The aviation industry is undergoing an unprecedented challenge to transition to a more sustainable way of flying. NLR is involved with national and international partners in developing technologies to make aviation more sustainable. The knowledge and tools from aerospace research can be used in other industries to contribute to a more sustainable world as well. This allows NLR to work together with Ampyx Power in the wind energy sector.

When the general public thinks about wind energy, the wind turbines which are a common landmark in the Netherlands come to mind. Those wind turbine blades essentially are aircraft wings: similar design, similar materials and manufacturing processes, and similar physical working principles. Thanks to those similarities, Royal NLR has been involved with projects supporting the deployment of efficient and safe wind turbines for a long time.
But in the wind energy sector, in recent years, new concepts have been flourishing to extract energy from wind in a more efficient manner. Solutions include bladeless wind turbines , smaller tulip-shaped wind turbines and wind tunnel towers, which seem to be inspired by ancient Persian cooling systems. One concept seems to be particularly promising: flying a kite high in the sky and using the tension in the kite cable to generate electricity. This is, in simple words, the principle behind the technology of Ampyx Power.

A kite in the sky
Ampyx Power uses a tethered aircraft that is connected to a generator on the ground. The aircraft flies crosswind in repetitive patterns from the ground up to an altitude of up to 450 metres, pulling the cable that drives the generator. After the reel-out phase, during which electricity is generated, the aircraft glides back towards the generator, reeling in the cable using a fraction of the power, and the process is repeated. This kind of power generation systems consisting of a ground tether and an aircraft or kite to harvest stronger wind at higher altitude, are referred to as Airborne Wind Energy Systems (AWES).
Even though the Ampyx Power concept is not a conventional aircraft, it can be seen as an aircraft generating energy. Thanks to the accumulated experience of 100 years of research and development in the aviation sector, NLR has been working together with Ampyx Power for several years, supporting the evolution of their prototypes.

“With the Royal Netherlands Aerospace Centre, we are developing the aircraft layout for our first commercial system (AP4) that needs to be able to supply more than 1500 households with renewable energy”, Eric Trottemant of Ampyx Power explains. “NLR applies its extensive expertise in the field of aircraft design, aerodynamic properties and analysis. The aircraft design is a novelty in the aviation industry as the considerably sized aircraft needs to be agile like a fighter plane.”

“We are developing the aircraft layout for our first commercial system to be able to supply more than 1500 households with renewable energy” – Eric Trottemant

Mario Verhagen* is the NLR project leader of one of joint projects with Ampyx Power. Within his project, NLR assists Ampyx Power to simulate the aerodynamic performance of the aircraft. “Working with Ampyx Power is an interesting crossover from conventional aircraft work at NLR. For our work with Ampyx Power, we are using a state-of-the-art computational fluid dynamics (CFD) tool that has been developed by NLR for fixed-wing and rotorcraft designs. In flight conditions which are typical of AWES, the accuracy of commercial codes’ predictions of the aircraft behaviour is not consistent”. Differences in results can be considerable, leading to radically different aircraft behaviours and performances. Those differences can lead to risky situations, such as the potential loss of an aircraft during flight tests or operations, with related safety risks for operators or population on the ground. Those risks can be minimized by using NLR’s expertise. Mario adds: “In addition to simulation work, Ampyx Power is supported throughout the entire process of a flight test campaign, starting at developing a test plan, then providing expertise in all aspects related to instrumentation, calibration, data acquisition and data processing, and finally during execution of the test campaign.”

Aerospace research applied to wind energy
The collaboration with Ampyx Power is just one, very innovative, example of how aviation research contributes to developments in the wind energy industry. According to Jos Vankan, senior R&D engineer: “The range of activities NLR has been involved in for the wind energy sector is broad. Aerodynamics and structural analyses similar to those done for Ampyx Power have been performed for blades of traditional wind turbines. In addition to this, the capabilities that NLR has developed in the aerospace sector, in terms of composite design and manufacturing, have been used towards the design of various types of AWES”.

The effective deployment of AWES technology requires further crystallization of various certification aspects, as regulations for those systems are being developed together with the technology. This paves the way for a more intense collaboration in the future between aerospace and wind energy sectors. A challenge is that airborne systems need to interact with the surrounding environment in a safe way and with minimal hindrance to the citizens, for example without noise or with no risks of collision with buildings, and minimal risk to local ecologies and aircraft in adjacent airspace. This can be managed through appropriate air traffic management or potentially through the application of GPS-location tracking solutions, as currently done for drones. NLR’s expertise in the fields of certification, safe operations aspects and noise, are a major resource for companies within the wind energy sector.

For Mario Verhagen, the work with Ampyx provides an opportunity to further validate NLR’s state of the art CFD tool in a novel aerodynamic domain of low speed high lift conditions. Validation of the CFD tool is of interest in this domain since also future aircraft concepts may operate in these aerodynamic conditions. Mario hopes in several years the longstanding and fruitful relationship between Ampyx Power and NLR will lead to a product that is commercially viable, generating clean energy for entire communities on a large scale.

*) While preparing this article Mario Verhagen has left NLR. For further information about this article you can contact Stevie Ray Janssen (

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