A system for using excess energy of a power generation system and an sCO2 (supercritical carbon dioxide) stream to store and generate power. An air separation unit uses the excess energy to cool and liquify ambient air into liquid nitrogen (L-N2) and liquid oxygen (L-O2). The L-O2 and L-N2 are stored until energy is desired. An L-O2 energy discharge path has an oxygen heat exchanger that vaporizes and heats the oxygen, a combustor that combusts the oxygen and fuel to produce exhaust, and a first turbine is driven by the exhaust to produce energy. An L-N2 energy discharge path has a nitrogen heat exchanger that vaporizes and heats the L-N2, thereby providing expanded nitrogen, and a second turbine is driven by the expanded nitrogen to produce energy. Heat for the heat exchangers on both discharge paths is provided by the sCO2 stream.

Techniques for subsurface thermal energy storage of heat generated by concentrating solar power enable smoothing of available energy with respect to daily and/or seasonal variation. Solar thermal collectors produce saturated steam that is injected into a producing or wholly/partially depleted oil reservoir that operates as a heat storage reservoir. Some of the saturated steam generated by the collectors is optionally used to generate electricity. Heat is withdrawn from the reservoir as saturated steam and is used to operate an active thermal recovery project (such as a producing thermally enhanced oil reservoir) and/or to generate electricity. Withdrawn heat is optionally augmented by heat produced by firing natural gas. The reservoir is optionally one that has been used for thermally enhanced oil recovery and thus is already warm, minimizing heat losses.