How Far Off Coast Are Wind Farms?

Offshore wind farms are an increasingly important source of renewable energy around the world. Located in bodies of water, usually oceans, lakes, or rivers, offshore wind turbines capture wind energy and convert it into electricity. While the first offshore wind farm was installed in Denmark in 1991, capacity has expanded rapidly in recent years as the technology has improved and governments have provided policy support. Offshore wind now represents a growing share of renewable energy production globally.

Defining Offshore

Offshore wind farms are typically defined as being located in bodies of water, most often oceans, lakes, or seas. The term “offshore” indicates that the turbines are placed a significant distance away from the shoreline. Nearshore wind farms are closer to the coast, usually less than 10 kilometers away. Farther offshore wind farms can be over 100 kilometers from land.

The distance from shore that constitutes an offshore wind farm varies by region and local regulations. In the North Sea off the coasts of northern Europe, offshore wind projects are frequently 50-75 kilometers from the coastline. In the nascent U.S. offshore wind industry, distances of 15-30 kilometers are more common so far. This is partly due to technological limitations of installing turbines and transmission infrastructure farther from shore.

Offshore wind farms utilize a range of support structure designs based on depth and seabed conditions, including fixed-bottom monopile foundations in shallow waters up to around 50 meters deep. In deeper waters, floating platforms become necessary. Sub-stations collect the power from multiple turbines and convert voltage for transmission to shore via submarine cables.

Industry terminology distinguishes between the nearshore area closer to shore, far-shore regions farther out, and the deep-water locations where floating technology is required. Precise definitions vary across different geographic regions and regulatory frameworks.[1]

Distance from Shore

Offshore wind farms are typically located a few miles from the shoreline. According to research from the University of Delaware, the majority of existing offshore wind farms are located within 20 km (12 miles) from shore at average water depths of around 30 meters. Some key factors that influence the distance from shore include:

Water depth – Deeper waters farther from shore can allow for larger turbines and foundations, but also increase costs. Nearshore shallow waters are cheaper to develop but limit turbine size. Many existing projects are in transitional depths of around 30 meters.

Wind conditions – Winds tend to be stronger and more consistent farther offshore, improving energy production. But this must be balanced with greater transmission costs.

Visibility – Projects closer to shore are more visible, raising public concerns over impacts to views. Distance limits visibility and public opposition.

Environmental impacts – Greater distance reduces visual and noise impacts to wildlife and coastal habitats like seabird and marine mammal migration routes.

Grid connection – Longer cable distances add costs, so distances must balance improved wind resources with greater transmission needs.

Overall, most offshore wind projects today locate turbines 10-20 miles offshore to leverage sufficient wind resources while containing development and connection expenses. But as technology improves, future projects may be sited even farther out to sea.

Regional Differences

There are significant regional differences in how far offshore wind farms are typically built. In Europe, offshore wind farms are often located between 10-30 miles from the coast. For example, the London Array in the UK, one of the world’s largest offshore wind farms, is located over 12 miles offshore. Many other major European wind farms are 15-25 miles out. Comparatively, offshore wind farms in Asia are often much closer to shore, frequently less than 10 miles offshore. For instance, major Chinese offshore wind projects in the East China Sea are only around 5-7 miles from the coast.

In North America, proposed offshore wind farms are generally further out, frequently over 20 miles offshore. The Vineyard Wind project off Massachusetts will be 15 miles out, while many lease areas for offshore wind are 20-30+ miles offshore. There are several reasons for these variations. In Europe, nearshore areas were developed first, but expansion is now further offshore due to limited remaining space close to shore. In Asia, technical constraints around constructing in very deep waters farther offshore has kept projects closer to shore. In North America, there are fewer technical barriers but more environmental concerns, so turbines are sited further away from coastlines and migratory bird paths.

Overall, regional differences in average offshore wind farm distances reflect factors like available nearshore space, public acceptance, technical feasibility, and regulatory requirements. But there are also examples bucking the trends, with some Asian projects further offshore and some European projects very close to shore.

Water Depth

The distance offshore wind farms are located from the shore is closely related to the water depth in that area. Generally, shallower waters are found closer to shore, while deeper waters are farther offshore.[1]

On the East Coast of the United States, the continental shelf descends gradually into deeper waters. This allows wind farms to be located within about 30 miles from shore while still being in relatively shallow waters less than 100 feet deep which is optimal for fixed foundations offshore wind turbines.[2]

In contrast, on the West Coast, the continental shelf drops off much more steeply into deep waters. Here, wind farms need to be located 50-100 miles offshore in order to be in water depths shallow enough for current wind turbine technology and economics, generally less than 1000 feet.[1]

Therefore, suitable locations for offshore wind farms are strongly dependent on the local bathymetry and how quickly the seafloor descends from the coastline. Closer proximity to shore is preferable for lower costs and easier grid connection, but sufficient water depth is also a key factor.

Foundations

The type of foundation used for offshore wind turbines depends on the water depth where the turbines are located. In shallower waters close to shore, turbines are typically mounted on solid foundations on the seafloor. Common foundation types in depths up to around 30-40 meters include:

Monopile foundations – These consist of a single, hollow steel tube piled into the seabed. Monopiles are the most common foundation used for offshore wind, accounting for around 75% of all installations. They provide a simple and cost-effective solution in relatively shallow waters (https://www.windpowerengineering.com/comparing-offshore-wind-turbine-foundations/).

Gravity base foundations – These are large concrete structures with a broad base and ballast to provide stability. They are lowered and founded on the seabed, typically in depths up to around 20 meters.

Jacket foundations – These are lattice structures anchored to the seabed with piles. The turbine tower passes through the center. Jackets allow installation in deeper waters of around 35-60 meters.

As water depth increases farther offshore, different foundation solutions are required that are anchored to the seabed rather than resting on it. These include tripod, tri-pile, and other multi-legged structures secured with driven piles. Floating foundations attached by mooring lines are also starting to be used in deeper waters over 60 meters (https://www.iberdrola.com/sustainability/offshore-wind-turbines-foundations). The choice of foundation type is a major factor determining how far offshore wind farms can be located.

Wind Conditions

Distance from shore is a key factor affecting wind speeds and consistency for offshore wind farms. According to data from the U.S. Department of Energy’s Windexchange, average wind speeds tend to increase further offshore, with wind power density over 500 W/m2 typically occurring 20-50 nautical miles from shore. However, there is significant regional variation.

Along the Atlantic Coast, average wind speeds range from 7.4-9.3 m/s based on analysis by S&P Global Market Intelligence (Assessing wind speeds, potential performance across Atlantic offshore wind portfolio). Wind speeds are generally higher along the North Atlantic compared to the Mid-Atlantic. In the Great Lakes region, average wind speeds are lower but more consistent nearer to shore due to less interaction with weather systems.

According to a 2010 NREL report (Assessment of Offshore Wind Energy Resources for the United States), wind consistency improves farther from shore, with capacity factors increasing from around 35% within 5 nautical miles to over 45% at distances greater than 15 nautical miles. This is likely due to less turbulence and steadier wind flows. Overall, locating wind farms 20-50 nautical miles offshore provides a good balance of strong, consistent winds while minimizing costs and technical challenges.

Grid Connection

Distance from shore is a major factor affecting the feasibility and cost of connecting offshore wind farms to the electrical grid. Farther offshore wind farms require longer undersea transmission cables to reach the onshore grid, which adds significant cost. According to one study, offshore wind farms located over 60km from shore can see their grid connection costs rise to over 30% of total project costs, compared to just 10-15% for nearshore wind farms.1

The length of transmission cables also brings technical challenges, as power losses and reactive power compensation requirements increase with distance. New long-distance HVDC transmission technologies are being developed to enable connections over 100km offshore, but these are expensive. Overall, distance from shore is a limiting factor on offshore wind farms and favors near-shore locations within around 60km of landfalls.2

Strategic grid planning and coordinated development of interconnected offshore transmission networks helps enable cost-effective grid integration even for more remote wind farms. Connecting multiple offshore wind sites through offshore transmission hubs can be more efficient than individual radial connections. But in general, farther offshore wind farms face higher grid connection barriers.

Environmental Impact

The distance of offshore wind farms from the shore can significantly influence their environmental impact. Nearshore wind farms, located within 10 km of the coast, face greater scrutiny due to their visibility from land and potential disruption of sensitive coastal ecosystems (Sullivan, 2013). Visual impacts are a major concern, as offshore wind turbines can be visible on the horizon up to 26 miles (42 km) away under clear conditions (Sullivan, 2013).

In contrast, wind farms located farther offshore, beyond 20-50 km, have reduced visual impacts but may disrupt migratory bird pathways over open water (Bailey, 2014). Underwater noise pollution during construction is another environmental concern that does not appear to significantly vary with distance from shore (Bailey, 2014). Overall, siting wind farms at intermediate distances around 20-50 km offshore appears to balance visual impact, ecosystem disruption, and transmission costs when evaluating environmental impacts.

Future Outlook

As offshore wind technology continues to advance, there is a trend towards siting wind farms farther from shore in deeper waters. According to the U.S. Department of Energy, newer turbine designs allow offshore wind projects to be built in locations with average wind speeds of over 7 meters per second, compared to 4-6 meters per second previously [1]. Access to stronger and more consistent winds farther offshore can increase energy production.

Floating wind turbine foundations are being developed that will allow wind farms to be located in waters over 200 feet deep, compared to current limits of around 60 feet [1]. This will enable wind farms much farther offshore. For example, the proposed Mayflower Wind project off Massachusetts will be over 20 miles offshore and utilize floating foundations in waters over 200 feet deep.

High voltage direct current (HVDC) transmission technology now allows efficient transmission of electricity over longer distances underwater. This enables grid connections from wind farms much farther offshore. Industry experts predict average offshore wind farm distances increasing from around 20 miles currently to over 40 miles in the future as technology improves [2].