Much of the power generation equipment now being installed is either for renewable energy (predominately wind and solar) or gas-fired combined cycles that leverage the historic low prices of natural gas. However, many of those gas-fired plants were built with an additional purpose: to serve as a fast-reacting source of electricity that can replace renewable generation when the wind stops blowing or the sun doesn’t shine. A better technology choice for “chasing” wind and solar is bulk electricity storage.
The system planning and grid operation complications posed by renewables were thoroughly discussed in an earlier article (“Energy Storage Enables Just-in-Time Generation,” April 2011, available at powermag.com). The key theme of that article was that utility-scale energy storage must be commercialized before renewable electricity can reach its full potential.
Today, most large energy storage projects appear to be little more than a shipping container filled with tens of thousands of small batteries. The value of energy storage is truly realized with utility-scale systems capable of “moving” bulk wind power produced chiefly during off-peak hours to on-peak hours when the value of the energy is significantly greater. The value is equally important to solar photovoltaic systems that wildly cycle when clouds pass overhead.
Several large-scale energy storage technologies in their early stage of development were discussed in the earlier article. Gravity Power LLC was identified as a company that stood out from the others, principally because its technology fully embraces the KISS principal (Google it) and should be deployable in the near term. In fact, the U.S. Department of Energy took a look at the technology and was unable to fund development work because no research and development was needed.
In the time it takes to read this article, even nontechnologists will intuitively understand how the system works and appreciate its simplicity.
According to Gravity Power CEO Tom Mason, the final design concept took shape in 2008, which was immediately followed by the company’s first round of funding in early 2009 from The Quercus Trust, followed by a series of other small investments. The company is currently raising a Series B round to fund final design and testing of the system. The first Gravity Power patent covering its technology was issued by the U.S. Patent Office in May 2012, and other global patents are pending.
The Gravity Power Module (GPM) is a simple machine (Figure 1). At the heart of the system is a reversible pump-turbine and motor-generator, much like those used in conventional pumped storage hydro (PSH) systems for 70 or more years. Sitting 40 meters (m) below ground is the top of an underground “water circuit” where the pump-turbine is located. The circuit consists of two sealed water-filled vertical shafts. The first is a large-diameter vertically bored shaft called the “power shaft.” The smaller is the “penstock” shaft. Within the power shaft rests a large piston that stores or returns energy when hydraulically moved up or down by water. The water is simply a hydraulic fluid. The water flows in a circuit in both directions: Either water flows within the system loop, pushing the piston up, or the water is being pushed in the other direction by the piston as it drops.
1. Gravity works. The piston (shown in red) moves up and down in the power shaft, depending on its operating mode. Power from the grid is used to pump water (the pump is shown in green) into the power shaft and raise the piston. When electricity is required, the piston drops, forcing water through the pump that now functions as a turbine, producing electricity from the motor that now functions as a generator. The penstock shaft is used to capture and return water to the system. Courtesy: Gravity Power LLC
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