# Why aren't gravity batteries everywhere? [closed]

The problem of energy storage, especially with regard to renewables, remains unsolved. Or at least there do not seem to be any popular solutions.

Why not gravity batteries?

I love the idea, and they look like a beautiful and effective approach to the problem. The low-tech, simple principles grant it so many different region-taylored forms. Many of these designs could easily be low-cost. I could go on.

Are there some shortcomings that I am missing here?

 Here is my link to the question on the Engineering stack exchange.

• Isn't Lake Meade one giant gravity battery?
– JEB
Sep 4 '19 at 2:52
• @JEB and all hydroelectric plants are using it , evaporation and the clouds doing the work. Sep 4 '19 at 3:46
• Sure, in the sense of it storing potential energy. I'm more specifically curious about technologies for storing energy from wind, PV, etc. renewables for the grid to address the supply-demand issue. Sep 4 '19 at 5:17

It's a good question. A while ago I computed this for fun, and was disappointed how little energy you could store with your 1000 kg lead ball of choice on a 10m tall pulley and gearbox in the garden. Let's quickly do some numbers: $$E \approx m \cdot g \cdot h \approx 1000 \ \text{kg} \cdot 10 \ \text{m}/\text{s}^2 \cdot 10 \ \text{m} = 100 \ \text{kJ}$$. That is not very much. Maybe one half of an apple. Won't really get you very far. Hot shower for 5 seconds? Have fun!^^ One can get the same storage capacity with ~0.3 litre of good old lead battery(!).

Now you can of course go wild and industrial scale (as these people at energyvault apparently did).

My view on this is that the energy density of the arangement is just too low, the gravitational field on earth is too weak. But as we can't change that locally :D I guess we're stuck here. So for the mechanical forces you have to deal with, you just get too little storage capacity out.

It can be attractive when by natural circumstances you have huge water reservoirs where you can pump up and down. Then the mechanical forces problem is solved by nature or additionally by heaps of concrete already. The round trip conversion efficiency can still be quite high.

The biggest problem is that gravity batteries are not easily scalable. They work okay for small-scale energy storage, and there are even several commercial products available aimed at powering small LEDs in developing countries with no widespread electricity supply. But, for any real industrial application (i.e. nation-wide electricity production) they would not be feasible.

• Take a look at this: energyvault.com. They claim they can provide a highly efficient energy storage and "20-35-80 MWh storage capacity; 4-8 MW of continuous power for 8-16 hrs". Sep 4 '19 at 2:04
• @anonymous cites a good case. In addition, the point you bring up has no reason why in principle they can't be scaled. Sep 4 '19 at 2:30

These batteries aren't easy to make from an engineering point of view. You need to be able to lift the counterweight up some height which you might not have, e.g. if you're trying to store the energy for your car, you can't exactly build a 100-meter tall car. You could compensate by increasing the mass that's lifted, but you can't exactly build a 100-ton car either (that would make the energy required by the car to move go through the roof, too).

Where possible this concept's already been used, e.g. an obvious one is in lifts and elevators:

In practice, elevators work in a slightly different way from simple hoists. The elevator car is balanced by a heavy counterweight that weighs roughly the same amount as the car when it's loaded half-full (in other words, the weight of the car itself plus 40–50 percent of the total weight it can carry). When the elevator goes up, the counterweight goes down—and vice-versa, which helps us in four ways:

1. The counterweight makes it easier for the motor to raise and lower the car—just as sitting on a see-saw makes it much easier to lift someone's weight compared to lifting them in your arms. Thanks to the counterweight, the motor needs to use much less force to move the car either up or down. Assuming the car and its contents weigh more than the counterweight, all the motor has to lift is the difference in weight between the two and supply a bit of extra force to overcome friction in the pulleys and so on.
2. Since less force is involved, there's less strain on the cables—which makes the elevator a little bit safer.
3. The counterweight reduces the amount of energy the motor needs to use. This is intuitively obvious to anyone who's ever sat on a see-saw: assuming the see-saw is properly balanced, you can bob up and down any number of times without ever really getting tired—quite different from lifting someone in your arms, which tires you very quickly. This point also follows from the first one: if the motor is using less force to move the car the same distance, it's doing less work against the force of gravity.
4. The counterweight reduces the amount of braking the elevator needs to use. Imagine if there were no counterweight: a heavily loaded elevator car would be really hard to pull upwards but, on the return journey, would tend to race to the ground all by itself if there weren't some sort of sturdy brake to stop it. The counterweight makes it much easier to control the elevator car.

(In particular point 3.)