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New, Cheap Electric Energy Storage System (Like Pumped Hydro, But Subterranean!)

Pumped Hydro Compressed Air Energy Storage (PHCAES) is a new system that can deliver stored energy at two to three cents per kilowatt-hour. This cost, far lower than that of lithium batteries, is similar to Pumped Hydro Energy Storage (PHES), a proven technology. Although it has many similarities to PHES, PHCAES has significant advantages over PHES — including lower capital costs and significantly less land space required. 

Like PHES, PHCAES uses a ground-level water reservoir and a power plant. The difference is in the pressure reservoirs. While PHES uses a high elevation water reservoir to create a water pressure head, PHCAES uses a depleted underground well (gas/oil/water) that contains a reservoir of water along with high-pressure air to create its water pressure head. Both PHES and PHCAES pump/reverse water flow between the surface water reservoir and their respective high- pressure head reservoirs through a power plant to either store or produce electric power. See the figures below.

PHCAES Water Reservoirs Discharged:

PHCAES Water Reservoirs Charged:

Capital costs will be lower for PHCAES because this system uses an existing well compared to constructing a sizeable elevated water reservoir for PHES. PHCAES stores energy at a much higher pressure, therefore requiring a five-times-smaller surface water reservoir than PHES for the same amount of energy stored.

When the air pressure increases during charging with the PHCAES system, a low amount of heat is created. The heat produced is far lower than the heat created by standard open compressed air systems (Isochoric vs. Adiabatic compression). This small amount of heat reduces round trip efficiency and does increase the cost, but the increase is offset by the savings in capital cost.

Using wells for storage is well known. For example, in the U.S., 4.8 billion cubic feet of natural gas is currently being stored with pressures between 500 to 3,000 psi in about 17,000 subterranean depleted oil/gas wells, aquifers, and salt formations. The volume of liquids a well can hold, as seen in fracking liquids, is also well known. These underground reservoirs already exist, are cheap to use, and take up virtually no land space.

If all 2.7 million of the abandoned oil and gas wells in the U.S. were used for PHCAES, they could store 60,000 gigawatts of power. This power storage is enough to store one hundred percent of over six hours of the per-hour power generated in the U.S. today. This storage does not include aquifers, which would add considerably to the total. In addition, the energy can be stored for far longer than the four-hour window of power storage that is typical of lithium batteries.

PHCAES will provide an attractive electric power storage solution as the world transitions from fossil fuels to intermittent renewables like wind and solar. With its potential low cost of two to three cents per kilowatt-hour and numerous, low-cost subterranean site possibilities, it could make the transition easier and faster.

Our company, PowerXpro*, LLC, does not have the capability to commercialize the patent-pending PHCAES technology. We are seeking an interested party to take over the whole project. Our goal is to see the project take fruition for the good of the world, rather than large profits.

If interested, please contact Bob Cantrell, president of PowerXpro LLC, at

*This article is supported by PowerXpro. All graphics provided by PowerXpro.

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