plant during the training course without endangering the test facility. [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]

Optimizing solar field operation and control is a key factor to improve the competitiveness of line-focus solar thermal power plants. However, the risks of assessing new and innovative control strategies on operational power plants hinder such optimizations and result in applying more conservative control schemes. In this paper, we describe some applications for a whole solar field transient in-house simulation tool developed at the German Aerospace Centre (DLR), the Virtual Solar Field (VSF). The tool offers a virtual platform to simulate real solar fields while coupling the thermal and hydraulic conditions of the field with high computational efficiency. Using the tool, developers and operator can probe their control strategies and assess the potential benefits while avoiding the high risks and costs. In this paper, we study the benefits gained from controlling the loop valves and of using direct normal irradiance maps and forecasts for the field control. Loop valve control is interesting for many solar field operators since it provides a high degree of flexibility to the control of the solar field through regulating the flow rate in each loop. This improves the reaction to transient condition, such as passing clouds and field start-up in the morning. Nevertheless, due to the large number of loops and the sensitivity of the field control to the valve settings, this process needs to be automated and the effect of changing the setting of each valve on the whole field control needs to be taken into account. We used VSF to implement simple control algorithms to control the loop valves and to study the benefits that could be gained from using active loop valve control during transient conditions. Secondly, we study how using short-term highly spatially-resolved DNI forecasts provided by cloud cameras could improve the plant energy yield. Both cases show an improvement in the plant efficiency and outlet temperature stability. This paves the road for further investigations of new control strategies or for optimizations of the currently implemented ones. [ABSTRACT FROM AUTHOR]

The integration of high temperature thermal energy storages into existing conventional power plants can help to reduce the CO2 emissions of those plants and lead to lower capital expenditures for building energy storage systems, due to the use of synergy effects [1]. One possibility to implement that, is a molten salt storage system with a powerful power-to-heat unit. This paper presents two possible control concepts for the startup of the charging system of such a facility. The procedures are implemented in a detailed dynamic process model. The performance and safety regarding the film temperatures at heat transmitting surfaces are investigated in the process simulations. To improve the accuracy in predicting the film temperatures, CFD simulations of the electrical heater are carried out and the results are merged with the dynamic model. The results show that both investigated control concepts are safe regarding the temperature limits. The gradient controlled startup performed better than the temperature-controlled startup. Nevertheless, there are several uncertainties that need to be investigated further. [ABSTRACT FROM AUTHOR]

Central receiver systems with open volumetric air receiver and packed bed thermal storage are an alternative to the currently deployed salt and steam receiver plants. The air receiver technology is being tested as a complete system at the 1 MWe test and demonstration plant Solar Tower Jülich since 2009 and shows high robustness and availability. Recent developments focused on increasing the thermal receiver efficiency. Changing the receiver geometry to a cavity type and further structural improvements resulted in an efficiency increase to above 85%, as experiments and CFD simulation models show. The high dynamic flexibility could be further enhanced. A commercial reference plant with 50 MWe power level was defined for a potential location in South Africa. An LCOE analysis based on an annual performance simulation was conducted for several design and operational alternatives. Under good conditions, results show LCOE values below 90 €/MWh for the single 50 MWe standard unit. Higher power levels can be reached by multiplying such standard units, leading to further cost savings. [ABSTRACT FROM AUTHOR]

The objective of this paper is the evaluation of trends when a power-to-heat system with electrical heaters is integrated to a molten salt solar tower to additionally charge the thermal energy storage system and to expand the capacity of the CSP plant. Therefore a techno-economic analysis was carried out considering several economical and technical boundary conditions. The results show a significant impact on the availability of excess electricity and its cost. Depending on system layout and the cost of the used electricity the overall LCOE of such plants can be reduced by up to 25%. Therefore such power-to-heat systems can offer an economic and strategic benefit to solar thermal power plants. [ABSTRACT FROM AUTHOR]