Offshore wind

Table of Contents

General

The best accessible global areas for offshore wind farms, reasonably close to shore and in shallow water, are the North Sea and the South China Sea.  See the Global Wind Atlas.

 At the end of 2020 UK was the global leader with 10 GW of offshore wind capacity installed, 37% of global offshore wind capacity.  

But in 2021 China installed a further 17 GW to take pole position, which it is now unlikely to lose.

Germany, Denmark, Belgium and the Netherlands propose to build 150 GW of offshore wind farms between them by 2050.  The proposal includes an energy island to enable power to be flexibly routed.

China is proposing a huge 43 GW offshore wind farm in the Taiwan Straits to provide power to the 13 million inhabitants of the city of ChouZhou, GuangDong province.  According to the local plan, work on this is expected to start by 2025.

Offshore wind turbines are typically far bigger than onshore wind turbines, constrained by the maximum turbine size that the industry can produce, rather than planning rules.  Whereas 3-4 MW is the limit for most onshore wind turbines, the largest offshore turbine on sale (5 December 2022) is the 16 MW wind turbine developed jointly by China Three Gorges Corporation and Goldwind Technology.  It is 270m tall with a rotor diameter of 252 metres, and a swept area of 50,000 square metres, the equivalent of around seven standard football pitches. However, (6 January 2023) CSSC Haizhuang Wind Power has now announced an 18 MW turbine with a rotor diameter of 260 metres and a swept area of 53,000 square metres.

There is very little NIMBY (“not in my back yard”) opposition from householders to offshore wind farms, but the needs of fishermen and other ocean users must be taken into consideration.  Environmental assessments are also required.

Wind turbine technology

There are two methods of securing offshore wind turbines to the sea bed.  Almost all current offshore wind farms use bottom fixed turbines, with foundations embedded into the sea bed. A handful use floating onshore wind in which the only connection to the sea bed is a cable and anchor.  The cable attaches to a float on which the turbine is mounted.

Other designs are possible, but not mainstream.

Offshore wind turbines are generally direct drive, with no variable speed gearbox.  The AC output of the generator is thus not tied to a mains frequency of 50 or 60 Hz.  If the onshore cable is HVAC, then offshore frequency converters are needed. 

Onshore connection

For distances under a few tens of km from shore, HVAC (high voltage AC) connections are used.  For more distant farms, HVDC (high voltage DC) connections have to be used.
Installation of world’s first unattended offshore HVDC substation on Dogger Bank
The issue is that undersea cables have a much higher capacitance than bare wires on pylons.  When carrying an AC electricity, this capacitance causes additional reactive (capacitive) currents in the cable that are not in phase with the voltage, which means no power is transferred by these currents.  For cable distances over a few tens of km of distance, most of the current carrying capacity of the cable (in amps) is taken up with the reactive power, leaving little for the useful power.  This phase mismatch can be corrected, but this must be done regularly over the length of the cable, not just at the ends.  This is far less straightforward underwater, as well as being required at much shorter distances compared with cables in air.

Output and Capacity Factor

Capacity factors offshore are much higher than onshore.  Offshore wind farms on the UK part of Dogger Bank (off the east coast of the UK) are expected to have a 60% capacity factor, because of the average wind speed of over 10 m/s.  The north and northwest of Scotland have significantly higher wind speeds than Dogger Bank, and should have capacity factors exceeding 60%, but will require floating offshore wind turbines because of the ocean depth.  

See the Global Wind Atlas, which can estimate the annual capacity factor for most of the likely locations for wind farms around the world.

Global pipeline

RenewableUK has analysed the UK and global offshore wind projects.  As of February 2023, the global pipeline of offshore wind projects is 1,100 GW (1.1 TW).  Renewable UK says this would provide 20% of global electricity demand.

According to Statista, in 2021, global electricity demand was 25,343 TWh.  That is an average of around 3 TW (3,000 GW) of demand.  If the average capacity of all offshore wind projects were 55%, then 1,100 GW of offshore wind capacity would result in average generation of 605 GW, which is 20% of 3,000 GW.

But by the time we get to 2030, electricity demand will be higher – maybe 50% higher or more.  However, the offshore wind pipeline is also likely to grow over time.

Cost

Initial offshore wind projects produced very expensive power.

After initial installations by Denmark, the UK chose to lead the construction of large scale offshore wind farms to achieve mass market status for offshore wind and economies of scale.  UK was assisted in this by Germany, Holland, Denmark and other northern European countries. This has reduced the cost of offshore wind considerably over the last seven years.

Since the 2014 CfD (contract for differences = fixed price) contracts were signed, the cost of UK offshore wind has come down 73%.  

Some Dutch and German offshore wind CfD (contract for differences) contract strike prices have been for zero euros.  However, these countries have a “one way” CfD contract whereby zero just means they will take the normal wholesale electricity price (never subsidised).  

The UK has “two-way” CfD contracts which means the electricity generator has to pay back any difference between the wholesale price and the contracted CfD strike price.  UK CfD auctions will be annual from now on, likely with 7 GW of offshore wind auctioned each year, as in the 2021/22 CfD auction.

The Dutch and German grids also pay for the cost of the connection from the offshore wind farm to the onshore transmission network.  Whereas, in the UK, the offshore connection has to be paid for by the offshore wind project up to the point of connection to the onshore transmission (or distribution) network.  So UK offshore wind prices are not strictly comparable with those in the Netherlands and Germany.

The US DoE (department of energy) says the US cost of bottom fixed offshore wind in 2021 was $84/MWh, expected to drop to $60/MWh by 2030.

In China the cost of offshore wind is estimated at $78/MWh, including cabling.  Coal power there is estimated to cost $76/MWh, so Chinese offshore wind is now challenging coal on cost.  

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