Why Airborne Wind Energy II

The real question is, Why not?  Both airborne wind energy systems and conventional wind turbines use the same aerodynamic principle: an airfoil, forced to move crosswind, harvests wind energy from the area, many times exceeding the area of the airfoil.  Then, the harvested wind energy is converted into electrical energy with an inexpensive electrical generator.

Before the 21st century, the only way to force crosswind motion of the airfoil was to attach it to a rotating axle, eliminating all degrees of freedom but one.  Various optimizations of this basic idea gave us the modern wind turbine.  Its design suffers from three main inefficiencies:

HAWT Shortfalls

1 – A nacelle with the main axle, gearbox and the generator must be installed on the top of a tall tower.  This tower resists pressure of the wind on the rotor by bending—the most inefficient way of resisting a mechanical force.

2 – The relative airspeed changes over the length of the airfoil from almost zero to the maximum at the tip.  The airfoil is also subject to the bending force.  The center of that bending force is at two-thirds of the airfoil’s length from the axle.  Thus, the airfoil must be in the form of a blade with characteristic variable profile, relatively high solidity close to the center, and high cost.

3 – The rotor of the conventional wind turbine rotates slowly, at 20 RPM or less.  This creates a huge torque in accordance with the equation Power = Torque x Angular Velocity.  Usually, the angular frequency has to be increased to 1,500–1,800 RPM using a gearbox capable of withstanding this enormous torque.  These gearboxes are tremendously expensive and not always reliable.  Some direct drive wind turbines do not use gearboxes, but they simply move the cost of low initial RPM from one place to another.  As of today, direct drive wind turbines are more expensive than similar models with a gearbox.

Thus, the conventional wind turbines remain too expensive to compete with other sources of electricity in most places.  Luckily, in the 21st century, we are not limited to the centuries-old conventional wind turbine design.  The modern synthetic fiber materials allow the airfoil to fly on its own, powered by the wind, while being attached to the ground by a tether.  The advances in the sensors and computing power allow controlling the flight of the airfoil in 3D trajectory with a full six (or even more) degrees of freedom.  Thus, a finger-thick Dyneema tether replaces a 300-ft steel tower, and a cheap nylon kite replaces an elaborate wind turbine blade.  Other expensive parts of the conventional wind turbine go away, too.  In particular, AWELabs shows designs that achieve high RPM without a gearbox.

Besides the costs, conventional wind power suffers from the problem of intermittency.  In any location, the wind at the ground level is sometimes stronger, sometimes weaker and sometimes absent.  The power of a conventional wind turbine fluctuates proportionally to the third power of the wind speed for most of the time.  In regard to the intermittency, AWES beat conventional wind power hands down.  At its altitude, the winds are stronger and more persistent.  Even more, AWES can dynamically change the wing’s altitude, selecting the best wind conditions.  Some locations are especially attractive because of the low-level jet streams.  While this topic requires further research, it is quite likely that well-sited and properly sized AWES can provide electrical power as reliably as a gas power plant: full nominal power for 85% of the time, with well-predicted interruptions for maintenance and extreme weather events, plus a quicker response to frequency regulation requests.

To summarize, airborne wind power is the most promising energy resource at this time.

Airborne Wind Energy Systems