> Most expensive component – the hole in the ground – can have a life of well over 50 years
Well well well, I must buy a well.
Gravitricity
51–60 of 222 posts
Re: Gravitricity
#52What 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
Gold is about 20g/cm^3, and although it sounds expensive, there's a lot of gold just sitting in storage. Why not store it underground here instead? This is in contrast to tungsten which has direct commercial value and would be wasted just being stored.
Re: Gravitricity
#53Earlier quoted context omitted.
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.
You're certainly losing torque with the gearing but that's not a problem. The problem is losing energy. Energy loss can be made arbitrarily small at increasing cost to build the transmission.
Re: Gravitricity
#54The 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
#55Earlier quoted context omitted.
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.
Dang, that's impressive. I had no idea electricalmechanical conversion was so efficient.
Re: Gravitricity
#56Although 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…
That's a good point. Say you fill half the hole with containers of water. Let's assume it's 1km deep. Say we have a circular hole that's 2m across, that should get you W = m * g * d = (3.14 * 1m^2 * 500m * 1000kg/m^3) * 9.8m/s^2 * 500m = 7.7 * 10^9 J or 7,700 MJ? That's a lot higher than the numbers being thrown around here.
Edit: just noticed the comment about 500,000 kg -> 1.2MWh. Read that as MJ at first... d'oh.
Re: Gravitricity
#57Seems like this would be more cost effective as a component of sky-scrapers. Generator on top, series of weights on rails down the sides.
Re: Gravitricity
#58Good 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."
Re: Gravitricity
#59Earlier 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.
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."
Re: Gravitricity
#60> Most expensive component – the hole in the ground – can have a life of well over 50 years Well well well, I must buy a well.
They missed a perfect opportunity to use "sunk cost".