Plugging in to a volcano: Geothermal power and the science that enables it

In Rotorua, New Zealand, the evidence of geothermal activity is everywhere. Often, the grates covering street drains will steam. Every now and again, a homeowner will wake up to find that their backyard has been replaced by a steaming hole in the ground. But all of this was nearly lost in my youth thanks to humanity's attempts to tap into it. Geothermal fields cannot be endlessly plundered, it turns out.

But this is a good-news story. Geothermal activity, in 2017, supplies some 17-18 percent of New Zealand's electricity. But there are many places in the world that have volcanoes, and many of them are more active than New Zealand's. In New Zealand, geothermal fields cover sleeping volcanoes, not the restless, ready-to-throw rocks volcanoes. Which raises the obvious question of why the islands' sleeping volcanoes can be tapped so effectively.

Finding out answers raises a couple more general questions: what does it take to have a good geothermal energy supply, and is tapping it really as simple as sticking a pipe in the ground?

Ingredients for a geothermal field

As you might have guessed, having a local volcano is not enough to ensure that you have a geothermal system. Auckland City has 53 volcanic vents that are all linked to the same boiling pot of magma. This volcanic field is reasonably large and has vents that reach the surface over a large area. Yet there is no geothermal field. It turns out that the hot rock is located rather deep, and eruptions occur when a small volume of magma is pushed from that deep reservoir to the surface.

For geothermal activity, you need to have large quantities of hot rock reasonably close to the surface. The type of volcano that meets this criteria is a caldera-forming volcano, often referred to as super volcanoes. These volcanoes host somewhere between 15-50 cubic kilometers of hot rock, located within about five kilometers of the surface. A typical cone volcano might have a magma volume up to a cubic kilometer, while volcanic fields also tend to top out at about the same size.

Depth also matters. While Taupo's massive volume of hot rock is less than 5km from the surface, Auckland's volcanic field is powered by a small volume of hot rock that is 80km deep.

In sum, you really need a lot of hot rock, and it needs to be in touching distance. That typically means a caldera. New Zealand just happens to have eight caldera-forming volcanoes.

To produce power from these caldera, you also need water. Rain has to come down as fast as the water boils off, because that is exactly what is happening. As rainwater seeps down or flows through the water table into the caldera, some of it heats up and rises to the surface, exiting as steam, boiling water, and boiling mud. This complicated dynamic between water flow and heating makes understanding and managing geothermal energy fields different from all other energy-generation processes.

An exhaustible supply of hot water

When I was about 10, a fight started to brew in Rotorua. The city, then as now, generated a huge portion of its income through tourism. Visitors came from across the globe to see spectacular geothermal activity, combined with a dose of Maori culture. It was a unique experience with the added attraction of taking place in a relatively safe country. But the geysers weren't playing as often or as vigorously. Mud pools were drying up, and it was feared the supply of visitors might go the way of the mud pools.

Funnily enough, motel operators that serviced the tourists were part of the problem. For them, heating and hot water was a question of sticking a pipe in the ground; they all did it without a second thought. Homeowners followed suit. Hot water was being sucked out of the ground at an ever-increasing rate.

Motels and homeowners were reluctant to give up their geothermal bores, but it was that or watch the town go broke. Since water extraction would be difficult to monitor, the authorities set up an exclusion zone instead of regulating use directly. No bores inside the zone. Homeowners and motel owners within the zone were furious, but their anger was ignored. Nearly three decades later, the geothermal field has not yet fully recovered. Yes, the field has improved, but it still has a long way to go.

That near miss is the cautionary tale that has been used to guide geothermal field use in New Zealand ever since. And what we learned from it has been adapted as geothermal energy ended up used in a wide variety of ways, says Dr. Greg Bignall, head of geothermal sciences at GNS Science in New Zealand.

Although Rotorua is the most visible example of geothermal field exploitation done without any thought for the future, it is not the earliest. In the 1950s, people realized that the steam was there for the taking. Drill a bore a few hundred meters deep, and high pressure water rises to the surface. At the surface, some portion of the bore flashes to steam with a temperature of about 250-300 degrees Celsius. The steam goes to a turbine, and suddenly there is clean, green electricity.

But what about the hot water (80 degrees celsius) that did not flash to steam? Luckily, the Waikato River was nearby and could always hold a bit more water. Sadly, the local wild life didn't think much of being gently cooked. Although nothing was done about this for many years, the relocated water wasn't just bad for the river. It also damaged the geothermal field.

Resupplying the field

Water drawn from the field has to be replaced. If no measures are taken, it is replaced with cold water that comes in from the edges. And, if extraction exceeds in-flow, the pressure will drop, which is highly undesirable. "Changes to the geothermal system, pressure decline as a consequence of all that fluid coming out of the system, [and] physical changes to the field, they [power station operators] realized that, if we put the fluid back into the ground, it maintains pressure, it has less impact on natural surface features and things like that," explains Bignall. Water pressure can be maintained by injecting the wastewater at the margins of the field.

New Zealand's oldest geothermal power station—Wairakei Power station, still operating but now modernized—did not, initially, inject any water. But that changed over time, increasing to 70 percent while the station was open. More modern power stations inject up to 98 percent of their wastewater back into the field. The Waikato River is much happier for it.

Even with injection, it takes time for water to enter the main part of the field, heat up, and return to the site of the bore. So there is a limit to how much hot water and steam can be extracted from the field. How much is too much?

That is a difficult question to answer, and it has to be thought about before the first bore is put down. As a result, the process has changed enormously since the river-boiling days of the '50s.

First of all, you have to realize that drilling for hot water is not like drilling for oil or tapping into aquifers for drinking water. Unlike oil, there aren't really any trapped reservoirs of water—the driving force of the underlying magma keeps the water in motion. And water that is flowing through the field is probably not all that hot. No, what you need to find are places where water is moving vertically: upwelling water is your friend.

Bignall outlines the general process as: "What you do is you look for the vertical connectivity. So, it's not a horizontal natural flow. You tap those fluids coming to the surface, whether [they are coming through] fault structures or fractures, bring that to the surface and then put [the fluids] back into the ground again."