Gravitricity

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.

Also, does the shaft has to be vertical? You could dig it with a sharp slope, and put your weight on rails. That would make the shaft longer for the same depth, but it would probably be easier to build and maintain.

Re: Gravitricity

#6

Post by KaiserPro » Sat, Nov 22, 2014, 10:47 PM UTC

So how many joules can it contain?

I mean, technically I can get a super cap the size of jam jar to kick out 1 kw, just not for very long.

A watt is a unit of how much energy is expended in a second, not how much energy is stored. There is a reason why hydrostations in wales use lakes to store energy, because you need a lot of mass at great height to be of any use.

Re: Gravitricity

#7

Post by KaiserPro » Sat, Nov 22, 2014, 10:48 PM UTC

gatehouse wrote: post #3

Is 90% efficiency really feasible for electrical -> mechanical -> electrical?

95% efficient each way, it does sound a bit much.

Re: Gravitricity

#8

Post by sz4kerto » Sat, Nov 22, 2014, 10:48 PM UTC

This came up a long time ago. It doesn't work well, the amount energy it stores is simply too low. I've seen a calculation somewhere, but it's too late here so can't reproduce - does anyone remember?

Edit:

here it is. https://news.ycombinator.com/item?id=6739349

Re: Gravitricity

#9

Post by bsilvereagle » Sat, Nov 22, 2014, 10:48 PM UTC

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.

Re: Gravitricity

#10

Post by elektropionir » Sat, Nov 22, 2014, 10:49 PM UTC

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 (immovable) hole to have the equivalent of a Roadster battery pack. I'd go with the battery pack.