Usually AWES are classified into two main categories: drag wings (for flygen) and lift wings (for yoyo). But numerous AWES doesn’t enter these two categories.
If the “two main categories” were defined in regard to the swept area instead of the wing features almost all AWES could be included. Let us try:
1.Stationary swept area (almost all methods excepted yoyo)
2.Swept area going downwind (yoyo).
Stationary swept area refers to torque systems like rotors (flygen like Sky Windpower, or groundgens like Daisy or Rotating Reel), carousels and also crosswind flygens (Makani) which should enter a subcategory as drag power systems, but not as one of the “two main categories”.
Swept area going downwind (yoyo) could remain one of the “two main categories” .
Some researches comprising that below could justify the classification with the swept area, as
Stationary swept area refers to some AWES reaching theoretically 16/27 Betz limit (with corrections in the details), while
Swept area going downwind refers to AWES reaching theoretically 4/27 Betz limit, so 1/4, involving in a possible lower power/land use ratio.
Indeed beside it the power/wing area ratio of a wing is certainly important, but the power/space-land use can be still more important, in my opinion.
I’ve reached the same distinction into two categories completely differently.
I’ve called them drag and lift loaded, and I’ve read these terms in the literature.
Drag and lift power AWE devices are mentioned categories from Loyd’s paper (“Crosswind Kite Power”) and are mentioned by the paper of which I provided the link.
Lift and drag categories are often used in scientific circles. But these terms can lead to some confusion as crosswind kites use their high lift to drag ratio to increase the power, and in the same time are also drag devices if the generators are onboard.
Groundgen and flygen categories are also used sometimes. Yet the groundgen Daisy system is closer to the flygen Makani system than a groundgen yoyo: that’s what this linked paper shows, rating the first two for 16/27 and the last for 4/27. Indeed Makani and Daisy have both a stationary swept area. Makani works like a wind turbine with secondary rotors, so by using torque. Daisy and SuperTurbine ™ use torque also, although it is torque transfer.
Making stationary and downwind AWES categories leads to a faster view about the efficiency per swept area unity (leading to power/land use ratio), and makes it possible to encompass almost all of the AWES, even the carousels.
The big red flags for kite-reeling yo-yo systems are:
After achieving lift-based performance, they use it as drag. (thrust)
After achieving rotation, they do not use it, but instead throw it away, and use drag (thrust) to eventually get back to rotation in a roundabout way, eventually driving a rotating generator.
The working surfaces do not hold position relative to the wind, but surrender to drag, moving downwind, reducing relative windspeed, like a savonius, known as inefficient.
Intermittent operation cycle adds more intermittency to a resource (wind) whose main problem was already intermittency.
power is used, rather than being generated, during the retraction cycle, further reducing overall efficiency, capacity-factor, and output…
And that’s just off the top of my head!
Reel-out/in yoyo systems would be one of the two categories as the swept area going downwind category. The yoyo system is the most studied system by both companies and universities.
The other category, as the stationary swept area category, deserves to be more represented. In it, besides crosswind flygens like Makani wings, there are stationary rotating devices.
The “yo-yo” idea is the first and most obvious concept that occurs to almost anyone flying a kite from a reel of string. It doesn’t require that much creativity or cleverness to think of the idea, so it is the most common idea, but the initial apparent simplicity quickly degenerates into complexity. In my opinion, not the most promising approach, but still happy to be proven wrong on that.