Can You Store Renewable Energy In A Big Pile Of Gravel?

As the world grapples with transitioning away from fossil fuels, engineers are hard at work to integrate new types of generation into the power grid. There’s plenty of challenges, particularly around the intermittent nature of many renewable energy sources. Energy storage projects are key to keeping the lights on round the clock, even when the wind isn’t blowing and the sun isn’t shining.

Conventional grid-level energy storage has long made use of pumped hydro installations where water is pumped uphill to a storage reservoir where it can later be used to run a generator. More recently, batteries are being used to do the job. When you consider the cost of these installations and their storage capacities, there is a gap between batteries and pumped hydro. A recently published whitepaper proposes Mountain Gravity Energy Storage — gravity-based energy storage using sand or gravel in mountainous areas — is the technology that can bridge the gap.

They’re Storing Energy On Mountains Now

A diagram of an representative example installation in Molokai Island, Hawaii.

The concept of Mountain Gravity Energy Storage, or MGES, involves storing excess energy from the grid by raising sand or gravel to a higher elevation. This is achieved using a pair of cranes, which load the material into storage containers, before pulling them up to height on a cable. The material can then be held in storage at higher elevation until power is requested by the grid. At this stage, the material can be reloaded into storage containers, and lowered to the bottom storage site, with gravity doing the work to pull the weight back down, turning a generator in the process. Interestingly, the same electric motors that lifted the gravel in the first place can also be used as the generators.

If this sounds familiar, you’d be right. It’s not dissimilar from the basic theory of pumped hydroelectric installations, where water is pumped into a dam, and then allowed to flow out through a turbine when energy is required. However, these installations are typically only economically viable in larger installations of 50 MW and above. MGES systems are intended as an option for smaller installations, on the order of 1-20 MW output. For small islands or other isolated areas, an MGES could be a great way to support the local power grid in combination with renewable sources of energy.

Can You Spare a Mountain?

A GIS data analysis was used to determine areas around the world that would be viable for MGES installations.

The viability of such an installation is dependent on the availability of suitable mountainous terrain. The higher the elevation difference between the top and bottom of the system, the more energy can be stored. An MGES would likely become viable for areas where the natural landscape enables an elevation gap of between 500-2000 meters.

Obviously, using sand or gravel as the energy storage medium brings its own set of challenges. It’s not possible to easily pump dry materials around like liquids, hence the need for cranes to move the material. Said cranes would need to be specially designed to carry heavy loads, and work reliably for high duty cycles during periods of rapid energy transfer. Loading and unloading of the sand or gravel would ideally be done with an automated system, with the paper’s authors suggesting this be handled by passing containers underneath the upper and lower storage areas. Thus, gravity could simply feed the material into the containers when required through a hopper. This adds a small loss of potential energy to the system, but minimizes the complication of the loading and unloading process.

On the other hand, pumped hydro has its own problems. One is the need for large amounts of water. Another is that storage reservoirs are prone to evaporation and freezing.

Is It Competitive?

While the basic physics of the system is sound, there are pitfalls to such a system. There’s a high degree of mechanical complexity involved, which not only complicates the design, but also adds to maintenance and running costs which could spoil the viability of a full-scale project. In large-scale electricity grids, pumped hydro is likely a more prudent choice, being an established technology that scales well to higher power levels. For smaller scale, isolated grids, MGES could have a place, but it is forced to compete with battery systems capable of delivering similar power levels. In these cases, batteries have the edge for short-term storage, while MGES could prove valuable where energy needs to be stored for weeks or months at a time. This has particular relevance to holiday spots, where there may be large seasonal changes in energy use.

If there’s a particularly bold company willing to invest in a pilot plant in a far-flung mountainous island, we may yet see such a system in action. It would likely make quite the spectacle, or eyesore, depending on your point of view. In the event that happens, we’ll be sure to cover it, but for now, MGES remains a novel and interesting concept that hasn’t quite reached fruition just yet.

112 thoughts on “Can You Store Renewable Energy In A Big Pile Of Gravel?

  1. How is this any more efficent than using pumped water? The friction increase in using a solid medium over a fluid would be a huge loss of “energy”. Pumped water does the same thing and doesnt require cranes or the energy to operate them.

    This sounds like snake oil or the monorail salesman from the simpsons

    1. From the paper:

      “This paper argues that gravitational energy storage could fill the existing gap for energy storage technologies with capacity from 1 to 20 MW and energy storage cycles of 7 days to three years storage.”

      1. there is no existing gap as PHS is viable below 20 MW:

        https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity#Small-scale_facilities

        That and PHS is mechanically more efficient and also requires less maintenance. Like i said, snake oil…

        The economies of scale when it comes to PHS will continue to drop as technology gets better and after actually skimming the paper, i am concerned with such assumptions as:

        “Assuming that the plant operates autonomously with a precision software,”

        I mean, if you are transferring the medium above ground then precision software means shit because nature will mess with it and if you transfer the medium below ground then you are in the same costs as PHS.

        Table 5 kind of tells it all, when considering that PHS has a cost of installed capacity and yearly storage cost half that of MGES and that in the table they state the minimum viability of 100MW for PHS while other places they state the minimum viability as 20MW, I would argue that this paper is guilty of selecting data to push an already pre concieved notion. This is not a comparison of energy storage methods, it is a paper to push something using chery picked data.

          1. Pumped hydro isn’t quite obsolete yet – the cost per MWh stored is about half that of batteries, and it can be left fully charged almost indefinitely with no adverse effects.

          2. China have batteries half the cost of batteries being installed in California and yet China is opting for pumped storage. Elon is a salesman and is selling his flavor. Pumped storage will beat batteries hands down on MWH costs capicity costs not so much. Energy stored is far more important than power capacity available

        1. as mentioned in the article.. having massive amounts of water to store is its own problem as well , the first part of which being that a lot of places may like power but not have excess water.

    2. There’s a rule in reporting that say something like; every headline that ends in a question mark can be answered in the negative. That’s because if the article talks about something that isn’t already proven, it’s most likely not true and they’re just running clickbait.

      Can you? No.

      The energy density of sand is at best 3-4 times that of lifting water up the same distance, so instead of square miles of water in a reservoir, you have slightly fewerd square miles of sand to store any meaningful amount of energy. Since sand doesn’t flow, it freezes solid in the winter, your excavators would wear out, cables would snap, pulleys would run out… etc. it’s just an engineering disaster and an exceedingly stupid idea.

      1. some remarks (not so much related to the linked comment):

        — In my book, sand density is at 1500..1700 kg/m³.

        — for water you could be fine with one set of turbines, that can also work as pumps, especially for small scale solution. So only one mechanic base station, some pipes and lots of levees.

        — why want small scale solutions? wouldn’t it be better to have a working power grid and some large scale storage units?

        1. Sounds about right for sand. All the little voids between the crumbly bits don’t add much mass being made of air.

          Mercury isn’t used all that much in industry these days. So there has to be a bit of surplus kicking about somewhere (check the ink wells in old physics labs). Mercury is much denser than water (13.5 times) and putting it to use for gravity storage would allow us to bottle the displaced water and sell it for a fat profit at train stations and other places where there are lots of thirsty people.

          1. “Mercury isn’t used all that much in industry these days.”

            Excellent idea if can be sealed and quarantined critically. I’m guessing with proper risk assessment and mitigation planning implemented in a valid design… the system will be considerably safer than nuclear storage and handling. Seems like a tandem project.

            Furthermore, this would be an excellent way to quarantine and still utilize the mercury vs vaporize into the air stream and atmosphere. Might even be some sort of subsidized incentive to use bromine or other scrubbing methods as a value stream for the waste streams at power generation facilities and other materials processing sites.

            Also, on the thought of nuclear materials again (depleted Uranium is ~18.7 times more dense than water)… might even be a way to utilize nuclear waste too if quarantined and stored critically. Then again… I’m guessing the mercury will cause stress in the materials used if not critically designed for the longest term and least maintenance cycles.

            Still, might be an advance and goal for materials science to consider advancing materials or methods used to process and store nuclear waste with the mercury for an even more effective working fluid to utilize most value added.

            Makes me wonder what the eutectic points and graphs of the mixtures look like in regards to fluid dynamics and states.

          2. A huge quantity of mercury like that is a disaster waiting to happen.

            The price of mercury is very volatile. If I’m doing the calculations correctly, it’s currently about $20 per pound.

          3. @jafinch78:
            Perhaps chemists could find a liquid uranium compound as high density storage fluid? :-) Then you have a use for the depleted waste uranium and don’t need the toxic mercury.

            P.S.: I know, that uranium is also a chemically toxic heavy metal :-) But Gallinstan is probably to expensive.

          1. Only if you do it stupidly/at minimum cost. You’d get similar problems from just ripping out a few acres to put down your sandpile which might blow off and dustbowl areas downwind if not covered.

    3. Oh man, Monorail power storage! Haul weighted cars up the mountain to store the power. use a controlled descent of the mountain to generate or “release” power. You are brilliant!

    4. Yes, it is the snake oil or something like that. Maybe the original “whitepaper” was sponsored by some company that produces excavators ….

      Water can be used for the smaller generation also, and if you have an island with a mountain, it should not be that difficult to find water around there….

      Their paper really looks like the depictions of those basic perpetual motion machines, with the weighs hanging around a wheel….

    5. Water requires pipes and scaffolding instead of cranes, and still electricity to operate the pumps.

      Not every place has the geological landscape of Quebec, in some places building a reservoir is impractical.

      If you’re not building a reservoir and instead planing on some kind of container, why not fill it with a material that has more weight than water, like gravel, or lead sand? Bonus point if you have some kind of industry at the top of the hill that produces the weight medium.

      1. Because water flows on it’s own with very little internal friction (power losses) and doesn’t require complicated equipment to (un)load it into/out of hoppers. It also doesn’t erode your equipment at the same alarming rate. The gains in density aren’t all that great either.

      1. The whole crane concept seems idiotic to me. Big, cumbersome, fragile, complicated, inefficient. Not that it makes sense either, but a tracked cable car system would be safer, cheaper, and more efficient. Don’t load and unload the gravel. just keep it in the carrier and have switchyards at top and bottom.

    6. It’s probably less efficient, BUT in terms of energy stored per unit of height/volume – using denser materials means you can store more energy with less height delta.

      Just like car engines are less efficient but still used because they have a superior power to weight/volume ratio to a combined cycle coal or gas electric plant.

  2. >”Another is that storage reservoirs are prone to evaporation and freezing.”

    Hydroelectric dams work just fine in the winter. Ice floats to the top, and running water doesn’t freeze.

      1. Why not combine the two mediums – liquid rocks!
        All the benefits of fluid water but with the density of rocks and, as a bonus, thermal energy.

        Just need to find an active volcano to setup next to.
        If you’re really lucky then you could (very slowly) feed the rocks, now at the bottom of the mountain, into a subduction-fault to recycle them! No need to even transport them back to the top – Free energy ‘init :-)

        1. How about liquid salt? Use mirrors to concentrate solar energy to heat molten salt that you store in an insulated tank, then pump the salt through a heat exchanger to create steam to drive a traditional turbine.

          If you get a leak, let it cool down then tidy up using an excavator.

          Already online in Australia and Morocco.

          1. “How about liquid salt?”

            Good call! Talk about a stressful operation for long term integrity to be designed. However, since liquid salt is already used and demonstrated successfully used as a heat transfer working fluid… why not try as a mechanical transfer working fluid?

            Talk about super challenging materials science hurdles or other forms of processing/handling that most likely will not be energy efficient. I think the materials science and maintenance protocols can be devised and will be interesting to review feasibility of proposed methods. The mechanical transfer process ideas will be interesting to observe.

            https://en.wikipedia.org/wiki/Solar_power_tower

            Why not however?

          2. @Martin et.al.:

            Other than mercury… I’m not sure thinking now what would be a room temperature, or really ground temperature, material that would be able to make a liquid form of depleted nuclear waste.

            Might be a saturated solution that doesn’t super saturate and crystallize unless the design wanted to take advantage of thermal storage heat exchange methods with the same system, i.e. store the thermal energy in the supersaturated state and crystallize to release the energy. Then store the thermal energy again to bring the crystallized supersaturated solution back to liquid state.

          3. That got me thinking in the last thought I just posted in using supersaturated solutions to store thermal energy also.

            Basically, when you want the heat released… crystallize the supersaturated salt solution. This design I’m thinking would be more likely useful only for pre-heating or maybe geothermal heating in some sort of facilities design unless there were highly exothermic solutions used that can be regenerated. Then have some sort of heat exchange tank design that would optimally exchange heat back into the system during the day in order to re-dissolve the crystallized supersaturated solution vessel.

            Makes me wonder about what other mixtures aren’t on the chemistry tables I’ve reviewed, if there are any others at STP. Wondering if there are more energenic mixtures as different pressures?

    1. “Running water doesn’t freeze.” If it’s cold enough long enough, it freezes from top to bottom. Of course, when it freezes it doesn’t run, but to conclude that water that’s running can’t incrementally freeze until its all frozen is just wrong.

  3. I really like the idea of using gravitational storage with a solid medium, but using sand or gravel seems strange or difficult to me.
    I am by no means studied or even slightly educated in this sort of thing, but do systems exist which would basically be:
    – A large solid mass on a rail trolley mounted to a slope (mountainside etc) (in my mind, like a furnicular etc)
    – A gear system / motor / generator mounted either to the trolley or to the top of the slope
    – When energy is to be stored, the trolley is moved up the mountain by the motor
    – When energy is to be regained, the trolley is moved down the mountain

    1. There’s a Swiss startup called Energy Vault that is doing something like that where they use cranes to stack up barrels of concrete as an energy storage medium. I think Hackaday even had an article about it sometime this year but I’m not sure when.

      1. You know, this is an interesting idea – and it could be easily adapted to solve the “flow” issue.

        Instead of using sand, use large steel balls. At the top of the hill, deposit them into a marble run – https://en.wikipedia.org/wiki/Rolling_ball_sculpture

        There will be some energy loss from having a small downhill tilt of the marble run, but it’ll be a fraction of a percent of the energy stored by raising the ball up the hill.

      1. While the discrete nature of the power does pose some problems its not insurmountable. If you ran this sort of system like a cable stay railway you can actually have multiple decent on the same track with very little additional engineering – each cart just grips the cable/chain and adds its mass to the decent. Which would allow a cycle of dropping wagons to keep power delivery sufficient as long as mass is at the top to be dropped.

        I do think this system makes less sense than pumped hydro in general. But there are places where liquid water will evaporate rather readily or freeze far to often, both of which will effect the efficiency and deliverable power. In those locations this works, could also work in highrise buildings. Have a power store elevator shaft to run the building for a while, with some cunning engineering the elevator shaft could potentially be used normally under/over this mass much of the time. The grid in the UK already controls power draw in many large buildings by turning the AC etc off at peak times when needed, adding in the small gravity stores to that system wouldn’t be difficult.

        1. One ton of material lifted up one meter stores roughly 10 kJ of energy or 0.0027 kWh. Even if you lift it up the Burj Khalifa, it would store only the equivalent energy of two car batteries.

          The specific energy density of any gravity based system is terrible any way you cut it – it works because you can have cubic kilometers of water dammed up in a high valley.

          1. Really depends on what you are looking for as to which technology make most sense. For short term grid smoothing even the small stores in an elevator shaft work well for a few reasons – Simplicity/cheapness, durability, scaling (lots of big buildings getting built) and safety considerations (also an elevator shaft power store would weigh much much more than 1 ton.. most every elevator I’ve been in recently is rated to lift more than that as the load on top of the steel box’s weight so just running a current elevator as a generator would do better than your numbers – And if you were building to store power you’d have a block much much more massive).

            If you want long term mass storage you need a much bigger system but like compressed air energy storage and hydro simple mechanical systems have some huge benefits to chemical batteries. The most telling being the cost build and run the system, a battery does not last all that long where a properly designed CAES system should last pretty much indefinitely so it becomes a more cost efficient and environmentally sain system (and even if you built cheaply a new rotor/bearing is much cheaper and easier to create and the old ones easy to recycle).

            Then you have the ever present problem of batteries needing to be constantly recycled and the multitude of toxic and/or rare elements required for them to exist. Which puts a hard limit on how many high energy density batteries can possibly be made. So the high energy density batteries should be reserved for places they are actually needed! Which is not to say there isn’t a place for battery stores on the power grid – but it shouldn’t be the default choice as many other options work out better in the longer term.

          2. You seem to be oblivious of the scale of energy storage needed.

            One American household uses 11 MWh a year, or 25 MWh if it’s heated by electricity. This is 30-70 kWh per day.

            The Burj Khalifa is almost a kilometer tall. If you take a more reasonable height like 100 meters, you need to lift up 110 – 260 tons up the tower to store enough energy for one household for one day. This is a ridiculous amount of material for a such a small amount of energy. This is the energy contained in approximately two gallons of gasoline.

  4. On an island with a mountain, why not just create a siphon pipeline? One line up with static volume n, turn around at top and make a convoluted line back down with volume 3n. Hydro generator at the bottom of the hill can also be the pump to prime the siphon.

      1. Tidal generation has been in use for decades. There is a system in France that changes the orientation of the turbine blades to use the flow in both directions. Sorry I don’t recall where. I was there almost 50 years ago.

    1. Mountainous islands are rarer than mountains with a pair of lakes or floodable valleys at two different altitudes.

      The siphon isn’t contributing anything to your system. What matters is the mass of water you can store at the top and the altitude difference to the turbine.

      You only need one pipe, you’re either draining or refilling.

      Having a longer pipe between your top valves and the generators means you’ve got bigger problems when you need to drain that pipe to maintain it or the generator.

      This is not a new idea. Look up Dinorwig, a pumped hydro facility near Snowdon. FullyCharged and Tom Scott both have good YT videos about it.

  5. I think the key to this system is the minimal infrastructure involved. No need to build a reservoir on top of a mountain (where they don’t really like to be) or build a bunch of rails up the mountain (which might lead to disaster if a car ever got away from you).

    And I believe the sand and gravel are stored in shipping containers, so easy to load and unload.

    I assume they aren’t really going to use cranes when the ski industry has had technology for years to connect and disconnect chairs from cables.

    A fully automated load and unload system that drops the shipping containers on giant furniture dollies and moves them around would actually make this thing viable.

    Viableish. I guess you’d need a giant concrete pad.

    From a hacker standpoint I think there’s space for a pea-gravel-based system if you happen to have property with a significant slope, but I haven’t completely thought it through.

  6. The idea isn’t bad for small scale systems, especially in arid environments, but I’m really dubious about the cranes. Why unload? Leave the containers full of sand and have lots more containers. Use forklifts if you have to, or short rail lines, and have your tramway snag and pull the containers up or down, the same way a ski lift does.

  7. Reminds me of a mine I read about where they were using electric trucks to move ore down a mountain. They arrived at the bottom of the mountain with more charge than when they left and the excess energy was used to power the rest of the operation.

    1. Not to mention operation and maintenance costs. All new power generation is being eclipsed by battery storage. The ‘energy revolution” is already over. They are closing combined cycle power stations. Wholesale markets have negative rates for solar (i.e. solar fields are being paid to curtail).

  8. And what wrong with just a traintrack and a train filled with gravel attached to a cable? The track goes uphill. You use an electrical engine to pull the train uphill with a generator+electric motor on top of said hill/mountain. When you want electricity back all you need to do is change gears to attach said cable to a generator and let down the train again downhill. With some proper engineering just use 1 motor that also functions as efficient generator (aka regenerative breaking). With the right gear ratio you won’t need brakes as it spins the generator quickly and has enough friction (from the generator/motor generating power). Also same gear ratio is most likely adequate for pulling back up ok there’s some losses but the amount of power you put in will be on the same ballpark as you’ll get out again. Also apart from some wear on the weels and the cable you’ve got no pollution or batteries that need replacing..

    Maintenance is low tech, just any regular train maintenance and the steel cable is like they have on ski-lifts. Just a regular track on a steep hill and an old train used as deadweight that rides smoothly on said track is going to beat efficiency of the above proposed way of working with sand and cranes. All electronics+motor and housing is on top of the hill (or on the bottom if you use twice the amount of cable). You can have multiple tracks on the side of the mountain and basically scale this to as much power you want to store and release at will.

    1. There’s already an older post where a large EV bulldozer charges itself to 100% by rolling down with rocks purely from regenerative breaking. So just image a train with weight about 100 times of that bulldozer you can scale this to store quite an amount of electric energy into kinetic energy just by pulling said train up a hill. This basically also solves the whole windmill curtailment situation where they shut down windmills because there is enough power generated already by gas+coal generators. Instead keep the windmills running and store the excess energy in kinetic energy. Once you scaled enough you can actually start to replace said coal and gas plants because you’ll be able to bridge the times there is low wind and high demand with all the trains that are on top of the hill that you can just let ride down slowly until there is more wind again.

  9. And of course you could just put in a small footprint gas turbine generator and eliminate all this hassle all together with much greater energy output. Simple :) . More hydro dams, or a nuclear plant where very large tracts of land aren’t needed to generate the same amount of power. Seems energy options are going backwards here with ‘unreliable power’ with ‘batteries/etc’ to back them up…. when we already have reliable good known power options to keep the lights on 24×7.

    1. He did the dumb barrels and cranes idea, this is the slightly less dumb sand and cranes idea. Still full of problems but it could at least be a terrible system that works poorly instead of a terrible system that doesn’t work at all and is a huge safety hazard. Ultimately water works really well, it’s developed, it’s cheap, it’s reliable. In small systems it can even double as a drinking water pressurization system. Sand can be more dense but your water tank doesn’t have to be as shallow, so even if space is an issue, capacity is still better with water. It’s another ‘save the earth’ scam. They might as well include solar roadways as a package deal.

  10. So, the round-trip efficiency of pumped water systems is about 70-80%. This scheme would be less (I would bet no better than 50%). The energy you’re storing is going to be from some renewable source that isn’t particularly efficient or dense to start with. Maintenance is going to be significant — probably at least as much trouble as a funicular railway. Your “working fluid” is subject to wetting, freezing, clumping and who knows what.

    Once you’re piling inefficiencies on top of other inefficiencies, you’re going to be nibbled to death by ducks. Remember, the energy that comes out of this beast should ideally be able to at least pay for its installation and maintenance (let’s say it’s run by idealists who don’t care about profit or the opportunity cost).

    Nope, unless you can get a billionaire to build it gratis, this is a disaster waiting to happen.

  11. Why not use rails? Install rails on the side of a mountain. Design a chassis for the rail system with a huge weight on it. It takes away so many moving parts and inefficiencies. Place a solar roof over the rail system. We could even use the motor generators as part of the weight.

  12. Why not sell the surplus power cheaply when it is available, then enable domestic and commercial consumers to automatically use the cheap electricity to do things like heat stored hot-water or cool fridges/freezers/air-con to lower temperatures. This can be done on the understanding that the same automation would induce those customers will perform less of (but not completely prevent) those actions when power is short.

    1. Commercial customers with large usage can already do this.
      For residential use, life doesn’t generally work that way. Heating water at the wrong time is probably not going to save energy in the long run. And finding your washing didn’t run Or food didn’t cook because there wasn’t enough wind today is a pain. Finding someone dead because their dialysis machine was plugged into the cheap energy socket and got turned off is bad. Or their fridge turned off and the food went off, or… there’s a lot that can go wrong.
      However, there’s already off-peak energy tariffs for consumers, which are usually used for storage heaters, though Charging EVs works too.

    2. These two things both already happen.

      In the UK, Octopus Energy offers a domestic supply with a tariff that follow the market’s wholesale price in 30 minute blocks. At 4pm, the smart meter receives pricing for the next 24 hours. (i.e the pricing isn’t tracking the live grid status, but it allows flexible consumers to benefit) **The price of electricity isn’t always positive.** Smart meters can start and stop power-hungry devices like EV chargers, so you can consume at tiny fractions of the average standard kWh price.

      Electricity supply contracts for industrial consumers can have arrangements whereby they’ll be paid to switch off if they can do so with about 15 mins notice.

    3. That’s the big problem of all the climate hysteria: We already have a very reliable power grid, where you can get power anytime you need it. The believers in the new religion of climate hysteria want to turn that back into the past, when you had to use power just when it happened to be available somehow. That is a big step back into the dark ages of the past. These people want to bring us back into an underdeveloped world.

  13. I’m trying to think of a fun Rube Goldberg way of doing the above. How about you use a solar concentrator to heat sea water into steam, the steam is raised up inside a pipe to a much higher elevation, where it is condensed into potable water (the temperature difference between steam and existing gravity circulated cold water is used to drive TEG’s). And the water is eventually used to grow corn, and the corn is used to make alcohol and the alcohol is used to store energy until it is needed. Oh and the corn husks and used mash (feed for poultry, pigs, and cattle) can be lowered back down to sea level for global shipping and the mass used to generate electricity!

    1. your solution is much more complicated than mine…

      I just wanted to replace the gravel with lithium batteries, hoist the charged ones up the mountain for storage, and then use them coming back down to generated electricity, and then plug them into the grid when they get to the bottom.

    1. You can Store almost nothing in contraptions like that. If you lift a thousand metric tons of mass 100meters high you have stored (with 100% efficiency in a perfect system) about 250 kilowatt-hours of Energy. One single Windmill in the 3MW-class would fill up such a thing in a couple of minutes.

  14. I reckon, places with excess energy with large amounts of bauxite ore should be used to turn it into aluminum and the aluminum shipped and energy recovered in aluminum fuel cells (Which can have aluminum regenerated from them with energy input)

  15. Adding in the time factor I could envision a scenario where 24/7 during low usage times, there is a constant raising of the gravel or sand slow but very constant – just like a water keeps flowing into a dam day and night at a largely steady pace. Suddenly when peak power is needed – the siphoned off bits of power are returned to the grid but at a much faster rate. Kind of like a bank account how you work your arse off and save up and then blow it all one big purchase. In the end you didn’t really gain anything, but saved up and then used what you saved in one big exchange – as it was needed. Kind of like compressing time.

    1. Yes. It’s totally amazing how little energy you can store by lifting mass. If you lift a metric ton a hundred meters high you can barely cook a couple of eggs with the stored energy.

  16. I hoped this was going to be about thermal energy storage in tanks full of hot gravel – thermal energy storage beats gravity hands down. My favourite fact about heat vs. kinetic energy: put one golf-ball sized rock in boiling water, fire one out of a cannon at the speed of sound and lift one from sea level to the top of Denali (the highest peak in North America, >20,000ft) – the one in hot water has the most energy imparted to it. An insulated box full of heat storage medium and some scroll compressor/expanders seems like a more practical solution than lifting boxes of rocks up mountains.

  17. yet another wacky idea in the flourishing industry of save-the-planet frauds and scams.
    they will of course get generous financing from the usual entities who spend someone else’s money, get rich, and retire.
    this will go down in history as the era of the BS

  18. “pumped hydro has its own problems. One is the need for large amounts of water.”
    That does not sound very convincing when looking at that map. The density of gravel is also very similar to water, so you need to store a similar volume and a similar weight, since that’s what holds the energy.

    For small installations you would want a system that is even simpler to maintain that pumped hydro, probably a li-ion battery system.

    This sounds similar to the idea of making a circular underground vacuum tunnel, and have a train drive around in circles to store energy. And then claiming that would somehow be cheaper than (very outdated) $1000/kWh battery prices.

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