In a short series of three articles Michiel Kruijff covers the Frequently Asked Questions for all aspects of our solution. This Part III is about the tether, ground systems and our certification approach.
May 4, 2017
The Technology of Airborne Wind Energy – Part III: Safe Power
The tether remains attached to the drone in all normal operations (including landing) and most contingencies. The tether is made of Dyneema DM20, a creep resistant version. The bottom part that is cycled onto and off the generator wears down the most. It is replaced once a year. The top part wears little. It is thinner to reduce drag and is replaced once every 5 years. It is coated for minimal wear and features a noise-reducing strake.
The loads that AP-3 must routinely undergo are quite phenomenal for a 350 kg drone. It is loaded by 4200 kg in nominal conditions. A test of the tether release system inside its test rig gives a feel of the magnitude of the tether tension.
Our team has been working with Dyneema for many years:
Five of our team had key roles in the YES2 space experiment that set a world record om 2007 by a controlled deployment of 32 kilometer (!) of tether at an altitude of 300 km in orbit around the Earth. In preparation of that experiment, more than 700 kilometer of Dyneema tether has been deployed under controlled conditions in various ground set-ups. For a detailed description of this work see http://bit.ly/2nVXmjI.
Ampyx Power has been flying since 2008 and completed over 125 flights with our current generation AP-2 aircraft. We have installed an extensive preflight validation process and follow a detailed operations manual for each flight. So far we have been incident-free with AP-2. Currently the AP-2 is flown in service of the AP-3 design effort, validating e.g. precision landing algorithms and power level control. With the recent extension of our operations team, we will be able to fly several times a week from now on.
The generator and grid connection
The winch/generator combination is all about efficiency. The reel-out for power generation is done at up to 10 m/s depending on the wind speed. The reel-in phase of the power cycle occurs however at up to 30 m/s, to minimize the time spent not generating power. In this phase, the aircraft glides towards the generator. Reel-in is performed at only 1% of the operational tension, mostly to keep the tether from sagging and touching the ground. This is a wide range of operation for a motor-generator system. The design challenge is to minimize inertia and to limit the size of generator and back-end electronics to what is strictly required. In fact, a single drone type could operate with different generator sizes and tether diameters depending on the local wind statistics for optimal performance. AP-3 and AP-4 each will have their own optimized architecture.
The AP-3 tether is rolled onto/from a carbon drum that is directly driving the generator. This keeps the system simple and avoids the additional wear and efficiency loss that would be caused by gearing.
The current is converted from alternating (AC) to direct (DC), and is then conditioned to be inserted into the grid as AC. A power storage solution on the DC-bus smoothen the output during reel-out and guarantee that no power needs to be drawn from the grid for reel-in. On AP-3 this power storage consists of a rack of supercapacitors. If the wind exceeds a critical value of about 14 m/s, a power capping algorithm is activated between drone and winch that reduces the pull on the cable to keep the power generated within limits of the back-end electronics. In the event the grid would be unintentionally disconnected from our system, the excess power generated can be diverted to the supercapacitors and down a back-up resistor bank to avoid overheating.
A park of about 100 commercial systems will be operated from a single Supervisory Control And Data Acquisition (SCADA), derived from conventional wind park solutions. One X-band radar per park integrated with satellite data performs local threat prediction and detection that can trigger preventive landing or unmanned inspection/maintenance. The flight of the multiple drones is coordinated within 2 seconds accuracy inside each power generation cycle. They are synchronized in groups, such that the units can be placed about 400-500 m apart without interference.
Engineering approach and certification
There has been a lot of discussion regarding the necessary safety levels for airborne wind energy systems.
For Ampyx Power, there is no question that aviation-style process-based engineering is the way to go.
Our considerations to follow a process-based approach are the following:
1. Only commercial aviation has as yet demonstrated the quality and reliability levels needed also for commercial viability of airborne wind energy systems. We must therefore adopt the lessons learned there. An off-shore park of 100 systems will accumulate about 600.000 flights hours per year. For comparison, a general aviation aircraft has an accident about once every 10.000 flight hours. A commercial airliner based on process-based engineering only once every 1.000.000 hours.
2. We are convinced process-based engineering will not only lead to better quality but in fact can be cheaper than an ad-hoc approach to design, even during the development stage. We can avoid unnecessary delays and expenses by making sure we know why we are designing something the way we do, and by making sure we all use the same data in our calculations.
3. Certification of our drone based on process-based engineering would allow production in large numbers without extensive verification of each single drone built. We will be able to provide evidence they are all exact copies of each other.
We therefore embarked on a certification path with the European Aviation Safety Agency (EASA). Note that all indications are that large rigid drones used for commercial applications in populated areas require aeronautical certification and process-based engineering (EASA E.Y013-01, EASA SC-RPAS.1309-01, EASA NPA 2014-09, EASA Concept of Operations for Drones).
Ampyx Power has invested significant time and resources to develop a quality management system tailored to the needs of highly automated drone development, based on a merging of aviation and space standards uniquely combined with lessons learned from skunkworks experience.
We believe we have found the necessary balance between rigor and pragmatism. We commissioned a dedicated single tool, Xignum, that implements this balance and supports all our process and project data management. With this approach we believe we can approach the quality of commercial aviation at the cost of general aviation.