Wind turbines continuously remove kinetic energy from the lower troposphere, thereby reducing the wind speed near hub height. The rate of electricity generation in large wind farms containing multiple wind arrays is, therefore, constrained by the rate of kinetic energy replenishment from the atmosphere above. In recent years, a growing body of research argues that the rate of generated power is limited to around 1.5 W m −2 within large wind farms. However, in this study, we show that considerably higher power generation rates may be sustainable over some open ocean areas. In particular, the North Atlantic is identified as a region where the downward transport of kinetic energy may sustain extraction rates of 6 W m −2 and above over large areas in the annual mean. Furthermore, our results indicate that the surface heat flux from the oceans to the atmosphere may play an important role in creating regions where sustained high rates of downward transport of kinetic energy and thus, high rates of kinetic energy extraction may be geophysical possible. While no commercial-scale deep water wind farms yet exist, our results suggest that such technologies, if they became technically and economically feasible, could potentially provide civilizationscale power. wind power | geophysical generation limits | offshore wind | atmosphere-turbine interactions | storm tracks E ach wind turbine in a wind farm extracts kinetic energy (KE) from the mean flow and converts it into electricity. However, many studies have shown that individual turbines in a wind farm cannot be treated as independent and that the amount of electricity generated per turbine decreases as the turbine density and geographical area of the wind farm increases. As KE is extracted, the mean flow wind speed is reduced. This becomes particularly apparent in large wind farms with high turbine densities, where a multitude of wind turbines are arrayed in close proximity. As KE is continuously removed from the atmosphere, the maintained rate of power generation in the wind farm is constrained by the extent to which KE can be restored to its free flow value over the wind farm area (1, 2). Previous estimates based on wind speed climatologies grossly overestimated wind farm power generation potentials as interactions between wind turbines and the atmosphere, and the resulting geophysical constraints on wind power generation were ignored (3-7). Near-surface mean flow wind speeds are constrained by the amount of KE dissipated into the boundary layer, which forms in the lowest part of the atmosphere, and are governed by turbulent dissipation generated by surface drag, surface heat, and moisture fluxes. Operational turbines in current onshore and offshore wind farms extract KE primarily at heights between 30 and 120 m and are, therefore, predominantly entrained in the surface and boundary layer. Furthermore, each turbine poses an additional source of drag and an increase in near-surface dissipation of KE, which leads to a reduction of the mean flow wind speed. Therefore, sustaining high levels of power generation in a wind farm consisting of multiple turbines depends on whether the increased KE dissipation by the turbines can be compensated for by sources of KE, which contribute to the regeneration of the mean flow wind speed. Near-surface KE is generated because of either near-surface pressure gradients or the downgradient transport of KE along wind speed gradients from the upper levels of the atmosphere. Both of these sources are ultimately driven by gradients in diabatic heating (8). In this manner, the energy cycle within the atmosphere imposes a limit on electricity generation by wind turbines, which acts at the scale of kinetic energy extraction (KEE) rates required to meet the primary power generation demands of the 21 st century. Several studies argue that the rate of electricity generation by large wind farms may be limited to 1.5 W m −2 or less, even if the installed capacity of the wind farm greatly exceeds this threshold (2, 9-13). The power generation potential of a large area wind farm is limited by the downward KE transport, but the extent to which this limit may be used depends heavily on the wind farm's geometric design and layout. Tight turbine spacing, the absence of turbine staggering, and suboptimal orientation of the wind turbines may further reduce the power generation potential of a wind farm below its geophysical limit. This has been the focus of multiple studies investigating the characteristics of individual turbine wakes and their superposition as a function of mechanical turbine characteristics, turbine positioning, the intensity of boundary-layer mixing, and boundary-layer stability (14) (15) (16) (17) (18) (19) (20) . Furthermore, generated turbulence by the spinning turbine blades may also impact wake recovery (18, 21) , although this effect is likely overestimated in mesoscale and coarserscale numerical models parameterizing turbulent KE generation caused by turbine blades (18). Individual turbine wake effects play an important role for wind farm optimization, but the total extracted power over a large area remains constrained by the efficiency of the vertical KE transfer from above the wind farm. It has been shown for onshore (22) as well as offshore (14) wind farms that boundary-layer stability may Wind speeds over open ocean areas are often higher than those in the windiest areas over land, which has motivated a quest to develop technologies that could harvest wind energy in deep water environments. However, it remains unclear whether these open ocean wind speeds are higher because of lack of surface drag or whether a greater downward transport of kinetic energy may be sustained in open ocean environments. Focusing on the North Atlantic region, we provide evidence that there is potential for greater downward transport of kinetic energy in the overlying atmosphere. As a result, wind power generation over some ocean areas can exceed power generation on land by a factor of three or more.
Moreover, a recent study focused on estimating the potential of wind energy over open oceans concludes that, with adequate technological development, the North Atlantic area alone could power the entire world via wind converters REF .
Geophysical potential for wind energy over the open oceans
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
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