(499b) Experimental Evaluation of a Continuous Oxidation Reactor for High-Temperature Discharge of Thermochemical Energy Storage Particles

Thermochemical Energy Storage (TCES) using metal oxide particles has emerged as a promising technology to facilitate dispatchability of renewable energy. TCES stores thermal energy in the form of chemical bonds in metal oxide particles for short or long-term periods, which can subsequently be released as heat when needed. Apart from providing energy storage, TCES has other advantages, including high energy density, decoupled charging and discharging steps for long-term storage, and the ability to deliver high-temperature heat (~1000 °C). On the one hand, the “charging” step involves exposing the particles to high temperature heat and an inert sweep gas where they undergo a reduction step releasing oxygen. Numerous studies recommend that the input energy should come from sustainable and low-cost sources such as concentrated solar or renewable excess electricity. On the other hand, the “discharging” or oxidation step requires that the particles are maintained at certain operating conditions of temperature and atmosphere that ensure that the exothermic oxidation step proceeds at a high reaction rate, and effective heat transfer to a heat transfer fluid that will be used downstream for applications such as power generation or industrial heat. Such operating conditions are often constrained by the need of maintained a coupled reduction and oxidation steps with intermediate hot storage and low efficiencies due to the intrinsic heat and mass transfer limitations of gas-particle contacting patterns. A proposed reactor concept has been put forward to facilitate the effective extraction of high-temperature heat from TCES particles while maintaining high oxidation reaction rates. The design involves a particle-gas counter-flow reactor with heat exchanger located between two moving beds. The moving beds allow for efficient transfer of sensible heat between the gas and solids, thereby enabling complete preheating and cooling of the gas and particles with minimal sensible heat losses. The construction of this reactor involved the use of high-temperature resistant materials such as superalloy Hastelloy X for the reactor and heat exchangers, top and bottom moving beds. Experiments using N₂ as a heat transfer fluid demonstrate that the heat exchangers can achieve sustained HTF outlet temperatures of ~ 900 °C with particle flowrates of 2.2 g/s and a reactor temperature of ~ 1000 °C. The main performance metrics are thermal efficiency and conversion. In this study, the thermal efficiency is defined as the useful energy extracted by the HTF divided by all the energy inputs at steady state such as heat of reaction, auxiliary external heating, and pumping work. Our experiments so far have obtained thermal efficiencies ranging from 30 to 50% with oxidation conversions of approximately 66%. Future work will involve testing key operating parameters such as HTF flowrate to maximize efficiency and conversion while maintaining adequate thermodynamic conditions for the oxidation reaction.

The presented reactor development for TCES using metal oxide particles represents a potential advancement in the field of energy storage. The implementation of the system in various thermal applications is enhanced by the decoupled charging and discharging steps, integration of different HTFs, and the ability to feed and remove solids and gas at ambient temperature. With further development and refinement, this reactor concept has the potential to advance the way we store and utilize energy from renewable sources.