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Comment by mrfusion | Hacker News Reader

I'd be curious to see gravitational energy storage built individually into each wind turbine. If you think about it you already have a lot of height from the tower.

Have the turbines directly lift a weight. And then lower the weight as needed to generate electricity.

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A typical wind turbine is 80m high and and generates 1.5MW [1]. Gravitational storage in Joules is (mass in kg) * (height in m) * 9.8 (gravity). Divide by 3.6 million to convert to kWh.

Suppose we raise a 30-ton weight (i.e. a big shipping container filled to capacity with rocks) all the way up that 80m turbine. We've stored 30,000 * 80 * 9.8 / 3600000 = 6.5 kWh of energy.

That's 16 seconds worth of energy from the turbine spinning at full speed.

[1] https://en.wikipedia.org/wiki/Wind_turbine#Design_and_constr...

Impressive way to put the power generation capability into perspective.

Also makes me wonder how a spring (instead of or in addition to those 30 tons) would scale to turbine tower size. Just don't think of the destruction when something breaks!

Catastrophic energy release doesn't care if you're a spring, a flywheel, a lithium battery, or a chemical bomb - in fact all those things start to look pretty similar at high enough energy levels.

But you must admit that an 80m (~262ft or ~24 stories) wind turbine blasting off like a giant pogo stick into the sky is by far the most comical.

Maybe gravitational doesn't make sense, but compressed air might. Big compressor directly driven by the turbine (so as not to waste energy converting to electricity to turn an electric compressor motor) being used to compress normal air into large tanks inside the wind turbine tower which can later be used to spin another turbine-based generator using the same air. Not a physics major, but this seems feasible. Not sure of the potential energy storage, but I feel like the direct conversion of rotational energy to spin the pump has to be much more efficient than converting it to electricity first, then using that [many miles away] to compress air or drive a train up a mountain.

Bonus points if you can spin the wind turbine with the discharge of the compressed air, on the lag edge of the propellers.

Dispatchable wind via compressed air storage seems to be quite a thing in the scientific community. Unfortunately, big subsidy schemes like the german one fail to acknowledge the importance of dispatchability, so they utterly disincentivize any development in that direction.

What about lead or a heaver element instead of rocks?

A 40' shipping container-sized box filled with lead would be roughly 90 metric tons, so multiply that number by three. Now you've got ~45 seconds worth of energy.

(An actual shipping container would break before you hit this point; a standard 40' shipping container has a maximum gross weight of ~30 metric tons-- you're not likely to fill it to its volumetric capacity with rocks or lead.)

And lead is a lot more expensive than rocks.

And rocks tend to be not toxic and can be sourced from near anywhere.

Good thing they're bringing SEVEN different 8600-ton trains up 4000 feet then.

5.5 Mile track * tan (8 degree slope) * 5280 feet per mile == ~4000 feet height.

But yeah, gravity storage as part of the Turbine itself doesn't seem too useful.

There's an old paper by Salter on the possibility of doing this through hydraulic pressure, having the turbine drive a pump, the weight being a piston within the turbine tower, and the output of the system being a hydraulic generator. This was back before modern developments in inverters meant that turbines had to have either a gearbox or a hydraulic torque conversion system like a car's automatic gearbox.

http://www.artemisip.com/wp-content/uploads/2016/03/1984-Sal... (SH Salter, M Rea, proceedings of European Wind Energy conference 1984)

You are referring to a hydraulic accumulator[0], such systems were built in the 19th century. There is a modern effort to use the technology in a massively scaled up fashion by Heindl Energy[1]. Their concept uses pumped water to raise a rock weight 100 meters or more in diameter.

[0] https://en.wikipedia.org/wiki/Hydraulic_accumulator

[1] http://www.heindl-energy.com/hydraulic-rock-storage/overview...

You need incredible amounts of mass to store the amount of power a turbine outputs using the small change in height available with a turbine.

This system doesn't need to fit that mass into a small structure and has a much longer working height.

What if you drill a hole 500 meters down and raise and lower the weight through that?

You end up with an incredible amount of mass divided by 4 or 5, which most likely works out to an incredible amount of mass.

In one of the patents, they work out an example system that can handle 30 megawatts, which might be the output of 10 or 15 turbines. The system absorbing the 30 megawatts for 30 minutes has a working mass of 1200 tons (conveniently, they use a working height of ~600 meters).

So all you need is a system capable of lifting 100 tons a distance of 600 meters (to store 1/2 hour of output). Which is probably something we are capable of building (there are cranes that lift much more significant distances), but it probably isn't something that would be economical.

What if the mass is water and you transport it back up with the capillary effect? Over a stairway?

It would make a lot more sense to fill that space with flywheels. Or better yet, good old Li-ion batteries.

There also is this idea: https://www.ecn.nl/news/item/floating-train-at-2000-kmh-set-.... It would store 20 GWh in a maglev train riding at 2000 km/hour on a 15 km circular track. More or less an enormous flywheel.

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