Minimum Viable AWES

(04/19/2014 Note: Rotary Lift may be even more attractive option for minimum viable AWES.)

An airborne wind energy system (AWES) with a fast motion transfer having a tether and a separate motion transfer belt is more complex than an AWES with a tether only.  Therefore, aspects of an optimal design of AWES with a tether only are discussed below.  The motivation is to develop a minimum viable AWES — an airborne wind energy system that has just marginally lower levelized cost of energy (LCOE) than a conventional wind turbine with the same power, in at least some locations.  After the initial commercial success, this AWES will be optimized further for even lower costs.  Eventually, the development will lead to AWES with a fast motion transfer having LCOE below that of natural gas plants and other power sources.  It is expected that the first minimum viable AWES will have nominal power of 1.5-2 MW and will easily scale to 10-20 MW.  Several choices are available in design of this minimum viable AWES.

Minimum Viable AWES

1.      Ground Generator

Its alternative—an onboard generator—is proven feasible, in principle.  Nevertheless, it presents a number of problems compared with a ground generator:

a)      the onboard generator must be proprietary and is expensive;

b)      the onboard generator makes the wing heavy;

c)      a conducting wire in the tether makes the tether heavy;

d)      because of its heavy weight, the system stops producing power and starts drawing power to stay aloft in light winds, increasing power production intermittency;

e)      the heavy wing together with the heavy tether present a danger for whatever is below them (cars, agricultural machines, buildings, boats, etc.); and

f)       a wing crash means loss of the whole system, so the wing must have aircraft grade reliability, with accompanying costs.

Thus, a ground generator is a clear choice.

2.      Single Wing

Multiple wing systems might have their advantages, but a single wing system must be chosen for the sake of simplicity.

3.      Single Tether

Some AWES prototypes control the wing through two or even three tethers, pulled from the ground.  Multiple tethers increase the tether’s drag, and the tether’s drag frequently exceeds the wing’s drag even for a single tether.  Another problem is that multiple tethers would rub each other in the air and fray.  Of course, a kite attached with a single tether would require a kite steering unit (KSU).  Single tether also allows wing trajectories other than a figure eight, like a flattened circle.

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.

5.      Belt & Sprocket for Higher Angular Speed

Using a drum from which the tether reels off to convert motion of the tether into rotation does not scale.  Instead, it is better to attach a flat perforated belt to the lower end of the tether and let the belt rotate a sprocket of as small a diameter as possible.  The smaller the diameter of the sprocket is, the higher the angular speed of the sprocket.  The system might still need a gearbox but with lower torque and lower ratios.  The belt drum is still present, but it is not subjected to large forces.

AWES Ground Assembly Details