A local energy source is desired for near-shore and offshore applications. Gas generators, diesel generators, and long-length submerged power cables tend to be expensive. A proposed solution is to use offshore wind with some type of energy storage mechanism for up to 1 GW-h. Energy storage in batteries is too expensive and massive, and subsea compressed air energy storage (CAES) has not been proven for very deep depths. Furthermore, CAES involves very great temperature changes that result in large inefficiencies.

In Pumped Subsea Energy Storage, compressed air or pressurized liquid can be stored in zero-pressure bags. Air can be expanded in a gas turbine, and high-pressure heavy liquid can feed a hydro pump, as per pumped storage.

A novel solution is to use inexpensive, environmentally friendly fluids of low or high density, such that moving the fluid to another depth requires energy that is stored, and returning the fluid to the original depth releases the energy as electricity to be consumed. This is similar to pumped energy storage for water behind dams. Pumping water back up the dam during periods of light energy use allows more energy to be used by utility customers during peak hours.

A calcium chloride solution has a density of about 1.4 g/cm3, compared to the ocean density of 1.02 g/cm3. The heavy fluid solution can be stored near shore or in an offshore tanker. The heavy fluid flows down to some depth, where it then powers a hydraulic generator and is stored in a deep sea reservoir. During windy days, the heavy fluid is pumped back up to the higher elevation. Some type of oil separator and/or flexible film separator keeps the fluid in the two reservoirs from mixing with the ambient ocean water. A depth change of 500-m is about equivalent in pressure to water behind the 200-m-tall Hoover dam.

Instead of generating energy at the bottom of the ocean, a light fluid, such as methanol, can be stored in a scuttled tanker at depth. When the methanol is transferred to the sea surface, it generates power above the surface of the ocean, thus making maintenance much less costly.

Natural or constructed contours in the ocean floor can be used to contain the upper and lower dense liquid reservoirs. This system would be good for near-shore applications to store wind or solar energy. Likewise, dense fluid from an upper reservoir (onshore or submerged) can flow down towards a lower basin. A lower mechanical hydraulic pump is thus powered by mechanical energy to pump a high-pressure fluid to an on-shore generator. This design allows the upper reservoir and all power generation electronics to be located safely and inexpensively onshore.

For offshore oil platforms, the dense liquid systems are completely self-contained (no consumables). They avoid the cost and mass of storing batteries on the platform, as well as having generators on the oil platform.

These designs provide a relatively inexpensive, high-efficiency means to store energy. Nearly all pumped energy storage using dams has already been utilized in the U.S., and no marine pumped storage systems have been built due to fear of salt water leakage and large ocean biota destruction with each power generation/pumping cycle. These designs are an alternative means to store very large amounts of energy beneath the ocean on subsea land that does not need to be purchased. All fluids (ethanol, brine, and calcium chloride solutions) are relatively benign to the ocean environment and readily mix with seawater if accidentally released.

This work was done by Jack A. Jones of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49117

This Brief includes a Technical Support Package (TSP).
Pumped Subsea Energy Storage

(reference NPO49117) is currently available for download from the TSP library.

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