Offshore wind turbines have problems that include high construction and maintenance costs and the risk of toppling in high seas or strong winds, especially for floating turbines. These problems can be eliminated or reduced by using verticalaxis wind turbines (VAWTs), say researchers at Sandia National Laboratory.
A research team there concluded that VAWTs are less complex and use fewer assemblies than horizontal-axis wind turbines (HAWTs). So construction and maintenance costs should be less. Regular maintenance is also easier because major VAWT components sit near the bottom of the structure. With HAWTs, technicians must carry replacement components and tools while climbing up into the nacelle.
VAWTs have a lower center of gravity than HAWTs, so they are more stable when floating and less likely to topple in rough seas or high winds. VAWT towers also support much less weight than those for HAWTs, incurring lower fatigue which could extend the life of VAWTs.
But there are downsides. VAWTs have curved blades that are complex and not easy to manufacture, especially for large turbines with blades over 900-ft long.
VAWT blades also cyclically load the drivetrain as each blade passes through the upwind and downwind position. So as they spin, VAWT blades create two torque and power pulses. The resulting torque ripple can prematurely wear out drivetrains. HAWT blades, on the other hand, generate smooth torque and power curves.
VAWTs also rely on mechanical braking, which is more difficult to maintain and less reliable than the aerodynamic braking used on modern HAWTs. HAWT blades can change their pitch, letting the blades use the wind to slow down their rotation and bring the blades to a full stop.
Over the next few years, Sandia researchers will be developing new blade designs and construction methods for VAWTs.