QMT Features: March 2012
Wind turbine testing
An overview of current small and medium wind turbine testing and certification in the UK and also farther afield. By Alistair Mackinnon, operations manager Wind Energy at NEL.

Wind has become the largest source of renewable electricity in the UK due to the maturity of wind turbine technology, the availability of wind turbines for deployment, and the favourable wind resource.

European Union (EU) figures show that today wind power supplies more than five per cent of its overall electricity demand. The European Wind Energy Association (EWEA) believes that by 2020, wind energy will meet 15.7 per cent of EU electricity demand, growing to provide half of Europe’s power by 2050. The latest statistics from DECC show that seven per cent of electricity generated in the UK in 2010 came from renewable resources of which 40 per cent was generated using wind.

Scotland is committed to generating an equivalent of 100 per cent of electricity demand from renewable sources by 2020, which is the most ambitious target in the EU.  Meeting 100 per cent of its electricity consumption from renewables in 2020 means that, together with 11 per cent renewable heat and 10 per cent renewable transport targets, Scotland's overall share of renewable energy will be at least 30 per cent by 2020, exceeding the EU's renewable energy target and doubling the UK's agreed EU target of 15 per cent.* As the popularity of small and medium wind turbines increases, particularly in the UK, so does the need for greater confidence in their ability to perform as designed, both safely and reliably. This has resulted in a complex and challenging testing and certification regime.

The testing criteria for small and medium wind turbines are largely driven by international standards, as well as an implementation regime specific to meet UK needs. In the UK, the MCS scheme places a heavy reliance on the British Wind Energy Association (BWEA) Small Wind Turbine Performance and Safety Standard, which in turn references a number of International Standards in the IEC 61400 series. A not dissimilar criteria exists in the US and Canada, but relies on the AWEA 9.1 Standard in place of the BWEA Standard.

Both the BWEA and AWEA Standards are underpinned by IEC 61400-2: 2006, which is the design criteria for small wind turbines.

The next edition of the Standard is due to be published shortly and is currently in a Committee Draft for Voting form. To deliver this next edition, an IEC Maintenance Team has been working very intensively and has reviewed many of the criteria in the light of lessons learned and current best practice. The resulting new version of the Standard will give greater guidance amongst other things on the use of the simplified load approach and dealing with extreme wind conditions. The presentation of results will also be more user friendly with the inclusion of a consumer label.

Reliable performance
The performance of a turbine is assessed against the criteria in IEC 61400-12-1: 2006, the power performance Standard, and IEC 61400-11: 2003, the acoustic performance Standard. Both of these are crucial in determining a turbine’s suitability for a particular wind regime, purpose and location.

The power delivered by a wind turbine, most usually to the grid or for battery charging purposes, varies with the third power of the wind speed and is reported as a headline figure at 11 m/s for comparison purposes, and as an Annual Energy Production (AEP) in terms of its yield. From a detailed knowledge of the local wind speed at a turbine’s specific location it is then possible to calculate the likely energy delivered by the system over a specific period of time, which is usually one year.

The acoustic performance can be a significant factor in the planning process in determining whether a turbine(s) are likely to infringe local amenity. The acoustic performance is usually quoted as a sound power figure (Lw) for comparison purposes which can then be used to determine the local sound pressure (Lp) that a receiver will be subject to. The sound power is the unique character of the specific turbine which then manifests itself in the local environment as a sound pressure. The sound pressure is influenced by the local conditions at the receiver.

The UK’s MCS scheme requires that all of these criteria are properly evaluated by qualified test organisations in accordance with ISO 17025: 2005. The scheme also requires that these results are certified by a suitably qualified or recognised certification body. Within Europe the main certification criteria is based on the requirements of EN 45011, whereas in other jurisdictions it is ISO Guide 65, but they are essentially the same criteria.

Size matters
At the time of writing, there is no formal certification scheme in the UK for larger turbines whose power exceeds 50kW and/or have rotor swept areas greater than the 200m2 limit. Historically, larger machines have been designed in accordance with the provisions laid out in IEC 61400-1. However, as large machines have increased in size, some believe that the ‘dash-1’ standard is now less appropriate for machines in the “medium-sized” category.

While medium-sized turbines are unlikely to be the mainstay of wind farms of the future, they are likely to play a significant part of distributed and community wind projects, which means that an appropriate Standard supported by government incentive is likely to benefit all stakeholders in the longer term.

In order to try and address this, the UK, through RenewableUK (formerly the BWEA – British Wind Energy Association), convened a Medium Wind Strategy Group whose brief was to look at options to address this apparent anomaly. Work has been ongoing within this group for around 18 months and there is now a UK proposal to take things forward.

Currently, the UK does not have any significant players which manufacture such medium-sized turbines, but the Group has taken input from a number of manufacturers of smaller machines, testing and certification bodies, trade associations and government bodies. It has also benefited from a wider international participation from overseas manufacturers which are active in this part of the sector.

This means that a UK Medium Wind Standard (developed under the auspices and governance of RenewableUK) is currently at a draft stage and it is anticipated that it will be published as a final version soon. The Standard has also had input and support from other trade associations, specifically AWEA in the US and CanWEA in Canada, as well as receiving support from a number of US States.

The proposal has also been put to the International Committee with responsibility for wind turbines, known as IEC Technical Committee 88 (TC 88), and it is anticipated that a new version of the dash-1 standard may take account of some of the issues with medium wind.

Evidence shows that as we seek to reduce our overall carbon emissions that there are exciting opportunities ahead for manufacturers in this sector as we are able to utilise some of the best technological developments. The future is sustainable, but to ensure the success of wind turbines projects up to date, internationally agreed standards must be implemented to assure a truly significant future.l
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