REQUIREMENT: horizontal spot landing
Launch & Land is one of the great technical challenges for airborne wind energy systems. Launch & Land can be vertical or horizontal.
Vertical take-off (quadcopter style) is accurate but requires a lot of power. If the propellers fail the drone may not be able to land safely. It may also be challenging in the hard wind and gusty conditions that you are likely to encounter over sea.
Horizontal launch (aircraft carrier style) is quite straightforward by catapult. Horizontal landing is very safe but to land properly in any wind and gust condition typically it requires a long landing strip. This is because, as the drone is gliding down almost horizontally, it is hard to control the exact location of the touch down point. To make such a strip accommodate any possible wind direction would require a massive structure.
Ampyx Power has designed a completely new approach to solve the problem, for which a patent is pending. In the Ampyx Power approach to automated horizontal landing, the drone does not aim to land onto the platform, but it purposefully will fly over it. See the above video for an impression. The platform will be only about 20 m long. A precision landing is achieved in the following steps.
The platform rotates to maintain an orientation into the wind before the landing initiates. The tethered drone flies upwind to approach the platform. The winch retrieves the tether at the same time, keeping it taut, and the winch pull is used to help control the drone speed. Just before the drone overflies the platform edge, at an altitude of between 0.5 and 4 m, the winch is slowed down. This creates a slack in the tether between winch and drone. As the drone flies on, the tether gets taut again. The drone passes over a pulley that can slide over a rail but only as it compresses a damper system. In this way, the kinetic energy of the drone is dissipated and the drone drops dead vertically onto the platform. The landing gear absorbs the impact. The tether is then slowly reeled in, funneling the drone into launch position for another run.
Should the drone need to land without the tether, it will be caught in a single-cable “net” erected over the platform, dubbed the ‘wire trap’. Following such an emergency landing, inspection and manual reattachment of the tether is required.
Note that the drone is expected to land only once every few days. The drone could in principle be kept aloft, even without any wind, through active pumping by the generator. This will consume power. Also, the tether and actuator will slowly wear down while the drone is being kept airborne. Therefore we will typically prefer the drone to land if the wind is below about 3-4 m/s. It will then stand on the platform perhaps half a day until the wind picks up sufficiently to launch again.
The below graphs plot the attainable capacity factor, fraction of time spent in the air and the yearly number of landings, for an actual site of about Wind Class II statistics. These parameters depend on the selected cut-in and drop-out wind speeds at pattern altitude. We can thus trade off the number of Launch & Land maneuvers against the time spent operationally aloft (wearing down tether and mechanisms) and the yearly energy produced.