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Hello.

Once you know the velocity distribution at a given location,

the power curves are essential to estimate

the electricity prediction from a specific turbine.

A power curve shows the relationship between the turbine power

and the flow speed at hub height.

It essentially captures the turbine performance.

However, we will see how the power curve could be impacted by the physical

or the dynamical characteristics of the upstream flow.

This figure shows the calculated power curve of a commercial wind turbine.

The blue curve corresponds to the upstream kinetic power of the wind,

assumed to be uniform, through the swept area of the turbine.

The red curve corresponds to the turbine power function

of the upstream wind velocity at the hub height.

Due to the Betz limit and several other technical limitations,

the electric power generated by the turbine is always below the available kinetic power.

A turbine power curve is characterized by few specific values.

At very low speed, there is insufficient torque exerted

by the flow on the turbine blades to make them rotate.

The speed at which the turbine first starts to rotate

and generate power is called the cut-in speed.

Above the cut-in speed, the level of electrical output power rise rapidly

with the flow velocity.

However, at some point, the power output,

which is the limit that the electrical generator is capable of,

and this limit of the generator output is called the rated power

and the wind speed at which it is reached is called the rated speed.

For this wind turbine, the rated power is about two megawatts

and the rated speed is around 13 meter per seconds.

Above the rated speed, the design of the turbine is arranged

to limit the mechanical power to the maximum level of the generator

and the electric power stays constant.

For strong winds, there is a risk of damage for the blades or the rotor.

Hence, a braking system is employed to bring the rotor to a standstill

and twist the blades to reduce both the drag and the lift forces.

This speed is called the cut-out speed.

It is usually around 25 meters per second for standard wind turbines.

It is also very interesting to plot on the same graph the power coefficient

and the power of turbine.

As for the previous graphs, the output power of the turbine correspond

to the red line while the black dash-dotted line corresponds to the power coefficient.

We can see that the optimal efficiency of this wind turbine

is which for a given wind speed around 10 meter per second slightly

before the rated speed.

Similar curves are obtained from marine turbines.

There is just a shift toward smaller speed values.

The cut-in speed is around half a meter per second and,

quite often, there is no cut-out speed.

Indeed, the highest speed associated to tidal currents can be predicted

with a correct accuracy.

Therefore, the design of the turbine could take into account

the maximum thrust exerted by the flow.

If now we perform real measurements, we get a large dispersion in the results,

especially for the power coefficient.

This dispersion could be due to some mechanical limitations of the turbine,

but it can also be induced by the physical

or the dynamical fluctuations of the upstream flow.

Thank you.