Wind-Tunnel Tests of Vertical-Axis Wind Turbine Blades

Abstract

An ongoing research project is investigating the potential of locating vertical-axis wind turbines (WT) on remote, off-grid cellular communication towers. The goal of the WT is to provide local power generation to meet the electrical needs of the tower. While vertical-axis devices are less efficient than their more traditional horizontal-axis counterparts, they provide a number of practical advantages which make them a suitable choice for the present situation. First, the direction of their axis is aligned with the existing tower and its rotation does not interfere with the tower structure. Second, vertical-axis devices are much less susceptible to the direction of wind and they do not require control-systems to ensure they are oriented correctly. Third, vertical-axis turbines have very low start-up wind speeds so that they generate power over a wide range of speeds. Fourth, since vertical-axis turbines rotate at a slower speed compared with horizontal counterparts, they impart a lessened vibration load to the tower. These facts, collectively, make the vertical-axis turbine suitable for the proposed application. The design process involved a detailed initial design of the turbine blade using computational methods. Next, a trio of designs was evaluated experimentally in a large, low-speed wind tunnel. The wind tunnel is operated by the University of Minnesota’s St. Anthony Falls Fluid Laboratory. The tunnel possesses two testing sections. The larger section was sufficient to test a full-size turbine blade. Accounting was taken of the blockage effect following the tests. The experiments were completed on (1) a solid-wing design (unvented), (2) a slotted-wing design (vented), and (3) a capped-and-slotted design (capped). Conditions spanned a wide range of wind speeds (4.5–11.5 m/s). The turbines were connected to electronics which simulated a range of electrical loads. The tested range was selected to span the expected range of resistances which will be found in practice. It was discovered that over a range of these wind speeds and electrical resistances, slots located on the wings result in a slight improvement in power generation. On the other hand, the slotted-and-capped design provided very large increases in performance (approximately 200–300% compared with the unvented version). This large improvement has justified commercialization of the product for use in powering remote, off-grid cellular communication towers.