4 ms·

I had to check if that realistic. 40 million gallons at 85 feet is 11MWh of potential energy. At 70% efficiency it would take 16MWh to pump that. That has to be

I had to check if that realistic. 40 million gallons at 85 feet is 11MWh of potential energy. At 70% efficiency it would take 16MWh to pump that. That has to be a small amount of energy compared to what ships take for traveling the detour.

londons_explore 3y ago
And remember each ship goes up as well as down - so you can theoretically recoup all the energy you used pumping.
- actionfromafar 3y ago
  I am sorry, how?!
  - londons_explore 3y ago
    Imagine one ship going upwards in a lock, while another goes down. Ships and locks are the same size. To start with, there is a difference in the water levels - and this can be put through a generator to generate electricity as the water flows from one lock to the other. When the water has moved half way, the levels are equal. You can now use electricity to pump the rest of the way - and theoretically, assuming lossless pumps and generators, you use the same amount you generated earlier. Unfortunately, low-head generators tend to be inefficient and expensive. Variable head generators are even harder to design to be efficient.
  - Mutttttioi 3y ago
    You pump the water out of it into a reservaour, than later on when you need to fill it, you let it run back and generate energy. Water power efficency is about 80-90%
  - rtpg 3y ago
    Gravity is a conservative force, so if you raise something up really high, dropping it will recover the energy used to pull it up. Of course in practice there's a lot of other things that cause energy loss, but there are lot of dams in the world that exist as "stored power", by pumping water uphill, then getting energy back as the water goes through the dam later. Big bucket high up is basically a battery. The question then becomes about the details of, like, using the power when the water gets pumped down (if you had two canals, you could probably time filling one with draining the other?). There's a lot of practical things to consider
    - ChrisMarshallNY 3y ago
      > Big bucket high up Reminds me of this lesson in gravity: https://www.youtube.com/watch?v=vFUj6LH4FSI https://www.youtube.com/watch?v=vFUj6LH4FSI
    - deleted 3y ago
      [deleted]
  - nemacol 3y ago
    There is a nice video (Practical Engineering) that covers this very well. https://www.youtube.com/watch?v=SBvclVcesEE https://www.youtube.com/watch?v=SBvclVcesEE
  - rjmunro 3y ago
    One way is like this: https://en.wikipedia.org/wiki/Falkirk_Wheel https://en.wikipedia.org/wiki/Falkirk_Wheel No water is used up. Virtually no energy is needed either, except to overcome friction. But scaling it up to the size of the locks on the Panama canal would be huge.
charlieyu1 3y ago
But if it is not pumped back it is free electricity
- stuaxo 3y ago
  Helloo externalities.
ok_dad 3y ago
It would take a long time to pump that amount of water out of the lock that you’re lowering, and over time you’ll be pumping saltwater into a freshwater system, which isn’t good. You’d have to do filtering or something, at great expense. Currently it takes a short time to drain the lock, so ships transit faster. It’s pretty fun to do that transit, but I was amazed that the pilot let me conn the ship the whole time instead of doing it themselves. I guess on military ships they prefer we do it maybe, but I was glad for that experience. My ship had two jet turbine engines that did 31 MW of propulsion, which was a small ship compared to most freighters, so energy wise it would be very efficient still.
- nashashmi 3y ago
  Saltwater mixing is a good point. But the canal has two lanes. One lane is for one direction. The other lane is for the other direction. I can imagine cases where the saltwater is never used.
  - ok_dad 3y ago
    The way locks work you can save some by using extra storage ponds but you can’t save it all. You have to move some water into the ocean, because that’s the cost for the physics that move giant ships up and down as easily as this.
- Veserv 3y ago
  The Panama Canal Authority derives ~3 G$ in fees over ~14 K crossings per year for a average cost of ~200 K$ per crossing. 40 M gallons is ~150 K*m^3. Modern bulk desalination turns seawater into potable fresh water at ~0.40 $/m^3, so only ~60 K$. The cost of filtering out any incidental saltwater mixing should be a tiny fraction of that cost. Even at full desalination it would only constitute ~30% cost increase. Given the minimal amount of filtering that should be required for incidental mixture, it is hard to call that a great expense except in relation to rainwater.
  - ok_dad 3y ago
    30% is a large increase to a canal that we use for a huge portion of our consumer goods, I think. I do agree that they soups build desalination plants now, so they can use them in twenty years when they’re finished and the drought is even worse!

londons_explore 3y ago

And remember each ship goes up as well as down - so you can theoretically recoup all the energy you used pumping.

actionfromafar 3y ago
I am sorry, how?!
- londons_explore 3y ago
  Imagine one ship going upwards in a lock, while another goes down. Ships and locks are the same size. To start with, there is a difference in the water levels - and this can be put through a generator to generate electricity as the water flows from one lock to the other. When the water has moved half way, the levels are equal. You can now use electricity to pump the rest of the way - and theoretically, assuming lossless pumps and generators, you use the same amount you generated earlier. Unfortunately, low-head generators tend to be inefficient and expensive. Variable head generators are even harder to design to be efficient.
- Mutttttioi 3y ago
  You pump the water out of it into a reservaour, than later on when you need to fill it, you let it run back and generate energy. Water power efficency is about 80-90%
- rtpg 3y ago
  Gravity is a conservative force, so if you raise something up really high, dropping it will recover the energy used to pull it up. Of course in practice there's a lot of other things that cause energy loss, but there are lot of dams in the world that exist as "stored power", by pumping water uphill, then getting energy back as the water goes through the dam later. Big bucket high up is basically a battery. The question then becomes about the details of, like, using the power when the water gets pumped down (if you had two canals, you could probably time filling one with draining the other?). There's a lot of practical things to consider
  - ChrisMarshallNY 3y ago
    > Big bucket high up Reminds me of this lesson in gravity: https://www.youtube.com/watch?v=vFUj6LH4FSI https://www.youtube.com/watch?v=vFUj6LH4FSI
  - deleted 3y ago
    [deleted]
- nemacol 3y ago
  There is a nice video (Practical Engineering) that covers this very well. https://www.youtube.com/watch?v=SBvclVcesEE https://www.youtube.com/watch?v=SBvclVcesEE
- rjmunro 3y ago
  One way is like this: https://en.wikipedia.org/wiki/Falkirk_Wheel https://en.wikipedia.org/wiki/Falkirk_Wheel No water is used up. Virtually no energy is needed either, except to overcome friction. But scaling it up to the size of the locks on the Panama canal would be huge.

charlieyu1 3y ago

But if it is not pumped back it is free electricity

stuaxo 3y ago
Helloo externalities.

ok_dad 3y ago

It would take a long time to pump that amount of water out of the lock that you’re lowering, and over time you’ll be pumping saltwater into a freshwater system, which isn’t good. You’d have to do filtering or something, at great expense. Currently it takes a short time to drain the lock, so ships transit faster. It’s pretty fun to do that transit, but I was amazed that the pilot let me conn the ship the whole time instead of doing it themselves. I guess on military ships they prefer we do it maybe, but I was glad for that experience. My ship had two jet turbine engines that did 31 MW of propulsion, which was a small ship compared to most freighters, so energy wise it would be very efficient still.

nashashmi 3y ago
Saltwater mixing is a good point. But the canal has two lanes. One lane is for one direction. The other lane is for the other direction. I can imagine cases where the saltwater is never used.
- ok_dad 3y ago
  The way locks work you can save some by using extra storage ponds but you can’t save it all. You have to move some water into the ocean, because that’s the cost for the physics that move giant ships up and down as easily as this.
Veserv 3y ago
The Panama Canal Authority derives ~3 G$ in fees over ~14 K crossings per year for a average cost of ~200 K$ per crossing. 40 M gallons is ~150 K*m^3. Modern bulk desalination turns seawater into potable fresh water at ~0.40 $/m^3, so only ~60 K$. The cost of filtering out any incidental saltwater mixing should be a tiny fraction of that cost. Even at full desalination it would only constitute ~30% cost increase. Given the minimal amount of filtering that should be required for incidental mixture, it is hard to call that a great expense except in relation to rainwater.
- ok_dad 3y ago
  30% is a large increase to a canal that we use for a huge portion of our consumer goods, I think. I do agree that they soups build desalination plants now, so they can use them in twenty years when they’re finished and the drought is even worse!