To the average non-scientist, especially those of us who grew up on cartoons and oversimplified grade-school graphics of the earth’s layers, “geothermal” heat seems like the logical answer to everyone’s energy needs. Why not just send a pipe down to that molten layer, pull up that lava, and get the geothermal rebate down to Augusta?

OK, good news and bad news. True geothermal is really hot, but not really available. False geothermal is really available but not really hot! Here, we want to try to remove some confusion between ground source heat pumps (often misleadingly referred to as “geothermal”) and true geothermal, where heat is extracted from the extremely hot ground and used for heat or electricity.

True geothermal heat is a combination of radioactivity in the earth’s crust (80%) and heat flowing up from the core and mantle of the earth (20%). Over most of the earth, in order to reach rock that is warm enough to directly heat our homes, we need to drill down two miles or so. To find temperatures high enough to generate electricity we have to drill even deeper–maybe 5 miles. Cool water is then sent down in one pipe and steam comes up another and is used to drive a turbine to generate electricity.

But this hot rock is not an unlimited source of energy. As heat is extracted from the rock, the rocks around your pipes cool—that is, ”the heat gets used up.” Heating sustainably would require a large underground network of pipes or fractured rock, as energy can be extracted only at the rate it is replenished from the surrounding rock. In our research, no one seems willing or perhaps able to quantify the requirements and rewards of this proposed technology.

But what about volcanoes and hot springs, you ask? They’re plenty hot and isn’t all that steam and lava just being wasted? Well, volcanoes, hot springs (and earthquakes) exist in regions that lie near the boundaries of tectonic plates. Like Iceland. These regions have high concentrations of geothermal energy that – yes, can be, and are, exploited to generate electricity economically. The problem is that these regions are very limited.

From what we were able to discover for this article, currently, the world electricity production from geothermal is around 15 GW (billion watts) and is only produced in the high concentration areas. This may sound big, but compared to total annual electrical production, it is only about 0.5% (five-tenths of a percent) of our electricity. The worldwide potential for geothermal in the future is speculated to range from around 5% to a maximum of 10% of our current usage, but apparently quite far in the future and probably far more expensive than other renewables and battery storage.

Some claim geothermal electricity is the solution to the “baseload” energy issue—that is, the continuous source of energy needed to compensate for fluctuations in wind and solar. We are regretfully skeptical about this. There just aren’t enough tectonic plate boundaries or geothermal growth potential to meet the baseload power need. It will help, though, sure!

Now for the “false geothermal.” Ground-source heat pumps (GSHP) are often referred to as geothermal. Literally, “geo” means earth and “thermal” refers to heat, so yes. But 45°F is not what most of us call hot, right?

The heat that the GSHP extracts from the ground is actually stored solar energy. Over the warmer seasons, the ground temperature is replenished, not from below, but from above–from the sun. In fact, at the surface of the earth, the energy from above is around 4000 times greater than from below. This is not geothermal. It is solar.

Heat pumps use the same technology as a refrigerator or air conditioner. They are able to extract energy from a relatively cooler region (your backyard ground or the air) and deposit that energy, at a higher temperature, into a warmer region that you are trying to heat. The cooler region gets cooler, while the warmer region gets warmer. The heat pump uses energy to leverage heat out of the cold region and into the warm region. Yes, it uses electricity, but you end up with several times the amount of heat in the house than if you used that electricity to run a normal electric heater.

In central Maine, the ground from which a GSHP extracts its heat is between 35 and 50°F at the depth of the pipe network (as little as 6 to 8 feet deep). It extracts heat from this network and puts it into the house at a higher temperature. The advantage of ground source heat pumps over air source is that the warmer and more uniform ground temperature means you get more for your money (investing more as well). These heat pumps will produce 4 to 4.5 times the heat you would get from electric resistance heat. These days, this translates to a 50% savings over oil heat.

So, true geothermal is a potent source of energy, but of limited potential, while “false” geothermal, not so potent, actually has much potential for growth. Remember – it’s numbers, not intuition.

Paul Stancioff, Ph. D. is professor emeritus of physics at the University of Maine Farmington who dabbles in energy economics on the side. Cynthia Stancioff pursues climate action and sanity. Their emails are pauls@maine.edu and cynthia.hoeh@gmail.com. Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/.

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