Fraunhofer Institute for Solar Energy Systems ISE
Fraunhofer Institute for Solar Energy Systems ISE

System and process technology

Development and optimization of storage concepts

The storage technology used in solar thermal power plants must be adapted to the respective heat-transfer medium. Commercial plants using thermal oil are generally fitted with a two-tank molten salt storage system. Latent storage units are the best solution for plants using direct steam generation, as their isothermal charging and discharging process is ideal with regard to exergy. In addition to working on new concepts with lower storage costs than the processes developed to date, we focus on sensitive stratified single storage tanks and the use of solid matter in storage systems.

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Innovative thermal power generators and processes

The Polygeneration Laboratory allows various consumers of heat to be characterized. These may include expansion engines, thermal cooling systems, thermal storage systems, sea water desalination plants and connections between these components. The heat is generated by a gas-fired thermal oil heater with a thermal output of 250 kW. A hardware-in-the-loop approach makes it possible for the behavior of fluctuating heat sources, such as concentrating collectors, to be simulated with a gas burner. The components may be characterized according to their efficiency, operating performance and how they interact within the system. Possible heat-transfer media are thermal oil (up to 300 °C), steam (up to 30 bar) and a water-glycol mixture (up to 90 °C).

© Fraunhofer ISE

Optimization of plant management and process control

Customary simulation programs for calculating annual reports on energy yields typically work with hourly time step sizes. However, real-life controllers generally have significantly shorter response times (e.g. PI controllers). Calculations based on these response times are necessary for the controller’s performance to be simulated realistically.

Fraunhofer ISE has developed the ColSim simulation environment to investigate and optimize control concepts in solar thermal plants. ColSim allows the same control algorithms that are used in real-life microcontroller units to be employed and enables the results to be exported thanks to an interface with the control hardware. A dynamic model for concentrating collectors is available, which can be used to simulate the collector’s behavior in the event of fluctuations in solar irradiance (clouds passing overhead). Furthermore, the influence of the collector’s dynamic range (as a result of direct steam generation, for example) on a solar-powered plant process can be investigated.

Using these short-term dynamic models as a basis, it is possible to create control concepts for actual collectors and plants which above all guarantee that the plant is operated safely. Operating strategies that ensure maximum possible yields are achieved, can also be developed.

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