Two recent studies report that the world (i.e., humans) could meet its entire electrical energy needs from several million wind turbines.
Is there not enough wind blowing across the planet to satiate our demands for electricity? If there is, would harnessing that much of it begin to actually affect the climate?
Two studies published this week tried to answer these questions. Long story short: we could supply all our power needs for the foreseeable future from wind, all without affecting the climate in a significant way.
The first study, published in this week’s Nature Climate Change, was performed by Kate Marvel of Lawrence Livermore National Laboratory with Ben Kravitz and Ken Caldeira of the Carnegie Institution for Science. Their goal was to determine a maximum geophysical limit to wind power—in other words, if we extracted all the kinetic energy from wind all over the world, how much power could we generate?
In order to calculate this power limit, the team used the Community Atmosphere Model (CAM), developed by National Center for Atmospheric Research. Turbines were represented as drag forces removing momentum from the atmosphere, and the wind power was calculated as the rate of kinetic energy transferred from the wind to these momentum sinks. By increasing the drag forces, a power limit was reached where no more energy could be extracted from the wind.
The authors found that at least 400 terawatts could be extracted by ground-based turbines—represented by drag forces on the ground—and 1,800 terawatts by high-altitude turbines—represented by drag forces throughout the atmosphere. For some perspective, the current global power demand is around 18 terawatts.
The second study, published in the Proceedings of the National Academy of Sciences by Mark Jacobsen at Stanford and Cristina Archer at the University of Delaware, asked some more practical questions about the limits of wind power. For example, rather than some theoretical physical limit, what is the maximum amount of power that could actually be extracted by real turbines?
For one thing, turbines can’t extract all the kinetic energy from wind—no matter the design, 59.3 percent, the Betz limit, is the absolute maximum. Less-than-perfect efficiencies based on the specific turbine design reduce the extracted power further.
Another important consideration is that, for a given area, you can only add so many turbines before hitting a limit on power extraction—the area is “saturated,” and any power increase you get by adding any turbines ends up matched by a drop in power from existing ones. This happens because the wakes from turbines near each other interact and reduce the ambient wind speed. Jacobsen and Archer expanded this concept to a global level, calculating the saturation wind power potential for both the entire globe and all land except Antarctica.
Like the first study, this one considered both surface turbines and high-altitude turbines located in the jet stream. Unlike the model used in the first study, though, these were placed at specific altitudes: 100 meters, the hub height of most modern turbines, and 10 kilometers. The authors argue improper placement will lead to incorrect reductions in wind speed.
Jacobsen and Archer found that, with turbines placed all over the planet, including the oceans, wind power saturates at about 250 terawatts, corresponding to nearly three thousand terawatts of installed capacity. If turbines are just placed on land and shallow offshore locations, the saturation point is 80 terawatts for 1,500 installed terawatts of installed power.
For turbines at the jet-stream height, they calculated a maximum power of nearly 400 terawatts—about 150 percent of that at 100 meters.
These results show that, even at the saturation point, we could extract enough wind power to supply global demands many times over. Unfortunately, the numbers of turbines required aren’t plausible—300 million five-megawatt turbines in the smallest case (land plus shallow offshore).