The view atop Haliade-X, 260 metres above the sea, is mesmerising (1). White-crested waves crash against the pylons of an army of giant windmills that would have excited the imagination of Don Quixote (2). This is Dogger Bank C, located in the North Sea between the United Kingdom and the Netherlands, the largest offshore wind farm in the world. General Electic’s Haliade-X is its largest inhabitant. This behemoth sports 107-metre-long blades and it can generate up to 14 megawatts (MW) of energy, enough to power 16,000 homes. The tallest and most powerful windmill in 2018, it is now dwarfed by bigger turbines, such as Dongfang Electric Corporation’s 18 MW installed in Xinghua Bay, Fujian province, and MingYang’s 22 MW built in Shantou City, Guangdong province, both in China. The largest windmills are getting even bigger at a quick pace, why is there such a race for the sky? The answer lies in economics and physics. Unlike on land, winds out at sea are stronger and more predictable. Besides, space is plentiful, and no objections are raised by private owners since they are few or none. Operations also benefit from the technologies established by the offshore gas and petrol industry over the last century, including logistics: ports, ships, trained staff, and capital. Of course, installation and maintenance are more expensive than on land. The cost increases which each turbine, hence there is a strong incentive to install fewer of them but more efficient. Here it is where physics come in handy. The power generated by a windmill increases as the cube of the wind speed and as the square of the radius of its blades. Since winds are stronger above the water surface, height is a big plus; and the longer the blades, the more energy they can capture. In short, it is more economical to install a single large tall turbine than two smaller ones (3). Offshore windmills are anchored to the bottom of shallow continental platforms, but many coasts such as the West side of North America lack these. An alternative consists of floating turbines. They are even more expensive to operate but can take advantage of stronger winds found in the large expansions of the oceans. New designs are being tested rapidly such as two turbines sharing a single stem (see picture). These large investments in capital underline a strong interest on developing wind energy; in fact, installed capacity has grown from 6 GW in 2001 to 117 GW in 2023, and it has been projected to reach 320 GW in 2030 (4). Often, they are helped by its green credentials, but what is the impact of offshore wind generation on nature? Although, wind energy does not produce carbon dioxide directly, there are emissions linked to its manufacturing and maintenance. These can be argued to be lower than those of fossil fuels-fired power plants, which they are meant to substitute. However, proof supporting substitution is thin on the ground. Then it is the impact to local aquatic ecosystems. First, offshore farms share space with marine birds which migrate through or use it as fishing grounds. Second, the structures and vessel traffic can alter the distribution of shoals of fish, algae and can lead to collisions with large marine creatures. Finally, the impact on sea floor communities has received very little attention and not much is known. On the positive side, windmill pylons can be used by marine creatures as artificial reefs and the ban on trawlers also protect them from fishing (5). The blades of Haliade-X shimmer in the sun as they move slovenly through the air. The buzz they make is akin to the one this nascent industry is creating. Riding on favourable economic and political winds it is expected to grow quickly contributing to the expansion of renewal energies. If this is coupled with studies on the aquatic ecosystems on which they rest, they will truly be “green”. Photograph: OceanX double turbine - MingYang Wind Power Read more: 1) https://www.nbcnews.com/mach/science/world-s-tallest-offshore-wind-turbine-will-tower-over-some-ncna853596 2) https://cvc.cervantes.es/literatura/clasicos/quijote/edicion/parte1/cap08/default.htm 3) https://www.bbc.com/news/business-58704792 4) https://gwec.net/global-offshore-wind-report-2024/ 5) https://doi.org/10.1038/s44183-022-00003-5