MECHANICAL POWER IN A TURBINE BASIC INFORMATION AND TUTORIALS

Mechanical power in a turbine is the amount of power that the turbine harnesses from wind.We can determine the available power based on the wind speed, air density, and the turbine size (blade diameter).

Out of this amount of power, the rotor of a turbine can harness up to a maximum theoretical value of 59%. In fact, a turbine rotor can harness between 0% and a maximum value, which cannot exceed 59%.

That maximum value depends on the quality of the rotor design. For instance, for a particular turbine it can be 52%. This is the energy on the shaft before the gearbox represents its efficiency.

Any device, such as a gearbox or a generator, has a definite efficiency. So, the final energy output from the turbine, aft er the gearbox and the generator, will be even less.

The efficiency determines how much of the input power is available on the device output. For example, in a generator, how many kilowatts of electrical energy can it deliver for each 100 kW of mechanical power input (certainly it is less than 100, since part of the power converts to heat during the process)?

Based on the preceding discussion, the energy grasped by the rotor of a turbine is further reduced in the succeeding components. In this section, nevertheless, we are not going to bring the effect of the efficiency of the components into picture.

The intention is to study the power grasped by a turbine rotor. In particular, we want to see if there is any difference between cases when a rotor rotates at different angular speeds.

When a turbine is stationary (it is not working), it grasps 0% of the wind energy. Th is is when a turbine is yawed out of wind and its blades are feathered. In this case, one wants a turbine not to grasp any power from wind.

The position when a turbine is yawed out of wind and its blades are feathered corresponds to the minimum power grasp. Any small amount of power that the rotor may grasp is canceled by the rotor brakes (in order to make sure that there is no rotor motion).

When a turbine is yawed into the wind, the blades capture the wind and a torque is created in the rotor shaft. In addition to the wind speed, the air density, the blade size, and the blade airfoil form, the magnitude of this torque depends on the pitch angle of the blades, if this angle can vary.

In fact, changing the pitch angle alters the design of the blade. So, we need to study
a. Th e effect of the angular speed change in a turbine, and
b. Th e effect of changing the blade pitch angle.