Analysis aerodynamics diffuser-augmented wind turbines

A complete and consistent, one-dimensional momentum theory is derived for the aerodynamics of slotted diffuser-augmented wind turbine (sDAWT). This theory does not require empirical relations. It results in a set of three-parameter relations for aerodynamic characteristics such as power coefficient C-p, rotor resistance k, rotor axial force coefficient C-T, axial induction factor a, wake-expansion factor beta, duct axial force coefficient C-T(,duct) and slot mass flux <(m)over dot>(duct). The theory predicts that the maximum achievable power coefficient C P of (s)DAWT's increases monotonically with increasing beta, surpassing the Betz limit of open-rotor wind turbines (ORWT's), already for modest (>2) beta's. The slot of an sDAWT feeds outside air into the diffuser, which for given beta decreases the flow through the rotor and therewith C-p. However, the flow through the slot delays the onset of flow separation in the diffuser, increasing the maximum achievable beta and therewith the power coefficient of sDAWT's beyond that of DAWT's. Based on a vortex model of the (s)DAWT, an expression is derived for the velocity induced at the rotor plane by the diffuser and for the corresponding circulation of the diffuser. The derived three-parameter relations for sDAWT's reduce to two-parameter relations for DAWT's and the familiar one-parameter relations for ORWT's.