When you’re trying to tap geothermal energy, for heating or electrical generation, you’ve got to consider a number of factors.
“The temperature is the obvious one. Flow rate is really important too,” says Gwen Holdmann, director of the University of Alaska’s Center for Power and Energy. She spoke at the recent Rural Alaska Energy Conference in Juneau.
“You might have a really high temperature resource and one good example that’s pretty close by here (Juneau) is Tenakee Springs. They have a fairly high temperature but they have an extremely low flow rate,” she says.
You also need to know the extent of the hot-water reservoir, its depth and the rate it recharges itself.
Another importation factor is location. Being close to a city or transmission lines make tapping power more affordable.
That’s why the Aleutian Islands city of Akutan, and the local Trident Seafoods plant, are looking at nearby Hot Springs Valley.
“We hit water as hot as 350 (degrees)-plus at 500 feet,” says Ray Mann, a consultant for the city of Akutan, northeast of Unalaska.
“But according to the studies that have been done that’s the outflow resource and we probably will not get the flow and the capacity we need. So we have to go further up the valley to the upflow zone. And the estimate is we could achieve anything between 15 and 100 megawatts, with a minimum of 8 megawatts, to provide power,” Mann says.
Outflow is where water comes from the ground. Inflow is the subterranean area where it travels to near the surface.
There’s been interest in the Akutan site for at least 30 years. New wells were drilled this and last year, one finding water up to 500 degrees. Deep water is under enough pressure that it does not boil off at those temperatures. And other studies further defined the resource.
Mann says the city is committed to building an approximately $60 million plant, including about 5 miles of road and transmission lines.
He says it could bring down power costs, from 66 cents a kilowatt-hour, without power-cost equalization, to around 13 cents per hour. Studies have shown it’s a better option than wind or hydropower.
“This has been the option because of the proximity of this resource and the size of the resource. There really aren’t that many other applications that are going to generate 7 to 8 megawatts for you the way this will do, because it’s right there and accessible,” he says.
Seven or eight megawatts is what Akutan needs, including the seafood plant. Trident is conducting its own study, and Mann says they’re working together.
He says the town already has about $10 million toward the project. It’s seeking another $15 million from government sources, and $45 million in private sector investments, which could include Trident.
Energy conference speakers say another area with a significant geothermal resource is Pilgrim Hot Springs, near Nome.
“We selected this site because we thought it was one that had potential to be developed to benefit the region,” says Gwen Holdmann of the Center for Power and Energy.
She says the former spa was first drilled in the late 1970s. She says the crew found two surprises.
“They didn’t hit bedrock, which is pretty interesting. They went down to 1,000 feet. And then the second thing that happened is that they drilled through a really shallow layer of super-hot water, up to 200 degrees Fahrenheit. That’s really hot. That’s barely below boiling temperature,” she says.
More drilling took place in the past two years. The work is not finished, but agencies think it’s worth developing. Holdmann says studies show the resource could produce more than enough electricity to power Nome.
“Right now the estimated potential for power generation is approximately 5 megawatts. We haven’t found anything to change that number at this point in time. That’s still a viable number,” Holdmann says.
A preliminary report, released about 4 years ago, estimated total cost at $50 million to $115 million, depending on the depth and generation system. A final report is due in about a year and a half.
A significant part of the expense would be transmission lines. That’s because the site is about 50 miles from Nome.
Holdmann says developers could tap the $4 or so million a year the town spends on diesel generation to help pay off construction costs.
Alaskans looking into geothermal energy are watching existing developments to see what they can learn. One point is the limits of some hot-water reservoirs.
“Renewable is not the same as sustainable and can’t be used interchangeably,” says Jo Mongrain of the University of Alaska Fairbanks Geophysical Institute.
“Geothermal is a renewable resource. You extract energy and it’s going to be replaced by a comparable amount of energy. But what renewable doesn’t tell you is over what kind of time scale and what are you actually doing to that resource,” she says.
Mongrain has studied Chena Hot Springs, east of Fairbanks. It’s home to one of the state’s best-known geothermal energy projects.
She says research have shown several underground hot water zones below the resort. And some have showed lower temperatures and pressure as more wells have been drilled and more water removed.
“We certainly have interzone mixing within the well, although some of that’s being addressed by filling the wells up with bentonite. We need to look at the data in more detail but we may also have some kind of mixing within the reservoir,” Mongrain says.
She says sealing exploratory drill holes, and lining open shafts, can help keep cold water out of hot zones. And systems where cooling water is returned underground to maintain flow need separate wells.
Further studies could lead to additional recommendations for making future systems more sustainable.