Why are hydropower plants always wheel-shaped and not flat?

Question: Why are there no flat power generators like in the picture below, that work on the surface of shallow, but steadily flowing rivers ? (As a floating micropower plant.)

The picture shows a conveyer belt with vanes/blades(?) attached to it. The water flow moves the conveyer belt. A generator could be attached to the front and back "wheel" of the belt.

Here's a video of something similar. I would just build it on a larger river.

There are much more flat rivers than waterfall-like structures on this planet. Using them looks like a much more non-nature-inversive, cheap solution. Having a longer surface should supply better drag by flowing water.

• Have you checked Francis turbines - as used for shallower rivers? Jan 16 at 8:40
• As far as I could tell, they are also round. Regarding their use, they are afaik highly optimized turbines, that can only be used with high fall and much water. Ie their usage is highly constrainted. Jan 16 at 9:44
• Pelton is high fall... any turbine to be efficient is optimised to the location. Seems like you need to check all three: Pelton, Francis and Kaplan... Jan 16 at 9:46
• I we go that route: The question is about, why not stretch the idea of a Pelton turbine? Maybe information on the internet is wrong. But all three are wheel shaped. (I tried to precise my question) Jan 16 at 9:59
• If you look at your design, there will be lower friction losses if you compress the length of your device so it has one axle instead of two... Do make the effort to check out the water wheels that have been in use for centuries under, over and breast shot come to mind. Jan 16 at 10:08

When you want to solve a problem, the best start is to look at previous attempts. To provide some perspective, I'm doing that for you now. You are not looking at a typical hydro power plant where a dam provides a high head, and the flow is ducted onto a a francis or pelton turbine. You are describing a microhydropower installation with a floating turbine.

Floating hydrpower allows capturing some power without building a dam. The turbine could be placed in or near the middle of the river, where the current is fastest. An installation with a damn will always harvest vastly more power from the same river.

Before electrical power transmission became widespread, there used to be boat mills - workshops with machinery driven by water wheels, placed on boats.

(Boat mill in Servia, 1900, Image from lowtechmagazine page on boat mills)

Improvised versions have also been used for electricity generation.

Floating hydro power is, AFAICT, an ongoing area of developement. The two most common turbine shapes appear to be a propeller hanging from buoys:

... Or some sort of flat paddle wheel:

Vertical axis turbines also exist.

I think Kamran explains quite well why a propeller or a paddle wheel is used, rather than a conveyor belt. I will just add this: Look at the water wheel in the direction of flow: You want to maximise area here.

The incoming stream of water flow momentum is transferred to the radial blade of the turbine wheel when it hits the slow blade of the turbine, giving it its kinetic energy, and pushing it out of the way making room for the tail stream to keep rushing in, continuing the momentum and the process.

$$W=\dot{m}v^2_{initial}-\dot{m}v^2_{final}$$

In linear track in your figure, the flow gets trapped in the buckets moving along with them. There is very little exchange of energy because the speed of the stream is more or less the same as the track, only the front wheel is working.

And the flow of the tail end of the stream gets slowed down and goes around the track.

• This is what you're trying to say?: In a round turbine (e.g. Kaplan) energy gets added through the blade. This allows the turbine to turn faster, than the river flows. However in my linear approach, energy is not added but set to the speed of the river. || But don't old water wheels have the same problem? Jan 17 at 15:07
• the trick is to catch the maximum possible energy od the stream. if your turbine sqeezez all the energy out of the stream it means the discharge kinetic energy is zero, meaning no mass coming out, not possible. if the difference in the speed of blade and the stream is too small, not an efficient turbine. if you google Carno principle you find what he proved about max energy available from a flow. Jan 18 at 0:56
• Thank you very much for the explanation. For me your and mart's answer together helped. I would've like to accepted both answers. As I needed mart's answer to get a better grasp of yours, I decided to take is as accepted. Feb 4 at 3:04