Windpower Technology

To be able to discuss wind energy, it is beneficial to understand more about wind itself. Some definitions are necessary to discuss wind power technology. Wind speed is the speed at which air flows relative to the earth’s surface. Wind power is the amount of energy that can be derived from the wind. It is proportional to the cube of wind speed. That is to say, if wind speed doubles, then wind power increases by a factor of 8.

Wind is created by the heating and cooling of air. The sun unevenly heats the earth’s surface, causing pockets of low and high pressure. This causes air to flow from areas of high to low pressure, creating wind. Not all wind is created equal in terms of its suitability for energy generation.

Wind turbines today are more complex than windmills and simple wind turbines of the past. The image below shows the major components of a modern wind turbine. 

[image from: Iowa Energy Center's Wind Energy Manual]

The rotor, which consists of wood or fiberglass blades, collects the energy of the wind. It is connected to hub which is connected to the main shaft, as shown. Most new turbines work on the principle of lift. Just as air flows over an airplane wing and causes the plane to lift, turbines used for power generation have wings that are shaped to allow air to flow over them in such a way that the wings are not pushed, but are caused to lift. This causes the rotor to turn.

Turbines use the principle of lift, which allows the rotational speed of the blades to actually surpass the wind speed. This is described quantitatively by the tip speed ratio: the ratio of the rotation blade speed to the wind speed. Turbines today that employ lift technology can reach tip speed ratios of approximately 10.

The generator is where the electricity is produced by rotating a coil of wires in a magnetic field. Depending on the turbine, either alternating current (AC) or direct current (DC) electricity is generated. A more in depth discussion of AC and DC is included later in the Converting Electricity part of this section.

There is some general terminology that is useful to know when discussing wind turbine technology. The cut-in speed, typically 7 to 10 mph, is the minimum wind speed required for the turbine to start generating electricity. Rated speed, generally 25 to 35 mph, is the minimum speed required for the turbine to generate at its rated power.

As technology has improved, turbine blades have grown wider and towers have reached higher and higher. Wind turbines can weigh hundreds of tons and reach over 100 meters into the air. The blades, which are similar in size to large airplane wings, are attached to a 50 ton box holding the generator, gearbox, and the controller. New variable speed turbines rotate at 10-20 RPM. New designs allow generation with lower wind speeds than in the past.

There have also been advancements in technologies such as solid state switches. A 2 ½ megawatt turbine is nearly 3,500 horsepower and requires an electronic switch that can handle that much power. Such switches did not exist 10 or 15 years ago. Improvements in technology have lead to more reliable turbines. In the mid-1980s, windmills were operating at 35% reliability; today, operating reliability has reached 98%. Increased reliability means more uptime, and less money spent on maintenance. This is especially important for offshore installations, where a maintenance issue can mean an expensive trip by boat or helicopter.

Advancements in technology have also been accompanied with a decrease in price-per-kilowatt-hour. In the early 1980's, wind energy cost approximately $0.30/kWh to generate. Due to improved technology and federal incentives, large wind systems are now in the competitive range at less than $0.05/kWh in the United States. However, wind turbines have a different cost structure than fossil fuel or nuclear facilities. Wind facilities have a large upfront cost for installation, and no costs associated with fuel. Fossil fuel and nuclear facilities cost less per kWh to install, but require a supply of fuel, which is subject to fluctuations in price, as well as costs associated with waste disposal, and environmental controls.

In the long run, wind power benefits from its free fuel, which allows a turbine to be run for the fairly stable costs of maintenance, and real estate costs.  In other words, it is insulated from the risk associated with a fossil fuel shortage or supply interruption.  It is however, suseceptible to the weather, but while we cannot perfectly predict the wind, it will never stop blowing completely.  Additionally, wind energy is free of the widespread environmental costs of other forms of generation.  Compared to fossil fuel pollution which can spread across oceans and continents, and the possible scope of a nuclear accident, wind energy's environmental effects are localized to the viewshed of the turbine.

Electricity on the grid in the United States has a frequency of 60 Hz. Before electricity generated from a wind turbine can be added to the grid, it has to be converted to this standard. In order to do that, it is first converted to DC and then converted back to AC at 60 Hz. Most of Europe operates at 50 Hz, so the same processed is used with the final AC conversion being to 50 Hz frequency.

One of the biggest questions concerning wind energy is "what do we do when the wind stops blowing?" The answer is to employ load leveling techniques, which store surplus generation for future use when the wind dies down.  The techniques and methods used for load leveling are still maturing, but are in operation around the world today.  

On Kings Island, between Australia and Tasmania, a vanadium redox battery (VRB), a type of flow battery, stores excess wind energy, and has saved 2,000 tons of CO² (and an unknown quantity of other pollutants) which would have been produced by the island's diesel generators. A flow battery is an electrochemical device like a common lead acid battery, except that it uses a larger supply of electrolytes. As the electrolytes are charged by excess generation, they are replaced with fresh uncharged electrolyte fluid, with the charged fluid stored in tanks. When electricity is needed, the process is run in reverse, with charged electrolyte fluids forming a battery. As they become drained, they are replaced with fresh, charged fluid. The wattage of the system is limited by the generation system, but the capacity is only limited by the amount of charged electrolyte solution storage.

There are many other ways of storing excess electricity from wind turbines supplying more energy than demand can use. Compressed air is often used to increase the efficiency of fossil fuel burning plants, water can be pumped uphill for use in hydroelectric facilities, flywheels can be brought up to speed, ice or hot water can be prepared for future use, or water can be electrolyzed to make hydrogen for use in combustion and fuel cells.

While these load leveling techniques can expand the use of wind energy as it does on Kings Island, the people of Kings island still rely on their diesel generators, although not as often. In most of the world, fossil fuel burning plants are required to be kept online as a spinning reserve, to quickly be brought into production when the wind stop blowing. This chips away at their clean image, as many groups who oppose wind energy argue the point that every megawatt of wind energy installed needs to have a traditional method of generation backing it up. Perhaps in time load leveling devices like the flow battery can be used to a greater extent in order to better utilize the clean nature of wind energy.

Wind turbines also cause interference with ground based aerial defense radar. Installations in the UK are attempting to work around this problem by deploying more advanced systems which either see through or filter out the radar clutter caused by the spinning blades while simultaneously tracking aircraft.