POWER February 2014 - 18

subsurface concentric rings of horizontal
wells inside which CO2
, nitrogen, and water
circulate to draw heat from deep below
ground up to the surface, where it can be
used to turn turbines and generate electricity.
" This well arrangement encircles
the injected fluids with a subsurface hydraulic
dam, functioning much like a hydroelectric
dam. The intent is to recover
the maximum energy benefit from fluid injection
operations, a major improvement
over conventional geothermal power systems, "
noted Tom Buscheck, an LLNL earth
scientist, as the team debuted an expanded
version of the design at a December
American Geophysical Union meeting.
The researchers say CO2
absorbs subterranean
heat more efficiently than water.
Extraction rates are 1.7 to 2.7 times larger
with CO2
than with water because CO2
mass
flow rates are up to five times greater
(given a fixed pressure difference between
injection and production wells). Computer
simulations suggest that a system of four
concentric rings of horizontal wells about
three miles below the ground, with the
outer ring being more than 10 miles in
diameter, has the capacity to produce as
much as 500 MW-much more than the
average 38 MW produced by a conventional
geothermal plant. A plant of that
design might also sequester as much as 15
million tons of CO2
per year.
The approach, which stems from a
design developed by Martin Saar of the
University of Minnesota, adds nitrogen to
the mix to enable highly efficient energy
storage for an unprecedented duration.
" Much of the energy required to drive
the hot fluids out of the deep subsurface
to surface power plants can be shifted
in time to coincide with minimum power
demand or when there is a surplus of renewable
power on the electricity grid, "
Buscheck explained.
Meanwhile, the fledgling technology
has a number of advantages over enhanced
geothermal systems (EGS), which
are essentially man-made reservoirs created
where there is hot rock but little
natural permeability of fluid saturation.
In EGS, fluid is injected into the subsur6.
Deep potential. Researchers say sequestered carbon dioxide (CO2) can be used to
boost power generation by at least 10 times compared to existing geothermal approaches. As
shown, the University of Minnesota-developed CO2 plume geothermal (CPG) system could be
established in deep saline aquifers or components of enhanced oil recovery. Developers say
the approach has a number of advantages over enhanced geothermal systems (EGS). Courtesy:
Saar and Randolph
face under controlled conditions, which
causes pre-existing fractures to open.
The new plant design doesn't require
hydrofracturing of the reservoir, and it
uses lower reservoir temperatures and
pressures. Plus, it would likely sequester
more CO2
with more emphasis on sequestration,
the researchers say.
There are a number of caveats, however.
One is that it would need to be connected,
likely by pipeline, to a large CO2
source, possibly a coal-fired plant fitted
with carbon capture. Buscheck added,
however, that a pilot plant based on this
design could initially be powered solely by
nitrogen injection, in order to prove the
economic viability of using CO2
. The study
, broadening
also showed that this design can work effectively
with or without CO2
where this approach could be deployed.
The research team is currently working
on more detailed computer model simulations
and economic analyses for specific
geologic settings in the U.S.
Startup company Heat Mining Co.,
which was spun off from the University
of Minnesota, holds the worldwide patent
to an earlier form of the researchers' approach,
dubbed " CO2
Plume Geothermal "
(CPG) and is looking to put an operational
project online by 2016. The South Dakota-based
company admits, however, that
its CPG power system, which works " like
a giant pressure cooker " to vent pressure
and reduce the risk of CO2
leaks, requires a
capped saline aquifer at least 2.5 kilometers
deep, a minimum aquifer temperature
of 50C, and a minimum of 1 million metric
tons of stored CO2
(Figure 6).
POWER Digest
EU's Highest Court Says French Onshore
Wind Tariff Is Illegal. The Court
of Justice of the European Union (EU)
ruled on Dec. 19 that a French regulatory
mechanism allowing network distributors-namely
Électricité de France and
non-nationalized distributors-to recover
from final power consumers additional
costs arising from an obligation to purchase
wind-generated electricity at higher
than market prices constitutes " an intervention
through state resources. " The association
Vent De Colère! (Wind of Anger)
and 11 other anti-wind groups urged the
French Conseil d'État (Council of State) to
annul the 2008 ministerial order that allows
full cost recovery from consumers instead
of through a public service fund, as
was previously required. The French council
asked the highest court that interprets
18
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POWER February 2014

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