Dear Ahmad,

I don't see any activity in this field and I think I can give a reason, why.

Energy content grows with V3windand if you take a look at current designs, the windmills already operate in power limitation mode from any wind speed >13m/s...25m/s.

Designing a wind mill has to take the average wind speed into account, used for energy harvesting. Designing the mill to operate at higher wind speeds will make all components involved far more expensive, yet will not lead to much higher energy generation as periods with high wind speeds only occur for short periods of time.

In sum, this makes is economically non feasible to go for higher speeds.

On the other hand, increasing the diameter will work at any time and any wind speed, making it the perfect way to increase the energy harvested per site.

Dear Martin,

Many thanks for your interesting answer. There are innovative ideas which suggest to concentrate wind speed, let say using a casing, to several times bigger values. Dealing with such high accelerated wind speeds may rise environmental concerns such as acoustic noise and also structural concerns due to several times bigger cut-out wind speeds. From economical point of view, these ideas can substantially increase wind energy yields even in poor wind sites but challenges remain on how technology can evolve around such ideas.

The German wind turbine manufacturer Enercon already produces wind turbines that keep producing power at wind speeds over 25 m/s. Their turbines gradually reduce the power output at wind speeds between 25 and 35 m/s. They call this "storm control". I think there are some patents involved so other manufacturers don't use this.

The advantage for the owner is that during gusty conditions there is no erratic on/off switching that costs yield and puts a strain on the turbine.  The advantage for the grid is that there is no sudden loss of several GW when a storm front moves over an area with many wind parks. I expect that in the future grid operators will start to demand this kind of grid friendly behaviour.

There is some research to increase cut-off speeds. See for example:

http://proceedings.ewea.org/annual2012/allfiles2/1159_EWEA2012presentation.pdf

http://orbit.dtu.dk/ws/files/64380852/Control_of_variable_speed.pdf

http://orbit.dtu.dk/fedora/objects/orbit:55471/datastreams/file_3742155/content

You can probably find more with google scholar.

Regarding wind concentrators:

Wind turbines with some concentrator system added to it have been proposed since at least three decennia. They never caught on. The reason is that you have to compare the cost of building a wind turbine with a small rotor plus concentrator  with the cost of a wind turbine with a big rotor.  Practice showed that it was just  easier and cheaper to increase the rotor size than to complicate things with heavy and clumsy concentrator rings.

You might be interested in this paper:

INVELOX: Description of a new concept in wind power and its performance evaluation

Daryoush Allaeia,

Yiannis Andreopoulosb, ,

doi:10.1016/j.energy.2014.03.021

The capacity factor can be increased by reducing the size of the generator for a given rotor diameter. This reduces the amount of electricity that can be generated per unit of land, but allows wind energy input to the grid for longer periods.

This is not a response to your question, but does show that compromises can be made to emphasize certain aspects.

Dear Jethro Betcke,

Many thanks for these great valuable information. The links was great!

Thanks so much

Dear H. Douglas Lightfoot,

Many thanks for the inspiring links to INVELOX. it was great to know about 

this design.

Thanks allot