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Gravitricity

gravitricity.com

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Re: Gravitricity

#21

Post by grondilu »

bsilvereagle wrote: post #9
grondilu wrote: post #5

What determines the depth of the whole? I mean, since E = mgh, you can get the same energy storage capacity with a less deep hole if you use a heavier weight. And you can get a heavier weight either by using more expensive material (why use a cheap one? it's not like it's going to wear or anything), or a larger hole. I'm not sure what determines the cost of digging a whole, but I suspect depth matters more than area.…

I suspect they plan on recovering the power with a generator. So the input shaft to the generator has to rotate. With a deeper hole, you get more 'clicks' on the generator so you can produce power for a longer period of time before having to pull the weight back up.

I suppose they plan on using a transmission mechanism that will turn one meter down to as many turns as you want anyway, so I guess the depth of the hole is at least partially determined by how good a transmission mechanism you can afford. But since they explicitly say the hole will be the most expensive part, I'm not so sure.

Re: Gravitricity

#22

Post by bsilvereagle »

grondilu wrote: post #21
bsilvereagle wrote: post #9

Earlier quoted context omitted.

I suspect they plan on recovering the power with a generator. So the input shaft to the generator has to rotate. With a deeper hole, you get more 'clicks' on the generator so you can produce power for a longer period of time before having to pull the weight back up.

I suppose they plan on using a transmission mechanism that will turn one meter down to as many turns as you want anyway, so I guess the depth of the hole is at least partially determined by how good a transmission mechanism you can afford. But since they explicitly say the hole will be the most expensive part, I'm not so sure.

You're losing torque with every gear reduction, and I believe that current out of a generator is a function of torque. So there's some happy medium between hole depth & transmission ratio for the amount of power they want to generate.

Re: Gravitricity

#23

Post by slashnull »

Animats wrote: post #13

OK, a typical mine hoist is about 10 metric tons. 10 metric tons descending at 1m/sec is very close to 100KW. So a 1000 meter deep hole can deliver 100KW for 1000 seconds, or 27 KWH. That's about $3 worth of electricity, and about 1/3 of the battery capacity of a Tesla Model S with the large batter option. Numbers not looking reasonable for this concept.

I don't know how heavy are the planned weights, but it sure as hell will be heavier than 10 tons.

Re: Gravitricity

#24

Post by awjr »

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…

It becomes viable when you use a 500,000kg weight...about 1.2MWh

This is not unachievable with some lifting systems currently exceeding this lifting weight.

Re: Gravitricity

#25

Post by grondilu »

gus_massa wrote: post #20
grondilu wrote: post #5

What determines the depth of the whole? I mean, since E = mgh, you can get the same energy storage capacity with a less deep hole if you use a heavier weight. And you can get a heavier weight either by using more expensive material (why use a cheap one? it's not like it's going to wear or anything), or a larger hole. I'm not sure what determines the cost of digging a whole, but I suspect depth matters more than area.…

I still not sure about the details, and I don't like how they use the word "cheap" there. Nevertheless, the density of Lead is 11.35 g/cm3 (the density of water is 1 g/cm3). The densest element in this table is Osmium with 22.6. So replacing if you replace a Lead weight with a more expensive weight you only gain x2. http://www.lenntech.com/periodic-chart-elements/density.htm

Tungsten is near 20g/cm3 and is much more reasonable as far as price is concerned (Osmium is basically a precious metal). I'm not sure sparing a few hundred meters of hole digging is worth gathering tons of tungsten, but I would not be surprised if it was.

Re: Gravitricity

#27

Post by p1mrx »

troymc wrote: post #17
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 true. Unfortunately, current Tesla battery packs use lithium, which isn't super-abundant in Earth's crust. That's why companies like Ambri are looking at using other materials in the batteries they're developing for grid energy storage.

Other materials, like lead and antimony?

http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%...

  Lithium: 33
  Lead: 37
  Antimony: 62

Re: Gravitricity

#28

Post by jostmey »

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…

Okay, so what if you built a huge artificial dam way above sea level? To store energy you would pump water uphill. Usable work could then be extracted when the water is allowed to travel back downhill.

Re: Gravitricity

#29

Post by timdiggerm »

p1mrx wrote: post #27
troymc wrote: post #17

Earlier quoted context omitted.

That's true. Unfortunately, current Tesla battery packs use lithium, which isn't super-abundant in Earth's crust. That's why companies like Ambri are looking at using other materials in the batteries they're developing for grid energy storage.

Other materials, like lead and antimony? http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%... Lithium: 33 Lead: 37 Antimony: 62

Well they are more common

Re: Gravitricity

#30

Post by mikeyouse »

troymc wrote: post #17
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 true. Unfortunately, current Tesla battery packs use lithium, which isn't super-abundant in Earth's crust. That's why companies like Ambri are looking at using other materials in the batteries they're developing for grid energy storage.

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 from a lifecycle and cost perspective but I don't think a lithium shortage is going to have any real impact on prices.

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