April 10-14, 2016
One of the key approaches to developing a supply of renewable energy is Concentrating Solar Power (CSP). However, the diurnal and intermittent nature of the solar resource necessitates efficient energy storage solutions if a reliable energy supply is to be expected. By increasing the temperature and energy density of thermal storage, improvements in power cycle efficiency and energy storage cost are possible and can result in increased economic competitiveness. Thermochemical energy storage (TCES) holds promise for enabling this goal. Metal oxides (MOs), with their elegantly simple reduction/re-oxidation (redox) chemistry, approach an ideal medium for TCES in many respects. For instance, the chemistry is typically highly selective, reversible, and the only gas phase species required for the reaction is oxygen. Also, metal oxides are generally robust, stable at high temperatures, and compatible with advanced falling particle receiver concepts. In fact, MO TCES can be envisioned as an augmentation to particle receiver concepts wherein the reduction enthalpy adds to the sensible energy being stored in the particle.
We seek to systematically develop, characterize, and demonstrate a robust and innovative energy storage cycle based on novel metal oxides with mixed ionic-electronic conductivity (MIEC) that can be directly integrated with Air Brayton power cycles. MIECs differ from more conventional oxides in that they exhibit a continuum of redox states over a large range of thermodynamic conditions (temperature and oxygen potential) rather than a single and discrete transition. Furthermore, the high atomic-scale conductivity of oxygen and electrons within these materials facilitates rapid reaction kinetics and full utilization of the redox capacity. Finally, MIECs are exceptionally tunable in composition, which allows optimization of the thermodynamics and the cost of the material.
In our system concept, particles are reduced in a solar receiver reduction reactor (SR3) and then flow into a hot storage bin. The particles flow out of the hot bin on demand into a re-oxidation reactor (ROx) where they are contacted counter-currently with compressed air flowing from the compression cycle of the Brayton system. The compressed air acts as both oxidant and heat transfer fluid. The heated air exits the ROx and flows to the turbine while the spent particles flow to cold storage pending recycle to the SR3.
MIECS comprised of earth-abundant materials such as calcium and manganese have been developed and characterized. Reaction enthalpies of up to 390 kJ/kg were realized over conditions of interest (Figure 1) and stability was demonstrated for up to 100 cycles.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This work is supported by the U.S. Department of Energy, SunShot Initiative, under Award Number DE-FOA-0000805.
Eric Coker, Sean Babiniec, Andrea Ambrosini, and James Miller, "Metal oxides with ionic-electronic conductivity for thermochemical energy storage" in "CO2 Summit II: Technologies and Opportunities", Holly Krutka, Tri-State Generation & Transmission Association Inc. Frank Zhu, UOP/Honeywell Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/co2_summit2/38