Geosciences Column

Geosciences Column: Why are jet streams not good wind energy sources?

Commercial airlines know jet streams well. Planes often hitch a ride on these strong, high-altitude atmospheric winds, which blow from west to east, to fly faster, and they are the reason why long-haul easterly flights (such as those between the US and Europe) are quicker than the corresponding westerly journeys.

Scientists are also familiar with these fierce and persistent winds, which occur at altitudes of 7 to 16 kilometres and have velocities from 90 to several hundred kilometres per hour. Some have even suggested we could harvest wind power from jet streams by developing appropriate airborne technology such as large kite-like wind-power generators. A group of researchers from the US and Australia estimated in 2007 that this potential renewable energy source could provide roughly 100 times the global demand of energy.

But research published this week in Earth System Dynamics, a journal of the European Geosciences Union, challenges this assumption. Lee Miller and collaborators from the Max Planck Institute for Biogeochemistry in Jena, Germany, calculated the maximum extractable energy from these streams to be about 200 times less than previously reported. They also warned that extracting wind power in this way can result in significant climate impacts.

Airborne wind-power generators: to remain science fiction? (Source: AlphaGalileo)

The scientists pointed out that the high velocities of jet streams are not the result of a strong power source but are consequence of the near absence of friction high up in the atmosphere, as it is well-known in meteorology. The group shows in their calculations that, in fact, it takes very little power to accelerate and sustain these winds.

“It is this low energy generation rate that ultimately limits the potential use of jet streams as a renewable energy resource,” said Axel Kleidon, the study’s leader, in a press release.

A maximum of 7.5 terawatts (7.5 trillion watts), less than half of the 2010 global energy demand of 17 terawatts, can be extracted from jet streams, they determined. Previous studies arrived at much higher values because they used the wind velocity to estimate wind power, a method the Max-Planck researchers claim is flawed.

As with other weather systems, jet streams are in part caused by the fact that equatorial regions are warmer than the poles, which are less strongly heated by the sun. The differences in temperature and air pressure between these regions drive the atmosphere into motion creating the strong winds. These differences, rather than wind speeds, are what controls how much of the generated wind can be used as an energy resource.

The authors also estimated the climate impacts of extracting energy from jet streams. Wind turbines build up resistance when harvesting energy, which alters the flow of the wind. This disruption can slow down the entire climate system of our planet when substantial amounts of energy are extracted.

If 7.5 terawatts of energy were extracted from jet streams “the atmosphere would generate 40 times less wind energy than what we would gain from the wind turbines,” said Miller in a press release.

“This results in drastic changes in temperature and weather.”

By Bárbara Ferreira, EGU’s Media and Communications Officer

Geosciences Column: Iceland spar, or how Vikings used sunstones to navigate

Nowadays, we can rely on GPS receivers or magnetic compasses to tell us how to reach our destination. Some 1000 years ago, Vikings had none of these advanced navigation tools. Yet, they successfully sailed from Scandinavia to America in near-polar regions where it can be hard to use the Sun and the stars as a compass. Clouds or fog and the long twilights characteristic of polar summers complicate direct observations of these celestial bodies. So how did they find their bearings? A new study published in Proceeding of Royal Society A shows that they probably used Iceland spar, a “sunstone”.

Centuries-old Viking legends tell of glowing sunstones that navigators used to find the position of the Sun and set the ship’s course even on cloudy days. In 1967, a Danish archaeologist named Thorkild Ramskou speculated that the Viking sunstone could have been Iceland spar, a clear variety of calcite common in Iceland and parts of Scandinavia.

This crystal has an interesting property called birefringence: a light ray falling on calcite will be divided in two, forming a double image on its far side. (This double image is easily seen by placing transparent calcite on printed text.) Further, the Iceland spar is a polarising crystal, meaning the two images will have different brightnesses depending on the polarisation of light.

Birefringence of Iceland Spar seen by placing it upon a paper with written text. Source: Wikimedia Commons.

Light is made up of electromagnetic waves with component electric and magnetic fields. If these components have a specific orientation, the light is said to be polarised, while in unpolarised light the orientation of these fields has no preferred direction. Calcite can appear dark or light depending on the polarisation of light that falls upon it.

Sunlight becomes polarised as it crosses the Earth’s atmosphere, and the sky forms a pattern of rings of polarised light centred on the Sun. Changing the orientation of calcite as light passes through it will change the relative brightness of the projections of the split beams, even when the Sun is hiding behind clouds or just below the horizon. The beams are equally bright when the crystal is aligned to the Sun.

It can be hard to determine when exactly these split beams have equal brightness. But the new study, led by Guy Ropars at the University of Rennes 1 in France, suggests Vikings could have built a simple device to better use the sunstone.

The technique consists in covering the Iceland spar with an opaque screen with a small hole in its centre and a pointer. As light passes through the hole onto the crystal, a dark surface below it receives the projection of the double image for comparison.

The authors of the Proceedings of the Royal Society study believe Vikings could have used a device like this to navigate. The crystal is inside, and the projection of a double image is seen below it. Credit: Guy Ropars. Source: ScienceNOW.

By rotating the apparatus and determining the direction at which the two images were equal in brightness, the team managed to pinpoint the Sun’s position on a cloudy day with an accuracy of one degree on either side. Researchers also conducted tests when the Sun was largely below the horizon. “We have verified that the human eye can reliably guess clearly the Sun direction in dark twilights, even until the stars become observable,” Ropars’ team writes in the paper.

Although archaeologists have not yet found Iceland spar among Viking shipwrecks, the new study adds credence to the idea that Viking seafarers used the crystal in their travels.

Further, the recent finding of a calcite crystal on a sixteenth century Elizabethan ship shows that navigators could have used Iceland spar even after the appearance of the magnetic compass. Cannons on ships could perturb a magnetic compass orientation by 90 degrees, so a crystal serving as an optical compass could be crucial in avoiding navigational errors and get sailors to a safe port.

By Bárbara Ferreira, EGU’s Media and Communications Officer