An energy storage system based on combined liquefaction and adsorption of carbon dioxide

Abstract

The paper describes a form of thermo-mechanical energy storage in which CO2 is liquefied during charge and adsorbed into zeolite at near-ambient conditions during discharge. Unlike some liquid-air and CO2-based systems, the exergy flows for this system are “in phase” such that potential to produce electrical work is simultaneously stored in both the adsorbent and the liquid CO2 tanks. The focus of the paper is a thermodynamic analysis to determine limits of efficiency and exergy density, but simple estimates of energy-capacity capital cost are also included in order to undertake realistic optimisation. It is shown that 13X zeolite is a near-optimal adsorbent for this application in terms of its desorption enthalpy. Nonetheless, external heat input is required during charge to attain a satisfactory round-trip efficiency. On this basis, a “generation-integrated” system is proposed, operating in conjunction with a thermal power plant. The effective round-trip efficiency for this is calculated to be in the range 50 to 58% (depending on assumed values for component parameters, and excluding electrical losses), and the estimated energy-specific cost is around 50 $/kWh. Although these figures may not be competitive relative to claims for some other thermo-mechanical storage methods, the analysis points to strategies for improvements and provides guidance for the development of similar systems.