The horizontal-axis wind turbine

technology · earth · global; offshore concentrated in the North Sea, China and East Asia

A machine that extracts kinetic energy from moving air through a lifting rotor, and the partial case in this Journey — half manufactured, half constructed. Two scaling relations govern its economics. Power available in the wind scales with the cube of wind speed, so a site with twenty per cent more wind yields roughly seventy per cent more energy; and captured power scales with swept area, hence with the square of rotor diameter, so doubling the rotor quadruples the capture. No rotor can take more than 16/27 — about 59.3 per cent — of the kinetic energy in the stream it intercepts, a result derived by Betz; good modern rotors reach power coefficients around 0.45 to 0.50, so the physics ceiling is not the binding constraint. Growth has therefore been growth in size: commercial machines of the early 1980s were rated in the tens or low hundreds of kilowatts with rotors under twenty metres, while offshore machines now in service are rated in the mid-teens of megawatts with rotors beyond two hundred metres, and larger prototypes have been erected in China. That is where the divergence from solar appears. The nacelle and blades are factory products and behave like factory products; the monopile, the installation vessel, the export cable and the grid connection are construction and marine operations, and they behave like construction — so offshore wind's costs rose sharply when interest rates, steel prices and vessel day-rates rose in 2022–2023, in a way solar module costs did not.