Below are some quotes from the document.
Page 1/6:
Airborne wind energy (AWE) is an upcoming renewable energy technology which aims at harvesting the steady and strong high-altitude winds that cannot be reached by conventional wind technology, at only a fraction of the resources.
Page 4/6:
• The tether length is bounded from above by a value lt,max = 700 m. For the M-AWES, this constraint is active at the end of the reel-out phase.
To begin with, this paper (and more broadly AWE as indicated in the first quote which makes sense to me), is about devices capable of reaching high altitudes.
Now about your observation Tallak, the table 1 page 6/6 replies in some way:
According to this table, a power density of 7.3 MW / km² is possible by using M-AWES flying a circle radius of 18.4 m, and comprising two aircraft of 5.5 m span each. In contrast only 0.2 MW / km² is possible for a S-AWES (a single aircraft) which flies a circle radius of 46.5 m. The same table indicates a value of (only) 8.4 MW / km² by using far larger aircraft of 26 m span in M-AWES (two aircraft) configuration.
From this a temporary conclusion could be that Scaling by size is not necessarily required. This is rather a case of Scaling in numbers rather than size. The wingspan of Kitemill KM1 is 7.4 m if I’m not mistaken, which is a little over 5.5 m. So if we take the final plant of the paper as a model, KM1 already has the required dimensions. This is an example of a set that can determine the characteristics of a unit. This may give good prospects for the rigid kites that have so much trouble scaling up.
I would like to make a comment on the statement on page 5/6, which may also affect the aforementioned Table 1, concerning a single aircraft:
The dual-aircraft configuration thus allows the system to fly extremely tight circles.
It is also possible with a single soft kite as I experimented it. For a single aircraft I don’t know exactly, but I guess that (as for a soft kite) a far smaller circle radius of flight would be possible for a slightly lesser efficiency. On the other hand, with either a soft or rigid kite, the main tether would wobble a bit, which is noticeable on the video.