Bob Hunt, a former nuclear engineer, is testing an innovative system for extracting geothermal energy. Can he tap the heat beneath our feet?
"I initially fell in love with nuclear power," Bob Hunt explained in his Mississippi drawl. "But I became disillusioned with it. Now I'm excited about it again, because we have a nuclear plant in the core of the Earth that's sequestered away, has no pollutants, and that we can tap from anywhere in the world. It's a true renewable energy."
Hunt hopes his new system for extracting geothermal energy out of spent oil and gas wells will catch on, and usher in a new era of clean, homegrown power. Hunt is a physicist, engineer, and inventor who has launched several companies. In 1969, he helped design the atomic systems for the U.S.S. Nimitz, a pioneering nuclear-powered aircraft carrier. Some years later, he left the nuclear industry over fears about its safety. Hunt ended up in aquaculture, and became the first person to successfully spawn redfish, in part because of his invention of a device that pumped pure oxygen into the water.
More recently, Hunt has worked with a number of renewable technologies, including solar building materials (through his company RenewableOne), wind turbines, and a wave-energy generator. He has tried his hand with compressed air-powered vehicles (which remain controversial among many green proponents), and he has promoted his idea for a "gravity plane" that would allegedly fly without fuels, instead taking advantage of the expansion and condensation of a sealed gas. "I don't know if I'll ever build one of those in my lifetime," he told me. "It requires a huge amount of money, far beyond my scope financially."
As far as his geothermal well concept is concerned, Hunt told me he is currently building some test units on oil and gas wells in the Houston area. He expects them to be running by later this year. He hopes to be able to produce about two megawatts of energy per well, roughly equivalent to one commercial wind turbine. "The U.S. only has 3,000 megawatts of geothermal energy installed, so doing 1,500 wells would double that," Hunt said. "This has potential for rapid expansion because you can build these modular units within a factory environment, and we've got thousands of abandoned oil and gas wells."
Hunt's systems currently cost around $1.5 million each (roughly comparable to a wind turbine), he says. At this point, test units are being financed by "Texas oil money." Hunt has also applied for federal funding, which is now available to the tune of hundreds of millions of dollars for geothermal projects (as my coauthor, Jay Egg, and I mention in our recent book Geothermal HVAC).
The Houston Advanced Research Center (HARC) has helped Hunt prepare a grant proposal to the U.S. Department of Energy. HARC's vice president, Jim Lester, calls Hunt's system "a very interesting idea." According to Lester, "There are multiple ways of extracting geothermal energy, and it's an important resource for this country, particularly for Texas, where we poke so many holes in the ground. In fact, there are 600,000 oil and gas wells in the state."
Lester explained that Hunt presented a prototype to his group, and that the team is impressed with the concept. He added that the real test will be whether the system can stand up to the high pressures in the field. "There are interests in the oil and gas industry who are interested in seeing what happens," Lester said.
How Bob Hunt's System Works
In Hunt's system, an existing oil or gas well is "tapped" and connected to his proprietary engine. When turned on, a rushing column of groundwater that escapes from the well drives the engine, which makes power. The water is then re-injected back into the same aquifer.
If a well already has geopressure, meaning water that wants to rush out as soon as you uncork the hole, "you can connect my machine and go," Hunt said. However, if you don't have that benefit, you could try what Hunt calls "gas lift" to create "artificial geopressure." This is done by pouring in liquids that have a lower boiling point than water, such as liquid carbon dioxide, propane, or refrigerants. When the fluid travels 9,000 to 10,000 feet underground, it comes into contact with temperatures around 150 degrees, and it turns into gas. It then displaces higher-density groundwater, making the whole mass lighter than the surrounding rock. As a result of the pressure difference, the groundwater shoots out with tremendous force. It's essentially the same process that makes natural geysers work, except Hunt carefully monitors how much fluid is introduced to keep the well's output constant.
The kinetic energy of the water column is harvested by the "linear engine" Hunt designed. The machine is analogous to an old-fashioned steam engine, although it lacks the levers and flywheel of past designs, and it doesn't have a crankshaft, which Hunt says makes it more efficient. The engine has two cylinders, which fire each other. "The more pressure you put in, the more it runs," Hunt explained.
Asked why he came up with this design, instead of using a water or steam turbine as currently found on hydroelectric dams and solar thermal plants, Hunt replied, "we tried various turbines and other designs, but we didn't find anything else that held up under the extremely high pressures [thousands of pounds] of an oil and gas well. Everything else disintegrated almost instantly."
Hunt's design can work on both small reservoirs and those wells that tap into vast underground oceans. "In some places it could essentially run forever," he said. "So long as you keep recirculating the water supply, it will continually be reheated by radioactive decay of the Earth, so it's a true renewable energy."
Hunt claims that his technology is unlikely to cause localized earthquakes, which have been blamed for property damage adjacent to geothermal power projects in Switzerland and California. "That can only occur with fracking, when they put in hydrologic pressure so powerful that it cracks rocks, in order to get exchange over a greater area," he said. "They do that in hot, dry rocks, but the type of technology we're doing, going into old oil and gas wells, means we already have permeable basins with good circulation."
To wring even more power out of the system, Hunt envisions eventually adding a heat exchanger on the surface, to extract thermal energy out of the hot groundwater after it passes through the engine.
But Will It Actually Work?
To get some perspective on Hunt's design, I contacted one of the foremost experts in geothermal energy from wells, professor David Blackwell of Southern Methodist University. Blackwell said that he has spoken extensively with Hunt, but he declined to comment on the feasibility of the inventor's ideas, saying he wasn't familiar enough with the details.
I also spoke with Professor Jack Hamilton, who teaches geology at the University of Utah and heads up the institution's Energy Innovation Commercialization Center. Hamilton said he is "a bit skeptical" of Hunt's concept, saying that, on one hand, wells would have to have "significant" pressure in order to overcome the weight of the water, while on the other, sand in high-pressure fluid can be extremely abrasive and damaging to equipment. Hamilton also said it may be harder to work with wells that have already been capped. To this point, Hunt responded that it may increase the cost a bit to work with capped wells, although he is currently testing sites that were recently producing fossil fuels.
Hamilton acknowledged that there could be valuable methane dissolved in the water brought to the surface by Hunt's process, which could conceivably be recovered for an additional source of revenue. "Clearly, Hunt is an interesting and clever guy," said Hamilton. "I know nothing about his surface equipment, and that may be the real value, if he has, indeed, invented a better mousetrap. It could have applications in conventional geothermal or various kinds of hydropower."
"Geothermal is 24/7, it never stops, and it's not intermittent like the wind or the Sun. That's what the power companies want," Hunt said at the recent TEDx conference in Austin. If his invention proves successful in the field, it could help breathe new life into old oil and gas wells. Eventually, it could help decrease our use of oil and gas, period.
Remy Chevalier of the Environmental Library Fund provided research assistance.
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