Here's a cool design for a shrouded wind turbine designed by the FloDesign folks. Not sure if it is as efficient but it encorporates some cool features. They claim the same output as the much larger and less stable 3 bladed designs which would be a big deal if true. Here's a cool video showing the airflow although you should read the article first to understand why.

The company's design, which draws on technology developed for jet engines, circumvents a fundamental limit to conventional wind turbines. Typically, as wind approaches a turbine, almost half of the air is forced around the blades rather than through them, and the energy in that deflected wind is lost. At best, traditional wind turbines capture only 59.3 percent of the energy in wind, a value called the Betz limit.

FloDesign surrounds its wind-turbine blades with a shroud that directs air through the blades and speeds it up, which increases power production. The new design generates as much power as a conventional wind turbine with blades twice as big in diameter. The smaller blade size and other factors allow the new turbines to be packed closer together than conventional turbines, increasing the amount of power that can be generated per acre of land.