Two years ago, the future of geothermal energy in the US did not look all that exciting. Although heatpumps (used for heating and cooling of buildings) and natural geothermal energy were certainly not uncommon, very large scale geothermal, which draws heat energy from deep down in the earth’s crust, seemed far in the future.
But then, MIT (Massachusetts Institute of Technology) published an exciting report titled “The Future of Geothermal Energy” with subtitle “Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century”. This changed the scene. The report got the intention of policy makers as well as investors, EGS received enough capital to start several new projects, and suddenly it became a potentially important future energy source. I wished that more scientific reports had such a positive outcome! The report itself can be found at http://geothermal.inel.gov .
To date there are 18 natural geothermal resources in the United States in California, Nevada, Hawaii and Utah. In total, they supply around 2,700 megawatts (MW) of electricity. A decent amount when viewed by itself. 1 MW can be seen to supply electricity to about 1000 homes, so this delivers enough energy for 2,7 million households. It is not that much as compared to the total capacity of the US which is around 1,100,000 MW. However, in California, geothermal energy supplies 5% of total electricity demand and 7% of that in Northern California, which boasts the site of the largest geothermal plant (the Geysers, 620 MW) in the world. Apart from these 18 geothermal plants, the USA has more than 1 million geothermal heatpumps, supplying an additional 3700 MW for heating and cooling.
Geothermal energy is not completely clean. Subsurface water, which is brought to the surface, can contain some carbon dioxide, for example, but it is a minor problem. It is estimated that a geothermal plants emits about 1/200 of the carbon dioxide of an equivalent coal-fired power plant.
The geothermal story gets much more interesting when we look at geothermal heat contained between 3 and 10 kilometer depth, which EGS taps into. The total energy in this layer is staggering. Just producing 1% of this energy would supply 1400 times the total energy that the USA needs per year! So, how can we tap into this resource? The idea is simple: we drill down to a depth at which the rocks are sufficiently hot (say 150-250 degrees centigrade). It is not a simple task to drill through hard granite to a depth of 20,000 feet, say, but in the oil industry we do this on a regular basis and so the technology exists. Then, we fracture the rock over a reasonable large area. We drill another well a bit removed from the first. This second well will serve as our production well. The first is used to pump water, or another liquid, into the rocks under high pressure. It will start moving through the fractures to the producing well under a pressure gradient. As it moves, it picks up heat from the surrounding rock. We then pump it back to the surface, extract the heat, and reinject the new cool liquid again. Sounds easy enough, and it certainly sounds like a great idea to tap into that huge heat reservoir down below.
EGS is not without its complications, of course. It is not trivial to create a good fracture system down below. We need to have a very large network of fractures so that the water that flows through this network touches a lot of the rock for better and prolonged heat transfer. Also, to make this economic, an EGS well must pump through at a rate of, say, 80 liters per second, the equivalent of 50,000 barrels per day. This has not yet been achieved in pilot tests.
However, EGS is certainly a promising technique. Estimates are that if it can be done at the high rate mentioned, then within 5-10 years a typical EGS project can be cost competitive in the current energy market of around 5 US cents per kilowatthour of energy. Not bad, huh! With sufficient investments, experts claim that we could produce around 30 times as much geothermal energy by 2030 as we do now, supplying around 5-10% of the electricity needs of the US in 2030. A substantial contribution. I think we should go for it. And I’m delighted that this MIT report established so much. It has pushed this energy technology forward at a much faster rate than I thought possible.
OK, so let’s now write a report on “The Future of Wind Energy” and “The Future of Solar Energy”. Maybe not by MIT, maybe by Stanford. Yep, we should be able to do that too, don’t you think?
See also this KQED show of November 18, 2008 on geothermal energy.