A sand battery could transform clean energy
bbc.com
bbc.com
Sand is generally the last option to choose for thermal mass, because it is so inefficient at storing heat; the very air between the grains acts as insulation.
Instead, clay (or clay mixed with straw to form cob) is the preferred medium for storing heat, as it is a significantly better conductor and stores much more heat energy than the air between the grains of sand.
It seems like they are using the air as a medium here- the article states that they blow air through the sand, and the resistive bands heat up the air, which then sheds some of the heat onto the sand (or picks some heat up off the sand, depending on if it is storing or discharging).
I guess the cheapness and simplicity of construction makes this a better option at the scale they're operating at, but it's pretty wild to imagine how much more efficient the system could be if it had been designed differently.
And the air is blown through pipes in the sand, which means you need something which will easily surround the pipes.
Solid materials would suffer from thermal expansion problems (like cracking), while liquids evaporate so sand seem to be a reasonable choice.
For their application the conductivity does not really matter and the cheap reject sand was chosen because it has the best mass to price ratio.
It is heated to 600°C so it would be not the best idea to use clay with straw ;)
I did overlook the 600*C bit at first- straw would definitely be a bad idea, lol. I still would guess that the additional cost of clay would be worth it. Clay is both more conductive AND has better capacity- the air between the grains of sand is an insulator, and has significantly lower storage capacity than the grains of sand themselves.
Another option some people use if they don't want a permanent installation (or just need occasional portability) is large stones with gravel for fill. It's quite a bit less efficient than clay, but the larger size of the gravel and rocks inside make it a better option than sand.
I imagine they have done the math on the cost of the sand and the efficiency they are getting out of it, it just goes against everything I have ever learned about thermal mass storage from the permie / rocket heater mass storage community.
I have not seen a picture of the filler but they said its the cheapest sand they could get (probably even free).
So i assume it is mostly rocks and gravel anyway.
Not sure about the math here, but it is surely cheaper to build a bigger tank with crappy sand than a smaller one with more expensive clay.
I would say rocket heaters have more constraints on space while for a large industrial tank you would not really care about it being large and heavy.
From an efficiency standpoint you will not run into many problems, since the efficiency of heat based systems is based on the thermal loss.
Thermal loss occurs on the walls and in processing (like the rest heat in the air venting out of the system).
The process losses are independent of your storage material and often unavoidable.
And since those are large installations, the mass to wall ratio is really really good (square–cube law).
If you make them big enough not much insulation is required and the overall efficiency is high.
For a one time capex on an industrial installation, what would it matter? As long as the material is not liquid gold, it seems like the material is a drop in the bucket for everything else you have to maintain (personal, pumps, generators, etc).
It's also interesting that the sand is heated with energy generated from wind turbines and solar panels, which means that as long as enough energy is being thrown into the thermal capacitor, it will heat up with little cost. From this point onward, it's an economics problem. Either money is thrown at the efficiency of these heating capacitors, which brings at most small gains, or money is thrown in building additional storage capacity for cheap along with increasing how much energy is fed into the system. Would it be more cost effective in replacing sand with more efficient materials, or spend the money in, say, another cheap thermal energy source?
It really depends on whether the air in the system is a closed loop or not while they are building up the heat in the device. Sand is, at best, going to be about 1/3 as efficient as clay in an open system (hot air goes in, travels through some pipes, then comes back out as waste exhaust). This number is based on experiments with rocket mass heaters I've seen. It could actually be worse- I honestly can't remember if the 1/3 number was for sand, or the large rock (think bowling ball sized) + gravel mix, which was more efficient than sand.
In this scenario, since your input energy is basically free, more storage capacity = more money, and switching to clay could triple your effective capacity.
If, on the other hand, air is blown in a closed loop, you really only risk burning out your fan and resistive heating elements faster (they don't get cooled down by fresh air). In this scenario, 100% of the energy eventually transfers into the sand, so you're really only losing money if you need to spend more on land for setting up additional units.
That's all fine and dandy, but the whole point is that projects are evaluated in terms of money and not joules. This means that if a material like clay is , as you claim, 3 times more efficient but ends up costing twice or more to replace and more to maintain, you are in reality looking at a prospective best-case scenario that lies somewhere significant losses or irrelevant gains.
Meanwhile, building the exact same sand-based prototype right next to the original one would literally double the capacity at a far smaller cost.
People want to get warm, not discuss whether clay or sand pack as much heat. Half the world could not care less about what goes into AA batteries.
Projects are evaluated in financial and economical terms. It matters nothing if there's a theoretical possibility that some other material or design is more efficient, if it's savings are irrelevant. The world rides Volkswagens, not Formula 1 cars.
A few tons of clay are not doing to cost more than the benefit from an operating lifetime of triple returns.
See this fun fluidized sand-filled jacuzzi if you're not grokking my too brief explanation:
That seems like a benefit if you're looking to stretch release of energy over many hours?
- rock / gravel mix (lowest preference unless portability is a requirement)
- clay / cob mix (super fine particles, little to no air)
- water
where water is held separately from the heat source and some form of heat exchanger is used. Aside from the mess caused by leaks, accidental pressurization turns water thermal mass storage into a bit of a bomb (much like a vastly oversized pressure cooker) so it's really only used in outdoor wood-fired boilers. Also, the temp is usually capped at 180F, so nowhere near what these guys are getting.
So it's slower to charge. And the charge speed is presumably adjustable by circulating more air (or moving it through faster). It's unclear whether this is a problem for its intended application. It may charge fast enough.
> and stores far less heat than the sand itself.
That would only be relevant if you were comparing an equal volume. The same mass of clay and sand should store roughly the same amount of energy, it's just that the sand one would be bigger to accommodate more air. And being bigger isn't even a drawback here, since bigger means proportionally less surface area to volume that you're losing heat through.
> That's why in residential mass storage, ...
"Residential" could imply that you don't want a giant sand silo in the middle of some housing units, so I agree there. You would need to put this somewhere that size doesn't matter much.
Can’t you build that silo underground? If so, size wouldn’t matter much. You could have a playground, communal garden or, if you must, parking spaces on top of it, so it wouldn’t really use any area.
Clay weighs significantly more than sand. For the same volume of material, clay will store significantly more thermal energy. The big question is- is the hot air on a closed loop or an open loop? I don't know of too many fans comfortable cycling 600C air, which means that you need an open loop of air (fresh air > fan > heater > sand > exhaust). If the sand isn't drawing heat off of the hot air fast enough, the heat won't actually "sink" in through the sand. If we are talking a closed loop of air (fan > heater > sand > fan) then the sand will get fully heated.
My reference to "residential" is the aformentioned rocket stove mass heater- see https://richsoil.com/rocket-stove-mass-heater.jsp or https://www.youtube.com/watch?v=fwCz8Ris79g and https://www.youtube.com/watch?v=8ptwncPImuo if you prefer videos. These types of things typically stay warm for about 24 hours, so they're burned once a day, maybe once every other day depending on how the house is insulated.
Since these are all wood fired, they must be open loop, and sand is almost universally acknowledged as a terrible idea to use for thermal storage.
I think that’s a good reason for them to not use water. _If_ you’re designing for 600°C, I would think using water is quite risky.
I would also think sand at 600°C stores more energy per volume or mass than water at 180°F.
Operating at large scale like this probably helps, but really the main benefit comes from the district heating part.
This has the double impact of a) letting you take advantage of cheaper electricity by modulating your load slightly, b) letting your heat pump run continuously at a lower heat output, maximising efficiency as it extracts heat from the continually replenished outside air.
I'm always impressed by how prevalent district heating is in parts of Europe, and wonder if anything like that has ever been done in the states, or if it seems too much like communism.
Also have people done the math on the transmission losses vs the efficiencies of a single heating location?
Nowadays heat pumps would be far more efficient, shifting 3-4x the heat for the same energy.
I note that even the figure provided in the article ($2,000/MWh) is out of date, and current prices are closer to $500/MWh and still dropping. I'd expect this to be a nice niche solution for them but have tepid uptake elsewhere.
A bit of Googling suggests that the combined energy per second that reaches Earth from all the visible stars other than the Sun is around 0.0000002% of the amount from the Sun.
I recall reading that the area needed with current solar panel technology to power the entire US would be 10000 square miles. With starlight being 0.0000002% of sunlight, that suggests we'd need 5 trillion square miles of panels to get the same amount of energy at night.
The surface area of the earth is a little under 200 million square miles.
That suggests that it isn't possible to overbuild enough to work off of starlight.
It would be even worse on cloudy nights. On cloudy days you still get a significant amount of sunlight coming through, because the Sun is giving us so much more than we need. Not so with stars.
Whether this is better than local storage is another question.
If that is the future -- I hope it is :D
Then there will be hours of the day where electricity is practically free, if not actually free. So any mechanism for time shifting energy consumption might generate a nice buck.
I guess the competition is PV or wind + long distance transmission loss.
It would be interesting to see exactly what kind of overbuild would be required to "fully accommodate" for the winter season with PV in Finland.
[1] https://research.tuni.fi/uploads/2019/05/0a103135-p086568.pn...
But chemical battery prices are also dropping as quickly.
At present the BESS (battery energy storage system) industry is (to a first approximation) a sideline for vehicle battery makers.
Relaxing engineering constraints imposed by vehicle use[1] means BESS prices can drop further. This is happening as the BESS industry splits off from vehicle batteries.
Add vehicle battery swapping like Ample's[2] to an urban BESS, you have two businesses in one, that can follow supply availability exactly.
1. Structural strength, vibration resistance, performance at extreme high and low temperatures, high mass energy density, high power/mass ratio, tolerance for overdischarge being the obvious constraints that can be relaxed a bit.
See this video which suggests somewhere in the region of 5x overbuild of renewables being the least cost option.
You may be aware that large swaths of Finland, Norway and Sweden sees the sun set and not rise again for weeks or months. And during this period it’s also not uncommon to see -20 C or colder. Even worse/ the colder it is, it’s also typical with very little wind inside large stationary high pressures.
Then it’s months when the sun barely sets, as well as windy periods in spring and fall. Using wind/sun only requires storage not just between night and day but between seasons. Luckily in these particular regions there is plenty of hydro so aren’t reliant on wind and sun only to be 100% renewable.
So this is more cost effective than batteries at the current price but also less flexible and requires more engineering up front, and the price of batteries keeps dropping. It makes sense today but is going to feel the squeeze in the long run.
Then there is the source of energy. Yes, PV is one source, but it is only 20% efficient. Evacuated solar tubes collect heat energy at around 60% efficiency, and last about a long as PV panels.
And I think you may not realise how most of our electricity is used. Most of it goes into heating and cooling. (I speak from experience here. I have a house battery. Turn off the hot water, ovens, stoves, dryers, air conditioners and the remainder of our electricity use could be handled by a 2.5kW battery. How do I know. Well we lost electricity, and our 5kWh battery easily handled all what was left, including refrigeration. I estimate it would take a 15kWh to handle everything.) Yes, the COP of absorption refrigeration is poor, but the 60% vs 20% makes up for it.
Finally, I suspect you haven't twigged to just how much energy this can store. Sand melts at 1700°C, so they could take it to 1200°C without hitting too many limits. At 1200°C, 1 cubic metre of sand holds 451 kWh. In comparison, right now a battery holding that much would cost AUD$300,000.
"Buy" cheap PV electricity off house owners during the day and "sell" them heat in off-peak hours.
You're basically competing with hybrid inverter + battery storage + heat pump which is not cheap per house but even if heat storage might be cheap, that's plenty of initial cost to install all the piping.
I imagine these could be used to buffer the output from wind turbines or solar plants?
Sounds like the battery is charged via electricity.
Ultimately its about energy transfer, efficiency, and storage.
Traditionally district heating systems use waste heat from thermal generation of electricity, which heat is free.
At the end of the day, a chemical battery is a reusable bomb that is designed to go off very, very slowly.
> Wood placed in an oven at 700°F. catches fire almost immediately. At oven temperatures of 450°-500°F., the wood gradually chars and usually ignites after several hours.
> “Pyrophoric carbon,” formed when wood slowly chars, absorbs and combines rapidly with oxygen. This produces heat which under certain conditions causes the charred wood eventually to catch fire at temperatures well below those required to ignite the original wood. Cases are recorded where wooden flooring in contact with steam pipes at 250°-300°F. has caught fire after years of EXPOSURE.-FACTORY MUTUAL RECORD.[1]
I disagree with most of the naysaying in this thread, but pretty much all energy storage that is easily accessed can also have a failure state that is difficult to handle. It would probably be impossible to save a structure with a 600°C blob resting against it.
[1] https://www.fireengineering.com/leadership/ignition-temperat...
Flywheel batteries are usually placed in small bunkers, because the failure mode of a giant rapidly-spinning concrete wheel jumping its bearings is not exactly pretty. Dams are usually designed with diversion channels and mechanisms to limit the damage if they burst. Etc.
Really, it's strange that we are so cavalier about lithium battery failures compared to the other sorts.
1. Sand has a lower heat capacity than water. Water is much cheaper than sand. Yes, you can't heat water to 600C, but that's not a problem if you're just wanting to heat houses with it. The only reason I can see that they want to use sand is so that they can deliver higher temperatures, which are necessary for some fairly rare demands, or so they can generate electricity, which their system is going to suck at anyway.
2. There are more efficient ways to get hold of heat. They're using electricity to heat it, using a resistive element. A heat pump (for example) can give you 3-5 times as much heat as the electricity you put in, so why not just heat the houses directly with much less electricity used? They are using solar electricity to heat the sand, but PV panels are typically only 21% efficient, and they're expensive. A concentrating solar water heater is very cheap and simple, and will give you your heat with a much greater efficiency.
You can heat a home with 600C and some circuitry that steps it down to a comfy 22C. If that's more efficient than heating water, then I can see why they do it.
The price of the storage material is probably almost completely irrelevant in the total cost of this solution, when comparing sand and water. They even mention using sand nobody else wants. But using water will probably increase the cost of everything else.
If you can heat sand to a much higher degree, the fact that it has lower heat capacity is irrelevant on its own. The fact that they explicitly mention that they were setting out to find if a solid material was better than water makes me think they've considered all these factors and found sand to be better in total.
"The fan is the only moving part and it's easy to replace if necessary"
Heat pump is a good idea, but it will add quite a lot of up front and maintainance cost. I've looked at heat pump water heaters myself, and it's hard to get the economics to add up in the short term. With a water heater or a system like this, the heat pump isn't going to be running constantly, and when you're running it you need a lot of power, so you need a large unit. Ideally a heat pump should be running almost constantly at a low rate to make sure it's operating at high efficiency and to pay for itself in a reasonable time.
I don't know if there even are heat pumps that can effectively heat things up to several hundred degrees?
I think it makes more sense to put the heat pump between the "battery" and the building it's heating, so you draw less heat from the battery to supply a large amount of heat to the building. Then you can use the same heat pump to extract heat from the air when the "battery" is empty. You'll get much better utilization of the heat pump.
Btw, I think significantly cheaper and better heat pumps would be a hail mary for solving the energy / climate change crisis. So many things like this would suddenly become more economically viable.
> A concentrating solar water heater is very cheap and simple
Sure, but that's kind of a different solution. This could take energy from the grid drawing power from many sources spread out over a wider area. A concentrating solar heater would require a lot of area at the exact place where you want the storage unit. That's not always viable. Using water would probably increase the total volume significantly too.
That’s not how heat pumps work. They move heat from one medium to another - so using a heat pump to move one Joule from the battery to the house will lower the amount of heat in the batter by one Joule, same as if you use any other means.
The efficiency gain that heat pumps provide when heating houses is that they essentially cool some medium (air, ground, water…) outside the building and move the heat into the building. Moving the heat uses less electricity than turning the electricity into heat via a resistive heating element. The outside medium gets warmed up by sun, or retains heat from a warmer season.
https://en.wikipedia.org/wiki/Energy_storage: “A device that stores energy is generally called an accumulator or battery”
“a group of two or more cells (see CELL sense 5) connected together to furnish electric current”
Battery is a generic term for collection of things working together, guns/generators/medical tests etc. This why batteries where initially called such, they are a collection of smaller cells operating together which doesn’t apply to say pumped storage.
Sometime battery is used descriptively Ex: “A Carnot battery is a type of energy storage system that stores electricity in thermal energy storage.” Note they said it’s a type of energy storage system rather than type of battery. https://en.wikipedia.org/wiki/Carnot_battery
Instead people try to chose words so their audience understand what was said because people aren’t telepathic.
This is a cheaper, lower maintenance option.
But I doubt 600°C is enough to make sand win.
You're looking at LCOS of $0.23 kWh-1 [1], which is higher than Li-ion. However, such a system may offer other advantages (better recyclability, no reliance on lithium, less risk of fire), or such systems may be cheaper in the future, which is why people are researching them.
Additionally, these systems work with existing district heating pipes, which is great because it mean's you don't have to expand the grid capacity. In places with district heating, that could make a lot of sense. (I don't think anyone is proposing building new district heating systems, just retrofitting old ones).
[1] https://www.sciencedirect.com/science/article/abs/pii/S01968...
Also, Carnot efficiency increases without bound as the temperature difference decreases, so lowering the temperature of the room or increasing the temperature of the outdoors make a huge difference. The same applies vice versa for air conditioning which is why people use cooling towers.
If there is going to be curtailment anyway then the energy is effectively free (or has negative cost). Or if you'd need to use something like pumped hydro or chemical storage then the heat pump only gives you a COP of 1-2 once you include storage losses.
This is probably a bit harsh to Matt Ferrel it's probably copycats I've got more of a problem with.
I'm glad to see someone thinks it isn't so stupid :-)
In this case I think new home builders are really conservative with trying new things due to the high price of houses and strict building codes.
And even though it’s a simple idea it’s the kind of thing HVAC companies would be happy to Charge you 20K for. Just like geothermal.
And I suppose it's really not so different from simply having a hot water heater like so many homes do? That you could program to only add more water to when electricity is cheapest? (And in-floor heating uses hot water pipes.)
But hot water tanks are very safe, they're only 60°C. I would think that 600°C sand (or rocks) is the kind of thing that's not particularly safe to have inside your home. That's really, really hot.
Have you done the math and figured out how many rocks a home would need, how much insulation, and so whether it's even viable in theory, setting aside practical engineering/safety concerns?
> Have you done the math and figured out how many rocks a home would need, how much insulation, and so whether it's even viable in theory, setting aside practical engineering/safety concerns?
No. But why wouldn't it be viable? And what could be cheaper than a box of rocks? No moving parts, other than a fan your HVAC system has anyway.
> But why wouldn't it be viable?
Because it would take up too much space at non-dangerous temperatures? Or would set your house on fire if compactly sized?
With a 30 degree delta in a system that uses 1MWh/month you could store 2 months of heat/cold in a 5mx5mx2m tank.
https://en.wikipedia.org/wiki/Trombe_wall
Coupled with a well designed ventilation system, a Trombé wall can also be used for cooling.
Basically commercial version of a box of rocks.
Cool, so we just need three million of these to take care of Europe.
If you look up Kankanpää on a map, you will find that it is a very small town in the middle of nowhere. And I haven't looked this up, but I'm suspecting it's got abundant sand production/shipping nearby from the Baltic. In those conditions, this sand-thermal storage might work well. But for most of the world, the space requirement, low energy efficiency, and costs associated with building large projects (and digging deep holes) in places where people actually, you know, live, will basically rule this out.
You don't want to be, like, the only person with a sand-heat-battery. Well, you might, as an individual, if you like to tinker, but as a municipality it's a pain. All of your problems are unique, you can't just hire a repairman to come in and swap out a standard part for another, you don't have a standard payscale for your Sand Management Technician, etc etc.
How many installations do you need before it becomes a good idea? 1% of the total? 50 total in a service area?
>with currently available technology the process of converting heat back into electricity only has an efficiency rate of 30%
Yes they're recovering heat as well, which is great when it works. But in general 30% is not great. This probably only makes sense to use when you have huge amounts of nearly-free power. Like if you massively overbuilt solar/wind (which could be a plausible thing to do!) or nuclear.
With nuclear power generation I thought there was no need to store energy, since it can generate at all times and ramp up/down pretty quickly.
Nuclear can be ramped, but typically is not. I haven't dug into why, but I get the impression it's partially economic - the plants are very expensive to build and the marginal cost of a few hours of fuel is so low that they prefer to just run all the damn time. I think there may be some technical reasons as well. In general if you look at how utilities operate, they will ramp down literally everything else before the nuclear plants.
That probably applies less to the Small Modular Reactors people keep talking about, but those are still not really a factor.
In some places where mid-winter PV output is 1/4 of summer, it *already* makes sense to overbuild by a factor of 4. Keeping 30% of the summer energy would double the winter output.
Of course this only makes sense if the electricity extracting bit is much cheaper than wind.
I'm very much pro-renewables and pro-storage, but that doesn't mean that all forms of storage are equal. This one has certain advantages (cheap, simple, tough), but the roundtrip efficiency kinda sucks compared to some other options.
And I'm guessing (without checking) that the 30% is for short-term storage (eg, overnight). In which it will drop over time, as the battery inevitably leaks heat into the environment. I'd guess that after 6 months (summer to winter, as you mentioned) it will lose a non-trivial amount of energy so the efficiency will be even lower.
[1] - Actual rates from deployed systems from the EIA: https://www.eia.gov/todayinenergy/detail.php?id=46756
[2] - https://www.energyvault.com/newsroom/energy-vault-announces-...
Your heat engine is where you lose all the energy.
Batteries are great and getting better. There's a reason most storage uses them, but they're ill suited to multi month storage due to cost. The best contender here is chemical fuels (which have similar efficiency penalties).
Energy vault is, at best, a very poor subsitute for batteries. A 1000 tonne block on top of a 500m tall stack can store $20 worth of solar energy and might sell it for $80. If you use it annually, then you get $60/yr. If you use it 1000 times a year it might be viable compared to current generstion batteries, but that will change quickly as there's no real way to make it more economical. Ignore it and tell anyone who suggests using it as anything other than 4hr storage they're probably being scammed.
Thermal batteries are the budget option. You can store about 5-20Wh/kg in whatever thing that's lying around that doesn't turn into a gas or become too corrosive. By every other metric they are terrible, but they are, in the most literal sense possible, dirt cheap (although you generally use low grade sand or maybe water because dirt is too valuable).
The use case is mostly providing very cheap heat with energy that is not valuable enough to store with other methods. Insofar as using the concept for work (where the efficiency penalty lies), the logic would probably be something like:
-- I have free energy noone wants (curtailed wind during midday summer)
-- I have free sand noone wants (someone dug a hole and wants to get rid of it), but it costs me a little more to move it and build a silo than I will get paid to dispose of it.
-- I have a free steam engine with its exhaust hooked up the district heating noone wants (a coal generator went bankrupt because renewables replaced it).
I'll put the heat in sand, then make steam with it in winter (extracting 20-30% of the original energy as work), then pump 50-60% of the original energy into homes as heating (losing about 20% in the pipes and steam engine and some during storage).
Also, doesn't thermal storage have the same problem with long-term storage of energy not producing a lot of revenue? Perhaps the capital investment is lower, but the yearly income would be order-of-magnitude the same.
I was pretty much agreeing with you about thermal being the cheap option, but thanks for the breakout.
Do you know of any examples of 10 GWh thermal storage? Just curious, I haven't really poked into that area too much.
> Do you know of any examples of 10 GWh thermal storage? Just curious, I haven't really poked into that area too much.
There are a few pilot projects in the 100s of MWh to GWh range. 100MWh is enough that your surface area is quite low.
Why not lift something REALLY heavy and let gravity spin a gearbox on the way down?
Like say lift a silo of sand? Up the side of an existing hill on a rail/track?
Alternately pump water uphill but in cold/hot climates that would be a problem and microbes, etc.
Gravity isn't going to leak or fail. And enough of an angle that weight is always coming down regardless of weather.
If you put the calculation into google "8MWh / 9.8m/s*2 / 100 tonnes", it tells you that you need lift those 100 tonnes 29.39km up, which is over 3 times the height of Everest. And the mechanics of the system are much more complicated, more expensive to build and maintain.
However, pumping water uphill is done in many places, and is more practical because a lake of water can easily weigh millions of tonnes. The problem is finding places with suitable hill top lakes.
Also, if you are using it for district heating output, storing it as heat is convenient.
There might scope to increase efficiency by using some sort of heat pump, but getting a heat pump to go up to 600°C is quite hard.
And a similar capacity lake at the bottom of the station to hold the water that you are going to pump up to the top.
Because lifting something that’s heavy doesn’t store much energy (or, stated alternatively, 1kWh is a lot of energy, compared to human muscles)
https://www.quora.com/How-much-mass-in-kg-can-I-lift-to-1-m-...:
Q: How much mass in kg can I lift to 1 m height, with 1 kWh of energy?
A: Let x kg be the mass to be lifted. The energy required = mgh
x * 9.8kgm²/s² or 9.8x Joules
1 KWH = 3.6 million Joules.
9.8x = 3.6*10⁶ => x = 3.6*10⁶ /9.8 = 3.67*10⁵ kg.
The great pyramid of Giza weighs about 6 billion kg (https://www.egyptabout.com/2019/02/facts-16-fun-facts-about-...), so lifting that by a meter would store less than 20,000 kWh.
I believe sodium ion batteries (CATL alleges mass production of 160 wh/kg) will be the killer app battery of grid storage that doesn't have pumped hydro as a geographic possibility, although that's probably 5 years out. A 160 wh/kg sodium ion battery (which should have superior cell-to-pack density than nickel/cobalt chemistries similar to LFP chemistry) can power a standard range model 3.
https://aresnorthamerica.com/gravityline/
which sounds plausible, since rail freight companies have spent 200 years micro-optimizing train efficiency.
https://news.ycombinator.com/item?id=17789456
https://news.ycombinator.com/item?id=32240546
https://news.ycombinator.com/item?id=29910248
Rail loop:
AFAIU heat losses through the piping aren't considered a huge problem.
Wait, I thought there was a sand shortage?
https://www.bbc.com/future/article/20191108-why-the-world-is...
https://www.reuters.com/business/environment/sand-crisis-loo...
I guess there are different types of sand though?
> "The company uses cheap, low-quality sand that's been rejected by builders instead of high quality river-sand which is used in vast quantities for construction, leading to a global shortage."
"The company uses cheap, low-quality sand that's been rejected by builders instead of high quality river-sand which is used in vast quantities for construction, leading to a global shortage."
That stuff is useless for construction but might still be good for this?
Or anything that will catch fire and then still be big and heavy once it's done burning.
tl;dr We already move enormous amounts of drinking water, and reservoirs are much better (and cheaper) candidates for batteries than sand/cement/etc.
(I do believe the article's design fares much worse, even on capex alone. I never saw some salt selection that melts at 600°C, but I imagine it would have better results even in a lower temperature.)
Iron flow is currently 'it will be dirt cheap later we swear, ask us about a demo project'. So probably substantially more expensive. No compelling reason not to believe them though.