Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
Storing compressed air in old oil wells and releasing it later to drive energy turbines looks promising as a more sustainable energy source according to the latest models from the USA.
The process is known as Compressed Air Energy Storage (CAES).
Pennsylvania State university researchers have turbocharged the performance of CAES, a struggling renewable energy storage technique, by co-opting the geothermal heat of old oil and gas wells
Renewables such as solar and wind are intermittent, which is why research is increasingly focussed on storage — putting the energy somewhere for when the sun isn’t shining, or the wind isn’t blowing.
Penn State researchers are now looking at tapping into geothermal energy to turbocharge a relatively new but struggling energy storage technique — Compressed Air Energy Storage (CAES).
“Without taking advantage of the geothermal setup, you could not get enough encouraging numbers,” says Professor Arash Dahi Taleghani, of Pennsylvania State University in the USA.
CAES uses electricity to compress air into sealed underground caverns or above-ground tanks, which is later released, through a turbine, to generate electricity.
Start-up costs limit CAES’s commercial development, says Dr Qitao Zhang of Penn State. Zhang, Taleghani and colleagues used computer simulations to model the compression of the air down into old oil and gas wells. The idea makes use of the earth’s heat, the ‘geothermal gradient’.
Temperature increases with depth into the Earth’s crust — by 25-30oC per kilometre, depending on the thickness of the crust, heat flowing from the underlying mantle and nearby volcanic activity. That’s the geothermal gradient. 5 km down, temperatures range from 125-150oC; at 30Km — the base of the crust — expect 600-900oC.
3.9 million depleted oil and gas well dot the USA, so there is plenty of scope, and modelling suggests well use could improve CAES by around efficiency by 9.5%, meaning more energy recovery and more electricity, says lead author, Zhang.
Air heats with compression — pumping it down into hot rock reservoirs adds more pressure, potentially meaning more stored energy, says Taleghani.
“Drilling new wells may not justify the economics of this type of storage. But by combining these two factors, and by going back and forth through modelling and simulation, we found this could be a very good solution.”
Simulations showed that geothermal heat would add about 160oC and about 0.5 MPa to air compressed down the old wells, says, Zhang.
Compressed air storage in Australia
“This improvement in efficiency can be a game changer to justify the economics of compressed-air energy storage projects,” says coauthor Taleghani. “And on top of that, we could significantly avoid the upfront cost by using existing oil and gas wells that are no longer in production. This could be a win, win situation.”
“If we use existing wells, we are basically hitting two birds with one stone,” Taleghani said. “First, we are sealing these wells. That stops any potential leaks. And then if we are repurposing these wells for energy storage, we are still using the infrastructure that is in place in these communities. It can potentially maintain employment in the area and allow communities to be part of the energy future.”
The researchers published their findings in the Journal of Energy Storage.
Are you interested in the energy industry and the technology and scientific developments that power it? Then our free email newsletter Energise is for you. Click here to become a subscriber.