Wind Power’s Growing Contribution
On a flight from Chicago to Boston, the plane flew over Lake Erie, near the north shore. Passengers on the left side could clearly see tall, white wind turbines standing in farmer’s fields, turning lazily. These machines were generating electricity for local consumers without burning fossil fuels or creating greenhouse gasses.
Wind energy is growing in attractiveness as an electricity hungry world looks for sources that service the needs of consumers while remaining conscious of our carbon footprint. Wind is an inexhaustible source of energy, but what are its practical limits and how does power get from turbine to grid?
|Wind turbines of various sizes use the same basic configuration with and adjustable prop, yaw control, speed increaser gear box, and generator.|
Wind power’s potential
While many people might think of wind power as a California phenomenon, it has potential for far wider deployment throughout North America. In fact, North Dakota has the highest potential for harvestable wind, and north Texas has the world’s largest installed wind farm. California is actually well down the list of possible states, with the Great Plains region our greatest potential resource.
Wind driven electrical generation is still a small part of U.S. capacity, but it is growing steadily. At the end of 2006 the total stood at 11,600 MW, but the American Wind Energy Association (AWEA) estimates that 3,000 MW more will be added in 2007, and the pace of installation is picking up. (Compare that to a typical coal-fired power plant at 600 to 800 MW per unit, and the total world installed base of fuel cell generation at less than 150 MW.)
At this point one of the greatest impediments to faster expansion is a lack of manufacturing capacity. Manufacturers of wind turbine equipment have been reluctant to add capacity given the uncertain nature of government policy toward renewable production methods. “We’re seeing many new wind farms come on line, and very exciting new investment in wind turbine manufacturing, but it’s only the tip of the iceberg compared to what needs to happen to meet the increasing demand for wind power,” says Randall Swisher, AWEA executive director. “What is critical at this juncture is for the U.S. government to put in place a full-value, long-term extension of the production tax credit and a national renewable energy portfolio standard requiring that utilities generate more electricity from renewable sources. These policies will give the clear, big picture signal of support for the renewable energy that this country urgently needs.”
As part of a capacity expansion project, Siemens expects to hit full production in its new turbine blade facility in Fort Madison, IA, this month. There they are building blades for 35 new 2.3 MW turbines that will be used at the Sweetwater Wind Farm near Abilene, TX. Each blade is 148 ft long and weighs about 12 tons. The new plant employs 400 people.
Texas is the leading wind energy state, with more than 3,000 MW now installed, including three of the five largest wind farms in the U.S. While California doesn’t have the highest wind potential, it does have 2,376 MW operating, with Iowa, Minnesota and Washington rounding out the top five.
One particularly interesting installation is in Lackawanna, NY, where eight new turbines have sprung from a brownfield where an old Bethlehem Steel mill once operated. This has brought new life to the superfund site where industrial waste contamination has left the area abandoned and blighted. Now the turbines generate 20 MW for the local community. There are other similar installations in the northeast, where wind farms now operate at former mine sites and near or at wastewater treatment plants.
The obvious limitation of wind turbines is that they only operate when wind blows within a specific velocity range. As a result, their performance varies with the weather. Turbine performance depends on knowing both wind direction and velocity, using wind sensors that can be mounted on individual turbines or centrally for a larger wind farm. A turbine can twist its head on the support column (yaw control) to match wind direction and adjust the pitch of its blades to achieve desired speed and torque characteristics. These functions can be performed with mechanical or hydraulic mechanisms.
If an individual turbine has stopped due to a lack of wind, it will go through a startup routine once wind speed reaches approximately 9 mph (4 m/s). The turbine begins by turning its blades toward 0° until rotation begins. At this point a speed regulator modulates the blades to keep the generator turning at the desired speed, usually around 900 rpm. Once this speed is reached, the control system cuts in the power converter and then the generator. This minimizes inrush problems to and from the grid.
The regulator sets speed to maintain a balance between rotor speed and power output. Since generator loading is relatively constant, the system compensates for variations in wind speed by minute blade pitch adjustments. This is a continuous process during the time the turbine is operating, and ensures the drive train experiences near constant torque while operating.
Stopping a turbine due to a mechanical breakdown or because of very high winds can use a combination of reverse blade pitching and a brake mechanism inside the nacelle. Given the high momentums involved in something that large and heavy, stopping is not immediate, however in an emergency units can usually come to a halt within a few seconds. When winds exceed safe speeds, the blades can be parked in a neutral position for safety.
Location and sizing
When located appropriately, a turbine should run 60 to 80% of the time. When sized appropriately for a given location, it will provide its full rated output at least 10% of the time. With that in mind, a typical turbine will average 30 to 35% of its full potential output during the course of a day, although the amount at any given time can range anywhere up to 100%.
For purposes of planning production levels for the grid, one rule of thumb is to count 20% of the installed nameplate capacity as firm generation capacity. In other words, a 10 MW wind farm will be counted as 2 MW on the grid. While this may seem low, it reflects a useful tradeoff of cost, efficiency and capacity. Blade size and generator capacity are the two main design variables that correspond to typical wind conditions for a given location.
The turbine itself only has yaw control and blade pitch adjustment to optimize output in changing conditions. Given these unique peculiarities, putting a group of wind turbines together under the same control scheme and delivering power to the grid provides some unique challenges. An operating wind farm that’s connected to local distribution will have the same demands put on it as a coal-fired or nuclear equivalent. As demand changes, output also must react, except in this case, operators cannot simply adjust a firing rate to follow the grid.
Sophisticated control required
With their variable output, integrating wind farms with traditional power plants requires sophisticated control functions. The Federal Energy Regulatory Commission (FERC) regulates generating facilities to ensure that a given plant cannot cause interference or pull down service in an area.
When wind dies down, turbines slow or stop turning. With traditional control systems, they simply trip offline and when they start turning again, they must be put back into operation manually. FERC will be tackling these low voltage and zero voltage episodes when it issues new regulations in 2008. Control packages are available that allow wind farms to ride through such periods while remaining online, and re-establish themselves on the grid automatically as output climbs again.
Wind farms have the capability of helping stabilize weak grids or areas where loading is variable and heavy. They can be equipped with control mechanisms that allow them to provide reactive power for distribution system maintenance, even when they are not generating active power. This capability allows a wind farm to help stabilize local voltage and support the grid through imbalance conditions. This cannot be duplicated by more conventional technologies, such as coal-fired or nuclear.
With these capabilities, wind farms become a more critical element to their regional distribution grid beyond their simple generating capacity. “Wind turbines and wind power plants equipped with reactive power and voltage regulation capabilities can provide superior voltage performance for weak systems,” says Victor Abate, vice president of renewable energy for GE Energy. “Expanding this capability to zero-power operations can produce performance benefits not possible with conventional power generation.”
The remote locations of wind farms makes electrical transmission a challenge. One extreme example is a new offshore facility under construction in the North Sea, more than 60 miles (100 km) off the coast of Germany. At 400 MW, this is the largest offshore installation, as well as the most remote. The project will use ABB’s HVDC (high-voltage direct current) Light transmission technology, giving the utility a high degree of control from the shore, which is critical given the unpredictability of wind turbine output. “Linking renewable sources of power to the grid can be challenging due to environmental conditions and the distance involved,” says Peter Leupp, head of ABB’s Power Systems division.
But not here
Wind farms are subject to the same “not in my backyard” mentality of plants and processes that neighbors consider undesirable. Typical arguments are:
They’re unsightly. This is a subjective observation, although manufacturers have done all they can to help appearances. Still, it’s hard to disguise something as tall as a 30 story building.
They’re noisy. They can make a slight “wooshing” noise depending on which side the listener is on, but the sound is very low and audible only near the structure.
They kill birds and bats. The National Academy of Sciences has published a study that says pesticides and collisions with cars or buildings kill more birds and bats. The Audubon Society has even endorsed wind turbines.
As an example of one problem situation, the municipality of Hannover Park, IL, near Control Engineering’s offices, is arguing a proposal for adding a 1.6 MW wind turbine on the campus of an elementary school. Power generated by the turbine would save the school an estimated $8 million over its operating life. Yet some elements of the community have expressed dismay at the thought of a structure 250 ft tall in town, and the issue will likely be decided in court.
On the other hand, some people welcome wind turbines. Farmers who allow them to be placed in a field can plant right up to the base and are paid around $3,000 per year per megawatt for operating rights. This provides a double benefit for rural communities.
|Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .|