The chapter focuses on describing the author's own multi-level vertical axis wind turbine concept, putting emphasis on its specific features, the scope of conducted analyses, as well as general knowledge important to wind industry specialists, other people with an interest in wind energy, and engineers aspiring to achieve innovative results without needlessly complicating their design. Current results show a reduction of the maximum bending moment during a rotation at the bottom of a two level turbine of up to 19.7% after optimisation; at the same time an optimised turbine can achieve a reduction of maximum moment jump during a rotation at the bottom of a turbine of up to 73.4%. Further studies are currently being conducted, as both the study presented in this chapter and its continuations might have a definitive influence on the future development of the wind-energy sector.
TopIntroduction
Industrial wind energy production is currently dominated by Dutch-type horizontal axis wind turbines (HAWTs).
H-type vertical axis wind turbines (HAWTs), in spite of a simpler built and lower noise impacts, have yet to make a strong impact on the amount of electrical energy produced from wind, as there are so far no records of large scale H-type HAWT farms or even a single unit of over 1MW installed capacity lasting for more than a few years. Sadly, neither producers nor investors feel the urge to report the reasons for low lifetimes of large-scale VAWTs. After conducting a study of both scale and pattern of bending moments at the bottom of a large VAWT tower, I am inclined to consider them a very probable reason for accelerated wind turbine wear. Moment direction oscillations would cause cracking of concrete foundations, with cascading effects, as a loosened tower can gain momentum before hitting the concrete instead of just pushing on it.
The proposed solution to the problem is straightforward: if bending moments and their direction shifts at the bottom of a VAWT are too great, they should, if possible, be reduced. The possibility of such a reduction due to dividing a H-type rotor into parts and shifting them in phase has been analysed with satisfying results. Current results show a reduction of the maximum bending moment during a rotation at the bottom of a two level turbine of up to 19,7% after optimisation, at the same time an optimised turbine can achieve a reduction of maximum moment jump during a rotation at the bottom of a turbine of up to 73.4%. Further studies are currently being conducted, as both the study presented in this chapter and its continuations might have a definitive influence on the future development of the wind-energy sector.
Figure 1. Concept drawing of a dual-rotor H-type vertical axis wind turbine
Figure 1 shows a conceptual drawing of a dual-level H-type VAWT. Proportions in it have been chosen to make the drawing easy to understand and not to provide optimal performance.
TopBackground
Wind turbines are an ancient technology, with traces of wind turbines in Persia dating back to around 400A.D. and claims of wind turbine use in China as far back as 12th century B.C. Wind turbines that produce electricity however are a much newer technology, with the first working examples having been built in 1887 by Prof James Blyth in Scotland (Price, 2005) and simultaneously in 1888 by faculty and students of a high-school in Jutland, Denmark(Jamison, 1997) and by Charles F. Brush and his company in Ohio, U.S.A.(Anon, 1890). It is interesting to note that the first one of those, and as such the first wind turbine used for production of electricity was a vertical axis wind turbine (VAWT). It is also very important to note that VAWT technology is still in its nascent stage, due to problems with reliability of large-scale units. If these problems could be overcome, then Vertical Axis Wind Turbines could become a very important part of the energy mix, due to a series of strong advantages over Horizontal Axis Wind Turbines (HAWTs). Rapid rotation of wing tips of a HAWT is responsible for high levels of noise pollution as compared to VAWTs. The very same quality of HAWT’s, allowing for formation of strong wing-tip vortices, results in comparably high levels of turbulence behind a working turbine and the necessity to space HAWTs far apart. The fact that H-type VAWTs have a less turbulent wake (Dabiri, 2011) allows for achieving greater energy production density over an area – one of important aspects taken into consideration while conducting Strategic Environmental Assessments and Environmental Impact Assessments for energy generating investments. Finally, it is also worth noting, that the Betz limit, commonly referred to as the upper theoretical limit of the kinetic energy of an airflow that can be extracted by a wind turbine refers only to horizontal axis wind turbines. The Betz limit operates on the assumption that there is no mixing between the area swept by the rotor and the surrounding airflow.
Figure 2. Contours of velocity magnitude of an analysed wind turbine level 2D simulation performed in ANSYS Fluent for a wind speed of 9 m/s