Storage is really the central energy problem our civilisation has. The fossil fuels that we consume release energy that has been stored for millions or years. Batteries are useful because we can release chemical potential energy in a reversible process on demand. And hydrogen fuel cells can be considered a type of open-to-the-air battery. Even Hydro-electric dams store energy in the form of gravity potential energy stored behind dams. They are reversible in that pumping water uphil to recharge the dam - so-called pumped storage hydro is a great way to smooth out the demands of the grid with the capacity of renewable generation.

Some storage and release systems are more efficient than others. Hydrogen fuel cells are notoriously inefficient as it is difficult to reverse the process. While lithium ion batteries are difficult to scale up - while being very useful to power small devices. Pumped-Hydro is a good way to store large amounts of energy but there are others.

The Guardian has a recent article How UK's disused mine shafts could be used to store renewable energy on a new project that uses weights on winches suspended in vertical mine shafts. To store electricity the weights are winched to the top of the shafts, and to release the energy the weights are released to slowly unwind the winch which acts as a generator. For motors and generators are helpfully the same thing just operating in reverse.

This opens up new notions for storage. Imagine a skyscraper that buys cheap off-peak electricity and raises weights within its lift shafts. These can then release the energy when demand requires it. Optimising the use of the lift to time of day might mean that the shaft is used at busy times - morning and evening rush hours. But at other times it is used for energy storage. This also begs the question - why don't lifts use a system where there are departure times so that falling lifts can power rising lifts in a balance of generation and utilisation? This might be best worked for mass transit systems like those in Hong Kong where large lifts are used to bring passengers to and from deep stations. I'm thinking of the MTR's HKU Station on the Island Line.

Gravitricity is commercialising the mine shaft notion, with its eyes on several countries with deep vertical mines, but might purpose built shafts also be an option?

I could imagine shafts that plumb the depths a very real option when the alternative of damming an ecologically sensitive area would be disruptive. Its likely that there would be a bottom limit to purpose built shafts - but some mines in South Africa reach down kilometres. Surely enough to store a high amount of grid-scale energy. One more option to the storage conundrum.