SYSTEMS AND METHODS FOR FOAM-BASED HEAT EXCHANGE DURING ENERGY STORAGE AND RECOVERY USING COMPRESSED GAS

1 .- 174 . (canceled)

175 . A method of storing and recovering energy, the method comprising:

at least one of compressing a gas to store energy or expanding a gas to recover energy in a plurality of discrete stages, each stage being performed over a different pressure range;

wherein (i) during one of the stages, heat is exchanged between the gas and discrete droplets of a first heat-transfer fluid introduced into the gas, and (ii) during another one of the stages, heat is exchanged between the gas and a second heat-transfer fluid, the gas and the second heat-transfer fluid being combined to form a foam.

176 . The method of claim 175 , further comprising, after the at least one of compression or expansion during the another one of the stages, separating at least a portion of the foam into gaseous and liquid components.

177 . The method of claim 176 , wherein the at least a portion of the foam is separated via at least one of mechanical separation or application of ultrasound energy.

178 . The method of claim 177 , wherein the at least a portion of the foam is separated via mechanical separation by at least one of a blade, a shear, a baffle, or a centrifuge.

179 . The method of claim 175 , wherein the gas is compressed, and further comprising, thereafter, storing the gas in a storage reservoir.

180 . The method of claim 175 , wherein the gas is expanded, and further comprising, thereafter, venting the gas to ambient atmosphere.

181 . The method of claim 175 , wherein the first and second heat-transfer fluids comprise the same liquid.

182 . The method of claim 181 , wherein the second heat-transfer fluid comprises a foaming additive.

183 . The method of claim 175 , wherein the discrete droplets of the first heat-transfer fluid are introduced into the gas by spraying.

184 . The method of claim 175 , wherein the heat exchange between the discrete droplets of the first heat-transfer fluid and the gas renders the at least one of expansion or compression during the one of the stages substantially isothermal.

185 . The method of claim 175 , wherein the heat exchange within the foam between the gas and the second heat-transfer fluid renders the at least one of expansion or compression during the another one of the stages substantially isothermal.

186 . The method of claim 175 , wherein the at least one of expansion or compression during the another one of the stages is performed within a cylinder.

187 . The method of claim 186 , further comprising generating the foam by mixing the gas with the second heat-transfer fluid within the cylinder.

188 . The method of claim 186 , further comprising (i) generating the foam by mixing the gas with the second heat-transfer fluid outside of the cylinder, and (ii) transferring the foam into the cylinder.

189 . The method of claim 188 , further comprising altering at least one of an average cell size or a uniformity of cell size of the foam before the foam enters the cylinder.

190 . The method of claim 186 , wherein the gas is expanded and the cylinder comprises a piston therein, and expanding the gas in the cylinder drives at least one of (i) a mechanism mechanically coupled to the piston or (ii) a hydraulic system coupled to the cylinder.

191 . The method of claim 190 , wherein expanding the gas drives the mechanism, and the mechanism comprises a crankshaft.

192 . The method of claim 191 , wherein a foam mass ratio of the foam is selected from the range of 1 to 4.

193 . The method of claim 175 , wherein forming the foam comprises transferring the second heat-transfer fluid into the gas.

194 . The method of claim 193 , further comprising controlling a void fraction of the foam by controlling a rate of transfer of the second heat-transfer fluid.

195 . The method of claim 194 , further comprising sensing at least one of a pressure or a temperature within at least one of the stages, wherein the rate of transfer is controlled in response thereto.

196 .- 451 . (canceled)