Annual Reviews: Autonomous Airborne Wind Energy Systems

[gordon_sp]

Challenges |…

I feel that the analysis of the three crosswind modes mentioned in the report is inadequate because it does not include the non-crosswind groundgen devices that have been developed. These include Daisy, MAR3, Parachute/Parasail, Superturbine(R), and Kiwee. These systems transfer the problem of autonomous launch and land to that of a lifter kite. The Kiwee system is particularly interesting, because it is the only system where the turbines are oriented to face the wind, eliminating cosine cubed losses. It is also the only system where the tether acts as a support system, as well as a high speed power transfer system using a cable drive.

[PierreB]

I have referred to this document numerous times, particularly Figure 15 of a large flexible crosswind kite from SkySails.

@PierreB

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”.

@PierreB

@PierreB

On the second paper: Figure 15 (average power 92 kW, with peaks of 300-400 kW.

The wind speed was measured a few meters above the ground, and the flights were carried out at several hundred meters altitude, if I was not mistaken in reading both documents.

The wind must therefore be much stronger at the height of the kite, which would considerably reduce the real efficiency such as measured with a wind speed close to the ground.

If an average of 92 kW has been obtained with a wind speed of 12 m/s at mast height, it could mean that at flight altitude, the wind speed might be 18 m/s, since it shifts from between 4 m/s and 13 m/s at mast height to between 6 m/s and 19 m/s at flight altitude, as specified in 4.1.1. Flexible-wing systems.

This would confirm what I think, that crosswind kites have been largely overestimated.

I myself have made a comparison that would give a power (traction force x reel-out speed = 1/3 wind speed x 4/9) of a crosswind kite being only 3 times that of a static kite of the same area.

@PierreB

Losses due to relatively large figure-eight trajectories can be explained by the fast variations of traction force within the flight window. The position of the kite is always changing in relation to the central line of the flight window. This leads to increasingly greater losses as the kite moves away from that central line.

In addition to these losses, there are losses due to the irregularities themselves.

@PierreB

the average traction force of the crosswind kite is only 3 times that of the static parachute kite

@PierreB

Sailing can operate at an altitude between 100 and 300 meters where stronger and more stable winds can be found, delivering 2 to 3 times more power per square meter than a conventional sail.

I agree that Kiwee is a success. As for the parasails, only 3 times less power than crosswind kites for a simpler flying mode makes them a possible candidate for high altitude winds which they seem to withstand.

[PierreB]

I just saw that this document has been discussed: “Autonomous Airborne Wind Energy systems: accomplishments and challenges” - Engineering - AWESystems Forum.