## 3 Results of Literature Review and Economic Assessment
### Replacing Air by CO\({}_{2}\) at Liquid-Vapor Equilibrium in a Hydro-Pneumatic UPHES
#### 3.2.3 Operation as Short-Duration or Medium-Duration Storage, plus as Heat-Pump  
Quite often, heat or cooling is highly valuable, that is, for district heating and district cooling, supermarkets, hotels, hospitals, industries, call-centres and so forth and gets produced by the UPHES working in a slow and near-reversible way as heat-pump with an attractive COP (Coefficient Of Performance, equal to the ratio of useful thermal energy produced over electric energy consumed). We can read from Figure 4 that to export heating at temperature higher than ambient, we need to pump-in hydro-water at a pressure higher than 7 MPa. We can read as well that to export cooling at temperature lower than ambient, we need to let-out hydro-water at the turbine at pressure lower than 6 MPa. From simple computation of mechanical work, a COP of up to 10 can be expected, depending on required temperatures for heating or for cooling, related to the decrease of hydroelectric energy recovered. For the particular case of cooling needs, it is important to stress-out that the cavern extracts heat at the same time it produces hydroelectricity at the turbine. Indeed, due to the CO\({}_{2}\) mass acting as a PCM (Phase Change Material) in the quite uncommon pair liquid-gas for phase change, the cavern stores significant amount of cooling at the same time it stores electricity and additionally, does not need power to discharge such cooling. This unique feature could prove very useful for complementing intermittent renewable energies such as solar and wind to buildings having high cooling needs, such as most buildings in tropical areas, on-grid as well as off-grid. By enabling the continuous supply of hybrid energy (electricity + cooling), this feature could bring a radical solution to the worrying trend of fast-increasing energy spent worldwide for cooling, especially in China, USA, India and Indonesia [2; 21]. The feature of CO\({}_{2}\) acting as PCM for heating storage does not allow for the same benefit, since the production of heating only exists during charging of electricity and cannot happen without electricity supply. However, this issue can be fixed by the provision of an ancillary thermal storage (i.e., an insulated tank of hot water) allowing to fulfil heating needs around the clock, including outside of timeframes of electricity charging. 2.4 Economic Evaluation as a Storage of Short and Medium Duration, with or without the Heat-Pump Function. Comparison with Electrochemical Storage  
Based on a system of power 100 MW and of energy capacity 400 MWh (4 h) and to calculations based on a compounded excavation cost of 300 USD/m\({}^{3}\) for pressurized LRC (main cavern + smaller CO\({}_{2}\)-dedicated cavern), of a cost of 30 USD/m\({}^{3}\) for the hydro-water reception pond, of a cost of 750 USD/kW for turbine + generator, of a cost of 350 USD/kW for pump + motor, a cost of 250 USD/kW for heat exchanger and of a cost of 150 USD/kW for building and ancillary equipment, the two features of using LRC cavern and of replacing air by CO\({}_{2}\) bring the Capex of this short and medium duration storage of pure electricity to 1500 USD/kW Elec. + 200 USD/kWh Elec. The unitary costs above come from estimations reached from discussions with professionals from underground civil works, from hydroelectricity and from chemical engineering. The power component of Capex is simply the sum of the 4 items related to power (750 USD/kW +350 USD/kW +250 USD/kW +150 USD/kW = 1500 USD/kW), while the energy component needs to be calculated in a table like Table 1 below. The limitation of this configuration appears immediately by realizing that stored energy is proportional to the high cost of excavation of cavern volume and that no significant effect of scale can be expected. For value of energy density in this configuration, it is assumed that water takes 70% of the cavern volume and that recovered CO\({}_{2}\) pressure gets limited to 90 bar, bringing a value of stored energy equal to 1.750 kWh per m\({}^{3}\) (before loss at the hydro turbine).