By Amy Berry
Traditional Wind Farm Site Disadvantages Are Actually Advantages in Urban Settings
For most who hear the words “wind power” the mind conjures up images of towering white propellers in a wide open rural setting. These large propeller turbines, also known as horizontal axis wind turbines (HAWTs), are the standard in the large wind industry due to their excellent efficiency in converting wind to power. However, what makes them ideal for large scale wind farms (large and remote open spaces with consistent wind direction) does not necessarily make them a great fit for urban sites. In fact, the traditional limitations of vertical axis wind turbine (VAWT) technology for wind farm applications can actually turn into advantages for more urban locations. Increasingly, homeowners and small businesses are considering VAWTs to help overcome the challenges associated with many small wind sites.
It is important to understand the difference between HAWT and VAWT technology to understand why each is well suited to particular applications. HAWTs have blades which rotate vertically around a horizontal axis, similar to a propeller on an airplane. Propeller turbines need to be oriented perpendicular to the direction of the wind to be efficient, and in variable or more turbulent wind conditions they need to constantly re-orient themselves, losing efficiency in the process. Propeller blades are designed to use lift to propel themselves around faster than the speed of the wind. The part of the blade near the hub turns at a reasonable speed, because of their rigid outstretched blades, but the tips whir around at greater speeds; this is known as tip speed ratio. Typically the tip speed ratio of a HAWT is seven to ten times the speed of wind.
While HAWTs are efficient in using lift to maximize energy transfer and electricity production, the main drawback in an urban setting is that their tip speeds can create high levels of noise which can be bothersome to neighbors. Some more modern HAWT designs seek to lessen noise by employing special curvature in the blades. And while wind direction in the open spaces of wind farms is fairly consistent, wind direction in urban settings is often changing. HAWTs are not able to adapt quickly to changing wind directions, and thus operate extremely inefficiently in more turbulent conditions, as compared with VAWTs.
VAWTs include two main classes: a tall vertical airfoil style (Darrieus), and a solid winged style (Savonius). Darrieus Turbines come in a few varieties. Some have rotors with curved blades that look like an eggbeater and rotate about a vertical axis. Another variation uses straight-sided airfoils and is called a Giromill. Like propeller turbines, Darrieus turbines utilize some lift to capture wind energy and operate with tip speed ratios in the lower-middle range. Savonius Turbines have rotors with solid vanes or “scoops” which rotate about a vertical axis (picture an anemometer), using “drag” to allow the wind to push them around. The principle drawback of Savonius turbines is that drag produces far lower energy efficiency than the other types of wind turbines.
Traditionally VAWTs are not recommended for large wind energy production because they are a little less efficient than HAWTs, and do not scale as well to very large applications. However, the ability of VAWTs to operate silently and efficiently in variable and turbulent wind conditions make them a viable option for urban locations in which these are common site characteristics. The fact that they operate at lower rpm’s and with tip speed ratios only 2-3 times the wind speed means that they can produce power without creating noise. VAWTs also readily capture wind energy from any direction, allowing them to work with the constant changing wind directions in urban settings.
At the end of the day, the most important factor is power output. If a VAWT is able to provide ample energy output in an urban setting, then it is a real option for homeowners, small businesses and governments to consider.
Amy Berry can be found on twitter @wind2power tweeting about small wind power and the Windspire wind turbine.