Wind Energy Harvesting
Wind power has been recognized as a viable source of “free” energy for hundreds of years. Since it pre-dates the petroleum economy or even the industrial revolution, it seems odd to refer to it as an “alternative” energy source. Wind turbines have been a significant player as a renewable energy source since the early 1980s. Wind power covers a wide range of applications and can be harnessed by large wind turbine farms providing up to 800 MW of power, and small residential wind turbines providing 3 kW to a home.
Broadly speaking, wind energy conversion systems can be classified into variable-speed and fixed-speed devices. Variable-speed systems employ an electronic interface for grid connection. Fixed-speed systems commonly use squirrel cage induction generators which are directly connected to the grid.
The first turbines were constant-speed designs with very little control or optimization. In simplified terms, the generator would remain offline until the wind reached a minimum operational velocity. Once this minimum wind speed created sufficient blade rotational speed, the generator would be switched on, thereby joining the power grid. From that point the so-called infinite bus effect would provide speed control for the generator due to the overwhelming difference between the one rotating mass of the wind turbine compared to that of the rotating machines supplying energy to the electrical grid.
Vertical-axis wind turbines (VAWTs) are a type of wind turbine where the main rotor shaft is set vertically and the main components are located at the base of the turbine. Among the advantages of this arrangement are that generators and gearboxes can be placed close to the ground, which makes these components easier to service and repair, and that VAWTs do not need to be pointed into the wind as they accept wind input from a full 360 degrees.
On the other hand, the horizontal axis design (Figure 1) needs to be properly aligned in order to capture the greatest amount of energy. Fortunately, turning in the direction of the wind (weathercocking) is a naturally occurring phenomenon, so it’s not that difficult--for smaller designs that is. For the massive turbines used in utility-scale wind farms, yaw control is added with motors and gears to orient the blades perpendicularly to the wind direction.
Since all grid-connected commercial wind turbines today are built with a propeller-type rotor on a horizontal axis (i.e., a horizontal main shaft), for the rest of the discussion let us assume a horizontal-axis design.