Onshore

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the so-called topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way make large differences to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output. Local winds are often monitored for a year or more with anemometers and detailed wind maps constructed before wind generators are installed.

For smaller installations where such data collection is too expensive or time consuming, the normal way of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently "cast" or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical data from a nearby meteorological station, although these methods are less reliable.

Wind farm siting can sometimes be highly controversial, particularly when sites are picturesque or environmentally sensitive (for instance, having substantial bird life).

Near-Shore

Near-Shore turbine installations are on land within three kilometers of a shoreline or on water within ten kilometers of land. These areas are good sites for turbine installation, because of wind produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind, depending on the prevailing wind direction.

Common issues that are shared within near-shore wind development zones are bird migration and nesting, aquatic habitat, transportation (including shipping and boating) and visual aesthetics. Residents near some sites have strongly opposed the installation of wind farms due to these concerns.

Offshore

Offshore wind turbines near Copenhagen

Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise can be mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and near-shore locations.

In stormy areas with extended shallow continental shelves, turbines are practical to install. Denmark, for example, has many offshore windfarms.^[4]

The United Kingdom plans to use offshore wind turbines to generate enough power to light every home in the U.K. by 2020.^[5]

Locations have begun to be developed in the Great Lakes — with one project by Trillium Power approximately 20 km from shore and over 700 MW in size. Ontario, Canada is pursuing several proposed near-shore locations in fresh water ^[6] and one on the Pacific west coast ^[7].

Offshore installation is more expensive than onshore but this depends on the attributes of the site. Offshore towers are generally taller than onshore towers once the submerged height is included. Offshore foundations may be more expensive to build. Power transmission from offshore turbines is through undersea cable. Offshore installations may use high voltage direct current operation if significant distance is to be covered. Offshore saltwater environments can also raise maintenance costs by corroding the towers, but fresh-water locations such as the Great Lakes do not. Repairs and maintenance are usually more costly than on onshore turbines. Offshore saltwater wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection, which may not be required in fresh water locations.

Offshore wind turbines will probably continue to be the largest turbines in operation, since the high fixed costs of the installation are spread over more energy production, reducing the average cost. Offshore wind farms tend to be quite large, often involving over 100 turbines.

Airborne

Wind turbines might also be flown in high speed winds at altitude, although no such systems are in commercial operation.

An airborne wind turbine is a design concept for a wind turbine that is supported in the air without a tower.^[1] A tether would be used to transmit energy to the ground. These systems would have the advantage of tapping an almost constant wind and doing so without a set of slip rings or yaw mechanism, without the expense of tower construction. The main disadvantage is that kites and 'helicopters' come down when there is insufficient wind. Also, bad weather conditions, such as lightning and/or thunderstorms, could temporarily suspend use of the machines, probably requiring them to be brought back down to the ground and covered. These schemes require a long power cable and, if the turbine is high enough, an aircraft exclusion zone. As of 2006, no commercial airborne wind turbines are in regular operation.

References