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Comment by bunderbunder | Hacker News Reader

I'm curious to know why other storage methods aren't taking off more. I would have assumed that the long-term costs of something like hauling something heavy up a hill (which could easily be a manmade earthworks in flat areas) would be quite a bit lower, on account of the equipment in question having a longer life span than batteries.

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The energy density of gravity storage is terrible. But there's still a few plans for it:

https://www.wired.com/2016/05/forget-elons-batteries-fix-gri...

> 9,600 tons of rock- and concrete-filled railcars up a 2,000-foot hill.

> $55 million

> The Nevada project has a power capacity of 50 megawatts and can produce 12.5 megawatt-hours of energy.

Using the 4 cents/kWh figure upstream, that's ... $500 of electricity per round trip.

I'm still curious about that project. Energy density is a consideration, but it may not be the only one.

Yeah, as-is, there's no way it can pay for itself. But how much room does this have to grow, and at what cost? If most the cost is down to laying the track and making these custom railroad cars, then maybe there are some economies of scale that could come into play. And then I assume that most of the equipment involved could easily last for decades.

Up-front cost for battery installations may be cheap, but don't lithium ion batteries typically only last a few years? That means you won't be able to amortize the cost of equipment over a very big stretch of time at all.

This makes me wonder, why don't elevators generate electricity on the way down?

Elevators are counterbalanced -- when they are going down, there is a counterweight going up. So there's less energy than you might imagine being generated.

I know, but I can imagine that they would still generate some energy in a busy building. Then again, I don't really know how much energy they consume.

Regenerative braking elevators exist: https://www.asme.org/engineering-topics/articles/elevators/w... has a good summary.

Having lived in a 17-story building with Thyssen-Krupp "green elevators", they're terrible! An elevator would break down and require days of maintenance every couple months, greatly increasing queueing times.

Moving the elevator down would only take potential energy out of the system when the elevator together with its load weighs more than the counterweight.

When it does, you might be able to use it to generate electricity. But I wouldn't be surprised if that isn't done simply because it wouldn't justify the added cost and complexity of building all that extra energy harvesting mechanism into the elevator.

The keyword here is energy density. Batteries are really good when measured in electrical energy stored per volume. Your proposed mechanical potential energy store (lifting mass) cannot compete. The amount of stored energy is aurprisingly low in comparison. I did the math over a decade ago because I had the same idea and the numbers were underwhelming.

Yeah, but energy density isn't really significant. You may get less energy out of a m^3 of water pumped to a reservoir than you get out of a m^3 of battery, but even the smallest reservoir has a usable volume that probably surpasses the volume of all batteries ever built. You can just keep pumping.

No, you can't just keep pumping. To build a reservoir at the scale that you seem to imagine, you depend on locations that provide a suitable topography. Very few such places exist.

A lot of Hydroelectric plants on stable reservoirs can be used as large batteries.

During peak hours they'll increase the flow rate to turbines and during off hours they'll use surplus energy to pump water back into their reservoir.

See https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

Biogenic methane emissions likely make these a non-option at scale.

http://journals.plos.org/plosone/article?id=10.1371/journal....

Or thermal mass storage for AC systems.

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