A Better Geothermal System

Two University of Minnesota Department of Earth Sciences researchers, Earth sciences faculty member Martin Saar and graduate student Jimmy Randolph have developed a new way for tapping heat beneath the Earth’s surface.

The pair has already named the method, called CO2-plume geothermal system, or CPG.  The research was published in the most recent issue of Geophysical Research Letters. The men have applied for a patent and plan to form a start-up company to commercialize the new technology.  Here’s why:

The CPG system uses high-pressure CO2 instead of water as the underground heat-carrying medium. Randolph explains, “This is probably viable in areas you couldn’t even think about doing regular geothermal for electricity production. In areas where you could, it’s perhaps twice as efficient.”

First, CO2 travels more easily than water through porous rock, so it can extract heat more readily. As a result, CPG can be used in regions where conventional geothermal electricity production would not make sense from a technical or economic standpoint.  Because pure CO2 is less likely than water to dissolve the materials when passing through, CPG reduces the risk of a geothermal system not being able to operate for long periods due to “short-circuiting” or plugging the flow of fluid through the hot rocks.

Another idea is the technology could be used in parallel to boost fossil fuel production from hot reservoirs by pushing natural gas or oil from partially depleted reservoirs as the CO2 is injected.

Add in another advantage – CO2 isn’t as corrosive as water, a major problem when using heated water.

The story is Saar and Randolph first hit on the idea behind CPG in the fall of 2008 while driving to northern Minnesota together to conduct unrelated field research. The two had been conducting research on geothermal energy capture and separately on geologic CO2 sequestration.

Saar explains, “We connected the dots and said, ‘Wait a minute – what are the consequences if you use geothermally heated CO2?’. We had a hunch in the car that there should be lots of advantages to doing that.”

The pair is exploiting the atmospheric CO2 craze as well.  With two motivators, the flash of insight on a northern Minnesota road trip was jump-started with $600,000 in funding from the University of Minnesota Institute on the Environment’s Initiative for Renewable Energy and the Environment (IREE).  After working the idea up, the pair applied for and received a grant from the IREE, which disburses funds from Xcel Energy’s Renewable Development Fund to help launch potentially transformative projects in emerging fields of energy and the environment.

The IREE grant paid for preliminary computer modeling and allowed Saar and Randolph to bring on board energy policy, applied economics and mechanical engineering experts from the University of Minnesota as well as modeling experts from Lawrence Berkeley National Laboratory. It also helped leverage a $1.5 million grant from the U.S. Department of Energy to explore subsurface chemical interactions involved in the process.

Saar points up the importance of the early small-scale support, “The IREE grant was really critical. This is the kind of project that requires a high-risk investment. I think it’s fair to say that there’s a good chance that it wouldn’t have gone anywhere without IREE support in the early days.”

Just recently they applied for additional DOE funding to move CPG forward to the pilot phase.

Randolph said, “Part of the beauty of this is that it combines a lot of ideas but the ideas are essentially technically proven, so we don’t need a lot of new technology developed.”

Saar takes the point further, “It’s combining proven technology in a new way. It’s one of those things where you know how the individual components work. The question is, how will they perform together in this new way? The simulation results suggest it’s going to be very favorable.”

The established method for harvesting the Earth’s heat to make electricity involve extracting hot water from rock formations several hundred feet from the Earth’s surface at the few natural hot spots around the world, then using the hot water to turn power-producing turbines.

Without the water already there for a project, some very cautioning questions arise such as how much water would be needed to get a cycling system filled.  Then the mineralization and corrosion issues come up.  These can be ‘bottomless pit’ kinds of things.

But gasses, especially ones in great volumes of supply would get around those risks.  It’s also much easier to pump gas down and back.  The idea, should more pilot work come up positive, would open a vastly larger areas to geothermal production.

In the other hand the mass of gasses compared to water is quite small.  One will have to move a lot and quickly – which is just what using a gas can offer.

Its not clear that drilling to the heat source would be any less or more costly. What is clear is getting the heat moved has a new method that’s been begging for research for years.  The gas heat moving system should offer a lot of opportunity and perhaps a lower cost of production for a longer period of time.

Rigging gas circulation in a heat source suggests a binary system that dumps the heat into a closed circulator to drive generators.  As shown with water systems, binary systems need not have such hot sources.  Near the point of boiling water is enough to start, even though the hotter the better.

The billion-dollar question is, where are the easily accessible dry hot rocks with existing passages to get the gas heated.

The Minnesotans have gotten the circulating gas technology some legs.  It’s a very high potential method that intuition suggests has very good prospects.  Go guys, and keep those papers coming.

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