Sometimes we have too much electricity. But more often, grid operators are carefully managing its production to be sure that we have enough.

So, a lot of work has gone into trying to store excess electricity, to use later when we need it.

The obvious solution, giant batteries, is still too expensive for most applications and has environmental implications. This has led scientists to look for other ways.

One method uses surplus power to compress air and pump it into old salt mines. The salt tends to seal cracks in the walls, making the mines airtight.

When needed, the compressed air can be released to turn a turbine. Or it can be used as the intake air for a natural gas power plant, making the plant more productive.

Another way to store excess energy is to pump water uphill, into existing reservoirs, and then release it through hydroelectric dams when power is needed.

This method was pioneered 100 years ago in Italy and Switzerland, and is used today around the world, and in many U.S. states like Michigan.

On the Chilean coast, they’re even experimenting with using solar energy to pump seawater up a cliff, where it could flow down to make power at night.

These solutions don’t make economic sense unless the electricity is very cheap, and the reservoir was already built for another purpose.

But when those two things are present, pumping air and water to store energy plays a valuable role in balancing the grid to meet our ever-changing power demands.