The Wind Energy Division at Risø DTU has designed a new load-bearing box girder, which is what we call the interior of the grand white aerodynamic shell. The load-bearing box girder is the skeleton that gives the blade its strength and flexibility, but unfortunately also helps to increase the blades weight. Three Risø patents now mean that scientists have been able to save around 40 % of the thickness of main laminate away.
Since 2009 Risø National Laboratory has participated in a EUPD project, titled “Demonstration of new blade design using manufacturing process simulation” – or short DMT. Here they have, together with partner SSP Technology A/S, worked to create an internal structure for wind turbines blades, which can improve both performance and longevity. SSP Technology A/S is a Danish company that develops, manufactures and markets the molds, tools and components in composite materials for the wind turbine industry.
In Risø Blade design group they have their daily work on blade structures, and it has led to no fewer than eight applied patents in the period 2007 to 2009, and three of them are built into this new box girder. The new design, intended to be built into a 40-45 meter long blade on a classical 2.2 MW turbine, could reduce the thickness of a wind turbine main laminate with 40 % - and thus bring the total material consumption and weight of the blade significantly down.
"The main laminate is part of the box girder that carries the biggest loads on the blade. You can compare it with the flanges of a steel girder. Main laminates are very heavy and are composed of many layers of fiberglass overlaid on each other. In a conventional box girder, there may be more than 40 layers in a main laminate, "explains Per Hørlyk Nielsen.
Photo: The inner structur of the box grider (photo: Robert Bitsche)
Unique testing facilities at Risø
In early June, the box girder was delivered at Risø. The construction is designed by the blade group at Risø, and was produced by SSP Technology A / S Kirkeby at Svendborg, in close cooperation with Development Engineer Per Hørlyk Nielsen from Risø Wind Energy Division.
"Today, when we develop new designs and ideas for strengthening the blades, we usually create numerical models to study selected items on the blade. But there are many limitations in the numerical models, which mean that we also have to test the blade structure, and compare the expected values to the actual. Therefore it is extremely important that we have an experimental research facility for blade structures here at Risø," explains Per Hørlyk Nielsen.
The knowledge obtained when scientists compare results from simulations against those obtained directly from tests done on the blade in the test facility is what lies behind many of the innovative patents. And the patents offer solutions to several types of structural as well as material related problems.
Results in autumn
The next step in the project is that the test box girder should be set up in Risø Experimental Research Facility for Blade Structures, replacing the blade, which is mounted there at the moment. And already in this autumn researchers expect that there will be results from the load tests to be performed on the box girder.
"The three patents that are implemented into the structure: a reinforcement of the blade designed to prevent the blade curvature going flat, a reinforcement of the blade that is to calculate for flat laminate panels, and, last but not least, a cross-reinforcement to help prevent the blade twisting unevenly transversely," explains team leader of the Blade Design and Test Group Find Mølholt Jensen, who is also ‘father’ to many of the patents and anxious to get started with the tests.
The box girder structure will be exposed to severe loads of up to 25 tons on the tip, but before that, a multitude of test equipment is to be placed on it, both on the outside and on the inside.
Affiliations
As already mentioned, researchers at Risø have designed the box girder structure, however, it is based on a SSP-designed shape and blade root. The project is also conducted in collaboration with the Materials Research Division at Risø, where Malcolm MacGugan takes care of the acoustic measurements during the upcoming test.
(Photo: Ulla Uldahl) |