How much energy can you store in a stack of cement blocks?
wired.com
wired.com
> assuming energy transfers are 100 percent efficient—which they aren't.
A startup doing a large scale version of this idea using commercial tower cranes claims 85% efficiency, but take that with a grain of salt:
> The round-trip efficiency of the system, which is the amount of energy recovered for every unit of energy used to lift the blocks, is about 85%—comparable to lithium-ion batteries which offer up to 90%.
https://qz.com/1355672/stacking-concrete-blocks-is-a-surpris...
Of course, the gold standard way of doing this is with pumped hydro, hovers around 80% efficient.
> The round-trip energy efficiency of PSH varies between 70%–80%, with some sources claiming up to 87%.
https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...
Thanks for making me aware that the author is a prof! But I'm afraid that makes things even worse. As I see it, the writing quality is barely acceptable for an intern: it's so colloquial it almost buries the core message, and it's full of very irrelevant details. Two examples: (1) We're talking about straight vertical lifting from the start, so why bother giving the formula that considers the angle of the force vector? (2) why analyze the energy flow of stacking barrels when the most straightforward implementation (as discussed later) deals with simply lifting barrels some height above ground level?
This article could have been more clear and more approachable after editing it down to half the text.
There's no reason "What? Yup." belongs as the third sentence in an article.
Really? Yes.
As others have noted, it's not very space-efficient. Compare with https://en.wikipedia.org/wiki/Cruachan_Power_Station : you'd need 355 of these to equal its capacity. I suppose you could start dotting them in groups around wind farms?
On the topic of energy density, other solutions - like the house-sized lead-acid battery - have been proposed on the grounds that energy density isn't as important for stationary power stations as other factors, like the vampire effect. Losing 1% of your charge per day is a big issue when storing 20 MWh.
The other unclear factor is "C" rate. Is it actually possible to do a full 20MWh cycle in a day?
I'm betting the economics of recovering gravitational potential energy don't really work out in most use cases.
I realize there may be some over compensation in play, but unless the counter weight's weight is actual adjusted for each load, there should be something to work with.
Any weight difference due to meatsacks in the elevator will be relatively small compared to what a drum full of concrete weights. Going off the math in TFA, and assuming your average meatsack weighs 75kg, lifting one of them 15 meters up (so, roughly, to the top floor of a 6 story building) stores about half a smartphone battery worth of potential energy. So you probably want to be lifting them much more than 15 meters - and be packing your elevator cars full of them - if you want to pay off the cost of the fancy regenerative kit in any reasonable amount of time.
There's also a big difference between doing it in an elevator, which operates intermittently, can't be scheduled to match grid supply and demand, and works with relatively light loads, and doing it with a dedicated system that can match the grid and use much heavier loads.
I wonder how it stacks up against the more commonly discussed approaches.
A previous discussion mentioned a heavy train being hoisted up and down a slope for the same effect - same problem. In addition to the weight you can shift being a fraction of a hydroelectric dam, you also have to deal with material maintenance and wear and tear, security, etc.
Water you can get it perfectly load matched. Here you get one concrete block lowered worth of energy and stopping midway has its own share of problems. Look at the mere 100MWh Tesla bank in Australia. They made millions per day and reduced power prices for consumers by millions as well. They did it by not only utterly cornered the power service market but managed to outrun the problem so well they weren't getting paid for all of them until the grid upped their sampling rate to deal with the unprecedented speed for a system that expected dispatched natrual gas turbines to be the fastest thing it would ever deal with.
I may be wrong and it may have a legitimate use but it would clearly need complimentary components to cover its flaws.
The problem is the linear relationship between the mass, the height, and the stored energy: energy = mass * height * gravitational-acceleration.
gravitational-acceleration is fixed at the earth's surface to ~10m/s2
So taking an example of 1,000,000 tons lifted up 100 meters:
energy = 1,000,000,000 (mass) * 100 (height) * 10 (gravity) = 1,000,000,000,000 Joules
This looks like a lot, but really isn't. It's equal to ~278 MWh (megawatt hours), which means it can supply 278 MWs for one hour. 278 MWs is equivalent to one small power station.
Note that the largest pumped-storage power station in the UK, which is of course constrained by exactly the same E = mgh formula, Dinorwig (https://en.wikipedia.org/wiki/Dinorwig_Power_Station) stores ~9,000 MWh.
Another way to consider this is to calculate how much mass needs lifting 100m to supply the whole of a country for a day.
As a very crude estimate the UK requires an average of about 30,000MW of electrical energy. Over a day this equals 30,000,000,000 * 24 3,600,000 Joules = 2.510^18 Joules per day.
The mass required to be lifted up 100m to store this is 2.510^18 / (100 10) = 2.510^15 Kg = 2.510^12 tons = 2,500,000,000,000 tons.
Which is many times more than the current global annual concrete production of 10,000,000,000 tons (ref: http://www.columbia.edu/cu/civileng/meyer/publications/publi...)
So... where do I sign up?
Why would the stresses be any greater than burying a turbine electric generator at the bottom of a hydroelectric dam? The forces would be similar, right? That's the whole point: it's just a crap load of "pressure" due to a bunch of stuff piled up on top.
Also concrete is not exactly environmentally friendly to make.
Anything heavy will work. I wonder about bags of stones & rubble, or earth itself. The challenge would be making such bags not break apart due to fall.
If they're falling and there's an impact at the end, something has already gone wrong.
When operating correctly, the descent is a smooth glide.
Think that through.
A dam probably holds 100 or 1000 tons of water for every ton of dam material.
A concrete weight holds exactly on ton of concrete for one ton of concrete weight material.
And "amount of weight helt", be it water or concrete, is Exactly what matters here.
You don't necessarily need a river to be involved at all for pumped storage. Just build two reservoirs at different heights.
Charge the cars during the day, and park them at the top at dusk. Then drive one downhill every hour at night, using regenerative braking to pull some of the potential-turned-kinetic energy into the battery as electricity. Then discharge the car into the grid. Descending the hills in east Auburn and Kent in WA State usually regenerate 400-500 Wh into my hybrid.
Gravitational potential batteries have less vampiric effects than chemical batteries, but still some possibility to suddenly discharge, in a landslide or similar event. And they are less efficient than hydroelectric, as another poster has pointed out.
So you would have a tall vertical tower and at the bottom it transitions to a horizontal conveyor belt. Each of the drums are attached to the chain that is connected to the motor. When you run the motor is pulls the barrels from the horizontal conveyor and up into the tower. Then when you need power you let the barrels fall and spin the motor, and collect on the horizontal conveyor.
Still not saying it is a practical battery, just seems more reasonable than using a crane.
I have to say, though, I think you might be onto something with your horizontal/angled conveyor idea. I would be worried about the energy lost to friction, though? The nice thing about a straight up/down solution is that it doesn't have that problem.
I ask because simple lead acid batteries can be hooked into control systems, with maintenance done via changes to electricity and fluid. With such batteries there are no cables or bearings to need changing, no moving parts. So I would assume that maintenance costs for oldschool batteries would be far less than for this crane.
> But that gives 2 million joules of stored energy with just 50 cement drums (assuming energy transfers are 100 percent efficient—which they aren't). That's not too bad. Of course the Tesla Powerwall can store about 50 million joules, so 50 drums might not be enough.
Basically, lifting and lowering cement is a poor battery.
Is there any form of energy storage/generation for which this isn't true? No one every talks about the environmental costs of Lithium batteries, but they're certainly not nil. Pumped storage requires substantial infrastructure as well.
How about lifting and dropping heavy lithium batteries instead?
Or lifting and dropping cars in a parking garage?
Or lifting and dropping self contained data centers?
Laws of fluid mechanics will apply then. And the size of the motors will be ginormous.
Personally I like the idea of moving water up a river or behind a dam to store energy.