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.

Understanding Energy

All energy is solar energy.

From a thermodynamic perspective, the Earth is, for the most part, a closed system. This means that matter stays constant and the heat and other energies may flow into and out of the system.  The odd meteor collision, and Einsteinian relativity not withstanding, no matter is added nor removed from the Earth.  The sole source of energy, also, is from the sun.

What?! You might ask, what about all that oil and coal and nuclear stuff in the Earth’s crust that we mine and convert to useful energy?  Isn’t that energy that doesn’t come from the sun? It’s already here, you might say.

But, the question is, how did it get here?  We will first consider the so-called fossil fuels.  There are two theories of the source of (primarily) oil and natural gas:  biotic and abiotic.  We will consider only biotic in this blog, turning to abiotic in a later blog. In biotic-produced petroleum and natural gas, it is considered that sometime between 260 million and 400 million years ago, bracketing the Carboniferous Period, vast amounts of biological material was generated.  The majority of that material was made through the process of photosynthesis.  This process can be performed in plants or in algae. 

Photosynthesis is the process of taking a photon of light, passing the energy from the photon through a series of membranes and chemical reactions to capture carbon dioxide (CO₂), combining it with water (H₂O) and converting these chemicals into sugars such as glucose. The plant or algae then uses the sugars to build other molecules including cellulose that is used for walls and other structures, proteins and even lipids (fats).    Carbon fixation and sugar production occur in a biological organelle called a chloroplast.  The process of capturing and converting CO₂ is called Carbon Fixation.  The waste product of the capture of CO₂ is oxygen.  So, the process of using light (energy) from the sun to generate biomass both consumes CO₂ and produces O₂!  And it’s all driven by the sun!!!

To recap, hundreds of millions of years ago the Earth had an explosion of biomass production, which over millions of years captured a whole lot of both light and CO₂. This material was then converted into oil, coal and natural gas over a period of hundreds of thousands of years and has remained dormant for the intervening years until Exxon comes along and pulls it out of the Earth. 

So, how did it all happen?  Coal and petroleum had two very different environments and starting materials.  Coal is currently found in areas where the trees, ferns and other higher order plants lived, usually in a swamp.  As the plants died, they fell to the bottom of the swamp and were degraded.  You can see this process in a compost pile, or on the floor of a forest.  The plant material is converted first to humus.  Humus is valuable for agriculture and is the organic material-containing portion of rich dark soil.  If some pressure, or removal of additional O₂  (anaerobic conditions) occurs the humus can convert to peat.  If the peat is further subjected to anaerobic conditions, pressure and heat, it can convert to coal.  The chemical processes for conversion from humus to coal can take hundreds of thousands of years and doesn’t always occur. 

What happens is that the pressure and heat, in the absence of O₂, causes the removal of hydrogen (H₂) and water from the carbons.  It also tends to shorten the length of the polymer chains.  So, the original plant material converts from extremely long chain cellulose to much shorter hydrocarbons and even just carbon.  If this process continues with higher and higher pressures and heat, diamonds result.  If the process equalizes, then coal is the result. 

It is thought by scientists that the different types of coal (anthracite, bituminous, lignite) are due to the amount of heat, pressure and time the coal was exposed.  The amount of ‘contaminants’ such as sulfur are dependent on the environment the plants existed in, when they died.  Those plants that were growing in shallow sea-type swamps which had a lot of sulfur in the water, then have an abundance of sulfur in the coal.  Those plants that died in more freshwater swamps have less sulfur.

Petroleum, now, it is thought, resulted from the death and settling of single celled photosynthetic organisms of algae.  Most of these are termed diatoms.  Diatomes are very interesting organisms.  If they died in dry conditions and didn’t get exposed to heat and pressure, they simply fossilized into small boxes of silica (silicon dioxide).  This material is good for filtration and abrasion.  If they fell to the bottom of the ocean and were eventually covered in sediment, subjected to heat, anaerobic conditions and pressure, then oil was produced.  The process acted similarly to the coal process.  The long polymers were dehydrated (water and hydrogen removed), and severed somewhat.  If you have ever played with wax, you know that it is hard when cool, and can be liquid if heated enough. This is due to the length of the single molecules making up the wax. The longer the chains, the more stable is the solid in the presence of  heat.  Most oils are relatively short chains.  And the longer the exposure to the pressure and heat, the shorter the chains.   You have heard of  “octane”. This is an 8 carbon long chain of hydrocarbon (oil).  Shorter chains than pentane (5 carbons) become gases at normal temperature and pressure.  You are familiar with some:  methane (1 carbon), propane (3 carbons), butane (4 carbons).  Between 5 carbon and 25 carbon chains you have the various components of oil.  Above 25 carbons, you have waxes and finally at 35+ carbons in the chain, you have asphalt.

Therefore, natural gas (composed of methane, ethane, propane and butane) is the result of continued reduction of the chain length to its shortest).  That is why natural gas is associated with oil deposits.

So, from a certain perspective, oil is solar energy.  Unfortunately it was energy captured a long time ago over a very long period of time.  It also sequestered a lot of ancient CO₂.  Releasing all that CO₂ in such a short time is likely to be deleterious to the modern environment.

Whew!  We’ve only just finished oil.  What about the other sources of energy?  Stay tuned to this space.  Next we’ll discuss geothermal and nuclear.