Grid gravity storage

[clv101]

This looks like it could have potential:
https://www.bbc.co.uk/news/uk-scotland-56819798.amp

https://www.bbc.co.uk/news/uk-scotland- ... s-50146801

Grid scale storage, significantly cheaper than batteries, energy can be stored indefinitely and released fast or slow.


Back of the envelope, gravitational potential energy = m g h

Say 12 million kg x 9.8 x 2000 m x 75% efficient = 48 MWh

So 100 MW for half an hour, or 1 MW for 100 hours. That's useful storage.

[BritDownUnder]

Definitely worthy of further investigation. I wonder whether the diameter of a typical mine shaft may be a limiting factor in making the weight very long. Also whether mine shafts in the UK were capped or filled with stuff. I can imagine it would have been very tempting to drop a lot of rubbish down a derelict mine shaft in a Thatcherite fit of pique in the years following the Miners Strike.

I have also heard about a system that will lift concrete blocks building a kind of tower to store energy and then dismantle it when the energy is recovered. Then a system that uses a kind of 'piston' arrangement using a huge chunk of rock excavated to form a cylinder. Then a system of railway carriages being pulled up a slope.

Thinking about the 12,000 tonne weight. A 2 inch (or 52mm if you prefer) diameter steel cable has a safe load of about 30 tonnes (note the breaking load is much higher, about 140 tonnes) according to a quick web search. This means a 12,000 tonne weight will require 400 such cables. I have seen thicker cables on a mining dragline lifting maybe 300 tonnes of rock overburden. Then the cable will weigh 10kg per metre so a 2000 metre cable will weigh 20 tonnes and have to support itself when fully lowered taking two thirds of the usable safe load, or if you think differently, the cable could partly be its own weight.
400 cables is quite a lot to go down a deep mineshaft and each having their own cable drum, bearings and winding mechanism. I suspect the 800 metre version will be a bit more viable.

All in all I think it will be a bit more costly and complex than people think but viable in an engineering sense.

[clv101]

Thinking about the 12,000 tonne weight. A 2 inch (or 52mm if you prefer) diameter steel cable has a safe load of about 30 tonnes...

It's not one big weight, lots of smaller ones dropping in series.

[adam2]

I suspect that the costs and complications might rule this out.
A 120 ton weight in a deep mineshaft would store about 480 KWh
A similar weight in a more typical depth mine shaft would store perhaps one quarter of that or about 120 KWh.

I am not convinced that the cost of the installation would be less than a 120KWh battery, which is only 2 or 3 EV batteries. The batteries could be placed near the load, rather than being restricted to existing mineshaft locations.

Also, do we have enough deep shafts already existing ? The economics of constructing new ones sounds even more doubtful.

[kenneal - lagger]

Thinking about the 12,000 tonne weight. A 2 inch (or 52mm if you prefer) diameter steel cable has a safe load of about 30 tonnes...

It's not one big weight, lots of smaller ones dropping in series.

They manage to hang a suspension bridge weighing many more tonnes than that off steel cables. You would need one hell of a winding wheel though to allow for the bend in the cable. But, yes, a series of weights would work better.

[BritDownUnder]

Thinking about the 12,000 tonne weight. A 2 inch (or 52mm if you prefer) diameter steel cable has a safe load of about 30 tonnes...

It's not one big weight, lots of smaller ones dropping in series.

Now I see. 24 weights so 500 tonnes each. So four 2 inch cables might just do it without considering the weight of the cable and close to the breaking strain - maybe 10 or 20 for safety or even larger cables. I think the economics will need to be worked out well. I see the blocks will have to be coupled and uncoupled and on the surface being moved aside and stored

Maybe this system could be used for instances where rapid charging and recharging are required. In Australia there is something called the FCAS (Frequency Control and Ancillary Services) Market basically to inject and withdraw small amounts of power into the grid many times a day to keep the frequency close to 50 Hz. The Tesla battery in South Australia makes most of its money that way and not as a bulk storage storing MWh for hours on end. It would have an advantage of having no battery memory effect either.

I see it as viable but a very 'niche' system that may have some limited use. Possibly more useful on the fringes of a long highly renewable grid or in microgrids where there happens to be an old mineshaft.