Geothermal Energy

Geothermal

As we have seen in the previous blog, fossil fuels were created by the capture of light from the sun by plants, the subsequent treatment of the plant material by heat and pressure to create coal, oil and natural gas.

Today we will explore the cleanest form of energy and what its relationship is to the sun.  Geothermal energy is energy derived from the naturally occurring heat in  the earth.  Geo is Greek for earth and Thermal is Greek for heat.  Therefore, geothermal means ‘earth-heat’. 

It is possible to use the geothermal heat close to the surface of the earth for heating and cooling of residences and commercial properties.  Within a few hundred feet of the surface, the temperature at any given point on the planet is roughly equal to the average annual ambient temperature of the air at that altitude and latitude.  So, if the air temperature where you live averages 60° F (by taking the average of all the high temperatures from Jan 1 to Dec 31 and the average of all the low temperatures for the same 365 day period, and then averaging those two), then any caves in the area, and the ground itself, will be more or less 60° F too.  In general, the temperature of the ground is between 50° and 70° F nearly any place on the planet.

You are probably now thinking that, sure, in a lot of places, it’s 50° - 70° but what about in Yellowstone National  Park, where the geysers and hot springs are boiling?! And what about the other natural hot springs (like Hot Springs, Arkansas and Pagosa Springs, Colorado)?  Those places are certainly hotter than 70° F if not quite boiling.  So why are they so much warmer?

For the most part, those places are warmer due to either the action of friction, such as happens at the interface of two tectonic plate (faults).  When the plates move against one another, heat is generated.  Another source of the heat is due to the action of pressure over certain types of rock.  This can create magma and the molten rock transfers its heat to the surrounding earth.  Finally, a majority of the heat in the planet is due to the radioactive decay of naturally occurring radioactive isotopes.  Uranium, thorium and potassium are the most abundant. 

But wait, you say, you said all energy is solar. Sounds to me like the earth makes its own heat.  And you would be correct, as far as it goes.  First, the top few hundred feet are heated by the sun.  The air temperature is directly related to the sun and the earth absorbs a large amount of that energy as well.  But now deeper, how does the sun affect that temperature?  It may or may not have been the sun we see in the sky today, but all the elements of our planet began inside a star somewhere.  You are probably aware that the sun (our star) is a large fusion reactor. It is made mostly of hydrogen which is fused with itself in the extreme heat and pressure in the center of the sun into helium.  This fusion process releases vast amounts of heat which spew into the solar system and comes to earth in the form of light and heat.  The process doesn’t stop at helium though.  It can continue, fusing molecules together into the other elements as well.  Eventually these elements also spew into space.  There was varying amounts of all the elements in our vicinity when the earth was formed.  The earth’s gravity sucked in enough material to generate the mass of our planet today.  Included in those elements were large amounts of silicon, sodium, potassium, oxygen, carbon, nitrogen, hydrogen, sulfur, chorine and many others.  And, to lesser extents, uranium, thorium and other elements.  Some (like the uranium and thorium) were radioactive.  The decay of these elements helps generate the heat of the planet.

So, you see, a sun or star, is responsible for the radioactive heat in the earth!

Also, the heat, from all this radioactive decay, goes up as you get deeper.  Geothermal maps (from Google for instance) show that at 10 km depth, the temperature is above boiling (100° C or 212° F) and can be hotter than 200° C (430° F).  A process called Enhanced Geothermal Systems (EGS) and also called Hot Dry Rocks, seeks to use the heat from these depths to heat water to create steam which can then be used to run an electric generator.

So, you see, the clean, geothermal energy is made from the same process that is used inside nuclear reactors.

Of course, nuclear power comes from the process of fission too.   In a nuclear reactor, however, the radioactive materials have been purified and concentrated to speed up the process of fission and therefore generate a whole lot more heat in a short amount of time. 

We will explore, in a later blog whether or not nuclear power is ‘clean’ and if it has a place in the future mix of sources of energy.