JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012 - 39

design

Innovative design procedures for large-scale wind turbine blades
Developing a large-scale blade is a key issue in the current competitive wind turbine market simply because a wind turbine with large-scale blades provides the most effective way of reducing the cost of energy (COE). Rotor blades up to 128 m in diameter for wind turbines with power ratings up to 2.5-6.15 MW are already available in the market, and are expected to grow more in future thanks to the development of innovative blade designs.

Sung Kyu Ha, Professor Department of Mechanical Engineering, Hanyang University

ne of the critical issues associated with large-scale blades is weight. According to the square/cubic law, weight grows as the cubic power of blade length while energy extracted increases as the square power. Over the years, the weight growth exponent has been lowered from 3 to 2.3 due to advanced composite materials and design improvements. However, there are still many potential weight-reduction opportunities related to large-scale blade technology development. In fact, a weight reduction in the blade becomes possible not only when using a low-density material such as carbon fibres but also with an innovative design using new lay-ups and new blade geometry, eventually reducing the overall cost of energy. Standard wind turbine blade systems are mainly characterized by a straight main axis with twist and tapered cord and a blade made of E-glass fibres with a fibre angle of 45 degrees. Innovation is required to reach more cost-effective large blades. New composite materials, new lay-ups and new blade geometry are three innovative approaches to building

cost-effective, lightweight, large-scale wind turbine blades. Material costs are continuously changing, and the use of glass fibres (E-, S- and R-glass) and carbon fibre should be explored considering density/cost, specific stiffness and strength. The various fibre angles should also be used to best meet the design requirements. Rather than just a straight blade, various blade geometries should be investigated to increase power efficiency and to alleviate aerodynamic loads and increase fatigue life. Pre-bending will certainly alleviate the use of expensive high-stiffness materials. A fully-coupled aerodynamic-structural iterative analysis can be useful for adaptive wind blade designs. Several iterations of the aerodynamic analysis followed by structural analyses are performed until a blade design is finalized. More particularly, this process is needed to include the bend-twist and/or sweep-twisting coupling behaviour in order to alleviate passively the aerodynamic loads. The computational fluid mechanical analysis mainly aims to increase aerodynamic efficiency through lift and drag forces. The structural analysis includes a prediction of deflection, ultimate strength, buckling load and fatigue life. However, full three-dimensional modelling does

not always lead to a better design. In order to effectively explore a new and different design realm, designers need a flexible, user-friendly design tool, providing an insight into the behaviour of blades. The tool should be flexible enough to easily perform coupled aerodynamic-structural iterative analyses as well as manual iterations between analyses. It also needs an optimization module or the capability to perform a parametric study, leading to a high-performance blade complying with the design requirements imposed by industrial standards. The effects of each design variable or parameter (materials, fibre angles and shape) on blade loads and strength need to be visualized. A lightweight blade would, in most cases, require high-specific-stiffness and strength materials, which generally increases material costs. However, such high performance should offset the cost increase: the overall system cost can be reduced by using a lightweight blade since this reduces the costs incurred in the tower and nacelle. Cost increases related to the use of diverse fabrics and innovative design can also be lowered by a business alliance between material suppliers, non-crimp fabric (NCF) and blade manufacturers, resulting in lower total material and manufacturing costs.

No70 January - February 2012 /

jec composites magazine 39

Feature Wind Energy

JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012