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This is the fourth article in my continuing series on renewable energy technology, which I published previously here on Associated Content. Today I’ll be talking about geothermal energy. Like wind and hydropower, most geothermal systems are electromechanical in nature: they generate electricity by spinning loops of conductive wire in the presence of a strong magnetic field. Unlike wind and hydro however, geothermal systems are also thermodynamic in nature: they rely on the coupling between temperature, pressure, and volume in a working fluid exposed to an external heat source. They’re actually much more similar to coal, nuclear, and natural gas burning power plants than other renewable technologies.

Have I lost half of you yet? If you’ve read my previous articles in this series, you probably stayed with me through the electromechanical and spinning loops buzzwords. But a thermo-what? A what-dynamic? A what-what? Let me take a step back. Just how do geothermal power systems work?

Geothermal power simplified

In order to understand geothermal power stations, you have to know a little bit about thermodynamics. I’ll keep it as brief and painless possible, I promise. Let’s start with a substance we all know and love: water. Water is liquid at room temperature and atmospheric pressure. So it doesn’t have a fixed shape, but does have a pretty fixed volume. To prove this to yourself, fill a balloon with water, squeeze out all the air, and squish one side of it between your fingers. It bulges out on the other side right? If you squeeze too hard, the balloon breaks: fixed volume, variable shape.

Steam, water’s gaseous phase, doesn’t have a fixed shape or a fixed volume. You can expand it into a large volume or compress it into a smaller one by playing with the pressure, or by heating or cooling the steam. These parameters are thermodynamically coupled. Heat the steam up while maintaining constant pressure and it expands. Conversely, if you cool the steam down at constant pressure it contracts.

What happens if you heat water to well past its boiling point but don’t allow it to expand? By sealing it inside a cylinder for example? You get extremely high pressure water that will turn into a jet of steam at the first opportunity. Now imagine that one wall of this cylinder is actually a piston. Can you see where this is going? If you increase the pressure inside the cylinder enough (by adding enough heat) the water will vaporize, expand into steam, and move the piston. If the piston is attached to a crank which produces useful work by turning a shaft, you’ve built yourself the first half of a simple heat engine.

Of course now your piston is full of steam at fairly high temperature but low pressure, which isn’t particularly useful. In the second half of the cycle the steam will be ejected from the cylinder and allowed to cool and contract to its original state so you can repeat the process all over again.

Traditional power stations, like coal, natural gas, and nuclear plants use this process to transform heat energy into mechanical energy. They then use electromagnetic induction (a loop of conductive wire spinning in a magnetic field) to transform that rotating mechanical energy into electricity to power our utility grid. Sometimes they use fancier stuff than water as a working fluid. And sometimes they use turbines instead of reciprocating pistons, but the underlying concept is the same.

1) Heat a fluid until it changes phase into a gaseous state.

2) Allow the fluid to expand through a controlled mechanism which will extract mechanical power.

3) Let the fluid cool and contract back to its original state (by exhausting heat somewhere convenient) and repeat.

4) Use the mechanical power to drive a magnetic induction generator.

In traditional power plants, the heat come from burning a fuel like coal or natural gas, or from a controlled nuclear fission reaction. Waste heat (an inescapable byproduct of any thermodynamic engine) is dumped into the air or into a large body of water near the plant. In geothermal systems, the heat comes from the earth. But even though geothermal power technology is very similar to the workhorse power stations modern utilities rely upon (coal, gas, nuclear) less than 0.5% of the world’s electricity supply comes from geothermal sources.

That’s largely because getting access to the heat in the earth’s crust is challenging. Traditional geothermal plants must be built near a hydrothermal resource: a geological formation where heat from the earth’s core rises close enough to the surface to heat a reservoir of trapped water. Formations like this tend to occur near the  edges of tectonic plates, regions where volcanoes, earthquakes and hot springs are commonly found. Power plants built on hydrothermal resources fall into three basic categories.

Dry steam geothermal plants

Dry steam plants are built where underground hydrothermal reservoirs are filled with steam rather than liquid water. These plants are called “dry” because the working fluid never changes phase. The steam remains in its gaseous state throughout the thermal cycle. High pressure steam is brought to the surface through a production well in order to drive a turbine directly. Once the steam has expanded through the turbine, it can be pumped back down into the reservoir through an injection well. This is the oldest type of hydrothermal plants, first used in Italy in 1904, and still used in The Geysers complex of power stations in northern California.

Flash steam geothermal plants

In flash steam systems, the water in the reservoir is heated beyond its boiling point but under such high pressure that it remains in its liquid state. This hot, high pressure liquid is brought to the surface through a production well and injected into an intermediate pressure tank. Some of the water vaporizes, and the resulting steam drives a turbine, which drives the generator. Sometimes the flashing is done in multiple stages to extract as much energy as possible from the thermofluid. As with the dry steam plant, after passing through the turbine, the low pressure steam can be pumped back down into the reservoir through an injection well. This is the most common type of hydrothermal plant in operation today.

Binary cycle geothermal plants

In a binary cycle plant, the hot water from the reservoir is brought to the surface through a production well and pumped back down into the injection well. But the geothermal fluid never passes through a turbine. Instead, it passes through a heat exchanger along with a secondary fluid that boils at a much lower temperature. The heat from the geothermal fluid flashes the working fluid into vapor which drives the turbine. This approach offers some advantages: the geothermal fluid, with its corrosive minerals deposits and pollutants, never comes into contact with the turbines or generators, and binary systems can be built on resources with lower reservoir temperatures than dry steam or flash systems. For these reasons, most new geothermal plants built on hydrothermal reservoirs will probably be binary cycle plants.

Enhanced geothermal systems

The hydrothermal systems described above all require existing underground reservoirs of hot water or hot steam. There aren’t that many places in the world where all the geological factors required to form such reservoirs come together. But there is abundant heat just a few kilometers below the surface all over the world. We just have to get to it. This is the aim of Enhanced Geothermal Systems, or EGS. Sometimes called Hot Dry Rock or Hot Fractured Rock geothermal, EGS projects drill injection bore holes into deep hot rock using techniques first developed for natural gas and oil field exploration. If the hot dry rocks aren’t porous enough, hydraulic stimulation techniques (also initially developed for gas and oil field exploration) can be used to fracture the rock. The result is a porous bed of hot rock which can be flooded with water through the injection wells, effectively creating an engineered hydrothermal system. Once the new reservoir is filled, a production well is drilled and a power station constructed at the surface. Depending on well pressure and temperature, the EGS plant may be a flash steam system, or a binary cycle system.

EGS may just be the future of the geothermal power industry. I’d be happy to see it knock coal out of the number one spot in the world’s energy portfolio, but that dream is probably still a decade or two away. There are technology hurdles to overcome, like affordable hard rock drilling techniques, and cost effective resource exploration, just to name a few. But EGS offers us possibilities no other renewable energy technology can: greenhouse gas free power that can be built anywhere, that can run 24/7, and that has the same production capacity as similarly sized coal, natural gas, or nuclear plants. Hey, what’s not to love?

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