As the United States transitions away from fossil fuels, its economy will rely on more renewable energy. Because cur-rent renewable energy. . Hydrogen (either as a gas, liquid, or within another mol-ecule like ammonia) may store a substantial amount of chemical energy. The subsequent use of that energy through electrical fuel cells or combustion is relatively clean compared to fossil fuel usage (Ofice of Energy Eficiency and. . Mine shafts— Gravity Hydrogen, Methane Aquifer— Thermal Purpose-drilled shafts— Coal mine— Methane, compressed air, and hydroelectric Borehole— Thermal Salt mine— Methane, hydrogen, and compressed air Hard Rock mine— Compressed air and hydroelectric. . Depleted gas reservoirs Solution-mined salt caverns Non-potable aquifers Abandoned mines
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