Makani Power is one of six recent US Department of Energy ARPA-E grant winners for their Airborne Wind Turbine (AWT), a tethered flying wing that flies in endless loops around its anchor point on the ground and generates electricity from propellers on board. Makani believes it will be able to produce wind power that is 40% cheaper than conventional wind power and, more importantly, at an unsubsidized real cost competitive with coal-fired power plants.
Think of this as a really big wind turbine. But, instead of needing a big blade going back to the hub, which requires a lot of strength and a lot of weight, only the tip of the blade is used, in the form of a flying wing kite on a tethered line. The kite is flying loops just like the tip of a turbine blade. The tips are the fastest moving and most energy productive part of the turbine; this approach simply gets rid of the rest of the bulky, less-productive blade. The propellers on the wing are turned as it moves through the air, generating electricity and slowing the speed of the wing. The tether serves as both an anchor for the wing as well as the conductor to bring power to the ground for distribution.
The wing is able to self-launch and, since it is not carrying fuel or batteries, it has a very high thrust to weight ratio. The wing is turned vertically and the power-generating propellers act like helicopter rotors to power the wing to its operating altitude of 200 meters (656 feet). When wind at operating altitude drops below 3.5 m/s (7.8 mph), the speed needed to generate power, the wing re-orients into a vertical configuration (hover mode) and is winched back down to its cradle.
You can see a set of animated clips showing just how this works.
Once in flight, the wing is controlled by computer systems which steer the wing to keep it in flight and maintain power generation. "The autonomous controller is responsible for maintaining a stable flight path, while also maximizing power output. To do this, hundreds of times each second the controller calculates the wing’s position and heading from sensor data and adjusts the control surfaces (aileron, elevator, and rudder) to maintain the correct flight path. This fast response allows the wing to easily handle disturbances such as gusts. The control system has been proven, both in simulation and reality, to fly stable and reproducible paths."
We've seen other tethered power generation concepts, spinning blimps, and different versions of giant kites, as well as underwater kites, which are quite similar to the AWT. But the Makani system really seems to have everything pulled together in a complete system. The company has been testing a prototype 10 kW prototype, and will next move to the development of a utility-scale 1 MW system.
Flying at a higher altitude than turbine towers reach means that the less likely to harm birds or bats. Its maximum altitude is 600 meters (1969 feet), which is comparable with tall buildings, radio towers, and other structures, and aviation safety will be maintained with signals and lights in a fashion similar to other tall ground structures.