7 ms·

The problem: Gravitational potential energy isn't a very efficient storage format. 1kg elevated 1m is only around 10j. A 10,000kg mass elevated 1km would be 98M

by mabbo 10y ago

The problem: Gravitational potential energy isn't a very efficient storage format. 1kg elevated 1m is only around 10j. A 10,000kg mass elevated 1km would be 98Mj- only 27kWh. And only with perfect conversions, which you won't get. Each Tesla powerwall holds 14kWh for around $8000 each, and that price may fall over the.

beamatronic 10y ago
If it's cheap to store the energy, and you can store it at large scale, the conversion factor starts to look less important. If anyone had looked at the amount of energy that goes into an ICE car versus what makes it to the rear wheels, would we have autos today?
- micheljones 10y ago
  Yeah, especially with the front drive ones! :P
zackmorris 10y ago
Just to put this in perspective - a gallon of gasoline holds about 33 kWh (about 10 kWh of electricity after thermodynamic losses). So lifting 10 tons a km vertically only gives you about 1-3 gallon of gas, give or take (however motor efficiency could be 95% which helps). There is merit to lifting large loads like this, but IMHO they should be concentrating on weight instead of distance. Which basically takes us back to pumped hydro because we just don't have anything on the order of a million tons or more that can be moved cheaply and easily. I could maybe see a large carpet rolling a layer of sand or stones up into a cinnamon roll so it's self supporting. Probably not the safest place to stand under though!
- sqeaky 10y ago
  What about using old shipping containers just filled with rock and dirt found on site? If someone started with a hill or mountain at an almost suitable grade, then what they dug out to grade the slope properly could be put in second hand shipping containers at the top of a hill. Bring the containers up or down one at a time to get the power in or out. Maybe have some one site solar or wind generation to try to bring and extra container up every once in a while. People generally don't want steep slopes for much anyway so even the property ought to be cheap, it could even be municipal property for maximum cheapness. The hardest part of any of this is the all electric trains with efficient regenerative brakes.
  - Baeocystin 10y ago
    Water is worth it in very specific locations, because we can build a dam and let the natural shape of the Earth do the heavy lifting. But a dry system, relying on weights? There isn't a material that is dense enough to make it worthwhile. Even a boxcar full of granite would need to be orders of magnitude heavier than it is for gravity storage to be economically useful.
lucaspiller 10y ago
I wonder if something using hydrogen as the buffer would work - it has a density of 37 kWh/kg (or 2.4 kWh/litre). Instead of fuel cells (which are still expensive and relatively low capacity), using excess electricity to electrolyse water. The oxygen and hydrogen can be stored in tanks, and when you need the power it can be combusted, generate steam and drive turbines. There isn't really any new technology here, so it should be relatively cheap and quick to setup too. What am I missing?
- philipkglass 10y ago
  The volumetric energy density of hydrogen is very low, so either you need enormous tanks or very strong tanks plus extreme compression. One way of making enormous "tanks" that can tolerate fairly high pressures is by solution-mining caverns out of natural underground salt formations. But this is geographically limited to where such salt formations occur, like parts of Texas and Louisiana. The other problem is that the round-trip energy efficiency isn't great. You might expect 75% efficiency for electricity-to-hydrogen from electrolysis and then 60% hydrogen-to-electricity if you burn it in an efficient combined cycle gas turbine for 45% efficiency overall. Oh, and you irreversibly lose energy as heat when you compress the hydrogen, for further losses. And it's pretty expensive to buy big industrial-scale electrolysis units but run them only during the few hours each day when there's excess renewable energy you want to store (though it becomes a more attractive idea as the penetration rate of renewables, and corresponding excess generation times, increase.)
  - imtringued 10y ago
    The conversion factor is not that important because without storage the excess energy would be lost anyway if we assume electricity demand doesn't shift from night to day.
logicallee 10y ago
Remember, the entire mass of the Earth @ 5.972 × 10^24 kg (which is 1.317 × 10^25 lbs) i.e. 5 septillion kilos or 13 septillion pounds) which is under you is enough to push you down with something like 150 pounds of force. A magnet with 150 lbs of holding power is able to fight all of that and takes up this massive amount of room: https://www.amazon.com/CMS-Magnetics-Holding-Neodymium-Countersunk/dp/B0195V41QS https://www.amazon.com/CMS-Magnetics-Holding-Neodymium-Count... Yes - that's a penny for comparison. Gravity is soooo weak.
- tzs 10y ago
  Your "Earth vs. Magnet" fight is not fair. For that magnet to hold up something of 150 pounds, the magnet has to be quite close to the object. The Earth in that fight, on the other hand, is doing it with its center of mass (which is the relevant distance) about 6.7 x 10^6 meters away. Squeeze the Earth down to the size of the magnet so you can move it close to your object too, and it won't seem so weak. The magnet would still win, I believe, but not by nearly as much.
  - logicallee 10y ago
    Ah, good point! I'm not sure that it would still win versus point-sized Earth - according to this [1], shrinking Earth to half its size would increase gravity 4x. And a point is obviously much smaller than half. But still, to see how incredibly weak gravity is consider that you don't even have to consider the gravitational pull of the magnet. Even if you walk next to a mountain you don't have to think twice about falling toward it. Gravity is negligible at even monstrously huge scales! It's just not a very large force at all, compared with magnetism, electricity, or e.g. solar radiation. It's truly weak. I've never had to think about the gravitational pull of any object other than literally the Earth or maybe occasionally the moon (which has a mass of 7.35x10^22 kg, or 1.2 percent of Earth's mass -- i.e. the Earth and the 5 septillion kilos I quoted is only 81 times as large.) The everyday pull of objects (for example the pull by which the building next to you right now is attracting you) is so negligible. It's weak! [1] https://www.quora.com/If-the-earth-shrinks-half-of-its-radius-and-its-mass-remains-same-what-would-be-the-weight-of-objet-on-earth https://www.quora.com/If-the-earth-shrinks-half-of-its-radiu...
fulafel 10y ago
It can be efficient in $$ terms since ballast is cheap. Ballast is > 100x cheaper than powerwalls by the kWh in your equation[1]. 10 tons of rock just 5 m^3, you can put hundreds or thousands of this size of rock in a train[2]. You load/unload ballast instead of having all of it statically bolted onto rails and cars, so you get by with 1/50..1/100 carrying capacity[3] per trip vs total ballast amount. The amount of raw materials (lithium mining and refining etc) required by the powerwalls is probably much higher too - good lithium deposits have ~ 1/100 of lithium content and turning ore into batteries is a heavy process consuming a lot of water, electricity, fossil fuels (ore transport/mining) etc. Not to mention limited battery service life. [1] 2x powerwalls = $16000, even building material grade gravel is < $300 per 10 tons (50x cheaper) but here you could use sand or something from the site. [2] big ore transport trains carry 10000-40000 ton loads, so 1000-4000 of our 10t rocks per train. You would probably use a smaller train for energy storage purpouse since the distance is small and you can have quick load/unload turnarounds [3] if aiming for 1-day charge capacity, you can fit 100 15-min round-trips in 24 hours, or 50 30-min round trips
- mabbo 10y ago
  But it costs far more than just the ballast. The rails, the car, the huge system for pulling and releasing the cars, the dozens of km of land needed to have a train car be elevated by any significant amount. Lithium systems are going to wind up being cheaper overall because of all of those costs, plus operating costs.
  - deleted 10y ago
    [deleted]
  - evgen 10y ago
    We already know how to cheaply and efficiently build rail, rail cars, huge systems for pulling and releasing the cars (they are called engines and overhead electrification :) and in fact you can pick up a lot of the materials used for less than half the cost of new. Dozens of km of land are damn near free in places in the southwest US where you can also dump lots of solar panels. Lithium may eventually be cheaper, but not within the next decade it won't.
rjvs 10y ago
Put the powerwall on the train as well and use both :)
08-15 10y ago
The lead-acid battery in my car holds 660Wh (12V, 55Ah) and costs less than $100. That puts lead acid at a quarter the price of the Tesla powerwall (admittedly without an inverter).
- philipkglass 10y ago
  If you tried to use it like a PowerWall it would fail before it had reached 1/4 of the PowerWall's rated cycle life. Lead-acid is not a good chemistry for grid tied storage.

by mabbo 10y ago

beamatronic 10y ago

If it's cheap to store the energy, and you can store it at large scale, the conversion factor starts to look less important. If anyone had looked at the amount of energy that goes into an ICE car versus what makes it to the rear wheels, would we have autos today?

micheljones 10y ago
Yeah, especially with the front drive ones! :P

zackmorris 10y ago

Just to put this in perspective - a gallon of gasoline holds about 33 kWh (about 10 kWh of electricity after thermodynamic losses). So lifting 10 tons a km vertically only gives you about 1-3 gallon of gas, give or take (however motor efficiency could be 95% which helps). There is merit to lifting large loads like this, but IMHO they should be concentrating on weight instead of distance. Which basically takes us back to pumped hydro because we just don't have anything on the order of a million tons or more that can be moved cheaply and easily. I could maybe see a large carpet rolling a layer of sand or stones up into a cinnamon roll so it's self supporting. Probably not the safest place to stand under though!

sqeaky 10y ago
What about using old shipping containers just filled with rock and dirt found on site? If someone started with a hill or mountain at an almost suitable grade, then what they dug out to grade the slope properly could be put in second hand shipping containers at the top of a hill. Bring the containers up or down one at a time to get the power in or out. Maybe have some one site solar or wind generation to try to bring and extra container up every once in a while. People generally don't want steep slopes for much anyway so even the property ought to be cheap, it could even be municipal property for maximum cheapness. The hardest part of any of this is the all electric trains with efficient regenerative brakes.
- Baeocystin 10y ago
  Water is worth it in very specific locations, because we can build a dam and let the natural shape of the Earth do the heavy lifting. But a dry system, relying on weights? There isn't a material that is dense enough to make it worthwhile. Even a boxcar full of granite would need to be orders of magnitude heavier than it is for gravity storage to be economically useful.

lucaspiller 10y ago

I wonder if something using hydrogen as the buffer would work - it has a density of 37 kWh/kg (or 2.4 kWh/litre). Instead of fuel cells (which are still expensive and relatively low capacity), using excess electricity to electrolyse water. The oxygen and hydrogen can be stored in tanks, and when you need the power it can be combusted, generate steam and drive turbines. There isn't really any new technology here, so it should be relatively cheap and quick to setup too. What am I missing?

philipkglass 10y ago
The volumetric energy density of hydrogen is very low, so either you need enormous tanks or very strong tanks plus extreme compression. One way of making enormous "tanks" that can tolerate fairly high pressures is by solution-mining caverns out of natural underground salt formations. But this is geographically limited to where such salt formations occur, like parts of Texas and Louisiana. The other problem is that the round-trip energy efficiency isn't great. You might expect 75% efficiency for electricity-to-hydrogen from electrolysis and then 60% hydrogen-to-electricity if you burn it in an efficient combined cycle gas turbine for 45% efficiency overall. Oh, and you irreversibly lose energy as heat when you compress the hydrogen, for further losses. And it's pretty expensive to buy big industrial-scale electrolysis units but run them only during the few hours each day when there's excess renewable energy you want to store (though it becomes a more attractive idea as the penetration rate of renewables, and corresponding excess generation times, increase.)
- imtringued 10y ago
  The conversion factor is not that important because without storage the excess energy would be lost anyway if we assume electricity demand doesn't shift from night to day.

logicallee 10y ago

Remember, the entire mass of the Earth @ 5.972 × 10^24 kg (which is 1.317 × 10^25 lbs) i.e. 5 septillion kilos or 13 septillion pounds) which is under you is enough to push you down with something like 150 pounds of force. A magnet with 150 lbs of holding power is able to fight all of that and takes up this massive amount of room: https://www.amazon.com/CMS-Magnetics-Holding-Neodymium-Countersunk/dp/B0195V41QS https://www.amazon.com/CMS-Magnetics-Holding-Neodymium-Count... Yes - that's a penny for comparison. Gravity is soooo weak.

tzs 10y ago
Your "Earth vs. Magnet" fight is not fair. For that magnet to hold up something of 150 pounds, the magnet has to be quite close to the object. The Earth in that fight, on the other hand, is doing it with its center of mass (which is the relevant distance) about 6.7 x 10^6 meters away. Squeeze the Earth down to the size of the magnet so you can move it close to your object too, and it won't seem so weak. The magnet would still win, I believe, but not by nearly as much.
- logicallee 10y ago
  Ah, good point! I'm not sure that it would still win versus point-sized Earth - according to this [1], shrinking Earth to half its size would increase gravity 4x. And a point is obviously much smaller than half. But still, to see how incredibly weak gravity is consider that you don't even have to consider the gravitational pull of the magnet. Even if you walk next to a mountain you don't have to think twice about falling toward it. Gravity is negligible at even monstrously huge scales! It's just not a very large force at all, compared with magnetism, electricity, or e.g. solar radiation. It's truly weak. I've never had to think about the gravitational pull of any object other than literally the Earth or maybe occasionally the moon (which has a mass of 7.35x10^22 kg, or 1.2 percent of Earth's mass -- i.e. the Earth and the 5 septillion kilos I quoted is only 81 times as large.) The everyday pull of objects (for example the pull by which the building next to you right now is attracting you) is so negligible. It's weak! [1] https://www.quora.com/If-the-earth-shrinks-half-of-its-radius-and-its-mass-remains-same-what-would-be-the-weight-of-objet-on-earth https://www.quora.com/If-the-earth-shrinks-half-of-its-radiu...

fulafel 10y ago

It can be efficient in $$ terms since ballast is cheap. Ballast is > 100x cheaper than powerwalls by the kWh in your equation[1]. 10 tons of rock just 5 m^3, you can put hundreds or thousands of this size of rock in a train[2]. You load/unload ballast instead of having all of it statically bolted onto rails and cars, so you get by with 1/50..1/100 carrying capacity[3] per trip vs total ballast amount. The amount of raw materials (lithium mining and refining etc) required by the powerwalls is probably much higher too - good lithium deposits have ~ 1/100 of lithium content and turning ore into batteries is a heavy process consuming a lot of water, electricity, fossil fuels (ore transport/mining) etc. Not to mention limited battery service life. [1] 2x powerwalls = $16000, even building material grade gravel is < $300 per 10 tons (50x cheaper) but here you could use sand or something from the site. [2] big ore transport trains carry 10000-40000 ton loads, so 1000-4000 of our 10t rocks per train. You would probably use a smaller train for energy storage purpouse since the distance is small and you can have quick load/unload turnarounds [3] if aiming for 1-day charge capacity, you can fit 100 15-min round-trips in 24 hours, or 50 30-min round trips

mabbo 10y ago
But it costs far more than just the ballast. The rails, the car, the huge system for pulling and releasing the cars, the dozens of km of land needed to have a train car be elevated by any significant amount. Lithium systems are going to wind up being cheaper overall because of all of those costs, plus operating costs.
- deleted 10y ago
  [deleted]
- evgen 10y ago
  We already know how to cheaply and efficiently build rail, rail cars, huge systems for pulling and releasing the cars (they are called engines and overhead electrification :) and in fact you can pick up a lot of the materials used for less than half the cost of new. Dozens of km of land are damn near free in places in the southwest US where you can also dump lots of solar panels. Lithium may eventually be cheaper, but not within the next decade it won't.

rjvs 10y ago

Put the powerwall on the train as well and use both :)

08-15 10y ago

The lead-acid battery in my car holds 660Wh (12V, 55Ah) and costs less than $100. That puts lead acid at a quarter the price of the Tesla powerwall (admittedly without an inverter).

philipkglass 10y ago
If you tried to use it like a PowerWall it would fail before it had reached 1/4 of the PowerWall's rated cycle life. Lead-acid is not a good chemistry for grid tied storage.