The Offshore wind data (ref a) reveals only a small wind speed difference between 100m (typical HAWT hub height) and 300m altitude (typical AWE height), i.e. 0.5m/s -> 1.0 m/s. The power law coefficient (friction exponent) is between ~0.03 and ~0.1, as opposed to ~0.14 Onshore (ref b). Offshore is a low wind shear environment.
Other points of interest (ref a), at higher altitudes :
wind veer increases
wind speed variability increases (opposite to AWE assumption)
The Scaled Up Makani M600 (from the prototype M30 Wing 7), has a more erratic flight speed profile (ref videos).
The power vs time plot for Makani (ref b) will become more erratic, and more inefficient, further reducing the average power output over the cycle (< ~50% i.e. ~40% used here).
The M600 has a maximum power output of 600kW, with a ~40% efficiency at the generator, the ‘rated power’ output will be ~250kW.
The M600 wing span is ~26m, the aspect ratio is ~18, giving a wing area of ~38m² (ref e).
A propeller blade is ~1m long, with an area of ~0.15m².
Mean altitude is ~255m (equivalent to hub height), with a ~250m diameter sweep, and ground clearance of ~130m.
Tether length is ~440m, giving a mean elevation of ~35°.
To match a conventional wind turbine (HAWT) of 10MW ‘rated power’ output, ~40 M600 units are needed.
AWE total :
generator size => ~24MW
wing area => 1520m².
blade area => 240m² (~1600 blades).
swept area => 730000m² (doughnut/annulus only).
A 10MW HAWT (Vestas V164) has a rotor blade length of 80m, and a blade area of ~280m².
HAWT total :
generator size => ~10MW
blade(wing) area => 840m² (3 blades).
swept area => 21000m² (disc), 164m diameter.
Low wind shear will result in roughly the same ‘Capacity Factor’ for AWE and HAWT at the same site. Increasing the tether length will not improve power output (tether drag dominating).
The M600 extracts very little energy from the wind it intersects (sweeps). The ‘power coefficient’ (extraction efficiency) is ~3%, compared to ~30% for HAWT (the Betz Limit is ~59%). The diameter of the supersize doughnut forces a high tether elevation of 30° -> 40°, and cosine loss of 35% -> 55% (ref b). Therefore HAWT has more fuel (i.e. a higher wind power input), and can downsize the blades for the equivalent power output.
The average ‘power harvesting factor’ (zeta) for the M600 is ~7 (and for HAWT also). The M600 could improve the average zeta by reducing the cosine loss, but would need a tower.
The M600 has a relatively low ‘rated wind speed’ of 11.5m/s, compared to a typical HAWT (~13+ m/s). If this can be increased, it will allow a correspondingly smaller wing to be used.
LandSpace (SeaSpace) for HAWT (with 60 rotor diameter²) is ~6MW/km², and for M600 (with tether length radius) ~0.4MW/km².
In summary, Makani need a bigger wing, a bigger generator, a much bigger swept area (higher altitude), more SeaSpace and a system to smooth the erratic power generated, in order to produce the same ‘rated power’ output as HAWT.
Additionally, most of the Offshore cost is for the installation, cables, and grid connection (CAPEX), along with the OPEX (maintenance costs), and not for the kite (ref c). This favours big MW units, so lots of M600 units will tend to increase both, and give a higher LCOE.
Finally, HAWT Scales with bigger and heavier blades, yet produces a lower LCOE, keeping the ‘square-cube law’ in check. Blades (wings) are not solid, whereas the tether is. AWE limits are set by the tether (ref d & ref e & video), with exponential growth in tether mass (kg) when scaling, reduced aerodynamic efficiency of the kite due to tether drag, and ‘rated power’ output constrained by the maximum tether tension. A comparison with other AWE solutions such as Ampyx or SkySails, would give a similar outcome.
Wind shear is greater Onshore, where ‘Capacity factor’ and ‘Annual Energy Production’ can clearly advantage AWE.
Onshore CAPEX is mainly the kite cost, and not the infrastructure, so small units make sense.
Utility Scale AWE is ‘Scale-Out’ (Solar) and not ‘Scale-Up’ (HAWT).
a) Understanding of the Offshore Wind Resource up to High Altitudes <= 315m (2019)
b) Springer AWE Book, R. Schmehl et al (2014)
c) Makani keynote presentation AWEC 2017
d) Economic assessment of small-scale kite wind generators, Ivan Argatov, Valentin Shafranov (2014)
e) Makani Response to the Federal Aviation Authority
Makani Power, 2012 Testing Program
Testing Makani’s M600 energy kite in Spring 2017