Wind Energy - Basic Theory


To understand wind energy, we subscribe to the theory of conservation of mass and conservation of energy. A duct shown below is assumed to represent wind flowing in and out of the blades of the turbine.


The velocity Va is assumed to be the average of V1 and V2. Kinetic energy at the mouth of the tube is given by −

KE = 1/2 mV2

KE of energy changed = 1/2 mV12 - 1/2 mV22

1/2 m(V12 - V22)

Since m = p.A.Va then KE change, Pk = 1/2 p.A.Va (V12 - V22)

On further simplification, the estimated wind energy is give as −

KE, pk = 0.5925 * 1/2pAV13

Blade Element Theory

The blade element theory assumes that the flow at a given part of a wind turbine blade does not affect the adjacent parts. This subdivision on the blade is called annulus. The momentum is calculated for each annulus. All the resultant values are then summed up to represent the blade and hence the entire propeller.

On each annulus, an equally distributed velocity is assumed to have been induced.

Dynamic Matching

The dynamic inflow model was incorporated to improve the estimations by the Blade Element and Momentum theory. The basic dynamic in flow theory concept helps estimate the effect of blade turbulence. The swept area is given a dynamic state to help in deriving estimate mean velocity.

The BEM theory gives estimates only at steady wind but it is obvious that turbulences must occur. However, this is accounted for by the basic dynamic inflow model to provide a more realistic estimate.

Wind energy produced, especially in the horizontal axis type, is known to be the product of tip speed, the total number of blades used and the lift-to-drag ratio of the side with an aerofoil. The readjustment to a new steady state of equilibrium is well explained by the Dynamic Inflow Method (DIM).

Dynamic Inflow Method

DIM is also known as dynamic wake theory and is based on the induced flow, which is normally not steady. It calculates the inflow vertical to the rotor taking into consideration its effect on the dynamic flow.

This simply takes into consideration the wake effect or simply the velocity of the air vertically aligned with the rotors caused by the turning of the blades. It however assumes the tangential velocity to be steady. This is referred to as the Wake effect and its drag lowers the efficiency of a wind turbine.

Electricity Generation

The kinetic energy in wind is converted to electricity by wind turbines. They use the ancient concept used in windmills though with inherent technology, such as sensors, to detect wind direction. Some wind turbines have braking system to halt in case of strong winds to protect the rotor and blades from damage.

There are gears connected to the rotor shaft to accelerate the blades to a speed suitable for the generator. Inside the generator, electromagnetic induction (the basic method of conversion from mechanical energy to electricity) occurs. The shaft rotates a cylindrical magnet against an electric wire coil.

All electricity from the turbines in a wind power station is assimilated to a grid system and converted to a high voltage. This is actually the conventional technique of transmitting electricity in the grid system.

Large surface-tipped blades are needed although this should be determined by the noise that results from wide blades. A wind farm may have up to 100 generators, which will result in more noise.