POWER April 2011 - 33

FUTURE POWER
Electrolyzed Water with CO2
Other startup companies are exploring similar
chemical processes that could be adapted
for electricity storage. Doty Energy is
pursuing an approach similar to ZSW's to
produce synthetic fuels in a process it calls
WindFuels. Doty proposes to use off-peak
wind energy to produce fuels such as gasoline
and diesel from carbon dioxide (CO2
)
and water. Electricity is used in an electrolysis
process to break the molecular bond of
water to form hydrogen (H2
) and oxygen
(O2). The hydrogen is then combined with
CO2 in a reverse water gas shift reaction to
reform CO2
into H2
(CO). Finally, the CO and H2
O and carbon monoxide
are chemically
reformed into liquid hydrocarbon products
in a process Doty Energy calls a Renewable
Fisher Tropsch Synthesis system.
The overall conversion efficiency is on the
order of 60% based on this combination of
processes with a long history in the chemical
process industry. Doty Energy states
that WindFuels are economic with oil prices
above $80/barrel and off-peak energy at $16/
MWh. The company is currently looking for
funding for a first demonstration project.
Electricity as Compressed Air
Compressed air energy storage (CAES) has
long been recognized as an effective means
of storing electricity generated off-peak
that can be released during high-electricitydemand
periods. The technology has been
well-thought-out since the early 1960s,
when combustion turbine technology progressed
to the point where the system was
technically feasible. However, plant construction
has not advanced nearly as far as
the technology. Today, there are only two
systems in commercial operation: The 290MW
Huntorf plant in Germany that entered
service in 1978 and the 110-MW Alabama
Electric Corp. plant in McIntosh, Ala., commissioned
in 1991.
Many other sites have been identified by
developers as potential CAES candidates, but
no projects are close to construction at this
time. (See " The Return of Compressed Air
Energy Storage, " October 2008 and " Could
CAES Answer Wind Reliability Concerns? "
December 2010 for the technical details of a
typical CAES system.)
After more than 20 years of CAES design
and construction inactivity, the Thayer
School of Engineering at Dartmouth College
in 2007 developed a modernized and
more efficient CAES plant design. The college
has recently spun off a development
company called SustainX, supported by
funding from the National Science Foundation
Small Business Innovation Research
Program plus equity investments from PoApril
2011 | POWER
laris Venture Partners, Rockport Capital,
and General Electric, as part of its GE
Ecomagination Challenge.
The new approach, which SustainX calls
ICAES (isothermal compressed-air energy
storage), uses electrical energy to compress
air near-isothermally, stores it aboveground
in commercial gas storage facilities, and expands
it near-isothermally to generate electricity
using no fossil fuels. To store energy,
SustainX uses off-peak wind energy electricity,
for example, to drive a reciprocating
compressor outfitted with a water spray to
keep the air at a constant temperature during
compression from atmospheric pressure up to
a maximum of 3,000 psig. During compression,
the sprayed water absorbs heat from the
air, effectively improving the compression
efficiency. The compressed air is then stored
in aboveground storage vessels and the heated
water is stored in a constant-temperature
reservoir (Figure 3).
The developers estimate a seven-fold reduction
in storage costs compared with conventional
CAES, which is not surprising,
because there is no massive solution-mined
salt dome to construct. To produce energy
from aboveground storage, compressed
air is used to drive the same compressor
in reverse, which in turn drives an electric
generator. Warm water from the reservoir
is sprayed into the air during expansion to
keep the air from cooling. Keeping the air
at a constant temperature during compression,
storage, and expansion yields a more
efficient storage cycle.
In a November 23, 2010, interview with
Smart Grid Today, SustainX CEO Tom
Zarrella estimated that the electricity storage
market will grow to a $200 billion
business in coming years. Zarrella also
noted that the SustainX technology approach
could cost less than half as much
as traditional CAES systems because it
" does not have to be paired with a secondary
heat source such as a combustion
turbine. " The SustainX design generates
electricity directly from the expansion
of the compressed gas itself, without any
external fuel source. The big advantage is
that any aboveground gas storage facility
can be used, eliminating the requirement
for conventional CAES to be adjacent to a
large underground salt cavern.
SustainX announced in November 2010
that the company will bring its technology to
market in 2012, although the particulars of
the first project were not revealed.
2. Two-step process. A team of German and Austrian researchers proposes producing
natural gas in a two-step process: electrolysis of water using surplus wind or solar energy followed
by methanation of the hydrogen with carbon dioxide to produce a synthetic natural gas.
The natural gas would then be added to the existing system of pipelines, effectively storing the
gas. Source: Center for Solar Energy and Hydrogen Research Baden-Württemberg
Electricity
network
Wind
Solar
Other
renewables
From fossil fuels,
biomass, waste,
atmosphere
CO2
Combined heat
and power
turbines
Power generation
Power storage
H2O O2
H2 tank
CO2 tank
Electrolysis
H2
CO2
Methanation
CH4
H2O
Renewable power methane plant
3. Aboveground compressed air storage. A key enabler of the SustainX technology
is isothermal (constant temperature) compression and expansion of the gas rather than the
adiabatic (no heat transfer) processes used by other CAES techniques. Thermal efficiencies in
excess of 90% have been achieved experimentally by SustainX, compared with ~50% thermodynamic
efficiencies for adiabatic techniques over the same pressure range. Source: SustainX
Electric
motor
Hydraulic
pump
Isothermal
compression
High-pressure
gas storage
Isothermal
expansion
Hydraulic
motor
Electric
generator
Natural gas
network
For heat and
for transport
Gas storage
Wind-methane
Solar-methane
Compression cycle
Expansion cycle
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POWER April 2011

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