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Comment by runeks | Hacker News Reader

Here's an idea: what if -- instead of digging a deep hole into the ground -- we use the tower of a wind turbine as the vertical space in which the weight is raised/lowered?

A wind turbine is already equipped with a generator, so it'd be a matter of building some sort of "switch" which would make it either: (a) generate electricity using the turbine, as per usual; (b) raise the weight using the turbine, thus not producing electricity; (c) generate electricity by connecting the weight to the rotor/generator, while lowering the weight.

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> Here's an idea: what if -- instead of digging a deep hole into the ground -- we use the tower of a wind turbine as the vertical space in which the weight is raised/lowered?

First, there's not really any room in there, wind towers are built for the job, we don't add twice the amount of steel and an elevator shaft for funsies.

Second, even if there were some room it would be inconsequential, you need a lot of crap very high up to get significant amount of energy.

As demo, let us take the largest turbine available right now and lift the entire thing up and down its tower.

The Haliade-X has a hub height of 150m, a 600t nacelle, and 165t blades. In normal operations it's rated for 14MWe.

765t at 150m is 250kWh. You can get electric buses with larger battery packs than that. For reference, US households use about 29kWh/day.

That's the issue with gravity: it's really not that strong, so to store significant amounts of energy you need either ridiculous amounts of weight (hence dams and pumped hydro which can manage unfathomable weights), or extreme height difference (hence… still dams, pumped hydro a bit less so I think).

As a point of comparison, Bath County (the largest pumped-storage station in the world) has a hydraulic head of 270~385m and the upper reservoir stores 44 million tons of water.

Now taking in account that you can't really empty the entire thing, that it's not perfectly efficient, etc… Bath has a storage capacity of "only" 24GWh, it's not actually moving 44 million tons up and down 400m.

What if we used load bearing batteries as part of the tower structure?

Turbine fires would turn into something really special.

You risk damaging the batteries and you make maintenance a pain in the ass. If you've got that amount of batteries, just put them on the ground the normal way.

I was thinking what if we used buildings built on jacks as our source of mass. Big one are largely built on pilings anyway, and even small ones on softer ground.

Going to be a lot of flexing though.

Nice thing is that some of the “inefficient” heat may be usable on-site: and you may get compressed air as an output that could be usable directly.

> I was thinking what if we used buildings built on jacks as our source of mass.

Then you've got no height. Plus buildings are not dense so it's a pain in the ass, and when (not if) the jacks fail you're out a building.

It’s amusing to think of whole houses getting jacked 100 ft into the air on giant stilts during the middle of the day when solar panels are operating, then slowly dropping to release the stored energy in the evening/morning.

:-) Perfect for quarantine.

I don't know how much a typical house weighs, but assuming 100t, raising it by 30m will give you potential energy of ... 8.2 kWh, or about $1.6 in today's electricity rate.

Gravity is weak.

I was estimating based on a 200-ton house, 30 kWh/day electricity use, and a need to store up to half of daily energy at any one time.

What about using batteries as the weight to lift?

> That's the issue with gravity: it's really not that strong

The weakest of all forces, isn’t it?

We should always hold out for some new discovery that is weaker.

Design forces are very weak...

btw anyone saying that gravity is weak should try getting of this planet...

anyone saying gravity is strong relative to the other forces should ask themselves why they don't keep sinking or get crushed when gravity pulls there feet back to the ground.

Yes, by many, many, many orders of magnitude.

Does that matter though? If it is weak to extract energy with gravity, then it should also be weak to “charge” the system against gravity. The weakness may push these systems to less “instantaneous” loads, but I don’t see why it would necessarily be bad to use gravity storage (as opposed to another mechanical method like pressure).

I’d love for someone to explain more though.

It's just impractical. You only need a 21x21 foot square of ground covered in solar panels to generate the same amount of electricity this does.

It doesn't make sense to invest so much land, machinery, and labor into something that generates and stores so little energy

While I don't disagree with your general sentiment of the viability of gravity batteries. I don't think its a very good comparison. One is all about electricity generation and the other is all about storage. If gravity batteries ever become used in practice it would probably be in tandem with solar. You would charge the battery during the day off solar and allow it to discharge at night when there is no sun to stabilize the grid.

Because gravity is so weak, in order to store a meaningful amount of energy, the lifted object has to be very heavy or very high. Building large tall structures is expensive, and we just don't have objects that are dense enough to make the system small.

Okay, so the question isn’t “can you get out what you put in”, but rather “is the amount of energy you can store worthwhile”?

Yes. The efficiency is OK (nothing amazing), it's really the scale requirements for anything useful which are problematic.

It depends on the distance

Wind turbines use the tower to take parts to the top for maintenance. Regardless, you'd need to dramatically increase the wall thickness of the tower along with the foundation for the additional load, to the point it probably wouldn't be worth it.

Bad math, let's say we can use a lead weight 2 meters in diameter, 3 meters tall (107,000 kg) with 120 meters of drop available.

All of that gets us 35 kWh, or 0.035 megawatt hours of energy. Compared to a 2 MW turbine, it's a negligably small amount of stored energy, even if we scaled every dimension up by a factor of 2 (getting us to 0.28 MWh).

Funny how people tell you that that this does not make any sense at all, but here it is: http://www.naturspeicher.de/en/naturstromspeicher.php

(Yes, I am aware that it is not storing the water at the top :)

Furthermore, there exists the opposite concept of replacing water with air deep in the sea: https://www.iee.fraunhofer.de/en/research_projects/search/20...

I think it'd be more efficient to use a massive flywheel for a wind turbine. That flywheel could then be the battery and each turbine would come with their own.

This could be something more like a planetary gearbox or a differential that can connect 3 systems (e.g. gas engine, electric motor, wheels in a hybrid car). But if requirements are too different 2 generators may be the better option.

The tower would probably need reinforcements for the additional weight, but I have no idea about the magnitude required.

I’ve always been surprised that there is no product wind turbine with a guaranteed power output, that is, a turbine with an embedded battery. Sure it would cost much more, but in some applications steady power is worth it.

> Sure it would cost much more, but in some applications steady power is worth it.

It would cost much more and be way more complicated & less reliable than… just getting the two separately and using battery grid storage on the output of the windfarm.

Even for a backyard turbine it wouldn't make much sense, for maintenance reasons you'd want to manage your turbine and storage separately, and if, say, you add solar panels, you want that to feed into your battery bank as well.

Why would it be more complicated and less reliable than getting them separately? I understand that it would be a more complex product for GE but for a turbine buyer it sounds extremely attractive

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