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Why are wind turbines constructed like reverse propellers? Would they not be more efficient if they were shaped like an actual turbine with a deep spiraling bl

Why are wind turbines constructed like reverse propellers? Would they not be more efficient if they were shaped like an actual turbine with a deep spiraling blade, placed inside a cylindrical or conical encasing?

jackmott42 4y ago
I believe that if you consider the money it takes to add the conical casing, and instead just make a bigger tri-blade without a casing with that same money, you come out ahead.
- mynegation 4y ago
  Not an expert, but that rings true. Probably environmental impact of a huge volume of air sucked into the cone and fed into a high-speed grinder should be factored in as well.
- deelowe 4y ago
  It's not that. It's because they are optimized for low pressure differentials.
  - s1artibartfast 4y ago
    I'm sure both are true. It doesn't make physical sense or economic sense. Encasing the entire turbine with a shroud the diameter of the blades would be astronomically expensive. However, there is no rule in physics that you can't 10x the windspeed with a focused inlet. Natural mountain ranges around some farms have a similar effect to improve power output, but we would never consider building a natural mountain to improve turbine efficiency
    - 5d8767c68926 4y ago
      > but we would never consider building a natural mountain to improve turbine efficiency Maybe we could? Mining operations already produce huge volume of material from tailings and overburden. Not an outrageous idea to be more strategic in how and where that material is placed. Could create some artificial ranges with better wind properties.
      - s1artibartfast 4y ago
        I mean I think it's outrageous but I would be willing to look at a cost benefit analysis proving me wrong
elil17 4y ago
This is a really good question. The reason regular turbines and wind turbines are designed differently is because regular turbines have a small volume of fluid experiencing a large change in pressure while wind turbines have a large volume of fluid experiencing a small change in pressure. You see the exact same thing in fans - big ceiling fans look like wind turbines and air compressors look a lot like regular turbines. I’m sure the cost of a casing plays into it, but its primarily about the energy efficiency of different blade shapes in different hydraulic conditions.
- Someone 4y ago
  And if it were more efficient, there’s the the challenge of rotating the casing when the wind direction changes.
  - elil17 4y ago
    I mean the whole point of the casing as I understand it is to contain a flow at above ambient pressure - not something that even makes sense for wind. You could have different (more turbine-like) blade geometry without a casing.
pjc50 4y ago
There's a bunch of maths around Betz' law that dictates how the efficiency works, but it turns out that the theoretically optimal structure is a single blade: http://www.wind-works.org/cms/index.php?id=543 http://www.wind-works.org/cms/index.php?id=543 For mechanical engineering reasons, mostly to do with evening out the load at the point of blade attachment, the industry has mostly converged on three. This is also linked to tip speed ratio: http://www.reuk.co.uk/wordpress/wind/wind-turbine-tip-speed-ratio/ http://www.reuk.co.uk/wordpress/wind/wind-turbine-tip-speed-... ; the tip speed is usually several times faster than the wind speed. Remember that the blades are aerofoils, effectively wings. They don't need to touch all the air in their swept area, their effect is given by redirecting the flow of the whole stream of air. Casings are only useful for small turbines operating at high pressures, where the energy lost to spilling over the tip of the blade would be high.
- DIARRHEA_xd 4y ago
  > but it turns out that the theoretically optimal structure is a single blade In Betz's own derivation, the ideal rotor is an "actuator disk", having an infinite number of blades, which have no drag.
- jacquesm 4y ago
  Single bladers have - as do twin bladed machines horrific tower thump.
jjk166 4y ago
For the same reason propellers aren't shaped like compressor blades inside a cylindrical encasing. The cylindrical encasing dramatically reduces air flow, but is necessary for large pressure changes. For a brayton cycle heat engine, like a jet engine, you need to maximize pressure gradient for optimal efficiency, but if you're not burning fuel then you want to minimize pressure gradient and maximize mass flow rate.
ansible 4y ago
You've already received some good responses (particularly from /u/pjc50), but one particular point may not be real clear. Generally speaking, the power a wind turbine can generated is proportional to the entire swept area. So, other things being equal, it is better to go for longer blades to increase the area, rather than trying to "capture all the wind" in a smaller cross-sectional area. So the most efficient use of your wind turbine's mass (which is proportional to cost) is to make it as big as feasible.
timbit42 4y ago
The Halcium Power Pod uses a casing to increase power generation by 40%: https://www.halcium.com/ https://www.halcium.com/

jackmott42 4y ago

I believe that if you consider the money it takes to add the conical casing, and instead just make a bigger tri-blade without a casing with that same money, you come out ahead.

mynegation 4y ago
Not an expert, but that rings true. Probably environmental impact of a huge volume of air sucked into the cone and fed into a high-speed grinder should be factored in as well.
deelowe 4y ago
It's not that. It's because they are optimized for low pressure differentials.
- s1artibartfast 4y ago
  I'm sure both are true. It doesn't make physical sense or economic sense. Encasing the entire turbine with a shroud the diameter of the blades would be astronomically expensive. However, there is no rule in physics that you can't 10x the windspeed with a focused inlet. Natural mountain ranges around some farms have a similar effect to improve power output, but we would never consider building a natural mountain to improve turbine efficiency
  - 5d8767c68926 4y ago
    > but we would never consider building a natural mountain to improve turbine efficiency Maybe we could? Mining operations already produce huge volume of material from tailings and overburden. Not an outrageous idea to be more strategic in how and where that material is placed. Could create some artificial ranges with better wind properties.
    - s1artibartfast 4y ago
      I mean I think it's outrageous but I would be willing to look at a cost benefit analysis proving me wrong

elil17 4y ago

This is a really good question. The reason regular turbines and wind turbines are designed differently is because regular turbines have a small volume of fluid experiencing a large change in pressure while wind turbines have a large volume of fluid experiencing a small change in pressure. You see the exact same thing in fans - big ceiling fans look like wind turbines and air compressors look a lot like regular turbines. I’m sure the cost of a casing plays into it, but its primarily about the energy efficiency of different blade shapes in different hydraulic conditions.

Someone 4y ago
And if it were more efficient, there’s the the challenge of rotating the casing when the wind direction changes.
- elil17 4y ago
  I mean the whole point of the casing as I understand it is to contain a flow at above ambient pressure - not something that even makes sense for wind. You could have different (more turbine-like) blade geometry without a casing.

pjc50 4y ago

There's a bunch of maths around Betz' law that dictates how the efficiency works, but it turns out that the theoretically optimal structure is a single blade: http://www.wind-works.org/cms/index.php?id=543 http://www.wind-works.org/cms/index.php?id=543 For mechanical engineering reasons, mostly to do with evening out the load at the point of blade attachment, the industry has mostly converged on three. This is also linked to tip speed ratio: http://www.reuk.co.uk/wordpress/wind/wind-turbine-tip-speed-ratio/ http://www.reuk.co.uk/wordpress/wind/wind-turbine-tip-speed-... ; the tip speed is usually several times faster than the wind speed. Remember that the blades are aerofoils, effectively wings. They don't need to touch all the air in their swept area, their effect is given by redirecting the flow of the whole stream of air. Casings are only useful for small turbines operating at high pressures, where the energy lost to spilling over the tip of the blade would be high.

DIARRHEA_xd 4y ago
> but it turns out that the theoretically optimal structure is a single blade In Betz's own derivation, the ideal rotor is an "actuator disk", having an infinite number of blades, which have no drag.
jacquesm 4y ago
Single bladers have - as do twin bladed machines horrific tower thump.

jjk166 4y ago

For the same reason propellers aren't shaped like compressor blades inside a cylindrical encasing. The cylindrical encasing dramatically reduces air flow, but is necessary for large pressure changes. For a brayton cycle heat engine, like a jet engine, you need to maximize pressure gradient for optimal efficiency, but if you're not burning fuel then you want to minimize pressure gradient and maximize mass flow rate.

ansible 4y ago

You've already received some good responses (particularly from /u/pjc50), but one particular point may not be real clear. Generally speaking, the power a wind turbine can generated is proportional to the entire swept area. So, other things being equal, it is better to go for longer blades to increase the area, rather than trying to "capture all the wind" in a smaller cross-sectional area. So the most efficient use of your wind turbine's mass (which is proportional to cost) is to make it as big as feasible.

timbit42 4y ago

The Halcium Power Pod uses a casing to increase power generation by 40%: https://www.halcium.com/ https://www.halcium.com/