POWER August 2013 - 66

ENERGY STORAGE
Let Gravity Store the Energy
Gravity Power LLC-a startup based in Santa Barbara, California-has developed
a low-cost, quick-start, and fast dynamic response energy storage
technology that competes with classical pumped storage hydro and gas
turbines for peaking and intermediate duty power generation. The system
is simple, yet its potential is profound.
By Dr. Robert Peltier, PE
M
uch 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.
66
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
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.
Simple Technology
The Gravity Power Module (GPM) is a simple
machine (Figure 1). At the heart of the system
www.powermag.com
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
POWER | August 2013

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POWER August 2013

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