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Underground Water Battery To Bust Energy Storage Dam Wide Open

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Pumped hydro already accounts for 93% of utility-scale energy storage in the US, and plans are in the works to build up from there.
ByTina Casey

The US Department of Energy has been eyeballing pumped hydro energy storage for a makeover, and it looks like they have a winner. The Texas firm Quidnet Energy has just won a $10 million grant to put the finishing touches on a new type of “water battery” that deploys underground rock formations instead of having to rely on elevated reservoirs.

Pumped Hydro Energy Storage 101
For those of you new to the topic, pumped hydro is a centuries-old, gravity-based energy storage technology that has gained new relevance in the age of wind and solar power.

The idea is to pump water to an upper reservoir whenever excess wind or solar power is available. When needed, water from the reservoir flows downhill to a power station, where it runs turbines to generate electricity.

Pumped hydro can run on a closed loop system, in which the same water cycles between upper and lower reservoirs. Pumped hydro systems can also be designed as open loop, in which water from a river or other natural water body is pumped to a reservoir, then discharged back after leaving the power station.

Water Batteries Go Underground
These “water batteries” are still an important part of the energy storage landscape in the US. Despite all the hoopla over new lithium-ion technology and other emerging energy storage systems, pumped hydro still accounts for about 93% of utility-scale energy storage capacity in the US.

The Energy Department counts 43 existing pumped hydro facilities in the US and is looking to double their collective capacity. The problem is that conventional water batteries involve a massive amount of above-ground infrastructure, and they require topography that provides for the difference in elevation. That makes it difficult and expensive to find sites for new facilities.

That’s where Quidnet comes in. The company first came across the CleanTechnica radar back in 2019, when the Energy Department tapped the company to share in a round of funding to develop new pumped hydro systems.

“The innovative technology can operate at higher temperature than traditional PSH, achieve 95% mechanical efficiency (each way), and has the potential to reduce capital expenditures and energy storage solutions in relatively flat areas where conventional PSH may not be possible,” the Energy Department stated.

Quidnet has not been asleep at the wheel since then. The new grant of $10 million comes through the Energy Department’s cutting edge ARPA-E funding office, which focuses on supporting new high risk, high reward technologies.

“Quidnet Energy, Inc. (Houston, TX) will scale its Geomechanical Pumped Storage (GPS) to a commercial system at CPS Energy, the largest U.S. municipal utility,” ARPA-E explained.

“GPS uses the earth as a mechanical battery by storing energy as pressurized water between layers of rock. The objective is to lower cost associated with long-duration energy storage by 50-75% to enable more reliable and cost-effective utilization of renewable electricity generating assets,” they add.

Long Duration Underground Energy Storage Solution To Kill Coal
That’s pretty much it in a nutshell. Although the concept is new enough to attract ARPA-E, the Quidnet model leverages established mechanical systems and whatever terrain is at hand, as long as an appropriate rock formation lies beneath the surface.

“Utilizing existing drilling and hydropower machinery supply chains, Quidnet Energy repurposes existing resources to quickly marshal implementable solutions to our most pressing energy and climate challenges,” Quidnet explains.

The company also points out that its underground system is a closed-loop model that minimizes water loss from evaporation.

If all goes according to plan, San Antonio’s CPS Energy will serve as a high profile showcase for replication. The ARPA-E funding will enable Quidnet to scale up its pilot-stage system to a 1-megawatt, 10 megawatt-hour commercial system that can deliver electricity for 10 hours or more.

That 10-hour time frame is an essential part of the Energy Department’s efforts to push utility scale energy storage systems beyond the capabilities of lithium-ion battery technology, which hits a wall after several hours.

With long duration energy storage, utilities can integrate more wind and/or solar energy into their grids, without sacrificing reliability.

CPS, for one, is counting on more renewables. Population in the area is growing, and CPS estimates that it will need to add about 115 megawatts in generating capacity annually in the coming years. Meanwhile, the utility aims to retire two of its oldest power plants by 2030, the 830-megawatt Sommers gas plant and the 1,345 megawatt Spruce coal plant. Three older gas units at the Braunig plant are also slated for closure.

The Water Battery Revolution Is Just Beginning
Over and above the development of innovative new systems, the nation’s existing fleet of pumped storage facilities has room to expand. In 2018, for example, Duke Energy upgraded a 30-year-old pumped hydro system to add another 300 megawatts in capacity.

Floating solar panels onto reservoirs is another way to get more clean power from existing pumped hydro infrastructure. In Germany, for example, the company Vattenfall has embarked on a program to install solar panels on several of its pumped hydro facilities.

In terms of sites for new pumped energy storage facilities, brownfield development can also come into play. In Kentucky, the hydropower firm Rye Energy is eyeballing old coal fields for new pumped hydro reservoirs.

The Swiss company Nant de Drance illustrates another approach to the site selection dilemma. They leveraged underground caverns to construct a massive 900-megawatt pumped hydro system. Use of the existing caverns enabled the company to trim the expense of construction for the underground powerhouse, though the project still took 14 years to complete.

Over here in the US, the Energy Department is also supporting the development of a new, compact underground powerhouse that can fit into a slim well-like structure.

The Energy Department is also counting on closed-loop systems of one kind or another to expand the nation’s pumped hydro capacity. That fleet of 43 existing pumped hydropower facilities consists primarily of open-loop systems. The Energy Department’s National Renewable Energy Laboratory recently ran a GIS-enabled mapping study of millions of potential sites for new closed-loop systems. After filtering for cost-optimization and other factors, the lab identified thousands of potential sites for new pumped hydro energy storage deployment in the US, including 11,769 sites in the contiguous 48 states and 3,077 sites in Alaska, Hawaii, and Puerto Rico.

Of course, most likely just a small fraction of those sites are actually developable. Not helping much is a coordinated effort by state-level office holders to stem the flow of investor dollars towards renewable energy projects. Still, the National Hydropower Association is holding out hope. In a report issued last year, the organization is looking to add 50 new gigawatts to the nation’s stock of pumped hydro energy storage capacity by 2050.
(photo courtesy of Quidnet Energy).