By George Harvey
The term “geothermal” is properly used to describe just about anything that uses heat energy from the Earth. This makes it rather confusing, because there are very different technologies used, for very different purposes, which are properly referred to as geothermal. There is geothermal heat, and there is geothermal power. There are different types of each.
Not far below the surface of the Earth, the temperature is fairly constant and close to the average year-round temperature of the area. In New England, this is typically above 50° F. As you go deeper below this, the temperature increases about 1° F. every seventy feet down in most parts of the planet. The heat energy from these areas can be used in various ways.
Geothermal heating can be a matter as simple as constructing a building in an earthen mound or into the side of a hill to reduce its heating requirements. If such a building is sufficiently well insulated, it may require very little beyond the body heat of its occupants and the residual heat of their activities. Cooking in such a building goes a long way toward heating it.
Geothermal cooling can be used instead of air conditioning. Green Energy Times had an article on the subject in June of 2013 called “Windcatchers – Ancient Persian A/C.”
Geothermal heating and cooling need not be passive, however. By burying heat exchangers, it is possible to heat or cool just about any building. For residences, the heat exchangers are usually either tubes buried a few feet down in trenches or wells that go deeper into the rock. In either case fluid is pumped through the tubes, providing a heat source for a heat pump, the source being much warmer and therefore more effective than outside air. Though such a system is usually rather expensive to install, it is typically very inexpensive to use for heat.
Another use of the Earth’s heat is geothermal power. This is a very different matter from heating buildings. It means extracting heat from parts of the Earth that are hot enough to cause a fluid to boil. There are a number of different ways to do this. In a place such as Iceland, where there are geysers and volcanoes, the heat is so close to the surface that it requires very little drilling to get down to it. In New England, it requires drilling deep wells, typically thousands of feet deep.
Typically, two wells are drilled, side by side. Water is pumped down one well. At the bottom, it passes through permeable rock, which heats it up. It makes its way to the second well, which brings it back to the surface. Under the pressure of the great depth, it usually stays liquid. But as it gets to the lower pressure at the surface, it boils. The boiling drives turbines to make electricity.
Some of the technology for geothermal power is fairly mature. Some, such as enhanced geothermal is not. Enhanced geothermal is done where the rock at the bottom of the well is insufficiently permeable for a sufficient quantity of water to go through from the one well to the other. In such a situation, cold water is pumped down, forcing the rock to cool and contract. This cracks the rock making it more permeable. This is very like fracking, except that it is on a much smaller scale and does not require any chemicals aside from water.
Geothermal power can have a higher carbon footprint than most other renewable resources, because it can allow gasses to escape from underground. Nevertheless, the carbon footprint is far below that of natural gas, even in the most efficient natural gas usages.
There is little potential capacity for traditional geothermal in New England. The potential for enhanced geothermal, however, is very high. We could get a multiple of our power needs met by geothermal – about 636% in Vermont; 958% in New Hampshire; 160% in Massachusetts, where the large population has a high power demand; and 3270% in Maine, where there is a lot of land area for the population. These percentages are greater than those for wind, but less than what we could get from solar PVs.
There is a downside for geothermal power we should be aware of, especially enhanced geothermal. It has been known to cause earthquakes. Those that have arisen from it have nearly always been so small that a human being could hardly feel them. Nevertheless, it is something to consider, before we bet too heavily on the system.