Investigation on an Axial Passive Magnetic Bearing System (APMBS) and its application in building integrated vertical axis wind turbines

Abstract

Obvious weather changes have been taking place in the world and global warming and greenhouse gas emissions is still a hot topic. However, for many people global warming is of less importance when faced with economic hardships. The link between the economic development and the consumption of fossil fuel of the past is analyzed first in this thesis by showing that a sustainable energy supply is crucial not only for reducing greenhouse gases emissions, but also for the economic development. The negative economic implication of the dependency on crude oil and other fossil fuels is introduced. The instability of the world economy has been caused partially by the crude oil price fluctuations. The only way to create a stable and sustainable economy is to minimize the consumption of fossil fuels, and the money that might otherwise be lost in future financial collapses could be used wisely now to initiate the move away from a petrol-based economy. To facilitate this move, huge investment is needed for the development of a smart utility grid, non-petroleum based transportation and renewable energy-based energy supply economy, as along the lines of the financial bailout packages and economic stimulus packages issued by the American Government after the financial crisis in 2008. The current situation has forced a number of governments to increase research and development investment in the renewable energy sector. As one of the well-known renewable energy resources, wind energy, which attracts a larger part of today's total investments, is now playing an increasingly important role, especially in China. The work developed in this thesis is focusing wind energy utilization in urban areas. The off-shore and on-shore wind farms are well known, but recently a new application for wind turbines has attracted significant interest from architects, engineers and developers, namely the building-integration wind turbine (BIWT). Several prototype BIWT projects have been developed in Hong Kong, mainland China and other countries, and it is estimated that future urban wind turbines can produce a substantial amount of energy if they are integrated into urban buildings. However, the integration of large rotating machines into buildings has some structural effects on the buildings, like noise and vibration transmissions. The purpose of this project was thus to develop a novel Axial Passive Magnetic Bearing System (APMBS) and to investigate its application in Building Integrated Vertical Axis Wind Turbines (BIVAWT) for wind power generation from buildings in urban areas. In order to get a good estimate of the vibrations of a VAWT, the air velocities and the rotation speed of the wind turbine must be known, therefore the air velocities surrounding a building in an urban area were investigated first in this study. A building in Hong Kong was chosen and its air velocities surrounding the building for a one-year period were simulated at the beginning of this research project. The results of the calculations were then used for wind tunnel tests of several Vertical Axis Wind Turbines (VAWTs), which were designed and manufactured on the basis of CFD simulations. Each constructed Savonius-type vertical axis wind turbine (VAWT) was tested with different overlap ratios, shift angles, and the previously found wind speeds. The wind tunnel test results produced the benchmarks of the rotation speeds for the development of the novel axial passive magnetic bearing system, an invention from this project. An axial passive magnetic bearing system was then invented, which is thought to be best suited for the VAWTs at inner city locations due to its vibration dampening character, low maintenance and low friction. This novel and special Axial Passive Magnetic Bearing System (APMBS) was developed specifically to minimize the transmission of vibrations to buildings. This permanent magnetic bearing is much cheaper and simpler than traditional magnetic bearing systems for achieving highly reliable vertical supporting functions. Many current systems adopt ring magnets to supply magnetic levitation force, but the current size of ring magnets produced is limited because of the difficulty of charging the magnet evenly to produce a uniform magnetic field. This new system consists of small, cuboidal magnets aligned along the rotation path of the bearing. The only problem was that the repulsion force was strong when the stator and the rotor magnets aligned, and weak when they did not align, which caused a higher torque and would induce vibration. This problem was overcome by introducing a unique configuration of the location of the magnets, in conjunction with a thin iron or mild steel sheet (mild steel is the most common form of steel), which was able to unify, strengthen the magnetic field and protect the magnets from aging. Using this method, thinner air-gaps are produced between the rotor and stator, which can increase the stiffness of the bearing. Besides that will the mild steel sheet also distribute the magnetic flux within the iron or mild steel plate more uniformly, which will lead to reduced vibrations. Furthermore, due to the enhanced strength of the magnetic field, cheaper magnets can be used, which makes the bearing desirable for many high performing applications. To optimize the magnetic block arrangement, countless simulations of the magnetic field of the bearing were made and a number of prototypes of different versions of such a bearing were developed from the study. A test rig was constructed for testing the prototypes. The tests found the invented system to be reliable during the wind tunnel test of the VAWT. A simulation using the Finite Element Method (FEM) was carried out to predict the torque of the bearing of any size and loading. This bearing was then tested extensively under different rotation speeds for different air velocities. The torque of the bearing and the vibration transmission form the rotating turbine to the structural frame were recorded and analyzed. The simulation and experimental results demonstrated the advantages of such a bearing. The test results showed that the bearing decoupled the wind turbine from the building. Overall, this new bearing system can lower rotational friction considerably, and minimizes vibration transmission as well. This innovative bearing system should not only be applied to the VAWTs, but also to other rotating devices like flywheels, which can benefit greatly from such a bearing system. The findings of this study have shown that the novel bearing is very well suited for decoupling the buildings from the turbines for renewable power generation in an urban environment. This development has been condensed into a patent application and a large VAWT with this bearing system has been designed and constructed for the Hong Kong Water Services Department (HK WSD) for future on-site tests. Another remarkable finding from the wind tunnel tests of the Savonius wind turbines is that a second performance peak at high Tip Speed Ratios (TSR) of the wind turbines exists, which has been reported only rarely and not been explained in the literature to date. The Savonius turbine has considerable lift properties, but the turbine is commonly considered as a drag driven turbine. The reasons for the existence of this second performance peak are explained in the thesis. The results of the study demonstrated that a wider range of rotation speeds has to be considered during the design of the bearing. For further development of the VAWTs, the concept of a double rotor motor for counter rotating VAWTs was also developed. This motor is based on the structure of a transfer flux machine, which was developed comparatively recently (1989) and has been used commercially in large horizontal axis wind turbines for power production. This new development of the double-rotor motor can be used in the VAWTs to solve the problem of different air velocities at different heights, as well as to eliminate the gear system. This system can be further developed in the future.