Comment by devrelm | Hacker News Reader

Comment by devrelm | Hacker News Reader

Has anyone worked out how much energy this could actually theoretically store given the number of weights as n, the mass of each weight as m, and the height of each well as h?

My physics is a little rusty, and I'm sure someone will come up with an answer before I figure it out.

EDIT:

If my math is right (using this [1] as reference),

  E = m * g * h (J)

gives the energy E in joules. 1 watt hour is 3600 joules, so:

  E = m * g * h / 3600 (Wh)

So, if this system were made up of 4 x 200kg weights suspended over a 50m well, it would hold

  E = 4 * 200 * 9.81 * 50 / 3600 = 109 Wh

109 Wh. That's hardly enough to run a few high-efficiency light-bulbs for an hour. I don't mean to be a naysayer, but this doesn't seem very efficient at small scales.

[1]: http://physics.stackexchange.com/questions/39281/needed-ener...

It's just mass * gravity * height.

So, for instance, 1000 kg with a working height differential of 1000 meters can store a theoretical maximum of 2.72 kilowatt hours (9.8 million joules).

You would have to factor the efficiency motors that raise the weights, and the generator that lowers them to get a more accurate estimate.

That's why I used a kilometer. There's little need for an accurate estimate, capacity for such a system is very expensive, either in dealing with huge masses or huge distances.

By way of comparison, a thousand dollars of lead acid batteries would have more capacity (and despite the issues with lead, such batteries are quite recyclable...).

Which is comparable to 10 kg of lithium-ion batteries - not terrible efficient.

Yeah, that doesn't seem very efficient. I see why this works at larger scales, such as water reservoirs and such, but there's no way this would be worth the cost to install in one's backyard. Especially when the price of equivalent battery-backup systems are only a few thousand dollars.

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