Introduction | Technology | Applications | Strengths/Limitations | Related Links

Introduction

Wind turbines are one of the oldest DE technologies, and have been useful for centuries for grinding wheat, pumping water, and a variety of other mechanical tasks. When electrical generation technologies were first developed, wind turbines were considered an obvious prime mover to drive a generator. While there are many different designs, all wind turbines capture wind to convert the wind's kinetic energy into mechanical energy (in the form of a rotating shaft). In electric power turbines, the mechanical energy drives an electric generator. Wind energy is a pure renewable resource and produces zero emissions.

Figure 1: Typical Wind Farm

Courtesy of NREL

Electric output for commercially available systems has grown dramatically over the last couple of decades. In the early 1980's, the largest turbines had a capacity of roughly 25 kW. Today, the largest commercial turbines have increased that capacity by a factor of 100 (over 2,500 kW). Wind energy systems are modular and can be clustered in areas with good wind resources to form wind farms of 50-100 MW, or larger. At the end of 2002, the United States had an installed capacity of 4,865 MW, or the equivalent serving more that 1.3 million households. Europe has an even larger installed base.

Currently there are three designs of wind turbines, with some innovative new designs currently being developed. The three are the propeller-driven turbines, the Darrieus turbines, and the helical generators. Each of these designs can be modified to be used in offshore wind farms.

• Propeller-Driven Turbines - These wind turbines are the most common, and represent the largest segment of commercial wind turbines. These turbines usually have two or three slowly rotating blades sitting at the top of a very tall post or tower. The turbine pivots at the top of the tower so that the propellers can always be aimed at the incoming wind. The propellers drive a horizontal shaft. These are commonly seen on mountain ridges, farmlands, and offshore.
• Darrieus Turbines - Unlike propeller turbines, these turbines incorporate a vertical drive shaft into the blade design so that the blades can capture the wind from whichever direction the wind comes.
• Helical Wind Generators - These turbines have vertical drive shafts like Darrieus designs, but the blades are integrated into one unit that looks like a large screw. Helical turbines are versatile and much less intrusive - physically or visually - than the larger Darrieus and propeller type designs.

Figure 2. Darrieus Type Turbine

Courtesy of NREL

The one major limitation wind turbines is that they are dependent on an intermittent source of power. Simply put, when the wind does not blow, the turbines do not generate electricity. This means that while wind turbines have the capacity to generate large amounts of electricity (especially when combined into wind farms), they are not reliable as primary power resources, unless they are accompanied by energy storage devices.

The following section provides a description of wind turbine systems and applications, along with the strengths and weaknesses associated with this DE technology.

Technology

Technology variations are based on the major wind turbine components. These components include:

• Blades/ Hub - For propeller-driven turbines, the blades are connected at the hub. Often several blade designs are offered for each model so that the turbine can better meet the site-specific wind conditions. Most designs consist of either two or three blade models attached to a hub. Using stronger and more lightweight materials has allowed manufacturers to create larger blades, increasing the capacity of the turbines.
• Nacelle - This is the name for the case or housing that contains the shafts, gearbox, generator, hydraulics and electronics and it is mounted on the tower.
• Gearbox - All wind turbines contain gearboxes, which convert the slow rotation of the shaft into the high speed required to generate electricity.
• Generator - In recent years, wind power has become very competitive in electrical cost production due to increased efficiencies and the increased size of the generators, with typical outputs over 500kW for newer, utility-scale systems.
• Auxiliary systems - Even though wind systems are based on old technologies, new turbines are sophisticated and need additional technologies - like cooling systems for the generator and gearbox, as well as breaking and hydraulic systems to stop, lock and unlock the turbine - to run efficiently.

Wind turbine blades act similar to an airplane's wing or a boat's sail. When air travels over the curved blade, a low-pressure area is created on the concave side of the blade (referred to as Bernoulli's effect) creating pressure. This pressure pushes against the blade, causing the rotational mechanical energy that drives the low speed shaft connected to the hub.

Figure 3: Typical Wind Turbine Design

Applications

In the U.S., wind power is typically associated with the large merchant energy wind farms found on mountain ridges and offshore facilities that provide grid power. However, wind turbines have many potential DE applications including:

1. Agriculture - For water pumping.
2. Business Parks - For providing power at peak times.
3. Homes - Installed at homes with storage systems to provide power.
4. Remote Locations - Excellent source of power for remote locations with proven wind resources.

California provides a great example of wind power as it has one of the world's largest installed wind power capacities. In the most favorable areas of California, the wind patterns are most often capable of supporting electric generation in the afternoon and late at night. On average, wind turbines in California provide electric power between four to six hours a day, coinciding with hours of peak electric usage. As peak power is always more expensive than base load power, the value of wind energy in California is magnified.

As the California example suggests, wind power must be combined with other technologies if it is to play a role in providing base load as opposed to peak power. One such option is to combine a wind turbine with an energy storage technology, such as batteries. When there is ample wind, electric power can be provided directly from the turbine to not only match the load, but also recharge a bank of batteries. During hours of the day when winds are either too low or too high, the energy stored in batteries can be used to provide electric power. Similarly, wind power can be combined with other DE technologies in hybrids that take advantage of the strengths of wind and combine them with the strengths of technologies such as reciprocating engines or solar panels (photovoltaic).

Not only has the size and capacity of wind turbines increased, as mentioned above, but the costs have also dropped dramatically over the last two decades. In the 1980's, wind power cost roughly $0.25 per kWh. According to the American Wind Energy Association, the cost today is about $0.048 per kWh. Advanced designs, larger systems, and increased manufacturing volumes have all contributed to cost reductions.

The siting of wind turbines is a critical aspect of their output and economic performance. The following factors contribute to turbine performance:

1 Height - Winds tend to be stronger and steadier as the elevation increases, so regardless of the turbine design, finding the optimal height is essential to maximizing the amount of wind power the turbine will capture. The turbine needs to be placed above any turbulence or eddies caused by surrounding obstacles.

2 Topography - Wind is created when there is a temperature difference across broad expanses of terrain. Typical areas with high winds include the following:

a. Flat Plains - Often the windiest areas due to less frictional drag than over uneven surfaces.
b. Slopes/Ridges - Act as towers and generate wind as a result of temperature differences in altitude.
c. Water - Large bodies of water such as oceans and lakes generally have higher winds when there are temperature differences between the water and land. The direction of the wind can change as land temperatures shift from being hotter to cooler than the body of water. Like flat plains, bodies of water have low frictional drag.
d. Obstacles - Buildings or natural obstacles can impact the wind patterns, negatively impacting the performance of the turbines.

Figure 4: Wind Farm Sited on a ridge

Strengths and Limitations

Wind turbines have become a competitive source of electric generation. They are not, however, suited for all sites and applications. Areas with little wind potential and high population densities reduce the viability of wind turbines. Bigger wind turbines tend to be more efficient, but as they become bigger, they create more noticeable visual impacts. On the other hand, wind turbines have zero fuel costs, have small footprints because the rotors are well above ground, and are completely emissions free. The following table highlights the strengths and limitations associated with wind energy.

Related Links

These links provide you supplemental information on wind energy. The vendor links will provide direct access to the Websites for manufacturers of wind energy equipment. Refer to the vendor sites to obtain product specifications. The general distributed energy links enables you to search other leading DE Websites for wind turbine analysis.