Environmental Engineering Reference
In-Depth Information
need widening of reinforcement. Importantly, planning approval may be easier to obtain for
offshore schemes due to their reduced visual and noise impact. Such advantages will become
increasingly important as the acceptable onshore sites are progressively used up. At present,
offshore wind energy is more expensive than on shore, primarily due to signifi cantly higher
foundation, installation and electrical connection costs. Complex and costly underwater cable
systems are required to link the turbines together and to make the connection to shore. In
some cases an offshore substation may be required. However, as more off-shore plant is built,
the technology will advance to deal with unanticipated problems and to reduce transportation
and installation costs. For example, while lightning tends to strike onshore wind turbines at
the blade end, in offshore applications it was observed that strikes are more frequent at the
centre of the blade. Blades are now redesigned to cope with such occurrences. To speed up
installation dedicated vessels are being developed and built to deliver and install the towers
nacelles and blades.
One of the major problems is the accessibility of turbines for maintenance. This is only
feasible by sea during relatively calm conditions, so the larger off-shore wind turbines are
sometimes fi tted with platforms on which personnel can be lowered from a helicopter. These
procedures are costly so designers are under pressure to develop turbines with exceptional
reliability and long maintenance intervals. As already mentioned, sophisticated condition
monitoring systems continuously assessing the turbine performance and fatigue state will
become increasingly common. Such systems were too expensive for onshore turbines of 1 to
2 MW but for multimegawatt units the systems represent a much smaller fraction of the
turbine capital cost; moreover the costs of instrumentation are falling making such approaches
more cost effective.
Electrical Integration
The growth of signifi cant offshore wind capacity has raised grid integration issues to a new
prominence. Increasingly large amounts of electricity will be feeding into national networks
at points not specifi cally designed for such infeeds. Consequently, signifi cant network rein-
forcement will often be required. In addition these time varying infeeds must be integrated
into grid management systems not previously required to cope with such non-dispatchable
sources. The costs of expected system adaptation will be signifi cant and new market mecha-
nisms may well be needed to ensure that the required infrastructural investments are made
in a timely manner.
Today's onshore wind turbines typically generate at around 700 V, which is commonly
stepped up to
35 kV by a transformer in the nacelle or the tower base. The fi rst offshore
schemes have tended to use a similar arrangement, although it is anticipated that as wind
farms become larger with longer distances between turbines and increased distance to shore,
the voltage will have to increase.
In the Danish 40 MW Middelgrunden wind farm a voltage level of 30 kV was chosen for
the collection of power between the 20 turbines and the short 3 km connection to shore. In
contrast for the 160 MW Horns Rev wind farm, the collection voltage is 36 kV but the voltage
is then raised to 150 kV at an offshore transformer substation before transmission over 15 km
to shore. In future large offshore schemes AC transmission may severely limit the distance
over which electrical power may be transmitted by undersea cable.
Search WWH ::




Custom Search