i live in a hot spot

John Denver proclaimed it "almost heaven", but a group of geoscientists are implying it's a little more like the opposite.  Most of the rest of the world doesn't think of West Virginia as a hot spot, but it is.  It's just been overlooked.  A recent study funded by Google (more on that) updated the Geothermal Map of North America that had been originally generated in 2004.  That map had been complied by researchers at Southern Methodist University, but it focused primarily on regions where they expected to find heat.  If you're feeling geographically competitive, download the graphical presentation of the data in Google Earth and see how your state compares as a function of depth.  The download is a really great visual presentation of three-dimensional data.

The temperature of the Earth increases as you move toward its center.  Although the surface of the Earth is relatively cool, if you burrow down 50-60 miles, the rock becomes partially molten and the temperature rises to between 1200 and 2200 degrees Fahrenheit.  The earth's center (about 4,000 miles deep) is estimated to be over 9700 degrees Fahrenheit.  The heat is produced via many mechanisms (some of which remain the subject of devate), but the primary contributants are the residual heat left over from the formation of the planet, and radioactive decay of elements within the Earth's crust. 

Since heat flows from high temperatures to low temperatures, the Earth is constantly emitting heat - 42 terrawatts (42 x 10¹² watts), which means 420 billion 100-watt light bulbs. (Thanks Left Coast Bernard.) That's a lot of energy being generated basically free of charge.  You don't need exceptional temperatures to do significant things.  Between 40 and 80 degrees Fahreneheit, heat can be used in a geothermal heat pump for warming dwelling.  Between 100 and 250 degrees Fahrenheit, heat can be used for dyring:  food, concrete blocks or even lumber.

So you don't have to go too deep into the Earth to find useful heat.  You do, however, have to find a way to get the heat to the surface.  Nature has given us some good models to build on.  In volcanic regions, where magma is close to the surface, water passing through the rock that is heated by the magma, creating "hot springs".  People have long attributed restorative properties to hot springs - President Franklin Delano Roosevelt was known to frequent Warm Springs, GA, where the water maintains a pretty constant 90 degrees Fahrenheit temperature.  The restorative properties of the warm water are attributed to the high mineral content common to the waters.  The solubility of most minerals in water increases with increasing temperature.  Warm or hot springs are more likely to have high concentrations of minerals like calcium or lithium (or radium).

Larger amounts of heat can transform the water into steam, which builds up pressure and eventually erupts to the surface as a geyser, like Old Faithful.  They word 'geyser' actually originated in Iceland, proving that there is no correlation between ambient temperature and geothermal heat.  Hot springs and geysers are much more likley to be found in volcanic regions, so it probably won't surprise you that that most of the geothermal energy in the U.S. is preferentially located in the western part of the country

Drilling thousands of feet into the surface of the Earth is expensive, so people don't just go around drilling holes in the Earth to check the temperature.  Much of the data the SMU group used is incidental - a byproduct of industrial drilling for oil and gas. While it's nice to have someone else provide data for you, you have to make some compromises.  If you were drilling with the express intent of measuring the temperature of the Earth, you would drill your hole, pump some water or other fluid down into the hole, wait for the water to come to thermal equilibrium (i.e. get to the same temperature as the rock surrounding it), and measure the temperatures.  Since most of these measurements were for oil and/or gas exploration, the only fluid going into the hole was water that was intended to cool the drill and clean off all the debris accumulating on the drill bit.  So it wasn't as simple as just copying down a bunch of temperatures.  The researchers had to come up with a method to reliably convert the temperatures into meaningful data.

Geothermal energy - and a variant called Enhanced Geothermal Systems, or EGS, are of special interest to Google.  I'll get to how you enhance geothermal systems in a moment. Geothermal energy is a much riskier way to harness energy than, say, mining for coal or drilling for oil.  We don't necessarily know where all that geothermal energy is located. Google provided the funding (a little less than a half million dollars) for the SMU team to expand the 2004 map by analyzing newly available data and previously unanalyzed data. 

Electricity can be produced by passing water through porous rocks in a geothermal 'hot spot', allowing the rocks to heat the water until it turns to steam, then passing the steam through turbines to produce electricity.  Warm water can also be used to heat directly, as it is in Iceland and in Idaho's state Capitol - the only one in the U.S. that is heated geothermally. 

Geothermal energy efforts are already underway.  Hot spots in Nevada reach 200˚C at 2 kilometers below the surface.  Recent stimulus funding efforts from the Department of Energy are focusing on developing a new 'green energy' industry around geothermal.  The Geothermal Energy Association says that Nevada (if it were a country) would ranks as the ninth biggest global producer of geothermal energy. The Blue Mountain 'Faulkner 1' project in Humboldt County Nevada is about 17 square miles and is rated at 49.5 MW, with a possible potential of 100 MW.  For comparison, a typical coal power plant might be rated at 500 MW. 

 

Traditional geothermal energy requires enough heat in the Earth, water and enough porosity in the rock so that water can travel down, be heated, and rise back up again as steam.  Once you've located a hotspot, water can come from rainwater or pumped in.  The problem is that many hotspots contain pretty solid rock, so there's no path for the water to travel down so that it can be heated.  Enhanced geothermal systems use artificial means to fracture the rock and create a path down (and up).  One way to break up rock is to use supercritical fluids.  A supercritical fluid is a liquid that has been heated and put under pressure - to the conditions at which the densities of its liquid form and its gaseous form are the same.  Carbon dioxide is a common supercritical fluid for this application - which requires heating to 88 degrees Fahrenheit and a pressure of 1,074 psi.

 

If you drill a small hole, you can pump the supercritical fluid into the rock.  When it heats up, it expands, which fractures the rock.  The warmed fluid is pumped out of the heat reservoir, the heat is used, and the fluid is pumped back down.  Although some carbon dioxode escapes, it is mostly trapped in the rock and diffuses slowly back inot the Earth.  Carbon dioxide is a readily available gas, it's not flammable and it has the advantage that most minerals are not soluble in supercritical carbon dioxide, so the minerals that might be brought to the surface in, say, a geyser of water, are left in the Earth when the fluid is supercritical carbon dioxide.

While geysers advertise hot spots, the places that might be most useful for enhanced geothermal energy don't call attention to themselves -- hence the need for studies like the Google-funded SMU project.  The USGS estimates that the potential geothermal energy in the Western US is equivalent to about half the current installed electric power production capacity in the U.S. 

 

Someone asked me how I was adapting to West Virginia.  Sure, it's not Dallas (or even Lincoln), but I'd have to say that geothermal energy is only one of the things that most people overlook when they scan the states. 

Comments

[Doctor Jay]

I once stayed the night at a spa in Japan's Hakone region which was built over a hot spring. To my astonishment, signs at the spa touted the "healthful radiation" of the hot spring waters.

[Left Coast Bernard]

"42 terrawatts (42 x 1012 watts), which means 42 trillion 100-watt light bulbs."

Don't you mean 42 X 1012 is 42 trillion watts or 42 trillion 1 watt light bulbs?

It is 420 billion 100 watt light bulbs.

[Michael Dunn]

Love your new unit "terrawatts" since these are "earth" or "terra" generated;)

[David Wescott]

this is pretty cool. There are private companies doing similar things - 3Tier gives people free access to resource maps for solar and wind power feasibility, and they're looking at hydro-electric power. (full disclosure - they're a client.) http://www.3tier.com/firstlook/

[Ned Wright]

The map should specify a depth for the temperature. Otherwise every spot will have temperatures reaching 1000's of C at some depth. I found a few other geothermal maps on the Web having either heat fluxes or with a depth specified. In any case, the 42 terawatts would keep the Earth at a "toasty" 35 K if the Sun went out.

[Mark]

Ned Wright said:
"In any case, the 42 terawatts would keep the Earth at a "toasty" 35 K if the Sun went out."

At least it'd make superconducting wire economically viable.