Modelling and Control of Transients in Parabolic Trough Power Plants with Single-Phase Heat Transfer Fluids

Optimizing solar field operation and control during transient processes, like passing clouds and solar field start-up, is key to improve the competitiveness of line-focus solar thermal power plants in comparison to other renewable energy technologies. Although Simulation tools play a significant role in the design and optimization procedures, common solar field computational models cannot predict the behaviour of such a system exposed to high degrees of, both, temporal and spatial variability in the energy input. Some very detailed tools can only simulate parts of the system within acceptable time and computational power, and hence, they are not utilized as platforms to test new concepts under realistic operation conditions. On the other hand, tests in pilot or full-sized solar fields not only are too costly to prove a concept, but it is also nearly impossible to reproduce a transient test case with exactly the same disturbances to provide fair comparisons. Thus, a novel transient solar field model, the Virtual Solar Field (VSF), is developed within the scope of this thesis. The model couples a static hydraulic flow model with a detailed dynamic loop model making use of the different time scales in the system. This results in an accurate and computationally efficient simulation tool for commercial power plants scale, such that, for example, 10 hours of Andasol-III solar field operation can be simulated in under 10 minutes. VSF has been validated against solar field operation data from Andasol-III as presented in the thesis. Also new control schemes that use direct normal irradiance (DNI) maps from nowcasting systems, as well as single loop valve control have been implemented and tested in VSF. A controller performance assessment scheme based on energy output and conversion efficiency has been developed to estimate the expected revenues of the solar field within the simulated time. This provides information to quantify the benefits of using one control strategy over another. The availability of the simulation tool also paves the way for developing model-predictive control strategies to optimize the field operation and production

Evaluating the Potential Benefit of Using Nowcasting Systems to Improve the Yield of Parabolic Trough Power Plants with Single-Phase HTF

Solar field developers include innovative solutions to optimize the energy production of their plants. Simulation tools play a significant role in the design and testing phases as they provide estimations of this yield in different conditions. Transient processes, like passing clouds and solar field start-up, are specifically challenging to optimize and estimate using such simulation tools. Solar fields are subject to high degree of both temporal and spatial variability in the energy input and a detailed estimation can be achieved by simulating subsystems within acceptable time and computational power. Hence, such simulation tools cannot be utilized for tests under realistic operation conditions. The Virtual Solar Field is a computationally efficient simulation tool that allows a detailed transient simulation of parabolic trough solar fields based on single-phase fluids. Using this tool, developers could reproduce a transient test case with exactly the same disturbances to provide fair comparisons between different configurations. In this paper, an evaluation process based on numerical simulations using the Virtual Solar Field is presented. The economic benefit of novel innovative control concepts can be assessed according to the presented scheme. This is demonstrated by evaluating the potential benefit of availability of spatial DNI nowcasts on the control of parabolic trough solar fields. Results show that nowcasting can increase the economic revenue of commercial power plants by up to 2.5% per day. This proves the feasibility of installing such systems

Modelling and control of transients in parabolic trough power plants with single-phase heat transfer fluids

Optimizing solar field operation and control during transient processes, like passing clouds and solar field start-up, is key to improve the competitiveness of line-focus solar thermal power plants in comparison to other renewable energy technologies. Although Simulation tools play a significant role in the design and optimization procedures, common solar field computational models cannot predict the behaviour of such a system exposed to high degrees of, both, temporal and spatial variability in the energy input. Some very detailed tools can only simulate parts of the system within acceptable time and computational power, and hence, they are not utilized as platforms to test new concepts under realistic operation conditions. On the other hand, tests in pilot or full-sized solar fields not only are too costly to prove a concept, but it is also nearly impossible to reproduce a transient test case with exactly the same disturbances to provide fair comparisons. Thus, a novel transient solar field model, the Virtual Solar Field (VSF), is developed within the scope of this thesis. The model couples a static hydraulic flow model with a detailed dynamic loop model making use of the different time scales in the system. This results in an accurate and computationally efficient simulation tool for commercial power plants scale, such that, for example, 10 hours of Andasol-III solar field operation can be simulated in under 10 minutes. VSF has been validated against solar field operation data from Andasol-III as presented in the thesis. Also new control schemes that use direct normal irradiance (DNI) maps from nowcasting systems, as well as single loop valve control have been implemented and tested in VSF. A controller performance assessment scheme based on energy output and conversion efficiency has been developed to estimate the expected revenues of the solar field within the simulated time. This provides information to quantify the benefits of using one control strategy over another. The availability of the simulation tool also paves the way for developing model-predictive control strategies to optimize the field operation and production

Modelling and Control of Transients in Parabolic Trough Power Plants with Single-Phase Heat Transfer Fluids

Optimizing solar field operation and control during transient processes, like passing clouds and solar field start-up, is key to improve the competitiveness of line-focus solar thermal power plants in comparison to other renewable energy technologies. Although Simulation tools play a significant role in the design and optimization procedures, common solar field computational models cannot predict the behaviour of such a system exposed to high degrees of, both, temporal and spatial variability in the energy input. Some very detailed tools can only simulate parts of the system within acceptable time and computational power, and hence, they are not utilized as platforms to test new concepts under realistic operation conditions. On the other hand, tests in pilot or full-sized solar fields not only are too costly to prove a concept, but it is also nearly impossible to reproduce a transient test case with exactly the same disturbances to provide fair comparisons. Thus, a novel transient solar field model, the Virtual Solar Field (VSF), is developed within the scope of this thesis. The model couples a static hydraulic flow model with a detailed dynamic loop model making use of the different time scales in the system. This results in an accurate and computationally efficient simulation tool for commercial power plants scale, such that, for example, 10 hours of Andasol-III solar field operation can be simulated in under 10 minutes. VSF has been validated against solar field operation data from Andasol-III as presented in the thesis. Also new control schemes that use direct normal irradiance (DNI) maps from nowcasting systems, as well as single loop valve control have been implemented and tested in VSF. A controller performance assessment scheme based on energy output and conversion efficiency has been developed to estimate the expected revenues of the solar field within the simulated time. This provides information to quantify the benefits of using one control strategy over another. The availability of the simulation tool also paves the way for developing model-predictive control strategies to optimize the field operation and production

Modelling and optimization of transient processes in parabolic trough power plants with single-phase heat transfer medium (Motivation)

The Federal Government in Germany adopted a long term “Energy Concept” with the goal to significantly reduce CO¬2 emissions by 2050. Renewable energy sources are considered the cornerstone to achieve this goal. As stated in [1], locally-produced and imported electricity from renewable energy sources are to account for 80% of the gross electricity consumption by 2050. Using molten salt single-phase heat transfer media in linear concentrating solar thermal power plants represents a very promising opportunity [2]. In addition to the increased temperature, as compared to oil-operated plants, the heated fluid could be directly stored to achieve longer operation periods. On the other hand, shut down and maintenance costs are significantly higher for solar fields with molten salt as the heat transfer fluid. That is due to the risk of fluid solidification or tube corrosion as the fluid temperatures drop or rise, respectively, beyond certain limits. In this PhD project, it is planned to develop a model to simulate whole fields and provide control information to account for changing irradiation conditions. Simulations of transient processes for single loops and subfields have already been sought and computed, for example in [3] and [4]. However, a more flexible, efficient and robust tool is required to better investigate larger fields in a timely manner. The tool could be used to optimize the system response to passing clouds and to improve the efficiency of startup procedures. The authors seek to simulate the field using a more detailed model for the hydraulic network and the mass flow distributions among the loops. The efficient calculation of mass flow distributions and pressure drops represent a challenge for the large systems of varying temperatures and fluid properties. The PhD project runs for the upcoming 3 years and is funded by DLR and DAAD. References: [1] Federal Ministry of Economics and Technology, Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Energy Concept for an Environmentally Sound, Reliable and Affordable Energy Supply, September 2010 [2] Wagner, P.H., Wittmann, M., Influence of different operation strategies on transient solar thermal power plant simulation models with molten salt as heat transfer fluid, Conference paper, SolarPACES 2013 [3] Hirsch, T., Feldhoff, J.F., Schenk, H., Start-up Modeling for Annual CSP Yield Calculations, Journal of Solar Energy Engineering, 2012, 134 [4] Giostri, A., Transient effects in linear concentrating solar thermal power plant, Dissertation, Energy Department, Politecnico Di Milan

Power-to-Heat in CSP Systems for Capacity Expansion

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

Virtual Solar Field - Validation of a detailed transient simulation tool for line focus STE fields with single phase heat transfer fluid

Simulation models enable designers and operators to test different settings of the real systems while avoiding the large costs associated with experimental or practical systems. However, creating models that resemble the real physical system with acceptable accuracy and computational time remains a challenge for developers of such tools. With this motivation, a new simulation tool, the Virtual Solar Field (VSF), has been developed for line-focus power plants with single-phase heat transfer fluid (HTF) to assist in plant control during transient processes. VSF is a whole-field model based on an efficient coupling of hydraulic and thermal solvers that take into account the flow distribution in the parallel loops, as well as the transient conditions in the field. In this paper, some validation cases for VSF using data from Andasol-3 power plant are shown. Five test cases are examined, which include normal operation, start-up and evening operation during clear-sky and strong transients. The results show very good agreement with the real plant and some discrepancies are discussed and studied. The main sources of discrepancies are associated with difficulties in modelling all fine details of reality using the current technologies in some commercial power plants. For example, small cloud passage are not detected by the few weather stations in the power plant, as well as knowing the exact loop valve settings is not possible in Andasol-3. In addition, an overview of the potential applications of VSF are mentioned and briefly discussed. VSF offers a suitable platform for testing novel control strategies and assessing the performance of the solar field