title: Evaluation of Self-Starting Vertical Axis Wind Turbines for Stand-Alone Applications
Creators: name: Kirke, Brian
Publisher: name: Griffith University
Year: 1998
Subjects: subject: Sustainable energy supplies
subject: Wind energy
subject: Savonius rotor
subject: Vertical axis wind turbines

description: There is an urgent need for economical, clean, sustainable energy supplies, not only in densely populated areas where electricity grids are appropriate, but also in rural areas where stand-alone power supply systems are often more suitable. Although electrical power supply is very versatile and convenient, it introduces unnecessary complexity for some off-grid applications where direct mechanical shaft power can conveniently be provided by a wind turbine. Wind energy is one of the more promising renewable energy sources. Most wind turbines are of the horizontal axis type, but vertical axis wind turbines or VAWTs have some advantages for direct mechanical drive applications. They need no tail or yaw mechanism to orient them into the wind and power is easily transmitted via a vertical shaft to a load at ground level. Blades may be of uniform section and untwisted, making them relatively easy to fabricate or extrude, unlike the blades of horizontal axis wind turbines (HAWTs) which should be twisted and tapered for optimum performance. Savonius rotor VAWTs are simple and may have a place where the power requirement is only a few Watts, but they are inefficient and uneconomical for applications with larger power requirements. VAWTs based on the Darrieus rotor principle are potentially more efficient and more economical, but those with fixed pitch blades have hitherto been regarded as unsuitable for stand-alone use due to their lack of starting torque and low speed torque. This starting torque problem can be overcome by using variable pitch blades, but most existing variable pitch VAWTs, variously known as giromills or cycloturbines, need wind direction sensors, microprocessors and servomotors to control the blade pitch, making them impracticable for stand-alone, non-electrical applications. A simpler but less well known concept is passive or self-acting variable pitch in which the blades are free to pitch under the combined action of aerodynamic and inertial forces in such a way that a favourable blade angle of attack is maintained without the complexity of conventional variable pitch systems. Several fonns of self-acting variable pitch VAWTs or SAPVAWTs have been described in the literature, several patents exist for variants on the concept, and at least two companies world-wide have attempted to commercialise their designs. However the aerodynamic behaviour of these devices has been little understood and most designs appear to have been based on nothing more than a qualitative appreciation of the potential advantages of the concept. This thesis assesses the potential of both fixed and passive variable pitch vertical axis wind turbines to provide economical stand-alone power for direct mechanical drive applications. It is shown that the starting torque and low speed torque problems of VAWTs can be overcome either by passive variable pitch or by a combination of suitable blade aerofoil sections, either rigid or flexible, and transmissions which unload the rotor at low speeds so that high starting torque is not necessary. The work done for this thesis is made up of a sequence of stages, each following logically from the previous one: 1. Several tasks have been identified which could be performed effectively by a self-starting vertical axis wind turbine using direct mechanical drive. These include, a. pumping water, b. purifying and/or desalinating water by reverse osmosis, c. heating and cooling using vapour compression heat pumps, d. mixing and aerating water bodies and e. heating water by fluid turbulence. Thus it is apparent that such a system has the potential to make a useful contribution to society. 2. A literature survey of existing VAWT designs has been carried out to assess whether any are suitable for these applications. 3. As no suitable existing design was identified, an improved form of SAPVAWT has been developed and patented. 4. To optimise the performance of the improved SAPVAWT, a mathematical model has been developed in collaboration with Mr Leo Lazauskas of the University of Adelaide (see Kirke and Lazauskas, 1991, Lazauskas and Kirke, 1992). As far as the author of the present thesis is aware, this is the only existing mathematical model able to predict the performance of this particular type of SAPVAWT, and one of only two worldwide which model SAPVAWTs. 5. In order to use the mathematical model to predict the performance of a given SAPVAWT, it is necessary to have lift, drag and moment data for the aerofoil profile to be used, over a wide range of incidence and Reynolds numbers. A literature search has revealed large gaps in the existing data. 6. Wind tunnel testing has been carried out to assess the effect of camber on the performance of one set of NACA sections at low Reynolds number, and performance figures for other sections have been estimated by interpolation from existing data. 7. Using the assembled aerofoil data, both experimental and estimated, the mathematical model has been used to predict the performance of both fixed and variable pitch VAWTs. It has been found to predict correctly the performance of known fixed pitch VAWTs and has then been used to predict the performance of fixed pitch VAWTs with cambered blades using newly developed profiles that exhibit superior characteristics at low Reynolds numbers. Results indicate that fixed pitch VAWTs using these blade sections should self-start reliably. 8. To validate the mathematical model predictions for self-acting variable pitch, a two metre diameter physical model has been built and tested in a wind tunnel, and acceptable agreement has been obtained between predicted and measured performance. 9. To demonstrate the performance of a SAP VA WT under field conditions, a six metre diameter turbine has been designed, fabricated, erected and tested. 10. Because a prime mover such as a wind turbine is of no use unless it drives a toad, particular attention has been paid to the behaviour of complete systems, including the wind turbine, the transmission and the load. It is concluded that VAWTs with the improved self-starting and low speed torque characteristics described in this thesis have considerable potential in stand-alone, direct mechanical drive applications.