Seems like this would be more cost effective as a component of sky-scrapers. Generator on top, series of weights on rails down the sides.
I wonder if you could somehow combine it the counterweight system.
Gravitricity
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Re: Gravitricity
#92Earlier quoted context omitted.
Why is the maximal mass of weights proportional to depth? It's more proportional to diameter than anything else. There's a practical limit to how much weight you can hang. Sure if you make a 10km deep hole and a 3km deep weight you could store a lot of energy. But a 3km deep weight might not hold itself together.
If we have enough volume that we can't use the most dense possible weight for all of it, we can find cheaper weight materials by dropping the density constraint. Couldn't you have the weight grip the sides of the tunnel with gears attached to a generator/motor (inside the weight) so the weight wouldn't rip itself apart? It would be the same machinery that ordinarily would operate the pulley at the top, just moved dow…
Technically that would work fine. Economically, it's a disaster. McMaster prices are at least 2x what's available if you buy direct from a source, and if you were buying in bulk it'd probably get cheaper still. But this is cheap, cheap stuff for not that much weight. You'd need something I dunno, 100x-10000x stronger to do what you're talking about: http://www.mcmaster.com/#racks-and-pinions/=upn9xl
The appeal of the cables is that they're cheap. And they're cheap because they're easy to make (all things considered). Once you start applying sophistication to the design the price gets higher, and it's already too high from having to drill the hole.
Re: Gravitricity
#93Earlier quoted context omitted.
What's the math look like for the side of a mountain or hill, where you pour a concrete channel and put wheels on the weight so it can be rolled up and down the hill. I'm just wondering if there are places like in the Rockies where this idea would be viable.
I don't understand why they need to dig holes to do this. Why not build something equivalent onto the side of a cliff?
The depth requirement is 500m to 1500m. Though cliffs can be much taller than this, they usually are not totally vertical and cannot accomodate this much depth.
Disclaimer, not a geologist.
Re: Gravitricity
#94I'm far from being even slightly knowledgeable in this topic, but would it be possible to build this in very deep waters? Like a massive column containing a tunnel? Seems cheaper than digging a 1km hole.
At the base of a 1000m shaft you'd have approximately 100 atmospheres of pressure. Not saying it cannot be done, but construction and materials cost would still be very high. Maintenance too. Plus you've lost the advantage of locating it anywhere.
Re: Gravitricity
#95Let's do out the math on this... A subway tunnel might have a diameter of about 6 meters, so cross section = 3 * 3 * pi = 28 square meters. Digging subway tunnel through rock costs about $100M per kilometer. On the one hand, these holes would be vertical, which is harder than horizontal; on the other hand, they wouldn't need ventilation and train tracks and stuff. Let's handwave and say it's $100M for a 1 km deep hol…
Can we get a source for the $100M per kilometer figure? The NYC subway system has 373 km of tunnels, 60% of which is underground, which would translate to roughly 22 billion dollars just for the digging... that just seems way too high to me.
Re: Gravitricity
#96Earlier quoted context omitted.
Your units got messed up there -- it's just mgh. There's no reason to square the distance. $ units Currency exchange rates from 2013-07-11 2562 units, 85 prefixes, 66 nonlinear units You have: 1000 kg gravity 1 km You want: J * 9806650 / 1.0197162e-07
That's what I get for doing intermediate thermo & fluids all day and then trying to make a comment on basic dynamics. Thanks for the catch.
Sure. This is also neglecting the compressed air part of this technology (which thermodynamics might actually be quite appropriate for modelling!).
Re: Gravitricity
#97Seems like this would be more cost effective as a component of sky-scrapers. Generator on top, series of weights on rails down the sides.
Not even close. First a 1KM tall skyscraper? Second they are talking quite a lot of weight - the skyscraper would have to be MUCH stronger.
Re: Gravitricity
#98Drilling into the ground should almost never happen.
Why?
Re: Gravitricity
#99Good thing HN wasn't around durning the development of most of humanity's great inventions. "So your telling me I'm going to have to hold my food over this fire for 15 minutes before I eat it? No thank you, I'd stick with my raw meat."
Here's an interesting reference, Drew Houston showing Dropbox to HN: https://news.ycombinator.com/item?id=8863 Definitely more negative comments on HN these days (although it's always been a skeptical audience).
To be fair, there are many, many more Show HNs which wound up going nowhere.
Re: Gravitricity
#100This is just rampant speculation - but what if they used depleted uranium as the weight? They could get paid to take the material off others hands (instead of buying lead) and it's almost 70% more dense.