## 3 Results of Literature Review and Economic Assessment
### Novel Use of Existing LRC Cavern for UIPHES
#### 3.1.2 Use of LRC Cavern for Hydro-Pneumatic UPHES  
LRC represent an attractive solution for hydro-pneumatic UPHES because they can be excavated in shallow grounds and close to area of electric production, of electric consumption or close to power transmission networks. Although there is not yet any LRC actually used for hydro-pneumatic UPHES, we do not anticipate particular constraints related to this specific usage. Although it exceeds the scope of this work, we also do not anticipate particular limitations in the further use of LRCs for most other pressurized storage utilizations such as CO\({}_{2}\) sequestration and storage, hydrogen storage, methane storage and so forth.  
In all cases, to limit rock failures to unavoidable low-amplitude tensile fracturing in the vicinity of cavern periphery and to avoid stepping into the domain of shear fracturing, it is recommended that the maximum pressure operated in the cavern does not exceed the uniaxial compression strength of the rock mass, which is usually higher than 15 MPa for most rock formation. In the case of soft rock mass, usually of low Young's modulus (i.e., lower than 10 GPa), large radial displacements of rock walls must be avoided to hinder a heavy tensile fracturing of the thin concrete shell protecting the liner, as well as to hinder a later buckling of the liner itself at depressurization [18]. This can be accomplished by a high-pressure cement-grouting of the rock mass to a radial distance of at least one diameter, creating a somehow synthetic rock mass of higher uniaxial compression strength and of higher Young's modulus.  
Issues related to fatigue of the concrete shell and liner are of course linked with maximum and minimum pressures during operations, to the quantity of cycles along the cavern lifetime (40 years expected), to the quality of the rock mass and finally to the shell and liner themselves. [PERSON] et al. have conducted thorough simulations on this subject [18]. Another issue is the liner compatibility with fluids contained in the cavern. For instance, CO\({}_{2}\) has a strong oxidizing effect on carbon-steel in presence of humidity. Moreover, CO\({}_{2}\) shows a high permeability to elastomer membranes as well as to several other polymer materials.  
Another consideration about LRC cavern, also not specific of use for hydro-pneumatic UPHES, is the high risk of damaging the liner by the hydrostatic pressure of ground water when the cavern gets depressurized for inspection or maintenance. For all caverns below ground water table, it is therefore compulsory to insert water drains between rock walls and the concrete shell. In that matter, taking benefits of available cliffs or hills, even of low-elevation, can be beneficial to excavate caverns above water tables.  
In a different respect, although completely untested for UPHES as well as for other storage use, it shall be possible to take benefit of the mass of high-rise buildings (25-100 floors) to build a cluster of small-diameter shaft-shaped LRCs a few tens of meters under a thick concrete slab placed at the lowest level of underground parking lots. Subject to further geotechnical studies, this configuration of underground storage could become of interest whenever local or national administrations would enforce regulations for on-site Net Zero Emission requirements based on renewables + storage, in urban areas when land surface is highly constrained.