# Where does the energy come from in a canal lock?

by amalgamma
Tags: canal, energy, lock
 P: 7 You can lift a zillion-ton ore barge 30 feet like magic, and nothing seems to have done the work. Help me get it.
 P: 1,404 There's a difference in water level.
 Engineering Sci Advisor HW Helper Thanks P: 7,284 Either the water that filled the lock flowed from a higher level and lost some potential energy, or else the water was pumped into the lock and energy was used to drive the pump.
P: 7
Where does the energy come from in a canal lock?

 Quote by willem2 There's a difference in water level.
And ...? The water would rise just the same if the barge weren't there.
P: 7
 Quote by AlephZero Either the water that filled the lock flowed from a higher level and lost some potential energy, or else the water was pumped into the lock and energy was used to drive the pump.
When the lock is full again, the water is back where it was, and in, say, a hydro dam, the water is slowed going through the turbine. In a lock , it simply flows there whether there is a load or not. And what kind of canal lock pumps water, when all you have to do is let the higher water in? What am I not seeing?
Mentor
P: 11,864
 Quote by amalgamma And ...? The water would rise just the same if the barge weren't there.
If there's no barge in the lock, more water has to flow in, in order to reach the same level. In fact, the mass of the "extra" water equals the mass of the barge, because the barge displaces a mass of water equal to its own mass.
Mentor
P: 22,313
 Quote by amalgamma When the lock is full again, the water is back where it was, and in, say, a hydro dam, the water is slowed going through the turbine. In a lock , it simply flows there whether there is a load or not. And what kind of canal lock pumps water, when all you have to do is let the higher water in? What am I not seeing?
Water that would have flowed through the turbine flows through the lock instead.
P: 7
 Quote by jtbell If there's no barge in the lock, more water has to flow in, in order to reach the same level. In fact, the mass of the "extra" water equals the mass of the barge, because the barge displaces a mass of water equal to its own mass.
I'm sure I'm being thick, but are you saying that the energy comes from the water that doesn't enter the lock? If you fill the lock with the same amount of water it would hold if the barge were there, its surface would be at a lower level, and more potential energy would have been lost than if it had lifted a barge. I'm hoping you can spot the idiotic misapprehension in what I'm thinking here.
P: 7
 Quote by russ_watters Water that would have flowed through the turbine flows through the lock instead.
What? At what point does the flow give up energy in a lock?
Mentor
P: 22,313
 Quote by amalgamma What? At what point does the flow give up energy in a lock?
When you open the entrance gate and let the water in the lock spill into the lower level.
P: 7
 Quote by russ_watters When you open the entrance gate and let the water in the lock spill into the lower level.
Right, which it would do anyway.
 P: 15,319 Wiki seems to have a pretty good description of the sequence: http://en.wikipedia.org/wiki/Lock_%2..._and_operation Note in particular, boat going upstream step 4-5: "The lock is filled with water from upstream." Ultimately, the energy is extracted from the river itself as the water level drops from upstream to downstream. This potential energy can be extracted.
Mentor
P: 22,313
 Quote by amalgamma Right, which it would do anyway.
....but through a turbine. You lose energy by just dumping the water instead of running it through a turbine.
Mentor
P: 5,490
 Quote by amalgamma You can lift a zillion-ton ore barge 30 feet like magic, and nothing seems to have done the work. Help me get it.
Ultimately the answer is the sun. Let's deal with a purely mechanical lock and a ship going upstream: A boat enters the lock and the gate closes. The gate before them releases water from upstream which raises the boat. The gate them opens and the boat proceeds.

This water must be bought back up at some point and this is achieved by the hydrosphere cycle i.e. the downstream water eventually evaporates and rains again on land.

Is this the answer you are looking for?
Mentor
P: 11,864
 Quote by jtbell If there's no barge in the lock, more water has to flow in, in order to reach the same level. In fact, the mass of the "extra" water equals the mass of the barge, because the barge displaces a mass of water equal to its own mass.
 Quote by amalgamma I'm sure I'm being thick, but are you saying that the energy comes from the water that doesn't enter the lock?
The point I'm trying to make is that it takes the same amount of energy to fill the lock so that the water line ends up at a certain level, regardless of whether there is a boat in the lock or not. With the boat in the lock, you have to lift x kilograms of boat and y kilograms of water. Without the boat in the lock, you have to lift x+y kilograms of water.

 If you fill the lock with the same amount of water it would hold if the barge were there, its surface would be at a lower level
The purpose of a lock is to get the water level up (or down) to a certain point, namely the level that matches the water level outside the lock at the exit end. So if the barge isn't there, you have to add more water if you're going upwards, or drain more water if you're going downwards.
P: 7
 Quote by Ryan_m_b Ultimately the answer is the sun. Let's deal with a purely mechanical lock and a ship going upstream: A boat enters the lock and the gate closes. The gate before them releases water from upstream which raises the boat. The gate them opens and the boat proceeds. This water must be bought back up at some point and this is achieved by the hydrosphere cycle i.e. the downstream water eventually evaporates and rains again on land. Is this the answer you are looking for?
I think I grok the water cycle. I'd like to apologize for getting turned around back there---I got the entrance gate and the exit gate reversed in my head somehow. I also grok a lock.

Your answer does help if I imagine using the same water over again. I'm still a bit baffled by the seeming effortlessness of the process. Therer are many unintuitive phenomena in nature, and I guess I'll have to accept this as one more. I never could get my head around potential energy, anyway---it always seemed like giving a name to the inscrutable, treating energy as a substance like an alchemist or something.
 Mentor P: 22,313 Good point, jtbell, and let me expand to help clarify. There are actually two different sources of loss being discussed: 1. In a real lock, water is just allowed to flow through when the gates are opened (or valves and pipes, more likely). This loss could theoretically be eliminated by putting turbines in. You'd have one turbine by the high side gate to recover energy from filling the lock and another turbine by the low side gate to recover energy from emptying the lock. This volume of water is the length and width of the lock times the lift height. At first glance, this appears to be all of the energy required, which would imply that the lift is free, but it isn't.... 2. When a ship enters the lock at the bottom it pushes out a parcel of water equal to its own displacement. When it leaves at the top, it allows back in the same volume of water. That volume of water sneaks past the turbines by going through the open gates instead of through the pipes and turbines. That's the energy that is really expended and "lost" to lift the ship.
 P: 901 Couldn't we stick a dam on the Panama canal and generate a lot of energy that way?

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