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Tom Schueneman headshot

Gravity Power Module Revolutionizes Pumped Hydro Energy Storage

Energy storage is arguably the Achilles heal of renewable energy.

Matching the diurnal nature of the wind and the sun with the on-demand, baseload nature of human energy consumption requires energy storage that is scalable to Gigawatts, reliable, efficient, economically viable, and environmentally benign.

The only method of energy storage with any significant deployment is pumped hydro, with more than 120,000 megawatts installed globally. But the last large scale pumped hydro project went online more than thirty years ago, and almost all of that capacity is used as "peaking" power for baseload sources like coal, oil, and nuclear power.

Other technologies, such as batteriesthermal energy, or compressed air storage are considered candidates for renewable energy storage. But these solutions can be costly, difficult to scale to grid-sized operations, and are fraught with environmental concerns. And so it is with conventional pumped hydro.

Typically, a large scale pumped hydro project requires an investment of up to $3 billion dollars before investors see a return, taking up to fifteen years, with an equally large environmental footprint requiring two large bodies of water at differing elevations.

There are some visionary plans for the pumped hydro concept, such as the Green Power Island from Danish architectural firm Gottlieb Paludan. But such grand schemes remain purely conceptual and thus far fail to address the core issue: pumped-hydro remains environmentally and economically costly.

Gravity Power LLC, a startup based in Santa Barbara, California, has a plan to turn pumped hydro on its head - literally.

Pumped hydro and the genius of simplicity

In all its elements, Gravity Power Module (GPM) is simple. It's the unique application of known mechanical engineering principals that is the innovative key to GPM.

At the heart of the system is a reversible pump-turbine and motor-generator sitting atop an underground "water circuit" consisting of two sealed water-filled vertical shafts - a main bore called the "storage pipe" and a secondary "return pipe" bore. Within the main bore rests a large piston called the "weight stack." The weight stack is the means of storing energy. Water flows through the two sealed bores at the top and bottom, circulating within the system around the weight stack.

Drawing on grid power the reversible drive turbine pumps the water down through the return pipe and up the main bore below the stack, lifting the stack to store energy

To release stored energy when power demand exceeds supply, the weight stack descends, forcing the water up through the return pipe, driving the pump-turbine, and generating electricity in the process. The speed the stack drops determines the power of the GPM; the length of the storage pipe and the mass of the stack determines the amount of energy stored in a GPM.

A "peaking" capacity GPM facility will have a drive shaft 10 meters (33 feet) in diameter and 1000-2000 meters deep (about 3200 to over 6500 feet deep). Easily clustered together, eight such shafts could provide up to 150 megawatts each for four hours. Up to sixteen GPMs clustered together could supply a total of 2400MW on 2.5 acres of land - essentially 1 gigawatt per acre.

Modular, scalable, and efficient

The pump-turbine can ramp from zero to full power in about 20 seconds. With a modest drop rate of 0.2 to 0.4 meters per second, piston friction is minimal. Efficiencies of 75 to 80 percent can be achieved, says Jim Fiske, Founder and CTO, over a broad power range, from smaller ancillary service applications to full-scale peaking plants. GPMs modularity makes it highly scalable and individual units can be immediately brought online once each GPM is constructed.

Using readily available, low cost materials (cement, iron ore, steel) makes for rapid construction, with standardized components manufactured offsite and requiring no additional factory capacity. The key determinants of cost are local labor and the ability to sink deep shafts quickly and efficiently. Shaft construction is a main focus of current project R&D and a key partner in the development of the GPM is the Robbins Company, pioneers in automated, high-speed tunnel boring technology.

Siting of GPM is highly flexible. With a small and relatively unobtrusive footprint, a multi-shaft installation can be built even in dense urban areas, or near sources of renewable energy, or wherever economics and engineering dictates is best.

Fighting institutional inertia and the myth of "shovel ready"

Interest in GPM is steadily growing, especially offshore in places like China, India, South Africa and the Middle East. But Jim Fiske admits to a certain amount of "indifference" from "institutional inertia" here in the United States. Fiske sees one reason is the clamor for "shovel ready" projects to fund - a misleading concept when funding for innovation and development of new ideas is what is needed before anyone can get out their shovel and move forward building a clean energy economy. "We just have to prove things to people," says Fiske.

Proving the concept

And Gravity Power is working hard for the opportunity to prove itself. Gravity Power received its first round of funding in early 2009 from The Quercus Trust and 21Ventures, LLC, and is currently raising a Series B round of financing.  A proof-of-concept 50kW test unit will be built near company headquarters in Santa Barbara. Fiske is looking for opportunities to build a commercial grid scale facility by 2013. There are tentative plans for a project in Texas, where the limestone geology is ideal and wind generation is abundant. But if not Texas, Fiske is confident a scaled-up version of GPM will be built by 2013, though it may be built in China, India, South Africa or the Middle East first, he says, where interest in the GPM is keen and inertia is less.

Get big

It is with economies of scale that GPM can address the needs of energy storage in the coming decades. When I spoke with Fiske about GPM, I asked about his vision for Gravity Power, LLC. "It needs to be big," he told me. "We want to be the next Calpine."

"I've always found it puzzling," says Fiske. "When I hear people say energy storage isn't cost effective." He aims to prove those people wrong with the Gravity Power Module. And the new big news for Gravity Power is that it has hired Thomas R. Mason, who was Executive Vice President of Calpine and who oversaw the engineering, construction, operation and maintenance of over 27 GW of power generation. ”My hope and expectation is that this technology will globally change the way that utilities plan and operate their systems,” said Mason.

Want to know more? Get the answers from the founders

I've barely scratched the surface with GPM.  To find out more about Gravity Power's technology, business model, pump-turbine design, FERC regulatory requirements (Fiske says GPM doesn't fall under FERC regulations), and ongoing R&D, leave your comments here. Jim Fiske, CTO, and Chris Grieco, EVP, are available to answer your questions.


Tom Schueneman has been a contributor to Triple Pundit since October of 2007. He is the founder of GloblaWarmingisReal.com, and a citizen-advocate of the clean energy economy. Tom also provides information for homeowners interested in building their own solar panels.

Thomas Schueneman headshotThomas Schueneman

Tom is the founder, editor, and publisher of GlobalWarmingisReal.com and the TDS Environmental Media Network. He has been a contributor for Triple Pundit since 2007. Tom has also written for Slate, Earth911, the Pepsico Foundation, Cleantechnia, Planetsave, and many other sustainability-focused publications. He is a member of the Society of Environmental Journalists

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