Wind Energy Physics

Intermediate

Inside this Article

Wind Energy Physics
Wind Energy Physics
The swept area of a turbine
The swept area of a turbine is its aperture window that captures the wind. The area is directly proportional to the amount of wind energy available to the turbine.
one-bladed turbines have the highest potential power coefficient
Believe it or not, one-bladed turbines have the highest potential power coefficient, but they are difficult to keep balanced. Two- and three-bladed turbines are the proven standard, balancing aerodynamic efficiency and stability during turbulent winds.
Ducts and shrouds are common attempted augmentations
Ducts and shrouds are common attempted augmentations to divert more wind into, or increase wind speed through, the turbine’s blades. However, the nature of fluid dynamics makes the wind adept at flowing around these obstructions rather than through them. Savvy turbine designers use larger swept area to increase power input instead of trying to trick the wind.
Multiple blades
Multiple blades only help in getting the turbine started in low-wind situations. After that, one blade’s wake quickly interferes with the blade following it.
“innovative” designs
Many “innovative” designs just complicate a system that would work better if simplified. Luckily, most kooky designs never leave the prototype stage.
Wind Energy Physics
The swept area of a turbine
one-bladed turbines have the highest potential power coefficient
Ducts and shrouds are common attempted augmentations
Multiple blades
“innovative” designs

Wind turbine design encompasses multiple disciplines, but perhaps the most important—and often the least understood, even by would-be turbine designers—is fluid dynamics. Understanding the physics laws that govern wind power, presented here, can help consumers make wise wind turbine technology choices.

The Energy Equation

The purpose of a wind turbine is to convert kinetic energy (energy of a moving mass) of the wind into electrical energy. Energy conversion is common to all machines because they must obey the law of energy conservation—energy cannot be created or destroyed, but only changed from one form to another. For example, your car converts the chemical energy stored in fuel (if it’s an electric car, batteries) to kinetic energy, moving it down the road. A wind turbine also obeys this law when it extracts the kinetic energy in the wind and converts it to electrical energy.

The amount of kinetic energy in any moving mass is calculated with this equation:

Kinetic energy = 1/2 × mass × velocity2

For wind energy, the velocity in the equation is wind speed. The mass is for a particular volume of air. Consider the example of wind blowing through an open window. The illustration describes how a volume of the air passing through the window relates to window area, wind speed, and time. This makes sense if you consider that the larger the opening, the harder the wind is blowing, and the longer the window is open, the more air volume will flow through it.

The mass of this volume of air is arrived at by multiplying the volume by the air density. Putting this all together, we write our equation for wind energy as:

Kinetic energy = 1/2 × air density × area × wind speed3 × time

Power in the Wind

Although a wind energy system’s final objective is to generate energy, it is more convenient to describe its size in terms of power. The relationship between power and energy is a simple one—energy is power multiplied by time. This is why power is defined in watts and energy in watt-hours. The terms power and energy are often confused and even used interchangeably in casual discussions, but in a technical analysis, it is important to make the distinction.

If we are interested in the power in the wind, we divide the energy by time. This gives us the governing equation for power available in the wind passing through an area as:

Pwind = 1/2 × air density × area × wind speed3

We see that the amount of wind power is dependent on three variables. The first is air density, a quantity defined by Mother Nature, over which we have no real control. Another is swept area, that is, the projected area perpendicular to the wind that the turbine intercepts. One thing is clear from the equation: everything else being equal, a larger swept area can generate more power.

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