Thermochemical energy storage based on the reversible reaction of metal oxides

Abstract

The application of thermal energy storage (TES) technologies enables dispatchable generation of solar thermal power and is crucial to improve the energy efficiency of industrial processes. Using the reaction enthalpy of reversible gas-solid reactions to store thermal energy represents a promising concept, allowing high energy densities, long term energy storage with minimal losses and a facile separation of the reactants. Specifically for the high-temperature range between 500°C up to 1100°C, multivalent metal oxides constitute promising storage materials for thermal energy storage. Those materials offer process technological advantages compared to other thermochemical storage materials, as ambient air can be used as sink and source of the reactant oxygen during the reduction and oxidation (REDOX) reactions. Accordingly, oxygen does not need to be stored in this open loop process. A lab-scale storage test rig with a packed bed storage reactor was set up to study the reversible reaction of metal oxides for thermochemical energy storage. This paper reports on the thermal reduction (storage charging) and oxidation reaction (storage discharging) of manganese oxide in a packed bed storage reactor. The concept uses air as a heat transfer fluid which is in direct contact with the solid storage material. Reaction progress and conversion of the manganese oxide redox reactions was determined by means of the measured oxygen concentration at the outlet of the reactor and temperature sensors within the bed, as well as gas inlet and outlet temperatures. We discuss in this paper the completeness of the conversion and duration of the reaction. Also, the thermal characteristics of the oxidation and reduction process are compared. The feasibility of the storage concept to store thermal energy in a metal oxide storage reactor using air as the heat transfer fluid and carrier of the reactant oxygen is demonstrated.