Optimization of an improved calcium-looping process for thermochemical energy storage in concentrating solar power plants

Abstract

The calcium-looping (CaL) process comprises an endothermic calcination reaction, where CaO and CO2 are generated from CaCO3, and its reverse exothermic carbonation reaction. CaL is promising for thermochemical energy storage (TCES) in concentrating solar power plants. The CaL-TCES process includes: a calciner where solar energy is transformed into thermochemical energy; a carbonator where the stored energy is released; turbines for electrical power generation; and tanks where the reaction products are stored. In this work, the CaL-TCES process is simulated and optimized using gPROMS Process. The innovation lies in identifying process improvements: make-up and purge streams to reduce CaO deactivation after cycling; a water separation process at the calciner outlet to allow calcination with water vapor as fluidizing gas; and the use of several degrees of freedom for process optimization. The goal is to maximize the thermal-to-electrical efficiency. By optimizing the carbonator and main turbine outlet pressures, the efficiency is improved from 38.1 % in the literature to 39.2 %. When make-up and purge streams are considered, the savings in power supply owing to the purged CaO allow improving the efficiency to 43.0 %. The water separation process reduces the thermal-to-electrical efficiency to 34.7 %, but allows a higher solar-to-thermal efficiency or a smaller calcination reactor.