Cost-effective wind power

For onshore installations the concentration on blade efficiency is logical but for deep water offshore it may make less sense

I am not an enthusiast of wind energy but I have to accept that our government is now committed to the investment of many billions of pounds in offshore windfarms. I hope this money will be spent in the most cost-effective way, but I have some misgivings.

Turbine manufacturers spend much effort in improving blade designs to ensure that the power extracted per metre of blade is as high as possible. This is one reason we have very large-diameter turbines with usually only three blades. For onshore installations the concentration on blade efficiency is logical but for deep water offshore it may make less sense.

One reason is that in the typical deeper water licence areas now being considered the turbine is likely to be as little as one third of the capital expenditure. The other two thirds will be mostly made up of the undersea power cabling and the turbine support structure. The priority may therefore be for turbine designs which minimise the cost of support structures and cabling rather than designs which maximise blade efficiency.

Current support structures being considered are usually of the gravity platform or steel piled structure type. A major factor in the cost of such structures is the need to resist the very high bending moments generated by the aerodynamic thrust on the turbine. The bending moments are directly proportional to the height of the turbine nacelle above the sea bed and therefore there is an incentive to investigate the possibility of smaller-diameter turbines with larger numbers of blades, which may be slightly less efficient but which having a lower aerodynamic moment could significantly reduce the support structure cost.

Because of the rapidly escalating cost with water depth, piled steel structures and gravity platforms are only viable in water depths up to around 45m and this precludes their use in the much greater water depths which exist in many licence areas.

Compliant semi-floating structures have been used as tanker loading platforms for many years and are of a size directly suited to wind turbines and offer a potentially cheaper solution than equivalent rigid structures. Compliant support structures could be economic in water depths up to 200m. If they were adopted for offshore turbines they could be located in deep water closer to land and could therefore save a significant part of the cabling costs as well as being cheaper.

Ian Crossley, Camberley, Surrey

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