"Aircraft propellers are driven by engine power and pull the aircraft forward through the air and force the air behind them at increased velocity, whereas wind turbines sit stationary and are turned by the air approaching them," says inventor Bill Khan, president of General Airfoil Dynamics of Glendale, California. "That got me thinking and I came up with the idea of a reverse-orientation design approach."
When a team of scientists and aerodynamicists tested a 2.3-metre diameter version in the Mojave Desert, the results were startling: an increase in capacity factor, which translates to a power gain in the range of 20% a year more kilowatt hours (kWh) for a typical turbine installation site.
To date, the tested rotor has been a two-blade model. Subsequent tests are planned for three-bladed rotors. Hopes are that these will uncover the working principals that explain the higher performance while also scaling up the technology for commercial usage in the megawatt range.The patent for this novel approach to blade design is pending.
Challenging conventional wisdom
The design basis of all standard blades is largely based on aircraft blades and wings — with an extra twist. Much research and billions of dollars over the last century have gone into bringing the conventional blade to the current state of the art.After all, the blade is the most important component as it converts the energy of the wind into mechanical energy before any other part of the turbine can do anything with it. For existing wind farms, any increase in blade efficiency is immediate profit and goes directly to the bottom line. However, studies have aimed mainly at making small adjustments to the existing design and the overall airfoil shape has not changed. In other words, modern blades can largely be characterised as having a rounded leading edge and a sharp trailing edge.
Rather than going with the existing mindset, though, Khan pondered the possibilities of a complete switcheroo — What would happen if the sharp trailing edge became the leading edge facing into the wind, and with the standard large radius leading edge moved to the rear? The inventor made a number of small-scale models using various contoured shapes. Initial testing indicated potential advantages to this concept. Figure 1 overleaf shows both the general shape and contour of a conventional blade and the contour of the reverse-orientation (RO) blade-leading edge.
"Reverse orientation blades consistently showed more power produced over the entire operating wind speed range of a typical wind-farm site," says Khan.
This led to the creation of a development team spearheaded by two aerospace engineers, Kim Aaron, president of Aeronomech Consulting and a doctor of aeronautics from the the California Institute of Technology; and Case van Dam, an aerodynamics professor from the University of California, Davis, and head of the California Wind Energy Collaborative. The development team also includes engineers and specialists in electronics, computers, instrumentation andconventional blade computer assisted design. Gary Kanaby, a consultant at Wind Energy Services Company, gave advice on blade design for manufacturability.
A 2.3-metre diameter blade was designed and manufactured for use in a field test. This blade size allowed operation in the l,000,000 Reynolds number range, which was close enough to full-sized blade values to allow reasonably accurate comparisons. Researchers mounted the blade on a truck and dynamically balanced it.They arranged access to the El Mirage State Park, north of Los Angeles.
Testing took the form of a number of passes on the sand bed at a speed in the approximate range of 25-30mph in order to record the power output of the design.
Wind conditions were ideal for the duration of the test programme; few strong gusts were experienced that could radically affect the basic test data. Once completed, comparative tests were carried out with a conventional design blade of the same diameter. A truck test bed was built with a forward-projecting frame to locate the reverse orientation (RO) blades in front of the truck wind-stream.
Aaron, the project’s chief aerodynamicist, was confounded by the result.All his aerodynamics training told him that its unconventional shape would cause its performance to be terrible in comparison with a more traditionally crafted rotor.
"The RO rotor outperformed all commercially available wind turbines in the 2.3-metre diameter class that I know of," says Aaron. "And this is without any adjustments or improvements."
He does not fully understand why this unorthodox shape performed so well. He recommends that further research be done to comprehend the mechanics involved. In particular, he wants the rotor tested in a wind tunnel with smoke flow visualisation to develop some understanding about how it does what it does.
"Our best guess is some kind of vortex lift, but without wind-tunnel testing to confirm this speculation it’s just a guess," says Aaron. "After we have a better understanding of what it’s doing, we’ll be in a much better position to make adjustment to take advantage of whatever the flow mechanism is and improve the performance even further."
Once wind tunnel testing is completed, optimisation can be done in various directions. Variations in rotor chord distribution, twist distribution, and minor changes to the airfoil shape could enhance performance.
While wind turbines are generally rated for maximum power output, this does not give much of an indication of annual energy production.This depends on the particular site’s wind velocity profile. Power in kilowatts is measured at any given moment in time. Maximum (rated) power is limited to a given wind speed.
Overall energy yield, however, is given in kWh and indicates the total output over a period of time. Therefore, a wind turbine can produce greater annual energy with a lower rated power than another of a higher rated power if it has a greater power coefficient over a wider span of wind speeds. This RO blade concept has thus far managed to maintain its efficiency over a range of wind velocities without the use of peak-power tracking.
"The shape of the graph of power coefficient plotted against tip speed ratio is much broader, allowing the RO rotor to produce power efficiently over a wider range of operating conditions," says Aaron. "We estimate that this rotor will produce about 20% more energy in a year than its closest rival." He emphasises that the 20% figure is based on the power coefficient measured during testing on the truck compared with data for conventional rotors of about the same size.
Figure 2 reflects analysis of a typical wind-power site, illustrating the extra power generated by the RO blade with larger rotors.The performance of conventional rotors increases with size slightly due to the effects of the Reynolds number, says Aaron. He adds that the RO rotor realises its largest benefit at smaller size and is expected not to scale up as fast from there because it is less affected by Reynolds number.
This article originally appeared in Wind Stats