Enough Wind to Power Global Energy Demand: New Research Examines Limits, Climate Consequences

There is enough energy available in winds to meet all of the world's demand. Atmospheric turbines that convert steadier and faster high-altitude winds into energy could generate even more power than ground- and ocean-based units. New research from Carnegie's Ken Caldeira examines the limits of the amount of power that could be harvested from winds, as well as the effects high-altitude wind power could have on the climate as a whole.

Their work is published September 9 by Nature Climate Change.

Led by Kate Marvel of Lawrence Livermore National Laboratory, who began this research at Carnegie, the team used models to quantify the amount of power that could be generated from both surface and atmospheric winds. Surface winds were defined as those that can be accessed by turbines supported by towers on land or rising out of the sea. High-altitude winds were defined as those that can be accessed by technology merging turbines and kites. The study looked only at the geophysical limitations of these techniques, not technical or economic factors.

Turbines create drag, or resistance, which removes momentum from the winds and tends to slow them. As the number of wind turbines increase, the amount of energy that is extracted increases. But at some point, the winds would be slowed so much that adding more turbines will not generate more electricity. This study focused on finding the point at which energy extraction is highest.

Using models, the team was able to determine that more than 400 terawatts of power could be extracted from surface winds and more than 1,800 terawatts could be generated by winds extracted throughout the atmosphere.

Today, civilization uses about 18 TW of power. Near-surface winds could provide more than 20 times today's global power demand and wind turbines on kites could potentially capture 100 times the current global power demand.

At maximum levels of power extraction, there would be substantial climate effects to wind harvesting. But the study found that the climate effects of extracting wind energy at the level of current global demand would be small, as long as the turbines were spread out and not clustered in just a few regions. At the level of global energy demand, wind turbines might affect surface temperatures by about 0.1 degree Celsius and affect precipitation by about 1%. Overall, the environmental impacts would not be substantial.

"Looking at the big picture, it is more likely that economic, technological or political factors will determine the growth of wind power around the world, rather than geophysical limitations," Caldeira said.

Length of Yellow Caution Traffic Lights Could Prevent Accidents, Researchers Say

A couple of years ago, Hesham Rakha misjudged a yellow traffic light and entered an intersection just as the light turned red. A police officer handed him a ticket.

"There are circumstances, as you approach a yellow light, where the decision is easy. If you are close to the intersection, you keep going. If you are far away, you stop. If you are almost at the intersection, you have to keep going because if you try to stop, you could cause a rear-end crash with the vehicle behind you and would be in the middle of the intersection anyway," said Rakha, professor of civil and environmental engineering at Virginia Tech.

He's not trying to defend his action. Rakha, director of the Center for Sustainable Mobility (www.vtti.vt.edu/csm.php) at the Virginia Tech Transportation Institute (www.vtti.vt.edu), is describing his research. Since 2005, his research group has been studying drivers' behaviors as they approach yellow lights. Their goal is to determine signal times for intersections that are safer and still efficient.

If a driver decides to stop when instead of proceeding, rear-end crashes could occur. If a driver proceeds instead of stopping, collisions with side street traffic could occur. Although observation-based research shows that only 1.4 percent of drivers cross the stop line after the light turns red, more than 20 percent of traffic fatalities in the United States occur at intersections.

"If the yellow time is not set correctly, a dilemma zone is eminent," Rakha said.

"The dilemma zone occurs when the driver has no feasible choice," he said. "In other words the driver can neither stop nor proceed through the intersection before the light turns red. This can also occur if the approaching vehicle is traveling faster than the posted speed limit and/or if the driver's perception and reaction time is longer than the design one-second value."

In most cases, the yellow time is set for 4.2 seconds on a 45 mph road. The time is longer for higher-speed roads. "These timings are based on two assumptions," Rakha explains. "Namely, the driver requires one second to perceive and react to the change in signal indication and that the driver requires 3.2 seconds to stop from 45 mph at a comfortable deceleration level, assumed to be 3 meters per second squared (3 m/s2 ) or 10 feet per second squared."

For his studies, Rakha's used Virginia's Smart Road, located at the Virginia Tech Transportation Institute. The Smart Road intersection has a signal that can be controlled for length of the red, yellow and green lights. "We can study driver behavior by changing the signal when the driver is a certain distance from the intersection."

Center for Sustainable Mobility researchers have determined that half of drivers make the stop-go decision three seconds before the stop line. Of those that go, few clear the intersection before the light changes to red. In Virginia, if you are in the intersection when the light turns red, you are not running a red light. However, there is still risk.

The specific findings from the Smart Road study are that 43 percent of drivers who crossed the stop line during the yellow time were not able to clear the intersection before the light turned red. At 45 mph, it takes 1.5 seconds to clear a 30-meter (98.4 feet) intersection. "If the all-red interval is the minimum conventional one second, then there is a potential risk that the legal yellow-light runners would not be able to completely clear the intersection at the instant the side-street traffic gains the right-of-way," the researchers reported at the Transportation Research Board Annual Meeting in 2010.

People over 60 years of age have a longer perception-reaction time, so they have to brake harder to stop. But they are more likely to try to stop, compared to younger drivers. However, if they keep going, they are unlikely to clear the intersection, the researchers report.

"Even if yellow timing is designed properly to avoid a dilemma zone, someone driving above the speed limit could encounter a dilemma because it takes longer to stop from a higher speed. They could speed up, but our studies show that drivers who keep going usually maintain their speed," Rakha said.

The research determined that the perception-reaction time is slightly longer than one second, but that driver deceleration levels are significantly higher than the deceleration level assumed for traffic signal design.

If the road conditions are poor, drivers react 15 percent slower because they are processing more information. Deceleration level decreases by 8 percent. "In such conditions, you need a longer yellow," Rakha said.

He and his team has developed a novel procedure to compute the yellow and change interval duration that accounts for the risk of drivers being caught in the dilemma zone. Using this procedure they have created tables for light vehicles at various speeds for dry roads and wet roads, so that traffic planners can set traffic signals that can be adjusted to roadway surface and weather conditions. They also created tables for the driving characteristics for different age groups.

One strategy to make intersections safer might be more use of caution lights that tell drivers a green light is about to change, so the driver has a longer time to react. Such systems now are used on high speed roads, where the stopping distance is longer, and when the lighted intersection is not visible until the last seconds.

A future strategy that researchers are investigating is in-car display systems that can be customized to each driver's reaction time. "So one person receives a four-second warning of a light change, and another person receives a five-second warning. Or, instead of a warning, the system might just tell you to stop," said Rakha.

Research to implement vehicle-to-vehicle and infrastructure-to-vehicle communication is ongoing, including at the Virginia Tech Transportation Institute Tier 1 University Transportation Center. Rakha's research group is studying vehicle-to-vehicle communication at intersections.

The research has been presented at the 2010 and 2011 Transportation Research Board annual meetings. Articles published in 2012 are:

"Novel Stochastic Procedure for Designing Yellow Intervals at Signalized Intersections," by Ahmed Amer, transportation engineer with Vanasse Hangen Brustlin Inc.; Rakha; and Ihab El- Shawarby, assistant professor at Ain-Shams University in Cairo and senior research associate with the Center for Sustainable Mobility at the Virginia Tech Transportation Institute. It appeared in the June 1, 2012, Journal of Transportation Engineering.

"Designing Yellow Intervals for Rainy and Wet Roadway Conditions," by Huan Li of Blacksburg, who has received his master of science degree in civil engineering; Rakha; and El-Shawarby. The article appeared in the spring 2012 issue of theInternational Journal of Transportation Science and Technology.

NASA's Kepler Discovers Multiple Planets Orbiting a Pair of Stars

Coming less than a year after the announcement of the first circumbinary planet, Kepler-16b, NASA's Kepler mission has discovered multiple transiting planets orbiting two suns for the first time. This system, known as a circumbinary planetary system, is 4,900 light-years from Earth in the constellation Cygnus.


This discovery proves that more than one planet can form and persist in the stressful realm of a binary star and demonstrates the diversity of planetary systems in our galaxy.

Astronomers detected two planets in the Kepler-47 system, a pair of orbiting stars that eclipse each other every 7.5 days from our vantage point on Earth. One star is similar to the sun in size, but only 84 percent as bright. The second star is diminutive, measuring only one-third the size of the sun and less than 1 percent as bright.

"In contrast to a single planet orbiting a single star, the planet in a circumbinary system must transit a 'moving target.' As a consequence, time intervals between the transits and their durations can vary substantially, sometimes short, other times long," said Jerome Orosz, associate professor of astronomy at San Diego State University and lead author of the paper. "The intervals were the telltale sign these planets are in circumbinary orbits."

The inner planet, Kepler-47b, orbits the pair of stars in less than 50 days. While it cannot be directly viewed, it is thought to be a sweltering world, where the destruction of methane in its super-heated atmosphere might lead to a thick haze that could blanket the planet. At three times the radius of Earth, Kepler-47b is the smallest known transiting circumbinary planet.

The outer planet, Kepler-47c, orbits its host pair every 303 days, placing it in the so-called "habitable zone," the region in a planetary system where liquid water might exist on the surface of a planet. While not a world hospitable for life, Kepler-47c is thought to be a gaseous giant slightly larger than Neptune, where an atmosphere of thick bright water-vapor clouds might exist.

"Unlike our sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question always has been -- do they have planets and planetary systems? This Kepler discovery proves that they do," said William Borucki, Kepler mission principal investigator at NASA's Ames Research Center in Moffett Field, Calif. "In our search for habitable planets, we have found more opportunities for life to exist."

To search for transiting planets, the research team used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars. Additional ground-based spectroscopic observations using telescopes at the McDonald Observatory at the University of Texas at Austin helped characterize the stellar properties. The findings are published in the journal Science.

"The presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is an amazing discovery," said Greg Laughlin, professor of Astrophysics and Planetary Science at the University of California in Santa Cruz. "These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary disks."

Ames manages Kepler's ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed the Kepler mission development.

Ball Aerospace & Technologies Corp. in Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's tenth Discovery Mission and funded by NASA's Science Mission Directorate at the agency's headquarters in Washington.