Packing heat: New fluid makes untapped geothermal energy cleaner

PNNL’s new geothermal stimulation fluid could make geothermal power production more environmentally friendly and less costly where conventional geothermal doesn’t work. The nontoxic fluid is designed to be used in enhanced geothermal systems, where fluids are injected into drilled wells that lead to underground geothermal reservoirs. The fluid expands when exposed to carbon dioxide underground, which creates tiny, but deep cracks in otherwise impermeable rock.

Nontoxic solution could cut water use in half for enhanced geothermal systems

More American homes could be powered by the earth’s natural underground heat with a new, nontoxic and potentially recyclable liquid that is expected to use half as much water as other fluids used to tap into otherwise unreachable geothermal hot spots.

The fluid might be a boon to a new approach to geothermal power called enhanced geothermal systems. These systems pump fluids underground, a step that’s called “reservoir stimulation,” to enable power production where conventional geothermal doesn’t work.

The new reservoir stimulation fluid features an environmentally friendly polymer that greatly expands the fluid’s volume, which creates tiny cracks in deep underground rocks to improve power production. This fluid could also substantially reduce the water footprint and cost of enhanced geothermal systems. A paper describing the fluid has been published by the Royal Society of Chemistry in an advance online version of the journal Green Chemistry.

“Our new fluid can make enhanced geothermal power production more viable,” said lead fluid developer Carlos Fernandez, a chemist at the Department of Energy’s Pacific Northwest National Laboratory. “And, though we initially designed the fluid for geothermal energy, it could also make unconventional oil and gas recovery more environmentally friendly.”

Natural power beneath us

Geothermal power is generated by tapping the heat that exists under the Earth’s surface to extract steam and turn power plant turbines. Conventional geothermal power plants rely on the natural presence of three things: underground water, porous rock and heat. Existing U.S. geothermal power plants generate up to 3.4 gigawatts of energy, making up about 0.4 percent of the nation’s energy supply.

Enhanced geothermal power can be generated at sites where heat exists, but isn’t easily accessible because of impermeable rock or insufficient water. A 2006 report led by the Massachusetts Institute of Technology estimates enhanced geothermal systems could boost the nation’s geothermal energy output 30-fold to more than 100 gigawatts, or enough to power 100 million typical American homes.

Interested in this potential, DOE has funded five enhanced geothermal system demonstration projects across the country. At one demonstration project in Nevada, enhanced geothermal methods increased a conventional geothermal plant’s productivity by 38 percent. But the use of enhanced geothermal systems has been limited due to technical challenges and concerns over their cost and heavy use of water.

Creating enhanced geothermal systems requires injecting millions of gallons of water – a valuable resource in the arid American West, where enhanced geothermal has the most potential. That water is sometimes mixed with a very small amount of chemicals to help the fluid better create and spread tiny cracks underground, which ultimately extends the life of a geothermal power plant.

Some geothermal reservoir stimulation fluids are similar to oil and gas hydraulic fracturing fluids in that a small percentage of their volume can include proprietary chemicals, according to a 2009 paper in Geothermics and other sources. These chemicals can be toxic if ingested, leading geothermal developers to retrieve and treat used fluids. This protects aquifers, but increases the cost of power generation as well. Environmental reviews must be conducted to receive permits for enhanced geothermal injections.

A better solution

PNNL’s fluid is a solution of water and 1 percent polyallylamine, a chemical made of a long carbon chain with nitrogen attachments that’s similar to well-understood polymers used in medicine. The fluid is pumped into a well drilled at a geothermal hot spot. Soon after, workers also inject pressurized carbon dioxide, which could come from carbon captured at fossil fuel power plants.

Within 20 seconds, the polyallylamine and carbon dioxide link together to form a hydrogel that expands the fluid up to 2.5 times its original volume. The swelling gel pushes against the rocks, causing existing cracks to expand while also creating new ones. The expansion is expected to cut in half the amount of water and time needed to open up an enhanced geothermal reservoir, which shrinks the cost of power generation.

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