Why use natural laminar flow airfoils in vertical axis wind turbine applications?

Abstract

Since the early eighties, Natural Laminar Flow (NLF) airfoils have been preferred to NACA airfoils on Vertical Axis Wind Turbine (VAWT) blades because their use normally lead to a lower Cost Of Energy (COE). This paper wants to illustrate and clarify the relative merits of using NLF airfoils in the design of VAWTs. This is achieved by the application of the doublemultiple-streamtube model of Paraschivoiu to the performance predictions of VAWTs equipped with conventional (symmetrical NACA airfoil series) and NLF (SNLA airfoil series of Sandia Laboratories) blades. The individual contributions of lift and drag to power are presented so that the aerodynamic efficiency of both types of airfoils may be compared. It has been demonstrated that NLF blades are aerodynamically more efficient than conventional blades only in a very low wind speed range so that the point behind the use of NLF airfoils in VAWT applications is not associated to their low drag characteristics present only in the very low wind speed range, but rather to their sharp and early stall characteristics which lead to a relatively flat power curve at high wind speeds.