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 Heat is withdrawn from the reservoir as saturated steam and is used to operate an active thermal recovery project (such as a producing thermally enhanced oil reservoir) and/or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 Withdrawn heat is optionally augmented by heat produced by firing natural gas.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.