Abstract
A new class of diffuser augmented wind turbines (DAWTs) is presented. The new diffuser concept exploits aero-dynamic principles for the creation of high-lift airfoil configurations known from the aircraft industry. Combining this with our objective of obtaining a compact power-efficient design has enabled creation of a family of DAWT designs with energy capture potentials which exceed the power efficiency based on the diffuser exit area by 50%. The paper presents the 1D momentum theory governing the DAWTs, and discusses upper limits for power extraction, similar to the Betz limit applicable for bare Horizontal-Axis Wind Turbines (HAWTs). Inviscid axisymmetric panel code calculations are then used to drive the diffuser design towards higher power coefficients. Axisymmetric actuator disk Navier-Stokes calculations reveal the types of stall that inhibit the functionality of the ideal inviscid optimum, leading the design towards the new class of DAWTs. DAWT performance has been differently measured over time, creating confusion. Proper comparison with performance of existing DAWT designs is therefore emphasized. This involves reference to established literature results, and recalculation of earlier DAWT designs in an attempt to project all results onto a common metric.
Highlights
The quest for affordable cost-of-energy competitive renewable energy has driven the wind turbine industry to the mature stage of today
From Equations (23)–(27) we note that the diffuser augmented wind turbines (DAWTs) power-optimal far wake velocity and disk thrust coefficients of 1/3 and 8/9 are independent of the diffuser and identical to the Horizontal-Axis Wind Turbines (HAWTs) power optimum
The tendencies observed in the analysis of the Hansen DAWT in the comparison of momentum theory and panel code results are largely repeated in the case of the circular cylinder DAWT
Summary
The quest for affordable cost-of-energy competitive renewable energy has driven the wind turbine industry to the mature stage of today. Betz limit of 16/27 (in the following referred to as Betz number) for bare propeller turbines are readily obtained This is not the case when normalizing the generated power with the diffuser frontal area. Surface stall from the inner (suction) side of the diffuser was identified as a performance limiting factor, and especially the Grumman Aerospace group targeted their focus towards boundary layer separation control This was achieved by the use of passive devises such as diffuser trailing edge flaps and boundary layer slots, which both inject higher-energy fluid into the adverse pressure boundary layer, with the purpose of stall suppression. A handful of household wind turbine manufacturers have commercialized the DAWT Most of these designs are single-ducted, some are double-ducted with a slot in-between to allow outside flow passing through to the inner-side duct to energize the boundary-layer in the adverse pressure gradient wake behind the rotor. This means that the streamline bordering the wake flow and the surrounding flow emanates from the trailing edge (TE) of the ring-shaped diffuser airfoil
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