Energy storage | Salt dome storage Cavern Energy Storage is pioneering underground pumped storage hydroelectric
technology in the Gulf Coast by utilizing natural salt domes. This innovative approach could provide the region with 50GW of long-duration energy storage, leveraging a resource lying beneath our feet, writes William M Taggart
Above: Cross section of a salt dome
Below: Salt dome cavern locations
LOOKING AT THE INCREASING amount of solar and wind in some utility markets, especially across Texas, there was clearly a need for energy storage on the Gulf Coast. And this has been recognised by the United States’ Department of Energy (DOE) which formed the Energy Storage Grand Challenge to encourage long duration energy storage (LDES) development, provided grant funding, and encouraged developers to participate in the LDES Consortium they formed with the national laboratories. To date, however, none of these efforts have been successful at filling the requirements. Cavern Energy Storage was formed over two years ago when I asked a basic question, “How do you provide LDES to Texas, Louisiana, and Mississippi?” I felt like the answer had to be something that would tailor understood technologies but deploy them in a way that was unique to the region. As it turns out the answer was under my feet all along. Pumped storage hydropower requires large elevation difference between the water reservoirs to make it economical. A requirement that the flat terrain of the Gulf Coast in Texas, Louisiana, and Mississippi can’t supply. But while the Gulf Coast lacks mountains, it does have salt domes – large geological formations of salt that are thousands of feet wide and miles deep. Since the 1940s, the DOE, oil companies, natural gas pipeline companies, as well as others have used salt domes to create underground storage caverns for oil, natural gas, hydrogen, and many other fluids. The Strategic Petroleum Reserve operated by the DOE is 60 salt dome caverns in four locations that can store over 700 million barrels of oil. And salt domes can provide a lot of energy storage. Cavern Energy Storage estimates that there are 160 salt domes on accessible land in Texas, Louisiana, and Mississippi with over 40,000 unused acres that could be developed into 50GW of capacity with twenty hours of storage. This could easily supply the bulk of the LDES requirement for the over 25 million people that live in this region. There are also salt domes in Europe, the Middle East, and other areas of the world. Underground pumped storage hydroelectric is
the combination of these two proven technologies, pumped storage and salt dome caverns. Cavern Energy Storage has patented a construction method that allows for an economical way to build the energy storage that we will need in the future.
Going underground Underground pumped storage hydroelectric uses
large wellbores drilled into the salt domes to two different elevations. Once the wellbores are in place, water is pumped down the interior casing and returns up the annulus of the wellbore as saturated salt brine. This is a process called solution mining by injecting water and removing the brine which dissolves the salt and forms the caverns. The brine is processed to remove the salt which can be sold as a commodity and then the water is returned to do more solution mining. Once the caverns are formed, compressed air is added to create the space in the caverns to move the brine back and forth. The compressed air also raises the brine pressure in the interior casing so that the pump/turbine can sit on the surface. The pump/turbine looks just like a conventional pumped storage one, but with saturated salt brine as the fluid rather than regular water. The brine is pumped
Salt dome caverns
Technicalities: Long duration – up to 20 hours of storage that does not degrade over time. 80% round trip efficiency. Off-the-shelf – wellbores, pumps, hydroelectric turbines, are all available right now. No regulatory barrier as salt dome cavern construction is ongoing for other purposes. Provides on-demand energy storage that is inertia based to help stabilize the grid. Reduce carbon dioxide emissions by removing inefficient peakier power plants from the grid.
Economics: Salt removed to form caverns can be sold to offset construction costs. Modular design allows for “LEGO” type approach, with repetitive construction and cost savings. Improvements in drilling and salt recovery allow for US$50/MWh levelised cost of storage.
Existing ERCOT market is seeing revenue potential ofUS $62/MWh through arbitrage alone.
36 | September 2024 |
www.waterpowermagazine.com
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