Houston-based Sage Geosystems has started construction on a 3 MW geo-pressurized geothermal energy storage system in Christine, Texas. The announcement follows a land-use agreement signed with the San Miguel Electric Cooperative Inc. (SMECI) enabling the location of the facility near an existing coal-fueled power plant. Sage will serve as merchant, buying and selling electricity to the Electric Reliability Council of Texas (ERCOT) grid.
The storage system, dubbed EarthStore, is based on Sage’s dry rock geothermal technology, which consists of a drilled well into which water is pumped and kept at ambient heat and pressure in subsurface rock formations. When electricity is needed, the naturally heated and pressurized water is released to run a Pelton-type hydroelectric turbine generator. The storage facility is expected to have six to 10 hours of capacity.
The SMECI project will be the company’s first commercial storage facility. Sage CEO Cindy Taff said the coal plant will not have any bearing on storage operations, except as a source of water, and that the idea is to buy electricity from ERCOT to run pumps when demand and prices are low. When ERCOT experiences high demand Sage will run its turbine and sell the power.
“We’ll be drilling the well in September and building the facility,” Taff told pv magazine USA. “We’ll have everything done by the end of December this year.”
Ideally, Taff says, the EarthStore system would serve as a long duration energy storage companion to solar and wind generation, where surplus energy is used to run the pumps. The amount of storage depends on the number of wells available: more may be drilled to increase capacity on site. The pumped water may be stored indefinitely and when released delivers a round-trip efficiency of 70-75% with a water loss of less than 2%, she said.
The EarthStore system is one of family of geothermal storage and baseload energy systems Sage is developing. A more ambitious geothermal generation technology drills a series of wells to depths of 9,000 to 20,000 feel, where ambient temperatures range from 218- to 485-degrees Fahrenheit. In such systems, pressurized steam is liberated to run Rankin-cycle turbines to generate electricity. A more advanced version will heat pressurized, supercritical CO2 to drive a specialized turbine with greater efficiency.
Sage has contracts with the Department of Defense to develop geothermal baseload generators and microgrids for its facilities. It is conducting feasibility studies at the Army’s Ft. Bliss and Air Force’s Ellington Field bases, both in Texas. A prototype geothermal plant is under construction at the latter site. In addition, Sage has a test site of its own in Starr County.
According to Taff, the primary advantage of Sage’s approach to geothermal storage and generation is that it uses existing drilling techniques from the oil and gas industry to produce renewable energy from rock formations that exist essentially everywhere. All of the 16 GW of existent geothermal energy is produced from hydrothermal locations linked to volcanic activity – relatively rare occurrences.
“If you look at the continental U.S., we can put storage even in the East, where the geothermal potential is a little bit more challenge,” Taff said. “We can do it in the West, where you have good geothermal potential. So, we can pair with wind or solar just about anywhere. We are actually looking at pairing with solar to provide off-grid, 24/7 power for data centers and other customers who need tons of power.”
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