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S. Flegkas:
"Modelling, Design and Profitability of Thermochemical Energy Storage Systems";
Supervisor, Reviewer: A. Werner, H. Walter; Institut für Energietechnik und Thermodynamik, 2019; oral examination: 2019-10-23.

This cumulative thesis consists of four papers that were published between 2016 and 2018, which address different aspects of the topic of thermochemical energy storage. The rapid increase of the global energy demand along with the heightened awareness for climate change and the shift towards sustainable energy systems demand the development of novel thermal energy storage concepts. Due to the volatile nature of renewable energy resources, especially wind and solar sources, as well as the discontinuous waste heat flows from industrial processes, the interest in storage methods in order to match intermittent output with consumer demand has increased rapidly. Amongst the existing thermal energy storage mechanisms, thermochemical heat storage shows great promise compared to sensible or latent heat storage. Thermochemical energy systems utilize reversible chemical reactions to store and release heat. During the charging process the heat is absorbed while decomposing the reagent into its products. Discharge occurs when the products are brought to contact again, releasing the stored heat. In the first paper, a method to model a fluidised bed reactor for thermochemical energy storage is proposed, which combines available knowledge of solid-state reaction kinetics and fluidized bed reactor technology. Also a process utilizing such a reactor is designed and its limitations and drawbacks were assessed. Based on these findings a process utilizing a reactor cascade with different storage materials was proposed in the second paper. This study was performed because, due to the isothermal operation of fluidized bed reactors and the thermochemical storage process, the cooling of a heat source beyond the temperature level that is defined by the storage material is not possible. Hence, it is advantageous to use a combination of materials to obtain high heat-recovery effectiveness and minimize exergy losses. Furthermore reactor design questions like the impact of horizontal heat exchanger tube arrangements on the reaction conversion were addressed in the third paper. Finally a profitability analysis and capital cost estimation of a thermochemical energy storagefacility utilizing fluidized bed reactors was performed in the fourth paper. The study estimate is based on the simulations of the previously published work. Transport, operation and maintenance as well as utility costs were estimated in order to evaluate the profitability of the system. A sensitivity analysis was also conducted, in order to identify the key process and economic parameters critical for a positive net present value.

Energy Storage / Modeling / Profitability