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Wind energy is a major source of clean, renewable energy that could be used to meet the needs of the world’s populations. If the demands of this booming market are to be satisfied, we need to develop enhanced wind-turbine generators capable of more electric output, i.e. with larger rotor diameters. Current composite technologies have limitations, however, that are an obstacle to such development.
(Published on March-April 2009 – JEC Magazine #47)
NARASIMALU SRIKANTH, RESEARCH AND DEVELOPMENT DEPARTMENT, VESTAS TECHNOLOGY R&D, SINGAPORE
Producing the stiff, strong and lightweight composites that are needed to prevent potential buckling, fatigue and creep problems is dependent on a number of things, such as:
Energy demand is increasing exponentially due to the growing world population. At the same time, more than 80% of our energy comes from fossil fuel, and these resources are being rapidly depleted. Increased global awareness of the resulting pollution is prompting the engineering community to look for renewable energy sources. Wind energy is one choice that can be viewed as a costeffective, predictable and reliable source of clean energy. Today, both industrialized and developing countries are focusing on wind energy as a reliable resource for the future, with plans to move from land to off-shore deployment. The future of the wind-energy market looks very good through the year 2012 and beyond.
Wind turbine performance
Due to the direct impact of variations in terrain on air flow patterns, traditional windmill designs have always featured elevated towers. Wind turbine manufacturers are pushing the technology towards larger and larger rotor-blade diameters, because the power derived from wind depends on the square of the rotor blade’s diameter. Their efforts have already enabled a radical increase in wind-turbine power generating capacities, from 3 KW in the 1980s to 3 MW in the 1990s. The research departments of leading wind turbine firms continue to seek more radical enhancements. Some of the keys to success are enhanced material characteristics such as low weight, high strength and stiffness.
Traditionally, wood was a material of choice for wind turbine blades. Major players like Vestas had mastered the technology to develop reliable powergenerating machines. However, the constant demand for enhanced performance has spurred along material innovations such as the composite concept, and paved the way for strong, stiff fibres such as glass and carbon, taking material limits beyond those of traditional metals and polymers in terms of stiffness and ductility. Additional improvements have also been achieved in terms of damping capacity, fatigue strength, and more. The wind turbine industry has now adopted composite materials, consuming thousands of tons of these every year. Wind turbines require large rotor blades, nacelles and other components manufactured with customized processes such as wet lay-up, VARTM, prepreg lay-up and other processes.
Composite technologies are key enablers
Technological change and market structure are co-evolving. Jovanovic’s research showed that major innovation triggers an industrial shakeup, forcing out firms that are unable to develop a follow-up on major innovations. William Abernathy and James Utterback described the evolution of products and processes as a transition from an early ‘fluid’ state where performance criteria are not well defined for new products (less standardization) to a highly ‘specific’ and rigid state where the performance criteria are clearly specified (more standardization) and production volume increases. Today, horizontal axis wind turbines (HAWT) have reached the design status where their major features (such as three-blade structures) have become standardized from supplier to supplier, due to a combination of factors such as aesthetics, dynamic stability, and optimum cost versus power generation.
Knowledge depth is what distinguishes market leaders from the rest. Such firms treat technology as a major source of competitive advantage. Patents can be seen as a measure of their explicit form of knowledge, but a firm also keeps knowledge in a tacit form which resides in the people, especially in process technologies, etc. These are reflected in the quality and reliability of the products the firm produces. Vestas wind turbines are one such example, boasting 20 years of outstanding reliability. Dynamic wind turbine systems like these, which operate in all types of conditions with minimal human intervention, constitute a unique engineering product.
As noted earlier, the power generated from wind energy is directly proportional to the rotor area, and hence to the square of the blade diameter. Figure 2 represents a typical power curve, which shows that wind power increases cubically with wind velocity. However, beyond a critical wind velocity, the power capture capability of a wind turbine is de-rated to avoid mechanical damage due to material limits. Rotor blade characteristics are related directly to length in a number of different ways, including:
Rotors are subjected to wind loads and other steady and unsteady loads that increase non-linearly with length. Two main types of loads are involved: 1) wind loads such as steady mean wind, vertical wind shear, cross winds, tower shadow and wind turbulence, and 2) non-wind loads, including centrifugal forces, gravity forces, gyroscopic forces and ground motion and shock loads.
Evolution of composite technology
An industry’s success clearly depends on the application demands and the technology’s push in what they can deliver. This enables innovation transfers from one industry to another. However, the selection criteria and performance expectations of a technology, as well as the resources it offers, enable enhanced growth in that particular technology compared to before migration.
New technologies undergo periods of evolution and revolution and can be created either by convergence or fusion of existing technologies. This can be compared to the evolution process of biological species, where competing views exists. Darwin’s theory assumes that evolution takes place gradually through natural selection, but is unable to explain why some species evolve in intermittent leaps. A punctuated equilibrium model can be used to explain how a geographic separation can cause disruption in a new species and a new species to arise.
A similar phenomenon can be observed when technology migrates from one application or market to another and must meet a different set of performance criteria/demands, as long as healthy resources are available to allow it to grow with unique characteristics. In some cases, the technology can come back to “conquer” the original application. Such technology drifts are encouraged, to create a surge in the overall technological development. But this can only come about with the collaboration of all partners all along the value chain, including scholars, researchers, and suppliers on the outside, and research and development teams, manufacturing staff, marketing and service personnel, among others, on the inside.
We need to focus on both technology development and market application to manage the development of emerging technologies. Intellectual property concerns, legal restrictions, sunk costs may hinder, but are not a reason to abandon an application.
Composite science is interesting for wind turbine generator (WTG) applications for several reasons. First, it makes it possible to increase the specific strength and stiffness of materials. The length of future blades and the power output of future WTGs can also be increased, even as WTG operating costs are lowered due to improved reliability. The size of components/systems can be reduced, and mass production with consistent quality becomes possible.
Wind turbine blade design is predominately driven by fatigue loading, good fatigue performance being essential for blade optimization. As turbine size increases, so does their fatigue loading. In operation, blades are subject to multiaxial loading (aerodynamic loading, gravity, etc.). At present, fatigue design is based only on uniaxial tests, and knockdown factors are applied to take multiaxial effects into account. More sophisticated methods are needed to factor in the multidirectional nature of stress.
However, different operating environments such as sub-zero conditions where ice and snow are common, desert conditions, offshore conditions, lightning-prone conditions and dirt-rich areas involve different sets of requirements that will lead to unique material challenges. Wind speeds in different regions also pose different levels of load and power delivery challenges, so that product differentiation becomes essential. Accordingly, companies develop unique product types to address the specific needs and demands of customers. Below a certain performance level, there is a wider choice of materials available for lines that are similar, but for longer blades, only advanced composites can meet the application requirements.
The main application demands and the challenges posed by the wind industry include:
There are also logistic challenges to be solved, due to the larger size of wind turbines.
Apart from product innovations, process innovations are key in manufacturing high-quality, high-volume components in a cost-effective manner. While wet lay-up was the main blade production method in the early days, firms like Vestas with the necessary know-how are now using enhanced methods such as prepregs.
Such new-generation processes should allow improved defect control during blade manufacture in terms of voids, debonds, delamination, impact damage, density variation, resin variation, broken fibres, fibre misalignment, wrinkles, resin cracks, cure variations, inclusions and moisture.
Today, Vestas wind turbines operate in a range of geographical locations and conditions that spans from hot desert with sand-storms to cold/frosting/snowy/ arctic climates. Wind turbines are now operating from land all the way out to sea, due to the better marine wind resources. The level of performance confidence involved for these regions and design requirements is high (as much as 95+% ), and the service life is 20 years. This raises the material challenge to a new dimension that exceeds even aviation requirements.
Composite technology is a key enabling technology that will directly enhance future wind-turbine capabilities, in terms of lower energy costs for mass production due to reduced loads, increased power, reduced weight and increased reliability. This encourages the use of new composite product and process innovations in wind turbine applications, such as nanomaterials, biodegradable materials, lightning-resistant materials, self-cleaning materials, and low-cost materials, in order to meet performance challenges and benefit from the booming market conditions. This can only be possible if key players such as material scientists, raw-material manufacturers and tooling manufacturers, among others, work together with wind-turbine market leaders such as Vestas.
Narasimalu Srikanth is the technical director of the research and development department of Vestas Wind Turbine. He received his first masters from the Department of Mechanical Engineering of the Indian Institute of Technology, Bombay, India in the area of design engineering and his second masters from National University of Singapore (NUS) with a specialization in materials engineering and further pursued his doctoral degree from the department of mechanical engineering of NUS. Dr Narasimalu has published in more than 100 international journals and conferences, and his areas of technical interests are mechanics & design, simulation, materials engineering and technology roadmaps & management. He is a member of ASME, ASM, ASPE, IEEE and MRS(S).