Power from Not-So-Hot Geothermal

Technology Review

By Prachi Patel-Predd

A large share of the geothermal resources suitable for power generation--those with temperatures higher than 300°F--are deep underground, beyond the reach of current technology. Lower-temperature resources, which are common across the United States, are generally used for heating, but could be a bountiful source of power as well, if researchers were able to find an economical way to convert them into electricity.

Engineers at the United Technologies Research Center (UTRC), a unit of United Technologies based in East Hartford, CT, say they have developed a low-cost system that can utilize low-temperature geothermal resources. The technology could be particularly useful in generating electricity from waste hot water generated at oil and gas wells.

The modular, 200-kilowatt power plant from UTRC can convert temperatures as low as 165°F into electricity. The technology is similar to steam engines, except that steam or hot water vaporizes a hydrofluorocarbon refrigerant that drives the turbine. And the refrigerant has a lower boiling point than water. "It's hard to run a steam engine at 165 degrees [Fahrenheit]," says Bruce Biederman, who leads the project at UTRC. "The size of the equipment would be enormous and your turbine would be very poor in efficiency."

The UTRC power plant can be thought of as a reverse cooling system, and the new turbine is essentially a refrigerator compressor running backwards, Biederman says. Instead of using power to create a temperature difference, like a refrigerator does, it converts a temperature difference into electricity.

The company is now testing a unit at a remote hot springs resort 60 miles northeast of Fairbanks, Alaska. Biederman expects a commercial power plant to be ready by early next year, after they've tested the reliability of the demonstration system.

According to him, the system could utilize the large amount of hot water pumped out of the ground at oil and gas wells. In Texas alone, more than 12 billon barrels of water are produced from wells. Oil companies usually discard the waste water by re-injecting it into the earth; but they could use it to generate electricity. Biederman is planning to set up demonstration projects at oil and gas wells in Texas and Nevada next year.

This reverse cooling concept isn't new; but until now no one has made an efficient turbine at a reasonable cost, he says. UTRC has kept down costs by modifying refrigeration units that its sister company, Carrier Corp., makes, and using its production line in Charlotte, NC.

The system's small size also keeps costs down, and makes it more usable, says Maria Richards, who coordinates the geothermal laboratory at Southern Methodist University in Dallas. "The fact that it can fit on the back of a flatbed truck and be driven to a well site makes it much more convenient and less expensive," she says. "It's [like comparing] a mainframe computer and a laptop." And, as with other renewables, increasing fuel costs are spurring interest in geothermal power units, she adds.

Gwen Holdmann, vice president of new development at the Alaskan hot springs resort where the technology is being tested, says they spent $2.2 million on the UTRC geothermal power plant, and that it should pay for itself in five years. "It could even be a quicker payback if the cost of fuel keeps rising," she suggests. Before the power plant was installed, the resort was burning $1,000 worth of diesel fuel per day to generate electricity, she says. The plant eliminates those costs and the harmful emissions from diesel generators.

Right now, geothermal power plants are located mainly in the western United States, where high-temperature steam or hot water appears naturally at the surface. Drilling wells to reach high-temperature resources deep underground can cost millions of dollars, yet still be cost-effective because they're efficient for power generation, Richards says. So far, however, it hasn't been economical to use lower-temperature geothermal resources for power.

But existing oil and gas wells, where electricity generated from waste hot water could run the oil pumps, would be the ideal location for the UTRC power modules, Richards says. "They're already drilling wells, the wells are already being used, and they're producing something that is a secondary source of energy."