A Numerical Calculation Tool Design for the Performance Assessment of a Bench-Scale Thermochemical Heat Storage System

Sammanfattning: Thermochemical heat storage (TCS) is a technology to convert the heat and cold energy into chemical energy, via reversible chemical reactions, to be stored for heating and cooling applications. TCS technology is gaining interest for its very compact energy storage densities offering attractive thermal energy storage (TES) alternatives to decrease energy-related greenhouse gas(GHG) emissions and contribute to sustainable development.  This thesis is part of the umbrella project “Neutrons for Heat Storage (NHS)”, funded by Nordforsk. The objective of the NHS project at KTH is to design, construct and operate a bench-scale TCS system using strontium chloride (SrCl2)-ammonia (NH3) as the solid-gas reaction pair for low-temperature heat storage applications (40-80 ℃). This system has been thus numerically designed, adapted to practical considerations, and is now being built at Energy Technology, KTH.  Within this background, this thesis, as its primary objective, designs a calculation tool for evaluating the experimental performance of the above described bench-scale TCS system. A thorough explanation of the thesis methodology is presented here, including preparing the composites (SrCl2 impregnated into expanded natural graphite), the system’s risk analysis, and the critical focus is on the systems’ performance evaluation parameters, and the mathematical design of the calculation tool. A review of relevant literature is also conducted to identify the most pertinent performance evaluation parameters of this TCS system. For the consideration of user-friendliness, simplicity, and effectiveness, the calculation tool is designed using Ms. Excel. Here, energy efficiency, reaction advancement, reaction advancement rate, the real thermal energy density per mass, and actual thermal energy density per volume are chosen as the parameters to best-represent the system’s performance (i.e., Key Performance Indicators (KPIs)), calculated based on mass balance and energy balance expressions, primarily. Using this calculation tool, concerning this experimental bench-scale system, the user can visualize the obtained experimental data, calculate the defined KPIs of the system, and seek the potential to improve the current system.  A group of test data is assumed (consulting the reaction equilibrium curve, thus ensuring that they fall within realistic experimental conditions) to check the calculation tool's accuracy and function. For the lack of experimental data, the results of the test data are not ideal. However, thanks to these assumed test data, it is proven that the calculation tool functions correctly. The calculation process can be finished in several minutes, saving a lot of time otherwise required for the data analysis after the experiments. It also functions as the test model to analyze the experimental data.  In conclusion, this project designed and presents a functioning calculation tool to evaluate the experimental performance of a bench-scale experimental TCS system (being built and commissioned at KTH) for the reaction between SrCl2 and NH3. Some suggestions related to future improvements are proposed as well. For instance, the calculation tool is not automatic enough because it involves manual operation at specific points. Therefore, one of the future tasks is to add the ability to identify the reaction pressure vs temperature curve against the equilibrium conditions and defining whether the process is absorption or desorption automatically. Besides, currently, much electrical equipment is employed in the system, which decreases the system's sustainability, whereas, in future work, the layout of the system can be improved. The system's exergy performance is not analyzed in the thesis report, which can be chosen as another future task.