## 3 Results of Literature Review and Economic Assessment
### A Novel Long-Duration Storage, Hybrid of Hydro-Pneumatic UPHES with PTES
#### 3.3.3 Economic Targets of the UPHES-PTES Hybrid Long-Duration Storage  
Target for RTE of our novel storage, at 60%-70% may seem an ambitious figure compared with results currently achieved for pilot plants of other PTES. Besides similar thermal irreversibilities due to imperfect heat exchanges, other existing PTES use high-speed circuits of a working fluid (steam or refrigerant) and need 4 complex turbine or piston machineries working only in gaseous and supercritical phases (1 compressor and 1 expander for the heat-pump mode, 1 compressor and 1 expander for the thermal machine mode). [PERSON] et al. show that, for a traditional vapor compression process bearing a typical back-work-ratio of 36.5% and assuming realistic grid-scale efficiencies of 80% for each machinery, the best RTE which can be attained in theory by high-speed trans-critical circuit PTES is 45% [24]. [PERSON] et al. realized in 2016 a model using butane as the working fluid and computed a maximum RTE of 27.3% only [8]. While the Siemens Gamesa company expects a full-scale deployed system to reach in the future round-trip efficiencies of 50%, a RTE of only 25% has been so-far obtained in June 2019 in Hamburg, Germany, after several years of development on a pilot plant of 130 MWh per week [9]. Although there is no reason why large efficient gas compressors and gas turbines could not be developed in a later future, the present lack of high-range RTE in pilot and demonstrator systems is probably the cause impeding the worldwide unfolding of grid-scale PTES. In the favourite configuration of the novel hybrid UPHES-PTES, only 1 operation of a hydro pump and 1 operation of a hydro turbine are needed for a conversion round trip. Thermal irreversibilities of the hybrid UPHES-PTES differs slightly from those of the simpler UPHES as described in paragraph 3.2.4. by the minor fact that hot water and cold water get stored and loose some energy to the ambient environment during storage time (i.e., 3% loss per month). To ascertain the value of RTE in the hybrid mode relatively to the 60%-70% target, is planned for 2020 the construction of an aerial prototype made out of steel vessels, of 35 mm metal thickness, for a total pressurized volume of a few cubic meters. Thanks to the low cost of atmospheric insulated thermal storages, assessed at 40 USD/ m\({}^{3}\) for the Hot Store pit [21] and evaluated at 100 USD/ m\({}^{3}\) for the Cold Store ice slurry pit, the expected Capex of a hybrid UPHES-PTES assisted by two large thermal stores can be evaluated, using the same other cost components than in paragraph 3.2.4. above, to a Capex of electricity equal to 2160 USD/kW + 10 USD/kWh (meaning a compounded Capex of 20 USD/kWh only, for a 216 h storage). Calculations can be counterintuitive because in this configuration the cavern and its receiving pond are not any longer components of energy capacity Capex but, as loci of energy conversion, components of power Capex. Based on the same required power of 100 MW but with an energy capacity increased to 100 h and to 10 GWh, based on two caverns in asynchronous parallel mode achieving a complete cycle in an arbitrary time of 2.5 h, the calculation results in 200,000 m\({}^{3}\) of caverns,  
Figure 11: Illustrations of scale and of energy density for the UPHES-PTES storage. 200,000 m\({}^{3}\) of water receiving pond, 987 000 m\({}^{3}\) of Hot Store and 525,000 m\({}^{3}\) of Cold Store to achieve the required power and energy capacity. The power component of Capex is therefore increased, still for 100 MW, of an extra cost of 200,000 m\({}^{3}\) X (300 USD + 30 USD), thus 660 USD/kW, raising this power Capex to 2160 USD/kW. The energy component of Capex needs to be calculated in a table like Table 2 below.