Utility-scale wind turbines have overtaken nuclear energy in global power-generating capacity, and outstripped solar by more than three to one in electrical energy production. The technology is mature, growing fast, and producing cheap electricity. Home-scale wind energy is less mature, and often doesn’t compete with solar as well.
This article will help you understand what small wind turbines can do, and what engineering approaches work best. Globally, small wind turbines represent less than 1% of the total wind-generating capacity, but they do have their place when properly sited and installed.
Wind turbines convert wind power to mechanical power. The earliest applications were windmills that performed essential tasks like grinding grain and pumping water. At first these were “panemones,” vertical shafts driven by crude sails that were blown downwind on one side and made their way back upwind on the other side of a central shaft. Next there was a huge technical breakthrough, and the sails were mounted on a horizontal shaft so that they worked by lift instead of drag, sailing across the wind instead of being carried before it. This meant that they could work all the time throughout the revolution of the shaft, with improved efficiency.
Nineteenth-century water-pumping windmills in the United States were designed to produce as much torque as possible at lower wind speeds, using many blades. This technology was enormously successful, with millions of units installed. It is not surprising, then, that the first electricity-producing windmill, built in 1888 by Charles F. Brush, was of the same design. The resulting machine was enormously heavy, with 144 blades and 1,800 square feet of surface area.
Although the turbine operated successfully for 20 years and supplied his home with electricity, it was an unwieldy and inefficient machine. A more-efficient turbine uses high rotational speed, not high torque. Fewer blades, moving faster, can capture more of the available energy in the wind. European windmills with four sails had the advantage over multiblade ones, and by 1918 there were more than 100 windmills of that classic type supplying electricity to Danish utilities.
The early 20th century saw the birth of the aeronautical industry, and with the knowledge to fine-tune the lift-to-drag ratio it became possible to capture the wind with only two or three blades, moving fast across the wind. This rotor type is more efficient than high-torque ones because it does not generate as much of an energy-wasting swirling wake. Tens of thousands of “propeller-type” two- and three-bladed wind generators brought electricity to ranches across the Midwest plains in the 1920s. Modern wind turbines are similar, but with blades more slender in relation to their ever-increasing length. This has proven to be the most successful blade design.