Pumped storage hydropower (PSH) is really the only large-scale electricity storage technology widely used today, with over 120,000 megawatts of capacity worldwide. There's just a few hundred megawatts of other energy storage technologies deployed globally in the form of compressed air storage, sodium-sulphur, lead-acid, nickel-cadmium, and redox flow batteries.  

Most pumped-hydro was originally intended to work with baseload power plants like coal and nuclear.

A new pumped-hydro installation can take more than a decade and cost billions before a single watt of power is banked. It also has daunting siting limitations -- you need two large reservoirs at different elevations and a willing utility commission and environmental community.  And lots of water resources.

I spoke with two entrepreneurs with a different take on pumped hydro, one that eliminates some of the siting and geographical limitations of the technology.  It's an enormous engineering project and not without its own spectacular challenges.

Jim Fiske is the CEO and Chris Grieco the EVP of Gravity Power, a venture-backed energy storage startup with about $2 million in funding from the Quercus Trust and 21 Ventures. (Quercus Trust has invested in more than 30 green firms, including Solar Tower firm EnviroMission.)

Instead of reservoir-based pumped hydro, Gravity Power is going underground.

Fiske was originally working on a project to develop high-speed flywheels for energy storage applications.  But in the words of the CEO, "As we got deeper into the project, we realized that high-speed flywheels are never going to be really cheap. Flywheels work right at the edge of the limits of materials."

"Because they are conceptually easy, it's easy to think they are actually easy [to develop] -- but they are not," he added.

Fiske also learned that the real market opportunity is in large-scale storage.  And he claims that Gravity Power's design is much higher in capacity than what most of the other technologies are capable of.

Fiske restates what we've covered numerous times: "Today we have the problem of integrating renewables. The problem is that renewables are too variable."

Fiske surveyed the technologies available to store large amounts of power.  "Batteries weren't a good choice for me. CAES has some utility but also has some issues.  And in the U.S., pumped hydro is very difficult to get permits and environmental approvals for."   He cited the Lake Elsinore Advanced Pumped Storage, a project started in 1987 on which construction has not yet begun.

He started looking for a way to use gravity and elevation to get around those obstacles.

According to the CEO, their proposed Gravity Power Module "circumvents most of the problems with pumped storage."  

Gravity Power's entire system is underground and doesn't require lots of land.  A seven-acre site can easily accommodate more than two gigawatts, according to Fiske.  When asked about seismic issues, Fiske said that the only time seismic is going to be critical is during construction. The finished system is a deep shaft lined with heavy reinforced concrete and then filled with water. He assured me that "It's going to be quite sturdy."

The Gravity Power Module (GPM) is a vertical column excavated hundreds of feet into the earth.  An immense weight rests on a column of water which is raised, like a piston, to store energy and lowered to discharge energy using an additional return pipe. Here's a link to the patent application.

The pressure depends on the vertical dimension of the "heavy-concrete" weight moved up and down, while the distance traveled dictates the storage time available.  When energy is cheap or available from variable sources, you pump, and then move the weight and water column upwards.  Releasing the weight drives water through a turbine and produces power when needed or when costly.  The energy coming in from the grid could be a wind farm or solar farm or nuclear power plant.

Round trip efficiency is about the same as conventional pumped hydro energy storage according to Grieco -- in the range of 75 percent to 80 percent. The cost of these machines is difficult to determine.  "Cost is very dependent on where we put these things" and "local labor," according to the CEO.  He envisions building them in clusters.
 
"If you look at hydropower systems, every turbine is a one-off."  Grieco believes that their design allows Gravity Power's suppliers to mass produce turbines with a similar cost structure to that of pumped storage.  We think we can get to $150 per kilowatt kilowatt-hour for installed capacity.  The startup envisions the largest of the single shafts to be able to store more than 50 megawatts for four hours, which equates to 200-megawatt-hours of storage.

Energy storage comes in a variety of flavors, ranging from ancillary services of short duration to diurnal energy arbitrage.  Gravity Power can serve a variety of these energy storage applications.

2011 could see construction start for a commercial prototype, possibly in Texas.

Gravity Power is collaborating with the Robbins Company, the inventors of the "mole machine," to adapt that technology for vertical tunnel boring. "Those machines are incredibly powerful and incredibly fast," said Grieco, "with tunneling speeds of up to 115 meters in 24 hours."

The CEO sees his competition as peaker plants.  As with all new approaches, this project will likely require more time, more money and reassessment of initial assumptions along the way.  Symbiotics is another firm working on pumped hydro in the more conventional sense.