Relatively high initial capital costs are another perennial issue when it comes to arguing the merits of wind energy.
Both efficiency and cost are greatly affected by the quality of maintenance that the turbines receive during their operational lifetime. A well-maintained turbine is better than one that is poorly looked after because, with components less likely to break down, it can offer a greater level of availability to generate when the wind is blowing. And, with components in optimal condition, there is minimal wastage of the wind energy captured by the blades. A poorly maintained gearbox, for example, can restrict the amount of wind energy available to be converted into electricity.
By being well maintained, the turbine is also more likely to operate well for the majority of its lifetime, and perhaps even beyond its initial predicted life span - usually assumed as 20 years.
This combination of maximising a turbine's availability and ensuring a long operational life can have a huge impact on calculating the capital costs of projects over their lifetime. Wind turbines' fuel is of course free, so when compared with more traditional power generation such as gas-fired plants or potential low-carbon rivals such as nuclear - and even renewable technologies such as biomass - over a project's entire lifetime, wind becomes highly competitive. Indeed, a recent report published by the Michigan Public Service Commission in the US found that average costs over the life cycle of renewable energy systems equalled $91/MWh while the cost for a new coal-fired power plant totalled $133/MWh.
In a world where public subsidies for wind energy are on a continual slide and the availability of bank debt to finance projects is also scarce, the entire wind industry has a vested interest in being able to make a strong case for wind based on lifetime costs. Such a case can only be made if the efficiency and longevity of the wind turbines can be demonstrated, and this can only be done through a sound operations and maintenance (O&M) regime.
On the efficiency side, many manufacturers have focused on improving turbine technology to boost both output and reliability. Central to this has been the growth in blade lengths, something that will continue to push the maintenance challenges, as the diagram of a blade and inspection points shows on page 10.
Also essential for efficient running of turbines is the ability to understand their performance, which requires analysis of data from remote monitoring systems and onsite inspections (page 9).
When it comes to longevity, a question already facing turbine owners in Europe and across the globe in years to come is what to do with old turbines. As our article on recertification (page 6) points out, many turbines in Europe that were installed in the early 1990s are approaching the end of their operational lives, if they have not done so already. Is it better to decommission those turbines or to look at ways of extending their operational lives, if only for a few years? The article looks at the assessment process for extending a turbine's life, "a complex engineering task" due to the huge number of variable factors likely to affect the decision. Questions that need addressing include the impact of the wind-farm operator's maintenance regime on the original life assumptions.
Responsibility for maintenance after the manufacturer's warranty expires is a hot topic, particularly in China, where successive years of major capacity installations have created a post-warranty service market worth $16 billion (page 17).
Whoever is responsible for the maintenance will need to ensure that an adequate supply of replacement components is easily accessible. The head of central maintenance for an operator of almost 4GW of wind projects in Europe explains how his firm calculates stock levels (page 13). For smaller parts such as sensors and filters the stock levels are first set using recommendations from the turbine manufacturer and then adjusted by the local wind farm manager. The procedure for calculating major component stock levels, meanwhile, begins while the parts are still under the manufacturer's warranty. Over this period of around five years the company will keep a record of all the failures and defects that develop for each major component, ensuring that, when it takes over responsibility for maintenance at the end of the warranty period, it has a database of when components may need to be replaced or repaired. Having parts available will ensure fewer delays before the turbine is back in operation again.
John McKenna is an associate editor of Windpower Monthly