Question about power curves

I am wondering why windmills have a nominal windspeed for max power at 12 m/s wind.

I would assume most sites have frequent winds beliw 12 m/s. One could generate the same power at lower windspeeds by adding an extra blade?

Once the nominal power is reached at 10-15 m/s wind speed, it is capped until a wind speed of around 25 m/s which corresponds to the stopping of the turbine. “One could generate the same power at lower wind speeds” by increasing the area swept by the wind turbine, so by using a larger rotor.

Yes. But why 12 m/s? Many sites dont get this windspeed very often. Could you not increase the output of a HAWT at lower windspeeds eg. by increasing the wing area (add blades)?

That way you would make less power more of the time. But it could be profitable.

What factors are limiting the minimum windspeed for a HAWT? Of course I am implying that I believe the calculation may be completely different for AWE, in a way that may be an advantage to AWE.

If the limiting factor for HAWT is the swept area it may not make sense to make a bigger blade without also increasing the power output. So the swept area is the limiting factor rather than anything else. For AWE, swept area is not limited in the same way, our utilization of the available wind is much lower. If the limiting factor is that you’d rather make more money at a lower capacity factor, then there is no difference between AWE and HAWT.

More info here: Power Density Function

I think the nominal power (10-15 m/s wind speed, generally 12 m/s) is studied for all components of the wind turbine, comprising the generator. Unfortunately for wind energy wind speed is varying a lot, leading to a reduced capacity factor. I think 12 m/s is a compromise allowing to optimize kW/h production during the desired time, by using as little material as possible. There are no often winds above 12 m/s but they lead to a significant part for generation.

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Do you mean to increase solidity, that is, the ratio of the area of ​​the blades to the swept area?
Increasing solidity does not necessarily increase efficiency: it is often the opposite. Efficient wind turbines have a low solidity, allowing the blades going fast (several times wind speed) at high efficiency (Cp about 0.45, Betz limit being 0.59). Unlike that drag turbines like Savonius type have a high solidity and both low efficiency (Cp about 0.1) and speed (wind speed for the best).

We can clearly see the difference in method of measurement between a HAWT and an AWES. On a side (HAWT) the swept area is considered, leading to possible variations of the blade area; on the other side (AWES) the wing area is considered, leading to possible variations of the swept area.

Currently AWES use a tiny part of the available frontal airspace of wind. As a result the wing will roughly keep its efficiency according to its area, unlike the blades of a wind turbine.

What I’m thinking: The 12 m/s evolbed as windmills were optimized for the design. It may not make sense considering price drops when its windy, even if you collect
more kWh in the windmills lifetime.

As we know, this optimum occurs for windmills when extracting the most amount of energy from the swept area. This is why HAWT are immensely efficient.

Now I am speculating: but in order to get more energy from the windmill in lower wind speeds, the only real option is to increase the swept area. But once you have done that, why cripple the larger HAWT at a lower max power output.

So there is no real possibility in decreasing the windspeed where you get max power.

Anyways, my point in the end is that for AWE, it probably doesnt make sense to optimize for the same nominal windspeed. We are extracting a larger area «for free». Scaling is limited more or less by selecting a power then selecting a kite and components to match that power.

The nominal windspeed is very dependent on the choice of these components. For sure, a lightweight kite with larger wing area moves the nominal wind speed towards zero. In theory you can select any nominal windspeed you like with AWE. Even landing at a high windspeed (eg 18 m/s) may make sense for AWE; this energy has negative price anyways, and AWE could land and did not have to endure harsh winds. The build thus may be lighter. HAWT must endure worst case, as @dougselsam is keen to point out.

To summarize, I believe AWE may be designed to optimize the power curve to get more power from lower winds and possibly at the expense of losing high wind capability.

This advantage may perhaps be described by a price factor to the LCOE representing the price advantage of producing more power in lesser winds. This in addition of course to generating more energy perhaps overall (as lower winds are more common).

The notion that there is no more energy to extract does not match well to AWE pricing. For AWE, it more boils down to component cost relative to a power output.

Id be glad if anyone could point me to sources describing why HAWT are optimized for 12 m/s. So I dont need to speculate so much…

Solution - Kitemill :

KITE

7.5m wingspan

4 propellers for vertical takeoff and landing

WINCH

30kW nominal power

7500 N nominal traction force

4 m / s nominal feed out rate

OPERATING PARAMETERS

5 m / s wind - start production (cut-in)

12 m / s wind - full power

It does not seem to be that different from the nominal power (also about 12 m/s) of a HAWT.

Well this is just an example of what Kitemill is doing right now. We are not optimizing for LCOE, rather development costs.

Also, the opinions in here are my own, not those of Kitemill’s. I am just stating my employer as a disclaimer.