Arranging wind turbines like a school of fish could reduce the amount of land they take up by 100-fold while maintaining their electrical output, say researchers. Wind farms based on the approach might also be considerably safer for migrating birds.
Whether it's Lance Armstrong bicycling behind his teammates in the Tour de France or a storm of fish slicing their way through the ocean, animals benefit from drafting. The leader breaks through the calm air or water, while the followers enjoy the reduced resistance in the leader's wake.
The same doesn't hold true for horizontal-axis wind turbines(HAWTs), the most common kind of windmill. Placing one HAWT in another's draft drastically reduces the efficiency of the trailing windmill. That's because the turbulent breeze created by the leading turbine's blades can't propel the trailing blades as well as an unobstructed airflow. So engineers spread the giant fans across hundreds of hectares of land--a practice that has created a backlash from people who find the turbines unsightly.
Turn a windmill on its side, so to speak, and the drafting benefit returns. That's what two fluid dynamicists from the California Institute of Technology in Pasadena have discovered. Robert Whittlesey and John Dabiri decided to study how a new type of generator, called a vertical-axis wind turbine (VAWT), stacked up against its more conventional counterpart. VAWTs resemble giant versions of the lawn ornaments that gardeners install to scare away birds and other veggie-loving critters (see picture). The researchers measured airflows at a prototype VAWT array in Glendora, California. They then compared that data with existing studies of how water flows through schools of swimming fish to see how geometrically arranging the arrays affected their performance.
Bunching up the vertical-axis turbines behind a leader pays off, Whittlesey and Dabiri reported yesterday at the annual meeting of the American Physical Society in Minneapolis, Minnesota. In fact, using a new mathematical model they developed from the Glendora data, the researchers found that arranging the VAWT arrays just like schools of fish produced the best results. Such tightly packed VAWT arrays can produce as much electricity as conventional windmills, all while using as little as one-hundredth of the land area. "I don't think I expected to see as great an improvement in the land use," says Whittlesey.
The study revealed that the most efficient arrangement involved alternating the rotational direction of the turbines: a clockwise rotation in the lead turbine, say, with two counterclockwise rotators next in line, followed by three clockwise rotators, and so on. Whittlesey notes that the models are preliminary, so even greater improvements might be possible with further study. And he says that the VAWT arrays could be much less deadly to birds because "the faster they spin, the more solid they appear," thereby allowing birds to see the turbines more easily and navigate around them.
The study shows "great promise," says mechanical engineer Lex Smits of Princeton University. Even if VAWT configurations achieve only half of the land-use gain as suggested, he says, "it would be a major improvement."