Abstract: A method of temporarily storing thermal energy via a thermal storage subsystem in a compressed air energy storage system comprising an accumulator disposed at least 300 m underground and having an interior configured to contain compressed air at an accumulator pressure that is at least 20 bar and a gas compressor/expander subsystem in communication with the accumulator via an air flow path for conveying compressed air to the accumulator when in a charging mode and from the accumulator when in a discharging mode.

Abstract: A hydrostatically compensated compressed air energy storage system may include an accumulator disposed underground, a gas compressor/expander subsystem in fluid communication with the accumulator interior via an air flow path; a compensation liquid reservoir spaced apart from the accumulator and in fluid communication with the layer of compensation liquid within the accumulator via a compensation liquid flow path; and a first construction shaft extending from the surface of the ground to the accumulator and being sized and configured to i) accommodate the passage of a construction apparatus therethrough when the hydrostatically compensated compressed air energy storage system is being constructed, and ii) to provide at least a portion of one of the air flow path and the compensation liquid flow path when the hydrostatically compensated compressed air energy storage system is in use.

Abstract: A method of transitioning a hydrostatically compensated compressed air energy storage system from an operating mode to a dewatered maintenance state may include a) charging an accumulator to a fully charged state where the air water interface is at a charge plane by conveying compressed air at a storage pressure into the layer of compressed air using a gas compressor/expander subsystem thereby displacing a corresponding amount of compensation liquid from the layer of compensation liquid out of the accumulator into the compensation liquid flow path and from the compensation liquid flow path into the compensation liquid reservoir until the accumulator is substantially free of the compensation liquid, b) fluidly sealing the compensation liquid flow path thereby isolating a residual amount of the compensation liquid, and c) depressurizing the accumulator interior to a service pressure that is lower than the storage pressure.