Our subject last week was solar energy. We'll continue the energy theme
this week with a look at three more sources; geothermal, hydroelectric,
and wind energy. These three sources share a common characteristic with
solar energy. They are categorized as renewable energy
sources, ones that are replenished indefinitely by natural processes. But
are these energy sources truly "renewable"? Hydroelectric and wind
energy are ultimately fueled by the sun. And the sun will consume
it's fuel (hydrogen) in another 4 billion years! The source of
geothermal energy will probably wither away in just a few billion years.
However, the non-renewable sources such as wood, coal, oil, natural gas and
fission reaction fuels will run out in the next few centuries. Clearly, the
renewable sources are a better bet for the long run.
Geothermal Energy is energy
obtained from the heat energy inside the earth. Old Faithful, the geyser
pictured to the right is one of most well known and visible demonstrations
of the intense heat that lies deep in the earth's mantle. The recent
volcanic eruptions on Montserrat are another indication of the immense heat
in the liquid core beneath the earth's crust. And "hot springs" resorts
featuring naturally heated water can be found all over the world. It was
originally believed that this heat was left over from earth's early molten state. But a
relatively straightforward estimation (that will have to wait until we've
covered thermodynamics) shows that the earth would have cooled long ago. The source is a bit more more exotic than most people would imagine.
How is the geothermal energy extracted from the earth? Although the
earth's core is composed of very hot molten rock, we do not presently have
the technology to use it. Instead, we make use of water in the crust that
has been heated by the core. (The thermal inertia of the local ground can also be use and is describe in the first two links provided.)
How the geothermal energy is utilized depends upon the temperature range of the water. Lower temperature water (around 300 oF and less) can be used directly to heat homes by transporting the hot water through heating pipes or by utilizing heat pumps. The really hot water sources (300 oF to 1000 oF ) can be used to run electrical power plants. These plants use the hot water to heat a working fluid that powers electrical turbines. One of the difficulties with this type of plant is the corrosive nature of the hot water. Because of the high temperature, the water is typically very high in mineral and salt concentrations. Very high grade steel and titanium pipes must be used to transport the water. There are three types of these geothermal power plants ; the Dry Steam, the Flash Steam and the Binary Cycle power plants.
Wind Energy has been used in various forms since man first took to
the sea. Windmills used to dot the land across America and can still
be found pumping water on some farmlands. (If you're curious, check out the history of windmills.) Today, wind energy is most
likely to be used for generating electricity; from the small wind
generators that can be seen on many of the boats anchored around St. Thomas
to the giant Altamont
Pass Windfarm located east of San Francisco, California. The idea is
simple. Let the wind rotate wind turbines (these are essentially the same
as the propellers on airplanes) which are connected to electric generators.
There are many countries now using wind power to supplement their electrical power
needs. But there are some different opinions of the windfarm at Altamount Pass.
Hydroelectric power uses moving water to turn the turbine blades of
electric generators.
The most common arrangement is to build a dam on a river, creating a lake
behind the dam. Water is allowed to "fall" onto the turbine blades.
Hoover Dam is probably the most famous hydroelectric dam. It is over 700
feet high and can generate over 2,000 MegaWatts of power! The details of
how Hoover Dam was constructed are fascinating. Click on the picture if
you'd like to learn more.
The source of the power is gravity and evaporation. Essentially, the gravitation PE of the water is converted to electricity.
The cycle of renewal is completed through evaporation of the water (the
sun's contribution) which falls as rain upstream. Carl Vizcarra of the California Polytechnic State University has created an interesting tutorial and discussion
for hydroelectric dams, including a bit of information on the latest idea of harnessing ocean waves to produce electricity.
For a long list of links to renewable energy resources, Alternative Links.
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Questions:
What is the source of geothermal heat energy?
What is the main difference between a Binary Cycle power plant and a Dry or Flash Steam power plant?
What are the average summer wind speeds at Altamont Pass?
What is the "head" created by a hydroelectric dam?
What is the purpose of pumped storage in hydroelectric facilities?
Geothermal Energy is energy
obtained from the heat energy inside the earth. Old Faithful, the geyser
pictured to the right is one of most well known and visible demonstrations
of the intense heat that lies deep in the earth's mantle. The recent
volcanic eruptions on Montserrat are another indication of the immense heat
in the liquid core beneath the earth's crust. And "hot springs" resorts
featuring naturally heated water can be found all over the world. It was
originally believed that this heat was left over from earth's early molten state. But a
relatively straightforward estimation (that will have to wait until we've
covered thermodynamics) shows that the earth would have cooled long ago. The source is a bit more more exotic than most people would imagine. 
Wind Energy has been used in various forms since man first took to
the sea. Windmills used to dot the land across America and can still
be found pumping water on some farmlands. (If you're curious, check out the 