Reducing the Cost of Geothermal Energy with Hydrothermal Spallation Drilling

Geothermal energy offers a great deal of potential for renewable energy. But, the costs of drilling can make geothermal energy less than cost-effective.

With conventional rotary drilling, deeper holes raise the costs exponentially. Now, at least two organizations are working on developing hydrothermal spallation, a drilling method which will allow for reaching greater drilling depths in a more efficiently and cost-effectively.

Researchers at ETH Zurich, a Swiss university, have developed a simulator that mimics the temperature and pressure conditions in a borehole deeper than three kilometers below the earth’s surface. In their experimental reactor they can ignite a flame underwater at a pressure of around 250 bars and 450 degrees Celsius.  The 1.2 million-Swiss-franc plant should demonstrate that it is actually possible to drill through rock by means of hydrothermal flames. The project is funded by the Swiss Federal Office of Energy, the industrial organization swisselectric research, ETH Zurich and the Swiss National Science Foundation.

In the U.S., Potter Drilling is developing a similar deep-drilling technology. According to a 2006 MIT study, such Enhanced Geothermal Systems (EGS) could potentially supply 2,500 times the country’s current energy consumption. Such statistics led internet giant Google’s philanthropic arm to invest $4 million in Potter Drilling as part of its green energy initiative.

Conventional geothermal power plants use underground hot water which is relatively close to the surface. The water creates steam that drives electricity-generating turbines. Suitable underground water in the U.S. is mainly in the western states. Technology that allows for deeper drilling would mean that geothermal energy could be generated nationwide.

But, deeper drilling, as much as six miles below the earth’s surface, is incredibly expensive; conventional geothermal developers spend millions of dollars to just drill test wells. At such depths, drill bits wear out more quickly; and a broken drill bit 30,000 feet underground can lead to the abandonment of a $10 million well. Such statistics mean that conventional drilling techniques are uneconomical: a 10 km borehole costs around $60 million to dig, and drilling a well can amount to half of a geothermal power plant’s cost.

The hydrothermal spallation method, however, drills not with bits but with extremely hot water. Potter Drilling’s device resembles a round garden hose nozzle with slotted openings at the end. Chemical reactants begin to heat up the water supply to 800 degrees Celsius and the hot water is forced through nozzle. The jets of water begin to fracture the surface of the rock.

“The heat from the flame causes the rock to crack due to the induced temperature difference and the resulting linear thermal expansion,” explains Tobias Rothenfluh a researcher with ETH Zurich. The expansion of the upper rock layer causes the rock’s pre-existing natural flaws to develop into cracks.

The method is particularly suitable for the hard, dry rock that is often encountered at depths greater than three kilometers. Drill bit wear is considerably less, because there is no mechanical contact with the rock, reducing the time and cost of drilling a well. “We think we can cut the cost by 50 percent,” says Jared Potter of Potter Drilling.

Later this year, Potter Drilling will test the process in the foothills of the Sierra Nevada, where the company plans to drill a 1,000-foot hole. It has applied for a U.S. Department of Energy (DOE) grant and also plans to raise venture funding to commercialize its technology.

It may be some time before the method is commercially viable. “It is for sure possible to speed up the project towards the industrial application”, Philipp Rudolf von Rohr of ETH says, “but we still want to focus on basic research at a university like the ETH Zurich.” Such knowledge, they hope, might be useful not only for geothermal energy, but also for other applications.

Image courtesy of Wired Magazine