A New Geothermal Heat Extraction Method

Scientists at the Department of Energy’s Pacific Northwest National Laboratory will determine this year if their innovative approach can safely and economically extract and convert heat from vast untapped geothermal resources.  PNNL’s Laboratory Fellow Pete McGrail says, “By the end of the calendar year, we plan to have a functioning bench-top prototype generating electricity.  If successful, enhanced geothermal systems like this could become an important energy source.”

PNNL’s system relies on a biphasic fluid, an intermediate carrier of the heat energy from the source heat to a thermal power unit such as a steam making system or other heated gas systems.  This offers another engineering plan.  Currently hot geothermal is trying to go from hot ground water directly to steam, or using binary systems where the heat is moved to a closed loop secondary fluid that expands to gas, drives a mechanical generator of some type and condenses back to a fluid.

Now PNNL is offering what might later be called a “trinary” system.  Heat is moved to their fluid, which is then used to transport the heat to the generation step.  All this research and engineering is needed to confine the hot geothermal materials away from the energy production side.  Most any hot geothermal is going to bring along some dissolved minerals, stuff one’s heat generation machinery isn’t going to react with well, like hot, mineral rich, corrosive or acidic or even radioactive material.  So having an intermediary or trinary system offers big savings for engineering and capital costs and surely will bring big savings in operating costs and maintenance.

PNNL’s conversion system will take advantage of the rapid expansion and contraction capabilities of a new liquid developed by PNNL researchers. To aid in efficiency, scientists have added nanostructured metal-organic heat carriers, or MOHCs, which boost the power generation capacity to near that of a conventional steam cycle. McGrail cited PNNL’s nanotechnology and molecular engineering expertise as an important factor in the development, noting that the advancement was an outgrowth of research already underway at the lab.

McGrail says, “Scientific breakthroughs can come from some very unintuitive connections.  Some novel research on nanomaterials used to capture carbon dioxide from burning fossil fuels actually led us to this discovery.”

“By the end of the calendar year, we plan to have a functioning bench-top prototype generating electricity,” predicts PNNL Laboratory Fellow Pete McGrail. “If successful, enhanced geothermal systems like this could become an important energy source.”  That is a good position to take, as the geothermal source needs some solutions to get more market traction.

The PNNL press release also offers another link to the technical and economic analysis (a downloading pdf file.) conducted by the Massachusetts Institute of Technology estimates that enhanced geothermal systems could provide 10 percent of the nation’s overall electrical generating capacity by 2050.  The analysis looks more and more dated as the maps show great swaths of the U.S. can utilize geothermal when the mechanical details are worked out such that its can be economical and competitive.

The PNNL work offers what at first glance is another costly step in harnessing geothermal heat energy. But such an assessment wouldn’t be correct.  Handling the source heat carrier is the major problem, not the power generation side.  Most any generating system can be designed at high efficiency with off the shelf technology now.

That shows the holdup is in the handling source heat.  Perhaps if the PNNL research has turns up a dirt cheap, long lasting trouble free intermediary the geothermal effort really will have great growth.  One can hope that a “three simple systems” from source heat to energy is better than one or two highly complex, made from exotic materials, and difficult or costly to maintain systems.

The work also gets the geothermal industry a little closer to “heat mining” a term we’ll be seeing more over time.  Heat mining is more about getting the heat from deep below up to use than simply putting to work the hot water as seen at Yellowstone, Iceland and other spots where the heat is close and the water is already there.  The drive to cheaper geothermal is on, and new interfacing fluids are likely to be part of the effort.

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