How can you determine the optimal height for a wind turbine?

Wind speed varies at different height. Turbine should be placed at a height where the wind speed is consistently high. Consider the wind shear, which is the change in wind speed with height. A site with a good wind shear means wind speeds increase significantly with height, making taller turbines more efficient.

NREL has some great interactive maps that show wind resources globally. It is called NREL Wind Resource Map. They have accounted for varying heights and show the different wind speeds for heights from 10 m to 200 m off the ground.

Of crucial importance is measuring the wind speed and profile over the complete rotor and in development as high as possible. Modern Light Detection and Ranging (LiDAR) wind measurement systems allow us to do this accurately for up to 200m today. Back in 2007 I worked on some projects in Greece, which only had very low heigh measurements, as it was simply not possible to get any taller measurements mast up into the mountains before roads were built. As a result it was not possible to detect that the mountains caused the wind shear to be negative in some heights and the wind potential was overestimated.

Too many factors are not considered here, especially economic factors. Basically, one has to determine the maximum energy output at the least cost of energy (COE). COE may include building new infrastructures (roads or a port) to accommodate for transportation of parts, initial cost and net annual costs. Rough terrain may play a part too.

The other problem with blade size is the impact of air pressure density across the total radius of the swept area. There will often be a difference between the pressure on the blades at the top and those at the bottom. Where the tip of the blade dips into the boundary layer with the ground (approximately 18m above ground) the wind sheer jumps dramatically. This means the air-pressure on the blade drops suddenly as the blade dips into the boundary layer - placing significant forces on the blades and hub-assembly. So the lowest part of the swept area needs to be kept well above the boundary layer at ground / sea surface.

As we know the power output from a wind turbine can be expressed by given below formula: P = 0.5 * ρ * A * Cp * v^3 where P is the power output, ρ is the air density, A is the swept area, Cp is the power coefficient, and v is the wind speed, wind speed varies (increases) as the height increases to a certain level. the maximum efficiency of a wind turbine can be defined by Betz limit. Real-world wind turbines typically operate at efficiencies below the Betz limit due to factors such as aerodynamic losses, friction, and other inefficiencies. Modern wind turbines are designed to operate efficiencies ranging from 30% to 45% of the Betz limit. According to Betz's law, a wind turbine can capture up to 59.3% of the kinetic energy in the wind.

It is important (And often overlooked) nuance of wind energy, that different turbines deal with different wind conditions more or less effectively. For example: vertical turbines are more effective at dealing with air-flows that are inconsistent - like the wind 'behind' a large wind-farm. So the overall mix of turbine types is important to consider when designing a wind farm. To gain the most energy from the resource it is ideal to use a mix of turbine types that compliment each other by reducing 'blade-wash' and/or by placing them in such a way that airflow is managed dynamically. Turbines at the front of the airflow might be best left off - if the wash they create reduces the output from all the other turbines.

On limiting factor in the height of a wind turbine is that once a tower goes over a certain height the base diameter required to support such a tall tower is wider than what can be transported on public roads from the factory to the site.