(156d) sCO2 Power Cycles with Integrated Thermochemical Energy Storage Using an MgO-Based sCO2 Sorbent in Direct Contact with Working Fluid for Grid Energy Storage Applications

With the increasing deployment of solar photovoltaic (PV) power, advanced energy storage technology is needed to balance the electrical power supply and demand curves, especially during evening hours where PV power output is dropping rapidly, but demand is increasing. This work describes a novel thermochemical energy storage (TCES) technology, and its integration with state-of-the-art supercritical carbon dioxide (sCO₂) power cycles for concentrating solar power (CSP), waste heat, and grid energy storage applications.

Current grid-scale energy storage mainly takes the form of fluid reservoir storage (pumped hydropower or compressed air, both of which are geographically limited), and thermal energy storage. For the first time an MgO-based CO₂ sorbent is considered for TCES applications, utilizing the reversible oxide-carbonate reaction to store and release thermal energy:

MgO + CO₂ = MgCO₃ + 108 kJ/mol (at 650°C) (1)

A key advantage of thermochemical energy storage systems is the small temperature range (ΔT) over which the charge and discharge cycle occurs, typically around 25-50°C. This small temperature range allows for high exergetic round-trip efficiency, and has the potential to couple well with power cycles that approximate the Carnot ideal of heat transfer at constant temperature. For high temperature CSP applications solid MgO and MgCO₃ reactants are contained in a packed bed reactor within a single pressure vessel. In the discharged state, the reactor is filled with MgCO₃ at a reduced temperature (650°C). To charge the reactor, CO₂ is heated in the CSP receiver to 720°C, and a portion passes through the reactor, which decomposes the MgCO₃ to MgO and excess CO₂ or “reactant CO₂”. During energy discharge, the CSP receiver is bypassed, allowing lower temperature (470-610°C) sCO₂ to enter the reactor. A portion of this CO₂ reacts with the MgO, and releases heat, which increases the temperature of the remainder of the CO₂, which then is expanded through the power turbine in the sCO₂ power cycle. The reactant CO₂ is stored as a relatively dense fluid near its critical point when the system is in the charged. A techno economic model is used to determine system configurations with the potential to deliver highly-efficient, low-cost energy storage on a utility scale. Preliminary results of capacity and durability of novel MgO-based sorbent are reported.