A radical new method of producing electricity from the Earth's inner heat has been devised by a power plant designer from Texas.
Doyle Brewington of ESOR Consulting Engineers in Houston has designed a long, self-contained turbine shaft called a Power Tube that could tap subterranean heat without relying on geysers and steam vents.
"I spent 25 years building power plants and I saw the damage they were causing," Brewington said. "The noxious gas being emitted by a lot of these steam turbines were causing a lot of acid rain around the world."
Brewington's Power Tube is a sealed tube, four-foot wide and 185-feet long, containing a vapor-driven generator. The idea is to bury the Power Tube deep enough to touch hot rock.
The tip of the Power Tube contains a pair of hydrocarbons, isopentane and isobutene, that turn to vapor when in contact with rock that's at least 220 degrees Fahrenheit (104 C). The vapor rises to drive a generator above. The vapor is then cooled back into a liquid by helium that is compressed and expanded using sound waves. The liquefied hydrocarbons are then pumped back to the tip to restart the cycle in an unbroken loop. Magnetic suspension, rather than lubrication, eliminates friction in the turbine.
Brewington has designed a half-size prototype that is just over two-feet in diameter and 85 feet long. It is expected to produce a megawatt of electricity, enough to power 750 homes. The first Power Tubes to go into operation will be in Hawaii and Costa Rica, Brewington said. He is still talking to authorities, and didn't say when it would go into the ground.
Brewington said full-size Power Tubes will produce 10 MW, enough to light up a small residential town. Unlike old-fashioned geothermal sites, which consume up to 10 acres of land, Power Tubes will have only a small maintenance shed on top. And because Power Tubes run silently, homes and offices could be built over them.
Working Power Tubes will be easy to assemble on site: Long ones will be transported in sections, Brewington said. And on-site maintenance of a Power Tube would be swifter than that for traditional power plants because the entire shaft can be removed and replaced in hours. The defective tube could then be retrofitted back at the plant.
If Brewington is correct, much of the energy needed for expanding industrialization could be provided without releasing greenhouse gases associated with burning fossil fuel or the risks associated with nuclear energy.
Many of the fastest growing economies sit on the Earth's "Ring of Fire," a circuit of volcanoes, earthquakes, and other manifestations of tectonic tension. Brewington claims that 48 countries in the Ring of Fire alone could be powered entirely by Power Tubes, a great stride over other geothermal systems.
"Anything that's got magma underneath it is great," he said.
But leading researchers are watching with a wary eye, and some have expressed strong reservations about Brewington's plans. They don't see how he's going to pull off his power plans without water.
In geothermal plants, water plays the vital role of a heat conductor. Designs for extracting electricity from hot, dry rocks have hit a very basic and frustrating wall — the rocks cool down too fast, noted John Lund, director of the Geo-Heat Center at the Oregon Institute of Technology.
"The process is feasible in the short term. There's no question about that," said Lund. But "rock is very poor at transferring heat. You're pulling heat out to turn the turbine, so the surrounding rock cools down and new heat is very, very slow to replenish.
"If not (in) a month, maybe after a year, and he'll be out of heat. I don't think the process he has in mind is going to work very well."
Lund said there are already geothermal power plants using a process called binary cycles that use hydrocarbon vapors in much the same way as Brewington's Power Tubes. The critical difference is that when these plants are situated in a dry area, they pump water down at high pressure to exploit fissures in the rock so that water can then conduct heat from a wider area.
A plant built for sustained operations needs to draw upon a heat well hundreds, if not thousands, of feet deep, Lund said. Considering the geology of the United States, Lund said nothing east of the Mississippi could meet a Power Tube's needs at the shallow depths proposed by Brewington. "So the economics defeats him and the heat transfer of rock defeats him," Lund concluded.
Ted Clutter, executive director of the Geothermal Resources Council, an industry advocacy group, would only say that Power Tubes are "not something we're interested in."
A Department of Energy spokesman said the agency's experts weren't familiar enough with Brewington's plans to offer an evaluation.
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