Joining for Wind Energy by EWI Engineers Bill Mohr and George Ritter Wind energy has been rapidly growing as a source of electrical energy, both in the US and around the world. Recent growth of installed capacity has been at about 25% per year in the US and even faster in some areas of the world.

Blades are multilayer composites that are designed to be long, thin and light, but also stiff enough that they avoid being pushed into the tower or twisted in high wind. The blades have evolved to complicated lay-ups of fiberglass, carbon fiber, epoxy and additional materials. Composite blades have been the first choice for the wind power industry since its beginnings. EWI’s experience in adhesive joining offers clients solutions for improving joining of additional material into the blades.

The hub poses particular challenges because of the switch from composite blade to steel shaft adjacent to the motors that control the blade angle. Composite to metals joining technology is an award-winning specialty of EWI. A six-year study into bonding composites to steel for marine structures was recently completed. In this program, it was shown that composite structures can be adhesively bonded into a steel receiver to produce load carrying ability exceeding that for mechanically fastened joints. Work of that type is continuing for other bonded marine ship structures, including increased ability to model bonded joints and to predict behavior based on the integration of mechanical tests and modeled predictions. This includes testing of joints, verification of performance against the models, and the effects of environmental exposure such as temperature and humidity.

Over the past five years, EWI has also gained increased experience in bonding to other metals, specifically aluminum and titanium. Bonded aluminum structure has been selected by one client to prototype a boom assembly for lifting applications. That assembly will also include strategically placed welds. Others have worked with EWI at bonding aluminum to titanium for maximum weight-strength efficiency. There is also a program underway involving bonding of advanced composite structure to titanium for aerospace applications. This approach can be expanded to bond composite honeycomb structure to metals.

The combination of adhesive bonding and welds into one assembly technique, called weldbonding, is another approach that maximizes joining efficiencies. EWI has worked in weldbonded structure using resistance welding, laser welding, ultrasonic welding, and friction stir welding. The advantage of weldbonding is it incorporates the high shear load capability of adhesive bonding along with the higher peel performance of welding. Together, weldbonded structure provides much improved fatigue resistance while giving better mechanical performance than either bonding or welding alone.

Not all of the joining technology is reserved for the blades. Equipment at the top of the tower must convert the energy from the rotor to another form of energy. For utility-scale wind turbines this is electrical energy, but other intermediate forms are possible. The position at the top of a tower causes a penalty for additional weight that can give additional advantage to welded stiff structures. The top of tower location gives not only a weight penalty, but also exposure to varying loadings that may be large enough to set the design life of equipment such as shafts, bearings, turbines and brakes. EWI can help to make sure these welded structures are sufficient for the lifetime of the turbine.

The tower itself is commonly a welded steel structure with a large proportion of it being shop fabricated and the remainder fabricated on site. Welding technologies that improve the productivity and dimensional accuracy are valuable both for the shop portion and the site fabrication. EWI’s knowledge of welding process allows it to suggest and demonstrate options to improve productivity while retaining the dimensional accuracy to allow rapid on-site construction and high strength.

Whether involving welding or bonding, joining is critical to the original construction, but will also be crucial for repair modification and refurbishment. Repair and maintenance techniques for structure or blades are essential to ensure long life in service. This market is certain to grow as both the number of turbines and their years of operation grow. Wind farms must be placed in areas with good wind flow but also near power grids. This makes site selection a critical factor in the overall economics of the installation. That means placement of towers cannot be limited by the construction methods or the inability to place the towers where the wind is. Larger units are more efficient based on installed cost and energy recovery. For offshore installations, new options for increasing unit size, wind farm size, and high reliability place extra demands on the joining technologies. In fact, the economics of future offshore units will require all three to make the locations economically viable for power production.

Being a member-based organization, EWI works to anticipate the needs of expanding technologies and to be ready with the joining methods that are necessary for our member companies to move forward. Whether it is welding, bonding, NDE, design, analysis, or testing, EWI strives to have the project experience and the team approach that is needed to meet the materials joining needs of a growing wind power industry.