New Scientist (pg. 44)  [Printer-friendly version]
September 29, 2007


A pocket of porous rock beneath the American midwest transform the way
we use wind energy?

By Daniel Pendick.

Daniel Pendick is associate editor at Astronomy magazine in Waukesha,

Just a short drive west out of Des Moines, Iowa, amid fields of corn
and soya, there's a dip in Route 44. Here, near the small community of
Dallas Center, a short gravel road runs north to a cluster of houses
and across the street there's a farm machinery dealer's yard. It seems
to be an unremarkable corner of the Midwest, yet almost a kilometre
beneath that dip in the road is something that could change the way we
use wind power. If all goes to plan, it could allow the world's most
appealing renewable energy source to compete head-to-head with fossil
fuels as a way of generating electricity.

My guide for the day is one of the architects of this project, the
Iowa Stored Energy Park (ISEP), and he is happy to pull off the road
for me to take a few snaps of the gently rolling terrain. Even his
name seems to promote the project: Thomas A. Wind. No, really, that's
his name. A consultant engineer whose family farm is close to
Jefferson, some 50 kilometres away, Tom Wind now leases out his land
so that he can devote his time to ISEP and other energy projects. He
is a consultant for the Iowa Association of Municipal Utilities
(IAMU), a consortium of more than 600 utility companies from across
the state.

IAMU plans to transform a sandstone aquifer beneath Route 44 into a
giant battery for storing energy from the wind. At night, when wind
turbines produce power nobody needs, the electricity will be used to
compress air and pump it into the aquifer, creating a huge pressurised
bubble. During the day, when demand for power rises, the compressed
air will be piped backed to the surface where its energy will be
converted into electricity.

If the project comes to fruition it will be a world first, capable of
delivering some 268 megawatts of electricity for 16 hours each day.
That's enough energy to satisfy the needs of about 75,000 homes. The
technology aims to tackle the big complaint that wind energy always
faces: the wind doesn't necessarily blow when you want it to. With
compressed air storage, it will be possible to store power from Iowa's
growing wind generation capacity and then turn it on and off like
water from a reservoir, available to customers when needed -- and when
they are prepared to pay the highest price for it. A power source that
the energy industry has till now viewed as fickle will become firm and

The energy park project grew out of a study Tom Wind conducted for
IAMU in 2002 to assess what sort of generating capacity the utilities
would need to serve future demand. The study found they would need
more power to fill in between daily peaks and valleys in usage -- so-
called "intermediate load".

"There's a certain amount of power you need 24 hours a day, 365 days a
year just to keep everything running," Kent Holst tells me. He is
development director for the Iowa Stored Energy Park Agency, which has
the task of managing the project and raising the funds needed to make
it happen.

In Iowa, as in most places in the US, coal plants supply the 24-hour-
a-day "baseload" power. To maximise the efficiency of these plants,
utilities try to keep them running at a constant rate. When demand
peaks -- as it does on hot days when everybody flips on their air
conditioning -- many utilities have to fire up expensive diesel
generators. That happens for about 200 hours a year. "In between,"
Holst says, "something has to meet a variable load for about 1000
hours a year."

The usual strategy is to hold "spinning reserve" power for that
intermediate load -- some form of generation such as gas turbines that
can be cycled up or down quickly to meet spikes in demand. But what
form of generation to use? IAMU and Tom Wind realised that what Iowa
would have more and more of is wind turbines. Driven by a combination
of federal tax breaks, concern over global warming and favourable wind
conditions, wind farms are sprouting all over the state. But wind
power is notoriously unreliable, and as a result the industry operates
on a rule of thumb that you shouldn't have more than about 20 per cent
of generating capacity as wind. Something else -- generally coal and
natural gas -- has to fill the gaps when the wind isn't blowing.
Install too much wind capacity and problems can arise. If the wind
drops unexpectedly, for example, energy output can fall rapidly and
the grid must be able to compensate for any variations in, say,
voltage or frequency that this causes.

The way round this problem, Tom Wind's report argued, is a technique
known as compressed air energy storage (CAES). "We figured we were
going to end up with a lot of wind energy in Iowa, so we thought we
would be needing something like this to use wind energy more
effectively." In the jargon of the power industry, CAES makes wind
"dispatchable". IAMU would use as much wind energy as possible at
night to compress air, store it underground, and then tap it during
the day to meet fluctuating demand. "What storage really does is let
you use more wind than you could otherwise," Tom Wind says. What's
more, CAES can be used to store cheap off-peak electricity from any
source and sell it on the market for a higher price when demand rises.

In principle, it ought to be possible to use the compressed air to
spin an electric generator directly, but in practice that is not the
most efficient way of exploiting it. To make the most of the stored
energy, the energy park will install two 134-megawatt gas turbines
adapted from conventional units used in gas-fired power stations. In a
conventional gas turbine, compressor fans squeeze air into the
combustion chamber at high pressure, where fuel is burned to produce
hot exhaust gases that spin a set of turbine blades at high speed. The
turbine in turn drives the electric generator, and also the compressor
that squeezes air into the combustion chamber.

Though the compressor typically consumes between a half and two-thirds
of the power available from the turbine, the high-pressure environment
makes the unit more efficient overall. In the CAES plant, the
compressed air from the underground store creates high pressure in the
combustion chamber without the need for a power-sapping compressor. As
a result, the turbine generates two to three times as much power from
a given amount of fuel.

Although CAES is not widely used, two large plants have between them
built up decades of operating experience. The first came on stream in
1978 in Huntorf, Germany. The 290-megawatt plant stores compressed air
in two deep salt caverns. Eight hours of compressed-air "charge" is
enough to run the generators at full power for 2 hours.

The second plant, in McIntosh, Alabama, was commissioned in 1991 by
the Alabama Electric Cooperative. It stores its compressed air in a
mined-out salt dome 80 metres across and 300 metres tall, lying 450
metres below ground, and can use the air to supply a turbine
generating 110 megawatts of electrical power continuously for some 26

Giant bubble

At the Iowa plant, the compressed air will be stored in a porous
sandstone aquifer rather than a cavern. This has the advantage that
the pressure of the stored air is kept constant, regardless of whether
the reservoir is full or almost empty. As air is pumped into the
aquifer it displaces water around it, and because this doesn't change
the hydrostatic pressure of the water the pressure of the air remains
constant too. "You can optimise your equipment for better efficiency
if you have a constant pressure," Tom Wind says.

There is a downside to using an aquifer, though: the porous water-
bearing rock needs to be deep enough underground to provide the
pressure needed to run a turbine, and be contained by a dome-shaped
cap rock that retains the bubble of compressed air.

Fortunately for the ISEP team, these conditions are identical to those
needed for storing natural gas. Northern Natural Gas and other utility
companies have made detailed maps of Iowa's geology that have allowed
the search to be narrowed to three candidate formations. Even so, the
ISEP agency has had to invest hundreds of thousands of dollars -
provided by power companies and the US Department of Energy -- to
narrow down the potential sites. Two proved unsuitable because they
lacked a containing cap over the water-bearing sandstone. Preliminary
seismic surveys of the third one -- the aquifer below Dallas Center -
look good, says Holst. In early 2007, the team got confirmation that
the site is large enough and deep enough to be useful. It is also
capped by a suitable rock structure .

Computer models based on the porosity of the rock show that 13
boreholes into the aquifer should be enough to get the compressed air
in and out fast enough. The next step will be to make sure the aquifer
doesn't contain minerals such as pyrites that could combine with
oxygen in the stored air, and so inhibit combustion in the gas

As with so many renewable energy projects, funding remains the
sticking point. A huge 2700-megawatt CAES project proposed in Norton,
Ohio -- using an abandoned limestone mine as the air storage reservoir
- has already stalled for lack of finance. ISEP has a $200,000
government grant to keep the project moving, but failed to get a grant
from the 2007 federal budget to help cover the $1.5 million funding it
needs to complete the study of the aquifer.

After that it will need $200 million to build the plant. The initial
design studies are planned for early 2008. Current plans call for 268
megawatts of generating capacity drawing on power from new wind
generators rated at 75 megawatts. The wind turbines do not have to be
on site. The electricity they generate can be imported over the grid
from anywhere in Iowa or beyond, and ISEP will also buy cheap off-peak
power from non-wind sources. The team suggest that for every megawatt-
hour of wind energy used to compress air and store it underground,
about 850 kilowatt-hours are recovered when the air is used to operate
the turbines. They say ISEP could deliver electricity to consumers for
about 4.5 cents per kilowatt-hour -- about the price of electricity
from conventional power stations.

Though there are several other CAES projects under consideration
across the US, ISEP remains a one-off: no one else is contemplating
storing wind power in this way. Though Tom Wind remains enthusiastic
about the project, even he admits that going it alone can sometimes
lead to doubts. "We ask ourselves all the time: if this is such a good
thing, how come nobody else is doing it?" he confesses. "You start to
wonder when nobody is lining up behind you."

Perhaps it is simply a case of nobody wanting to be first to take the
plunge, as a number of recent analyses suggest that wind farms
combined with CAES should compete favourably with conventional energy
generation systems. One report on the potential for wind energy with
CAES in Texas, Oklahoma and New Mexico calculates that the operational
costs of a CAES plant in the region could be less than that of a
conventional gas or coal-fired unit. The economics could be even more
attractive in future if the government starts to tax carbon emissions.
"That's where our project starts to shine," Tom Wind says.

An analysis published in Energy Policy (vol 35, p 1474), suggests that
if emitting greenhouse gases is made costly enough, wind energy
combined with CAES will become an economical way to supply baseload
power to the grid . It estimates that this combination could provide
over 80 per cent of the energy on the grid, while cutting greenhouse
gas emissions by three-quarters compared with typical gas-fired power

Aquifer storage might not be a suitable solution everywhere. In
densely populated areas, where the need for energy is greatest, the
water stored in aquifers is a precious resource in its own right,
especially in the American Midwest. Fortunately, this is not a factor
for ISEP because it is using an aquifer that is not required for
drinking or irrigation, at least for now. However, Holst says there
could be problems elsewhere if the CAES portion of an aquifer were
placed close to existing wells. When air is stored or removed, it
could affect flow in the wells. Faced with plans to use local aquifers
for a CAES scheme, communities might end up having to weigh their
demand for power against the need to safeguard water supplies.

Before I leave Dallas Center, Tom Wind drives me north to a line of
seven wind turbines that have recently gone up near his hometown of
Jefferson. Standing more than 100 metres tall from the ground to the
tip of the blades, they are visible across the fields from a full 10
kilometres away. He then points to a concrete cistern next to an old
farmhouse. Farmers, he tells me, used small windmills initially to
pump up groundwater and, more recently, to power radios. They stored
the water in cisterns for use in times when the wind didn't blow, and
charged lead-acid batteries to keep their radios running. So ISEP, he
says, is just a modern take on what farmers have been doing for more
than a century. This time round, perhaps, the idea could provide the
gateway to clean energy for all.


Daniel Pendick

Off-peak electricity is cheap and plentiful, so it pays to store it
for use when supplies are scarce and it can be sold at a good price.
There are several tried and tested ways of doing this:

PUMPED HYDRO STORAGE: Off-peak electricity is used to pump water into
reservoirs. When demand peaks, the water drives generator turbines. A
single site can store gigawatt-hours of energy.

FLOW BATTERIES: Electricity is stored as chemical energy in solutions
held in large tanks. The technology is scalable and can store more
than 100 megawatt-hours of energy at a single site.

FLYWHEEL ENERGY STORAGE: Using electric motors to spin up a flywheel
to as much as 80,000 rpm can store up to 150 kilowatt-hours as kinetic

magnetic field, generated by large currents circulating in a
superconducting coil. Superconductors need to be held at low
temperatures so the technology remains expensive to build. Less than
100 megawatt-hours of SMES storage is installed worldwide.

HYDROGEN: Electricity is used to split water molecules to produce
hydrogen, which is then burned to generate electricity when needed.
Projects in the UK and on Prince Edward Island in Canada will use wind
turbines to generate hydrogen, which will then be stored in tanks for
use as fuel.

Daniel Pendick is associate editor at Astronomy magazine in Waukesha,

Copyright 2007 Reed Business Information, UK, a division of Reed
Elsevier Inc.