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\"Soft kites have advantages of relative cheapness, ease of prototyping, higher crash survival rates and lower crash potential liabilities.

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Fabric is cheaper to transport and build with than solid materials.
It’s fast to stitch together another kite, use kitesurfing kites or have a kitesurfing company build specific kites for you as UDelft does.
When fabric hits the ground it crumples into fabric as opposed to stray shards of shattered wing. Combined with the slower speeds, this increases the likelihood that a fabric wing that crashes can be used again without problem. That said, inflated spars pop, wings rip and battens break or bend.
When fabric hits something it crumples. Combined with lower speeds, the likelihood of causing significant damage is reduced substantially.

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However, soft kites don’t fly as fast, don’t fly in as wide a range of conditions, don’t last as long before replacement and are hard — perhaps impossible — to reliably auto-launch.

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Lower speed reduces the potential power output for crosswind generation.
Reduced viable flight conditions due to less structurally robust wings reduces the capacity factors annually because of weather-related grounding.
Reduced life span reduces capacity factors annually due to maintenance-related grounding and increases costs due to regular replacement (balanced by lower cost of the fabric kites). As a data point, the highest quality paraglider wings must be replaced within 400 hours of flight time due to fabric degradation.
The last one, auto-launch, is a real problem for soft kites. To launch, a soft kite has to be furled correctly and there has to be sufficient wind to loft it. Given turbulence near the ground and lower wind speeds near the ground, this is a non-trivial (by which I mean very hard) problem. No one has really solved this problem as far as I can tell. The three most complete systems — Skysails, Wind Lift and Kitegen — all require manual intervention and stronger winds near the ground for launches. Kitegen has two or three ideas for auto-launching in lower ground winds, none of which appear to be viable.

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Hard wings have the advantage of higher speed in the air and with that higher speed comes greater energy potential.

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Higher air speed comes with airfoil rigidity and reduced drag.
Energy potential increases with the cube of velocity of air, so rigid wings have significant potential advantages.

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However, hard wings require onboard aileron controls, are more likely to break if they hit something and more likely to break the things that they hit.

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While soft wings can be controlled by dual tethers — although several are not –, hard wings require onboard power and intelligence to fly. This increases complexity and weight in the air. Complexity increases the chances of failure. Weight reduces efficiency.
Hard wings, if they hit something such as the ground or tethers of other hard wings, are much more likely to suffer catastrophic failure. Failures which soft wings shrug off turn hard wings into splinters. This requires hard wings to have much more built-in crash avoidance, which increase initial engineering and costs.
Hard wings, if they hit something such as people or buildings, are much more likely to cause damage. This increases liability risks, and hence insurance.\"

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from http://barnardonwind.com/2014/01/06/airborne-wind-energy-a-collection-of-challenging-compromises/

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Rigid vs. flexible/textile

a continuum, not either-or choices... As one example TwingTec uses an inflatable rigid beam to give structure to a rigid wing comprised of very soft materials. As another, many of the pure soft wings have inflatable spars adapted from kitesurfing, or directly use kitesurfing kites.

Corwin Hardham | Makani Power

Endurance of materials is only one of the reasons why we moved away from fabric structures. While there is good anecdotal evidence that fabric structures can last, the peer-reviewed, published data on rigid structures provides a much more reliable framework to make estimates of life.

Nonetheless, the main reasons why we moved away from fabric structures are: controllability (repeatability), safety and performance. A few qualitative remarks on controllability and safety: 1) We found curvature control on textile wings was highly sensitive to windspeed and generally completely uncorrelated to control input, and 2) textile wings operating in crosswind flight demonstrate survivability on impact similar to rigid structures. These observations stem from many hours of flying in a large range of wind speeds from 2-25 m/s. Making power requires speed and repeatability in control which are two things that rigid structures provide to great extent.

In terms of performance, the estimate of wing size required for the MW system that I listed earlier was highly optimistic in favor of textile wings. The estimate was based on doubling the measured drag of Wing 7 which is a highly evolved and clean airframe. In 2008, we tested every textile wing that we could find (kitesurfing kites from nearly every manufacturer; custom kites from Pete Lynn, Don Montague and myself; and bridled ram air kites). We never measured a wing performance within a factor of 3 of the performance I describe here. Hence, the 533 m2 is likely a gross underestimate of the scale needed.

I recognize that other groups have claimed to have reached better performance with textile wings. To this, I would welcome a chance to see the power over the full winch out/in stroke. We were able to easily show very favorable power peaks on the reel-out stroke, but the time and power required reeling back to the same point did not make favorable average power. Our results were with fully active angle of attack control, fully autonomous (adaptive power tracking) flight control and a brushless winch drive with a high quality gear box.

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4. Wing Type: Rigid (glider) vs Flexible (kite)

This question is open. Both types can be used, and neither of them has a clear advantage.

Parameter \\ Wing Type

Rigid Wing

Flexible Wing

Cost

Higher

Lower

Longevity

High

Unknown

Glide ratio estimate

12 (better)

6 (worse)

Safety on crash

Unsafe

Safe

Wing restoration on crash

Unlikely

Likely

Aerodynamic stability

Stable

Varies

Flyback maneuver

Easy

Difficult

Control actuators

Plane-like (more available)

KSU + lines (less available)

Launch & landing

Varies

Varies

Glide ratio incorporates both wing and tether drag. The tether drag eliminates most of the rigid wing’s advantage in lift/drag ratio. An aerodynamically streamlined tether can eliminate most of the tether drag, but such tethers exist only on the drawing boards. While the rigid wing is less safe for objects on the ground than a flexible wing, it is still much safer than a wing of any kind with onboard generators.

It should be noted that there are multiple kinds of the kite wing, and some companies (like TwingTec, Switzerland) propose specialized AWES wings that do not fall into either of these categories.

http://www.awelabs.com/minimum-viable-awes/

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Advantages of flexible structures are:

Can crash without having damage to the system
More safe to operate for the people on the ground
Lightweight
Small in volume for transportation
Cheap materials

Cheap in production

Disadvantages of flexible structures are:

Shorter lifespan
Modeling the aerodynamic behavior is challenging

A lower Lift-to-Drag ratio compared to a rigid wing in general

http://www.kitepower.eu/technology/2-core-concepts/45-flexible-structures.html

Systems have been developed to increase the rigidity of soft wings eg. by (auto) inflating them:

e.g. Kiteboat\'s kite Inflator: http://project.kiteboat.com/category/misctests/

[IMG Source] http://barnardonwind.com/2014/01/06/airborne-wind-energy-a-collection-of-challenging-compromises/

AWE Map