Cheaper Geothermal

Tapping geothermal: A molecular representation of a nanomaterial developed at Pacific Northwest National Laboratory that can improve the efficiency of geothermal power plants.
Credit: PNNL

Energy

Cheaper Geothermal

Fluid extracts more heat out of low-temperature wells.

Researchers at Pacific Northwest National Laboratory in Richland, WA, say they've developed a superior type of heat-extracting fluid that could dramatically improve the economics of producing renewable power from low-temperature geothermal resources.
Lab fellow Pete McGrail says the liquid is used to absorb the heat from hot water that's been pumped from underground into a geothermal plant's heat exchanger. The liquid can potentially boost the rate of heat capture by 20 to 30 percent. Researchers engineered proprietary nanomaterials made up of metals linked by organic molecules. They found that adding the nanomaterials to a fluid such as hexane or pentane significantly enhanced the heat-trapping properties of the liquid.
"The hope here is that by improving the efficiency as much as we think we can, a project can become economic at much shallower depths," says McGrail. "You'd be able to deploy in what would now be considered marginal or uneconomic areas."
There's no shortage of geothermal energy under our feet. Drill deep enough and the heat is there. An MIT-led study from 2006 concluded that geothermal power systems have the potential to supply 100 gigawatts of power to the United States by 2050, but only if new drilling and rock-fracturing technologies and advanced plant designs emerge that could lower development costs.

Improved technologies are required because most economical geothermal plants today generate electricity by using steam or hot water directly from naturally formed high-temperature reservoirs, such as the Geysers field in California. The wells are relatively shallow, the water is 360 degrees Fahrenheit or hotter, and the rock is porous enough to sufficiently circulate water. Tapping geothermal resources in less-ideal locations requires drilling deeper and forcing fractures in rock, both of which add immense cost. It also means making the most of lower-temperature heat resources, which is accomplished using binary-cycle plants that extract and repurpose the heat from underground hot water rather than using the hot water directly to spin a turbine.
In these plants, water pumped into an injection well absorbs heat from hot rock and is pumped back up through a separate extraction well at temperatures ranging from 150 degrees Fahrenheit to 300 degrees Fahrenheit. The hot water is then passed through a heat exchanger, along with a fluid with a low boiling point. This fluid, which flows in its own closed loop within the plant, absorbs the heat from the water and flashes into vapor under high pressure. The vapor passes through a turbine, generating power, and is then condensed and recycled back through the loop.
McGrail and his research team stumbled on a way to boost the energy-conversion rate as the two loops pass through a heat exchanger. Initially, they had developed proprietary materials for another project to improve the capture of carbon dioxide emitted from a fossil-fuel plant. They realized that the materials had remarkable thermodynamic qualities when added to an organic fluid. The new fluid has the potential to capture up to 30 percent more heat from a closed water loop, and, because of its rapid expansion and contraction capabilities, it can achieve higher pressures for driving the turbine.
"It's one of those moments in the lab where you look at the data and say, 'Wow!'" says McGrail. His group has received a $1.2 million grant from the Department of Energy's geothermal technologies program to build a benchtop prototype that shows the properties of the fluid in action.
"Hopefully we'll get a test loop system together by the end of the year. We'll put together a complete working unit with heat exchanger, compressor, pumps, and a turbine system so we can see the whole process working," he says.
The lion's share of the cost of geothermal is in drilling and preparing production wells, says Susan Petty, chief technology officer of Seattle-based AltaRock Energy, a developer of enhanced geothermal systems. "If you're going to get a 20 percent or higher improvement in efficiency, that's 20 percent less well," she says. "That is really, really significant."
There are potential showstoppers, however. Ron DiPippo, professor emeritus of mechanical engineering at the University of Massachusetts Dartmouth and a coauthor of the MIT report, warns that the vaporized fluid must pass through the turbine without affecting performance. "You have to really view these things skeptically and do a careful analysis of the properties of these fluids," he says. "You may have a gain on one side and a sacrifice on the other end."
Testing how the nanomaterials pass through the turbine will be a priority once the prototype is developed, says McGrail. "We don't know if it will be an issue yet."