Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Gravitricity

gravitricity.com

Post reply on HN
Print view View original

151–160 of 222 posts

Re: Gravitricity

#151

Post by HarryHirsch »

cdoxsey wrote: post #144
Coincoin wrote: post #59

Earlier quoted context omitted.

"The greatest shortcoming of the human race is our inability to understand the exponential function."

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.

Re: Gravitricity

#152

Post by mtdewcmu »

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…

The hole for this has different requirements than holes for water and petroleum: you'd not want to drill one of these holes into an aquifer, because then it would fill up with water. Oil and gas wells have to be drilled in specific places where the oil and gas are, naturally. In this case, you'd want nothing but a dry, empty hole. A dry, empty hole would be the kind of hole least likely to interact with anything underground.

I'm not sure this has much to do at all with water and energy drilling. They're talking about using abandoned mine-shafts. Those are much wider and shallower than the kind of holes used to reach petroleum.

Re: Gravitricity

#153

Post by mtdewcmu »

sskates wrote: post #89
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."

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.)

Re: Gravitricity

#154

Post by jakobegger »

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.

If those physicists hurried up with cold fusion that helium shortage wouldn't be such a problem.

Re: Gravitricity

#155

Post by tempestn »

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…

My understanding is that peak power availability is at least as important as total energy storage. (The main use of grid storage is to handle spikes in usage, or similarly to fill in for dips in generation - like from wind turbines. Either way, peak power available is important.) One big weight would give you significantly more peak power than several smaller ones. Yes it would require more or stronger cables, motors, and other infrastructure, but the cost of all those is small compared to the shaft, so I imagine a single weight would maximize peak power / cost.

Re: Gravitricity

#156

Post by mtdewcmu »

elektropionir wrote: post #10

The energy density for gravity is just immensely small, that's why you need dams holding back rivers to use them to generate electricity. For a 1km hole (that's in the middle of their 500m - 1500m range) you have an energy density of 10kJ/kg of the weight that stores the energy. The energy stored in a Tesla roadster battery pack is around 50kWh which is 180MJ. This means that you need a 18,000kg weight in a 1km deep…

That's a devastating critique. The idea seemed a bit too good to be true, but I couldn't quite put my finger on why. Thanks for posting that.

I did a little more research, and I'm not sure anymore that it's a complete deal-breaker. But it's definitely going to put limits on how far this can scale.

Re: Gravitricity

#158

Post by darkmighty »

cperciva wrote: post #69
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…

Digging subway tunnel through rock costs about $100M per kilometer. On the one hand, these holes would be vertical, which is harder than horizontal I would have thought that a better comparison would be oil wells, which cost about $500 per ft of depth, or $1.5M per km. our total volume of mass will be about 25 m^2 1000 m = 25,000 cubic meters. If the weights are made from lead, that's a total mass of ~280,000 tons* Y…

Actually 1/2 the shaft filled by weights is optimal independently of material, considering only raw energy. (E=p A h(H-h), respectively density, area, payload height and shaft height, Eopt=p A H^2/2 ). Pretty surprising to me that the energy increases quadratically with depth.

Re: Gravitricity

#159

Post by ggchappell »

FTA:

> The key requirement is a deep hole in the ground; it could be a disused mineshaft brought back into use, or it could be a purpose drilled or sunk shaft.

So, apparently, "sinking" a shaft involves making a hole using some technique other than drilling. What technique is that?

Dictionaries are no help. Wikipedia[1] says:

> Shafts may be sunk by conventional drill and blast or mechanised means.

"Mechanized means" is pretty vague. Can anyone clarify?

[1] https://en.wikipedia.org/wiki/Shaft_mining

Re: Gravitricity

#160

Post by k__ »

aetherspawn wrote: post #157

If we had a dyson sphere around something heavy then the crank part could be in space with the weight dangling into the atmosphere: https://en.wikipedia.org/wiki/Dyson_sphere

How about the Moon? :D
Post reply on HN