Carbon dioxide could be turned into a huge underground battery

What if we transformed carbon dioxide from being a waste product into being a huge battery to help even out our energy supply? We could make carbon storage pay off, while solving problems of intermittent energy supply from renewables.

So say Tom Buscheck from the Lawrence Livermore National Laboratory in California and his colleagues who presented a design for this type of energy storage at the European Geosciences Union general assembly last week in Vienna, Austria.

Their design would be able to store the excess energy produced by renewable and conventional power sources when demand is low and, at the same time, lock up the major cause of global warming – carbon dioxide.

Carbon capture and storage has been slow to develop, in part because it is an extra cost for energy producers that provides little direct pay-off. "There's no business case to do it," says Jim Underschultz from the University of Queensland in Australia.

"CCS hasn't been utilised because no one has come up with a viable use for that storage," says Buscheck. But if stored CO₂ could be used to hold surplus energy, it may give such technology the economic boost it needs.

"The only way you can decarbonise the fossil-fuel energy systems is if you can devise an approach where the economics makes sense," says Buscheck, who thinks their design, which is funded by the Geothermal Technologies Office at the US Department of Energy, does just that.

Supercritical storage

Buscheck's team proposes storing that excess energy in two forms: pressure and heat. Excess electricity would power a pump that injects supercritical CO₂ – a hybrid state of liquid and gas – into underground brine in sedimentary rocks between 1 and 5 kilometres below the surface. Supercritical CO₂ can drive turbines much more efficiently than steam and can take a lot of squeezing and heating – improving its capacity to store energy.

Another set of pipes tap into the brine in the sedimentary rocks. As the CO₂ is pumped in, it will displace some brine, which is collected at the surface. Surplus energy can also be used to heat the brine and circulate it down into the deep rocks, which are able to store the heat effectively.

When the heated brine comes into contact with the CO₂, it causes it to expand, thereby increasing the pressure of the stored CO₂. The heat energy can be gathered by allowing the CO₂ to depressurise, spinning supercritical CO₂ turbines, which are 50 per cent more efficient than the steam equivalent. The team's modelling suggests that the system could regather up to 96 per cent of the heat stored.

Their approach could help solve a major problem with renewables: intermittent power. Solar and wind can fail to produce power when there is high demand. Similarly, sometimes they produce plenty of energy when demand is lower, and in this case, sources like nuclear, coal and older gas power stations can produce energy at a loss, or simply waste the heat they produce, never turning it into electricity.

The massive batteries that would be required to store the excess are still expensive and not very effective. Storing the energy by using it to pump water uphill – a current state of the art – can also waste a quarter of the energy in the process.

Getting bigger and better

"There is no doubt in my mind that we need to consider hybrid technologies of the sort proposed here," says Peter Cook from the University of Melbourne, Australia. He says the proposal takes a lot of existing ideas and integrates them in a new way, meaning that most of the technology is already proven.

But while this could contribute to reducing atmospheric carbon dioxide, it is unlikely to become a major carbon sink, says Cook.

One site could only store about 8 million tonnes of CO₂ each year for 30 years – about the same amount as produced in one big coal-fired power station, says Buscheck, whose group is now looking for power companies to partner with on a pilot project.

Whether it is possible to scale-up the design remains to be seen, say Cook and Undershultz. Given its complexity, Undershultz says that costs and inefficiencies could add up as they scale it up. And Stuart Haszeldine from the University of Edinburgh, UK, says it would require a really good knowledge of geology to ensure carbon is sealed and does not escape.

Correction 21 April 2015: When this article was first published on 20 April 2015, the depth at which supercritical CO₂ would be stored was wrong. This has now been corrected.

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