> 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%.
85% at least seems fairly reasonable. Your main losses will be in the motor/generator, and that sort of roundtrip efficiency is doable there. Hydro will be a bit worse as you'll also suffer from turbine losses, but the coupling between a motor and a concrete block will be nearly 100% efficient.
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.
Cost is listed as "less than $2m" for a capacity of 20MWh, charge/discharge rate not specified but probably a low "C" factor of that. $100/kWH? That's actually very favourable when compared to batteries; I see Tesla Powerwall prices in the >$400 range.
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?
Driving an down some hills can regenerate 400-500 Wh, at least in my hybrid, so it would take 40 million electric cars to convert 20 MWh in this way.
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.
Not sure about that charge loss question; it depends on how often you're cycling the battery, if it's charged and discharged every day then self-discharge losses may be much smaller than inefficiency losses or the wearout cost on the battery. 1% of 20MWh is 200kWh or about £30 of electricity.
The other unclear factor is "C" rate. Is it actually possible to do a full 20MWh cycle in a day?
If the math works out so well then why don't elevators and other apparatus that lift things with electric motors use batteries and regenerative braking on the way back down in order to mostly offset the energy cost of operating them?
I'm betting the economics of recovering gravitational potential energy don't really work out in most use cases.
Elevators are not cranes. There is always a counterweight, so "dropping" empty cabin is not generating energy. During that time you're lifting counterweight.
Wouldn't the counterweight only balance (a guess) an empty elevator? Therefore any added weight would be useful for energy recapture?
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.
Regenerative elevators do exist. I've no idea what they cost, but, shooting from the hip, I'm guessing they aren't worth the added price on any but the biggest buildings.
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.
Counterweights are typically 40% of the a loaded elevator car weight, minimizing the overall work that needs to be done to lift any given car, while also limiting the maximum power required to lift a full car.
Based on a quick search, it looks like regenerative braking for elevators is available, albeit not universal.
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'm a little embarrassed that in all of the discussions I've heard about ways to store solar energy for dark periods, I never noticed that this kind of solution was omitted.
I wonder how it stacks up against the more commonly discussed approaches.
It's not omitted, it's just proven to be terribly inefficient. Hydroelectric pumping is being used in practice, being relatively cheap and you can store a huge amount with little materials - millions of tons if need be. Imagine having to lift up blocks of concrete millions of tons heavy and the machinery it would require.
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.
Even if the blocks are technically efficient the logistics and lack of granularity would keep it from being more than a curiosity like recharging your phone via a thermoelectric generator stuck in a campfire. Technically it works and is neat but it isn't going to be used much.
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.
I mean, they basically in the article that the energy density is abysmal. You can't take advantage of the space filling properties of pumped water, either. I'm sure there are valid use cases, but I'm guessing they're specialized.
The energy density really is abysmal, which is why the "lifting heavy things up" techinique of energy storage is essentially never discussed outside of pumped-storage.
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:
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.
I live in an extremely hilly area, made of soft rock (limestone), but very little water. Carving out a hill to 100m down, with a 100x100m cross section is totally doable—we have larger projects just to make flat land to build houses on. If the weight was only 10m high, it’d be ~200k tons. By your calculation that’s ~54MWh/day, or enough energy to power ~1500 homes. There’s a hill the right size next to my village of ~1200 homes. We have excess wind & solar power.
But what kind of machine do you need to raise a hill up and down by a few meters ? Even if breaking the load in small bits, the cost and maintenance of equipment would likely be a few magnitude higher that the energy stored or saved.
An electric motor moves it up. An electric regenerating break (an electric generator) keeps it from going down too quickly and/or not at the right time. The particulars of maintenance and long term robustness are pretty straightforward engineering efforts. The largest steam hammers are 125 short tons, and they can operate for decades.
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.
Sorry, I should have been more specific. The category of solution I didn't seem to be hearing much about was "move a dense, solid mass uphill". I wasn't thinking specifically about concrete.
> 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.
This actually seems like the more ecological design, as the concrete used for weight here would be less than the amount used for a dam, and you wouldn't also have to pervert the flow of a river in order to build it. Besides, one time costs are trivial compared to running costs of industrial processes in terms of the environment, especially in terms of power production.
It's like storing energy in electric cars by parking them at the top of big hills.
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 instead of using a crane, I think you could make something similar to the chain that pulls a roller coaster up.
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'm pretty sure they just used a crane to prove the concept. It would be pretty dumb for them to use a bunch of expensive cranes for a permanent installation.
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.
Definitely would have a good amount of friction losses, the issue I have ran into is that I am not sure how you would raise blocks and add them to a stack with low friction.
I was wondering if it would be possible, perhaps, instead of using cranes to stack blocks of stones, to elevate an entire building by some small amount every day when there is enough excess energy. Lowering the building in the night to recover some of the stored energy. This would definitely be a very challenging engineering problem but the thought of transforming each house in a city into a battery using only its own potential energy without being dependent on chemical means of storage is intriguing to me.
I was about to post the exact same idea. I don't know if it would be efficient (plus mechanicaly lifting up a building is an engineering nightmare to do) but it would be a very elegant way of storing solar energy, or energy recovered from lost calories.
What are the maintenance costs of such a crane, remembering that it runs 24/7 at its max capacity?
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.
I've seen this tower energy storage discussed a few times, but ... what happens in a wind storm? Does the system dump all the blocks to the ground to protect against toppling? How about if a block at the base structurally fails? Are these storage facilities only suitable for wide open areas with nothing around the falling radius of the tower?
> 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.
The article leaves me with more than a few doubts with this strategy of storing power. One issue that concerns me is the amount of energy that will need to be spent in manufacturing all the concrete and steel drums as well as the amount of pollution this will generate. There will also be the pollution in transporting the drums onto the battery locations. I get that this isn't a unique problem and other forms of "green" power generation also suffer from this such as wind turbines or even photovoltaic panels. Once the drums are onsite there is likely minimal maintenance needed so once the cost of producing is done, there should be a way to figure out how many years or months it would take to offset that.
> I get that this isn't a unique problem and other forms of "green" power generation also suffer from this such as wind turbines or even photovoltaic panels.
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.
Here is another idea: concrete blocks on rails plated with solar panels, sometimes stored vertically, sometimes stored horizontally, with rails shaped as an "L", utilizing the most unusable land.
Laws of fluid mechanics will apply then. And the size of the motors will be ginormous.
