The Answer Is (Not) Blowing In The Wind

Scientists with dreams to harness energy from the upper atmosphere may have their head in the clouds…

High speed winds in the upper atmosphere, also known as jet streams, have in recent years gained considerable attention as a potential source of energy. It seemed like the only hurdle (granted a big hurdle) to overcome in this area was creating effective wind turbine technology at these altitudes. If it could be developed feasibly and cost effectively to reach those heights, it was thought that the energy potential would be sufficient for most if not all of the world's energy needs.

An artificial image depicting what jet stream turbines might look like. Image Credit: Max-Planck-Institut

The Science Behind Jet Streams

The enormous potential of high-altitude wind energy is from jet streams, which are regions of continuous wind speeds in excess of 25 m/s (55.9 mph) which occur at altitudes of 7-16 km. Like any other weather or wind system, jet streams ultimately exist because the sun heats certain regions more strongly than others (regions around the equator are heated much more than the poles). The resulting temperature and air pressure differential drives movement in the atmosphere, also known as wind. The upper limit on the amount of wind generated is set by this differential heating.

Visible jet stream. Image Credit: Shuttershock

The fact that jet streams in the upper atmosphere move much faster than winds near ground level is due to the near absence of friction. Due to low air density and the lack of contact with the surface, jet streams encounter little flow resistance. With help from Coriolis forces (due to the Earth's rotation), these winds require little energy to sustain themselves.

What This Means

Because it takes little power to generate and sustain jet streams, the amount of energy that can be captured is also small. "It is this low energy generation rate that ultimately limits the potential use of jet streams as a renewable energy source", says Dr. Axel Kleidon, head of the independent Max Planck Research Group 'Biospheric Theory and Modelling'. In other words, high velocity winds do not necessarily equal strong and powerful winds. Through climate models based on atmospheric energetics, Kleidon's group estimated the total amount of energy which can be extracted from the atmosphere. The maximum of 7.5 TW (1 TW = 10¹² W) is 200-times less than previously reported, accounting for about half the 2010 global energy demand of 17 TW.

Graphics depicting the map of energy transport (top) and extraction rates (bottom). Image Credit: Max-Planck-Institut

In addition, if a large portion of this energy was extracted from the jet streams, their natural flow and shape would be altered, depleting the atmospheric pressure gradient between the equator and the poles. "Such a disruption of jet stream flow would slow down the entire climate system. The atmosphere would generate 40 times less wind energy than what we would gain from the wind turbines", explains Lee Miller, first author of the study. "This results in drastic changes in temperature and weather".

What this means is that jet stream wind power may not be the promising energy solution of the future it was expected to be. Its potential to be harvested economically as a supplement to other energy sources is now, more than ever, very much up in the air.

Sources:

Max-Planck-Institut

ScienceDaily