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Fiber reinforced composites have tremendous potential in wind energy despite disposal issues

Fiber reinforced composites have tremendous potential in wind energy despite disposal issues

14-May-10
The development of alternate energy source has provided growth potential for the wind industry. The global wind industry is growing fast, in terms of both number of turbines and their sizes. The modern turbines are 100 times the size of those in 1980 according to Global Wind Energy Council (GWEC). Over the same period, rotor diameters have increased eight-fold, with turbine blades surpassing 60 m in length as per reinforcedplastics.com. By the end of 2007, around 20 GW of capacity had been installed, bringing the world total to almost 94 GW. In its report Global Wind Energy Outlook 2008, GWEC predicts that wind will supply 12% of the world�s energy needs by 2020 and could supply 30% by 2050. Wind turbine blades typically consist of reinforcement fibers, such as glass fibers or carbon fibers; a polymer such as polyester or epoxy; sandwich core materials such as PVC, PET or balsa wood; and bonded joints, PU coating and lightning conductors. As the turbines grow in size, so does the amount of material needed for the blades. For a 1 kilowatt (kW) wind power plant, 10 kg of rotor blade material is needed. For a 7.5 megawatt (MW) turbine, this would translate to 75 tons of blade material. Wind turbine blades are predicted to have a lifecycle of around 20-25 years. The wind-turbine industry is relatively young. There is only a limited amount of practical experience on the removal of wind turbines, particularly in respect of offshore wind turbines. At the moment, there are three possible routes for dismantled wind turbine blades: landfill, incineration or recycling. The first option is largely on its way out with countries seeking to reduce landfill mass. Germany has introduced a landfill disposal ban on glass fiber reinforced plastics (GRP) in June 2005, due to their high (30%) organics content such as polymer and wood. The most common route is incineration. In so-called combined heat and power (CHP) plants, the heat from incineration is used to create electricity, as well as to feed a district heating system. However, 60% of the scrap is left behind as ash after incineration. Due to the presence of inorganic loads in composites, this ash may be pollutant, and is, depending on the type and post-treatment options, either dumped at a landfill or recycled as a substitute construction material. The inorganic loads also lead to the emission of hazardous flue gasses in that the small glass fiber spares may cause problems to the flue gas cleaning steps, mainly at the dust filter devices. Wind turbine blades also have to be dismantled and crushed before transportation to incineration plants, placing further strain on the environment in terms of energy used and emissions. The alternative is recycling - either material recycling, or product recycling in the form of re-powering where old turbines are replaced by newer, more efficient ones. At the moment, however, there are few established methods for the recycling of wind turbine blades, and only 30% of fiber reinforced plastic (FRP) waste can be re-used to form new FRP, with most going to the cement industry as filler material. Several studies are in progress to find the best solution to disposal of blades after their service life. It will take some time before their results will bring out a winner for the disposal system. The most promising and mature renewable energy technology appears to be wind power and will contribute to securing energy independence and climate goals in the future, and could turn a serious energy supply problem into an opportunity in the form of commercial benefits, technology research, exports and employment.

To meet challenges of the energy sector, 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. As per Ole Thybo Thomsen, 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, the largest modern wind turbines have rated power outputs of 5 MW or more and rotor diameters of more than 125 m. larger wind turbines have larger energy output per unit rotor area due to increased mean wind velocity with height. Though larger wind turbines are more expensive to install and operate than smaller ones, the total production cost/kilowatt hour of electricity produced has generally decreased with increasing wind turbine size. Hence, 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.
 
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