This week industry trade body RenewableUK published new research on the subject of wind energy acoustics. The study was produced to explore the issue of Other Amplitude Modulation’ – this is a phenomenon which affects a small minority of wind turbine installations.
The research was carried out in partnership with the University of Salford, the University of Southampton, the National Aerospace Laboratory of the Netherlands, Hoare Lea Acoustics, Robert Davies Associates and DTU Riso in Denmark. The aim of the research was to determine the causes behind Other Amplitude Modulation and to investigate solutions to the issue.
Firstly it should be noted that Other Amplitude Modulation is entirely different to Normal Amplitude Modulation. Normal Amplitude Modulation is the sound commonly heard from wind turbine installations – the ‘swishing’ sound generated as the turbine blades spin through the air. Other Amplitude Modulation is a far more infrequent and uncommon sound which lasts for a few minutes.
The research reveals that Other Amplitude Modulation is caused by sudden and unexpected variations in both wind speed and direction. When this occurs the wind hits different parts of the turbine blade at different speeds causing momentary stalling of the turbine blades and a ‘whooshing noise’.
At this point it should be emphasised that the research has revealed that Other Amplitude Modulation is no louder than the more commonly occurring and accepted Normal Amplitude Modulation. Both cause noises at a level of around 35-40 decibels it is, however, the case that Other Amplitude Modulation occurs at a deeper pitch. The sound produced by these sudden variations of wind speed and direction has been likened to the noise produced by a single-carriage A-road at a distance of 1 kilometer.
Interestingly the research also revealed that Other Amplitude Modulation affects only around 3% of wind turbine installations in the UK. Very much a minority. These findings were based upon a 2007 research paper produced by the University of Salford. This paper found that only 4 of 135 turbine sites (as were installed in the UK as of 2007) were affected by Other Amplitude Modulation. From this the RenewableUK study concluded that 15 of 521 (currently operational) turbine sites would be affected by Other Amplitude Modulation. Other Amplitude Modulation could be then rightly described as affecting a very small minority of the United Kingdom’s wind turbines. However that does not stop it from being an issue that the UK’s wind industry is keen to address.
As such RenewableUK also published the solutions to the issue of Other Amplitude Modulation which were identified by their research. A software solution is sufficient to deal with the issues involved. Software systems already present in wind turbines can be adapted to change the angles of turbine blades at times when Other Amplitude Modulation could occur. This would avoid the problem of turbine blades momentarily stalling entirely.
Additionally RenewableUK has also entered into partnership with the Institute of Acoustics to produce planning conditions and guidance for the issue of Other Amplitude Modulation. This would ensure that occurrences of Other Amplitude Modulation would be minimized. It would be up to developers to measure instances of Other Amplitude Modulation and set a threshold in decibels above which they would be required to act immediately to change blade angles to minimise the noise. This would occur as part of the planning process. Given the already low level of Other Amplitude Modulation occurrences such planning procedures could virtually eliminate the issue entirely.
Speaking at the publication of the research, RenewableUK’s Deputy Chief Executive Maf Smith commented:
“It’s right that the wind industry should take the lead in investigating issues like this when they arise. As a result of the in-depth research we’ve commissioned, we’ve identified the causes of OAM, and, most importantly, the industry has identified a way to deal with it effectively.
“On the limited and infrequent occasions when OAM occurs, we can address it by using software to adjust the way turbines operate, changing the angle of the blades.
“Beyond that, the industry has worked with members of the UK’s leading acoustics institute to develop a planning condition for local authorities to use, which we’re publishing today alongside this work. This states that if OAM occurs, it’s up to the wind industry to resolve it.
“We’re proud to have commissioned this ground-breaking research as it pushes the boundaries of our knowledge of wind turbine acoustics considerably further forward. It’s a tangible example of the wind industry acting in a responsible manner, demonstrating that we’re continuing to be good neighbours to the communities who host wind farms in the UK”.
In other news this week UK company Blade Dynamics announced it’s plans to move forward development of a ‘very long’ and highly efficient turbine blade. This follows the successful completion of the design stage.
The project is receiving support from the UK Government, the industry-supported Energy Technologies Institute (ETI) and the wind turbine manufacturer Siemens. Blade Design will now proceed to the prototype stage of the project and hopes to begin testing the new 80 metre long turbine blade before the end of 2014. The blade will be lighter, longer and more efficient than those currently in use.
News of the commencement of the next stage of the project was greeted enthusiastically. Henrik Stiesdal, chief technology officer at Siemens Wind Power commented; “Subject to successful conclusion of the tests it is clear that the potentials of the split-blade technology of Blade Dynamics would be expected to become even more interesting as we move to even larger turbine platforms.”
Andrew Scott, program manager for offshore wind at the ETI stated:
“The ETI’s vision is to support the development of next-generation blade technology because improved rotor performance is fundamental to achieving the goal of reducing the cost of offshore wind energy.
“We greatly appreciate the ongoing commitment of Siemens to this project with Blade Dynamics. As leaders in this field, this partnership for the development of next-generation rotor technology has the potential to have a huge impact on the cost of offshore wind energy in the future.”
These two announcements indicate the progress which is being made by the UK wind industry. Whether it be the development of new and more efficient technology or the addressing of existing (if extremely infrequently occurring) issues it can be seen that the UK’s wind industry is ending 2013 as it began it; with an eye to the future.