April 18, 2017

The Technology of Airborne Wind Energy – Part I: Launch & Land

In a short series of three articles Michiel Kruijff covers the Frequently Asked Questions for all aspects of our solution.

By: Michiel Kruijff, Product development Ampyx Power

OBJECTIVE: pre-commercial demonstrator of safety and autonomy

STATUS: construction started

In short, AP-3 is a 250 kW system, using a 350 kg drone with a wingspan of 12 m. AP-3 is designed to be a demonstrator of safety and autonomy. It is the pre-commercial prototype. It is designed to be certifiable according to aviation safety standards and features the exact same avionics that the commercial AP-4 will use. We have already started building AP-3. It should roll out late 2018. Follow the progress and find more technical specs here (click “EN” for English).

With AP-3 we will showcase full functionality and autonomy. This means, 24/7 automatic operation without human intervention. The full cycle will be automated: launch, power generation, landing, repositioning and relaunch. Including a safe automated response to any off-nominal condition, such as a sudden drop in wind or a failure of one of the systems. We will develop gradually the software and control algorithms to be certified.

AP-3 will safely land if the tether breaks or if an actuator of a control surface gets stuck. The automatic landing has to be spot-on: it will touch down within meters from the target. In this way we can afford to have a very small landing platform.

We will fly AP-3 in Ireland, on the site we will develop with E-ON. There we will build up AP-3 flight hours to prove the avionics. We should eventually be able to fly the AP-3 at night and in extreme weather for days in a row. We aim to fly sufficient hours to gain meaningful experience on operations and maintenance aspects.


OBJECTIVE: commercial prototype

STATUS: design started

AP-4 is a 2 MW system with about double the wingspan of AP-3. AP-4 is the commercial prototype. The avionics and most of the architecture of the drone and of the launch & land system will be inherited from AP-3. The ongoing conceptual design focuses on the structural layout of the drone, as well as on the generator design and on the actuators, all much larger than AP-3. AP-4 will be certified for commercial operations with EASA (the European Aviation Safety Agency).

AP-4 will be optimized for the business case of early offshore repowering. This means, we will place it on old poles in the North Sea that already exist and are already equipped with cabling, but that are too old to sustain any longer the tall towers of conventional wind turbines. Hundreds of such poles will become available the coming decade.

The same sizing of 2 MW is also cost-optimal for on-shore deployment. Our system performs best in a park configuration (5 or more units), so we need open space. That’s why, at least in Europe, we are looking for offshore deployment first.

With the early AP-4 prototypes we plan to gain experience in offshore operations and maintenance, such that we can substantiate how we will provide energy at half the cost of conventional wind turbines.

Launch & Land

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.

Landing will very rarely be forced by the presence of too much wind. Our system is designed to operate at wind speeds above 25 m/s (50 knots) and can make nominal spot landings in gusty winds up to 20 m/s (8 Bft).

Our performance claims are based on simulation of AP-3 landings with Monte Carlo analysis that include severe gusts and sensor error models. Our AP-2A1 prototype, that we operate from Kraggenburg in The Netherlands (see picture above), serves as a test platform for the AP-3 algorithms and routinely performs automatic precision landings, within meters of error typically.