Combined enhancement of thermal and chemical performance of closed thermochemical energy storage system by optimized tree-like heat exchanger structures

The full exploitation of thermochemical energy storage potential is still hampered by several technical limitations, such as the poor heat and mass transfer in the reactive beds. This paper addresses the need of enhancement of heat and mass transfer in solid-gas reactive beds by proposing, investigating and optimizing the use of branched fins made of high conducting material. Two reactor configurations in closed-system operation are examined, and the optimal fin designs are derived by means of surrogate models based on high-fidelity finite elements predictions. The results indicate the use of optimized branched fins to increase the amount of discharged energy up to +9.1% compared to literature benchmarks. However, in the instance of reactive beds where transfer of heat and mass mainly occur along the same predominant direction, fins bifurcations might also hamper the transfer of reactants, leading to diminished thermal performance particularly during the latest stages of the energy discharge process. As a consequence, the optimal number of bifurcations is smaller for reactors that require to operate for longer discharge time. Conversely, such trend is not present in the instance of reactive beds where transfer heat and mass mainly occur along district and mutually orthogonal directions. In this case the typical heat transfer maximization design guidelines can be adopted, indicating a negligible influence of mass transfer on the optimal fins architecture.