Diffuser Augmented Wind Turbines (DAWTs) are one of many concepts to have been proposed to reduce the cost of renewable energy. As the most commercially viable, they have been the focus of numerous theoretical, computational, and experimental investigations. Although intimated in these studies to be able to augment the power output of a wind turbine, the extent of this power increase, or augmentation, the factors influencing DAWT performance, the optimal geometric form and their economical benefit remained unanswered. It is these issues that have been addressed in this investigation. In reviewing historic investigations on DAWTs it has been identified that excessive wind tunnel blockage, inappropriate measurement technique, varied definitions of augmentation, and the inclusion of predicted performance based on incorrect assumptions have in general led to the overstatement of DAWT performance in those studies. In reassessing the performance of the most advanced of those DAWT designs, Grumman's DAWT 45, it has been calculated that the actual performance figures for the 2.62 exit-area-ratio and 0.488 length-to-diameter ratio DAWT were an available augmentation of 2.02, a shaft augmentation of 0.64 and a diffuser efficiency of 56%. By contrast, the development of the Mo multi-slotted DAWT in this investigation has yielded a design whose shaft augmentation of 1.38 was achieved by a diffuser with exit-area-ratio of only 2.22 and overall length-to-diameter ratio of 0.35. Such performance improvement has been obtained by gaining both an understanding of the flow characteristics of DAWTs and the geometric influences. More specifically it has been shown that: the velocity across the blade-plane is greater than the free-stream velocity and increases towards the rotor periphery; that the rotor thrust or disc loading impacts upon diffuser performance by altering the flow behaviour through it; and that DAWTs are able to maintain an exit pressure coefficient more negative than that attainable by a conventional bare turbine. The net result is that DAWTs encourage a greater overall mass-flow as well as extract more energy per unit of mass-flow passing through the blade-plane than a conventional bare turbine. The major drivers of DAWT performance have been shown to be the ability of the design to maximise diffuser efficiency and produce the most sub-atmospheric exit pressure possible. Parametric investigation of the various DAWT geometric components has shown peak performance to be obtained when: the external flow is directed radially outward by maximising the included angle of the external surface in conjunction with a radially orientated exit flap; by applying boundary-layer control to a trumpet shaped diffuser via a pressurised cavity within the double-skin design of the multi-slotted DAWT; having an exit-area-ratio of the order of 2.22; and by employing an inlet contraction with inlet-area-ratio matched to the mass-flow passing through the DAWT under peak operating conditions. To translate the available augmentation into shaft power a modified blade element method has been developed using an empirically-derived axial velocity equation. The resulting blade designs whose efficiencies reached 77%, twice those of Grumman, highlight the accuracy of the modified blade element method in calculating the flow conditions at the blade-plane of the multi-slotted DAWT. It was also noted that the rotor efficiencies remain below 'best practice' and therefore offer the potential for further increases in shaft augmentation. However, in order to achieve such gains, a number of limitations present in the current method must be addressed. In assessing the likely commercial suitability of the multi-slotted DAWT a number of real-world influences have been examined. Shown to have little if any effect on DAWT performance were Reynolds number, ground proximity and wind shear. Turbulence in the onset flow on the other hand had the beneficial effect of reducing separation within the diffuser. Finally, DAWT performance was assessed under yaw misalignment where it was shown that the multi-slotted DAWT performed favourably in comparison to that associated with a conventional bare turbine. The major drawback identified in the DAWT concept by this investigation was its drag loading and the fact that drag and augmentation were interdependent. The result is that the cost of a conventional DAWT is dictated by the necessity to withstand an extreme wind event despite the fact that augmentation is only required up to the rated wind speed. The overall conclusion drawn was that in order to optimise a DAWT design economically, and therefore make the DAWT concept a commercial reality, a creative solution that minimises drag under an extreme wind event would be required.
| Identifer | oai:union.ndltd.org:ADTP/277894 |
| Date | January 2003 |
| Creators | Phillips, Derek Grant |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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