Simulation and experimental analysis of a thermochemical reactor for high temperature heat storage

Kurzfassung

Thermal energy storage has recently attracted much attention as a promising option to decouple power generation from the availability of renewable energy sources. Especially solar thermal power plants in combination with thermal energy storage systems offer a dispatchable, efficient and cost-competitive energy production system among all the different renewable options. Additionally, thermal energy storage can lower the production costs and consumption of fossil fuels in energy intensive industries. Although thermochemical heat storage is still at an early stage of development, the method offers several advantages compared to sensible or latent heat storage. For example, the endothermal dehydration of calcium hydroxide to calcium oxide combines low material costs and a large gravimetric energy storage density within a favorable temperature range from 400 to 560°C. The feasibility to store heat with this material system has already been demonstrated in a pilot scale reactor at DLR. Nevertheless, to evaluate the benefits of a thermochemical storage system, an efficient integration into the process, especially of the reaction gas supply, still needs to be investigated. Not until then the performance of the overall process can be determined. Within this thesis, a mathematical model of the calcium hydroxide storage reactor has been developed in order to develop efficient process integration concepts. By means of experimentally gained results this model was validated and integrated as a new component in the commercial simulation tool for thermodynamic cycles, ―Ebsilon Professional‖. Consequently, a reference plant was modeled where available excess heat was incorporated to charge the thermochemical storage system. The maximum possible amount of converted storage material within the reference configuration was determined by varying the heat input of the solar field. Based on these results, the control system of the plant was optimized whereby the possible heat input from the solar field was increased up to a factor of two, compared to the reference plant without storage system. With this plant configuration, 1100 tons of Ca(OH)2 could be converted during an eight hours charging cycle. Finally, the discharge process was simulated where the necessary heat to run the power plant was only supplied out of the storage system. The steam supply to drive the hydration reaction of the storage in turn was realized through a low pressure steam extraction from the turbine. Overall, the plant operation could be maintained for an additional 7.7 hours at a part load power of 77% for the applied system configuration.