Solar-Powered Rankine Cycle Assisted by an Innovative Calcium Looping Process as an Energy Storage System

Solar energy is an intermittent resource, and thus an energy storage system is required for practical applications of the collected solar irradiance. This work deals with the integration of a thermo-chemical energy storage (TCES) system based on the calcium looping (CaL) process with a concentrated solar tower power (CSP) plant. The objective of this work is the integration of a conventional 320 MWe Rankine cycle with a direct calcination for energy harvesting. Particularly, this work addresses the use of CO2 as the working fluid of a compressed-gas energy storage (CGES) system for hybrid energy storage with the CaL process. The hybrid TC/CG-ES (thermo-chemical/compressed-gas energy storage) system can increase the competitiveness of the CSP with respect to conventional fossil-based power plants leading to a reduction in CO2 emissions. The thermal integration with the calcium looping (CaL) system is optimized by means of the pinch analysis methodology. The obtained results show a reduction in the electrical efficiency of about four percentage points with respect to the conventional Rankine power cycle without the CSP unit: the net electrical efficiency reduces from 43.7% to 39.5% while the global (thermal and electrical) efficiency of the plant reaches the peak value of 51.5% when low enthalpy energy is recovered (e.g., district heating network, district cooling network). This paper highlights the importance of the thermochemical CaO based material. With a conversion of CaO to CaCO3 of 80% the storage efficiency is defined as the ratio of the energy released during the carbonation and the CO2 expansion to the energy collected by the solar field and required during the CO2 compression, which is 87.3%.