October 1st, 2010 by Tom Schueneman
Gravity power module — a step toward utility-scale energy storage.
The grid — that seemingly amorphous structure that delivers power on demand — requires instantly available power to light up our world. Renewable sources of energy are inherently cyclical and intermittent, making energy storage a primary factor for future development of reliable utility-scale renewable energy sources. Of all the storage systems tried and tested, pumped hydro is the only method in wide commercial use, with more than 120,000 megawatts deployed worldwide. But there is a problem.
A better alternative to energy storage
According to Chris Grieco, executive vice president of Gravity Power, LLC, it has been thirty years since the last large scale pumped hydro project went online. Requiring two large bodies of water at differing elevations, conventional pumped hydro projects carry with them a significant environmental footprint, seriously limiting development and siting options. With permitting and construction of a pumped hydro project taking eleven to fifteen years, investors typically spend up to $3 billion before seeing any return on the investment (or energy stored). All this makes pumped hydro economically risky and practically undesirable for any rapid and aggressive expansion of renewable grid power.
While to date the most widely-used option for renewable grid-scale energy storage, pumped hydro is, in many respects, tapped out. There have been bold and visionary variations on the idea, like the Green Power Island from Danish architectural firm Gottlieb Paludan we recently featured here in PlanetSave. But that doesn’t address the serious environmental impact of large-scale pumped hydro. Grieco sees a much better, simpler way – the Gravity Power Module.
How the Gravity Power Module works
The Gravity Power Module is a closed system using reversible turbine technology and advanced shaft bore construction methods to create a modular energy storage system with a greatly reduced footprint. Siting issues plaguing conventional pumped hydro is eliminated and projects can be deployed much quicker, says Grieco.
The gravity power module begins with a large borehole sunk deep into the ground — thousands of feet for a utility-scale project. Enclosed in the borehole is a sealed, water-filled “storage pipe” within which rests a large (very large) piston called the “weight stack.” Next to this main storage pipe/weight stack assembly is a thinner “return pipe” linked at the top and bottom to the main storage pipe. It is the weight, not the water, that stores the energy. The water volume remains the same within the module, either above or below the weight stack. Tightly sealed within the tube, buoyancy does not determine the position of the weight stack.
At peak demand, electricity is generated as the weight stack pushes water down, like a plunger, to the bottom of the storage pipe. Water is pushed through the link at the bottom and up through the return pipe, driving the pump turbine at the top and supplying power to the grid. As the water pushes through the pump turbine, it is then returned through the top link into the storage pipe above the weight stack.
During low demand, excess power reverses the pump turbine, pushing the stack back to the top, storing the potential mechanical energy for later conversion back into electricity. The concept is scalable, adding modules as needed.
Gravity Power Module utilizes common principals of mechanical engineering. The main challenge in construction is the shaft, which must be built quickly and efficiently to maintain the economic advantage of the concept. At thousands of feet, the geology of the site does play a factor in how easily such a hole can be bored. Limestone is optimal, Grieco explained, but with advanced boring technology, the Gravity Power Module can potentially be built virtually anywhere. A key partner in the development of the project, as one might expect, is an experienced shaft boring company who has developed an advanced boring technology for use in construction the Gravity Power Module.
Scale by 2013
A test unit is currently in operation in Santa Barbara, where Gravity Power, LLC is headquartered, with plans to have a grid-scale commercial project online as soon as 2013, possibly in Texas, where the geology is ideal. Interest in the Gravity Power Module is growing, with firm investment commitments and key partnerships in place to easily meet the current production schedule. With interest from countries like China, India, and South Africa, Greico is confident the Gravity Power Module will find an enthusiastic market (despite the odd coolness he admits GPM has thus far received from the US Department of Energy).
Realism and vision
Grieco and his colleagues are “realist” engineers and businessman. “We’ve fought long and hard,” he says, making the Gravity Power Module an environmentally and economically viable solution for renewable power energy storage.
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