Breaking the Betz limit: High-altitude wind energy toward future decarbonization
Wind power is a crucial component in the transition to carbon-neutral energy systems.1 By the end of 2025, global wind capacity exceeded 1,346 GW, representing the highest growth since 2020.2 To harness more energy, turbine capacities and efficiencies have steadily increased (Figure 1A). However, larger turbines introduce significant challenges, including increased structural demands and the efficiency constraint known as the Betz limit. First proposed by Albert Betz in 1919, this limit states that a wind turbine can capture at most 59.3% of the kinetic energy in wind. Due to land-use limitations for onshore wind deployment, attention has increasingly shifted toward far-offshore resources. However, the highly concentrated offshore wind supply chain results in prolonged construction timelines and elevated costs, particularly in Europe and the US.
High-altitude wind energy: Elevating wind energy from planar to spatial perspectives
Since wind speed generally increases with altitude, the atmosphere contains abundant exploitable wind energy resources. Consequently, high-altitude wind energy (HAWE) has emerged as a promising solution. As early as 1985, Loyd demonstrated that tethered wing-based HAWE systems could achieve power outputs up to five times higher than conventional wind turbines. HAWE systems typically operate at altitudes of 300–15,000 m and consist of an airborne device connected to a ground station via a tether. In terms of energy conversion, HAWE can be classified into two canonical configurations, namely, ground- and fly-generator systems. The former is predominantly represented by kite-based systems, which generate electricity by driving ground-based generators through tether tension. In contrast, the latter employs buoyant platforms, such as helium-filled aerostats, to lift wind turbine rotors to high altitudes for operation.
Transformative advantages in capacity and efficiency
HAWE systems offer significant advantages in both capacity and efficiency. Since wind power density is proportional to the cube of wind speed, higher-altitude winds enable substantially greater energy generation. Based on the fifth-generation ECMWF reanalysis dataset, wind resources in East Asia are increasingly dominated by the subtropical jet stream at higher altitudes, as topographic and frictional influences diminish. At altitudes of approximately 10 km, the climatological mean of maximum wind speeds approaches 40 m/s. On average, wind speed increases by ∼1.7–3 m/s for every 1 km rise in altitude. The upper-level jet stream is most pronounced over eastern and central China, the Korean Peninsula, Japan, the Yellow Sea, the East China Sea, and the Sea of Japan, exhibiting strong west-to-east zonal meandering. In these regions, wind power density is particularly high, averaging up to 29,881.8 W/m2.
