Feb 10, 2009
Wind turbine blades: the bigger, the better?
The days of cheap and abundantly available energy are over. The industrialized world is running out of fossil fuels at a time when a paradigm shift in energy prices is occurring. It’s clear that this century will be characterised by intensified competition for energy and this will inevitably push up prices, lead to periodic scarcity and precipitate a scramble for reserves among the world’s main economic blocks.
What’s more, dependency on imported fossil fuel has become a threat to economic stability because of the impact of increased fuel prices on the long-term cost base. Perhaps most importantly, there is a growing awareness of the irreversible and potentially disastrous effects of climate change, to which the burning of fossil fuels contributes.
The European Union’s Green Book estimates that imported energy to the EU will increase from 50% today to 70% in 20 to 30 years. So it is essential that internal energy resources are developed as much as possible, and that energy efficiency is promoted as well. It is an EU goal to have 20% sustainable energy in 2020.
At the moment, the most promising and mature renewable energy technology appears to be wind power. Wind energy will not only be able to contribute to securing energy independence and climate goals in the future, it could also turn a serious energy supply problem into an opportunity in the form of commercial benefits, technology research, exports and employment.
To meet these challenges, the number and size of wind turbines has increased strongly in recent years. This development is expected to expand significantly, especially with the installation and operation of very large numbers of wind turbines in offshore wind parks. These will effectively serve as large power plants that produce electric power directly to the grid.
But wind energy faces a wide spectrum of challenges itself, from the social and marketing-related significance and control of wind-turbine-produced energy, to the design and analysis of technically-advanced wind turbine components, to controls and electronics-related problems.
A number of different concepts for wind turbines have been proposed over time. The first classification refers to wind turbines with either vertical or horizontal axis rotor systems. Among the most well know vertical axis systems are the Darrieus and Savonious concepts. However, most modern commercial wind turbines are of the horizontal axis type, and all major wind turbine manufacturers have focused their commercial programmes on upwind three-bladed aero generators. They’ve made this choice as a compromise between many factors, including energy output efficiency, robustness and reliability, manufacturing, installation and service costs, and generated noise levels.
A modern wind turbine is an advanced power plant consisting of a large number of parts and subsystems. Starting from the ground these include the foundation, tower, nacelle with electric generator and drive train, and finally the rotor system made up of the hub and rotor blades – wind turbine blades or wings. The rotor system makes up a substantial part of the cost of a modern wind turbine – typically 20-30% of the acquisition costs depending on whether the turbine is for onshore or offshore use.
Wind turbine blades are being manufactured using polymer matrix composite materials (PMCs), in a combination of monolithic (single skin) and sandwich structures. A sandwich structure is a special form of laminated composite material composed of two thin, stiff and strong face sheets (PMCs in this context) separated by a relatively thick, compliant and lightweight core material. The resulting assembly provides a structural element with very high bending stiffness, strength and buckling resistance as well as very low weight.
Today’s wind turbine designs are mainly based on glass fibre reinforced composites (GFRPs), but for very large blades carbon fibre reinforced composites (CFRPs) are being introduced in addition to GFRP by several manufacturers in order to reduce the weight.
Over the last 25 years wind turbines have become significantly larger, from a rated power of 50 kW in the late 1970s to the multi-megawatt power plants of today. This trend is expected to continue for at least another decade. The largest modern wind turbines have rated power outputs of 5 MW or more and rotor diameters of more than 125 m. The driving motivation is that larger wind turbines have larger energy output per unit rotor area due to increased mean wind velocity with height. What’s more, even though larger wind turbines are more expensive to install and operate than smaller ones, the total production cost per kilowatt hour of electricity produced has generally decreased with increasing wind turbine size.
So it is anticipated that wind turbines with a rated power output in the range of 8-10 MW and a rotor diameter from 180-200 m will be developed and installed within the next 10-15 years. But current design methods and available components and materials do not allow this amount of up-scaling of blade size (or other turbine components). Also, gravity loads on wind turbine blades increase as the rotor disk diameter increases. It is to be expected that these loads will become more dominant than the wind loads, which again will lead to a significant increase in the weight and cost of the rotor system. The "optimal" wind turbine dimensions (or selection of design parameters) are not known at the present time, and it’s unlikely that a global or universally meaningful optimal design solution exists, but it is safe to say that the "best/optimal" design will depend on the material, design and operational characteristics of any given wind turbine concept.
Several research projects involving wind turbine manufacturers, service providers, universities and research organizations are addressing the technological barriers associated with the design, manufacturing, installation and reliable operation of very large turbines with a rated power about double that of today’s largest specimens. These research projects aim to explore and resolve design limits for future wind turbines. Critical issues for the future development of wind turbine blades include: innovative materials with a sufficiently high stiffness – and strength (both static and fatigue) – to-mass ratio; structural and material design of rotors; damage-tolerant design principles; cost-effective materials systems and cost-effective manufacturing; embedded health monitoring systems and smart structure technologies; and finally development of environmentally-neutral material and design solutions where the full life cycle involving manufacturing, service life and decommissioning is accounted for.
The EU’s UpWind project (Upwind 2006 – http://www.upwind.eu/default.aspx), is a five-year multinational research effort involving some 40 manufacturers, service providers, universities, research organizations and other professional organizations and is a good example of research into these areas.
Similarly, the Blade King project aims to develop radical and innovative technologies to increase manufacturing speeds (i.e. productivity) for the production of wind turbine blades. This should enable an increase in the overall production rate by a factor of at least two, and a much higher value is expected. How this overall objective is to be met is proprietary information for the time being, but I can reveal that it will involve the development and implementation of innovative fibre reinforcement, polymer resin and processing/manufacturing technologies. The project is co-funded by the Danish Advanced Technology Foundation, a Danish government research funding organisation, together with Risø DTU National Laboratory for Sustainable Energy, Denmark, Aalborg University, Denmark, LM Glasfiber and Comfil and will run until 2013. The total project budget is about £7.6 million.
About the author
Ole Thybo Thomsen is head of the Department of Mechanical Engineering and Professor of Solid Mechanics at Aalborg University in Denmark. He has led numerous research projects funded by the EU Commision, the European Space Agency (ESA), US Navy – Office of Naval Research (ONR), the Danish Research Council for Technology and Production Sciences (FTP), the Danish Agency for Science, Technology and Innovation (an institution under the Danish Ministry of Science, Technology and Innovation) and by industry and charitable foundations. In recent years many of the research projects have been conducted with leading wind turbine and blade manufacturers. Ole Thybo Thomsen is a fellow of the Danish Academy of the Technical Sciences.