Thermochemical Energy Storage Employing Fluidized Bed Technology: Experimental Investigations with CaO/Ca(OH)2 on a 21kWh Reactor

Thermochemical energy storage (TCES) represents one of the most promising energy storage technologies, currently investigated. It uses the heat of reaction of reversible reaction systems and stands out due to the high energy density of its storage materials combined with the possibility of long-term storage with little to no heat losses.

Amongst possible reversible reaction systems, gas-solid reactions, especially the reaction system CaO/Ca(OH)₂, have come to the fore.Until now, the focus for the technical implementation of TCES has been pointed towards fixed bed reactors. However, due to the limitations of fixed bed reactors, the application of fluidized bed reactors for TCES has gained increased attention. Fluidized bed reactors do not only enable the favorable option of operating the system continuously, which allows for a decoupling of storage power and capacity. They also offer significant advantages for the scale-up of the system due to the improved heat and mass transfer. The heat transfer between immersed heat exchangers and the fluidized bed is of particular interest for the herein investigated reaction system CaO/Ca(OH)_2, as preliminary experiments have identified it to be the main aspect limiting the hydration/dehydration reaction. In addition to the importance of the heat transfer characteristics, a deeper insight into the fluidization behavior of the storage material itself is of major importance. Even though gas-solid fluidized beds are nowadays implemented in a wide range of industrial operations, the fluidization of cohesive materials, such as the aforementioned metal oxides/hydroxides, still represents a sparsely investigated field. The consequent lack of knowledge of physical, chemical and technical parameters of the processes on hand is currently a hindering aspect for a proper design and scale-up of fluidized bed reactors for MW-applications of TCES.

Preliminary experimental work at TUM on a lab scale fluidized bed reactor has been carried out to gain first insights into the topic of chemical cycle stability of storage materials. In addition, the fluidization behavior of the bulk material has been investigated in a fluidized bed cold model, containing a heat flux probe and operating at ambient conditions.

In order to provide in depth information on the operation of a fluidized bed reactor for TCES at industrial operation conditions, a state-of-the-art pilot fluidized bed reactor has been set up at TUM’s research facilities. While being able to be operated continuously, the initial experiments presented herein focus on batch operation at conditions of up to 700°C and -1 to 6 bar_g. In this mode, the system itself possesses a thermal storage capacity of up to 21 kWh. The storage material used for the first experiments holds a mean particle diameter of about 350 µm. Solid samples, for further characterization of the process, are taken throughout operation with an online sampling system. This characterization is focused on changes in particle size distribution due to effects such as attrition, breakage and agglomeration, on conversion rates, and on heat transfer coefficients. The initial batch experiments focus on low cycle numbers, where one cycle is defined as the execution of one hydration and one dehydration step of the material. The results, obtained from these experiments, help to prepare for multiple cycle studies and give first insight for a successful scale-up to MW-scale.