Experimental Analysis of a Counter-Rotating Wind Turbine

Increases in wind turbine efficiency have helped to provide cost-effective power to an ever-growing portion of the world. This paper explores the possibility of increasing power production using two counter-rotating sets of wind turbine blades. A review of design characteristics, such as number of blades, blade angle of twist, chord length, and generator efficiencies, resulted in the design of a counter-rotating wind turbine incorporating three different National Renewable Energy Laboratory (NREL) cross-sectional blade profiles along the span of the blades. A three-blade front system and two different three-blade rear systems were studied. The blade prototypes were modeled in SolidWorks^®, produced using a Dimension SST 3D printer, and then tested using two Parallax™ four-pole stepper motors as generators in a model 406B ELD wind tunnel. Testing was performed between 15 mph and 40 mph in 5-mph increments. Preliminary results indicate that a counter-rotating assembly is promising for increasing energy extraction from a column of air. The counter-rotating system reached its optimum operating efficiency in wind tunnel testing at 25 mph using an exact reflection for the rear fan. At these test conditions 0.40% of the energy in the air column was converted into usable power. This outcome compares to a 0.21% power conversion when testing only the front-blade system. Additional testing will be completed using flow visualization in a ELD 502 water tunnel along with CFD analysis. The purpose of this testing is to discover air column behavior behind the upstream and downstream blade systems to optimize the design and increase total system efficiency. An appropriate scaling method must also be found. Currently, an energy model is being used to scale from the wind tunnel to the water tunnel. These tests would make it possible to design blade sets to create a maximum total efficiency at a specific wind speed. It would also be valuable to determine if counter-rotating systems could expand the range of possible turbine locations by lowering the required wind speed for significant power generation.