OVERVIEW
If you were to travel down into the Earth, you would notice that it
gets hot, very hot. On average, the temperature goes up about 30 degrees
Celsius for every kilometer you go below the earth’s crust this
is called the geothermal gradient. There are 3 ways that heat can be
produced: the sun, collisions from meteors (of which we thankfully have
few) and the decay of radioactive isotopes. Radioactive isotopes have
been in the Earth from the beginning, and as time went on, they decayed.
One by-product of the decay is heat. There are still some radioactive
isotopes heating up the planet now. Uranium, Thorium, and Potassium
all have long lived radioactive isotopes, which means they take a long
time to decay. Any short lived isotopes like Aluminum 26 have all ready
decayed (Basu). So its hot down there, and electricity can be formed
by heat. Coal and nuclear power plants produce electricity by boiling
water to turn turbines, which makes electricity. Inside the Earth, there
are all ready the 2 key ingredients in making electricity- heat and
water. Geothermal energy utilizes the hot water and steam naturally
present in the Earth to run generators.
HISTORY
Geothermally heated water has been used by people all over the world
for more than 2,000 years. Originally, the hot springs were used only
for bathing and washing clothes. People came to believe that the water
from these hot springs had medicinal value. Roman spas could be found
throughout their vast empire. Hungarians were the first to drill their
own, more than 100 years ago. By 1900, drilling for hot water was a
common practice in many countries. Shortly afterwards, Japanese farmers
were using hot springs to heat their greenhouses. By the 1930’s,
this water was being used in Budapest to heat homes. In the early 1940’s,
industries in New Zealand and Iceland started using the hot water. As
time progressed, people found more and more uses for this source of
heat. In addition to heating homes, recreation, agriculture and fish
farming, fumaroles have been used for recovering drinking water as well
as reclaiming minerals trapped in the water. The recovery of boric acid
in Larderello, Italy was one of the first operations in place for recovering
minerals. Starting in 1812, people boiled water from the hot springs
with wood fires. In 1827, they realized that they could use the fumarolic
steam to fuel the process. The real breakthrough came in 1904, engineers
in Larderello managed to light 5 light bulbs with a ¾ H.P. generator,
run on the steam there. That was the true beginning of geothermal energy
(Halacy, 1977). By 1913, Larderello had a 250 kw station in service
which produced a constant flow in electricity. 1922 saw America’s
first attempt at geothermal electricity, it was at a site in California
called “The Geysers.” They only managed to heat the resort
there before the steam corroded the engines, it was a failure. We were
not the only country to follow in Italy’s footsteps. Japan dug
their first geothermal well in 1919, 2 years before America. New Zealand
and Mexico both had small geothermal plants in operation shortly before
us as well (Kruger and Otte, 1973).
LOCATION
Most major geothermal areas are near plate margins. Areas of recent
volcanism are ideal, whereas places with thick sedimentary rock deposits
away from plate margins are generally not very productive areas to look.
Geothermal systems can be found where the geothermal gradient is at
or slightly above normal or in regions around plate margins. In the
regions around plate boundaries, the geothermal gradient can be more
than 10 times the average- more than 300 degrees Celsius per kilometer.
A geothermal system contains a heat source, a reservoir and a fluid.
The heat source can range from the normal Earth temperature (which depends
on depth) to over 600 degrees Celsius when a magmatic intrusion gets
to a depth of 5 - 10 kilometers. The reservoir is hot permeable rock,
the fluid can circulate through the rock, getting very hot in the process.
Usually, the reservoir will have a layer of impermeable rock above and
below it. There is often a superficial recharge area- a gap in the impermeable
rock on top, where water can seep into the system from rain or streams
(Dickson and Fanelli). The permeable rocks at Larderello are limestone,
dolomite and anhydrite. The capping rock is a mixture of carbonates,
argillites and amphibolites (Kruger and Otte, 1973). The geothermal
fluid is water, as a liquid or a gas. It often carries chemicals and
gasses with it, carbon dioxide and hydrogen sulfide are common (Dickson
and Fanelli). A geothermal hydrothermal system includes a geothermal
reservoir, wells, and a power plant. Hydrothermal means that there is
a lot of steam or water in geothermal system that can be brought to
the surface (EREN).
FINDING IT
There are several methods used for actually finding the productive areas.
These range from aerial photography to studying the water in surrounding
areas. Obviously, fumaroles and geysers are good indicators of a geothermal
area, but its usually not that easy. There are lots of locations which
do not have boiling water shooting out of the earth, so people have
come up with more sophisticated prospecting techniques. One method involves
infrared aerial photographs of the land. The infrared will show hotspot
in the ground (Halacy, 1977). Geology and hydrogeology are the starting
point for exploration. Information obtained in these studies can later
help the reservoir and production engineers. Generally, these studies
should be done by an geothermal geologist. Geochemical surveys are also
very important. These surveys can yield vast amounts of important information,
including: if the fluid is dominantly water or vapor, minimum temperature,
what kinds of minerals are likely present in the water, the homogeneity
of the water, and the source of the recharge water. These surveys are
relatively cheap, they involve chemical and isotope analysis of the
water and steam in fumaroles and other areas of geothermal activity
at ground level. Wells can also be dug and surveyed. Geophysical surveys
are done at or close to the surface. They include thermal, electrical,
electromagnetic, seismic, gravity and magnetic surveys. Many of these
techniques were developed by people searching for oil. Geophysical surveys
can help determine the shapes, size and depth of the reservoir, but
do not give any indication to whether there is any fluid in there. Drilling
exploratory wells are the final, and most expensive step in an exploration
program. It is also the only way to actually know what is down there.
Before any field work is done, all of the preexisting data is studied
to help cut down on the cost, as well as increase the effectiveness
of the prospecting (Dickson and Fanelli).
METHODS
There are 3 main ways of approaching the problem of turning hydrothermal
fluids (hot water and steam) into electricity: dry steam, flash steam
and binary cycle. The method used depends on the state (gas or liquid)
and the temperature of the fluid. There was an unsuccessful project
started in the 1970’s called the Hot Dry Rock project. Bore hole
were drilled down into the earth, then pressurized water was put down
there, making cracks in the rock, these were called hydraulic fractures.
The hope was that the water they pumped down there would seep into the
fractures and heat up, another well would be drilled into the reservoir,
through which the hot water could be pumped, and…voila- an artificial
geothermal system. It was too expensive, and didn’t work (Dickson
and Fanelli).
DRY STEAM
Dry steam is the oldest method, it is used in Larderello, Italy. This
uses steam to turn a turbine. A hole is drilled down into the Earth
to where there is steam present, this is called a production well. A
pipe is put in this hole. The steam is less dense than the surrounding
rock, so it rises. It turns the turbines at the top of the pipe, some
what like how a windmill works. The steam is then condensed and put
back into the ground through the injection well, so it will (hopefully)
re-circulate through the impermeable rock, get hot again, and find it
way back into the reservoir (EREN). Dry steam plants emit steam and
some gasses into the atmosphere. The Geysers, in northern California
uses dry steam, it produces more electricity than any other geothermal
power plant in the world. Dry steam is great for powering a plant, D.S.
Halacy states “Dry steam is the power engineer’s dream.
It is easy to handle, does not corrode equipment, and produces more
power than lower temperature wet steam or hot water” (Halacy,
1977). The only problem with dry steam is that those geothermal systems
are rather rare (Dickson and Fanelli).
FLASH STEAM
The flash steam method is the most common in practice today. High temperature
water is pumped from under ground, where it is under a lot of pressure
to the surface. It is kept pressurized until it gets to a tank at the
surface with a much lower pressure. Some of the water is flashed to
steam, turning turbines. The remaining brine is often sent to a second
tank where the process is repeated. Any brine left after this is injected
back into the ground via the injection wells. Double flash plants, those
with 2 flash tanks, typically use 18-25% of the mass of the fluid in
the reservoir for steam. The other 75-82% is injected back into the
ground. Geothermal brine often contains a lot of silica, which can form
on the walls of the equipment as hard scales. Another problem with flash
steam is that it requires very high temperature water, at least 200
degrees Celsius (EREN).
BINARY CYCLE
Many people believe that the future of geothermal energy lies in the
binary cycle (INEEL 2). The binary cycle system uses the earth’s
hot water to heat a secondary liquid with a boiling point lower than
that of water, usually an organic compound (Dickson and Farreli). The
hot water follows a pipe from the production well, up to the heat exchanger,
to the injection well. The water is kept under pressure, so there is
no buildup of silica scales. The secondary liquid is heated in the heat
exchanger, boils, and the gas turns the turbines. Then the gas is cooled,
either by air or water, and returned to its liquid state, ready to boil
once again. There are a couple advantages to the binary cycle. First
of all, once the power plant is in place, it is very cheap to operate
because there is little wear and tear on the engines and plumbing. It
can also run efficiently on lower temperature water that dry steam or
flash steam can (EREN). With the proper secondary fluid, a binary cycle
plant can be run with water temperature as low as 85 degrees Celsius
(Dickson and Farreli). Freon was successfully used by the Russians as
a low temperature secondary fluid in Paratunka (Halacy, 1977). Geothermal
areas often provide water warm enough for a binary cycle where as it
is difficult to find much water warmer than 200 degrees Celsius (INEEL
2). Binary cycle plants are easy to install, they can be ready for energy
production in less than one year. Recently, there was a break through
in binary cycles, it is a new system called the Kalina cycle. It uses
a mixture of water and ammonium as the working fluid. This mixture has
a very low boiling point, which allows the Kalina cycle to be up to
40% more efficient that regular binary cycles. There are virtually zero
emissions from binary cycle plants, making them an excellent source
of environmentally friendly energy. This is not to say that the other
methods (dry steam and flash steam) are dirty, they are also very clean
forms of energy, but do have more emissions (Dickson and Fanelli).
CONCLUSION + SOME FUN FACTS
Well, not a whole lot to say in the way of a conclusion, I must admit
that before I started researching this topic I was biased in favor of
geothermal energy because of the environmental factors, but now I am
even more convinced that geothermal energy is a place that deserves
a whole lot of research as a means of helping to solving our energy
problem. I’m a huge fan of it, especially the binary cycle, which
is the cleanest and the most useful in most areas. Direct use of geothermal
energy (using warm water to heat and cool houses, for aquaculture, agriculture,
etc.) saves millions of barrels of oil every year, and has the potential
to save many million more. It can also be up to 80% cheaper than using
fossil fuels. The cost of geothermal energy is usually between 5 and
8 cents/kWh, but as low as 3 cents. Plants put in now would have to
charge about 5 cents/kilowatt-hour to be cost efficient because they
generally have to drill into lower temperature areas (the really good
spots are taken.) The cost of energy from natural gas is about 3 cents/
kWh. The extra price of geothermal energy can be partially compensated
for by the fact that some plants are able to recover large amounts of
minerals, such as zinc and silica (CREST). How many natural gas plants
can sell their emissions? Geothermal energy emits no nitrous oxides,
tiny amounts of sulfur dioxides, and a little bit of carbon dioxide.
Fossil fuel burning plants emit between 1,000 and 2,000 times more carbon
dioxide than geothermal plants. Geothermal plants take up significantly
less land than fossil fuel and nuclear plants do: 1-8 acres per MW versus
5-10 acres/MW for nuclear and 19 acres/MW for coal. Another thing is
that they are productive more than nuclear or fossil fuel plants, 90%
of the time in comparison to only 65%-75% (INEEL 1)
Unfortunately, geothermal energy only accounted for 0.4% of the U.S.
electrical generation, and .26% of the global electrical generation.
The good news is that in the U.S. alone, this prevents the emissions
of 22 million tons of carbon dioxide, 200,000 tons of sulfur dioxide,
80,000 tons of nitrogen oxides, and 110,000 tons of particulate matter
every year (INEEL 1). Well that’s about all I’ve got, but
I’d like to say that I really think that geothermal energy is
a win-win situation. Although I hardly mentioned direct use in this
paper, I think that shows possibly even more promise than using it as
a fuel for electricity Any way you use it, its clean and cheap, and
who doesn’t want that?
Sources
1.) Basu, Abhijit
Info taken out of various lectures from his class “G121 meteorites
and planets”
2.) CREST (Center for Renewable Energy
and Sustainable Technology)
Geothermal power : FAQs
http://www.crest.org/articles/static/1/995653330_5.html
3.) Dickson, Mary H. and Fanelli, Mario
What is Geothermal Energy?
Instituto di Geoscienze e Georisorse, Pisa, Italy http://iga.igg.cnr.it/documenti_igaenergia/Geothermal
Energy.pdf
4.) Energy Efficient and Renewable Energy
Network (EREN)
Geothermal Hydrothermal
http://www.eren.doe.gov/power/pdfs/geo_hydro.pdf
5.) INEEL (INEEL 1 in the paper)
Environmental and Economic Impacts of Geothermal Energy
http://geothermal.id.doe.gov/xweb/other/framed.shtml?http://www.eren.doe.gov/geothermal/
6.) INEEL (INEEL 2 in the paper)
Geothermal Power Plants and Electricity Production
http://geothermal.id.doe.gov/x-web/other/framed.shtml?http://www.eren.doe.gov/
7.) INEEL (INEEL 3)
What is Geothermal Energy?
http://geothermal.id.doe.gov/what-is.shtml
8.) Halacy, D.S.
Earth, Water, Wind and Sun, Our Energy Alternatives
Harpor & Row Publishers 1977
9.) Kruger, Paul and Otte, Carel
Geothermal Energy: Resources, Production, Stimulation
Stanford University Press, 1973
The diagrams come from INEEL’s page What is geothermal energy?
http://geothermal.id.doe.gov/what-is.shtml
The map comes from the Oregon Institute
of Technology’s geo-heat center website
dusys.htm
http://geothermal.id.doe.gov/x-web/other/framed.shtml?http://www.eren.doe.gov/geothermal/