When most people think of wind turbines, they imagine the large, utility-scale turbines seen on “wind farms,” which can be from 500 to 5,000 kilowatts. Small wind turbines, rated at 100 kilowatts or less, have been used for decades, often on farms and rural homes to pump water or provide power. Now new “micro” or “urban” wind turbines are being designed for residential and commercial buildings with less space and lower wind speeds.
Like their larger counterparts, the mechanics of small wind turbines are actually quite simple. The energy in the wind turns propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. Some urban wind turbines look like scaled-down versions of traditional propellers, and are called “horizontal-axis” turbines, while others, known as “vertical-axis” turbines generally use darrieus or savonnius designs, which use lift and drag, respectively, to spin the blades. Vertical-axis turbines are generally less efficient than horizontal turbines, but offer some advantages for urban areas, including lower wind start-up speeds, better ability to handle winds from various directions, and possibly fewer impacts on wildlife.
To better understand San Francisco’s urban wind opportunities, former Mayor Gavin Newsom and then-Supervisor Tom Ammiano created the Urban Wind Power Task Force in July 2008. The Task Force’s mandate was to explore the potential for small-scale wind generation in San Francisco and develop recommendations for advancing City policy to encourage the expansion of local wind power generation.
Though San Francisco has only a “moderate” on-shore wind resource for medium- to large-scale turbines, the City’s small-scale wind resource is not yet fully understood. Conditions in some parts of the City may be suitable—perhaps optimal—for urban wind applications, for example at the top of tall downtown buildings, along certain city streets where winds are funneled by the surrounding buildings, or neighborhoods along the coast with unobstructed Pacific winds.
While much remains unknown about the use of small wind turbines in urban environments, including real world performance and structural impacts, urban wind has the potential to offer several benefits. Besides mitigating climate change and reducing the need for fossil fuels, urban wind has the potential to provide homeowners and businesses with a clean, distributed energy option for managing their energy needs and savings while increasing their property values. Urban wind may also provide an opportunity to create new green jobs.