Producing power is the same as it ever was: you need to find a source of energy that can be made to do work. Geothermal power, as the name suggests, relies on heat from the earth. The earth’s core is quite hot, but it’s a long way off – thousands of miles below us, in fact, far beyond our reach technologically and economically. This means you need to find a high-enthalpy reservoir (see my previous post for an explanation, here) reasonably close to the surface: ideally, no more than two or three kilometers (about 1-1/2 to 2 miles) underground.
Of course, you can’t actually see the reservoir at those depths, and that’s probably a good thing. The conditions at the bottom of a reasonably productive geothermal well – say, one that emits steam capable of producing 5 megawatts (MW) of electrical power if harnessed to a turbine – include temperatures that easily exceed 200o Celsius (about 400o Fahrenheit) and pressures of 30 bar (roughly 435 pounds per square inch). A geothermal reservoir exists at temperatures that would burn the skin from your body and pressures that would crush the crispy residue into a fine, oily paste.
Fortunately, you can find a reservoir without being cooked or crushed. The presence of nearby subterranean heat – below the surface of the earth but not so far down as to be effectively beyond reach – can be inferred from the existence of geologic formations produced by high-temperature events. Volcanoes are a good example. Heat can linger for centuries after a volcanic eruption, because rock and soil are excellent insulators. In addition, new influxes of magma can add heat to nearby layers of rock, preventing them from cooling off.
The Hengill volcano near Reykjavik last erupted 2,000 years ago, but it still retains enough heat to meet the residential electricity needs of more than 300,000 people – coincidentally, just a little less than the entire population of Iceland.*
Heat is important, but it’s only part of what you need. You also need water, a lot of it. The turbines at Hellisheiði, Iceland’s largest geothermal power plant, use 500 kilograms (1,100 pounds) of steam per second. To keep up that pace hour after hour, week after week, year after year, requires a reservoir that’s really large, or being replenished at an extraordinarily rapid pace … or both.
The presence of subterranean water can be inferred from geologic formations, but finding geothermal waters can be more of a challenge than finding water for more common uses. The aquifers people tap for drinking water and irrigation typically are at fairly shallow depths, less than half a mile below the surface; but geothermal reservoirs several miles below the earth are harder to detect. Occasionally, heated waters escape the reservoir and reach the surface in the form of geysers and hot springs, and these sorts of phenomena are excellent indicators that there’s a useful reservoir somewhere underneath.
The presence of nearby bodies of water, like lakes or glaciers, often correlates with underground water, because the water on the surface seeps into the earth. Sometimes, the water can flow many miles from its source while underground; the Hraunfossar waterfall in western Iceland is an example. Meltwater from the Langjökull glacier flows underneath an ancient lava field, making its way under a cap of solid rock to pour from cliffs overlooking the river Hvita. Sometimes, water works its way much farther into the earth along deep fractures, like fault lines, to form geothermal reservoirs.
The best reservoirs are contained in hot, porous, and permeable rock. Porosity is a measure of how much water can be held in the rock; often, rocks will have tiny holes that can hold minute quantities of water, especially at the sort of pressures found deep in the earth. Permeability is a measure of the amount of water that can flow through the rock; it’s a concept related to porosity, but not quite the same.
You want the rock to be porous, so that it contains a lot of water that will turn to steam when it’s taken from the high-pressure environment found under miles of rock to the low-pressure environment of the surface. You want it to be permeable, so the water you take can be easily replenished, either by the natural flow of water or by reinjection of the water you extracted to drive your turbines. And you want the rock to be hot, so that the inflowing water will reach temperatures high enough to continue turning the turbines for a long, long time. The lifespan of a geothermal plant should be measured in decades – and if the reservoir is large enough, and managed carefully, perhaps even longer.
So we know what we need: a large reservoir of pressurized water, contained in hot, porous, and permeable rock. We know roughly where to look: in places where there have been high-temperature events such as volcanic eruptions, within the past few centuries; and/or where there are currently manifestations of subterranean heat, like geysers or hot springs.
That narrows things down a bit, but volcanoes and hot spring fields can still cover hundreds of square miles. You don’t want to just start drilling at random in an area that large, not at a cost of millions of dollars per hole. Fortunately, it’s possible to actually map the structures under the earth without ever seeing them. I’ll discuss that in my next post.
* Residential electricity use varies considerably depending on economic and environmental factors: for example, per capita residential use in Hawaii totals about 6,000 kilowatt hours (kWh) per year, while in Louisiana, it’s over 15,000 kWh/year.