A team of researchers followed the impact of the proposition of arranging the blades of a 1,540 KW capacity wind power plant in May, 2024. 5-megawatt wind turbine at NREL’s Flatirons Campus downwind. Credit: Pietro Bortolotti, NREL
Historically, turbines turn in to the direction of wind and follow the Danish concept which was set in the1980s. It is a horizontal design with three blades that face the wind as it turns; also known as the upwind design.
This spring, however, the scientific team from the National Renewable Energy Laboratory in Golden, Colorado, in cooperation with the Technical University of Denmark (DTU), denied this approach. They sought to answer: What is the position of wind turbine blades during low wind speed conditions or turning the turbine away from the wind?
“Scientists have had very roasted argument for a long time as to whether all turbines must be upwind or not” said Pietro Bortolotti, a wind energy researcher from NREL and the project manager. “The turbines from the 1980s that comprised this standard were less in size and thicker both in terms of blade and tower when compared to the modern day turbines. ”
The recent experiment was carried out with the intention of giving factual information regarding the efficiency of the upwind design.
Turning the Tables
In order to perform the experiment, the researchers from NREL and DTU turned the rotor of a 1. New 5-megawatt research wind turbine at the Flatirons Campus of NREL in Colorado. These measures comprised of making the wind vane, nacelle and rewiring the generator such that it spins in the opposite direction. “Plus, a million other things that had to be done in order not to break anything,” Bortolotti added.
They were pressed in the efforts of transforming the turbine, Pressure belts and microphones were fitted at different intervals.
“We intended to investigate the techno-economic viability of down wind turbines and used the newly developed instrumentation at DTU which gives pressure distribution on a blade of the turbine,” added Bortolotti.
Assembling other wind energy researchers and technicians such as Jason Roadman, Mark Iverson, Chris Ivanov, Jon Keller and Derek Slaughter the team was able to achieve the physical move and commence data gathering.
A Techno-Economic Analysis
This experiment is one of the last tasks in the framework of the so-called Big Adaptive Rotor (BAR) project and was supported by the U. S. Department of Energy’s Wind Energy Technologies Office. BAR is in favour of onshore wind turbines, and aims at the cost-specific measures and, for example, light and flexible blades.
“We questioned if it would improve flexibility and reduce the costs by going down wind,” Bortolotti stated.
The downwind scenario offers some promising aspects: blades are inevitably deflected outward to turn away from the tower making the constructions lighter and more flexible, and the inclination of the rotor helps to redirect the turbine’s wake towards the ground thus having minimum interaction with other units. Research held also indicates that downwind rotors may be useful in increasing power coefficients particularly in large wind turbines.
However the major demerit is shaft reversal when a blade revolves behind the tower, a sound like a thump is heard and fatigue affects the blade. This fluctuation was recorded when receiving data for 11 hours.
Measuring the Impact
This was achieved since the oscillation caused detectable sound and pressure, notable by special belts placed on one blade and the turbine tower obtained from DTU. These belts gave knowledge to the additional fatigue loading on downwind rotors and gave credence to Aeroelastic numerical models that were created by Open FAST tool by NREL.
“Such devices as pressure belts listed above are essential for confirming the accuracy of the numerical models,” said Bortolotti. These belts the team intends to use in the future experiments on a conventional rotors.
Drawing Conclusions
The conclusion to be made for downwind operations, as per the initial findings of the study that shall be published formally in the last quarter of 2024, is that the advantages do not offset the disadvantages.
“Basically, the research community had limited data from the 1980s and anecdote of this phenomenon,” Bortolotti said. It will be possible to state now that although the next generation of the land based wind turbines will be even larger and more flexible the Danish concept will stay leading.
The experiment can be considered as the success as far as collecting the essential data can be coupled with the safe spinning of a large wind turbine downwind. “We have achieved something that no one believed we can achieve,” Bortolotti summed up this conversation. While this 1. 5 MW turbine might be small by today’s standards, the fact of reducing the rpm of this turbine safely, situated downwind and doing this without breaking a single bolt is also significant.