Two years ago, Risø DTU had the opportunity to set up measurement masts on the small peninsula of Bolund at Roskilde Fjord. Within a few months unique data on wind flow in complex terrain was collected. The data can be used to check the different wind calculation programs around the world. In the autumn of 2009 users from Europe, the USA and Canada participated in a blind comparison and a workshop where the measurements were published.
The world has not seen any measurement experiments like the Bolund experiment, so many wanted to register when the Wind Energy Division at Risø DTU was going to do a blind comparison based on the Bolund measurements.
The task was to predict the wind conditions around the steep slope of Bolund based on wind data from a mast some distance away out in the water.
"When the deadline expired 1 November 2009, we had received 54 responses from wind turbine manufacturers, consultants and academics from most countries in Europe, the USA and Canada, "says Research Scientist Andreas Bechmann. "The participants had used many different computer programs, and some had even built models and done measurements in wind tunnels."
During the month of November Risø scientists tried to get an overview of the results that were to be presented at a workshop at Risø in December. They also made their own prediction using the EllipSys program, which has been developed in collaboration between Risø DTU and DTU Mechanical Engineering.
"Of course we aimed to be completely objective, but the other participants were perhaps a little sceptical, as we took third place," says Andreas Bechmann. However, when we corrected all the mistakes in the submitted results after the workshop, we "only" took fourth place.
The programs are okay, but users could become better
At first glance, the results seem surprisingly different; however, the results are more alike when you leave out of account the replies where the task has been misunderstood. Yet it is striking how different results you can get, even when using the same tool.
"Our conclusion is that many of the tools work fine, but users may be better to use them. Many simply lack the experience, "says Andreas. "But more importantly, they also lack measurements like those from Bolund to verify their calculations."
"It was also striking that one of the most expensive and advanced models, LES (Large Eddy Simulation), did rather badly in the test, but it's probably just because people lack experience in using it. Moreover, it is still being developed."
There is a need for more measurement experiments
In early December, 80 people gathered at Risø for a workshop where the results of the blind comparison and the actual measurements were presented. They spent some very exciting days with discussions, says Andreas.
"Everyone wanted to share their results with each other and also with the rest of the world, even though you could have expected secretiveness before this workshop. The group wanted to continue to work together and maybe make new comparisons. "
Currently, scientists in the Wind Energy Division are working at full throttle to write articles about the results and conclusions.
The Bolund measurements can be used in standard tests
As mentioned, the Bolund measurements and the blind comparison are unique. The blind comparison is inspired by NREL/NASA-Ames, which 10 years ago did wind tunnel measurements of a model wind turbine with a subsequent blind comparison. Today these measurements are used to verify calculation tools for wind turbines. So it would be natural to use the Bolund data as a reference when new calculation tools for terrain are to be certified.
Perhaps the blind comparison could be used in new computer programs and help researchers predict the wind in new locations for wind turbines.
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