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Energies 2020, 13, 420 59 of 96 storage capacity [738]. Drawbacks are the loss of volume and structural stability due to steady-state and transient creep provoked by constant fluid pressure or rapid pressure change, respectively [739]. The underground gas storage in aquifers is a porous gas reservoir in a low permeable rock filled with water (typically saline) and a caprock to prevent leakages [740]. The storage capacity is affected by the injection flow rate, pressure and salt precipitation. The storage volume increases with the increase in the injection pressure [741]. Further, the storage capacity is affected by the distribution of pore and throat in the reservoir and water lock [742]. Moreover, the salt precipitation in the porous medium reduces the injectivity and induces damage [743]. Finally, the retention time affects the storage process because the gas is trapped in water, and only a small fraction can be recovered [744]. Depleted gas or oil reservoirs are the most suitable way for underground storage due to the broad availability and assured geology [745]. The reservoirs are surrounded by porous solid with a caprock to avoid vertical leakage [746]. Natural gas is injected through wells into depleted gas reservoirs that are at a great depth. The injection requires the gas pressurization to force the gas to permeate in the porous and permeable reservoir. Moreover, after the compression, the gas is cooled [747]. The advantage of the use of depleted oil reservoirs is the existence of wells, gathering systems and pipeline connections [748]. The storage availability is verified case-by-case as a function of the geological and petrophysical characteristics [749]. Indeed, gas storage involves safety issues. The gas injection provokes intergranular stress due to cyclic compression and expansion of the ground, leading to rock fractures and microseismic activity due to reactivation of existing faults of the reservoir [750]. Hydrogen Hydrogen injected in the natural gas blend will also reach underground gas storages used for seasonal gas storage (gas consumption is indeed higher during the heating season than during summer). Salt caverns are compatible with hydrogen storage, ensuring long-term stability and great tightness [751]. The storage in aquifers and depleted oil and gas reservoirs is also possible, but hazards are related to chemical reactions with minerals, residual oils and biological microorganisms (bacteria and archaea) [752,753]. In 2013, the energy company RAG (Rohöl-Aufsuchungs Aktiengesellschaft) and its project partners started the project Underground Sun Storage to verify the feasibility of gas storage with up to 10% of hydrogen content into underground storage facilities [754]. Results demonstrate that the hydrogen storage is possible, microbial processes can be handled and leakage out of the reservoir and alteration of rocks were not detected [755,756]. 6. Discussion Several routes for hydrogen generation via water splitting are available. Electrochemical processes include photoelectrochemical and electrolysis cells, whereas thermochemical processes involve thermolysis and thermochemical cycles. Table 3 outlines the technology readiness level, the energy conversion efficiency—defined as the ratio of the energy stored in the fuel (low heating value) and the primary energy required to produce the fuel—the fuel production cost and CO2 emissions of hydrogen and syngas generation routes. The most mature way to produce hydrogen is the electrochemical conversion of renewable power through alkaline (TRL 7–9) and proton exchange membrane electrolyzers (TRL 4–8). Solid oxide electrolysis cells are at an earlier research stage and suffer for thermal cycles due to high operating temperatures. Therefore, SOECs are not well matched with solar or wind power supply that is intermittent and fluctuating. The energy to fuel conversion efficiency of thermochemical cycles (TRL 4–7) enhanced by concentrated solar power is comparable to that of electrolyzers coupled with wind turbines. However, the production cost of hydrogen through electrolysis cells is lower. Instead, thermolysis technologies, photoelectrochemical cells and microbial electrolysis cells were studied only at the laboratory scale (TRL 1–4). Thus, their efficiencies and production cost are not comparable with those of more mature technologies.PDF Image | Green Synthetic Fuels
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