December 8, 2013
Are We Really Building the Best Wind Turbines?
A new turbine design uses old-fashioned know-how to reduce total capital costs, generate more power, and lower maintenance costs. Is this the future of wind turbine technology?
Ever since the Wright brothers used cables to strengthen the wings on their early planes, this approach has been perceived as outdated, or simply put, old fashioned. However, the Glen Lux designed, built, and patent pending Vertical Axis Wind Turbine (VAWT) is unique in that it uses this highly effective technology to reinforce its multiple blades. For example, the Lux VAWT design with a one-megawatt power generating capacity has one-third the total weight of a comparable Horizontal Axis Wind Turbine (HAWT), mainly because it does not rely upon a tower or central column. The Lux VAWT also uses the same blade profile along the entire blade length, thus the reduced construction inputs drastically reduce the capital costs. Table 1
The Lux VAWT has six evenly spaced blades that are curved into an elliptical shape. The chord length of the blades is shorter than previously used blades, but the structural part of the rotor is reinforced by the use of several small diameter cables. By strategically attaching the cables to the blades, the aerodynamic drag is minimized. The cables follow a near circular path between their blade-to-blade connections. This results in a small amount of lost power; however, the structural integrity of the rotor, which consists only of blades and cables, is significantly enhanced. There is no need for an expensive central column or struts.
From the 1970s to the 1990s researchers from Sandia Labs in Albuquerque New Mexico, and the National Research Council in Ottawa, Canada worked on VAWT or Darrieus-style turbines, with two or three blades rotating around a vertical axis. The more common type of wind turbine is the HAWT that rotates around a horizontal axis. At that time it was unclear which style was more economically feasible. Both types had their limitations.
As these two organizations experimented with two and three bladed turbines, they tried different blade profiles, adjusting solidity ratios, height/diameter ratios, and blade curvature, offset blade angles and strut positioning. The power outputs were comparable to the HAWT, however, several of these demonstrator prototypes, including the largest VAWT ever built, a four-megawatt turbine in Quebec, encountered structural problems. A five hundred kilowatt turbine located in Prince Edward Island had a premature structural failure, primarily due to fatigue and corrosion issues. In addition, these two bladed designs suffered from large fluctuating torque excursions that caused poor power quality.
For the past few years Canada’s National Research Council analyzed a Lux Turbine model with a diameter of 40-meters and a height of 65 meters with a one-megawatt power output. The Aerodynamic Lab used the Double Multiple Stream Tube Model computer program to predict the power output and wind loads along the entire length of each blade as the VAWT was rotated. These aerodynamic lift and drag loads, along with the centrifugal and gravitational forces were then applied to the six blades of a Finite Element Model of the Lux VAWT by the Structural and Material Performance Lab. The computer simulation program was used to determine the locations with the most damaging stresses and deformations. From the operating range of rotational stress data and using rainflow analysis they were able to predict the levels of damage to the blades, caused by these forces. The analysis performed by NRC, was for an International Electrotechnical Committee class II wind regime. Using a simulated annual wind spectrum, NRC predicted a life expectancy for the modeled blades to be measured in centuries instead of years. Deformations, even at extreme wind speeds that would be encountered once in 50 years, were exceptionally small. Further analysis on blade natural frequencies, temperature gradients, extreme wind loads and buckling were also achieved without any concerns to their long-term performance. The multiple blade design eliminated the torque excursions, a major source of structural fatigue loading, and achieved minimal torque ripple resulting in improved power quality. Table 2
The NRC also scaled the turbine to a remarkable 160-meter diameter with an approximate output of 16 MW. Surprisingly, the blade stresses remained relatively constant. It appears the maximum turbine size is only restricted to the economies of scale. As an added plus, large VAWTs are able to take advantage of the higher winds above the earth’s surface without the use of towers, making them ideal for use off shore. Table 3
VAWTs built in the past have certainly had their problems. The government program in Canada was folded in the late-1980s and Sandia suffered a similar fate with their vertical wind turbine program. Compared to the horizontal wind turbines, the vertical turbine has had relatively little research; however, in our opinion, it has far greater potential. In the quest for the perfect turbine, the vertical axis model has been overlooked. Some of the advantages of the Lux VAWT turbine include:
• Lower capital cost
• High power output
• Low maintenance with few moving parts
• Low overall center of gravity
• Easy access to ground level power generation
• Gearboxes and generators can be contained in sound proofed ground level facilities to reduce external noise
• Higher power output/land area by using counter rotating turbines in close proximity
• A simple Friction Drive system reduces or eliminates gearbox problems
VAWT are wind direction independent and the turbines offer uncomplicated scalability to smaller or larger sizes with excellent longevity. These are important issues, especially since the warranty period for newly designed and manufactured HAWTs has been decreasing in recent years, despite the fact that the bulk of research money over the years has been put into complex HAWTs. Isn’t it time to rethink the direction we are headed?
BSc. Eng. is CEO of Lux Wind Power Ltd., Saskatoon, SK, Canada. For more information visit www.luxwindpower.com or call 306-221-3297.
, MASc PEng is with the Structures and Materials Performance Laboratory, Institute for Aerospace Research National Research Council Canada, in Ottawa, ON, Canada. For more information visit www.luxwindpower.com or call (306) 221-3297