4 ms·

I hate to be a naysayer, but this is by no means a viable option to store energy. Let's see how much energy can be stored in this sphere with a diameter of 3m.

I hate to be a naysayer, but this is by no means a viable option to store energy. Let's see how much energy can be stored in this sphere with a diameter of 3m. E = VP where E is the energy stored, V is the volume and P the pressure. V = (1.5m)^3 * 4 * PI / 3 = 14m3 P = 100 * 1e4 N/m2 = 1e6 N/m2 thus E = 14*1e6J = 3.9kWh Imagine that, the capacity of 4 big car batteries that I can buy for 100€ each! It uses 20 tons of concrete and is a highly delicate and sophisticated machinery. Luckily, the energy stored scales with the cube of the diameter, right? So a 30m sphere will have a 1000 times higher capacity! 4MWh! However, it will be almost impossible to construct such a sphere out of concrete. Also, 700m depth? That is the limit for saturation divers and current nuclear submarines can't go deeper than 500m before getting crushed! I have no idea who reviewed the application to this fund. It must have cost millions to build a storage system that stores as much energy as 400€ worth of car batteries.

joantune 10y ago
You're probably missing an important number there, not that I'm sure that it will make a difference, but: How about longevity? one of those will probably hold that capacity for much longer than a Lithium battery, no?
- nabla9 10y ago
  Tests have been performed for the the durability of concrete spheres underwater over ten year period and found that concrete under the sea behaves the same way as concrete on the land in same pressure. In other words, very durable if you make it durable.
- dsr_ 10y ago
  There are two forms of longevity. One is, how long will it hold a charge? And the other is, How many times can it be charged and discharged before losing capacity or failing? The uncharged state is at pressure equivalence to the outside environment, so there's no reason not to think that it can last as long as concrete structures in water can last: decades to centuries. The generator parts will need more maintenance. The charged state is empty, with the interior water pumped out, and holding it will be a mechanical and materials engineering problem. However, the normal call will be for a daily cycle to replace energy that would otherwise come from solar. Reasonable management should prevent any particular cell from being held at charge for more than a few days at a time. Lithium chemistry cells have a limited lifespan in number of charge cycles; 600-1000 or so for full discharge, up to 10-15,000 for very light discharge cycles. The water storage system won't exhibit those problems unless they fail to filter the water adequately.
breischl 10y ago
I thought the idea was to use the difference in potential energy between the surface level and the sphere as the storage. ie, you'd store energy by emptying the sphere, and release energy by dumping water into it through a turbine. If that's the case, I don't get where the pressure comes into it.
- donquichotte 10y ago
  You are right, this is basically a gravity energy storage. It's the same idea as moving a 14m3 blob of water 100m up and down. Unfortunately, gravity storages have really bad energy densities. Let's see. E = mgh E is the energy, m the mass, g the gravitational acceleration, h the difference in height. m = 14e3kg g = 10m/s2 h = 100m E = 14e6J = 3.9kWh The only gravity storages that have been proved to be scalable so far are hydroelectric. And they have huge lakes behind them.
Hermel 10y ago
> The Fraunhofer Institute for Wind Energy and Energy Systems Engineering envisions spheres with inner diameters of 30m, placed 700m (or about 2,300 ft) underwater. That would be 5600 times more energy, or the 20 MWh from the article. That's 2 million USD worth of car batteries...
- donquichotte 10y ago
  Well, the energy scales with the diameter to the third power (we go from 3m to 30m, so x1000) and linearly with the depth (we go from 100m to 700m, so x7). So in total, a factor of 7000 if everything is ideal. What I mentioned in the post above (4MWh) is a 30m sphere at 100m. I agree with your point, but am unconvinced that the system is scalable.
Arizhel 10y ago
>current nuclear submarines can't go deeper than 500m before getting crushed! How exactly do you know that? The maximum depths of nuclear submarines are classified information, just like the top speeds of any military ships. The numbers they put on Wikipedia aren't real, those are the "public" numbers just so you can have an idea of how they perform in normal peacetime operation.
theptip 10y ago
Although your first-principles approach is admirable, it's also an order of magnitude out from what the original researchers are presenting (for a 30m sphere): > Assuming the spheres would be fitted with existing 5 MW turbines that could function at that depth, the researchers estimate that each sphere would offer 20 MWh of storage with four hours discharge time. Looks like you are assuming a 1.5m wall thickness, and then scaling that linearly to the 30m version, neither of which are correct. > Also, 700m depth? That is the limit for saturation divers and current nuclear submarines can't go deeper than 500m before getting crushed! I'd be surprised if they were making a claim like this without some justification that they can build at that depth. As a counter-anecdotal number, oil pipelines can be constructed at 2km depth [1]. These spheres are neither pipes nor submarines; I'm guessing they know (at least roughly) what pressure their spheres can tolerate. FWIW, the depth was chosen based on the depth limits of the turbines[2]. [1]: http://www.offshore-mag.com/articles/print/volume-71/issue-8/flowlines-__pipelines/designing-large-diameter-pipelines-for-deepwater-installation.html http://www.offshore-mag.com/articles/print/volume-71/issue-8... [2]: http://forschung-energiespeicher.info/en/projektschau/gesamtliste/projekt-einzelansicht/95/Kugelpumpspeicher_unter_Wasser/ http://forschung-energiespeicher.info/en/projektschau/gesamt...

joantune 10y ago

You're probably missing an important number there, not that I'm sure that it will make a difference, but: How about longevity? one of those will probably hold that capacity for much longer than a Lithium battery, no?

nabla9 10y ago
Tests have been performed for the the durability of concrete spheres underwater over ten year period and found that concrete under the sea behaves the same way as concrete on the land in same pressure. In other words, very durable if you make it durable.
dsr_ 10y ago
There are two forms of longevity. One is, how long will it hold a charge? And the other is, How many times can it be charged and discharged before losing capacity or failing? The uncharged state is at pressure equivalence to the outside environment, so there's no reason not to think that it can last as long as concrete structures in water can last: decades to centuries. The generator parts will need more maintenance. The charged state is empty, with the interior water pumped out, and holding it will be a mechanical and materials engineering problem. However, the normal call will be for a daily cycle to replace energy that would otherwise come from solar. Reasonable management should prevent any particular cell from being held at charge for more than a few days at a time. Lithium chemistry cells have a limited lifespan in number of charge cycles; 600-1000 or so for full discharge, up to 10-15,000 for very light discharge cycles. The water storage system won't exhibit those problems unless they fail to filter the water adequately.

breischl 10y ago

I thought the idea was to use the difference in potential energy between the surface level and the sphere as the storage. ie, you'd store energy by emptying the sphere, and release energy by dumping water into it through a turbine. If that's the case, I don't get where the pressure comes into it.

donquichotte 10y ago
You are right, this is basically a gravity energy storage. It's the same idea as moving a 14m3 blob of water 100m up and down. Unfortunately, gravity storages have really bad energy densities. Let's see. E = mgh E is the energy, m the mass, g the gravitational acceleration, h the difference in height. m = 14e3kg g = 10m/s2 h = 100m E = 14e6J = 3.9kWh The only gravity storages that have been proved to be scalable so far are hydroelectric. And they have huge lakes behind them.

Hermel 10y ago

> The Fraunhofer Institute for Wind Energy and Energy Systems Engineering envisions spheres with inner diameters of 30m, placed 700m (or about 2,300 ft) underwater. That would be 5600 times more energy, or the 20 MWh from the article. That's 2 million USD worth of car batteries...

donquichotte 10y ago
Well, the energy scales with the diameter to the third power (we go from 3m to 30m, so x1000) and linearly with the depth (we go from 100m to 700m, so x7). So in total, a factor of 7000 if everything is ideal. What I mentioned in the post above (4MWh) is a 30m sphere at 100m. I agree with your point, but am unconvinced that the system is scalable.

Arizhel 10y ago

>current nuclear submarines can't go deeper than 500m before getting crushed! How exactly do you know that? The maximum depths of nuclear submarines are classified information, just like the top speeds of any military ships. The numbers they put on Wikipedia aren't real, those are the "public" numbers just so you can have an idea of how they perform in normal peacetime operation.

theptip 10y ago

Although your first-principles approach is admirable, it's also an order of magnitude out from what the original researchers are presenting (for a 30m sphere): > Assuming the spheres would be fitted with existing 5 MW turbines that could function at that depth, the researchers estimate that each sphere would offer 20 MWh of storage with four hours discharge time. Looks like you are assuming a 1.5m wall thickness, and then scaling that linearly to the 30m version, neither of which are correct. > Also, 700m depth? That is the limit for saturation divers and current nuclear submarines can't go deeper than 500m before getting crushed! I'd be surprised if they were making a claim like this without some justification that they can build at that depth. As a counter-anecdotal number, oil pipelines can be constructed at 2km depth [1]. These spheres are neither pipes nor submarines; I'm guessing they know (at least roughly) what pressure their spheres can tolerate. FWIW, the depth was chosen based on the depth limits of the turbines[2]. [1]: http://www.offshore-mag.com/articles/print/volume-71/issue-8/flowlines-__pipelines/designing-large-diameter-pipelines-for-deepwater-installation.html http://www.offshore-mag.com/articles/print/volume-71/issue-8... [2]: http://forschung-energiespeicher.info/en/projektschau/gesamtliste/projekt-einzelansicht/95/Kugelpumpspeicher_unter_Wasser/ http://forschung-energiespeicher.info/en/projektschau/gesamt...