Abstract
Diffuser augmented wind turbines (DAWTs) can increase mass flow through the rotor substantially, but have often failed to fulfill expectations. We address high-performance diffusers, and investigate the design requirements for a DAWT rotor to efficiently convert the available energy to shaft energy. Several factors can induce wake stall scenarios causing significant energy loss. The causality between these stall mechanisms and earlier DAWT failures is discussed. First, a swirled actuator disk CFD code is validated through comparison with results from a far wake swirl corrected blade-element momentum (BEM) model, and horizontal-axis wind turbine (HAWT) reference results. Then, power efficiency versus thrust is computed with the swirled actuator disk (AD) code for low and high values of tip-speed ratios (TSR), for different centerbodies, and for different spanwise rotor thrust loading distributions. Three different configurations are studied: The bare propeller HAWT, the classical DAWT, and the high-performance multi-element DAWT. In total nearly 400 high-resolution AD runs are generated. These results are presented and discussed. It is concluded that dedicated DAWT rotors can successfully convert the available energy to shaft energy, provided the identified design requirements for swirl and axial loading distributions are satisfied.
Highlights
Wind energy today has become the most significant provider of renewable energy
Up to peak power thrust loading, the power performance is not affected by the type of nacelle/rotor, only exception being the centerbody w. cylinder root for high tip-speed ratios (TSR), where the drag force from the cylinder inner part of the rotor blades causes a CP loss of about 0.12
The only distinct power drops are noticed for case 3, where the pressure drop across the inner part of the highly loaded rotor leads to a steep pressure recovery along the centerbody surface, causing centerbody surface stall
Summary
Wind energy today has become the most significant provider of renewable energy. This is the outcome of a reliable robust concept, and the commercial feasibility for up-scaling the 3-bladed lift-driven front-runner “Danish concept” to large wind applications approaching 10 MW in rated capacity. Jamieson and Werle and Pretz reach similar results Both identify the speed-up factor of the axial velocity through the rotor disk at zero thrust as the augmentation factor of the available power efficiency. Werle and Pretz further identify the proportionality shroud coefficient between the axial force on the diffuser and the rotor thrust to equal the speed-up factor at zero power take-out minus one. Hjort and Larsen relax the earlier applied 1D-assumption such that the diffuser-induced speed-up at the rotor plane is no longer constant but is allowed to vary radially, leading to rotor area-averaged but otherwise quite similar expressions They show that Werle and Pretz’ shroud coefficient can only be regarded as constant in the linear proximity of the zero-thrust operational point, and demonstrate through AD.
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