Are global wind power resource estimates overstated?

Abstract

Estimates of the global wind power resource over land range from 56 to 400 TW. Most estimates have implicitly assumed that extraction of wind energy does not alter large-scale winds enough to significantly limit wind power production. Estimates that ignore the effect of wind turbine drag on local winds have assumed that wind power production of 2–4 W m**-2 can be sustained over large areas. New results from a mesoscale model suggest that wind power production is limited to about 1 W m**-2 at wind farm scales larger than about 100 km2. We find that the mesoscale model results are quantitatively consistent with results from global models that simulated the climate response to much larger wind power capacities. Wind resource estimates that ignore the effect of wind turbines in slowing large-scale winds may therefore substantially overestimate the wind power resource.

Discussion

Results from the mesoscale model described above show that at spatial scales larger than about 100 km2, extraction of energy by wind turbine arrays is limited by the physics of atmospheric energy transport. The results suggest that the maximum energy that can be extracted by turbine arrays at these scales is about 1 W m**-2. This result is surprisingly independent of array size from the small array at 2:7103 km2 to the 14  106 km2 array used in the GCM study of Keith et al [11] to global coverage in Jacobson and Archer [12], and it holds for two different boundary layer parameterizations in the WRF model. We caution against over-interpreting the specific numerical result, however, as it may well depend on factors such as the mean wind speed in the free troposphere overlying the turbine array allowing larger power densities to be extracted (in principle) from regions with stronger winds such as the southern polar ocean.

An immediate implication of this result is that regional analyses of large-scale wind power production, such as Kempton [30], that implicitly assume that capacity factors are independent of total wind capacity will need systematic reevaluation. Similarly, work that considers how wind resources may change under different climate change scenarios, such as Pryor and Barthelmie [31] will need to take account of the climatic effects of future wind energy deployment.

What then is the global wind resource? There is no simple non-arbitrary way to compute the global wind resource because the presence of large-scale collective effects means that wind resources are not additive. Extracting power in one location alters winds and therefore wind power production in ways that may be non-local and non-linear [11]. A robust estimate of the global wind resource might therefore require
one to find the optimal placement of wind power resources in the presence of long-range interactions, a daunting task.

These results suggest that estimates of global wind resource that ignore the impact of wind turbines on slowing the winds may substantially overestimate the total resource. In particular, the results from the three studies [2–4] that estimated wind power capacities of 56, 72 and 148 TW respectively appear to be substantial overestimates given the comparison between model results and the assumptions these studies made about power production densities as shown in figure 4. To cite a specific example, Archer and Jacobson [2] assumed a power production density of 4:3 W m**-2 yet the results shown in figure 4 (including results from Jacobson and
Archer [12]) suggest that production densities are not likely to substantially exceed 1 W m**-2 implying that Archer and Jacobson [2] may overestimate capacity by roughly a factor of four.

The total wind power capacity can—of course—be very large if one assumes that turbines are placed over the entire land surface or even over the land and ocean surface [12, 13], but while these geophysical limits are scientifically interesting their relevance to energy policy is unclear.

As we discuss in section 2, more policy-driven wind power capacity estimates have restricted the area considered by factors such as wind energy density (e.g., wind power ‘class’ which impacts production cost), distance from energy demand, or competition with other land uses [2–4]. Yet as shown in figure 4, these estimates have used power production densities that are several times larger than the wind power production limit of around 1 W m**-2 found in our study and in the global model studies [12, 13]. It is therefore plausible that wind power capacity may be limited to an extent that is relevant to energy policy. Addressing the question demands a new generation of studies that combine realistic physical limits to wind energy extraction with realistic economic and social constraints on wind power siting along with quantification of the climate impacts of wind power extraction.