Review and Design Adaptations of a SrCl2-NH3 bench-scale Thermochemical Heat Storage system

Sammanfattning: Thermochemical heat storage (TCS) is a thermal energy storage (TES) technology used to store thermal energy for later use. TCS can provide heating or cooling services from intermittently available thermal energy, often low grade waste heat. The system studied here stores and releases the energy in the form of chemical energy by utilizing reversible chemical reactions. TCS has potential to reduce greenhouse gas emissions, increase infrastructure system efficiency, lower society-wide energy system costs and by that contribute to sustainable development. This thesis is part of a joint TCS research project named Neutrons for Heat Storage (NHS), involving three research institutes. The project is funded by Nordforsk and KTH Royal Institute of Technology. KTH´s objective in the NHS project is to design, build and operate a bench-scale TCS system using strontium chloride (SrCl2) and ammonia (NH3) as a solid-gas reaction system for low-temperature heat storage (40-80 ℃). Here, absorption of NH3 into SrCl2⋅NH3 (monoammine) to form SrCl2⋅8NH3 (octaammine) is used for heat release, and desorption (of NH3 from SrCl2⋅8NH3 to form SrCl2⋅NH3) for heat storage. Prior to this thesis project, this TCS system, as well as its reactor+heat exchanger (R-HEX) units, were numerically designed at KTH, and the R-HEX units were manufactured. This system is now being built at the laboratory of Applied Thermodynamics and Refrigeration division at the Department of Energy Technology, KTH. The initial system is comprised of a shared storage tank, expansion valve, ammonia meter and an R-HEX (absorption path); and an R-HEX, ammonia meter, gas cooler, compressor, condenser, and the storage tank (desorption path), to accommodate absorption, desorption, and NH3 storage. This thesis was originally planned to include commissioning, operation and experimental data acquisition, and performance evaluation of this system. However, due to various delays and shortcomings discovered at the beginning of the project, its objectives were then redefined to review the system and its components and propose necessary design adaptations of the initially designed (and partially built) system. This thesis project was partly a joint project, where Harish Seetharaman performed various tasks in the overarching NHS project as part of his own thesis project, performed alongside the work described in this report. For various information and results, referring to Harish´s report therefore will be necessary. A literature review of the research into SrCl2-NH3 systems was conducted, with emphasis on performance evaluation, kinetics, and reaction paths. TES performance evaluation is discussed concerning the TCS key performance indicators, with the 2018 IEA's Annex 30 as a guideline and 2013 IRENA´s E17 technology brief as a comparative reference. Much progress and refinement has been made in the 5-year span between the publications of these documents, but some adaptations and interpretations still need to be made to the Annex 30 approach for a good approach to a TCS system of similar nature as the one studied in this report. Review of the latest research on the kinetics and reaction path of the SrCl2-NH3 reaction pair revealed that the 100-year-old single-line-and-path reaction expression is an oversimplification of the actual chemistry. The reaction path seems to be dependent on the kinetics of the reaction, and varies with heating rate, temperature, and pressure. Various literature was found and compared, which show that the reaction enthalpies and entropies are not settled science. This demonstrates the necessity for further research into the SrCl2-NH3 reaction pair before application-scale product design and commercialization can take place. A comprehensive equipment and system review was conducted, whereby multiple issues were found and addressed, that if gone unnoticed, would have caused difficult setbacks for the project.  Consequently, the previous purchased ammonia flow meters and ammonia compressor, were exchanged for new and better suiting equipment. The original ammonia flow meters were undersized due to miscalculations of converting flow units of NLPH (Normal Liters Per Hour) to the project units of g/s, while wrongly using the density of compressed ammonia to convert to g/s, instead of it at the defined normal conditions. Furthermore, these flow meters were of the wrong type, as they had no digital output for data acquisition. The original compressor was also severely undersized, only capable of evacuating 7-14% of the expected maximum desorption flow. This was due to a similar miscalculation during conversion of NLMP (Normal Liters Per Minute) to g/s, as well as an unrequested compressor stroke reduction. New solutions and additional equipment were then required to accommodate the operational limitations discovered in the final chosen equipment and system configuration. These include limiting the compressor inlet pressure to a maximum of 1.1 bar(a); avoiding risk of NH3 condensation at them inlets of the new mass flow meters and compressor; and maintaining the flow meter and compressor inlet temperatures below 40 °C. The pressure limitations required considerable design adaptations. Firstly, an ammonia by-pass is introduced to keep feeding ammonia into the compressor during low desorption flows. The inlet pressure limitation necessitated active pressure management in the form of pressure reduction valves, which were thus introduced. Secondly, the condensation regulation and temperature management required a new approach, as the cooling and condensation temperatures in the original design were too low, causing risks of far too low temperature and pressure in the desorption path, as well as counter-acting simultaneous heating and cooling between the condenser and the storage tank heating sleeve. As a solution, a shunt pump is proposed, where constant cooling water temperature provides condensation on a tight temperature range using an infinite cold wall approach. Along with reviewing the equipment and the system design, new procedures concerning investigating and confirming homogeneous heat transfer properties of the reactors are proposed. Furthermore, improvements are suggested concerning the commissioning of the experimental rig, that include equipment testing with N2-gas and stepwise changes in temperature in sequential cycles to gain a good understanding of the likely behaviors of the system before it is run at the extremes of the operating range. In conclusion, a new and improved process flow diagram, showing all these adaptations, additions, and changes from the original diagram is presented herein as the final key contribution to the overarching NHS-project. This is complemented with an instruction manual, to allow the next researchers a smooth continuation, in terms of the system build, and later commissioning and operation. Finally, some suitable next steps in the project are suggested. These include a conceptualization of descriptive functions for the performance and behavior of the specific system and reactors. These functions are proposed with temperature and pressure as independent variables, as these are two main variables influencing the kinetics of the reaction in the given system. As no experimental data exists yet, the form of the proposed functions is generic. Furthermore, a suggestion is made for a future adaptation for achieving the phase change from NH3(g) to NH3(l) (which is the storage form of ammonia in the system) by deep cooling at the desorption pressure, resulting in only a liquid pump being required to raise the pressure of the NH3(l) in the storage tank.