Gravitricity • Hacker News Board

Re: Gravitricity

#181

Post by Throwaway12928 » Sun, Nov 23, 2014, 9:00 AM UTC

malandrew wrote: post #80

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…

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.

Or what about the ocean? It's 4km deep on average, so why not just drop the weights there? No need for setting up tracks or carrying the weight up a mountain.

Re: Gravitricity

#182

Post by mtdewcmu » Sun, Nov 23, 2014, 9:10 AM UTC

HarryHirsch wrote: post #151

cdoxsey wrote: post #144

Earlier quoted context omitted.

Except all those dire predictions turned out be wrong. We didn't run out of food or resources, and birth rates have now declined in almost every country in the world. Population growth is basically now a complete non-issue. In retrospect, ignoring the Paul Ehrlichs of the world turned out to be the right move - a far cry from mankind's greatest shortcoming. If we run out of lithium we'll figure something else out, ju…

Except that then we have run out of available lithium, which means that solutions that require lithium will be inaccessible. Running out of chemical elements is a problem. The helium shortage is a serious problem. High-energy physics is made from helium.

Why is helium found underground with natural gas? My theory is that the helium underground comes from radioactive decay and gets trapped along with methane.

-- I guess there's no need to speculate when you have Google: https://www.google.com/search?q=where%20does%20helium%20come...

Re: Gravitricity

#183

Post by mtdewcmu » Sun, Nov 23, 2014, 9:15 AM UTC

jamesaguilar wrote: post #132

photon137 wrote: post #125

Earlier quoted context omitted.

Peak-power output would be less for an inclined rail (vs the vertical shaft solution).

Yeah, but if they're a quarter as efficient and cost a tenth as much, then we might have a winner.

It may have to do with maintenance.

Re: Gravitricity

#184

Post by mtdewcmu » Sun, Nov 23, 2014, 9:18 AM UTC

p1mrx wrote: post #180

mtdewcmu wrote: post #164

Earlier quoted context omitted.

That table uses a mass-based unit, so wouldn't that tend to create the misleading impression that lighter elements are more scarce? Energy storage capacity depends on the number of atoms, not mass (as you can intuit by comparing lithium batteries to lead or nickel-cadmium batteries -- heavier doesn't mean more powerful).

Well, if Pb and Sb are more scarce than Li by mass, then they're even more scarce when you're counting atoms. If Ambri's technology does pan out, it will be due to a cheaper manufacturing process, not because they're avoiding lithium.

I thought those numbers were showing that lead and antimony are more common than lithium.

Re: Gravitricity

#185

Post by tormeh » Sun, Nov 23, 2014, 9:20 AM UTC

bodyfour wrote: post #166

andrewflnr wrote: post #55

Earlier quoted context omitted.

Dang, that's impressive. I had no idea electrical mechanical conversion was so efficient.

Think about how little cooling is needed on a big electric motor. Even motors made 100 years ago are very efficient. By contrast anything that qualifies as a "heat engine" (including internal combustion) is limited by Carnot's law to be low-efficiency. If it weren't for the extraordinary energy density of combustible fuels they wouldn't be competitive.

Yes, the theoretical efficiency limit on electric engines and generators is 100%.

Re: Gravitricity

#186

Post by glassdoor » Sun, Nov 23, 2014, 9:22 AM UTC

May be a look at second 2nd law of thermodynamic would help. Electric motors are efficient but there are still more losses that this company claims.

Re: Gravitricity

#187

Post by SolarNet » Sun, Nov 23, 2014, 9:27 AM UTC

bequanna wrote: post #126

michaelbuckbee wrote: post #100

This 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.

Wouldn't the initial cost savings be offset by the ongoing challenges of dealing with a big moving chunk of hazmat?

Well at least if it fell it would be buried in a giant hole.

Re: Gravitricity

#188

Post by chipsy » Sun, Nov 23, 2014, 9:36 AM UTC

msandford wrote: post #65

andrewliebchen wrote: post #58

Good 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."

The majority of humanity's great inventions didn't have basic physics saying "this isn't brilliant" long before a prototype got built. Most of humanity's great inventions were things that were difficult because human beings thought they were difficult, or complicated or intricate or whatever. On their webpage they say " The biggest single cost is the hole, but it is expected that firstly this will have a very long li…

It wouldn't surprise me at all if the cost structure does work out but the complete picture can't be disclosed for competitive reasons. There are a lot of "obvious ideas" that have only come to fruition recently where the technology was already basically proven, but only entered productive use after several generations of systemic improvements made it a competitive choice. For example: LED lighting was around for several decades but only had niche application until gradual cost/quality improvements pushed it ahead of older lighting tech - and now every major city is rushing to replace all their old street lights. With the Gravitricity tech there are factors like the cost of drilling, the strength, cost and density of the materials used, and lifetime maintenance costs relative to other solutions. There's lots of room for expertise to know for sure what the issues and bottlenecks are.

I'm looking through all the dismissive comments to see if any of them are pinpointing it as a scam - because this is, after all, the kind of idea that is simple enough to captivate laypeople and lift dollars out of their pockets. But judging from this kind of response, it seems like it's also the kind of idea that is simple enough to be dismissed as a "they would have done it already if it worked," rationalized via fuzzy back-of-the-envelope math.

Ultimately, since I have no stake in this one, I'll just wish them luck.

Re: Gravitricity

#189

Post by Joeri » Sun, Nov 23, 2014, 9:36 AM UTC

mtdewcmu wrote: post #153

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).

I thought Dropbox was a good idea until all the files I put there mysteriously disappeared. (I think what may have happened is that they started cleaning up the machine I used at a former company and it was still connected.)

Online syncing is not backup, just like RAID mirroring is not backup, but that doesn't make either bad ideas. Dropbox does however archive deleted files for 30 days, and you can pay extra to extend it to a year: https://www.dropbox.com/help/296

Re: Gravitricity

#190

Post by tormeh » Sun, Nov 23, 2014, 9:41 AM UTC

tanzam75 wrote: post #172

tormeh wrote: post #47

Earlier quoted context omitted.

Not really. Electrical engines and generators are incredibly efficient. It's not the transition there that is the problem. In fact, if it was only for that loss the efficiency would be more like 97%. The problem is the friction in the mechanical parts; the wires, gears and so on.

Amtrak still uses motor-generators on some parts of the Northeast Corridor to convert 60 Hz AC grid power to 25 Hz AC traction power. It's exactly what it sounds like: a motor that is mechanically coupled to a generator. The motor runs on 60 Hz AC and turns the generator at the correct speed to produce 25 Hz AC.

Haha, wow, a mechanical engineer was on duty that day.