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Gravitricity

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Re: Gravitricity

#111

Post by pm90 »

revelation wrote: post #90
Zikes wrote: post #4

I look forward to Elon Musk's proposal for unconventional energy storage, because he wouldn't dare call it anything other than Eccentricity.

This is just a pun, but of course his proposal for energy storage already exists, and (surprise surprise) its lithium ion batteries. Significant parts of the projected gigafactory capacity are reserved producing cells for SolarCity home energy storage appliances and other storage systems. In fact, right now the Tesla factory has 4GWh of lithium ion batteries installed for smoothing energy demands and they are activel…

Yep, and as the maker of the most efficient of those batteries, he can make a nice clean profit. Good on him though, he (and the whole Telsa team really) did put in a lot of effort to get there.

Re: Gravitricity

#112

Post by sskates »

yen223 wrote: post #99
sskates wrote: post #89

Earlier quoted context omitted.

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.

That's the hard part though, isn't it? Telling the difference between bad ideas and good ones. Even professionals at it (AKA venture capitalists) get it wrong all the time. So the questions is- knowing that you don't know whether you're looking at a bad idea or a good one, how should you behave?

Re: Gravitricity

#113

Post by Pxtl »

There's interesting variations here - at first glance a single big weight is the same as many small ones, but you have to think of the engineering challenges of cabling and motors for a 1 kilotonne mass. So you could use many smaller objects in a stack and move them one at a time, but then you've got the problem of decoupling/recoupling the cable and reaching through the ones that are at the top while lowering/raising the ones at the bottom.

Now, the next obvious consideration is using mountains instead of a pit. The rockies are full of 4000M peaks. But a mountain means suddenly we have so many new considerations - mountains aren't exactly a constant slope from peak to foot. But extreme loads on rails are a solved problem - the world's heaviest single fully-loaded rail-car was about a kilotonne (a special schnabel car carrying a reactor up to the Alberta tar-sands).

Of course, then you've got a new construction problem - building a train-track that's a near-straight-line up a mountain and can support incredibly heavy trucks.

It's probably not workable because of the energy-density concerns, but it sure is neat.

Re: Gravitricity

#115

Post by Pxtl »

Sorgam wrote: post #50
tasty_freeze wrote: post #35

Although I think the idea isn't workable (energy density is too low, cost of boring the hole is tremendous), most of the other commenters here seem to think they'd just have one weight, whether it is 1000 KG or 50,000 KG. Any sane plan would be to have more than one weight. When the first weight hits the bottom, it would release from the cable and another weight up top would grab the cable and start dropping. To stor…

What advantage does this provide over combining all your weights together into one big one? If you want to limit the tension on the cable, just use more cables. You shouldn't be so confident that you know the only "sane" way to design such a thing. It's not a field anyone has experience in. These are only guesses.

Well, it does mean your cables and dynamo can be smaller. But then you've got a new problem of coupling the cables to the load at the bottom and somehow storing loads at the top that the cables must reach past/through.

Re: Gravitricity

#116

Post by jacquesm »

ars wrote: post #97
phacops wrote: post #57

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.

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.

http://en.wikipedia.org/wiki/Kingdom_Tower

Re: Gravitricity

#117

Post by PhasmaFelis »

apsec112 wrote: post #61

Let'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…

> Now, you can't fill the whole tunnel perfectly or the air can't escape, so our total volume of mass will be about 25 m^2 1000 m = 25,000 cubic meters.*

You seem to have accidentally assumed that the weight will stretch the full height of the hole. Obviously this would leave it unable to move vertically.

Re: Gravitricity

#118

Post by lukifer »

sskates wrote: post #112
yen223 wrote: post #99

Earlier quoted context omitted.

To be fair, there are many, many more Show HNs which wound up going nowhere.

That's the hard part though, isn't it? Telling the difference between bad ideas and good ones. Even professionals at it (AKA venture capitalists) get it wrong all the time. So the questions is- knowing that you don't know whether you're looking at a bad idea or a good one, how should you behave?

Even if it's a bad idea, I still prefer a community that encourages people to try anyway and make things for its own sake. HN culture felt much more positive and constructive when I joined a few years ago. While that ethos is still present, there's an unfortunate tendency for the snark to rise to the top.

Re: Gravitricity

#120

Post by PhasmaFelis »

tomp wrote: post #83
apsec112 wrote: post #61

Let'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…

How are you going to hang a 280 000 ton weight, and more importantly, how are you going to move it up and down?

You're not. Apsec's math is based on the weight being as tall as the hole is deep, which obviously doesn't make sense.
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