Matthew McArdle
Steven Pinelli
Chris Scaffa
Environmental Physics
Presentation Paper
Geothermal Power
Societies around the globe have used geothermal energy for thousands of years.
For the most part, the energy was is the form of heat contained in water that had made its
way to the surface on its own. This hot water was mostly used for bathing and cleaning,
but was also used to heat living spaces, for instance, the Romans used hot springs in the
first century AD to warm the water of large bathing pools open to the public. The
scientific study and measurement of geothermal energy first began in 1740, when a
researcher measured temperatures at various locations along a mineshaft in France. He,
and others began to notice that, generally, the deeper one goes, the higher temperature
one finds. It was also in France that, in the 14th century in a town called Chaudes-Aigues,
the first district heating system was implemented, and it continues to operate to this day.
Geothermal district heating is also found in the US and was first implemented in Boise,
Idaho in 1892 and the system continues to heat about 450 homes.
Geothermal energy was first used to produce electricity in Italy in the early 20th
century. The first working prototype was small and constructed by Prince Gionori Conti
in 1905. This experimental unit paved the way for the first commercially viable unit,
which in 1913 began producing 250kWe for the citizens of Larderello, Italy. Electricity
production from geothermal sources began to take off in the second half of the 20th
century. Locations in New Zealand and Mexico in the 1950’s raised the bar in terms of
efficiency and economic viability, while the 1960’s saw power plants constructed at the
Geysers in the US and at Matsukawa in Japan. These new plants were more efficient and
could produce more electricity with lower levels of heat. The Paratunka unit in the
former USSR, could, for example, produce 680kWe from only 81C ground water.
Detailed understanding of the nature of heat below the Earth’s surface occurred
when scientists began to understand the various origins of subterranean heat. One
important step towards understanding geothermal energy occurred when, in the twentieth
century, advances in the field of nuclear physics allowed for the discovery of radiogenic
heat. This radiogenic heat is generated by the decay of radioactive isotopes of uranium,
potassium, and thorium, which are found deep under the Earth’s surface, and
significantly contributes to the presence of subterranean heat. Once radiogenic heat was
understood, along with other sources, the creation, dissipation, and movement of
underground heat was better understood.
Geothermal energy, being heat energy, reaches the Earth’s surface through
convection, where fluid rock moves from the hottest areas of the inner Earth to the
surface. This heat can be used directly, or converted into power that can perform work.
When used as a heat source for living spaces, there are a few ways that this heat energy
can be extracted. If there is hot groundwater that naturally circulates heat up to the
surface, this water will be pumped through heat exchangers, which transfer the heat to a
new working fluid, which can be pumped through the radiators of a home. If there is no
water naturally carrying heat from the geologically active site, then water is often forced
into these areas. A cyclical flow is established where cool water is pumped downward,
forcing the newly hot water towards the surface, where it can be collected. When
geothermal energy is used to create electricity, the process is often similar to more
common heat engine-powered electricity generating plant. If the ground is hot enough,
water can be applied directly to the rock, where it boils and creates steam. This steam is
then fed into a turbine, which converts heat energy into mechanical energy, which, in
turn, drives a magneto, which converts mechanical energy into electrical energy. The
process is slightly more complex in binary applications, which allow for electrical
production from water temperatures of less than 175C. In these applications, hot ground
water is pumped through heat exchangers where the heat is transferred to a working fluid
with a relatively low boiling point, such as alcohol. The working fluid then boils and the
gas produced is forced through a turbine, then cooled in a radiator and made ready for the
process again, raising the theoretical Carnot efficiently of the operation.
A typical modern geothermal power plant would charge about $0.05 per kWh. At
The Geysers, a geothermal power plant north of San Francisco, power is sold at $0.03 to
$0.035 kWh. The costs of a geothermal plant are from mostly upfront costs as no fuel is
needed to run these power plants. The initial construction of the wells and pipeline
contribute the bulk of the initial costs as possible areas need to be confirmed as potential
places to construct a geothermal power plant. The fields of wells and pipeline along with
the power plant itself usually cost around $2500 per installed kW in the United States. A
small 1 MW power plant might cost $3000-$5000 per kW. Typical operations and
maintenance costs run around $0.01 to $0.03 per kWh. This amount depends on what
percentage of the time is the power plant producing energy. A geothermal power plant
will usually produce energy 90% of the time. However if higher percentages of 97 – 98%
are wished to be achieved the costs for operating and maintaining the facility rises
drastically. Residential geothermal systems typically costs about $2,500 per ton of
capacity. A typical residential size system is a three ton unit which is around $7,500. An
existing comparable HAC system would cost around $4,000. The geothermal system’s
higher efficiency would lessen the amount of time before the investment pays for itself.
The typical pay back period for a residential geothermal system is anywhere from two to
ten years depending on the circumstances.
There are three types of hydrothermal power plants, dry stream power plants,
flash stream power plants, and binary-cycle power plants. Dry Stream power plants use
hydrothermal fluids directly from geothermal wells that arise usually as steam. This
steam goes directly to a turbine, which is used to drive a generator to provide electrical
power. This is the first type of geothermal power plants used. The output of these plants
is excess steam and very low amounts of other gases. A flash steam power plant uses
hydrothermal fluids about 360oF or 182oC that are held in a tank at a much lower pressure
than the fluid caused the fluid to vaporize quickly, or “flash”. This vapor is then fed to a
turbine, which drives a generator producing electricity. Any fluid that has not vaporized
could be sent to another tank at a different pressure to be “flashed” again. The third type
of geothermal power plants is the binary-cycle power plant. In this process energy is
extracted from a hot geothermal fluid, which is transferred to a secondary fluid through a
heat exchanger. The hotter geothermal fluid causes the secondary fluid to flash and
vaporize which is then fed to a turbine, which drives a generator which produces power.
This is an example of a closed loop system, meaning that nothing is emitted to the
atmosphere as the secondary fluid is reused again and again. All three geothermal power
plant types are used for commercial and industrial processes as these systems can be quite
large in size and costs.
Other uses include greenhouse and aquaculture, where the geothermal systems are
used to provide a heat source for both of these agribusinesses. The geothermal heat pump,
a system that uses the planets constant temperature to act as a heat source in winter and a
heat sink in summer, is used for bother residential and commercial buildings. Geothermal
heat pumps are mostly used for space heating and cooling. It can be also used for water
heating depending on the type of system and how hot the area being tapped is. The
geothermal heat pump takes advantage of this by transferring heat stored in the Earth or
in ground water into a building during the winter, and transferring it out of the building
and back into the ground during the summer. A geothermal heat pump system could exist
as a closed looped system or an open looped system. The closed looped system relies on
a heat exchanger to transfer the heat from the earth to where it will be used. An open
looped system uses two wells, one to collect the heat from the Earth and the other to
release previously tapped water back into the environment to recharge the well.
The utilization of geothermal power poses a remarkably low environmental impact,
relative to the drawbacks of other fossil fuel and renewable energy sources. Now, more
than ever, technological advancements have elevated this prehistoric resource into a
viable, flexible, environmentally-friendly solution to power the modern world. However,
even this proven energy source presents unique consequences, which must be addressed.
The primary extent of geothermal power’s environmental impact includes: pollutant
gases released into the atmosphere during extraction, the concentration of toxic
compounds at surface level, and land instability due to subsidence and earthquakes. A
secondary factor depends on the impact of the electricity sources needed to power water
pumps, compressors, and associated geothermal equipment.
First of all, the practice of extracting fluid from deep earth can cause dissolved gases,
including carbon dioxide, methane, ammonia, and hydrogen sulfide, to escape into the
atmosphere. These released (greenhouse) gases and pollutants are well-known
contributors to climate change and acid rain. Fortunately, geothermal electric plants are
equipped with emission scrubbers that reduce the exhaust of these harmful products.
Additionally, current geothermal electric plants on average emit 122 kg of CO2 per
megawatt-hour. This value is overshadowed by the 1020 kg/MWh emitted by coalfueled thermal power plants, 758 kg/MWh emitted by oil-fueled power plants, and 514
kg/MWh emitted by natural gas-fueled power facilities.
Another environmental consequence of geothermal power generation is the
possibility for trace toxins such as mercury, arsenic, boron, antimony, and salt to
precipitate out of the extracted fluids and concentrate at earth’s surface. Despite the
established toxicity of these chemicals, modern geothermal power facilities are able to
minimize the impact of toxins by reinjecting extracted fluids back into the earth – a
closed-loop system.
Thirdly, critics often blame geothermal power generation techniques for causing land
instability in the form of local subsidence and as the cause of increased seismic activity
(i.e. earthquakes). The effect of geothermal power plant subsidence has been especially
well-noted in the Wairakei field in New Zealand and in Staufen im Breisgau, Germany.
This city in Germany suffered damage to some historical buildings due to deviations in
the ground by up to eight millimeters.
In recent news, the future of geothermal power generation has been threatened, at
least in the U.S., by evidence of related earthquake risk. The United States Energy
Department enacted new safety measures, including permitting and community
education, in January 2010 in order to minimize the risk of drilling-induced seismic
activity. This effort follows in the wake of the AltaRock Energy company’s failure to
properly disclose the earthquake risk to local residents. After drilling beyond two miles
into bedrock at the Geysers (a site 100 miles north of San Francisco), consequently,
seismic activity in the region sharply increased. Unfortunately, the project collapse at the
Geysers signifies an early setback for the Obama administration’s initiative to propagate
geothermal power.
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