Mohamed Osama
IEEE Senior Member & EGWEA Vice-President
Date: 14 October 2008
Scaling to larger size is the primary approach used to improve the wind turbine cost per kWh.
Advances in generators, gearboxes, blade designs, blade materials and controls allow wind turbines to improve in performance and grow in size. Over the past 20 years, average wind turbine ratings have grown almost linearly, with current commercial machines rated at 1.5 MW.
Wind Turbine Major Components:
1] Wind turbine Rotors:
Existing technology: Carbon fibers have been incorporated into wind turbine blades for some years to provide localized strength.Technological Challenge: higher structural stress due to larger turbine diameters
Possible technological solutions under development:
1- Lighter stiffer blades incorporating large volumes of carbon fabrics for both tolerating higher blade loads as well as reducing overall machine loads by reducing rotor weight.
2- Blade planforms that incorporate unique inboard airfoils using truncate airfoil shapes, providing a greater structural cross section to react to outboard loads.
3- Active designs that can reduce loads. The concept of aeroelastic coupling, the change in shape of the blade as it bends under load, offers possibilities for loads alleviation.
4- On-site manufacturing and segmented blades to help reduce transportation costs.
2] Drive Train (Generator + gearbox)
Existing Technology: three-stage gearbox with induction generator (either squirrel cage rotor N43, Wound rotor V47, or Doubly-fed G52).
Technological Challenge: increased size and weight of drive train for higher ratings.
Possible technological solutions under development:
1- Direct-drive synchronous generator either wound rotor or PM (permanent magnet) rotor.
2- Single-stage gearbox (e.g. 1:6 speed ratio) with low speed generator.
3- Distributed drive train to drive several parallel generators.
3] Towers
Existing Technology: Average turbine hub heights are now 65 meters for a 1.5 MW turbine, but a 2.5 to 3.5 MW turbine will demand a hub height of 80 to 100 meters with hub heights increasing for larger machines.
Technological challenge: Higher tower heights resulting in a significant cost impact due to need for extremely large cranes and the transport premiums for large tower sections.
Possible technological solutions under development:
1- Concepts that would eliminate the need for cranes for very high, heavy lifts.
a.Telescoping or self erecting tower. This concept allows assembly of the nacelle and rotor at close to ground level and then utilizes hydraulics to jack the tower and nacelle to its operating height.
b.Lifting dollies or tower climbing cranes that use tower mounted tracks to lift the nacelle and rotor to the top of the tower.
2- New Construction materials for towers.
a) Concrete-steel tower hybrids, possible cost reductions that may be realized by limiting the size of steel tower sections.
b) Composites.
Reference:R. Thresher and A. Laxson, “Advanced Wind Technology: New Challenges for a New Century”, Presented at the European Wind Energy Conference, Athens, Greece, February 27–March 2, 2006