Numerical Investigations of a Counter-Current Moving Bed Reactor for Thermochemical Energy Storage at High Temperatures

High temperature storage is a key factor for compensating the fluctuating energy supply of solar thermal power plants, and thus enables renewable base load power. In thermochemical energy storage, the thermal energy is stored as the reaction enthalpy of a chemically reversible gas-solid reaction. Metal oxides are suitable candidates for thermochemical energy storage for solar thermal power plants, due to their high reaction temperatures and use of oxygen as a gaseous reaction partner. However, it is crucial to extract both sensible and thermochemical energy at these elevated temperatures to boost the overall system efficiency. Therefore, this study focuses on the combined extraction of thermochemical and sensible energy from a metal oxide and its effects on thermal power and energy density during discharging. A counter-current moving bed, based on manganese-iron-oxide, was investigated with a transient, one-dimensional model using the finite element method. A nearly isothermal temperature distribution along the bed height was formed, as long as the gas flow did not exceed a tipping point. A maximal energy density of 933 kJ/kg was achieved, when (Mn,Fe)3O4 was oxidized and cooled from 1050 °C to 300 °C. However, reaction kinetics can limit the thermal power and energy density. To avoid this drawback, a moving bed reactor based on the investigated manganese-iron oxide should combine direct and indirect heat transfer to overcome kinetic limitations

A Moving Bed Reactor for Thermochemical Energy Storage Based on Metal Oxides

High temperature thermal energy storage enables concentrated solar power plants to provide base load. Thermochemical energy storage is based on reversible gas-solid reactions and brings along the advantage of potential loss-free energy storage in the form of separated reaction products and possible high energy densities. The redox reaction of metal oxides is able to store thermal energy at elevated temperatures with air providing the gaseous reaction partner. However, due to the high temperature level, it is crucial to extract both the inherent sensible and thermochemical energies of the metal oxide particles for enhanced system efficiency. So far, experimental research in the field of thermochemical energy storage focused mainly on solar receivers for continuously charging metal oxides. A continuously operated system of energy storage and solar tower decouples the storage capacity from generated power with metal-oxide particles applied as heat transfer medium and energy storage material. Hence, a heat exchanger based on a countercurrent moving bed concept was developed in a kW-scale. The reactor addresses the combined utilization of the reaction enthalpy of the oxidation and the extraction of thermal energy of a manganese-iron-oxide particle flow. A stationary temperature profile of the bulk was achieved with two distinct temperature sections. The oxidation induced a nearly isothermal section with an overall stable off-gas temperature. The oxidation and heat extraction from the manganese-iron oxide resulted in a total energy density of 569 kJ/kg with a thermochemical share of 21.1 %

Experimental Investigation Of Continuous Heat Extra Of Metal Oxides In A Moving Bed Reactor

Concentrated solar power (CSP) plants play an important part in climate-friendly power and heat generation. The volatile production during the day and the absence of solar radiation at e.g. high demand evenings can be met by combining CSP with thermal energy storage. Thermochemical storage materials, e.g. metal oxides, show high storage densities at temperatures suitable for CSP. Metal oxide granules play the central role in the investigated concept, where they act as heat transfer medium and thermal energy storage material. A 3 kW moving bed reactor for continuous heat extraction of metal oxide granules was constructed and put into operation. Inert operation proofed the general functionality of the reactor. The variation of inert particle mass flow indicated a direct influence on particle temperature during moving bed operation. The redox reaction of metal oxide was successfully observed with metal oxide as moving bed material

Techno-Economic Analysis of Thermochemical Storage for CSP Systems

The use of thermochemical materials, like redox oxides , for hybrid sensible/thermochemical storage in CSP plants can potentially reduce the LCOE and make such plants more competitive. For the techno-economic system analysis, three candidate redox materials were analyzed for their cost reduction potential: cobalt-based, manganese-based and perovskite-based oxide materials. As reference process the use of inert commercial bauxite particles (sensible-only storage) was considered. A CSP plant with a nominal power of 125 MWe and a storage capacity of 12h was assumed for the analysis. Cost factors influenced by the material selection are storage cost, steam generator cost and particle transport system cost. Based on total system cost and annual electricity generation, the LCOE was calculated. The results of the analysis show that some redox materials can significantly reduce the required storage mass and volume, while others lead only to a marginal improvement. More important is the specific cost of the redox material. Expensive cobalt-based materials result in significantly higher LCOE, while perovskite materials show potential for reduced LCOE when these particles can be manufactured at low cost. Therefore it is recommended to focus future work on this material clas