The mining industry is always looking for ways to rehabilitate their abandoned operations so that there may be a public use for them. This could entail leaving behind recreational lakes, building golf courses, creating nature parks or using empty pits as public landfills. Another rehabilitation idea being studied is using old underground mines as a means of green energy storage. If successful, we do have a lot of abandoned mines in all regions of the country.
Compressed air can store energy
I was at the 2019 Progressive Mine Forum in Toronto and a presentation was given on underground compressed air storage. The company was Hydrostor (https://www.hydrostor.ca/). They were promoting their Advanced Compressed Air Energy Storage (A-CAES) system.
It is a technology that addresses the power grid need for power transmission deferral services. The A-CAES system can theoretically provide low-cost, long duration bulk energy storage (i.e. hundreds of MWs, 4-24+ hour duration).
The idea is to store off-peak or excess power from solar, wind, or other generating source. Then the system can release this power back into the system during peaks or low generation capacity. Solar and wind power normally don’t work as well at night.
Flood the mine
The system uses excess electricity to run a compressor, producing heated compressed air. Initially heat is extracted from the air and retained inside a thermal store. This preserves the heat energy for later use. Next the compressed air is stored in the underground mine, keeping a constant pressure.
While charging, the compressed air displaces water out of the mine, up a water column to a surface reservoir.
On discharge, water flows back down forcing air to the surface where it is re-heated using the stored heat and expanded to generate electricity.
Imagine an underground mine beneath an open pit, and seeing the open pit water level rise and fall daily as the compressed air is recharged underground and then released.
Hydrostor is currently building a $33 million 5-MW project in Australia at the Angas Zinc Mine site. I asked Hydrostor if they had any white papers describing the economics for a typical abandoned mine we might see here in Canada. Unfortunately they don’t have such a case study available.
Update: A Canadian example recnetly came to light; “How an old Goderich salt mine could one day save you money on your hydro bill“.
No doubt there would be capex and opex costs to build and operate the plant, but these would hopefully be offset by the power generation. It just not clear over what time horizon this payback would occur. Many abandoned underground mines are already in place; they are just waiting to be exploited.
Permitting is still an issue
Converting an abandoned mine into a power storage facility will still have its challenges. Cost and economic uncertainty are part of that. In addition, permitting such a facility will still require some environmental study.
At Hydrostor’s proposed Australian operation, a fairly extensive environmental impacts assessment still had to be completed (see the link here).
Noise, vibration, air quality, ecology, traffic, surface water, groundwater impacts, visual impacts, employment, and indigenous consultations are aspects that would need to be addressed. However, given that this would be a green energy application, one might be able to get all stakeholders on board quickly.
Conclusion
I think governments are going to have to re-think their electrical grids and come up with something more appropriate to include green power. Some grids may be too small on their own, like Nova Scotia or New Brunswick or PEI. They may have to re-organize into a regional grid system to take advantage of the various power generation sources currently in place or planned. For example, PEI has only wind power plus a cable to the mainland for when the turbines aren’t turning. NB has a nuclear power source. NS has coal that will have to be discontinued and replaced by 2030. Both NB and NS have some hydro, which is an excellent “on/off” power supply to pick up the slack when power needs change.
In looking at Ontario Hydro, they have nuclear as their steady power source, supplemented by hydro, gas, wind and solar. Their power demand increases from around 14 Gwhr in the early morning, at low demand to about 18 or 19 Gwhr in the late afternoon/early evening. Their nukes run constantly at about 10 Gwhr and the other sources vary to pick up the rest of the load. Less wind means more hydro and vice versa. This is probably one of the best mixes of power in North America right now.
When a grid can deliver power like Ontario Hydro there is no need to store power for future use. Systems like the one you describe will be too expensive to develop and operate. (There is no such thing as spare electricity.) Five Mw for $33 million is over $6,000/Mw. That’s more expensive than even nuclear to build. If sites are available it’s better to develop a hydro site or two at a much lower capital and operating cost.
Here is a link to Ontario Hydro’s hour by hour power demand.
https://sygration.com/gendata/today.html
What about in isolated northern communities that right now operate on diesel? They could go wind/solar and store it for night time use. There is an unstoppable push for more and more wind/solar and the drawback is one needs big storage or backup power supply. A lot of work being done on industrial size vanadium batteries but they are not there yet, as far as I know. Not sure if large battery storage is cheaper than compressed air mine storage in those situations. One thing being discussed is that, in the future, all the electric cars parked 90% of the time will become a major power storage component in the system.
I’m not saying there is zero application. There may be isolated areas where storage can be useful, particularly battery storage. I looked at it for a camp at one project in Oz but the owner wasn’t interested in extra capital where payback wasn’t extremely high. (But the payback was reasonable in my mind.) But overall this part of the energy market is tiny compared to total demand.
Compressed air storage systems can also be useful to level out demand…Kidd Mine used an entire drift on one level for that purpose. Systems where a good part of the energy is wasted won’t be tolerated in the future because energy efficiency will be a prime motivator. Just my opinion.