As yet, all new ideas in wind energy technology are too small or too expensive to challenge the dominance of the horizontal-axis triple-blade. But on the way - to a 100% renewable future, there are many niches that existing wind turbines are struggling to exploit. A look into the crystal ball of alternative wind power designs.

Nano wind power for low wind speed

The Triboelectric Nanogenerator (TENG) produces renewable electricity at wind speeds as low as 1.6 meters per second - a value at which three-bladed turbines still take their
rotors still out of the wind. Developed in 2020 at the Chinese Academy of Sciences in Beijing, "WindTENG" consists of two thin plastic strips (polyvinylidene fluoride (PVDF) and fluoroethylene propylene (FEP)). When they flap in a light wind and touch each other, they exchange electrons and become differently charged. According to study authors, the triboelectric effect can be stored into electrical energy with a conversion efficiency of 3 percent.

Undoubtedly, despite maximum yield at wind speeds of 8 m/s, this is still a niche that is more likely to charge cyclists' lights and cell phones than to power households. However, scaling up to several square meters could be suitable as a quiet, low-maintenance energy source in cities and nature reserves. But it goes further: The next target is already 1,000 watts.

Vertical axes for self-powered operation

Wind turbines with vertical axes are usually considered ineffective small wind turbines. In 2020, however, the technology advanced to economically relevant sizes for the first time. Agile Wind Power erected a prototype in Grevenbroich with 750 kW output. However, a rare combination of a highly turbulent gust with a change in wind direction caused a rotor arm to break off and set back commissioning until the fall of 2021. At over 100 meters tall, the WTGs are no longer front yard wind turbines. The Swiss company builds its turbines primarily for captive power for large companies, P2X projects or sewage treatment plants. "Permitting is easier there. Normal turbines often don't get approval because of noise or bird protection - it's different with our vertical axis turbines," says Agile Wind Power CEO Patrick Richter. The slow-turning turbines are very quiet, birds can fly around them easily and the assembly on lattice towers saves the complex logistics. The first models are expected to be sold in late 2022, with series production planned for 2024. The difference from earlier vertical axes: Real-time pitch control prevents stall even during slow rotation. Electricity costs should be pushed from the current 6-8 cents per kWh to below 6 cents with the series, Richter said. This should make the turbines worthwhile for industrial customers who want to use them to produce some of their own electricity.

Kites in the high-altitude wind for base load

Fluctuations in photovoltaic and onshore wind power prevent a 100 percent renewable future without sufficient storage. The more hours the turbines provide power per year, the better for the grid. That's why wind kites with 5,000 to 6,500 full-load hours per year could convert the relatively constant high-altitude wind into highly predictable energy. Last year, Skysails brought a 200-kW kite to market, and in 2021 RWE announced a collaboration with the Hamburg-based company. In two years, Skysails then plans to offer a 1-MW flyer on the market. Electricity costs are currently 5-7 cents per kWh; starting with the 1-MW model, Skysail wants to be "well below 5 cents," says Stephan Wrage, CEO of the Skysail Group. The principle is not new: The kite's kinetic energy is transferred to the generator on the ground via a string. The relatively lightweight setup is particularly suitable for islands and offgrid solutions. The kite is capable of flight from wind speeds of 3 m/s, and from 5 m/s it produces more electricity than the turbine itself consumes. "In places where you can't get a permit for a conventional WTG, a kite is still easy to install," Wrage says. "We were even able to solve the use of airspace well in Schleswig-Holstein with a shared space concept." In combination with radar systems, the kite can land quickly from an altitude of about 800 meters when rescue aircraft approach, he adds.

Multirotors for manageable large-scale systems

According to the Square Cube Law, the material volume of a wind turbine grows cubically, while the yield increases only quadratically with diameter. Constant innovations in lightweight construction, for example, have so far been able to solve this problem. But at some point, the physical limit is likely to be unavoidable. Transportation, accessibility, and resident acceptance also limit further size growth.

Multirotors are supposed to be the permanent solution to the problem, explains the Hamburg-based research project X-Multirotor. 20-MW turbines consisting of several small rotors on a single frame could also be cheaper to maintain. However, development has been stalled here since the 2018 Vestas prototype. With the Nezzy2 model from EnBW and aerodyn alone, there was an offshore trial of a twin turbine in 2020. At the beginning of 2022, the original size with twice 7.5 MW is to go into test operation.

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