Offshore Wind Energy: The Next North Sea Oil

Sea Technology, Apr 2008 by Jeng, Dong-Sheng

Potentials and Assessments of Offshore Wind Energy Systems: Where are the Best Sites for Offshore Wind Farms in Australia?

Offshore wind energy is the next North Sea oil. It is one of the most important renewable energy sources, and it has attracted growing attention from researchers and the general public all around the world. In general, offshore wind has huge potential to both reduce carbon emissions and create economic prosperity, as well as increase the security of the world's energy supply.

Offshore wind farms are different from onshore installations for several reasons: The wind turbine generators have, on average, larger diameters and rated power; installation and maintenance are more expensive; the submarine electrical connection to shore increases the investment costs; and offshore wind energy requires more advanced technology, especially in the coupling design of wind farms.

An important factor is the amount of energy production. Due to the large wind power-generating capacity, the energy production of offshore wind farms may be significantly higher than onshore installations. Moreover, the wind turbines used offshore are usually larger with higher production capacities than onshore facilities.

In addition, onshore wind farms block sunshine and produce considerable noise, to the annoyance of local residents, while offshore wind farms are, more or less, invisible. Most offshore sites are constructed about 10 to 20 kilometers offshore, eliminating both shadow and noise problems, so the influence on onshore populations is minor.

However, cost is a major shortfall of offshore wind energy generation. The cost of an offshore wind farm is considerably greater than onshore. Offshore wind farms currently are installed in shallow water, in maximum water depths of up to 30 meters. This requires higher costs and more advanced technologies to build the foundations and the submerged parts of the monopile. Additionally, power has to be transmitted from the wind turbines to the onshore power stations through undersea cables-an expensive transit, especially with the higher voltage required. Another extra cost for offshore wind farms is maintenance. As offshore wind farms are in a saltwater environment, and most of the wind turbines are designed and tested on land without considering saltwater corrosion, the repair and maintenance costs are generally much higher.

Existing offshore wind energy projects have been mainly distributed in Europe and North America. Although the inland wind energy systems are active in Australia, there are no offshore wind energy projects currently available. Therefore, an assessment of appropriate sites for an offshore wind energy system (OWES) in Australia is desirable.

Assessment of OWES Sites

Generally speaking, three types of sub-structures have been used for offshore wind energy systems: monopiles, torpid and gravity-based caissons. The most commonly used substructures are monopiles, which can currently reach up 4.5 meters in diameter, but they are limited to a water depth of less than 30 meters. Recently, another option-floating offshore wind farms-has been proposed. However, the main disadvantage of this concept is the cost, which could be double that of seabed-mounted structures.2

Based on existing offshore wind energy projects in Europe, together with fuzzy analytic hierarchy processes, researchers identified dominant factors of determining an appropriate site, for an offshore wind energy project.

Water Depth

Water depth and distance to shoreline are extremely important criteria for the selection of offshore wind farms. All offshore wind turbines are currently installed with monopiles, which can only be used in shallow water (water depths of less than 30 meters). Current trends indicate that offshore wind turbines will increase significantly in size, and as wind speed increases markedly with distance from shore with less visual impact and less competing use of the seabed, developers will be attracted to deep water. It is likely that substructures will change from the current monopiles in shallow water to floating structures in deep water.

Wind Speed

Wind speed is another dominant criterion for the selection of sites, as it is the main resource for offshore wind energy. Researchers have compared potential power production at onshore and offshore sites, pointing out that the average power production of the offshore site was a constant 900 kilowatts per hour-double the peak value of the onshore site.1

Biological and Environmental Aspects

The biological and environmental state of the proposed site needs to be thoroughly investigated, as minor disturbances to ecology may cause devastating effects. If there is not a full assessment and appropriate planning, environmental groups and the population living close to the proposed site could become angered.

Leisure

The level of boating and other marine recreational activities may lead to complications for the construction of offshore wind farms.

If the selected area has active boating and recreational use, public disapproval could result.