We may be inching closer to a post-turbine wind energy future if a grant awarded to a University of Bristol boffin for wind-harvesting, ground-tethered drone research is any indication of things to come. 

Bristol flight dynamics and control lecturer Dr Duc Nguyen was awarded £375,000 ($479,000) from the UK's Engineering and Physical Sciences Research Council recently for his work into airborne wind energy systems (AWES), which he hopes will move the emerging concept into the commercial market.

"Airborne wind energy has enormous potential and is anticipated to generate €70 billion [$76 billion] per year worth of electricity by 2050," Nguyen said, but noted that AWES systems have had a problem in living up to their supposed potential.

"New designs have been rapidly deployed for test flights before their flying characteristics are fully understood," Nguyen said. "This has prevented many AWES prototypes from achieving full capacity in operation, leading to early termination of the programme and hindering commercialisation."

Just what's so AWESome about these drones?

The AWES concept is relatively simple: Put a rather hardy drone on a tether, let the wind pull it skyward, and a resistance mechanism in the base collects all the mechanical energy.

There are a few different AWES concepts. One involves the aforementioned glider yanked against its tether, a second involving a drone with rotors used to harvest air energy while flying in a steady pattern, and a third proposes a rotary kite that spins in the air and sends the energy back to a ground cable.

In all three cases, the advantage is that AWES can be sent far higher into the sky than a ground-based turbine, allowing them to catch higher winds and generate energy more quickly. Because the footprint of an AWES system - regardless of the type - is quite small, they're also portable and deployable in remote locations. The drones' flying patterns are generally autonomous as well so they can typically stay aloft on their own for several days with monitoring.

In other words, it's a great concept for expanding the use of wind energy, but with a big fatal flaw, according to Nguyen: No one is bothering to optimize AWES drone design.

The power of mathematical modeling

Nguyen's grant, and a major focus of his research at Bristol, is on perfecting the design of AWES drones. He proposes, and was funded to research, methods known as "bifurcation and continuation," a set of numerical techniques used in aircraft studies to predict oscillation, flutter, and spin that could be applied to AWES systems to improve their performance.

"I aim to use bifurcation and continuation to better predict the flying characteristics of these drones, thereby preventing them from crashing and improving efficiency," Nguyen told The Register. "These techniques will not replace anything at the planning stage, but will complement existing test flights and improve the design of flight control systems."

Nguyen's commercial partner in the research, Norway-based Kitemill, is working on an AWES design of the first type mentioned - the passive drone that generates energy by pulling against its tether. Kitemill's drone has the unique feature of a VTOL system that uses propellers to get off the ground on calmer days, but aren't used to generate wind energy.

Nguyen told us he's working with existing Kitemill AWES hardware to test his bifurcation/continuation method, which will model performance of the system to be compared to real-life tests. 

AWES systems like Kitemill's rely on intricate flight patterns to generate energy quickly, which means the drone's onboard systems have to be perfectly tuned to keep it aloft and responsive to rapid changes in wind patterns without crashing.

"Results from my project will be compared against existing flight test data from Kitemill to see if any undesirable flying characteristic predicted by bifurcation/continuation is reflected in real life," Nguyen told us.

The Bristol lecturer said that current AWES systems being tested are rated for around 25 kW - roughly equivalent to a small turbine. He believes systems available in the next one to three years - like Kitemill's forthcoming KM2 - will be rated for 100 kW, roughly equivalent to a "medium" sized commercial wind turbine.

The hope is that well-tuned AWES systems could supplement the UK's net-zero transition, but even if able to compete with a turbine Nguyen still thinks AWES will remain more useful in edge applications and to supplement turbines.

"The final use of AWES is still under research," Nguyen told us, adding that current predictions estimate it could be used in remote locations where ground-based wind isn't practical. ®

https://www.theregister.com//2024/06/05/airborne_wind_energy_grant/