Note: This is the third article of a three-part series covering plastic in wind energy (1) trends, (2) material/process advances and (3) applications.
Wind energy provides significant growth opportunities for composite plastic materials.
The global market for composite materials in wind turbine production is projected to reach $4.7 billion by the end of 2015. Carbon fiber and other advanced composites are expected to play an increasing role in wind blade production, owing to the expansion of offshore installations and the adoption of larger-scale turbines that call for stiffer and lighter materials.
Offshore wind energy is extending the breadth of wind generated power. Land-based wind farms are limited by their impacts on the landscape, large seasonal wind variability and fewer places available for construction. Winds over the ocean are more reliable, attain higher speeds and are less turbulent than winds over land, and no landforms block accessibility of the wind over the ocean.
Even though offshore winds generally offer a better wind resource, installing and operating turbines in harsh ocean environments is challenging. However, intelligent blades that adapt to wind conditions can provide a leap in energy yield.
Let's start by comparing vertical-axis vs. horizontal-axis wind power. Vertical-axis wind turbines (VAWTs) have some appealing features. The vertical orientation accepts wind from any direction and the heavy generator and gearbox equipment rests on the ground instead of on the tower (much easier to access).
However, compared to advances in horizontal-axis wind turbines (HAWT), VAWT disadvantages included the longer total blade length per swept area, cyclic loading, structural resonance, lack of aerodynamic braking and higher operating costs. As a result, HAWT have been the dominant technology for land-based wind systems, especially over the last two decades.
VAWT architecture, however, could transform offshore wind technology. VAWTs may become the favored technology for larger offshore systems in the 10- 20 megawatt scale, where the vertical-axis rotor architecture offers potentially large reductions in cost of energy (COE) for operations and maintenance, substructure, installation and infrastructure.
The economics of offshore wind power are different from land-based turbines due to installation and operational challenges. VAWTs offer three big advantages, namely:
- Lower turbine center of gravity: A lower center of gravity means improved stability afloat and lower gravitational fatigue loads. Additionally, the drive train on a VAWT is at or near the surface, potentially making maintenance easier and less time-consuming.
- Reduced machine complexity: VAWTs have fewer parts because they don't need a control system to point them toward the blowing wind to generate power. Yaw systems for the large rotors in multi-megawatt turbines are expensive both to build and to operate.
- Better scalability to large sizes
Sandia National Laboratories
Horizontal axis (left) versus vertical axis (right) wind turbine offshore power comparison.
The question remains: Do the advantages of the VAWT outweigh its disadvantages when they are configured for offshore sites? Sandia National Laboratories, under a research grant from the U.S. Department of Energy, is investigating the feasibility of VAWTs for offshore deployment. Research is focused on different rotor configurations, novel load control/braking concepts, new materials and manufacturing techniques for large VAWT blades
Next, let's look at transportable wind turbine blade development. In recent years, more and more turbines have been equipped with oversized rotors that increase the annual energy production (AEP) of a turbine in a cost-effective way. This is increasingly difficult onshore because of the high cost and difficulty of road transportation of these large blades.
Blade Dynamics' remote blade completion system is a containerized version of the same assembly technology used in the blade factory. Blades can be transported to the site in two segments in standardized 40-foot containers and permanently consolidated into a seamless blade in a location more local to the wind farm. This feature adds little mass to the structure and has been fatigue tested. The system is available as an option on every onshore blade.
The "Dynamic 49" wind turbine blade, which weighs only 6,150 kilograms, is the lightest blade of this scale in the world and leads others in manufacturing accuracy, lightness, quality and durability. Like all the company's rotors, the D49 can be transported in two separate sections and finished locally, saving significantly on logistics costs.
The shorter components, which are easier to manufacture at high quality, are assembled in laser-aligned jigs. The hybrid blade is made of epoxy, glass fiber and carbon fiber, with the company's Bladeskyn surface coating.
Core to the design is an inner-spar technology built from many multilayer, carbon-fiber-reinforced epoxy sections. The outer shell elements are built predominantly in glass-fiber-reinforced composite, providing aerodynamic cladding and also contributing to the blade's structural integrity.
Finally, let's review progress in wind turbine towers. Wind turbine towers must be strong enough to carry the weight of the turbine, which can reach 200,000 pounds (90.7 metric tonnes) and resist buckling under the stress of the rotating machinery.
An increase in utility-scale wind turbine tower height is shifting tower material toward advanced plastic composites. Wind power developers are pushing for higher towers to capture the high-quality wind found at higher elevations.
These towers likely will surpass the current height standard of 80 meters for 3 MW turbines, growing to between 100 and 150 meters in height for turbines of 5 to 7 MW capacity. Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight.
Composite tower sections would be significantly lighter and more easily transported. Typically, a steel monopole for an 80-meter wind turbine tower prefabricated in sections 4.3 meters in diameter and 21.3 meters in length for trucking to the wind farm site are already at highway load maximums. Composite plastics can:
- lead to substantial saving in transportation/erection
- lower maintenance costs
- improve dynamic damping characteristics
- extend fatigue life
University of Dayton Research Institute
Transporting turbine tower sections (top left), workers install wind tower (bottom) and on-site tower interior inspection (top right).
For offshore installations, corrosion-resistant composites offer advantages over steel in the harsh saltwater environment. Composite tower sections could also be built on site. Shipping raw materials and constructing the towers at the wind farm would save on logistics costs and allow for tower installation in harder-to-reach, more remote terrains.
Funded by the Ohio Department of Development, a consortium has designed, analyzed, built and tested a 100-meter glass-fiber-reinforced plastic composite tower. The research project fabricated a composite wind turbine tower from resin supplied by Ashland Performance Materials. Collaborators in the project funded by a $1.1 million grant from the Ohio Third Frontier Advanced Energy Program led by Ershigs Inc. include Ashland Inc., the University of Dayton Research Institute, WebCore Technologies Inc, and the Edison Materials Technology Center (EMTEC).
