With the power of the wind
107 meters (350 feet). This is the length of a single rotor blade of the world’s most powerful wind turbine. When all three blades are installed, they describe a circle in the sky with a diameter of 220 meters (722 feet). To put these dimensions into perspective: the world’s largest Ferris wheel, the High Roller in Las Vegas, is “only” 167 meters (550 ft) tall.
The record-breaking Haliade X wind turbine from American multinational conglomerate General Electric (GE) is currently being tested at the port of Rotterdam in the Netherlands. It has a total height of 260 meters (853 feet), which makes it just a little smaller than the Eiffel Tower. Most impressive, though, is its rated capacity of twelve megawatts. Not too long ago, such behemoths were regarded as mythical creatures. Today, they’re reality – and far from marking the end of their evolution. Other manufacturers have already announced 15-megawatt machines. According to GE, Haliade X generates electric power for up to 16,000 households and it takes just three rotor revolutions to charge the battery of a Tesla Model 3 – providing the car with a range of more than 500 kilometers (311 miles).
Offshore double pack
Aerodyn engineering based in Rendsburg, Germany, puts two wind turbines on a floater. This twin concept reduces costs and increases the power rating, according to the manufacturer. Due to its floating structure, the system can be used in in deeper waters. Its name is Nezzy2. The towers of its two wind turbines are separated from each other by 90 degrees, which is reminiscent of the fork of a tree branch. The two self-aligning rotors turn in opposite directions and are controlled so that they won’t get in each other’s way. This rotational concept prevents slipstream between the twins and stabilizes the floater. However, it’s still uncertain when the first twin system will be deployed offshore. A 1:10-scale prototype with a height of 18 meters (60 feet) was tested this fall in the Greifswalder Bodden lagoon of the German Baltic Sea and proved its seaworthiness even in hurricane-like winds. China has already expressed an interest in commissioning a 1:1 version with a height of 180 meters (600 feet) for testing. This full-scale system is supposed to be able to withstand even waves of 20 meters (65 feet) in height. Steel mooring cables in the seabed hold the system in place.
Obviously, such giants are installed only on the high seas, far away from coastal waters, where they do not bother anyone. Offshore systems are also better for wind yield, delivering up to 4,500 hours of maximum power per year compared to just around 2,000 supplied by those onshore. This perceived megalomania has a logical background. The larger the systems the more power they generate – plus, the simpler the logistics chain: “Larger systems have massive advantages,” says wind power specialist Manfred Lührs from the consultancy firm 8.2. “They optimally exploit the sites.”
Wood meets wind
Before wind power systems can provide renewable electricity, they generate a negative climate footprint: The installation of the huge masts and foundations of concrete and steel results in 2,000 and more metric tons (2,200 short tons) of climate gas emissions, depending on the size of the systems. Due to the utilization of wood, which binds CO2 during the growth stage (notably, one metric ton per cubic meter/63 lb per cubic foot), wind power systems can be established in climate-neutral ways. Plus, wood as a material offers other advantages:
- The modular design and the material reduce the weight of the towers. The composition of the modules does not presuppose any special quality of the access routes because the modules are assembled on-site, which eliminates the need for heavy haulage.
- A wooden tower has longer fatigue life and is not prone to corrosion.
- Wooden systems are much easier to dismantle and dispose of than systems of concrete and steel. Appropriate combustion of end-of-life system components will not cause any additional environmental burden and the energy can be used sensibly.
- Hub heights of up to 200 meters (650 feet) can also be achieved with this natural material.
- Wood is even suitable as a material for foundations and rotor blades.
In spite of these advantages, wood has not been able to crack the dominant position of steel and concrete with large-scale wind power systems because these materials are established in the marketplace at high levels of vertical integration. The 1.5-megawatt system from German manufacturer Timber Tower that was commissioned near Hanover in 2012 has so far remained a pilot project in spite of announcements that it would be followed by others. In Sweden, Modvion, an engineering and industrial design company, commissioned an initial prototype using prefabricated glued laminated timber modules in April 2020. The first projects with heights of up to 150 meters (490 feet) are planned to be implemented for Swedish energy corporations starting in 2022. Modvion expects that wooden wind energy towers cannot just be constructed in more climate-friendly but also in clearly more cost-efficient ways than those of steel. Wood is a work in progress – but still waiting to achieve its breakthrough in the wind energy sector.
Europe (including the UK) – or more precisely, the North Sea – is the epicenter of the offshore wind industry with a currently installed rated capacity of around 22 gigawatts. The water is shallow, and the demand for electricity high. Shallow in this case means 40 meters (131 feet), which is ideal for offshore wind power. Large steel foundations can be placed on the bottom of the sea here or gigantic steel tubes, so-called monopiles, be driven into the seabed to stabilize the systems.
Other oceans are as shallow as the North Sea only in very few places. Nearly everywhere in the world, the water is too deep for large tripods supporting the wind turbines. About 80 percent of the worldwide wind resources are located above waters that are at least 60 meters (200 feet) deep. But there’s a solution to this profound problem: floating wind turbines.
Dozens of prototypes are currently being tested worldwide, some of them with capacities of up to eight megawatts. In Asia, Europe and the United States, commercial projects in the gigawatt range are in the planning stage. By the end of this decade, floating wind turbines with a combined capacity of 6.2 gigawatts could be built, according to the Global Offshore Wind Report. This roughly corresponds to the capacity of eight coal-fired power plants.
Due to the huge rotor diameters of modern wind power systems of 200 meters (650 feet) and more, the rotor blades during one rotation cycle are exposed to heavily varying wind speeds, especially on the tips of the blades. A control concept that takes this phenomenon into account cannot just improve the electricity yield of a turbine but also avoid fatigue-induced damage due to pitch misalignment. So far, turbines have typically been using control systems that simultaneously adjust all pitch angles to the prevailing conditions, which may significantly reduce energy yield. With modern large-scale systems, individual blade pitch adjustment has been gaining traction. Its potential benefits: higher energy yield, reduced fatigue, lower noise emissions and a better flow field behind the rotor.
European countries are pushing the technology. According to Kimon Argyriadis from the consulting firm DNV GL, Europe’s western coast and the Mediterranean Sea are deep waters with good wind conditions and close to major consumers. What’s more, Europe was previously the technology driver of ground-based offshore wind power and is not inclined to miss out on the floating wind turbine business. This comes as no surprise because experts expect the technology to become standard with incredibly low kilowatt hour prices of just a few Euro cents. The twin-rotor floater Nezzy2 from North German engineering company Aerodyn (see info box on page 83) is such a European project. Klaus Ulrich Drechsel, an offshore expert at electric utilities company EnBW that acquired interest in Nezzy2, sees major potential in such systems because they can be used in virtually all coastal regions of the world, even those with steep drops that soon lead to great depths.
High in the wind
Although numerous airborne wind energy systems have already been deployed, all of them – strictly speaking – are still prototypes. Some of the technologies they use exhibit major differences. Two basic approaches have emerged: One of them features an airborne generator producing in-flight electricity which is then conducted to the ground via special tethers that simultaneously transport the electric power. The other approach prefers power generation on the ground. While the kite or wing rises, it reels off a rope that in turn drives a generator producing power in the process. This principle is also referred to as “yo-yo” because the wing keeps being retracted – without generating energy during this phase. Master of the airways is the U.S. company Makani that was supported by Google for many years. At the moment, the Americans’ M600 flyer features a carbon fiber wing with a length of 26 meters (85 feet) hanging on a rope. Ideally, in other words, in adequate wind conditions, eight generators installed on the wing deliver an impressive output of 600 kilowatts.
Harnessing upper-level wind
The floaters’ potential can be driven to even greater heights because the wind blows more intensely and persistently at higher altitudes. That’s why modern wind turbines are installed on towers that are as tall as possible. But there are limits to how tall they can be, so different ideas are needed. One of them is airborne wind power generation (see right-hand column). These machines look completely different than the customary three-bladers. Just like kids’ kites, airborne wind turbines are tethered and climb high into the sky. The tethers, however, are up to 600 meters (1,970 feet) long – and have to resist awesome forces. Although the commercialization of airborne wind energy systems is not yet in sight, its advantages are impressive, especially their minimal use of materials. They neither need a tower nor lengthy blades, and the foundation can be a lot more light-footed, too. Best of all, though, is the fact that, thanks to their higher yield, two-megawatt airborne wind power systems could achieve higher annual electricity outputs than conventional three-megawatt wind turbines. This is precisely the argument Fort Felker, CEO of airborne wind power pioneer Makani, uses to promote the technology as one that enables more energy to be harvested with less powerful systems.
Resisting the elements
Rotor blades consist of fiber-reinforced polyester or epoxy resin. Rotational speed is particularly high in the area of the blade tips (70–80 m/s; 230–260 ft/s). The force multiplies when rain – or worse yet, hail – hits them, which causes the surfaces to be attacked. This in turn increases friction drag that reduces the system’s efficiency. Intrusion of water in the blades is even worse. If lighting strikes, which is not uncommon in such exposed high-rise structures, the water trapped in the blades can cause the overstretched outer skin to crack and burst. Even if this should not occur, the water deposits change the distribution of weight. The resulting imbalances and vibrations stress the gearboxes and bearings, which reduces the life of the entire system. In an approach to solving this problem, the Norwegian research organization SINTEF has successfully experimented with diverse nano particles such as carbon nano tubules and silicon dioxide particles to make coatings more resistant against the impact of precipitation.
Energy for green hydrogen
All the wind turbines, whether standing on land, floating on the sea or even flying through the airways, could supply huge amounts of green electric power. Green electricity could charge all the battery-electric vehicles or – by means of electrolysis – produce the coveted green hydrogen that may either be used in the steel and cement industry or be reconverted into electric power in fuel cells. In this way, wind power might lend wings to the planet’s decarbonization. In any event, wind as an energy source is available in abundance. Experts tells us that 18 terawatts of installed capacity would be enough to cover the world’s primary energy requirement – the space available for this purpose would suffice for generating 400 terrawatts.
90 percent of global trade is handled by ship. The potential of reducing emissions by revitalizing sailing cargo ships is correspondingly high. Although the planned Oceanbird (pictured) is making slower headway than current motor ships, it does so with 90 percent less CO2. While the Oceanbird uses metal telescopic sails, other ideas feature huge kite-sails or sail-shaped high-rise hulls. At the same time, the search for maritime routes with favorable wind conditions is in progress.
Wind energy in mobility
There are other industries besides the energy sector that can use the power of the wind, and not necessarily just when converted into electricity. Examples include sea trade and aviation. Ocean-going cargo ships haul about 90 percent of all the goods we consume – from the aluminum foil we wrap our lunch in to industrial chemicals to tooth brushes. Plus, what used to work in the old days, still works today: sailing. Reliable winds blow around the globe, so more and more freighters are now putting to sea powered by the wind or at least using its force for assistance. Some of them rely on special sails like the Dyna-Rigg or kite rigs such as the SkySail. The planned cargo vessel Oceanbird with a length of 200 meters (650 feet) features five wing sails, each up to 80 meters (260 feet) tall. Due to using wind power, the Oceanbird’s carbon savings potential amounts to 90 percent. Others rely on the more than 100-year-old Flettner principle (see also page 37) to save fuel. Instead of sails, huge cylinders rotating with wind power provide propulsion in this case.
Horizontal instead of vertical
In a test bed in the western German town of Grevenbroich, a vertical machine rotates and converts wind energy into electrical power. The principle of vertical axis machines is as old as the hills. Some 1,700 years BC, the Persians set up the first mills with flat-rotating wings. Obviously, their modern-day descendants are clearly more powerful – and taller, with 750 kW of rated capacity achieved by a design height of 105 meters (345 feet). Even so, modern systems with upright propellers and similar hub height deliver about 30 percent higher output. The vertical axis systems have the major advantage of being three times quieter than conventional systems and can thus be established in closer proximity to residential areas without violating legal requirements.
Surfing the stratosphere on lee waves
Commercial aviation could drastically reduce its emissions as well by making efficient use of the wind. Gliders are masters of this art and have long been deemed to be pioneers, both in terms of flight maneuvers and materials. Record-setting pilots glide over distances of more than 3,000 kilometers (1,850 miles) and up to altitudes of 23 kilometers (14 miles) – without burning a single drop of fuel. They use huge lee waves forming above mountain ranges in conditions of strong wind. That’s what the pilots of large airliners aspire to do, too – and therefore learn from the gliders.
Even race cars made of high-tech carbon fibers with sophisticated hybrid powertrains use the awesome power of the wind to pick up speed, while so-called wind cars look like wind turbines on wheels and are driven right against the wind. Strangely enough, they reach higher speeds than the opposing wind itself. This is the principle of the annual Racing Aeolus competition that has been held on a dike in the north of the Netherlands since 2008 and in which teams from all over the world participate with race cars they’ve developed and built themselves, using the wind as their fuel.
3 questions for …
… Rudolf Walter, Vice President Business Unit Wind at Schaeffler.
Wind power is already playing a crucial role in the global energy transition process today. In your view, will its importance increase even further, going forward?
Yes, the global energy transition process is unthinkable without wind power. Its share in the energy mix has prospects of further growth. Take China, for example, which is already the world’s biggest market for wind energy. The rate at which its expansion is being accelerated in order to achieve climate neutrality by 2060 is awesome. In the next five years, new systems with a total capacity of 50 gigawatts on average are planned to be installed there per year, and even more in the subsequent years. This more than doubles the current growth rate of 20 gigawatts per year. Of course, that’s an interesting prospect for Schaeffler’s Industrial Division as well.
With what innovations is Schaeffler planning to achieve further efficiency increases of wind power systems?
Among other things, we’re using special technologies to enhance the robustness of larger bearing solutions which can then achieve greater output and, as a result, reduce power generation costs. For instance, that’s why we use an induction curing technique allowing us to produce bearings with an outer diameter of more than 2.5 meters (8 feet) more economically.
In terms of predictive maintenance, Schaeffler is planning to field another innovation. What exactly is this?
That’s right. We’ve developed technologies enabling us to permanently measure the water content in oil as well as detecting and feeding back a bearing’s exposure to electrical current. Both are warnings that indicate so-called white etching cracks, in other words, structural changes below the surface of bearings. As a result, we’re able to reduce cost-intensive bearing failures. At the moment, we’re in the prototype operation stage. The plan is to start production next year.