How deep and how far

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In Europe, most countries could meet at least 20% of their current electricity consumption by offshore wind power. How great the percentage can be depends on a number of natural constraints, particularly distance from shore and, crucially, depth of water.

A study off the north-east coast of America by the Long Island Power Authority discovered that 99% of the potential 2250 MW at sites with wind speeds of 8 m/s and above disappeared when a five kilometre from land restriction was imposed. When maximum allowable depth, however, was increased from 15 to 30 metres, a worthwhile resource of about 5200 MW floated to the surface.

As yet, there are no firm rules on optimum depth, but most wind power stations are being built in waters between five to 30 metres deep. In shallow waters below five metres there may be problems with breaking waves. In waters deeper than 30 metres, foundation costs become progressively more expensive.

The distance from suitable onshore grid connections is also a crucial economic parameter. Conveniently, conventional thermal power stations are often sited near coasts and provide suitable high voltage connection points, allowing the wind project developer to avoid the considerable cost of grid reinforcement. Minimal upgrading was needed for Denmark's Horns Rev 160 MW; even so, the grid connection cost amounted to EUR 250/kW -- about 15% of the cost of installing each megawatt.

An indication of how much grid reinforcement could multiply that percentage lies in the estimates of the UK's Department of Trade and Industry that full utilisation of the wind resources off the north-west coast of the UK could amount to more than EUR 300 million in reinforcement costs. At that price it seems sensible to leave some of the resource undisturbed.

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