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Vertical Kaplan turbines are often used for hydropower plants. They can have an efficiency of more than 90%.

Vertical Kaplan turbine

Why don't we see similar designs for wind power? I mean one could imagine to redirect wind from horizontal to vertical direction and then let the wind go through a wind rotor. This would be similar to a https://en.wikipedia.org/wiki/Wind_lens, but in vertical direction. I imagine it could combine the advantages of horizontal wind turbines with the advantages of vertical wind turbines. Is it that redirecting wind from vertical to horizontal direction is so inefficient? Or are there additional issues with such a design for wind power. Update: So basically my question is: Why does a vertical Kaplan wind turbine have a much lower efficiency than for water?

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  • $\begingroup$ What vertical turbine designs have you found so far? If the flow through the Kaplan turbine was horizontal would it make a lot of difference? $\endgroup$
    – Solar Mike
    Feb 19, 2022 at 17:22
  • $\begingroup$ if the exhaust was pointed downward, then the ground would present resistance to air flow ... if the exhaust was pointed upward, then gravity would present resistance to air flow $\endgroup$
    – jsotola
    Feb 19, 2022 at 18:12
  • $\begingroup$ @SolarMike I have found several designs, but none of them is redirecting the flow from horizontal to a vertical turbine. The closest thing I found is INVELOX (phys.org/news/2013-05-sheerwind-invelox-turbine-power.html). I think the vertical design would allow to always catch the wind without a mechanism to direct the turbine into the right direction. The mechanic stress on the blades and bearing should be less because the wind force and gravity point in the same direction. $\endgroup$
    – asmaier
    Feb 19, 2022 at 20:41
  • $\begingroup$ If wind and gravity act in the same direction, then the forces add don't they? $\endgroup$
    – Solar Mike
    Feb 19, 2022 at 20:44
  • $\begingroup$ @SolarMike I meant mechanical stress, so basically shear forces in the material caused by forces not pointing in the same direction, not the static force downwards. $\endgroup$
    – asmaier
    Feb 19, 2022 at 21:29

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Another reason, is that with hydroplants the water's natural motion is downwards, and its flow can naturally continue.

In the case of air the natural flow is horizontal. Diverting the air would be equivalent to a massive change in momentum and therefore very large forces on the duct structure. Also, as the air exited from the bottom it would need to change again direction (again very large forces and moments), and could also create other disturbances in the flow which would create vortices or other unexpected phenomena.

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  • $\begingroup$ Hmmmm your answer made think...is the OP asking about dams or oceans? I thought oceans but you seem to think dams. I guess it doesn't really matter on my end since the OP ultimately is asking why wind turbines don't use a design which involves ducts whether or not the Kaplan turbine is mean for a dam or the open ocean. $\endgroup$
    – DKNguyen
    Feb 20, 2022 at 7:25
  • $\begingroup$ Probably you are right because the pelton is usually employed at dams. Although for oceans the motion of the water is near the surface and you can divert vertically the water without issues. $\endgroup$
    – NMech ♦
    Feb 20, 2022 at 7:49
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I am coming at this from the aspect of what I've read about propellers so I'm just focusing on the fact that there is a duct present; Forget the vertical and Kaplan aspects.

The problem with anything that involves a spinning blades surrounded by a duct is the duct limits the length of the blades. A properly designed duct does improve the efficiency of an airfoil by reducing tip losses for a given size. But just making longer blades also reduces tip losses and is more effective in improving efficiency. It's also easier, cheaper, and lighter.

So for a duct to have a net efficiency benefit, there usually has to be some constraint limiting the length of the blades, and this constraint has to be overwhelming enough to shorten the length enough that the duct can be of a reasonable size. In air, blades can be made so long that the size of the duct required to accommodate them becomes unreasonable.

For example, you do find situations in airplanes that limit the blade length: ground clearance, tips approaching the speed of sound, drag at high airspeeds. But ducts are never used because the duct is not of a practical size (or weight). The only time you see ducts are experimental aircraft, electric radio control models (for aeshetics since they actually perform worse than with a larger propeller), and on turbofans and turbines (but that is more because a combustion chamber is necessary).

It rather makes sense for hydropower plants in the ocean to use ducts since water is a much thicker medium and would result in higher stresses on the blades which limits their length. Water also also limits blade length so the tip speeds does not produce cavitation. If the length is limited to a point where a duct is of a reasonable size, then you add a duct and that just tends to not be the case in air.

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    $\begingroup$ What I still don‘t get is why Kaplan turbine for water has 90% efficiency whereas a similar design for wind has a much lower efficiency? $\endgroup$
    – asmaier
    Feb 19, 2022 at 20:25
  • $\begingroup$ @asmaier That's probably the Reynolds number at work which was alluded to when I mentioned water is a much thicker medium (though don't mistake it for density). It sounds like wind turbines in general have lower efficiency than water turbines. $\endgroup$
    – DKNguyen
    Feb 19, 2022 at 20:29
  • $\begingroup$ en.wikipedia.org/wiki/Betz%27s_law#Concepts says that it applies to all Newtonian fluids. That includes wind and water and seems to be independent of Reynolds numbers. So why does Betz law not limit the efficiency of the Kaplan turbine, but only limit the efficiency of wind turbines? $\endgroup$
    – asmaier
    Feb 19, 2022 at 21:35
  • $\begingroup$ @asmaier This is really out of my area of expertise so I couldn't say. Are you sure that the 90% efficiency of a Kaplan turbine isn't marketing speak and really means really 90% of the theoretical max efficiency? Not of the total kinetic energy present? Because I'm reading real wind turbines are usually 75%-80% of the Betz limit. $\endgroup$
    – DKNguyen
    Feb 19, 2022 at 21:40
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    $\begingroup$ @asmaier The 90% probably refers to the hydraulic efficiency. Which is different to the overall efficiency which is lower than the 59% of the Betz limit. I can't really expand on this comment but there are also definitions volumetric and mechanical efficiency. (some authors consider mechanical and hydraulic the same). If you are interested you should research first those definition and their relationship (i.e. overall, hydraulic, volumetric, mechanical). $\endgroup$
    – NMech ♦
    Feb 20, 2022 at 6:41
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"Some manufacturers and inventors have made claims of exceeding the limit by using nozzles and other wind diversion devices, usually by misrepresenting the Betz limit and calculating only the rotor area and not the total input of air contributing to the wind energy extracted from the system."

https://en.m.wikipedia.org/wiki/Betz%27s_law

Betz law works for a free flow. Turbine you showed does not work in free flow to achieve the efficiency you showed, it works with a large vessel, that stops the water from just flowing all around the turbine. By Betz law you need to account all of that structure, not just thr rotor. Then efficiency will be comparable to what betz law suggests.

Why dont we build such turbines in free flow? Because they only work with a large concrete structure around it. That is costly. When alone, 'efficiency' will drop to well below of what Betz law suggests.

Why is it so? Because as you create larger pressure drop, more fluid goes around, and you loose power in a free flow. In restricted flow this is not an issue, flow doesnt get around.

We dont use such turbines in ocean free flow either. We use turbines that are very similar to wind turbines. They are optimal for a free flow, where fluid has an option to go around.

Also specifically your design: fluid will also just go through the intake horizontally, because flow is from one side, nothing will push it vertically, so it will not work in a whole new way as well.

water turbine

P.S. it has nothing to do with fluid being gas or liquid. Its just that rivers of water are more common than rivers of air.

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