Fish Schooling as a Basis for Vertical Axis Wind Turbine Farm Design
Thursday, July 15 2010
When a HAWT extracts energy from the free stream, the speed of the flow behind the turbine is reduced. A well-known aerodynamic result is that the maximum amount of power a rotor can extract is proportional to the cube of the free-stream speed. Horizontal axis wind turbines downstream of other HAWTs have a reduced incoming flow speed and thus are capable of extracting significantly less power than that of a spatially-isolated turbine. Theoretical analysis, computational simulations, and experimental results indicate that decreasing the downstream spacing of a row of multiple HAWTs from ten rotor-diameters to five rotor-diameters can decrease the power generated by roughly 40%. Consequently, most modern HAWT wind farms space turbines five to ten rotor-diameters downstream and three or more rotor-diameters laterally.
An alternative paradigm for wind energy extraction is found in vertical axis wind turbine (VAWT) designs. Studies have shown that a single spatially-isolated VAWT has a significantly lower power coefficient and costs approximately a third more than a HAWT with comparable power output. Some sources indicate that the typical spacing constraints on HAWT wind farms do not apply. Furthermore, a recent U.S. patent suggests that aerodynamic interactions between a pair of counter-rotating VAWTs contribute to a higher power coefficient per turbine. Previous work suggests an increase of up to 4% in the average turbine power coefficient of an array of VAWTs, due to steam-tube contraction (induced higher flow speeds around neighboring VAWTs). In contrast, a separate study shows no change or a mild reduction in power coefficient for a pair of closely-spaced VAWTs. However, neither of these studies considered the possibility of alternating the directions of rotation of neighboring VAWTs.