Nearly 150 km off the coast of Norway, in one of the windiest places on Earth, stands Hywind Tampen, the world’s largest floating wind power farm. Each weighing 8800 t and 190 m tall, with roughly one-third of that height sitting below the waterline, Hywind Tampen’s 11 turbines can generate 94.6 MW (
Fig. 1) [
1]. The blades—each 80 m long—travel at a blistering 290 km∙h
-1, yet because of their size they complete only about 10 to 15 full rotations per minute. A single rotation provides enough electricity to power an average home for a day. However, Hywind Tampen, which became fully operational in August 2023, will not power houses. Instead, the installation built by energy company Equinor (Stavanger, Norway) will provide 38% of the annual electricity consumed by five nearby offshore oil platforms in the North Sea [
1].
While Hywind Tampen is just the world’s fourth commercial deep-water offshore project to come online, the wind power industry is scrambling to manufacture and deploy more, and considerably bigger turbines. But supply chain issues, interest rate hikes, and a lack of infrastructure have delayed or derailed many recent billion-dollar projects.
It has been more than three decades since Denmark built Vindeby, the world’s first offshore wind farm, in 4 m depth of water and 2 km off the Danish island of Lolland. Since then, most offshore wind farms have been installed near countries, such as Denmark, the United Kingdom, Germany, and China, that have large expanses of shallow coastlines [
2]. Putting turbines on floating platforms allows developers to tap into stronger winds typically found over the ocean far from shore and opens markets where the continental shelf drops off just beyond the shore in places like Scotland of the United Kingdom, Ireland, the United States, Chile, the Taiwan region of China, Japan, and the Republic of Korea.
Floating wind turbines “give you many more locations to pick from,” said Finn Gunnar Nielsen, emeritus professor at the University of Bergen’s Geophysical Institute in Bergen, Norway, and co-designer of the Hywind floating turbine. He pointed out that moving turbines far from shore also avoids some of the common disadvantages of offshore wind farms, such as impacts on the coastal scenery, environment, and fisheries. “More than two-thirds of the offshore wind resources around the world are in deeper water that can only be accessed with floating turbines,” Nielsen said. “There is room for major growth in this space.”
In general, offshore wind power has seen significant growth. While still representing less than 1% of global electricity generation, offshore wind grew at nearly 30% per year over the last decade and has become a major factor in global power generation [
3], attracting 26 billion USD in investment over that period [
4].
According to a 2019 International Energy Agency report [
5], one of the most probable pathways for the European Union (EU) to achieve carbon neutrality by 2050 includes offshore wind becoming the most important part of the region’s power portfolio by 2040. The report also estimates that if floating wind-power technology were widely adopted across the world, the industry could theoretically supply the equivalent of 11 times the world’s estimated demand for electric power in 2040 [
5].
In the United States, the government has called for 30 GW to be generated annually from fixed-bottom turbines and 15 GW from floating offshore turbines by 2035 [
6]. “The US government, as well as leaders in the EU and elsewhere, recognize that offshore wind is a key part of meeting our global climate and energy goals,” said Gregory Nemet, professor of public affairs at the Wisconsin Energy Institute in University of Wisconsin-Madison, WI, USA. “There are efforts all over the world right now to harness it.”
However, accessing that potential using floating wind turbines involves massive feats of engineering. Though small in number, Hywind Tampen and the three other floating wind farms located near Portugal and Scotland of the United Kingdom are exercises in enormity. Each of their giant turbines, with rotors taller than the Great Pyramid of Giza, produces almost 20 times as much power as the first offshore wind turbines deployed in 1991 at the since-decommissioned Vindeby.
Hywind Tampen’s turbines were assembled dockside in a deep-water harbor. Ships then towed the turbines out to sea, and huge cranes stood them up on massive concrete stanchions. Chains heavy enough to secure a battleship hold them in place, with the ends of the chains attached to the sea floor by “suction anchors” that burrow into the sand inside of wide tubes and then unfurl as the tube is withdrawn from the seabed. It takes a network of 19 anchors to hold down the 11 wind turbines [
2]. While the ocean depth at Hywind Tampen ranges from 260 to 300 m, the theoretical maximum for siting a floating turbine is more than 1000 m [
7]. According to Nielsen, just how deep depends on the location and cost efficiency of the mooring system.
Manufacturing improvements lowered Hywind Tampen’s initial estimated cost per megawatt generated by one-third compared with Equinor’s Hywind Scotland floating offshore wind farm, which went online in 2017 as then the world’s first floating offshore wind farm [
8]. But Hywind Tampen eventually cost 200 million USD over its initial estimated cost of 500 million USD. According to Equinor, the increased cost resulted from inflation and supply chain disruptions, especially for steel, that were exacerbated by the coronavirus disease 2019 (COVID-19) pandemic and war in Ukraine [
2].
Other offshore wind projects are facing similar financial headwinds. Doubt has been raised about the financial viability of another Equinor project under construction and on pace to take the title of world’s largest fixed-bottom, offshore wind farm, the Dogger Bank Wind Farm to be situated about 130 km off the northeast coast of England in the North Sea [
9]. Independent research suggests the project may not be profitable due to weaker-than-expected, government-guaranteed electricity prices [
10].
In late
2023, leaders at Orsted (Frederica, Denmark), the largest energy producer in Denmark, warned that supply chain problems, high interest rates, and fewer-than-anticipated tax incentives might force the company to write off as much as 2.12 billion USD in losses related to three giant offshore wind installations off the United States’ east coast [
11]. The estimated initial overall cost of those installations was 10 billion USD [
11].
In 2022, the world’s largest maker of offshore wind turbines, Siemens Gamesa Renewable Energy (Madrid, Spain), reported a loss of 965 million USD, while General Electric (Boston, MA, USA), maker of the 13 MW Haliade-X, currently the world’s largest commercially deployed wind turbine [
9], said its renewable energy unit was likely to lose 2 billion USD in 2022 [
12]. “In the near-term, the high interest rate environment is a massive challenge that is putting many projects on hold,” Nemet said. “However, I do not expect this hiatus will last for long, as the advantages of standing up these projects are just too great.”
Habib Dagher, professor of structural engineering at the University of Maine in Orono, ME, USA, is also sanguine about the recent setbacks. “As we try to quickly grow the industry, there has been huge global demand for turbines, blades, towers, floating hulls, and so forth,” he said. “And the supply chain really is not ready yet at this scale. It will be, though. It is just a matter of time.”
At the moment, Chinese companies are meeting much of the demand, due primarily to their ability to manufacture turbines less expensively than their Western counterparts [
12]. In addition to building its own substantial offshore wind industry, China may have produced as much as 70% of the components in the turbines used in the West [
12], which accounted for nearly half of the 64 GW global offshore wind capacity in 2022 [
13].
When experts look at factors that impede a smooth-running supply chain for wind turbines, they often point to the constant drive to increase turbine size [
9]. While each Hywind Tampen unit outputs 8.6 MW, five companies have developed turbine prototypes that generate between 14 and 16 MW, while General Electric and Chinese manufacturers China State Shipbuilding Corporation Haizhuang Windpower (Beijing) and Ming Yang Wind Power (Guangdong) are all developing 18 MW turbines [
14]. Ming Yang is also reported to be working on a 22 MW design [
15].
“The industry has seen that as turbines have become bigger there is a reduction in the cost of energy produced,” said Dagher, who led a team that developed the first grid-connected offshore wind turbine in the United States. “For example, if you wanted to produce 15 MW of power, you could fabricate, tow, moor, and maintain 15 units of 1 MW. On the other hand, if you had a single 15 MW turbine, then you would have fewer hulls and mooring lines and just one offshore installation trip with fewer operations and maintenance costs.”
Nielsen suggested that one way to stabilize supply chains would be for industry leaders to agree on a set of manufacturing standards. “If the industry could keep a certain turbine size for five or ten years, they could make them more efficiently,” he said. With such an agreement, companies would not need to increase the size of their installation equipment every couple of years. Many offshore wind farm operators, for example, are already feeling the impact of the global scarcity of installation vessels large enough to transport 10 MW turbines and cranes robust enough to erect them. There are also few ports in Europe and other regions big enough to accommodate sufficiently sized installation vessels [
16]. Standardization does come with a downside, however. “Of course, the problem with standardization is you may also freeze innovation,” Nielsen said.
Eventually, though, manufacturers will reach a limit to how large they want to build. Nielsen said that blade weight, and therefore cost, increases with diameter faster than the power that can be generated. “You would expect that at some point these two curves will intersect, and you should not go further,” he said.
As the industry experiments with bigger and bigger turbines, it is also exploring novel wind power designs [
6]. These include semisubmersibles that seat the turbine toward the center of the structure, reducing the platform’s weight and thus its cost [
6]; bladeless cylinders that generate power by being wobbled back and forth by a wind phenomenon called vortex shedding [
9], [
17]; vertical-axis wind turbines [
17]; and devices that transfer the forces on wind-pulled kites to ground-station generators [
9], [
17], [
18], [
19].
Manufacturers are also exploring adding two or more rotors to a single tower. Such multirotor designs can produce greater average power by leveraging the interaction of wake vortices of closely spaced turbines. The smaller turbines are also easier to install, maintain, and transport than larger units. The startup Wind Catching Systems (Oslo, Norway) has used the multirotor concept to develop a 126-rotor floating design with help from General Motors (Detroit, MI, USA) (
Fig. 2) [
6].
Ultimately, floating wind power could become completely untethered to the seafloor. Several teams worldwide are now developing wind ships, a concept first suggested in 1972 [
6]. These tetherless, self-propelled floating platforms would capture wind power, use it to generate hydrogen, and store that fuel for delivery to shore. The tetherless design means wind ships would not be depth limited and, therefore, could migrate to areas of the ocean with the highest wind speeds. The design also eliminates the need for power cables and mooring chains used by floating offshore platforms and could also avoid some of the concerns over offshore wind’s potential impacts on fisheries and wildlife [
6]. Regardless of the design, “there may eventually be more deep-water wind installed than fixed-bottom wind,” Dagher said. “It is going to take some time, but floating will become more mainstream.”
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