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Gravitricity

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Re: Gravitricity

#41

Post by pbhjpbhj »

awjr wrote: post #15

Is a 500m-1500m shaft is pretty much going to fill with water? I could see a well designed weight being able to work in water (although water turbulence would erode the shaft walls. I don't see how air compression would solve this. The principle however of storing energy by raising a weight could also be used anywhere with a steep enough hill/cliff/montain and the weight could in theory be on a rail not just suspende…

>Is a 500m-1500m shaft is pretty much going to fill with water? //

So you're saying you get a free well as part of the deal?

Re: Gravitricity

#42

Post by jberryman »

_greim_ wrote: post #38
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…

This doesn't make sense. Gravity doesn't have an energy density. The energy density of this system is going to be directly related to the density of the material being raised/lowered.

Isn't that the math that the GP posted...?

Re: Gravitricity

#43

Post by toomuchtodo »

Retr0spectrum wrote: post #34
slashnull wrote: post #23

Earlier quoted context omitted.

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

Even if it was 1000 tons, it would still only be about $300 worth of power.

Depends on the spot price of power. Power on the open market (where utilities have to buy it to provide to customers) changes every minute. Its usually cheap at night until the early morning, starts to increase, and then spikes around 3-4pm (+_30-60 minutes, depending on the country).

If you delivered power when it was most expensive (ie high demand) and consumed it in the middle of the night, the arbitrage may work out.

Re: Gravitricity

#44

Post by [deleted] »

_greim_ wrote: post #38
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…

This doesn't make sense. Gravity doesn't have an energy density. The energy density of this system is going to be directly related to the density of the material being raised/lowered.

[deleted]

Re: Gravitricity

#45

Post by DanielBMarkham »

jostmey wrote: post #28
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.

I live near a twin-lake hydro system that does just that.

Every night the lights dim for a bit when the grid starts being used to pump water back up.

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

Re: Gravitricity

#46

Post by elektropionir »

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.

I think it's more likely we'll see an asteroid mining economy bringing lithium for batteries on Earth than millions of 1km holes with 20 ton weights storing energy for single charges of electric vehicles.

Whatever local (i.e. small scale) energy storage we have in the future you can probably bet (from first principles) with 99% certainty it will be based on the electromagnetic force. Gravity is too weak to be practical for smaller scale storage and cannot give mobile storage units since you have to deal with huge weights. For large scale see the "pumped storage" system already mentioned in this thread. Just for illustration, the "Taum Sauk Hydroelectric Power Station" can produce 175MW of continuous power. If you dropped the 18T weight in the 1km shaft you'd get about 13MW averaged over the 14s fall. Then you have to lift it up again if there are still enough pieces left.

Even if you found a way to store energy in nuclear interactions (i.e. "charge a nuclear battery") you don't want to have a bunch of containers full of radioactive material all over the place. If one thought it through, I wouldn't be surprised if there are in-principle issues for the charging part similar to the ones with gravity.

Since we don't know of any others, this leaves only the electromagnetic force to store energy with sufficient mass (or volume) density in practical ways. Whatever it ends up being (chemical batteries, supercapacitors or something else) the future local energy storage unit will separate and hold charges apart.

Re: Gravitricity

#47

Post by tormeh »

KaiserPro wrote: post #7
gatehouse wrote: post #3

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

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

Not really. Electrical engines and generators are incredibly efficient. It's not the transition there that is the problem. In fact, if it was only for that loss the efficiency would be more like 97%. The problem is the friction in the mechanical parts; the wires, gears and so on.

Re: Gravitricity

#48

Post by madaxe_again »

This is neat, but not novel. I used to live in a house (UK, middle of nowhere) that had a deep borehole that was used for this purpose 130 years ago. The weight and winding were long gone, but the dynamo was still sat there. Oh, and it wasn't raised by water, rather, servants, back in the day.

Re: Gravitricity

#49

Post by bsilvereagle »

schoen wrote: post #40
bsilvereagle wrote: post #12

Earlier quoted context omitted.

Work = Force*distance, so theoretically the number of Joules would be mgh^2.

Your units got messed up there -- it's just mgh. There's no reason to square the distance. $ units Currency exchange rates from 2013-07-11 2562 units, 85 prefixes, 66 nonlinear units You have: 1000 kg gravity 1 km You want: J * 9806650 / 1.0197162e-07

That's what I get for doing intermediate thermo & fluids all day and then trying to make a comment on basic dynamics. Thanks for the catch.

Re: Gravitricity

#50

Post by Sorgam »

tasty_freeze wrote: post #35

Although I think the idea isn't workable (energy density is too low, cost of boring the hole is tremendous), most of the other commenters here seem to think they'd just have one weight, whether it is 1000 KG or 50,000 KG. Any sane plan would be to have more than one weight. When the first weight hits the bottom, it would release from the cable and another weight up top would grab the cable and start dropping. To stor…

What advantage does this provide over combining all your weights together into one big one? If you want to limit the tension on the cable, just use more cables.

You shouldn't be so confident that you know the only "sane" way to design such a thing. It's not a field anyone has experience in. These are only guesses.

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