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Gravitricity

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Re: Gravitricity

#191

Post by throwaway_yy2Di »

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…

Actually, the majority of the weight would be in steel rope.

For one type [0], the safe limit is a tensile load of about 134-150 MN/m^2 (= MPa) [1]. At a density of 8 g/cm^3, the limiting length of a uniform cable is ~1.7 - 1.9 kilometers. If you have a stationary, suspended cable of this length, its own weight puts it at its maximum load; it can't lift anything else.

Steel rope on McMaster is around $10,000/ton [of rope]. So, these assumptions are a dead end.

You can't solve this by using shorter cables, because that decreases your energy capacity at the same rate. If you use cables of 1/10th the length (~100 m), you get only 1/10th the potential energy storage per ton. The cable thickness per lifted ton is constant.

[0] http://www.engineeringtoolbox.com/wire-rope-strength-d_1518....

[1] The breaking limit of rope is far higher (~700 MPa), and the breaking limit of a single wire strand -- the tensile strength -- is higher still (1,770 MPa according to [2])

For the cross section area, I'm assuming a circular rope (not accurate).

[2] http://www.gabaswire.com/en/overview/grades-of-wire-rope.htm...

Re: Gravitricity

#192

Post by benbojangles »

001sky wrote: post #123
sskates wrote: post #112

Earlier quoted context omitted.

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?

The responses in that Dropbox thread were pretty solid though. To all the naysayers, I mean. It's sort of a ritual format to test any new idea with the back and forth. That in and of itself isn't a problem. Real problems: Dismissive people who don't/can't/wont listen to good responses. Good founders/ideas but just really bad at communicating. Those are basically the type i and type ii errors of this format.[1] _____…

Why does it look like a Fracking drill? It looks like a Trojan horse.

Re: Gravitricity

#193

Post by jmadsen »

ggchappell wrote: post #159

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"…

This may be of interest:

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

Re: Gravitricity

#195

Post by throwaway_yy2Di »

throwaway_yy2Di wrote: post #191
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…

Actually, the majority of the weight would be in steel rope. For one type [0], the safe limit is a tensile load of about 134-150 MN/m^2 (= MPa) [1]. At a density of 8 g/cm^3, the limiting length of a uniform cable is ~1.7 - 1.9 kilometers. If you have a stationary, suspended cable of this length, its own weight puts it at its maximum load; it can't lift anything else. Steel rope on McMaster is around $10,000/ton [of…

"You can't solve this by using shorter cables, because that decreases your energy capacity at the same rate. If you use cables of 1/10th the length (~100 m), you get only 1/10th the potential energy storage per ton. The cable thickness per lifted ton is constant."

This is a simple "figure of merit" for cable in this problem,

    cost / (length * load capacity (N))
The is the same as the cost / energy stored. The denominator is simply the work equation (distance * force) -- the mechanical work the cable can do before it runs out of length.

    = cost / energy
This is actually sort-of constant, since the denominator is ~proportional to the cable volume. (The load capacity is ~ the cross sectional area d^2).

For steel rope from [0], it looks like a lower bound of about $1,200/kWh.

[0] http://www.mcmaster.com/#standard-wire-rope/=upui3o

(E.g. item "3440T68", 5/8" plain steel, $5.16/foot for 9,080 lbf lifting capacity;

$5.16 / (9,080 lbf * 1 foot) = $1,509/kWh)

Re: Gravitricity

#196

Post by joshvm »

brute wrote: post #175
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…

You just need more mass. Have a look at this concept: http://eduard-heindl.de/energy-storage/index-e.html (unfortunately the english version is very sparse, the german version has some more images and videos: http://eduard-heindl.de/energy-storage/ ) The idea is to cut a 1km diameter cylinder into the ground (solid rock), and then lift it up up to 500m by pumping water underneath it. The power stored can power a coun…

But presumably you'd need more than a day's worth of power to lift the damn thing? That's probably doable, but still, I suspect that'd be the main problem.

Re: Gravitricity

#197

Post by transfire »

I thought of this decades ago, and I am sure many others have too. And no the math does not come out favorably. While technically feasible, the cost is prohibitive. The reason is simple, you need LOTS OF WEIGHT to get any appreciable storage capacity.

Try it yourself: http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

Note that 1 joule = 2.77777778 × 10-7 kilowatt hours

Re: Gravitricity

#198

Post by Panoramix »

Anechoic wrote: post #105
malambic wrote: post #95

Earlier quoted context omitted.

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.

2nd Avenue subway [0] requires about 8.5 miles worth of tunnels (several hundred feet of which were already dug back in the 70's) and it's budget is $17 billion. Of course that whole cost isn't just for digging tunnels, but it's a clear indication that digging tunnels (especially in dense urban areas) can be extremely expensive. [0] http://en.wikipedia.org/wiki/Second_Avenue_Subway

There is a very large difference between building complex tunnels and a simple hole in the ground. The cost for drilling a straight exploratory/mining hole was 10 Million/km in the 70's and 80's[0]. I can easily imagine this being cheaper by a factor of 2-5 with current advances. Not to mention this concept has a lot of synergy with mining companies.

[0] http://facstaff.gpc.edu/~pgore/geology/geo101/interior.htm

Re: Gravitricity

#199

Post by [deleted] »

cperciva wrote: post #165
darkmighty wrote: post #158

Earlier quoted context omitted.

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.

Yes, if you're considering only energy. But in practice there are other considerations, such as the cost of the weight, the strength (and thus cost) of the cables, the amount of torque which can be produced for lifting the weight, etc.

[deleted]

Re: Gravitricity

#200

Post by Retric »

jmadsen wrote: post #133

I love how every Tom, Dick & Harry with a bit of math & engineering thinks he can "dispel the myth" of HN Idea X with two minutes of off-the-top-of-his-head equations. Read about these two: http://www.gravitricity.com/#people Do they seem like complete morons? Do you think they haven't studied this just a little bit more than you have? It doesn't mean their idea is good or will work - just that you aren't going to ri…

I saw a similar idea using weights suspended between two mountain peaks but it was to expencive without drilling a hole.

Intelligent people often run the best scams. Pumped storage can vary cheaply move a lot of mass up and down significant heights and it's still to expencive without a natural aquifers. The problem is power is rediculusly cheap and lifting crap takes far less power than you might think.

"The main problem with gravitational storage is that it is incredibly weak compared to chemical, compressed air, or flywheel techniques (see the post on home energy storage options). For example, to get the amount of energy stored in a single AA battery, we would have to lift 100 kg (220 lb) 10 m (33 ft) to match it. To match the energy contained in a gallon of gasoline, we would have to lift 13 tons of water (3500 gallons) one kilometer high (3,280 feet). It is clear that the energy density of gravitational storage is severely disadvantaged." http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

For useful amounts of grid storage you need to be lifting on the order of 100,000+ tons up 1 km which starts to get really expencive just for cables.

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