Comparison between Onshore & Offshore wind turbines

Onshore wind turbines :

Onshore turbine installations tend to be along mountain ridges or passes, or at the top of cliff faces. The change in ground elevation causes the wind velocities to be generally higher in these areas, although there may be a lot of variation over relatively short distances (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.
Sea shores also tend to be windy areas and good sites for turbine installation, because a primary source of wind is convection from the differential heating and cooling of land and sea over the course of day and night. Winds at sea level carry somewhat more energy than winds of the same speed in mountainous areas because the air at sea level is denser.

e.g. Wind turbines near Walla Walla in Washington

Wind farm siting can sometimes be highly controversial, particularly as the hilltop, often coastal sites preferred are often picturesque and environmentally sensitive (for instance, having substantial bird life). Local residents in a number of potential sites have strongly opposed the installation of wind farms, and political support has resulted in the blocking of construction of some installations.

Offshore wind turbines :
Offshore wind turbines cause less aesthetic controversy since they often cannot be seen from the shore. Because there are fewer obstacles and stronger winds, such turbines also don’t need to be built as high into the air. However, offshore turbines are more inaccessible and offshore conditions are harsh, abrasive, and corrosive, thereby increasing the costs of operation and maintenance compared to onshore turbines.

Wind blows briskly and smoothly over water since there are no obstructions. The large and slow turning turbines of this offshore wind farm near Copenhagen take advantage of the moderate yet constant breezes here.

Advantages:

• Wind is free, wind farms need no fuel.
• Produces no waste or greenhouse gases.
• The land beneath can usually still be used for farming.
• Wind farms can be tourist attractions.
• A good method of supplying energy to remote areas.

Disadvantages:

• The wind is not always predictable – some days have no wind.
• Suitable areas for wind farms are often near the coast, where land is expensive.
• Some people feel that covering the landscape with these towers is unsightly.
• Can kill birds – migrating flocks tend to like strong winds. Splat!
• Can affect television reception if you live nearby.
• Can be noisy. Wind generators have a reputation for making a constant, low, “swooshing” noise day and night, which can drive you nuts. An entire wind farm makes quite a racket! Having said that, the small modern wind generators used on boats and caravans make hardly any noise, and as aerodynamic designs have improved, modern wind farms are much quieter.