Thursday, 12 November 2009
Geothermal – just add water
With a $25 million influx of federal stimulus money, two companies are planning to try a different way of generating geothermal power just west of Newberry National Volcanic Monument.
But to test this method, Newberry Geothermal and AltaRock will need to add something more to the equation — an estimated 77 million gallons of water or more.
“The water question is clearly one that we all know is there, and that we will have to address,” said Doug Perry, the president of Newberry Geothermal.
In a traditional geothermal system, a company drills a deep well and taps into a pent up supply of water or steam, heated by the surrounding rocks. But when Newberry Geothermal drilled test wells in 2008, they found hot rocks but no water or steam.
So the idea with the new method, called enhanced geothermal systems, is to pump outside water down into a system of hairline fractures, have the rocks heat up the water, then pump it back to the surface. The hot water or steam will be harnessed to turn turbines, then cooled a bit and sent back down to be heated up again.
But the rocks could be leaky, said Asante Riverwind, with the local chapter of the Sierra Club, and people don't yet know how much water one of these plants would consume to generate power.
“Where would they get this water, given the water concerns already existent here?” Riverwind said.
Newberry Geothermal already has a temporary OK to use up to 100 gallons of water a minute, which it can get from any of four groundwater wells.
The company had to make up for the water it removed from the groundwater aquifer by buying mitigation credits, which pay for other projects that put water back into streams and rivers of the Deschutes Basin.
“Some farmer, someplace in Deschutes County or the Deschutes basin, is not irrigating, and in return the groundwater mitigation buyer is using that water instead,” said Scott McCaulou, program director with the Deschutes River Conservancy.
The Newberry project doesn't need to mitigate for that much water, compared with irrigators and other water users, he said — it's about the equivalent of watering 2 or 3 acres of alfalfa.
If the company needed to use more water, it could either apply for a new temporary license to do so, which would require more mitigation credits, McCaulou said, or buy up new water rights.
And the geothermal project will probably need to use more water, Perry said.
Water is needed for several steps in the enhanced geothermal system. Crews use water to drill a well, and then pump pressurized water down into that well to fracture a network of tiny cracks in the rocks thousands of feet below the surface.
“It takes a lot of water for a short period of time” to create that network of cracks, Perry said.
The company will also use water to drill another well or two that will eventually bring hot water back to the surface. And it will use more water to test how well the whole system works — how easily the water flows from the original well, through the network of cracks, and back up to the surface.
Because this is a relatively new way of doing things, especially in the United States, geologists don't know exactly how much water this will take, Perry said.
But based on a similar project in France, Perry estimated that drilling the wells, creating the cracks and testing the system would take between 77 million and 121 million gallons of water — more than the 53 million gallons they currently have the OK for.
In comparison, on the peak summer day in 2007, the city of Bend used 27 million gallons of water, according to a city publication.
Creating the cracks would also take an OK from water officials to use more than 100 gallons a minute, he said.
Perry added that most of the water used in these tests remains in the different rock layers. Some of the fluid, however, is lost to evaporation — and that has to be mitigated for.
After the testing phase for enhanced geothermal systems, if the company decides to build a power plant, additional water would be needed to keep flow circulating through the system.
Project planners don't yet have an estimate for that volume, Perry said, noting that a lot depends on how leaky the rocks are and how much water needs to be replaced.
There could be alternatives to groundwater as well, he said. Project designers could end up piping in gray water, or treated wastewater, and cycling that through the underground reservoir. Or, some new technology uses liquefied carbon dioxide instead of water, he said.
But it will be several years before Newberry gets to the power plant stage, he said, and the company hopes by that time the enhanced geothermal tests will reveal more about how much water would be necessary.
“We'll know enough so it will be a lot more factual based, (rather) than by analogy,” Perry said.