As everyone knows “Crosswind Kite Power” is the title of Miles L. Loyd’s seminal paper, and is the basis of a good part of AWE projects.
“Crosswind Kite Power” has been discussed in many threads but among other considerations.
This is why it may be interesting to return to the basics and compare its supposed efficiency with the results of two test reports that I had the opportunity to comment on but in topics for which the efficiency of the “Crosswind Kite Power” was not the first consideration. These two publications are, to my knowledge, the only ones reporting crosswind tests concerning flexible kites in yo-yo mode (pumping mode) flying by figure-eight. Although almost 10 years apart, the results were very similar, all things being equal. A comment and two quotes from another comment are therefore placed just after the two documents, and relate the two test reports.
The main power equation is summarized in the page 3 of
Page 3:
The power P that can be generated with a tethered airfoil operated either in drag
or in lift mode had under idealized assumptions been estimated by Loyd [8] to be
approximately given by P = 2/27ρAvw³CL (CL/CD)²
where A is the area of the wing, CL the lift and CD the drag coefficients, and vw the
wind speed.
Subsequently, to begin refining the basic formula, we multiplied it by a coefficient integrating the cubed cosine loss, for example 0.65 for an elevation angle of 30 degrees.
See also the second paper, figure 15, and page 20:
During a specific measuring campaign, an uninterrupted automated flight of 42 hours featuring day and night flight with an average power production of 62 kW has been achieved, in wind conditions varying between 4 m/s and 13 m/s (one minute average measured at 10m height) and estimated wind speeds at flight altitude of 6 m/s to 19m/s, showing the robustness or the implemented flight and power cycle automation algorithms and all system components at relevant load condition.
The measured power is based on the height of the mast (10 m), with an average wind speed of 12 m/s, among wind speeds varying “between 4 m/s and 13 m/s”, and apparently not the “wind speeds at flight altitude of 6 m/s to 19 m/s”.
The main question (hence the topic name):
But what are measurements taken a few meters from the ground worth when the kite is flying hundreds of meters from the ground, where the power (wind speed cubed) is of the order of 3 times greater or even more?
Usually the wind speed of a conventional wind turbine is taken at the nacelle level.
It would therefore be more judicious, if possible, to place the anemometer on the kite, take the wind speed when stationary and in the middle of the flight pattern. In these conditions the efficiency would then likely drop. And knowing the reasons could help. I myself had noted the irregularity of figure-eight flight, preferring by far the more regular Low radius loop one. But there may be other reasons: perhaps the winch loses a lot of its effectiveness as the kite accelerates and slows down, unless the unwinding speed is stabilized in spite of kite speed variations.