Studies on Metal Hydride Based Multifunctional Coupled Thermal Battery Systems

Abstract

Thermal energy storage systems (TESS) based on chemisorption/thermochemical reactions have several advantages over conventional thermal storage systems based on sensible and latent heat. The advantages include high energy storage densities, wide operating ranges, portability, and, most importantly, energy storage at ambient temperatures without incurring heat losses. Typical materials for thermochemical TESS include metal hydrides (MHs), salt hydrates, and ammoniated salts. Among thermochemical-based TESS, metal hydrides have received significant attention. In these applications, the decomposition of MH into metal/alloy and hydrogen gas is an endothermic reaction that can be used to store heat, whereas the formation of MH is an exothermic reaction that can be used to recover the stored heat. Two distinct MH reactors are often required, one for thermal storage, referred to as an energy storage bed (ESB), and another for hydrogen storage, referred to as a hydrogen storage bed (HSB). These two reactor beds are dynamically coupled such that the hydrogen released from the ESB at high temperatures can be stored in the HSB during the charging process. For recovering the stored heat from the ESB (discharging process), hydrogen is released from the HSB using ambient heat and supplied to the ESB. The primary objective of this thesis is to investigate various design and operational aspects of coupled MH-TESS. The first important task for the development of a coupled MH-TESS is the selection of thermodynamically compatible pairs of MHs. The MH for ESB is selected based on charging temperature and operating pressure. The corresponding MH for HSB is selected based on thermodynamic compatibility criteria. The criteria for the selection of a suitable pair of MH dictate that for the cyclic operation of a coupled MH-TESS, the pressure of the gas inside the ESB corresponding to its temperature should be higher than the pressure of the gas inside the HSB corresponding to its temperature during charging and vice versa during discharging. Thermodynamic compatibility is tested using the van’t Hoff equation within specified temperatures (charging temperature, discharging temperature, and ambient temperature). The minimum charging temperature and the maximum discharging temperature for a given pair of MHs at the specified ambient temperature are determined using the compatibility criteria. Following that, the mass and heat transfer characteristics of coupled MH-TESS through simulations are analyzed. MH-TESS can be operated in long-term and buffer modes, depending on the requirements. In long-term mode, the reactors are cooled down to room temperature, and heat is recovered from the ESB whenever required. Whereas, in buffer mode, the heat recovery process occurs without much time gap for its subsequent application, usually to cater for load fluctuations. The performance of a coupled MH-TESS in long-term and buffer modes is investigated using a pair of Mg2Ni-LaNi5 hydrides for high-temperature applications, where the former MH is used in ESB while the later MH is used in HSB. In addition to hydrogen storage and release, the low-temperature MH can also simultaneously produce a heating or cooling effect depending on charging or discharging. This multifunctionality of a coupled MH-TESS is also investigated using a medium-temperature MH (LaNi4.25Al0.75) for ESB, coupled with a low-temperature MH (LaNi5) used for HSB. Based on theoretical and computational studies, the development of MH storage devices, the development of a hydrogen loop, and experimental investigations on individual and coupled reactors are carried out. As the effective thermal conductivity of hydride alloys is very low, typically in the order of 1 W.m-1.K-1, the design of the hydride beds becomes crucial for the performance of MH-TESS. A novel cartridge-type reactor is developed that offers a thin annular MH bed, a large surface area, and overall compactness. A test rig consisting of a heat transfer fluid (HTF) loop, a hydrogen gas loop, and a test section for mounting the cartridge reactors is fabricated. A silicone-based oil (Julabo-Thermal H20S) is used as HTF. The initial equilibrium state of coupled cartridges is crucial because, at this point, the ESB must be nearly saturated with hydrogen, whereas the HSB must be nearly free of hydrogen. A systematic method to arrive at the optimum initial equilibrium condition of coupled hydride beds is provided. After testing the performance of coupled MH-TESS, parametric studies are performed on the coupled reactors to study the sensitivity of the alloys to various operating conditions. Experimental investigations on the coupled reactors reveal that the charging temperature and the ambient temperature (where the low-temperature alloy in HSB is intended to operate) significantly affect the performance of the system. Also, it is found that the flow rate of HTF controls the heat delivery temperature. It is observed that the coupled MH-TESS with LaNi4.25Al0.75-La0.75Ce0.25Ni5 pair, delivers 227.7 kJ of heat at an average heat transfer rate of 130.7 W at the heat transfer fluid inlet temperature of 25℃, while also producing a cooling effect of 171.8 kJ with an average cooling rate of 103.5 W. Based on the above performance of a single module of a coupled MH-TESS, the system can be scaled-up for any practical application by arranging a required number of such cartridge-shaped reactors in a ‘shell-and-tube’ configuration.