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Gravitricity

gravitricity.com

325 pointstempestn209 comments

Good 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."

The majority of humanity's great inventions didn't have basic physics saying "this isn't brilliant" long before a prototype got built. Most of humanity's great inventions were things that were difficult because human beings thought they were difficult, or complicated or intricate or whatever.

On their webpage they say " The biggest single cost is the hole, but it is expected that firstly this will have a very long life and secondly, as the technology rolls out, the costs of drilling will reduce significantly. So the economics will improve in time."

Drilling will not get cheaper without a serious reform of the law, and that's unlikely to happen. There are two primary kinds of drillers; water and energy. Folks involved in water have VERY protective rules in all states. The reasons are 1. good lobbying and 2. if you screw up you destroy the water supply.

Energy drilling is no cheaper. Drilling rigs are huge machines that aren't moved easily and cost at least $50k a day on land and $500k a day for seagoing. That's before you pay another $20k per day in staff and god knows how much for fuel for everything; a rig will produce at least 5MW of power.

Could they custom build a rig just for drilling for their idea and would that work? Sure. But they'd probably spend several million and then you've got to have it running 24/7 for it to pay off.

The regulatory hurdle is going to be non-trivial as well. They're going to case it which helps. But the casing would probably need to be cemented in place which is a non-trivial cost as well. And convincing lawmakers that this doesn't quack like some other kind of well is going to be no easy feat.

The hole for this has different requirements than holes for water and petroleum: you'd not want to drill one of these holes into an aquifer, because then it would fill up with water. Oil and gas wells have to be drilled in specific places where the oil and gas are, naturally. In this case, you'd want nothing but a dry, empty hole. A dry, empty hole would be the kind of hole least likely to interact with anything underground.

I'm not sure this has much to do at all with water and energy drilling. They're talking about using abandoned mine-shafts. Those are much wider and shallower than the kind of holes used to reach petroleum.

Please tell me where I can drill 1500-4500 feet deep without running into an aquifer.

http://waterdata.usgs.gov/nwis/current/?type=gw

Sadly what you say is true. Despite the fact that humanity has been drilling holes into various types of soil for over 8000 years, we don't really have a good way to do it. The situation is not quite as bad as you're describing though. It is not $50k per day, more like $5k. That is for machines that can only do soil, not rock, and aren't as much drilling as they're making a hole. But in places where you can do agriculture, you'd have at least 5m of soil above rock, and more usually 50+ meters. I wonder how much power this system would have with a 10 meter deep hole (like is regularly installed for heating water in homes these days).

These guys may be on to something. Scaling down seems to me to be the key. Full gridless operation with solar panels (at least in earthquake-free zones).

If you want to go deep enough for the system scale to be reasonable, you have to get a big drilling rig. You'll hit rock within a couple hundred feet or perhaps a thousand, but I'm not sure of anywhere in the world with 5,000-10,000 feet of dirt.

The problem with going shallower is that the system capacity is directly proportional to depth and weight. Too shallow and you have to have huge weight and that means a large diameter hole and really big cables and such.

But going deeper requires drilling through rock, and aquifers which are located within the rock. That means big regulatory hurdles because nobody wants to poison the aquifer on accident. Plus the large costs of drilling through rock.

I really like the idea from a theoretical standpoint. It's very elegant. But it doesn't seem practical from an economic or regulatory standpoint. If there are places where you can get the diameter and depth for free, it's clearly genius. But I don't think purpose-drilled holes for this will become a thing -- at least barring some kind of immense breakthrough that I can't even imagine.

It is quite easy to calculate how much energy can be stored and how much power can be generated for how long.

The potential energy of a mass in joule is simply

  m * g * h

where m is in kilogram, g is 9.8 m/s^2, h is the height in metre.

So, for example, 100 ton lifted 100 metre gives 98MJ.

That's 1MW for 98 seconds.

To make this very concrete my house uses, in the summer, about 40kW-hr per day for water heating, cooking, and the rather large amount of computer equipment we have.

That is 40000 * 3600 = 144MJ. So assuming that your solar power system can supply that over a 12 hour period we need to store half of it, 72MJ, to keep us going after dark. Your ten metre deep hole would need a 734 ton weight: 735000 kg * 9.8 m/s^2 * 10 m = 72MJ

Average energy use in the Netherlands is 3340 kW-hr per household per year, which amounts to about 9.15kW-hr per day (and seeing that these are averages, I gather that the type of household to pioneer this sort of tech will be sufficiently energy-conscious to use quite a bit less than that).

Either way, I think you'd have to round your estimate to order-of-magnitude anyway, because of the rather big assumption "that your solar power system can supply that over a 12 hour period we need to store half of it, 72MJ, to keep us going after dark", because getting max solar power for half the day and no solar power for the rest is not exactly how it works, nor does one use the same amount of energy during day and night (which is why I can opt in to an energy plan that gives you discount on electricity at night, because there's a surplus then).

3340 kW-hr is a remarkably low figure. Anyway the point was not to be accurate but to show the kind of working that gives the answers.

Even if we accept your figure the peak daily use is probably at least double your 9kW-hr/day (winter for example) and even if you use less at night you still need probably about 12MJ storage to allow solar to work per household, etc., etc. So, yes, in the Netherlands you need a smaller mass but it is still in the order of 100 ton.

The point of my reply was to show the person I was replying to how easy it is to find out how much mass and height is needed, you just have to plug in your own energy consumption and generation figures.

It wouldn't surprise me at all if the cost structure does work out but the complete picture can't be disclosed for competitive reasons. There are a lot of "obvious ideas" that have only come to fruition recently where the technology was already basically proven, but only entered productive use after several generations of systemic improvements made it a competitive choice. For example: LED lighting was around for several decades but only had niche application until gradual cost/quality improvements pushed it ahead of older lighting tech - and now every major city is rushing to replace all their old street lights. With the Gravitricity tech there are factors like the cost of drilling, the strength, cost and density of the materials used, and lifetime maintenance costs relative to other solutions. There's lots of room for expertise to know for sure what the issues and bottlenecks are.

I'm looking through all the dismissive comments to see if any of them are pinpointing it as a scam - because this is, after all, the kind of idea that is simple enough to captivate laypeople and lift dollars out of their pockets. But judging from this kind of response, it seems like it's also the kind of idea that is simple enough to be dismissed as a "they would have done it already if it worked," rationalized via fuzzy back-of-the-envelope math.

Ultimately, since I have no stake in this one, I'll just wish them luck.

There are a many places in the world where this approach would work and where regulation is not so onerous. Not everyone lives in the United States. And, as your comment seems to indicate, that is a good thing :)

A lot of bad ideas happen in between humanity's greatest inventions. When you look at the past, you are only looking to the successful stories. Maybe a lot of people with unrealistic or bad ideas would have benefited from HN.

Here's an interesting reference, Drew Houston showing Dropbox to HN: https://news.ycombinator.com/item?id=8863

Definitely more negative comments on HN these days (although it's always been a skeptical audience).

To be fair, there are many, many more Show HNs which wound up going nowhere.

That's the hard part though, isn't it? Telling the difference between bad ideas and good ones. Even professionals at it (AKA venture capitalists) get it wrong all the time. So the questions is- knowing that you don't know whether you're looking at a bad idea or a good one, how should you behave?

Even if it's a bad idea, I still prefer a community that encourages people to try anyway and make things for its own sake. HN culture felt much more positive and constructive when I joined a few years ago. While that ethos is still present, there's an unfortunate tendency for the snark to rise to the top.

The responses in that Dropbox thread were pretty solid though. To all the naysayers, I mean. It's sort of a ritual format to test any new idea with the back and forth. That in and of itself isn't a problem.

Real problems: Dismissive people who don't/can't/wont listen to good responses. Good founders/ideas but just really bad at communicating. Those are basically the type i and type ii errors of this format.[1]

_____

[1] A grey area exists in some other areas too (some people just get 'lucky'; others for good reason can't disclose everything the key answers/insights for competitive reasons)

Why does it look like a Fracking drill? It looks like a Trojan horse.

YC still uses that Dropbox application as a case study. Not sure if they also reccomend their show HN, tho.

I thought Dropbox was a good idea until all the files I put there mysteriously disappeared. (I think what may have happened is that they started cleaning up the machine I used at a former company and it was still connected.)

Online syncing is not backup, just like RAID mirroring is not backup, but that doesn't make either bad ideas. Dropbox does however archive deleted files for 30 days, and you can pay extra to extend it to a year: https://www.dropbox.com/help/296

I love how every Tom, Dick & Harry with a bit of math & engineering thinks he can "dispel the myth" of HN Idea X with two minutes of off-the-top-of-his-head equations.

Read about these two: http://www.gravitricity.com/#people

Do they seem like complete morons? Do you think they haven't studied this just a little bit more than you have? It doesn't mean their idea is good or will work - just that you aren't going to rip it apart with two hastily written paragraphs.

People who ask questions about the costs, process, etc. are in the right spirit. People who think they can show off how clever they are really make me /facepalm

You don't need to be a carpenter to tell if a house is built poorly.

You don't need to be a carpenter to tell if the doors are hung straight.

You DO need to be one to actually assess the overall build and structure.

I think you need to re-read the point.

Anyone can tell if a really really shoddy building is NOT structurally sound.

What is hard to tell is if a building IS structurally sound. That is why engineers exist. Not for the previous.

The picture they're drawing isn't very straight, as many have demonstrated.

Until these "experts" provide evidence otherwise, it's useless to berate readers for pointing out the obvious.

Bduerst, I have almost never downvoted anyone over anything. But I must say you have no right to put "experts" in quotation marks. Given their bio, they are experts. Which makes your comment a troll comment. (Unheard-of on HN.)

Really, what do you think the definition of an expert is? "Just because he teaches the stuff at the University of Edinburgh, has sold a company to Siemens that created the world's only installed MW-scaled marine turbines (with the sale described by Bloomberg as a 'blow to the UK'[1]), and received an MBE for his contributions, does not make him an energy expert."

Of course they're experts, even if they're ones you disagree with.

[1] http://www.bloomberg.com/news/2012-02-17/siemens-to-take-ove...

I should have been more specific because I don't think what I was trying to communicate came across. I did not mean to imply that these people are not experts at anything, just that they should publish findings, test results, or at least a white paper on proof of concept.

As it is, this website reads like they did the napkin math for the idea and are using only their credentials to legitimize it. Its an appeal to authority.

Those are valid points, but I think too many here are assuming they don't have anything more substantial & this website is their entire speil.

I don't think people like these two would risk their considerable reputations publishing something that every one of their peers could take apart at a cocktail party. More likely, there's more behind this website.

HN readers rarely give that obvious benefit of the doubt.

There's a lot of things more complicated than building a house.

I saw a similar idea using weights suspended between two mountain peaks but it was to expencive without drilling a hole.

Intelligent people often run the best scams. Pumped storage can vary cheaply move a lot of mass up and down significant heights and it's still to expencive without a natural aquifers. The problem is power is rediculusly cheap and lifting crap takes far less power than you might think.

"The main problem with gravitational storage is that it is incredibly weak compared to chemical, compressed air, or flywheel techniques (see the post on home energy storage options). For example, to get the amount of energy stored in a single AA battery, we would have to lift 100 kg (220 lb) 10 m (33 ft) to match it. To match the energy contained in a gallon of gasoline, we would have to lift 13 tons of water (3500 gallons) one kilometer high (3,280 feet). It is clear that the energy density of gravitational storage is severely disadvantaged." http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

For useful amounts of grid storage you need to be lifting on the order of 100,000+ tons up 1 km which starts to get really expencive just for cables.

Let's do out the math on this...

A subway tunnel might have a diameter of about 6 meters, so cross section = 3 * 3 * pi = 28 square meters. Digging subway tunnel through rock costs about $100M per kilometer. On the one hand, these holes would be vertical, which is harder than horizontal; on the other hand, they wouldn't need ventilation and train tracks and stuff. Let's handwave and say it's $100M for a 1 km deep hole.

Now, you can't fill the whole tunnel perfectly or the air can't escape, so our total volume of mass will be about 25 m^2 * 1000 m = 25,000 cubic meters. If the weights are made from lead, that's a total mass of ~280,000 tons or 2.8 * 10^8 kg, at a mean depth of 500 meters, so our total potential energy is 2.8 * 10^8 * 500 * 9.8 = 1.4 TJ, or 1.3 TJ net assuming you get the efficiency they claim. 1 kWh is 3.6 MJ, so you can store ~400,000 kWh at $100M capital cost (ignoring for the moment the cost of weights, generators, etc.), which is $250 per kWh installed capacity.

That's pretty good... but you also have to pay for weights and a bunch of other stuff. Bulk lead costs about $2,000 per ton on the current market, so that's $560M for the weights, which puts you back in the $2,000 per kWh range which doesn't beat lithium batteries. So you have to use iron or some cheaper material... but then you don't have as much storage capacity because the density is lower, and even with iron you're paying $400 per metric ton or $80M for all your weights. So this isn't obviously impossible like Solar Roadways, but even in the best case it won't make storage dramatically cheaper.

One major advantage this would have over chemical batteries (Lithium-Ion or otherwise) is that it should allow for many, many more cycles. You would have to replace cables and motors and such at some point, but the expensive components - the hole and the weight - should last near-indefinitely.

It certainly appears more expensive than pumped storage, but as they say, it doesn't require a convenient mountain and lake. That makes for much more flexibility in placement, and the closer you can place the storage to the demand, the less is wasted in transmission, and the less storage you ultimately need. That said, the pumped heat storage design[1] that hit the homepage yesterday would have all those same advantages, and although the efficiency would be slightly lower, it looks much cheaper to build.

[1]: http://www.windpowerengineering.com/featured/business-news-p...

Now that you mentioned a mountain... Why again is a hole needed?

Edit: I mean that an inclined rail could be just as good as a hole, and way cheaper.

For the gravity-based design, you need to lower a weight directly up and down in a controlled manner (ie without swinging etc.) That basically requires a shaft in the ground. If you have a mountain and a lake handy, pumped storage would be more economical, but the advantage of this design - as I understand it - is it can be deployed in places that don't have those things.

Actually an inclined plane and a hole in the ground are identical in terms of their energy storage, both are a function of their highest and lowest points. Which suggests a very simple implementation which is to run a pair of rails up the side of a mountain and but the winder/generator at the top and tie it off to the end of a rail car filled with rocks. Sure you would give up some efficiency with the friction of rail on steel but it would be pretty cheap to implement. On some of the alluvial fans in the Mohave it would make for an interested 'excess energy' storage mechanism for solar.

That said, how are those gravity fed porch lights doing? That seems like it would be an interesting proxy here.

I think the important factor here is the possibility of placing these things anywhere one wants. If mountains are available, I don't see why not (except maybe they would ruin the natural beauty and hence tourism...but I don't know how much impact that has). But if they are not, you can always just dig a big hole.

External rails on a mountain don't allow for the optional compressed air part of the system that (according to the page) can double the efficiency of the system at less than double the cost.

Also it uses way more valuable surface area. Especially when you consider the safety measures that would be required.

I have seen the 1830s-sh remains of such a system in Tasmania. They sent coal in buggies down a ~200m inclined railway to the dock below, using it as counterweight to haul goods up the hill. Win-win!

Suppose you have standard rail tracks and use a surplus freight train. Congratulations, you can now store the amount of energy needed to push a single train up a single mountain. That's not nothing, but it probably isn't enough to be useful on a utility scale.

That implies that you want a weight orders of magnitude more than a train (or many trains per charge "cycle" with a stockyard at each and of the mountain to store them) You're now going to need a very tough track and wheelset. They're also going to wear quickly. Probably enough to make it impractical to build and operate.

Peak-power output would be less for an inclined rail (vs the vertical shaft solution).

Yeah, but if they're a quarter as efficient and cost a tenth as much, then we might have a winner.

It may have to do with maintenance.

wouldn't this last a lot longer than lithium batteries?

Maybe? But what if it cost way, way more? You actually have to look at the time value of money for anything that takes more than a year or two to pay off to be sure that there isn't some kind of false economy.

If you assume that the thing lasts forever, well, you divide $x/infinity so it's free. EXCEPT that you've got $x tied up that could be earning interest instead. So it's not $x/infinity, it's the opportunity cost on $x versus anything else out there like batteries, the grid, etc.

If the capacity is proportional to the depth of the tunnel and the mass of the weights, and the maximal mass of the weights is proportional to the depth of the tunnel, then wouldn't the capacity grow quadratically with the invested money?

Edit: You assumed that they can both fill most of the tunnel up with lead and move that up or down 500 meters. The volume that they can fill with that is thus only half of what you say.

Why is the maximal mass of weights proportional to depth? It's more proportional to diameter than anything else. There's a practical limit to how much weight you can hang. Sure if you make a 10km deep hole and a 3km deep weight you could store a lot of energy. But a 3km deep weight might not hold itself together.

If we have enough volume that we can't use the most dense possible weight for all of it, we can find cheaper weight materials by dropping the density constraint.

Couldn't you have the weight grip the sides of the tunnel with gears attached to a generator/motor (inside the weight) so the weight wouldn't rip itself apart? It would be the same machinery that ordinarily would operate the pulley at the top, just moved down into the weights (cause with this, you would need no more pulley). (To illustrate why it wouldn't rip itself apart, imagine gaps on the weight every 100 meters.)

Technically that would work fine. Economically, it's a disaster. McMaster prices are at least 2x what's available if you buy direct from a source, and if you were buying in bulk it'd probably get cheaper still. But this is cheap, cheap stuff for not that much weight. You'd need something I dunno, 100x-10000x stronger to do what you're talking about: http://www.mcmaster.com/#racks-and-pinions/=upn9xl

The appeal of the cables is that they're cheap. And they're cheap because they're easy to make (all things considered). Once you start applying sophistication to the design the price gets higher, and it's already too high from having to drill the hole.

Digging subway tunnel through rock costs about $100M per kilometer. On the one hand, these holes would be vertical, which is harder than horizontal

I would have thought that a better comparison would be oil wells, which cost about $500 per ft of depth, or $1.5M per km.

our total volume of mass will be about 25 m^2 1000 m = 25,000 cubic meters. If the weights are made from lead, that's a total mass of ~280,000 tons*

You seem to be assuming that the entire depth of the shaft is filled by weights. My impression was that the weight was much smaller than the shaft it fell down.

Oil wells are also only a foot in diameter. Makes the economics worse, really.

Actually 1/2 the shaft filled by weights is optimal independently of material, considering only raw energy. (E=p A h(H-h), respectively density, area, payload height and shaft height, Eopt=p A H^2/2 ). Pretty surprising to me that the energy increases quadratically with depth.

Yes, if you're considering only energy. But in practice there are other considerations, such as the cost of the weight, the strength (and thus cost) of the cables, the amount of torque which can be produced for lifting the weight, etc.

I feel this idea would mesh well with the mining industry. Dig straight down, reinforce the walls on the way, separate out the desired minerals, pull up the drill, put in a pendulum and move the rig over and start again. You already need to setup giant motors to move the pendulum, so getting the ore up would be easy. An ideal site would have an abundance of lead, which would cut down on the pendulum cost. Unused rock could be mixed into cement for the walls. The small footprint combined with the benefits of being able to store solar or wind energy for long periods of time could make it the first form of mining that environmentalists approve of.

What's the math look like for the side of a mountain or hill, where you pour a concrete channel and put wheels on the weight so it can be rolled up and down the hill. I'm just wondering if there are places like in the Rockies where this idea would be viable.

I don't understand why they need to dig holes to do this. Why not build something equivalent onto the side of a cliff?

The depth requirement is 500m to 1500m. Though cliffs can be much taller than this, they usually are not totally vertical and cannot accomodate this much depth.

Disclaimer, not a geologist.

True, although I wonder if doing it over even a 100m cliff, but with a much larger weight, would be cheaper than the drilling operation.

It looks as though compressed air is a key component.

It's not.

It's not a key component, but it is a significant benefit of their system. The main reason though is that the primary benefit of this system is flexibility of placement. By locating grid storage close to where it's used most, you save significantly on transmission losses (and infrastructure). Finding sufficient vertical cliffs would be even more difficult than finding suitable locations for pumped hydro.

Also if you expose the weight to open air and wind the whole system would be much trickier than using an enclosed shaft.

Or what about the ocean? It's 4km deep on average, so why not just drop the weights there? No need for setting up tracks or carrying the weight up a mountain.

How are you going to hang a 280 000 ton weight, and more importantly, how are you going to move it up and down?

We can use steel cables. I can't find concrete reference atm, but I'm sure it can be done

You're not. Apsec's math is based on the weight being as tall as the hole is deep, which obviously doesn't make sense.

Can we get a source for the $100M per kilometer figure? The NYC subway system has 373 km of tunnels, 60% of which is underground, which would translate to roughly 22 billion dollars just for the digging... that just seems way too high to me.

2nd Avenue subway [0] requires about 8.5 miles worth of tunnels (several hundred feet of which were already dug back in the 70's) and it's budget is $17 billion. Of course that whole cost isn't just for digging tunnels, but it's a clear indication that digging tunnels (especially in dense urban areas) can be extremely expensive.

[0] http://en.wikipedia.org/wiki/Second_Avenue_Subway

There is a very large difference between building complex tunnels and a simple hole in the ground. The cost for drilling a straight exploratory/mining hole was 10 Million/km in the 70's and 80's[0]. I can easily imagine this being cheaper by a factor of 2-5 with current advances. Not to mention this concept has a lot of synergy with mining companies.

[0] http://facstaff.gpc.edu/~pgore/geology/geo101/interior.htm

There is a difference, but it's not necessarily large for deep drilling. Once you start drilling large diameter holes deep into the ground, you have to shore up that hole to prevent it from collapsing in on itself. For wells or expiatory holes, water or slurry is used, but that's not an option in this case, so there will need to be a casing involve. Installing that casing so that the pieces line line up perfectly is a complex endeavor (not impossible, it's been done, just not straight forward as "drill a hole")

> Now, you can't fill the whole tunnel perfectly or the air can't escape, so our total volume of mass will be about 25 m^2 1000 m = 25,000 cubic meters.*

You seem to have accidentally assumed that the weight will stretch the full height of the hole. Obviously this would leave it unable to move vertically.

>On the one hand, these holes would be vertical, which is harder than horizontal

I don't think this is the case. We dig vertical holes all the time for wells, geothermal, and pilings for buildings: http://www.bdonline.co.uk/Pictures/web/r/r/g/CCTV_17_foundat...

Look at all of them! For one building!

What about depleted Uranium? 19G / cm^3, 1.7 the density of lead. No idea what the cost is, but I can't imagine that waste material would be that costly.

This is wrong.

1. Digging mine shafts is dramatically cheaper than structal tunnels. They dig exploratory shafts frequently to roughly these depths for well under $1m

2. Using a disused mineshaft would have a negative cost associated with it because the security associated with keeping it safe is non zero.

Actually, the majority of the weight would be in steel rope.

For one type [0], the safe limit is a tensile load of about 134-150 MN/m^2 (= MPa) [1]. At a density of 8 g/cm^3, the limiting length of a uniform cable is ~1.7 - 1.9 kilometers. If you have a stationary, suspended cable of this length, its own weight puts it at its maximum load; it can't lift anything else.

Steel rope on McMaster is around $10,000/ton [of rope]. So, these assumptions are a dead end.

You can't solve this by using shorter cables, because that decreases your energy capacity at the same rate. If you use cables of 1/10th the length (~100 m), you get only 1/10th the potential energy storage per ton. The cable thickness per lifted ton is constant.

[0] http://www.engineeringtoolbox.com/wire-rope-strength-d_1518....

[1] The breaking limit of rope is far higher (~700 MPa), and the breaking limit of a single wire strand -- the tensile strength -- is higher still (1,770 MPa according to [2])

For the cross section area, I'm assuming a circular rope (not accurate).

[2] http://www.gabaswire.com/en/overview/grades-of-wire-rope.htm...

"You can't solve this by using shorter cables, because that decreases your energy capacity at the same rate. If you use cables of 1/10th the length (~100 m), you get only 1/10th the potential energy storage per ton. The cable thickness per lifted ton is constant."

This is a simple "figure of merit" for cable in this problem,

    cost / (length * load capacity (N))

The is the same as the cost / energy stored. The denominator is simply the work equation (distance * force) -- the mechanical work the cable can do before it runs out of length.

    = cost / energy

This is actually sort-of constant, since the denominator is ~proportional to the cable volume. (The load capacity is ~ the cross sectional area d^2).

For steel rope from [0], it looks like a lower bound of about $1,200/kWh.

[0] http://www.mcmaster.com/#standard-wire-rope/=upui3o

(E.g. item "3440T68", 5/8" plain steel, $5.16/foot for 9,080 lbf lifting capacity;

$5.16 / (9,080 lbf * 1 foot) = $1,509/kWh)

Here's a related idea: take a piece of land made of granite. Cut a 1km diameter plug of granite, one km deep. Store power by pumping water under the plug, raising the whole thing half a kilometer in the air. It would store enough energy to power Germany for a day.

http://www.solarserver.com/solar-magazine/solar-energy-syste...

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.

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

That table uses a mass-based unit, so wouldn't that tend to create the misleading impression that lighter elements are more scarce? Energy storage capacity depends on the number of atoms, not mass (as you can intuit by comparing lithium batteries to lead or nickel-cadmium batteries -- heavier doesn't mean more powerful).

Well, if Pb and Sb are more scarce than Li by mass, then they're even more scarce when you're counting atoms.

If Ambri's technology does pan out, it will be due to a cheaper manufacturing process, not because they're avoiding lithium.

I thought those numbers were showing that lead and antimony are more common than lithium.

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.

"The greatest shortcoming of the human race is our inability to understand the exponential function."

That's a fine saying but even at 20% YoY increases in lithium usage, we'd still have 35 years or known reserves -- without any increase in exploration or recycling.

As oil & gas have shown, if there's exponential demand for a naturally occurring element, we'll definitely find ways to pull more of it out of the ground.

Increases in known reserves can also be exponential.

The known reserves are 13 million tonnes. However lithium is the 25th most abundant element and you get 20mg of it for every kg of crust, so there is a lot more to find.

"Everybody who has ever claimed that something is doomed because of an exponential function was wrong."

"... or dead."

"The greatest shortcoming of exponential forecasting is the S-curve."

Except all those dire predictions turned out be wrong. We didn't run out of food or resources, and birth rates have now declined in almost every country in the world. Population growth is basically now a complete non-issue. In retrospect, ignoring the Paul Ehrlichs of the world turned out to be the right move - a far cry from mankind's greatest shortcoming.

If we run out of lithium we'll figure something else out, just like we would for any other resource.

Except that then we have run out of available lithium, which means that solutions that require lithium will be inaccessible. Running out of chemical elements is a problem. The helium shortage is a serious problem. High-energy physics is made from helium.

If those physicists hurried up with cold fusion that helium shortage wouldn't be such a problem.

Why is helium found underground with natural gas? My theory is that the helium underground comes from radioactive decay and gets trapped along with methane.

-- I guess there's no need to speculate when you have Google: https://www.google.com/search?q=where%20does%20helium%20come...

We will never "run out" of anything, including oil or lithium. We'll stop wasting these resources the minute their prices rise to a level where something else becomes more economical. Regardless of when that happens, there will be plenty of lithium left for any potential fusion sources that might be developed later.

For peak storage, vanadium accumulators are well established. Lithium cells give you an advantage in density, but for peak storage you are looking at capacity first, and then you consider the footprint. With vanadium you are only limited by the size of your electrolyte tank. The additional advantage is that vanadium reserves are much greater than lithium reserves.

He's not suggesting that we use lithium battery packs. He's suggesting the idea is just plain ludicrous.

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.

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.

Also lifting 500 tons is a lot easier when you don't have to move it horizontally. 1.2MW isn't as much as a wind turbine though which is a pity because they conceivably could be a good pairing.

In trying to ascertain how much weight is practical to lift, I came across this list of the strongest cranes, which can lift millions of kilograms (thousands of metric tons): http://enr.construction.com/products/equipment/2012/0925-the...

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.

This is what is currently done.

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

Well known as "pumped storage". They're competing with it, but dry and upside down. Is it more economical? Maybe. Is it safer? See http://en.wikipedia.org/wiki/Taum_Sauk_Hydroelectric_Power_S...

At least if this contraption breaks, there's nothing important below it.

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

I would think you could find an elevated lake that replenishes naturally to avoid pumping the water up. But then maybe that is just "energy".

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...?

18,000 kg is just 1.58 cubic meters of lead.

You can have a much much heavier weight.

That's a devastating critique. The idea seemed a bit too good to be true, but I couldn't quite put my finger on why. Thanks for posting that.

I did a little more research, and I'm not sure anymore that it's a complete deal-breaker. But it's definitely going to put limits on how far this can scale.

You just need more mass. Have a look at this concept: http://eduard-heindl.de/energy-storage/index-e.html (unfortunately the english version is very sparse, the german version has some more images and videos: http://eduard-heindl.de/energy-storage/ )

The idea is to cut a 1km diameter cylinder into the ground (solid rock), and then lift it up up to 500m by pumping water underneath it. The power stored can power a country for a day.

I am somewhat sceptical, but this guy is totally serious about it.

But presumably you'd need more than a day's worth of power to lift the damn thing? That's probably doable, but still, I suspect that'd be the main problem.

The point is to have a way to store the excess energy we generate at off peak times like the middle of the night.

The efficency is said the be 80%, just like any other pumped storage system. It just uses considerably less space than a water reservoir (actually close to none, as the top of the cylinder remains untouched) and is scalable by adjusting the radius.

I guess the main problem is to keep the water confined (200bar of pressure) while keeping the mass movable. Maybe someday they build a small testing facility, so we know for sure.

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 suspended.

Efficiency would potentially not be on par, however linked into solar/wind systems this is less about efficiency and more about creating a 1MW long term battery with a lifetime of 50+ years.

Guessing cost of digging and maintaining a hole compared to installing a guide rail is significantly higher.

>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?

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.

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.

I think the coolest thing about this concept (for me) is viewing it as a "whole-system" energy storage procedure.

It is effectively 100% renewable, 100% distributable, using 100% commodities (ie: rocks in a hole).

As a thought experiment: if on average you can meet 110%+ daily power expenditure captured from renewables (solar, wind, whatever), and store it by lifting up these weights, then you've broken into the "free energy" loop.

More specifically, don't look at the power input or storage, look at the power output / usage. If your input + storage capacity is greater than your output rate then energy effectively becomes "free forever".

Simulate it on a small scale. Get a pinwheel to run a small motor that winds something up. Attach a small LED to it that you only run occasionally. Basically, just so long as you have a really small output draw compared to your input rate and storage capacity, this "battery" will give you energy when you want it with minimal maintenance costs and minimal consumables.

The key figure of merit for comparing energy storage is not $/KWh, but rather $/KWh*Number of cycles. Li-ion only has about a 1000 cycles. Assuming one cycle per day (solar charge during day+discharge during night) in 50 years there are 18000 cycles.

The figure of merit for Li-ion is 250x1000=2.5e5. My estimate (and those of others) is that this costs up to about $2000/KWh. So the figure of merit for this is 1000x18000=0.9e7. Two orders of magnitude better than Li-ion.

Edit: I am ignoring the cost of capital, interest rates etc. Somebody should do this analysis.

Sandia National Labs has done the math but it's quite the PDF at 340 pages describing just about every variety of energy storage:

http://www.sandia.gov/ess/publications/SAND2013-5131.pdf

I did some quick analysis comparing the proposed project against Li-ion, adjusting for storage and lifespan, and the result is very hopeful. I used values for $/KWh and lifespan from abdullahkhalids' comment. The analysis shows that if you can get more than one cycle per day out, the proposal offers fantastic value.

Model visible here - http://imgur.com/KmQPb8P

Finance uses something called 'equivalent annual cash flows' (EACFs) to compare projects of different lifespans. Using EACFs makes this analysis very simple.

Using abdullahkhalids' figures and assuming a single cycle per day, the equivalent annual cash flows per KWh are equal when the cost of capital is about 4.35%.

Something magical happens when you assume can get more than one cycle per day. At two cycles per day, your funding costs could be 10% and the project still feasible! At four cycles per day and with a 5% cost of capital, Li-ion costs almost four times as much as the proposed project.

Could someone shine some light on how many cycles per day is realistic for both Li-ion & the proposal?

PS if you find this sort of analysis interesting and want to hit me up to talk about such things, feel free to send me an email (available in my profile).

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

This is just rampant speculation - but what if they used depleted uranium as the weight? They could get paid to take the material off others hands (instead of buying lead) and it's almost 70% more dense.

Wouldn't the initial cost savings be offset by the ongoing challenges of dealing with a big moving chunk of hazmat?

Probably... but it's still an intriguing idea! Especially since their pressurized gas proposal already calls for a sealed cavity; maybe there actually is potential there. (Of course, there would probably be a significant PR cost too.)

That said, they can already get more weight by making the weight taller, so density likely isn't a huge benefit. Whatever's the cheapest cost/kg (and without the other issues of Uranium) would probably be best.

Well at least if it fell it would be buried in a giant hole.

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 store energy, the top weight would get winched up, and when it hit the top it would lock into place somehow and the next weight at the bottom of the shaft would engage the lifting cable, etc. The cable would have to follow a circular track, rather than having 1KM of cable for each weight.

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.

Well, it does mean your cables and dynamo can be smaller. But then you've got a new problem of coupling the cables to the load at the bottom and somehow storing loads at the top that the cables must reach past/through.

You have a shaft 1km or 1.5km deep. You can afford to stack a lot of weights vertically. Imagine each has a hole running through the middle, where the cable runs, and a mechanism whereby the weight can engage/disengage from the cable. The cable forms a circular loop that passes through all the weights from the top of the shaft to the bottom, then circles around past a large pulley and back to the top, completing the loop.

At the top of the shaft there would be hefty prongs that retract when the weight needs to start dropping, and when the weight returns during a recharge cycle, the prongs reinsert themselves in to the shaft when the weight is lifted back into position.

Most likely each weight would have a "C" cross section with a nearly closed mouth -- just a slot from the edge of the weight to the larger central opening so that the weight can be removed/replaced from the cable if needed.

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.

My understanding is that peak power availability is at least as important as total energy storage. (The main use of grid storage is to handle spikes in usage, or similarly to fill in for dips in generation - like from wind turbines. Either way, peak power available is important.) One big weight would give you significantly more peak power than several smaller ones. Yes it would require more or stronger cables, motors, and other infrastructure, but the cost of all those is small compared to the shaft, so I imagine a single weight would maximize peak power / cost.

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.

Funny, I just finished Peter Thiel's book. In the final chapters, he discusses the cleantech flame-out companies of the 2000's and how many were run by old suits. He contrasts them with true technology innovators (like Elon) who are "t-shirt and jeans" people.

Interesting to scroll down and see suits at this site.

Seems like this would be more cost effective as a component of sky-scrapers. Generator on top, series of weights on rails down the sides.

I wonder if you could somehow combine it the counterweight system.

Not even close. First a 1KM tall skyscraper? Second they are talking quite a lot of weight - the skyscraper would have to be MUCH stronger.

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

No, the additional load is not worth it. It would be one of the least sustainable things you could do due to all of the heavier columns you'd have to use.

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.

Dang, that's impressive. I had no idea electrical<->mechanical conversion was so efficient.

Think about how little cooling is needed on a big electric motor. Even motors made 100 years ago are very efficient.

By contrast anything that qualifies as a "heat engine" (including internal combustion) is limited by Carnot's law to be low-efficiency. If it weren't for the extraordinary energy density of combustible fuels they wouldn't be competitive.

Yes, the theoretical efficiency limit on electric engines and generators is 100%.

Amtrak still uses motor-generators on some parts of the Northeast Corridor to convert 60 Hz AC grid power to 25 Hz AC traction power.

It's exactly what it sounds like: a motor that is mechanically coupled to a generator. The motor runs on 60 Hz AC and turns the generator at the correct speed to produce 25 Hz AC.

Haha, wow, a mechanical engineer was on duty that day.

Today, the majority of grid energy storage is done using pumped-storage hydroelectricity (PSH). "PSH reported energy efficiency varies in practice between 70% and 80%, with some claiming up to 87%." [1]

[1] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

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.

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.

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.

Does it really matter?

You should be able to get millions of rotations from a single centimetre hole, given that the mass is large enough and you have a good gear ratio..

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.

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.

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.

You have to add the cost of the armed guards, because a big block of gold will be a nice target of a robbery.

Well, yes. To be fair, if there was like a farm of these devices, it might makes sense to swallow that expense.

I wish gold wasn't so rare! Its such an amazing material, and our economy is not based on precious materials anymore (except the commodity markets or course), so it seems like a really sad thing that we can't have more abundant gold just yet

I thought of this decades ago, and I am sure many others have too. And no the math does not come out favorably. While technically feasible, the cost is prohibitive. The reason is simple, you need LOTS OF WEIGHT to get any appreciable storage capacity.

Try it yourself: http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

Note that 1 joule = 2.77777778 × 10-7 kilowatt hours

There's interesting variations here - at first glance a single big weight is the same as many small ones, but you have to think of the engineering challenges of cabling and motors for a 1 kilotonne mass. So you could use many smaller objects in a stack and move them one at a time, but then you've got the problem of decoupling/recoupling the cable and reaching through the ones that are at the top while lowering/raising the ones at the bottom.

Now, the next obvious consideration is using mountains instead of a pit. The rockies are full of 4000M peaks. But a mountain means suddenly we have so many new considerations - mountains aren't exactly a constant slope from peak to foot. But extreme loads on rails are a solved problem - the world's heaviest single fully-loaded rail-car was about a kilotonne (a special schnabel car carrying a reactor up to the Alberta tar-sands).

Of course, then you've got a new construction problem - building a train-track that's a near-straight-line up a mountain and can support incredibly heavy trucks.

It's probably not workable because of the energy-density concerns, but it sure is neat.

I have this idea in my mind for a while now and it seems great subject to share it with you.

Imagine an elevator going kilometers into the oceans dept. A huge tank is mounted on the top of it. It gets filled in with pressurized air. Because its heavy it sinks to the bottom. Then an air is released. Air travels into the surface but on its way it is actually captured into a little traps. When enough air is trapped, the entire structure built from hundreds of traps is lifted into the surface, together with the tank. of course during this trip, it triggers friction and the dynamo mounted on the surface translates this movement into electricity.

Once on the surface, the tank is filled in with air, and the process starts all over.

With long enough elevator in deep enough ocean, the electricity produced thanks to the travelling elevator would be greater than electricity used to put air into the tank.

If you ignore the rules of gravity and that trapped air travels to the surface of a water, this could be a perpetuate mobile.

What's wrong with my idea?

The deeper you go, the higher the pressure. The higher the pressure, the smaller the (trapped) air bubbles. The smaller the air bubbles, the lower the upward force.

As a result, the deeper you go, the more air you will need to pump into your tank.

Or in other words: with a given amount of air inside your tank, there's a maximum depth you can go. If you go deeper, the air bubbles will no longer lift the tank.

Of course, and that's what happened to Kursk.

But:

The released air would go up, regardless how pressured it is. While going up it would decompress taking up more space and therefore having greater force to push the tank up (as I mentioned, released air would be capture in a construction similar to a tree with leaves. Imagine leaves upside-down capturing the air.

I'm sure you can find different type of mix that would be less-compressible and overal better for this experiment.

If the bubbles are captured higher up, say 500m above the tank, so that the increase in deplaced water can lift the tank, you couldn't lift the tank all the way up to the surface; it would be stuck at 500m below surface.

It's the basic law of conservation of energy. You need to provide the energy required to lift the tank by compressing the air. The energy required to lift the tank is mgh (mass of tank, gravitational constant, height to lift the tank). The energy stored inside the tank is proportional to p*V (pressure times volume). You can't lift the tank further than the amount of energy you have available, so you can never have a surplus of energy.

If you drill a hole 1500m deep, and you use concrete as the material of the object used for energy storage, and the volume of the object is V [cube meters], when the object is at its maximum level you have stored an energy of:

E = 10 x V KWh

This means that with 10 cube meters of concrete you can store 100 KWh (more or less the energy consumed by 10 households per day). If what they claim about doubling the energy storage is true, with the same amount of concrete you can store 200 KWh. 10 cube meters of concrete would weight just 24 tons, not so much after all.

Assuming a hole 1600m deep, and a system of cables capable of handling 250 tons (100 cube meters of concrete), you can store up to 2000 KWh (so you can serve 200 households for a day).

I don't know the price of drilling a hole 1600 meters deep, but with these premises, if the price is in the range of 1-5 millions $, a medium neighborhood can afford the operation.

> 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".

I really hope these guys succeed, even if they don't replace conventional batteries everywhere. One thing I've been thinking for a long while is that: the future of humanity is in how advanced drilling equipment we can make. Think about it: asteroid mining, colonizing planets or even just conventional mining...all require drilling. If this technology catches on, there will be so much research done in finding better drilling techniques. In fact there is so much resources to be found in our Earth itself, if we drill deep enough.

Also, imagine if we have a base on the moon powered by solar cells: having the technology to drill quickly and cheaply would be indispensable in storing energy captured during lunar "days". Although I imagine you would need deeper holes because of the smaller g.

So basically clock-type counterweights. PSH seems like it would be far more efficient - there's surely a lot of mechanical losses in the sort of system in the OP?

If the hole could be used for some sort of heatpump too then maybe that would weigh off [no pun intended!] some of the problems.

If we had a dyson sphere around something heavy then the crank part could be in space with the weight dangling into the atmosphere: https://en.wikipedia.org/wiki/Dyson_sphere

How about the Moon? :D

Yeah, I thought about that, but there wouldn't be enough gravity and it might be a bit too far to beam the energy back.

Okay, so energy density is low - but what are the costs like?

If you can build one of these cheaply, and the running costs are trivial, then it's worth doing.

How many of them would you need to smooth out the energy of a wind farm, for instance?

I'm far from being even slightly knowledgeable in this topic, but would it be possible to build this in very deep waters? Like a massive column containing a tunnel? Seems cheaper than digging a 1km hole.

You'd be anchoring a buoyant tube to the ocean floor, trapping all that energy. Instead, float a barge in deep enough water, drop a weight to generate power, raise it to store.

At the base of a 1000m shaft you'd have approximately 100 atmospheres of pressure. Not saying it cannot be done, but construction and materials cost would still be very high. Maintenance too. Plus you've lost the advantage of locating it anywhere.

FTA:

> The key requirement is a deep hole in the ground; it could be a disused mineshaft brought back into use, or it could be a purpose drilled or sunk shaft.

So, apparently, "sinking" a shaft involves making a hole using some technique other than drilling. What technique is that?

Dictionaries are no help. Wikipedia[1] says:

> Shafts may be sunk by conventional drill and blast or mechanised means.

"Mechanized means" is pretty vague. Can anyone clarify?

[1] https://en.wikipedia.org/wiki/Shaft_mining

This may be of interest:

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

Been attempted before. Its interesting that below ground is preferable to above ground. Water tables in many places will be a problem.

http://www.launchpnt.com/portfolio/energy/grid-scale-electri...

http://www.greentechmedia.com/articles/read/Gravity-Power

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.

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.

Sure. This is also neglecting the compressed air part of this technology (which thermodynamics might actually be quite appropriate for modelling!).

I was really hoping it would keep scrolling down nearly forever, with something neat waiting at the bottom :-)

He's managed to get a patent for it http://worldwide.espacenet.com/publicationDetails/biblio?CC=...

At least you'll have to appreciate that these guys have started a viable enough business before for Siemens to buy it. However, when they started out it appeared equally unfeasible at the time..."impossible is an opinion not a fact"

I'm probably missing something, but what's the advantage of drilling a hole as opposed to "simply" building a kind of drain or sky scraper. I doubt it would be more expensive.

Sure, the pressure-aspect would be away, but besides that...

If this catches on everywhere we'll run out of gravity in no time.

Keep in mind, we currently remove entire mountains to extract coal. If there is one thing we can do at scale it's dig holes, move concrete, and make steel cable.

What about springs for energy storage?

https://www.youtube.com/watch?v=YCs002BywAE

I look forward to Elon Musk's proposal for unconventional energy storage, because he wouldn't dare call it anything other than Eccentricity.

Eclectic energy

This is just a pun, but of course his proposal for energy storage already exists, and (surprise surprise) its lithium ion batteries. Significant parts of the projected gigafactory capacity are reserved producing cells for SolarCity home energy storage appliances and other storage systems.

In fact, right now the Tesla factory has 4GWh of lithium ion batteries installed for smoothing energy demands and they are actively building 400kWh storage units:

http://www.greentechmedia.com/content/images/articles/straub...

I'm sure that the end game idea in his mind is using the 85kWh+ batteries of Teslas everywhere for distributed energy storage.

Yep, and as the maker of the most efficient of those batteries, he can make a nice clean profit. Good on him though, he (and the whole Telsa team really) did put in a lot of effort to get there.

This sounded so much more interesting before I knew what it was.

They are using the wrong approach. It's the difference between Karate and Aikido.

Can one use potential energy to, well, store energy? Duh. This, however, is the wrong way to do it.

Digging that hole seems expensive.

Good to see these's still a sucker born every minute.

Intelligent people on HN are pointing out huge holes with this idea but some people are still defending their Nigerian princes, because... they want it to be true?

The warning signs here are huge. It's an incredibly simple idea, if it was possible it'd be already done. Nothing here really seems to rely on scale either.

Where can I buy one? I'll start digging in my backyard ;)

Drilling into the ground should almost never happen.

Why?

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