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Gravitricity

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Re: Gravitricity

#71

Post by msandford »

the_cat_kittles wrote: post #63
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…

wouldn't this last a lot longer than lithium batteries?

Maybe? But what if it cost way, way more? You actually have to look at the time value of money for anything that takes more than a year or two to pay off to be sure that there isn't some kind of false economy.

If you assume that the thing lasts forever, well, you divide $x/infinity so it's free. EXCEPT that you've got $x tied up that could be earning interest instead. So it's not $x/infinity, it's the opportunity cost on $x versus anything else out there like batteries, the grid, etc.

Re: Gravitricity

#72

Post by mikeyouse »

Coincoin wrote: post #59
mikeyouse wrote: post #30

Earlier quoted context omitted.

In 2011, we mined 0.2% of known lithium reserves.. This would provide over 350 years worth of lithium at current rates. Obviously usage is dramatically increasing but so will our known reserves as the overall level of demand increases. This is also without developing a method to pull the relatively abundant lithium from sea water and without recycling any lithium we're currently disposing of. Ambri is interesting fro…

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

That's a fine saying but even at 20% YoY increases in lithium usage, we'd still have 35 years or known reserves -- without any increase in exploration or recycling.

As oil & gas have shown, if there's exponential demand for a naturally occurring element, we'll definitely find ways to pull more of it out of the ground.

Re: Gravitricity

#73

Post by msandford »

Gurkenglas wrote: post #67
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…

If the capacity is proportional to the depth of the tunnel and the mass of the weights, and the maximal mass of the weights is proportional to the depth of the tunnel, then wouldn't the capacity grow quadratically with the invested money? Edit: You assumed that they can both fill most of the tunnel up with lead and move that up or down 500 meters. The volume that they can fill with that is thus only half of what you…

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.

Re: Gravitricity

#74

Post by msandford »

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…

Oil wells are also only a foot in diameter. Makes the economics worse, really.

Re: Gravitricity

#75

Post by wklauss »

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."

A lot of bad ideas happen in between humanity's greatest inventions. When you look at the past, you are only looking to the successful stories. Maybe a lot of people with unrealistic or bad ideas would have benefited from HN.

Re: Gravitricity

#76

Post by ortusdux »

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…

I feel this idea would mesh well with the mining industry. Dig straight down, reinforce the walls on the way, separate out the desired minerals, pull up the drill, put in a pendulum and move the rig over and start again. You already need to setup giant motors to move the pendulum, so getting the ore up would be easy. An ideal site would have an abundance of lead, which would cut down on the pendulum cost. Unused rock could be mixed into cement for the walls. The small footprint combined with the benefits of being able to store solar or wind energy for long periods of time could make it the first form of mining that environmentalists approve of.

Re: Gravitricity

#77

Post by Stately »

I'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.

Re: Gravitricity

#78

Post by Gurkenglas »

msandford wrote: post #73
Gurkenglas wrote: post #67

Earlier quoted context omitted.

If the capacity is proportional to the depth of the tunnel and the mass of the weights, and the maximal mass of the weights is proportional to the depth of the tunnel, then wouldn't the capacity grow quadratically with the invested money? Edit: You assumed that they can both fill most of the tunnel up with lead and move that up or down 500 meters. The volume that they can fill with that is thus only half of what you…

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 down into the weights (cause with this, you would need no more pulley). (To illustrate why it wouldn't rip itself apart, imagine gaps on the weight every 100 meters.)

Re: Gravitricity

#79

Post by abdullahkhalids »

The key figure of merit for comparing energy storage is not $/KWh, but rather $/KWh*Number of cycles. Li-ion only has about a 1000 cycles. Assuming one cycle per day (solar charge during day+discharge during night) in 50 years there are 18000 cycles.

The figure of merit for Li-ion is 250x1000=2.5e5. My estimate (and those of others) is that this costs up to about $2000/KWh. So the figure of merit for this is 1000x18000=0.9e7. Two orders of magnitude better than Li-ion.

Edit: I am ignoring the cost of capital, interest rates etc. Somebody should do this analysis.

Re: Gravitricity

#80

Post by malandrew »

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.
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