Joule-Heated Perovskite Metal Oxides for Thermal Energy Storage

Kurzfassung

High-temperature thermochemical cycles based on redox metal oxides offer great potential to utilize heat from renewable sources such as concentrated solar irradiation or produced from excess renewable electricity and industrial waste heat. All these have in common that the metal oxides are heated by an external heat source. In this work an innovative approach to power-to-heat conversion by joule heated perovskite metal oxides is presented. While typical thermal storage units required a heat transfer fluid such as air to be charged and discharged, joule-heated storage units neither require a heat transfer fluid nor an external source of heat to be stored in the unit. Instead, all it required is electric power and current flowing through the storage units. This concept can not only serve as a means to overcome the inherently fluctuating power supply by PV and wind, but also as solution to electrification of large scale heat supply to industry with high-temperature heat demand. Perovskite metal oxides of the general formula ABO3 are a versatile material class, which is considered for various applications due to its tunability and flexibility. Substituting elemental fractions on either A- or B-site can not only govern thermodynamic and kinetic properties, but also electrochemical properties and can improve structural and mechanical stability. Perovskite metal oxides are capable of fully reversible cyclic reduction and oxidation during thermal cycling at high temperatures, which they undergo non-stoichiometrically without any major phase transition. The large variety of possible composition allows precise fine-tuning of desired properties in multi-cation perovskites, including the electric conductivity. This allows perovskite metal oxides to be directly heated by electric current flow through the material, so called joule heating. The presented work investigates Mn- and Fe-rich perovskite compositions as functional materials for such a combined power-to-heat and thermal energy storage approach. From Density Functional Theory (DFT) calculations, a sophisticated doping strategy is derived and chosen material compositions are synthesized and characterized for validation. Electric conductivity, thermodynamic redox activity, heat capacities as well as reaction kinetics are determined experimentally in the desired temperature range of RT – 1100°C and are used to validate the previous theoretical calculations and evaluate the performance of novel material compositions for the described power-to-heat approach. Finally, based on this works results the potential of this novel concept is evaluated and outlook on follow-up work is drawn.