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Controversy And ConcernsPrecautionsWindmills create unique hazards. Just like with a fan, don't put your fingers between the blades because you never know when a strong gust of wind might come. Don't climb the mast unless you really know what you're doing. Old style windmills like the 750 kW Legerwey in Toronto turn at a constant speed, to match line frequency (60 Hz in North America). Newer windmills often turn at whatever speed generates electricity most efficiently. Such newer turbines therefore output at an arbitrary frequency. The AC is then rectified with a three-phase bridge rectifier with six diodes, and the resulting DC is inverted back to AC at exactly line frequency (e.g. 60Hz). This process of converting to DC and then back to AC allows the windmill to turn at any speed. That gives greater efficiency but be forewarned that the blades can spin at any speed, in particular, the blades can spin very fast, suddenly. Older windmills have heavy steel blades, whereas newer units often use lightweight aerospace materials for low inertia design. In older designs that try to synchronize directly to line frequency, the inertia was advantageous, but in newer designs it's preferable to have low inertia to optimize the energy capture from sudden gusts of wind that are typical in urban settings. Thus urban wind turbines in particular often have low inertia, and can therefore suddenly change speed and direction. The most modern windmills typically use dynamic braking, rather than mechanical braking, to limit the speed in a storm or similar high-wind condition. Old style wind turbines have a mechanism to engage the wind less during high winds, or to turn away. Modern designs are less "timid"; they keep producing power no matter how fast the wind blows. Dynamic braking systems use a "dummy load", typically with a PWM (Pulse Width Modulator) control so that when slowing down is desired, the dummy load is connected in varying duty cycles depending on how much slowing is desired. Such brakes kick in when the power produced is greater than the power needed or consumed by the ordinary load. A typical modern windmill has two brakes: (1) a switch that shorts together the three phases to prevent the windmill from turning; (2) the dynamic braking system described above. The brake switch is actually more analogous to the "park" setting of an automobile transmission. Closing the brake switch should only be done when the windmill is already stopped. Otherwise dangerously high currents can result. Most notable, however, is the fact that perhaps the most dangerous aspect of such windmills is an open circuit. Unlike most other electricity we use (such as that provided by the electric grid), a short circuit in the windmill is OK when, for example, it's "parked". But the worst thing that can happen is if the wires get disconnected, and there is nothing to slow down the wind turbine. High speed of rotation can result in centrifugal forces that tear the system apart. A runaway windmill would break the sound barrier quite quickly and fortunately the sound might serve as a warning that one must quickly bring it under control. Ordinarily, though, one prefers to keep a windmill under control at all times, and therefore it is important that the "dummy load" never becomes disconnected. The "dummy load" is usually on the DC side, and therefore the integrity of the six diodes on the bridge rectifier that is typically used must be intact. Regular safety tests should be done with a reduced load to make sure that the dummy load "kicks in". As a failsafe mechanism, it is also desired to have two sets of wires going to and from the "dummy load". Also, it should be noted that the braking resistor can get quite hot when the braking takes place, whereas most of the time the braking resistor can be cold. Combustible material should not be piled up on the braking resistor even though it may feel quite cold to the touch, because it may simply be that the wind turbine is not producing more than is being consumed,but that the situation may change at any time and the resistive brakes may kick in without warning. Braking resistors should be located in areas where there is good airflow. Sometimes brake resistors are mounted out in the open, in the middle of a mechanical room, like a pot-belly stove, so that they also usefully heat the mechanical room in the winter time. Brake resistors are also sometimes used to heat water, to use the "waste energy" to supply hot water to the building. Using the braking resistor to heat a mechanical room can also help to prevent batteries from freezing and bursting in the winter. Many wind turbines charge batteries in addition to being tied to the electric grid. Batteries can be used to provide backup power when the grid goes down. It is important to have good ventilation for the battery, but venting the mechanical room increases the chance of freezing. However, a braking resistor in the mechanical room can help by providing more heat when the wind blows more strongly, which is exactly the time when more heat is needed (because a strong wind is what would otherwise make the mechanical room cool off faster). ControversiesThe debate around wind energy is heated and often emotional. Arguments of both parties are listed below. Arguments for OpponentsThere is resistance to the establishment of windfarms owing initially to perceptions they are noisy and contribute to "visual pollution," i.e., they are considered to be eyesores. The large installations of a modern wind facility are typically 100 m high to the tip of the rotor blade, and, besides the continuous motion of the 35-m-long rotor blades through the air, each time the blade passes the tower a deep subsonic thump is produced whose regular beat many people claim resonates through their homes and even makes them ill. The large number of turbines required for a viable wind plant, and the huge number of plants required to meet the ambitious goals of the wind industry and governments, ensures that more people will be affected by them. The construction of a large facility is also far from ecologically benign in previously undeveloped locations. It requires wide straight flat roads, a large hole filled with tons of steel and concrete to secure each giant assembly, clearing of trees in wooded areas, a transformer for each turbine, and power lines. Siting them offshore can address these objections in some cases, while raising other issues, such as dangers to navigation and the possible adverse effect of low-frequency vibration on ocean mammals. Another important complaint is that windmills kill too many birds and bats. Siteing generally takes into account bird flight patterns, but most paths of migration, particularly for birds that fly by night, are unknown. Most critics support the goals of renewable energy to reduce reliance on fossil and nuclear fuels, reduce the emission of greenhouse gases and other pollution (such as that causing acid rain), and establish a sustainable source of energy, but they question wind energy's ability to significantly move society towards these goals. They point out that 30% average output is considered high for wind facilities, that besides low output they provide electricity in response to the wind rather than consumer demand, and that this intermittency ensures that no "conventional" power plants can be shut down, particularly less efficient plants that are able to switch on and off in a matter of seconds. Another charge is that output figures, such as "Denmark produces over 20% of its electricity from wind," do not account for the electricity used by the plants themselves or electricity that is simply absorbed by the international grid because it is produced when demand is already being met by other sources. It is also noted that because electricity production uses only part (about a third) of society's energy, wind power does nothing to mitigate the effects of most of our energy use. For example, despite aggressive installation of wind facilities in the U.K., that country's CO2 emissions continued to rise in 2002 and 2003. It is often pointed out, e.g., by the UN's Intergovernmental Panel on Climate Change, that continued improvements in efficiency -- in building, manufacturing, and transport -- will achieve the desired mitigation goals to a much greater degree and at much less cost than wind power can. Arguments for Supporters
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