Using gravity to store energy

Edinburgh-based Gravitricity are starting work on a scaled down demonstrator tower to test the response of their design for a gravity driven energy storage system.

It’s not exactly a ‘new’ idea – I keep thinking of my dad’s fabulous 18th century grandfather clock – but it is deliciously simple and elegant.

Gravitricity plan to re-purpose disused mine shafts by installing a wind-up weight system, just like our grandfather clock, that can be used to even out the variable supply of renewable energy sources such as wind and solar.

The idea is, that like a battery (Elon Musk’s Powerpack for example) surplus electricity from the grid can be stored. In this case it is used to wind the weights up, and when there’s a shortfall of energy the weights can be released to provide near instant power from the winches which become generators in reverse.

Design visualisation of the test rig
Gravitricity test rig

The speed of that response to demand is what they are hoping to demonstrate on their test rig.

“Our demonstrator will use two 25-tonnes weights suspended by steel cables. In one test we’ll drop the weights together to generate full power and verify our speed of response. We calculate we can go from zero to full power in less than a second – which can be extremely valuable in the frequency response and back-up power markets,” explains Gravitricity Lead Engineer, Miles Franklin.

“We’ll then run tests with the two single weights, dropping one after the other to verify smooth energy output over a longer period, alongside a programme of other tests to demonstrate and refine the full capabilities of the system. This two-month test programme will confirm our modelling and give us valuable data for our first full-scale 4MW project which will commence in 2021.”

And, although they can repurpose any type of mine shaft (or dig their own), you have to admire the symbolism of putting old coal mine shafts to use as green energy machines!

So, why use gravity and not a battery?

Well, partly because you’re not having to dig up expensive minerals to make batteries, and because battery efficiency declines over time. The weights in the shaft can keep delivering the same power output for decades or longer… just like dad’s grandfather clock!

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