Sure, that's an idea that may not make sense initially. But bare with me, and i'll explain.
We're all familiar with the idea that solar panels don't produce electricity when there's no sun, and wind farms don't do a lot when there's no wind. This is the ongoing conundrum with many renewables, particularly at scale.
Over 3 million Aussie households have a rooftop solar system installed, however only a tiny proportion of these have any kind of capacity (such as a Tesla Powerwall) to store the power they're generating throughout the day. Instead, they feed it into the grid, and draw from the grid.
The same issue is still challenging commercial-scale renewables such as wind and solar farms. Large battery systems are already online in a meaningful way, such as Hornsdale Power Reserve in South Australia. However, the 150MW battery installation is the largest battery installation in the Southern Hemisphere, and not nearly as common as renewable production around the country.
As far back as the very first Issue of DIYODE, I wrote about the potential for using gravitational potential energy for storing renewable energy. My own article was focused primarily on small scale functionality, essentially to complement or replace a conventional battery, or even a generator which is often used as a backup in off-grid systems already.
As we continue to try to find ways to draw our energy from something other than coal, a somewhat ironic proposition has been suggested.
The International Institute for Applied Systems Analysis (IIASA) has released an idea to repurpose disused mining sites as an opportunity to resolve this energy storage issue.
As with many of the ideas I have seen proposed, this one still uses grid power, but is designed to rebalance the grid demand to some degree.
The crux of the proposal is this: fill the mines up with sand when electricity is expensive, and remove the sand when it's cheap.
The Idea
This is the idea being propsed by IIASA, named Underground Gravity Energy Storage (UGES).
Essentially, when electricity prices are high and generation is required, large amounts of sand would be sent down the disused mine shaft. Generation essentially occurs via regenerative braking (essentially, stopping it from free-falling on its descent).
The sand is then stored underground in the mine until a suitable time to retrieve it. When electricity prices are low, or renewables are available, electric motors reverse the process, with the loads of sand being returned to the surface, ready to start again.
WHY ABANDONED MINES?
According to the published paper, there are over 50,000 closed or abandoned mines within Australia alone, and that number is over half a million in the USA.
This clearly illustrates that if proven practical, there is an amazing resource of sites which could be used for this operation. Naturally, not all sites would be suitable, but the numbers are still a great starting point.
WHY SAND?
Sand is seen as a great option compared to batteries, because there's no risk of chemical contamination of underground water systems. It's also easier to manouvre than say, large boulders.
ARE THERE CHALLENGES?
One thing that we did notice with this proposal (and even the published diagram of how it works) was with reference to jobs when explaining the benefits, since many towns struggle when a mine closes if it's a primary employer.
There is the ultimate question of how difficult would it be to stabilise an abandoned mine to make it suitable and safe for this kind of operation?
WHAT ARE THE REWARDS?
Their number-crunching seems to have yield some interesting insights, however, it's highly variable based on the mine, its depth, and storage capacity (though arguably this could be enlarged).
Some of the theoretical generation figures from the paper are between 10 and 100MW, with over 100GWh storage capacity in some cases. That's some serious power!
One thing is clear, there's potential here. The energy could be stored in perpetuity, though reality is regular generation / discharge cycles would be the foundation of a business model.
Check out the paper on the link below. It makes for very interesting reading, indeed.