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I'm wondering what the best method is for generating electricity in a wide river. The assumptions here is that it's a strong flowing river. It is very wide and we're allowed to put an electricity generating device in the middle. It may not restrict the flow like a dam.

I myself have reduced it to two methods. One is a boat propeller. The other is a water wheel.

an 'artists' rendition of a propeller and water wheel in a river stream

My physical knowledge doesn't allow me to eliminate one or the other. A propeller seems great as it is fully submerged, isn't limited to the speed of the water and all blades can produce motion at all times. Still I'm worrying about drag and cavitation. A water wheel is only partly submerged, allowing the 'return stroke' to happen in lower resistant air. Though I'm worried it only works well with restricted flow, forcing the water past the scoops. I think the same reason propellers are now used on boats (efficiency) is the same reason a water wheel wouldn't work for generation. Water wheels would also cause a lot of turbulent flow which might hamper efficiency.

The question is which one of these is more efficient if you can't restrict the stream. If a different solution is better I'm all ears as well.

Trioxidane
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  • See if https://waterotor.com/ gives you any ideas. – Transistor Aug 01 '22 at 16:23
  • The submerged propellors turn slowly - not fast enough to produce sushi... – Solar Mike Aug 01 '22 at 16:24
  • @Transistor thank you for the suggestion. Unfortunately their best claim I could find was using high lift and high drag to generate power, but that is frankly the standard. There is no reasoning to be found why it is better, other than their claim it is better. – Trioxidane Aug 01 '22 at 17:30
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    @SolarMike If you have something to add to the question I'm glad to hear it. Unfortunately yiur comment is out of scope of the question. – Trioxidane Aug 01 '22 at 17:32
  • I got the impression that WaterRotor solved some of the problems of fouling of the mechanism. – Transistor Aug 01 '22 at 17:36
  • Also, don't get hung up on efficiency. An inefficient, cheap mechanism that doesn't foul can be scaled up to give the required output and still be better than "the most efficient". Efficiency is important when burning fuel. In the case of your water wheel your power is free and efficiency may not matter unless you really need to get "all" the power you can from it. – Transistor Aug 01 '22 at 18:11
  • is this a school assignment? – jsotola Aug 01 '22 at 19:28
  • @jsotola I left the university quite some years ago. "School assignments" even further. That being said I think I understand your approach. If you have accessible sources where I can find my own answer I would appreciate it. I can find documents about screw ships vs paddle boars, but comparing generating electricity from one or the other is more difficult for me to find. The technical side even more so than a bland comparison. Whats more important is I would like to understand the why. – Trioxidane Aug 01 '22 at 20:12
  • @Transistor I agree with your statement, but for my use case I do want the more efficient one regardless of cheapness and scaling. – Trioxidane Aug 01 '22 at 20:14
  • not blocking the river is a huge barrier to efficiency. Anything you do to extract the most energy will almost by definition block the river, just not with a dam. – Tiger Guy Aug 01 '22 at 20:25
  • @TigerGuy and yet it is explicitly not what I want answered. – Trioxidane Aug 01 '22 at 20:41
  • So, Ireland was looking into a project - not going to chase it down though - you can do that. do search tidal power. – Solar Mike Aug 01 '22 at 22:07
  • @Trioxidane, taking any energy from the stream will restrict it. Do you mean not restricting via a dam or weir? – Tiger Guy Aug 02 '22 at 14:23
  • @TigerGuy it seems my restrictions lead to such questions that I do not get the answer I want. As by the answers this is seen as a certain question I'll make a new question so people answer the question at hand. – Trioxidane Aug 02 '22 at 14:34
  • Propellers are used on boats because they work effectively when the boat is stationary and also when the boat is moving. Paddle-wheels can be more efficient at speed, but less efficient at start-up. – david Aug 03 '22 at 06:01

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the thing you sketched on the right is the opposite of a pelton wheel; the pelton device extracts power from a small amount of water flow that is moving under the influence of great pressure. You need a turbine which is designed to extract power from a great amount of flow (in cubic feet per second) at a negligible pressure difference. This is traditionally done with an undershot wheel which you mistakenly called a pelton wheel in your sketch. This works just like the stern wheel on a old-fashioned steamboat.

FYI here in the pacific northwest the very large Columbia river flows from the interior of Washington state to the Pacific ocean over a relatively small elevation difference. To efficiently extract power from a huge flow at negligible pressure head you need a truly gigantic turbine blade that looks like an enormous ship propeller, mounted inside an equally huge pipe to contain the flow.

For this extraction process to be reasonably efficient, a dam needs to be built to back up the river to an extra depth of at least 20 feet, which is the minimum pressure head needed to make the turbine pay for itself. So engineers have built a series of dams most of the way up the Columbia that are spaced 20 feet of elevation change apart from each other, converting the river into a series of slack-water reservoirs with turbine halls inside the dams.

Great for making electricity, but very bad for salmon migration.

niels nielsen
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  • Thank yiu for the correction of the pelton wheel. It had been a while so I forgot a pelton wheel is an impulse type. From your answer it seems a propeller is best? It isn't stated clearly. – Trioxidane Aug 01 '22 at 17:35
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All "run of the river" devices are pretty much hopeless. But I guess there must be a least worst one.

The propeller is a lift device. The water wheel is a drag device. Propellers don't like to be near the surface. The free surface wave energy is difficult to estimate in these circumstances. Probably the best open channel device is the Gorlov turbine.

But just about anything that isn't in the run of the river would be better. Divert the river into a sluice and run it downstream until you have a decent head.

Locating the mechanicals a couple of feet above the record high water mark is prudent, and build the structure so that a house floating down in a flood won't dislodge it.

Phil Sweet
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  • So 9.9m^3/s and 212m of head should do? Do you think that qualifies as "decent head"? – Solar Mike Aug 01 '22 at 23:50
  • I wouldn't turn my nose up at 150m head ;) But seriously, 10m is sufficient for small system, and might be above high water mark. My little creek topped out at 26 feet once 25 years ago. – Phil Sweet Aug 02 '22 at 00:12