POWER April 2011 - 34

FUTURE POWER
Electricity as Thermal Energy
A number of R&D development paths have
as a primary goal the storage of thermal energy
as an electricity equivalent. The main
differences between the technologies are
whether the thermal energy is stored as lowtemperature
ice or in higher-temperature
forms of energy.
One interesting low-temperature technology
is distributed ice-based energy storage
that uses less-expensive off-peak electricity
to produce and store thermal energy for use
during peak demand periods. The ice-production
technology is familiar and has been
around for many years. What's unique is
that ice production and conventional air conditioners
have been combined into a single
system. And it is just recently that the technology
has reached utility scale.
Ice Energy is currently constructing a 53MW
project, the largest ice-based energy
storage project of its kind, for the Southern
California Public Power Authority (SCPPA),
a joint powers authority consisting of 10 municipal
utilities and one irrigation district.
The goal is to " permanently reduce California's
peak energy demand by shifting as
much as 64 gigawatt hours of on-peak electrical
consumption to off-peak periods every
year, reducing exposure to costly peak power
and improving the reliability of the electrical
grid, " according to a company fact sheet.
The project is called " distributed " because
it consists of more than 6,500 " Ice
Bear " energy storage units installed at approximately
2,000 sites throughout Southern
California. Each ice storage unit operates
in tandem with a standard air conditioning
unit. It essentially replaces on-peak electricity
demand, when air conditioning is usually
required, with up to six hours of air
conditioning provided by melting ice that
was produced using off-peak electricity. Ice
Energy states that daytime energy demand
from air conditioning-typically 40% to
50% of an average commercial building's
electricity use during peak hours-can be
reduced by as much as 95%.
According to the SCPPA, all the systems
can be centrally controlled to " provide a
single manageable utility-scale resource that
can permanently reshape system load. " The
units are being installed over a 24-month period
that began in mid-2010.
Underground Pumped Storage
Gravity Power, an energy storage spin-off
of LaunchPoint Technologies, is proposing
to avoid the resistance to constructing conventional
pumped hydro storage facilities by
going underground. CEO Jim Fiske says that
by doing so, the company's proposed Gravity
Power Module (GPM) " circumvents most of
the problems with pumped storage. "
4. Pumping weights. Gravity Power proposes boring holes in the ground, perhaps thousands
of feet deep, and filling them with water and a large weight. Surplus electricity would be
used to raise the weight, creating a form of underground pumped storage. The weight would be
dropped during peak periods to produce electricity in a turbine. Source: Gravity Power
Generation
Substation
Motor/
generator
Pump-turbine
water flow
Ground level
Weight
Return
pipe
Return
pipe
generator
Substation
Storage
Pump-turbine
Motor/
The concept is brilliant in its simplicity.
GPM construction begins with a large borehole
drilled straight down into the ground,
perhaps thousands of feet for a utility-scale
system. At the bottom of the shaft is a large
concrete piston fitted to the shaft, called the
" weight stack. " Also bored into the ground is
a parallel but smaller-diameter " return pipe "
that is connected to the main shaft at the top
and bottom. Finally, the entire volume is
filled with water and tightly sealed-air is
compressible and its presence reduces the
system effectiveness. In essence, the position
of the weight stack in the shaft determines
the amount of energy stored.
During the energy storage process, off-peak
water flow
Deep
storage
pipe
Deep
storage
pipe
Weight
electricity is used to power a pump that pushes
water down the return pipe that will raise the
weight stack from the bottom of the deep storage
shaft. During a peak electricity demand period,
the weight stack is released, which pushes
the water up the return pipe, reversing the direction
of rotation of the pump-turbine and producing
electricity, much as in a typical pumped
storage hydroelectric plant (Figure 4).
According to Fiske, a 7-acre site can accommodate
more than 2 GW of installed
power storage, depending on the depth and
diameter of the storage shaft. The conversion
efficiency of the system is described as
in the 75% to 80% range. The installed cost
is estimated by the company as around $150/
kWh for a system capable of storing about
200 MWh by using a standard pump-turbine
design to reduce costs. (The installed cost of
a conventional pumped hydro storage facility
is around $100/kWh, based on a reservoir
capacity of around 10 hours and a first cost of
$1,000/kW. Compressed air energy storage is
also estimated at about $100/kWh, and this
is a capital cost, not a cost of electricity at
the meter.) The typical product is envisioned
to have a 6-meter (m)-diameter storage shaft
and a 2-m-diameter return pipe, 500 m deep,
with an 8,000-metric ton storage mass for an
8.5-MWh module. Fiske envisions the systems
to be installed in clusters to produce the
amount of energy desired.
Development of the GPM appears to be
progressing quickly. A small test unit is said
to be operating in Santa Barbara, Calif., and
the company says a utility-scale project is expected
to be online as early as 2013, possibly
in Texas, where the geology is ideal. Fiske
says that key investors are in place for this
project, and there has been much interest in
the technology by countries such as China,
India, and South Africa, although the U.S.
DOE showed little interest.
Pumped Heat Electricity Storage
A potential electricity storage gamechanger
is United Kingdom startup Isen34
www.powermag.com
POWER
| April 2011

http://www.powermag.com

POWER April 2011

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