Carbon capture, the technology widely deemed vital to saving the planet from a climate disaster – but frustratingly slow to gain traction – could have a friend in geothermal power.

For a few years, scientists have kicked around the idea of exploiting a possible synergy between the two technologies. Instead of pumping carbon captured from fossil-fuel-fired generating stations or industrial plants underground for storage, they’ve suggested taking advantage of CO2’s favorable properties and using it to replace water as the heat transfer mechanism in geothermal power plants, improving their efficiency and making geothermal power more widely viable geographically.

image via Energy Pathways video
Imagining how wind, geothermal and carbon capture could all work together (image via Energy Pathways video)

Well, that idea is back in the spotlight this month, with a twist: A team of scientists is proposing to use nitrogen, in addition to carbon dioxide and water, in some kind of network of “subsurface concentric rings of horizontal wells” to draw underground heat to the surface.

The researchers, from the Lawrence Livermore National Laboratory, Ohio State University and the University of Minnesota, said this past week that “in computer simulations, a 10-mile-wide system of concentric rings of horizontal wells situated about three miles below ground produced as much as half a gigawatt of electrical power – an amount comparable to a medium-sized coal-fired power plant – and more than 10 times bigger than the 38 megawatts produced by the average geothermal plant in the United States.”

The distribution of stored nitrogen in the underground geothermal reservoir system is shown after 10 years of energy storage and production operations. (image via LLNL)
The distribution of stored nitrogen in the underground geothermal reservoir system is shown after 10 years of energy storage and production operations. (image via LLNL)

But why the nitrogen addition?

“Nitrogen has several advantages,” Tom Buscheck, earth scientist from Lawrence Livermore National Laboratory and leader of the research team, said in a statement. “It can be separated from air at lower cost than captured CO2, it’s plentiful, it’s not corrosive and will not react with the geologic formation in which it is being injected. And because nitrogen is readily available, it can be injected selectively. Thus, much of the energy required to drive the hot fluids out of the deep subsurface to surface power plants can be shifted in time to coincide with minimum power demand or when there is a surplus of renewable power on the electricity grid.”

So this is how the technology becomes both more economical and an aid to solar and wind or any other variable clean energy source — the nitrogen can be pressurized when, say, the wind is blowing in the middle of the night, thus preserving its energy-producing potential.

As for the thinking behind the concentric ring wells, in a paper earlier this year, Buscheck and his collaborators wrote:

A well pattern consisting of a minimum of four concentric rings of horizontal producers and injectors is proposed to conserve pressure from injection operations, minimize loss of supplemental fluids, generate large artesian flow rates that take advantage of the large productivity of horizontal wells, and segregate the supplemental fluid and brine production zones.

The broader idea that this scheme piggybacks on is known as CO2 plume geothermal, or CPG, hatched a few years ago at the University of Minnesota by Martin Saar, a professor in the school’s College of Science and Engineering, and Jimmy Randolph, a research associate at the university (and who was in on this latest research).

At the heart of the Saar-Randolph vision is the idea to pump CO2 a few miles or so underground into salty aquifers, where it would be heated and pressurized into a supercritical state. In a 2012 release, the University of Minnesota described the process that would then take place:

The supercritical CO2 would flow through porous bedrock more easily than water. Becoming far less dense than water as it warms, the CO2 would rise quickly through the brine-soaked bedrock and pool beneath a virtually impermeable caprock, such as shale. The now hot, low-density fluid would buoyantly rise through a production well without pumping. At the surface, the CO2 would drive a turbine— more efficiently and vigorously than water drives conventional steam-generation turbines. After cooling, the CO2 would be pumped back down the injection well, flowing in a closed, geothermal heat self-powered thermosyphon loop that would let none escape to the atmosphere. The geothermally generated power could help run the CO2 injection pumps that provide the initial CO2 captured from the CO2  emitter. In addition, revenue from any additional power generation could help defray the cost of carbon capture and sequestration.

Saar and Randolph have spun the CPG idea off into Heat Mining Company, and in October Finance & Commerce reported the startup “expects construction to begin next year on a pilot plant that would produce electricity” in the first commercial application of the technology. Presumably the Heat Mining Company project will incorporate this latest research.

While some believe that renewables and energy-efficiency efforts – or all that plus nuclear power – could be an adequate response to the climate change threat, the consensus view is that fossil fuels remain too cheap and too entrenched to be pushed aside quickly. Thus, for all our wind and solar and other clean-energy efforts, “Current short, mid- and long-term projections for global energy demand still point to fossil fuels being combusted in quantities incompatible with levels required to stabilise greenhouse gas (GHG) concentrations at safe levels in the atmosphere,” says the International Energy Agency.