The cross-sectional subject process optimization includes the analysis, estimation and optimization of technical processes in solar thermal power plants and process heat systems. Regarding the applied methods and tools, there are differences in the consideration of steady-state and transient processes. In both cases, the objective is to adjust the parameters of the thermodynamic processes in order to result in an improved cost-benefit ratio.
Optimization of steady-state operation
For steady-state simulations, the efficiencies of single conversion steps are analyzed and compared with the corresponding costs on the component side. Besides the nominal load point (blue sky and good solar irradiation), also all partial load points (reduced solar irradiation) are to be considered. The stationary modelling is done using the commercial cycle simulation tool EBSILON®Professional, which has been expanded by a library for solar heat components with support from DLR.
Optimization of transient processes
Due to the varying intensity of the solar irradiation, the operation point for solar thermal power plants and process heat systems is changing continuously.
Besides the stationary operation points, also the transition processes must be considered in order to optimize the annual profits. This mainly includes the startup and shut down of the plant, as well as the controlling of the solar components for constant outlet conditions.
The large expansion of the pipe system in a solar field of parabolic troughs or linear Fresnel collectors during startup leads to a non-homogenous heating of the single collector rows. The link between irradiation intensity, fluid mass flow in the solar field, collector focus and heat delivered to the consumers is very complex and therefore, detailed simulation tools are required for the analysis. These tools shall reflect the inertia of the heat transfer medium and the pipe walls, as well as the throughput time of the heat transfer fluid in the collector loops. Depending on the technology, also the connection to the storage and the power plant process is to be considered in the analysis of the solar field startup process.
For the modelling of collector loops, in-house component libraries in the programming language Modelica are available at the institute. They can be connected to components from other libraries in order to create a technical model. During the simulation, the process is controlled and regulated by control algorithms which have been directly implemented in Modelica. Alternatively, external tools such as Matlab/Simulink or LabView can be connected over an interface for control purposes.
Moreover, a detailed program for transient simulations of large solar fields has been developed at the institute. This creates new opportunities for the investigation of locally varying effects on solar fields. As an example, the motion of the clouds over the solar field can be simulated in detail, and the required control reactions can be studied.
The application of model predictive methods for the solar field controlling is studied at the institute. With precise local irradiation forecasts for the solar field, so called DNI maps (DNI = Direct Normal Irradiation), as well as detailed solar field models as a base, the optimal control reaction for the short-term variations in the local irradiation can be determined. The primary objective of this development work is the stabilization of plant operation for all possible boundary conditions. The fundamental methodology is applicable for all parabolic trough technologies and only appropriate adjustments are required for oil troughs, salt troughs or direct steam systems.
Optimization of the operation schedule
A further field of application for the process optimization is the planning of electricity generation. With an increasing percentage of renewable energy in the grid, the importance of flexible electricity generation is increasing. The available optimization methods at the institute are used for determination of the optimal operation time of a solar thermal power plant, in which forecasts for electricity price as well as for irradiation are taken into account.
In the following projects, advantages in the operation of solar power plants could be elaborated with the previously explained methods: