Underground heat gives light
A new kind of power plant could take advantage of widespread low-temperature geothermal sources in the U.S.
A new geothermal power plant at an Alaskan hot springs resort harnesses the lowest-temperature underground heat source ever used to generate electricity. The plant, which went on-line this summer, could make geothermal power more common across the U.S. and lead to the use of low-temperature waste heat to produce electricity.
Typically, geothermal sources at temperatures greater than 300 °F (177 °C) are used to generate electricity on a large scale, powering whole cities in a few cases, whereas lower-temperature sources provide heat, but not electricity, to single buildings. But the system created by the United Technologies Corp. (UTC) extracts electricity from the 165 °F (74 °C) water at the Chena Hot Springs Resort, 60 miles northeast of Fairbanks.
“That’s amazing, because that beats the record for lowest temperature for a geothermal power plant,” says Roger Hill, a geothermal power researcher at Sandia National Laboratories. Hill, who was not involved in the project, says that naturally occurring low-temperature geothermal sources crop up across the U.S., and the new technology will make them accessible for electricity generation.
The system also paves the way for using a byproduct from drilling oil and gas wells. “Ninety-five to 98% of what they pump out of the ground in Texas is hot water,” says Bruce Biederman, who leads the UTC project. “They either reinject that hot water or discard it. So, currently there’s a tremendous amount of resource that’s not being used,” amounting to billions of gallons. Biederman expects to have demonstration projects running at oil and gas wells in Texas and Nevada by early next year.
The power plant is essentially a cooling system that runs in reverse: instead of using energy to cause a temperature difference, like a refrigerator does, it converts that difference into energy. In this case, the refrigerator compressor becomes a turbine that is driven as heat vaporizes a refrigerant.
Although this reverse-refrigeration concept has been around for a long time, costs to build a power plant based on it have remained a stumbling block, according to Gwen Holdmann, a mechanical engineer who is the vice president of new development at the Chena resort. But UTC has kept prices low by using commercially available parts from its sister company, Carrier Refrigeration.
“At the same time, the cost of fuel is going up,” Holdmann says. “Those are the two main driving factors” that are making the technology more attractive.
At Chena, the power plant eliminates diesel generators, cutting expenses and emissions. “We’d been burning diesel fuel at the cost of about $1000 a day to keep the lights on here,” Holdmann says. The resort invested $2.2 million in the UTC plant, which should pay for itself in 5 years or less. Moreover, because the plant’s capacity is 400 kilowatts (kW)—in the form of 2 standard 200-kW modules—and the resort needs about 250 kW to operate, Chena will have surplus energy.
“Right now, our resort is basically operating off close to 100% renewable energy,” Holdmann says, with plans to apply geothermal power to other uses as well. The resort already uses it for heating lamps at a new 5000-square-foot greenhouse and will start generating hydrogen to replace propane for cooking and fireplaces before the end of this year.
