Building pathology: Wind turbines

Wind turbines will be one of the technologies eligible for feed-in tariffs when they become available this April. The level of tariff decreases as the scale of the turbine increases, so small wind turbines of less than 1.5kW output would be paid 34.5p/kWh of electricity produced and large-scale wind turbines of 1.5MW to 5MW would receive 4.5p/kWh.

The output of wind turbines varies, depending on many factors, which can be grouped in two categories. First, are those relating to the wind resource such as wind speed and pattern. The second set includes the mounting of the turbine – on a building or freestanding – height, type and blade span. All these factors need to be appraised during installation, and investigated if there are problems.

For example, the recent Energy Saving Trust report analysing the performance of domestic wind turbines indicates that small-scale building-mounted wind turbines in urban or suburban areas generated no more than 200kW in a year. At 34.5p/kWh this gives a maximum likely return of £69 per year.

The best performing turbines are freestanding, in rural areas, near the coast or on exposed land. In this situation a 6kW turbine operating a 30% load factor – actual output compared with maximum output – may generate 18,000kWh/y, which would result in an income of £4,800/y with a 26.7p/kWh feed-in tariff.

What these figures do not show is expected reliability, downtime owing to repair and maintenance and malfunctions. Failures in wind turbines vary enormously in extent and consequence. Failures resulting in danger to life are, fortunately, rare, but risk should be managed within the existing framework of health and safety regulation, structural codes and wind turbine standards to BS EN 61400.

Investigation of such problems is a specialist field. Blade failure may be the result of lightning strike. A fault in the gearbox or brake mechanism might result in it rotating uncontrollably, which leads to damage or destruction. Blade damage has also been associated with storm and ice conditions.

Failure rates for wind turbines vary between 0.6 and 6.4 per year. That is the average component and mechanical failure a year from several thousand installations. Maximum rates for individual wind turbines might be greater. Components that account for about half the failures include: hydraulics, brake, generator, blade, yaw system, shaft and bearings, electrics, inverter, gearbox, instrumentation and controls.

The challenge with wind turbines is to find the optimum balance between reliability, loss of income from downtime and cost of replacement. Failure mode, effect and criticality analysis is a useful framework within which to assess failures.

Gearboxes, in particular, have a variable record and when they fail, downtimes are long with high replacement costs. Faults may arise owing to loads in-use being larger than design loads, manufacturing faults, misalignment of components, damage during transport and poor maintenance. However, the majority originate with bearing failures. Design and specification of gearboxes to ISO 81400–4 for power capacities between 40kW and 2MW benefit from field experience to achieve a life of 20 years. Increasingly, online monitoring of assemblies provides data about temperature and vibration. This can be correlated with power output, generator and wind speed when abnormal patterns of data start presenting maintenance interventions.

Cost may be more of an issue for large turbines in remote locations or offshore where a call out may require the use of a helicopter or specialist access equipment. Strategies for managing these risks include the consideration of incorporating access and hoisting equipment within the wind turbine and installing remote monitoring equipment. To save downtime, one strategy is to hold in stock replacement components or assemblies, replace the complete assembly and take the defective assembly out for repair off site. Assemblies which result in high downtime include the electrical system, rotor or blades, generator and gearbox.

What can go wrong?

Failures in wind turbines vary, but the most common problems are:

1) Blades – may become damaged as the result of lightning, storm and ice conditions, or failure to restrain the blade from rotating uncontrollably

2) Gearboxes – faults may arise because of higher than expected loads, misalignment of components, inadequate initial service and poor maintenance, such as waiting too long for oil changes.