tackling climate change
Can Geothermal Power Replace Coal?
;e resource exists. ;e challenge is tapping into it.
By CHUCK KUTSCHER
Chuck Kutscher is a
principal engineer and
manager of the Thermal
Systems Group at the
National Renewable
Energy Laboratory.
He is a past ASES chair
and was chair of the
SOLAR 2006 conference, which resulted in
the ASES report, “
Tackling Climate Change in
the U.S.” (Free download at ases.org/
climatechange.) He
taught a course at the
University of Colorado entitled “Climate
Change Solutions.”
The opinions expressed
here are solely those of
the author.
Enhanced Geothermal System
production
well
energy
conversion
plant
injection
well
engineered
fracture
system
NREL
Approximately half of U.S. electricity is generated by coal-;red power plants. A typical pulverized coal plant emits about two-and-a-half times the amount
of carbon emissions per kilowa;-hour of electricity generated as a modern combined-cycle natural gas plant. While
carbon capture and storage o;ers promise, signi;cant challenges exist, and developing it may take decades. If we are to
have any chance of avoiding the worst impacts from climate
change, we must stop building new coal plants and begin to
phase out the ones we already have.
One challenge with replacing coal, besides its low cost,
is that coal plants provide most of our nation’s baseload
power, de;ned as the minimum production needed 24
hours per day. Solar and wind power are variable and thus
cannot easily replace coal plants one-for-one. Not so for
geothermal energy. It has been reliably providing the U.S.
grid with nearly 3,000 megawa;s (MW) day and night,
rain or shine, with approximately 4,000 MW more under
development. In fact, geothermal power plants operate at
a capacity factor (the ratio of average annual power output
to rated power output) of greater than 90 percent, whereas
coal-;red plants typically operate in the range of 70 percent
to 90 percent capacity factor due to regular maintenance
outages. Although geothermal energy is not renewable in
the immediate sense, the heat resource will typically last the
life of a properly operated plant and will eventually replenish a;er a plant is decommissioned.
Geothermal power plants are similar to coal plants in
that they generate electricity by producing a high-temper-ature, high-pressure vapor that passes through a turbine,
which spins an electric generator. ;ere are a few ideal places in the world where dry (superheated) steam exists near
the surface. A;er drilling, that steam can simply be routed
directly to a steam turbine. Direct steam plants are used
in Northern California at ;e Geysers, the world’s largest
geothermal power plant complex. More
commonly, geothermal wells tap pressurized hot water. If the temperature of
that water is higher than about 350˚F (or
about 175˚C), it can be rapidly boiled in
a low-pressure ;ash tank where a fraction
of the water becomes steam. ;e steam is
then routed to a steam turbine.
For lower temperature resources, the
hot geothermal ;uid is passed through
heat exchangers, where it boils a secondary ;uid having a lower boiling point
than water, like pentane or isobutane.
hot rock
;e resulting vapor spins a specially designed turbine.
;ese are called binary-cycle plants. While all geothermal
power plants have very low emissions, binary-cycle plants
have virtually none, because all of the geothermal ;uid is
returned to the ground. Binary-cycle plants also have the
advantage that they do not result in any water drawdown
from a geothermal reservoir, although they can decrease
reservoir temperature over time.
;e best geothermal resources, so-called hydrothermal
resources, have three qualities: high near-surface temperatures (preferably at least 240˚F, 116˚C; the higher the better), ;uid content in the form of pressurized water or steam,
and permeable rock. All of the existing geothermal power
plants in the world are sited on hydrothermal resources.
Enhanced geothermal systems could
potentially provide all the electricity
needed in the United States.
But these resources are limited. Worse, we don’t even
have a good handle on how much we have. A 2008 U.S.
Geological Survey (USGS) study estimated that we have
power production potential from identi;ed hydrothermal
resources of between 4,000 MW (95 percent con;dence
level) and 13,000 MW ( 5 percent con;dence level). When
the USGS a;empts to include potentially undiscovered
resources, the range jumps to 11,000–90,000 MW (with
a mean of about 40,000 MW). ;at’s a big range, and this
resource uncertainty is a key question mark for the future
exploitation of geothermal.
;e resource picture gets a lot brighter if we ease our
expectations for the three qualities of good geothermal
resources. If we are willing to drill down 3 to 10 kilometers,
there are wide areas throughout the Western United States.
with adequate temperature. And although those areas tend
to be dry and low in permeability, we can potentially inject
water at high pressure and fracture the rock, as is o;en done
to enhance oil and gas recovery. ;is is the concept referred
to as enhanced geothermal systems, or EGS. When the
same USGS study estimated EGS resources, the range of
power production potential jumped to 350,000–720,000
MW with a mean of about 500,000 MW, which would be
su;cient power to provide virtually all the annual kilowa;-hours of electricity needed in the United States. A 2006
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