## 3 Results of Literature Review and Economic Assessment
### A Novel Long-Duration Storage, Hybrid of Hydro-Pneumatic UPHES with PTES
#### Moving the Storage Capacity outside of UPHES, toward Thermal Stores
This split in two adjacent cycles with the use of the environment could also prove being handy to initialize the storage system by charging independently the Cold Store and the Hot Store for a first time with the highest flexibility. An additional benefit for this UPHES-PTES hybrid storage to work in the \"low temperatures\" domain is that this feature provides to the system a larger occurrence to increase its RTE, when very-low-grade geothermal energy or very-low-grade industrial heat gets available from the nearby environment (very-low-grade being hereby defined from being in the 50-65 \({}^{\circ}\)C temperature range). Because very-low-grade heat is considered \"end-energy,\" it comes \"for free\" and goes unaccounted-for in the definition of system efficiency. Consequently, RTE of the novel storage could exceed 100% in conditions where very-low-grade heat is available at sufficient power and temperature. When an external source of low-grade energy is freely available (low-grade being hereby defined from being in the 65-95 \({}^{\circ}\)C temperature range), the same \"low temperatures\" feature of the working fluid shall allow the UPHES to work as an autonomous thermal engine, producing net-work without depending on thermal stores nor on external electricity source. Examples of such net production of electricity are the availability of low-grade geothermal energy or low-grade industrial heat. When the low-grade source of energy is of suitable temperature however of low power (i.e., of weak flow),the procedure is to include this inexpensive energy in the heating process of the CO\({}_{2}\) mass or to store it in water tanks to operate only a few hours per day in net-work mode. Operational modes taking benefit of very-low-grade or low-grade energy usually require the use of the ambient environment as a secondary thermal source. With or without the availability of external thermal sources supplementing the two thermal stores, in discharge mode the effect of applying a hot source and subsequently a cold source, to a stand-alone UPHES cavern creates a near-reversible thermal engine, slowly expanding/contracting CO\({}_{2}\), pushing-out hydro-water to the hydroelectric turbine and subsequently sucking-in hydro-water back. The favourite configuration to maintain steady power is to rig two caverns A and B asynchronously in parallel to a single turbine set and to use two receiving ponds or one larger receiving pond, located at the ground surface. While cavern A pushes-out hydro-water at high pressure (9.5-15 MPa) thanks to the expansion of CO\({}_{2}\) heated by 95 \({}^{\circ}\)C water at its evaporator heat exchanger, cavern B gets its condenser heat exchanger connected to a \(-20\)\({}^{\circ}\)C coolant, reducing gradually its inner pressure down to 2 MPa and allowing hydro-water to be sucked-in from the surface with close to no pumping work. Before cavern A gets entirely emptied, cold CO\({}_{2}\) of cavern B had been pre-heated or had been swapped with an external mass of pre-heated CO\({}_{2}\) and get ready to take-over the pushing-out of hydro-water to the same turbine set thanks to the expansion of CO\({}_{2}\) heated by 95 \({}^{\circ}\)C water, while cavern A gets prepared to suck-in the fresh water needed for the continuous operation of the turbine set. In charge mode, for transferring heat from Cold Store toward Hot Store in a steady way for the power network while the storage is in heat-pump mode, the favourite configuration is also to rig two UPHES caverns asynchronously in parallel to a single pump set and to use one or two receiving ponds located at ground surface.