## 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
UPHES for storage of short and medium duration uses the same components, except for the absence of the two large thermal stores. Figure 8: Main components of the hybrid UPHES-PTES for long-duration storage (here in stand-alone configuration and without the smaller pressurized cavern dedicated to CO\({}_{2}\)). UPHES for storage of short and medium duration uses the same components, except for the absence of the two large thermal stores. Figure 9: Operation of hybrid UPHES-PTES for long-duration storage, illustrated on the Pressure-Enthalpy diagram related to CO\({}_{2}\). A few companies are presently assisting to determine whether it could be valuable, instead of using pure CO\({}_{2}\) at every occasion, to consider using binary/ternary mixtures of CO\({}_{2}\) with selected molecules in low molar fraction, in order to displace the critical point of the CO\({}_{2}\) mixture. If results of simulations and experiments were to prove positive, the novel storage could benefit of several types of CO\({}_{2}\) mixtures, stored in several smaller dedicated caverns, to fulfil different situations (production of hydro mechanical energy only, co-generation of cooling, co-generation of heating, operation as hybrid UPHES-PTES). It is pointed out as well that, compared to most other PTES, thermal stores temperatures selected by this novel storage are classified as low. Several other PTES are using temperatures of 500-750 \({}^{\circ}\)C for the Hot Store in pursuit of a higher storage density, at the cost of higher Capex of energy and of sophisticated operating procedures. Other PTES using CO\({}_{2}\) as working fluid for energy conversion, albeit in continuous high-speed circuits of vapour compression differing greatly from the discontinuous expansion/contraction of CO\({}_{2}\) in closed caverns, use temperatures of 120-140 \({}^{\circ}\)C for the Hot Store in order to gain in energy density [24]. Our analysis is that the challenge of lowering CO\({}_{2}\) emission worldwide can only be achieved with the simultaneous higher penetration of low-cost and low-emission district heating and district cooling to residential buildings, office buildings and to some industrial developments, such as pulp and paper, pharmaceuticals, agro-food industry, textile and so forth. In particular, our vision objects to the growing installation of stand-alone electric heaters and stand-alone air-conditioners because in most countries those low-efficiency devices will remain powered by electricity of high carbon intensity, often from coal power plants, for many decades to come. Therefore, we foresee that the low temperatures selected for our thermal stores shall allow utility operators to readily integrate those stores, as well as the UPHES conversion caverns, for the production of low-cost and low-emission district heating and district cooling. From a single transcritical cycle as drawn in Figure 9, one can also exploit the alternative of operating two adjacent cycles by drawing a third horizontal segment placed between the low-pressure and the high-pressure horizontal line, in such way that this segment is positioned at the outright saturation vapour pressure of the CO\({}_{2}\) fluid for the ambient temperature (i.e., an horizontal segment, from 100% liquid phase till 100% gas phase, at a pressure of 58 bar when ambient temperature is 20 \({}^{\circ}\)C). By doing so, the storage operator gets, in discharge mode, the flexibility of prioritizing the thermal energy of the Hot Store towards heating networks or of prioritizing the thermal energy of the Cold Store towards cooling networks, while still being able to generate electricity from one of the two adjacent cycles, by using the environment to replace the missing store. Similarly, in charge mode, the operator enjoys the flexibility of using the environment to restore whatever imbalance by storing electrical energy in one of the thermal store whenever the second thermal store gets close to maximal capacity.