Partial Commissioning and Experimental Performance Characterization of a Bench-scale Thermochemical Heat Storage System

Sammanfattning

Thermal Energy Storage (TES) is pivotal in modern energy systems. Among TES techniques, Thermochemical Heat Storage (TCS) stands out for remarkably high energy storage densities and temperature adaptability. This master’s thesis is a continuation of a former collaborative project between Technical University of Denmark (DTU), Institute for Energy Technology (IFE), Norway, and KTH, Sweden, followed by several previous thesis projects. This project concerns a bench-scale TCS system designed for reversible NH3-SrCl2 reaction, i.e., of absorption and desorption. The first ammine (SrCl2·NH3, the monoammine) is required only for system commissioning, while the TES would happen between SrCl2·NH3 and SrCl2·8NH3 (to octaammine). In previous projects, the foundation was laid by simulating a thermochemical storage system, composed of two identical reactors A and B. Following this, the design of these reactors was started and after several iterations finally a configuration was chosen, where SrCl2-expanded natural graphite (ENG) composites were packed into cylindrical blocks and sandwiched between horizontal perforated aluminum fins connected to vertical aluminum tubes carrying heat transfer fluid (HTF). Afterwards, the system was advanced by focusing on system construction and upgrading devices. Subsequently, a data acquisition system was designed and partial NH3 absorption commissioning was conducted in reactor B. Despite these efforts, delays in components and disconnected compressor prevented full commissioning, leaving further analysis to this project. This project initially aimed for the complete commissioning and performance analysis of the system, which means both desorption and absorption path. However, unforeseen challenges emerged, including extensive re-debugging of certain instruments and strict inlet limitations of the compressor. To still maintain the master’s thesis timeline and scientific outcomes, the project’s aim was revised to focus on partial commissioning of the absorption path using reactor A, with a deeper performance evaluation. The TCS system’s two reactors A and B are each equipped with thermostat baths that serve as heat sources or sinks (per operational mode). The discharging process involves the absorption path, where NH3 is absorbed by SrCl2, releasing heat. Conversely, NH3 is released from SrCl2 on the desorption path, storing heat as the charging process. Once each reactor goes through one of the reactions, it is ready for the reverse reaction. The absorption path includes an expansion valve, a tube-in-tube (TinT) heat exchanger (HEX), and a mass flow meter. The desorption path is equipped with a mass flow meter, coolers, pressure-regulating valves, a compressor (with its by-pass path). The two paths share a NH3 storage tank as a common component. After desorption, NH3 is compressed and condensed into liquid state using a cooling system linked to the building's district cooling, with the NH3 storage tank connected back to the absorption path. This integrated setup ensures precise and extensive control over reaction dynamics andenhances energy storage and release efficiency. The selection of KPIs for performance evaluation was informed by extensive literature review. These were then short-listed per capabilities of the existing data acquisition systems. The final chosen KPIs include reaction time (hour), total amount of heat from absorption reaction (kJ), energy storage density per mass (kJ/kg) and volume of composite (kWh/m3), reaction advancement, reaction advancement rate, discharging power (kW) and heat discharging efficiency to the external environment (%). In the experiment, reactor A absorbed NH3 for 69 hours (where the reaction time excluding idling was ~22 hours), including monoammine and octaammine reactions at 95 °C and 65 °C (as thermostat bath A setpoints), respectively. 1587 g of NH3 passed through the mass flow meter, assumed as fully reacted with SrCl2, yielding a SrCl2 density in composite in reactor A of 0.35 g/cm3. This gives a total amount of heat from reaction of 2788 kJ and an energy storage density in reactor of 1190 kJ/kg composite (147 kWh/m3). The total time for monoammine to octaammine reaction was 16 hours despite intentionally employed restricted reaction conditions, with a typical power of 1-2 kW, and a peak power of 5 kW. The efficiency of discharging, which is the ratio of useful heat to the external environment compared to total reaction heat, was about 40%. Other KPIs, such as reaction advancement and its rates are illustrated and discussed as well in the thesis. Comparisons with theoretical expectations indicate potential variations in energy storage density based on composite density assumptions, ranging from 1617 kJ/kg (200 kWh/m3 ) to 1660 kJ/kg (205 kWh/m3 ), with the corresponding released heat of the exothermic reaction found to be 3786 kJ and 3887 kJ. Additionally, the experimental findings are juxtaposed with literature data, demonstrating promising system performance and exothermic functionality. Moving forward, optimization efforts could target: coping with a limitation that exists in the compressor inlet pressure (to 1.1 bar(a)), realization of the desorption reaction, and exploration of effects of more favorable temperatures and pressures on system performance. These endeavors aim to enhance system efficiency and advance this TCS technology in energy storage applications. Besides, the design of a dynamic uncertainty model for the KPIs could be considered, as in-depth uncertainty analysis was omitted for this thesis scope due to the complexity from dynamic nature of the temperature. Overall, the TCS system presents a promising option for TES applications due to its high energy storage density in thermal management. However, challenges related to temperature and pressure management, system complexity, and chemical safety must be addressed. With the suggested improvements, the TCS system has the potential to lean more positively towards practical and sustainable application in TES.