(698f) Process Design of Gwh Level Renewable Energy Storage and Supply Using Liquid CO2

The push towards electricity generation from renewable energy sources like solar energy is motivated by the need for reducing the use of finite-fossil fuels and minimizing CO₂ emissions. However, a grand challenge for the large-scale deployment of grid-based renewable electricity is the intermittent nature of the energy source, which warrants the need for energy efficient storage systems. Here, we outline a novel, energy-efficient solution for large-scale renewable energy storage involving a closed loop cycle that transforms carbon atoms between liquid carbon dioxide and carbon-based fuels like liquid methane and methanol.

Carbon-based fuels offer an attractive storage option owing to their high volumetric energy density and the well-established technology and infrastructure available for their utilization. Yet, the use of carbon-based fuels for energy storage is limited by the availability of renewable carbon and hydrogen sources. Hence, it is worth considering a closed system where little additional carbon or water sources are needed.

Our proposed storage cycle operates in energy storage and energy recovery mode, depending on whether the renewable energy is available or not. During energy storage mode of the cycle, renewable energy is stored as a carbon-based fuel, synthesized from stored water and liquid CO₂ using any of the well-known thermochemical CO₂ to fuel routes, and later liquefied to be stored onsite. Any energy input necessary for fuel liquefaction can be partially or fully met by the refrigeration available from vaporizing liquid CO₂. In the energy recovery mode of the cycle, the stored liquid fuel is vaporized and oxidized to generate electricity via systems like high temperature Solid Oxide Fuel Cells (SOFC). The oxidation byproducts, CO₂ and water, are separated and liquefied for storage. Here, the energy penalty of CO₂ liquefaction can be met partially or fully using the available refrigeration from vaporizing the liquid fuel.

We present two preferred examples of the proposed cycles designed for producing >100 MW electrical power round the clock, using methane and methanol respectively as the energy storage media. The methane cycle relies on a reversible SOFC, CO₂ methanation and synergistic liquefaction schemes involving: 1) liquid CO₂ vaporization and vapor methane liquefaction and 2) vapor CO₂ liquefaction and liquid methane vaporization. The methane cycle electrical energy storage efficiency is estimated to be ~55%. The salient features of the methanol cycle include a reversible SOFC, methanol synthesis, ease of methanol liquefaction and flexibility to store methanol-water mixtures rather than pure methanol to balance the storage volume and energy requirements for separation of water. When using a 50:50 methanol-water mixture for storage, the electrical energy storage efficiency of the methanol cycle is found to approach that of the methane cycle. However, methane as an energy storage media, in addition to its higher per carbon energy content, requires~ 59% lesser storage volume and 98% less carbon make up than methanol.  Both these proposed cycles avoid the large energy storage volumes associated with using alternatives like H₂ and current batteries.