One of the pillars of the efforts to reduce carbon footprint in energy production is increasing the share of renewables. Among them, solar energy, being virtually endless and fairly well-distributed, is an attractive resource that can be converted into electricity in both Photovoltaic (PV) and Concentrating Solar Power (CSP) plants. However, such technologies are limited by the intrinsic day/night alternance, and therefore some system for energy accumulation is needed to fully exploit solar energy. In principle, storing heat is more cost effective than using batteries, providing a competitive edge for CSP technologies. However, future CSP plants will operate a higher temperature and they will require efficient and inexpensive new media for storing the energy produced. In this regards, ThermoChemical Storage (TCS) systems can directly store solar heat produced by the CSP plant and reuse it on demand during off-sun operation mode. TCS system has higher energy density and longer-term storage duration respect to other thermal energy storage technologies.
TCS systems is based on the exploitation of the reaction heat (ΔH) interchanged in a reversible chemical reaction. Different kind of substances can be used in a TCS system, but in the last years a growing interest focused on metal oxide reduction/oxidation (redox) pairs due to their reversibility, higher temperature of operation and the use of air as reagent. However, a TCS material that satisfies simultaneously the following ideal requirements has still to be identified:
1. high energy density
2. complete reversibility for many charging/discharging cycles
3. fast kinetics to facilitate the energy charging and discharging
4. easily separated and stably stored reaction products
5. non-toxic, non-corrosive, and safe reactants and products
6. large-scale availabilities and inexpensive
The temperature storage range in which a TCS system coupled with a CSP plant will operate depends on the technology used. For tower technology temperatures >700 °C are achieved. However, operating temperature exceeding 1000 C are required for increasing the efficiency of future plants and for coupling TS with Air Bryton turbines.
Recently, perovskite oxides have drawn interest as potential candidates for TCS systems. Perovskites are solids with the crystal structure of CaTiO3 and general formula ABO3, where A and B are the two cations of the structure. They exhibit a continuous oxygen release/uptake within a very wide temperature range, through the creation/destruction of oxygen vacancies in the crystal lattice. The working principle of a TCS system based on perovskite consists in the following reaction:
ABO3 (s) ↔ ABO3-δ (s) + δ/2 O2 (g) (1)
The reduction is the heat storage step, while oxidation releases heat when it is required. The amount of reversibly exchangeable oxygen, δ, is a function of temperature and oxygen partial pressure. Chemical composition of perovskites can be conveniently modified to tune their behavior, allowing to the extent of reduction (δ) varying broadly. Many of the perovskite types until now studied contain rare earth elements, which makes them costly for large-scale applications. The OVERALL OBJECTIVE of SESPER was to study perovskites with more earth abundant elements (i.e. Ca, Fe, Mn-based) for identifying the most promising candidate storage medium. Additional specific objectives are scale-up the synthesis and testing the perovskites under realistic conditions.
Investigation carried out in SESPER has allowed to determine that, under technologically relevant conditions, the material CaFe0.1Mn0.9O3- presents ideal conditions for storing heat at future CSP plants. Specifically operating over a temperature window of 400 to 1200 C and under an oxygen partial pressure of 8 10-3 atm, CaFe0.1Mn0.9O3- oxide offers a total heat storage capacity of ~ 984 kJ/kgABO3, of which ~ 344 kJ/kgABO3 is due chemical heat storage. These attractive characteristics are complemented with good stability over multiple heat charge/discharge cycles. These properties warranted further evaluation in large scale reactor systems. The material showed remarkable performances being able to reach its full heat storage capacity, under technologically relevant conditions, even at a larger scale.
