On-off relationship

Is offshore wind energy worth the extra cost?

Offshore wind has a number of attractions, including large wind resources, higher wind speeds than on land and fewer environmental constraints. The wind characteristics at sea are also more favourable, with lower turbulence and less wind shear, which means less severe dynamic loads on blades.
There are opportunities offshore to build very big arrays, so that some overheads can be spread between large numbers of machines. These include site meterological measurements, and legal and financial fees.
However, construction costs are significantly higher due to the need for more expensive foundations to cope with wave and wind loadings, plus measures to protect machinery and external surfaces from the harmful salt spray (see below). The electrical cabling and grid connection to shore will also be more expensive, while the higher wind speeds necessitate the use of more robust wind turbines.
The need for appropriate vessels means operation-and-maintenance costs are also higher offshore. As the vessels may not always be able to gain access to repair faults due to adverse weather conditions, machine availability may be lower.

Dissecting the costs
Everything costs more offshore compared with onshore. The graph, left, shows typical data. Onshore wind turbines can now be procured for less than €1,000/kW, while offshore turbines cost roughly 15-20% more. Onshore foundations cost around €200/kW, compared with €500/kW or more offshore. Electrical cabling between machines, plus a grid connection, typically costs around €100/kW onshore, but up to six times as much offshore. Onshore installation costs are generally around €100/kW, but typically around €450/kW offshore.
Overall, total onshore installed costs average around €1,500/kW of energy produced — offshore that figure is roughly doubled. It should be noted, however, that there are wide variations in the constituent costs, and in the totals, with onshore costs ranging from around €1,300/kW to €1,700/kW, and offshore costs ranging up to around €3,300/kW.

Operation and maintenance costs
Operation and maintenance (O&M) costs are inevitably higher offshore but there is a shortage of data to allow definitive comparisons. Onshore O&M costs are around €15-20/MWh, and offshore levels are roughly two to three times as much. However, the precise scope of any particular estimate needs to be scrutinised, as not all the figures include rent, rates and insurance. This data is crucial to deriving a reliable figure for O&M costs.

Productivity
In submitting projections as part of their National Renewable Energy Action Plans, EU member states suggested the capacity factor of offshore wind would 24-45%, as shown below. On average, the offshore capacity factor, at 33%, was nearly 50% higher than the corresponding onshore figure of 24%. However, a 50% increase in productivity does not cancel out the 100% increase in capital cost, and so offshore wind is significantly more expensive than onshore wind.

Cost of energy comparisons
With installed costs offshore 70-100% higher than onshore and O&M costs also higher, generating costs are generally 30-80% higher offshore, despite greater productivity. Windpower Monthly’s annual review of generating costs (Windpower Monthly, January 2012) suggested that €90/MWh was a typical onshore figure at a site with a mean wind speed of 7 metres per second (m/s). With offshore wind at €3000/kW, the generating cost at a site with a mean wind speed of 9 m/s will be around €120/MWh.
Sites with higher wind speeds are being developed — particularly for the German and UK offshore programmes — but these are a considerable distance from shore and therefore construction costs are likely to be higher.
There are expectations that installed costs may come down in future, but the differential between on- and offshore is likely to remain significant.

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