Abstract: Excess energy generated from renewable energy (solar or wind sources) is used to heat a liquid which is injected into a naturally-occurring permeable, porous subterranean reservoir where it heats constituent reservoir grain matrix, thereby storing energy and modifying the reservoir's storage capacity and transmissibility, and energy is recovered, as demands require, by producing hot reservoir fluids whose heat is transformed into electric power. By using water heated to at least 250° F., thermal stresses are induced to create a measurable increase of units of permeability (at least double, but up to tenfold or more), as well as an increase in porosity (up to 10 volume %) between a pair of wells located more than fifty meters apart as the reservoir is heated to in excess of approximately 500° F.