Estimating maximum global land surface wind power extractability and associated climatic consequences

Abstract

Abstract. The availability of wind power for renewable energy extraction is ultimately limited by how much kinetic energy is generated by natural processes within the Earth system and by fundamental limits of how much of the wind power can be extracted. Here we use these considerations to provide a maximum estimate of wind power availability over land. We use several different methods. First, we outline the processes associated with wind power generation and extraction with a simple power transfer hierarchy based on the assumption that available wind power will not geographically vary with increased extraction for an estimate of 68 TW. Second, we set up a simple momentum balance model to estimate maximum extractability which we then apply to reanalysis climate data, yielding an estimate of 21 TW. Third, we perform general circulation model simulations in which we extract different amounts of momentum from the atmospheric boundary layer to obtain a maximum estimate of how much power can be extracted, yielding 18-34 TW. These three methods consistently yield maximum estimates in the range of 18-68 TW and are notably less than recent estimates that claim abundant wind power availability. Furthermore, we show with the general circulation model simulations that some climatic effects at maximum wind power extraction are similar in magnitude to those associated with a doubling of atmospheric CO 2 . We conclude that in order to understand fundamental limits to renewable energy resources, as well as the impacts of their utilization, it is imperative to use a "topdown" thermodynamic Earth system perspective, rather than the more common "bottom-up" engineering approach.

Two methods for estimating limits to large-scale wind power generation

“…An overall increase in the downward kinetic energy flux at larger deployment scales seems unlikely to occur, because climate model simulations performed at continental and global scales do not predict such an increase for present-day radiative forcing conditions (10,13). Although these studies did not include a full analysis of the energetics, their predictions broadly agree with the predictions of the VKE method in terms of a maximum of 25-27% of the natural dissipation rate that could be used for electricity generation (10) and a slowdown of hub-height wind velocities by 51% globally, 50% over land, and 51% over the ocean (13).…”

“…The more kinetic energy wind farms use, the greater the shift in the balance and the reduction of wind speeds should be. This trade-off between greater utilization and lower wind speeds results in a maximum in wind power generation from the vertical flux of kinetic energy (10). This maximum yields a potential for wind power generation of a region that is independent of the technological specifications of the turbines.…”

“…However, a growing body of research suggests that as larger wind farms cover more of the Earth's surface, the limits of atmospheric kinetic energy generation, downward transport, and extraction by wind turbines limits large-scale electricity generation rates in windy regions to about 1.0 W e ·m −2 (8)(9)(10)(11)(12)(13)(14). Ideally, these inherent atmospheric limitations to generating electricity with wind power could be considered without scenario-and technology-specific complex modeling approaches, be easily applied to "preturbine" climatologies, and yield spatially and temporally variable generation rates comparable to the energetically consistent atmospheric modeling methods.…”

The problem of the second wind turbine – a note on a common but flawed wind power estimation method

“…These two studies also show significant climatic impacts of wind power removal on the climate system. Additionally, Miller et al (2011) included these consumptive effects and estimated the maximum global landbased wind power availability from the atmospheric boundary layer (ABL) to be 18-68 TW, which is much lower than previous estimates and unavoidably associated with significant climatic differences.…”

“…Advocates of the common method usually argue that imprecisions in general circulation models (GCMs) and the associated representation of wind turbines therein cause the differences in climatic effects, and therefore the results should be disregarded (e.g. see open discussion on Miller et al, 2011). In our opinion the assumption of an unaltered wind velocity outside the turbine wake is the main reason for this difference, because it neglects the effects of turbine-related momentum removal from the atmospheric boundary layer.…”