The earth’s natural heat has tantalized seekers of new energy resources for centuries, and in selected areas, it has redeemed its apparent promise: San Francisco generates a fair share of its electricity at what’s known as The Geyser fields to its north.
In most places, though, the heat’s too far down. MIT’s Jefferson Tester says the temperature range needed for productive heat mining is 500-600 degrees F. “To get to those levels in a wide range of fields, you’d have to drill 30,000 feet-plus,” he notes – too far to go with today’s technologies.
Why? “You’re crushing and grinding the rock, and eventually it will defeat your drill,” explains Tester, a chemical engineering professor.
That’s okay if you’re going down a few thousand feet; you can simply raise the drill and replace its bit. But at great depths, the costs can be prohibitive – not least because it can take 24 hours to pull a drilling pipe out of, say, a 10,000-foot well. And if you want to get to 30,000 feet, says Tester, “wells can cost up to $200 million each.”
He hopes to cut such costs drastically. Among his concepts: instead of drilling through the rock, use a mini-version of a rocket engine to blast through it. “There’s a phenomenon called spallation,” notes Tester, “where if you apply heat in the right way to a rock like granite, it literally flies apart.” As an alternative, high heat can also let you melt your way through some types of rock.
One of Tester’s ideas for thermal drilling is a system in which you’d use electricity generated above ground to separate water into its hydrogen and oxygen components through a process called electrolysis. You’d then pump the gases down the drill hole to fuel the flame below.
In trial runs, Tester’s group has shown that applying high heat is a good way to penetrate rock. “We’ve drilled 1,000-footplus holes about 12 to 16 inches in diameter – the size you need for heat mining,” he says.
But while collaborating with a major energy company to develop this system, Tester’s also refining a more radical concept: a drilling system where almost everything happens underground.
Instead of piping gases down, you’d have an energy-conversion system mounted on the end of your drilling assembly. Electricity generated above the system would drive the electrolytic conversion process, so that hydrogen and oxygen would be produced right on the spot.
This approach could open areas as deep as 60,000 feet to heat mining, but there are big obstacles. An example is how to keep the system’s sheer weight from causing the piping in the hole, known as the drilling string, to collapse. Tester’s solution would take advantage of hydrogen’s lighter-thanair character: “We’d use some of the hydrogen to buoy up the string; it would basically be weight neutral.”
He admits designing this and other elements of the system won’t be easy, but adds that potential payoffs are great. “If we can go this route,” he says, “we’re basically paving the way for geothermal energy exploitation on a worldwide basis.”