Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 383

top in the water. They include concepts such as SWAY, Blue-H and Hywind from
StatoilHydro, etc. For the details please refer to [16].
5.2 Rotor design for offshore wind turbines
Rotor design for the offshore wind turbines requires new rotor structural concepts
and aerodynamic models for blade geometry optimization.
5.2.1 Structural concept
Structural design for large offshore wind turbine blades calls for development of the
hybrid carbon fi ber/fi berglass blades at system ratings in the multi-megawatt to 5-7
MW range. Structural performance needs to be evaluated for various arrangements of
the carbon blade spar. Critical performance aspects of the carbon material and blade
structure need to be addressed. This type of rotor blade design will use carbon stra-
tegically. The goal is not just to reduce the weight and cost of such large blades, but
to maximize the benefi ts of the introduction of aeroelastic tailoring, i.e. twist-bend
coupling. These features combined will allow the blades to shed peak transient loads.
Earlier studies conducted by GE Wind Energy showed signifi cant potential for
relieving fatigue and extreme loads using aeroelastic tailoring. Research at TU
Delft [17] in the Netherlands has further shown potential to use carbon to substan-
tially reduce blade cost. The gains are particularly signifi cant for large rotors with
reductions of up to 10% in blade cost and 30% in weight when it is compared to
the current practices.
As wind turbine technology evolving, the industry's optimal turbine size has been
steadily increasing. For turbines to grow into the 5-7 MW range and beyond eco-
nomically, rotor blades longer than 60 m will be needed. Longer blades will help
make lower wind speed locations close to shore more economically attractive since
rotor diameter is the single biggest design parameter affecting the amount of energy
capture for a given wind speed. In such conditions, it may be possible to put an ultra-
long blade on a conventional turbine without exceeding its design load capability
offshore. Few fundamental barriers have been identifi ed to cost-effectively scale the
current commercial blade designs and manufacturing methods over the size range
of 100-140 m diameter. Turbine designs with low specifi c rating need to be stud-
ied for the lower wind speed sites. As specifi c rating is decreased, i.e. blade lengths
increase at a given rating, blade stiffness and the associated tip defl ections becomes
increasingly critical for cost-effective blade design. The WindPACT rotor study
[18] predicted added costs of transportation and assembly adversely affect the cost
of energy (COE) for machines rated above 1.5 MW. Constraints for transportation
cost should be considered in all projects. An option for offshore may be to place
the blade fabrication plant at a harbor, which is advantageous with cheaper blade
shipment using barge only.
5.2.2 Aerodynamics and blade geometry optimization
A lower importance of noise emission, expected for offshore wind turbines,
offers new opportunities in rotor aerodynamics and blade geometry optimization.