Once a mine has been exhausted of its ore, there's really no use for it anymore – it just becomes an abandoned hole in the ground. According to a new study, however, the shafts of such mines could serve in energy-storing gravity batteries.

First of all, just what is a gravity battery?

Well, in a nutshell, it's a system in which electricity is generated by releasing a heavy load, allowing it to drop. That electricity can then be used at times when demands on the municipal grid are high. At other times, when there's excessenergy in the grid, the gravity battery system uses some of that energy to pull the load back up, effectively storing the energy for later use.

One of the most common types of the technology is what's known as a pumped-storage hydroelectric system. In this setup, water is released from a high elevation, generating electricity by spinning up turbines as it flows downhill. When excess energy is available, that water is pumped back up to the starting point.

Last year, scientists from Austria's International Institute of Applied Systems Analysis (IIASA) proposed a different type of gravity battery. The basic idea was that the elevators in high-rise buildings would use regenerative braking systems to generate electricity while lowering weighted payloads from higher to lower floors. Autonomous trailer robots would pull the loads in and out of the elevators, as needed.

That brings us to the mine-based Underground Gravity Energy Storage (UGES) system, recently proposed by the same researchers. It would likewise utilize elevators, but these ones would be in existing disused mine shafts, and they'd be raising and lowering containers full of sand.