POWER November 2013 - 14

reverses and the air expands, driving a generator. According to
the company, the isothermal gas compression is thermodynamically
ideal-that is, it requires the least possible work-and by
definition avoids temperature extremes. An isothermal expansion
process is also ideal, recovering the most possible work from the
compressed air.
However, while " perfectly isothermal " compression or expansion
is not practical, the company says, a gas can be expanded
or compressed near-isothermally if heat exchange occurs quickly
enough relative to density change. That's why the company uses
an aqueous foam-based heat exchange in its ICAES system. The
patented two-phase heat transfer process enables near-isothermal
gas expansion and compression between 1 atmosphere and
200 atmospheres with only two stages and at scales and speeds
appropriate for large-engine reciprocating machinery, it says.
" Rapid heat exchange between liquid and air has allowed development
of a megawatt-scale compressor/expander with [more
than] 95% isothermal efficiency over a large operating range and
at large-engine stroke speed, keeping the temperature change
of the liquid-air mixture to under 50C across the full operating
range of the system, " it claims.
SustainX's isothermal process was developed by the Thayer
School of Engineering at Dartmouth College in 2007 with funding
from the National Science Foundation Small Business Innovation
Research Program plus equity investments from Polaris Venture
Partners, Rockport Capital, and General Electric, as part of its GE
Ecomagination Challenge.
Study: Wind Power Curtailment More
Cost-Efficient Than Storage
A new study from Stanford University suggests that, if the overall
amounts of fuel and electricity required to build and operate
energy storage technologies are factored in, grid-scale batteries
make sense for storing surplus solar energy, but not for wind.
The study published in the online edition of the journal Energy
and Environmental Science compares the energetic costs of
building and maintaining several emerging technology systems,
including five battery types: lead-acid, lithium-ion, sodium-sulfur,
vanadium-redox, and zinc-bromine. In a previous study conducted
by lead author Charles Barnhart, a postdoctoral scholar at
Stanford's Global Climate and Energy Project (GCEP), found that
lead-acid batteries have the highest energetic cost and lithiumion
the lowest. " We calculated how much energy is used over the
full lifecycle of the battery-from the mining of raw materials to
the installation of the finished device, " Barnhart said. " Batteries
with high energetic cost consume more fossil fuels and therefore
release more carbon dioxide over their lifetime. If a battery's energetic
cost is too high, its overall contribution to global warming
could negate the environmental benefits of the wind or solar
farm it was supposed to support. "
What's more, while wind turbines and photovoltaics were both
found to deliver more energy than it takes to build and maintain
them, the scientists' calculations showed that the overall
energetic cost of wind turbines is much lower than that of conventional
solar panels, which require energy from fossil fuels for
processing silicon and fabricating other components. And when
the energetic costs of curtailment-the practice of shutting
down solar panels and wind turbines to reduce the production of
surplus electricity on the grid-were compared to the energetic
cost of grid-scale storage, the scientists found that the amount
of energy required to create a solar farm is comparable to the
14
energy used to build each of the five battery technologies. The
researchers' calculations were based on a formula known as " energy
return on investment " -the amount of energy produced by
a technology, divided by the amount of energy it takes to build
and maintain that technology.
However, the results were quite different for wind farms (Figure
4). The scientists found that curtailing wind power reduces the
energy return on investment by 10%. Storing surplus wind-generated
electricity in batteries results in even greater reductions-
from about 20% for lithium-ion batteries to more than 50% for
lead-acid batteries, they found. " Therefore, it would actually be
more energetically efficient to shut down a wind turbine than to
store the surplus electricity it generates, " said GCEP postdoctoral
scholar Michael Dale, a coauthor of the study.
One way to improve the energetic performance of storage technologies
is to increase the cycle life of a battery, the scientists
found. Conventional lithium-ion batteries last about four years, or
6,000 charge-discharge cycles. Lead-acid batteries only last about
700 cycles. To efficiently store energy on the grid, batteries must
endure 10,000 to 18,000 cycles, said Barnhart. Another option is
to use pumped hydroelectric storage. " Pumped hydro is used in
99% of grid storage today, " Barnhart said. " It works fantastically
from an energetic perspective for both wind and solar. Its energy
return on investment is 10 times better than conventional batterwww.powermag.com
POWER
| November 2013
4. Stashing wind. Stanford scientists find in a new study that
curtailing wind power reduces the energy return on investment by
10%, but storing surplus wind-generated power results in even greater
reductions-from about 20% for lithium-ion batteries to more than
50% for lead-acid batteries. This image shows AES Energy Storage's
2011-inaugurated Laurel Mountain energy storage project in Belington,
W.Va., which provides frequency regulation in the PJM market to help
manage the rapid rate of change in output that can occur with fluctuations
in wind conditions. The Arlington, Va.-based company in September
began operation of a 40-MW grid storage resource at Dayton
Power and Light's gas-fired Tait Generating Station in Moraine, Ohio.
Courtesy: AES Energy Storage
http://www.powermag.com
POWER November 2013

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