• Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
• The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.
• High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
HAWT disadvantages
• The tall towers and blades up to 90 meters long are difficult to transport. Transportation can reach 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
• Massive tower construction is required to support the heavy blades, gearbox, and generator.
• Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
• Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.
• Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
• HAWTs require an additional Yaw drive control mechanism to turn the blades toward the wind.
VAWT advantages
• A massive tower structure is less frequently used, as they are more frequently mounted with the lower bearing mounted near the ground, making it easier to maintain the moving parts.
• Designs without yaw mechanisms are possible with fixed pitch rotor designs.
• They have lower wind startup speeds than HAWTs. Typically, they start creating electricity at 6 m.p.h. (10 km/h).[citation needed]
• They may be built at locations where taller structures are prohibited.
• VAWTs situated close to the ground can take advantage of locations where mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.[citation needed]
• They may have a lower noise signature.
VAWT disadvantages
• Most produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind.
• A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guy wired models.
• While the parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts nearly impossible without dismantling the structure if not designed properly.
• Having rotors located close to the ground where wind speeds are lower due to wind shear, they may not produce as much energy at a given site as a HAWT with the same footprint or height.
• Because they are not commonly deployed due mainly to the serious disadvantages mentioned above, they appear novel to those not familiar with the wind industry. This has often made them the subject of wild claims and investment scams over the last 50 years.
