Despite the fast acceleration of wind energy implementation, wind turbine noise is still one of the major hindrances for the widespread use of wind energy in Europe. For modern large turbines, aerodynamic (in particular trailing edge) noise is considered to be the dominant noise source. In a number of European and national projects in the last decade, the understanding of aerodynamic noise from wind turbines has increased significantly, and new noise reduction concepts have been developed. In the recent EU project DATA ('Design and Testing of Acoustically Optimised Airfoils for Wind Turbines') these concepts resulted in a noise reduction of 3-6 dB on a model scale wind turbine in a large wind tunnel. Such a noise reduction offers enormous potentials, not only with respect to noise regulations, but also in terms of e.g. a smaller distance to the nearest building, an increased tip speed, or increased public acceptance. In this way, the implementation of wind energy in Europe can be increased greatly. Furthermore, silent rotors will improve the position of the European Wind Turbine Industry with respect to its competitors. Therefore, the objective of SIROCCO is to obtain a noise reduction of 3-6 dB on full-scale wind turbines, without a reduction in power performance.
      In order to achieve this goal, the project will start with acoustic field measurements to characterise the noise sources on two existing (baseline) wind turbines. A new measurement technique, developed in DATA, will be extended and utilised to localise and quantify noise sources on the rotating blades. Parallel to the field measurements, a combined acoustic/ aerodynamic design methodology that was developed in DATA will be extended to design low-noise airfoils. Airfoils shall be developed, which in contrast to the DATA project exhibit this noise reduction in the polluted (rough) state, the normal one for a wind turbine under real-world conditions. Subsequently the new airfoils will be tested in small scale, two-dimensional acoustic and aerodynamic wind tunnel tests. If the results are satisfactory, low-noise blades will be designed. These desings will be assessed analytically and if the outlook is promising, the blades will be manufactured to validate the acoustic and aerodynamic performance on full-scale wind turbines.

The project will be carried out by a balanced consortium, which brings together the major European experience and knowledge in the required fields. The consortium consists of a major blade and leading wind turbine manufacturer, a wind energy consultancy company, and Europe's leading research institutes in the field of wind turbine acoustics and aerodynamics. The project is aimed at two different (baseline) turbines, for which acoustically optimised blades will be designed, manufactured and validated in field tests. Both baseline turbines are pitch controlled and have a diameter of about 60 m. The activities performed in SIROCCO can roughly be divided in the following phases.       In the first phase, acoustic field measurements are carried out to characterise the noise sources on both baseline turbines. Using the acoustic array measurement technique, it will be verified whether indeed trailing edge noise is the dominant noise source. If other noise sources are present (e.g. gear box, tip noise, holes/slits), it will be attempted to reduce or eliminate these noise sources.      In the second phase, a combined acoustic/aerodynamic design methodology will be utilised to design acoustically optimised airfoils. The new airfoils should be compatible to the inner blades of the baseline turbines, while maintaining the aerodynamic requirements. The optimised airfoils are tested in two-dimensional aerodynamic and acoustic wind tunnel tests and compared to the reference airfoils, for varying conditions.      In the third phase the design of full-scale rotor blades with the new airfoils will be performed. Using analytical tools, it will be assessed how promising the new designs will be.      In the fourth phase the full-scale optimised blades will be manufactured, after which their acoustic and aerodynamic performance with respect to the baseline blades is verified in detailed field measurements for varying conditions. On the basis of the experimental results a final evaluation and assessment of the business potential of the new blades will be performed for both turbines.

 
Figure 1: Potential noise sources on a wind turbine

The problem addressed in the present proposal is wind turbine noise, which is one of the major obstacles for the application of wind energy in Europe. Numerous examples are known of wind energy projects that were prevented by public concern on noise or by the fact that noise regulations could not be met. In other cases wind turbines have to operate below optimum conditions in order to fulfil the noise regulations. The major goal of SIROCCO is a noise reduction on full-scale wind turbines of 3-6 dB, which offers enormous potentials not only with respect to noise regulations, but also in terms of e.g. a smaller distance to the nearest building, an increased tip speed, or increased social acceptance. To achieve a 3-6 dB noise reduction, a number of intermediate results must be realised which form formidable challenges in itself. Some of these objectives are described below.       Wind turbine noise may be caused by a number of potential sources, such as the gearbox, the tips, slits or holes in the blades, or inflow turbulence. On modern large wind turbines, all or most of these noise sources can be eliminated by a proper design of the turbine (although not all mechanisms are completely understood). The remaining, dominant noise source on a modern wind turbine is therefore considered to be trailing edge noise, which is caused by the interaction of turbulence in the boundary layer with the blade trailing edge (see Figures 1 and 2). Trailing edge noise will necessarily be present and represents a lower bound on the noise level of a wind turbine. One of the scientific objectives of SIROCCO is therefore to design airfoils for which the boundary layer is modified such that trailing edge noise is reduced, while the aerodynamic capabilities are maintained, for varying conditions on a full-scale wind turbine. For this purpose a combined acoustic/aerodynamic airfoil design methodology, developed in DATA and validated on a model rotor for controlled conditions, will be improved and extended to the full-scale situation.

 
Figure 2: Mechanism of trailing edge noise

A second challenge lies in the design and manufacturing of the full-scale rotor blades. Since trailing edge noise is mainly generated at the outer part of the blades (where the speeds are highest), the new, low-noise airfoils will only be applied at the outer 20% of the blades, while the inner parts remain unchanged with respect to the baseline blades. This necessitates the definition of a transition region, which should not be detrimental to the aerodynamic performance of the rotor. Furthermore, the optimum chord and twist distribution should be determined as a function of the radius. Besides the aerodynamic and acoustic aspects, aero-elastic, structural, and load issues also need to be considered, requiring a multidisciplinary approach that involves all project partners.
     A third technical/scientific objective is related to the measurement techniques. In order to reduce wind turbine noise, a detailed characterisation of the noise sources is required, both in the wind tunnel and in the field. The acoustic array technique, which has made great progress in aeronautics research over the last years, is perfectly suited for this purpose. However, the application of this technique to moving noise sources is only possible since a few years. In fact, the DATA wind tunnel test in 2000 was the first in which noise sources on rotating blades were located (Figures 3 and 4). However, these experiments were performed in the controlled environment of a wind tunnel. A major challenge in SIROCCO will therefore be to apply the acoustic array technique in the field measurements, and enable for the first time the identification of noise sources on the rotating blades of a full-scale wind turbine. Furthermore, the interpretation of acoustic array measurements in terms of absolute noise levels of different sources is still far from straightforward. This issue will also be addressed within the framework of SIROCCO.

 
Figure 3: Test set up for wind tunnel test in DATA.
The noise sources in the rotor plane are projected onto the picture.

 
Figure 4: Close up of model rotor (diameter 4.5 m).
The noise sources on the rotating blades are projected onto the picture.

Although wind energy is the fastest growing renewable energy source, noise from wind turbines is still a main obsacle for the full exploitation of wind energy. At present many examples are already known of wind energy projects, which were prevented by public concern on noise, or by the fact that noise regulations could not be met. Other examples are known where turbines should operate below optimal conditions in order to fulfill the noise regulations. The noise problem is of particular importance for Europe, since many European areas belong to the most densely populated ones in the world.
      At the end of the year 2000, some 13.000 MW windpower was installed in the EU. The EWEA’s targets for wind power in the EU assume a total of 150.000 MW in the year 2020, of which the majority (100.000 MW) will be installed at on-shore locations. Such enormous expansion of wind power will bring wind turbines, and in particular the noise from wind turbines, even stronger into public awareness, by which the public acceptance of wind energy may slump. 
      The present project aims to reduce the noise emitted from present state-of-the art wind turbines with 3-6 dB. Such noise reduction offers enormous potentials, i.e.

• Improved public acceptance of wind energy projects; 
• Noise regulations can be met more easily; 
• The distance to the nearest building can be decreased by about 50%;
• The tip speed can be increased to more optimal values with a consequent potential to increase the power production.

Hence the project results will help to accelerate the implementation rate of wind energy in Europe and to achieve the European goals for wind energy implementation. In this way the levels of CO2 reductions as set forward for the EU in the respective Kyoto obligation can be met. Furthermore wind energy helps to reduce the environmental pollution from other gases, i.e. SOx, NOx. On a longer time span, it is estimated that in the year 2020 a reduction of more than 134 Million Tonnes/year CO2, 480.000 Tonnes/Year SO2 and 400.000 Tonnes/year NOx is possible. These numbers are based on the conservative estimate of only 100.000 MW installed wind power capacity in the EU in the year 2020. 

The environmental friendliness of wind energy also has important economic effects. In this respect the economic effects of acid deposition upon human health, building materials and commercial forestry can be mentioned. In [1], the damage costs of acidification per tonne for both SO2 and NO are estimated to be 6000 EURO. The economic advantages from the reduction of greenhouse gas emissions are more difficult to estimate but it may for example lead to less protection measures against the rise of the sea level, which obviously has large economic impacts. 

Apart from the environmental and financial impacts, the installation of wind farm power plants improves the energy supply prospects of the EU since the dependency of finite energy sources and non-EU energy suppliers is reduced.

In the present project silent, but efficient wind turbines are developed, using new airfoils. The aerodynamic and acoustic performance is measured through advanced measurement techniques. Such project requires a combination of expertises. The project is performed by a consortium, which is well balanced and complementary and brings together the major European experience and knowledge on the required fields. The consortium consists of a major blade and a leading turbine manufacturer, a wind energy consultancy company, and Europe’s leading research institutes on the field of wind turbine aero-acoustics and wind turbine aerodynamics (with close links to aerospace acoustics). The knowledge and experiences, which are required to perform the present project are spread over different European countries. Therefore such project cannot (and should not) be the aim of only one EU member state but it should be performed on a European level.       

The market for wind energy has shown very large increases recently. European industries, consultancy companies and research institutes have a leading position on this fast increasing market, with the main competition coming from Japan and the US. It is important for the European Union to remain and even strengthen this competitive position. The proposed project plays an important role in this perspective and improves the position of the entire European Wind Energy Community: A lower noise level from European wind turbines increases the implementation rate of wind energy in Europe (the home market). Furthermore, the resulting low noise level of European wind turbines is an important sales argument, which makes them more competitive compared to the turbines from the competition.        

At present, the research institutes involved in the present project are still the mondial leaders in the field of wind turbine acoustics. However, research parties at other parts of the world (i.e. the USA) have initiated similar activities to close the gap with the European research institutes on the field of wind turbine aero-acoustics. In summary, the present project is meant to contribute significantly to the objectives of the 5th Framework Programme, specifically to item 5.3.3 (Improving the acceptability of renewables). The project aims to reduce the noise level from wind turbines with 3-6 dB at similar power production, which fully complies with the targets of item 5.3.3, where problems related to the acceptability of renewables, i.e. noise, are asked to be solved. In a similar way it contributes to point 5.2.2 (Wind Energy Optimisation). Targets of this point are to overcome bottlenecks, which hinder the full exploitation of onshore wind energy and to reduce the environmental impact. The project results obviously fulfil these targets, since noise is one of the main bottlenecks for wind energy implementation.      

Alternatively it may be argued that silent rotor blades have the potential to reduce the cost/kWh of wind power. In order to meet the noise regulations, present wind turbines often operate at rotor speeds, which are below optimal. Hence silent rotor blades make it possible to operate at optimum rotor speeds. Hence power production is increased, at constant costs.

Wind turbine noise may have several causes, which can be divided in mechanical and aerodynamic noise sources. Mechanical noise, originating from e.g. the generator or the gearbox, can be reduced efficiently by well-known engineering methods. As a result, aerodynamic noise is dominant for modern large turbines. A number of different mechanisms can contribute to aerodynamic noise. Some of these represent tonal sources, which can be avoided relatively easy (e.g. blunt-trailing-edge noise or noise from holes/slits). The remaining, broadband aerodynamic sources that determine wind turbine noise are tip noise, inflow-turbulence noise (due to atmospheric turbulence), and trailing edge noise. Although the mechanisms are still not fully understood, the problem of tip noise appears to be solved by the application of 'silent' tip shapes. As a consequence, attempts to reduce the noise from modern large wind turbines should focus on inflow turbulence and trailing edge noise, the latter being the more important of the two.
      In the 1980's much effort was spent to understand aerodynamic noise from wind turbines, which has led to the development of various noise prediction models. However, these models did not take into account the exact airfoil shape and required empirical, turbine dependent input. Therefore these models were not suitable for low-noise airfoil design. In the 1990's, aerodynamic wind turbine noise again received considerable attention in a number of European and national research projects (e.g. STENO, DRAW). For trailing edge and inflow turbulence noise prediction models were developed that did consider the actual airfoil shape. Noise reduction concepts were investigated, which yielded important design guidelines for low noise airfoils. Reductions of 3-6 dB were obtained in wind tunnel tests on two dimensional airfoils, for both inflow turbulence and trailing edge noise, by application of an optimised airfoil shape and trailing-edge serrations respectively. In the latest European project on wind turbine noise, DATA, it was investigated whether trailing edge noise can also be reduced by an optimised airfoil shape, as an alternative (or addition to) trailing edge serrations, which have some practical complications. Furthermore, for the first time all noise-reducing concepts were tested in case of blade rotation and unsteady flow, on a model scale rotor in a large wind tunnel. For the purpose of this test, a new acoustic array technique was developed which enabled the location of noise sources on the rotating blades. The test showed trailing edge noise reductions up to 6 dB for various flow conditions, with no significant loss in aerodynamic performance.
      The main innovative aspect of SIROCCO is a generally validated design methodology for full-scale 'rough' rotor blades, which integrates all noise reducing concepts, as well as aerodynamic and structural issues, in order to obtain a noise reduction of 3-6 dB with respect to the present state of the art. The design methodology is based on aero-acoustic and aero-elastic design tools, supported by wind tunnel measurements. To achieve this goal, a number of intermediate results are required which will also advance the state of the art. For example, the application of the acoustic array technique to locate and quantify rotating and stationary noise sources on a full-scale wind turbine has never been performed before. Also, the combined acoustic/aerodynamic airfoil design methodology developed in DATA, which represents the current state of the art, will be extended to design low-noise airfoils for the full-scale blades, while maintaining the aerodynamic requirements for various flow conditions.
       The project is organised in a number of phases (see section 2.1), all of which represent a certain technical risk. To minimise these risks, the results of each phase will be quantified through detailed, high-quality experiments, or by thorough theoretical analyses and the progress will be assessed in milestones between the phases. On the basis of this assessment a go/ no go decision will be taken for the rest of the project, in consultation with all partners. In the following, the main technical risks in the project are described. In the first phase, the noise sources on the two baseline turbines will be characterised using advanced measurement techniques, and it will be assessed whether trailing edge noise is indeed the dominant source. Although it is expected that trailing edge noise from the outer blades is dominant, it may turn out that other noise sources are present, which are difficult or impossible to eliminate using the low noise blade design methodology foreseen. In the second phase, low noise airfoils will be designed and tested in two dimensional acoustic and aerodynamic wind tunnel tests. Depending on the properties of the airfoils currently used on the baseline turbines, it may be difficult to find airfoils that show a significant noise reduction with respect to the reference airfoils (say at least 3 dB) while having similar aerodynamic capabilities. In the third phase, the airfoils designed and validated in two-dimensional tests will be used to design full scale rotor blades. It may be that the acoustic airfoils are not optimal for the present wind turbines and that it is necessary to find compromises between acoustic, aerodynamic, and structural requirements, which may endanger the overall performance of the blades. This will be assessed by means of advanced analytical tools. In the fourth phase, full-scale, low-noise blades will be designed and manufactured, and all noise reducing concepts will be tested on full-scale wind turbines for the first time. In the full-scale validation tests, a number of circumstances will be different from the two dimensional wind tunnel tests, such as (obviously) scale, Reynolds number, rotation, inflow conditions, and the blade planform and twist distribution. All these factors may lead to unexpected effects that reduce the acoustic or aerodynamic performance of the blades.

In summary, SIROCCO will advance the state of the art significantly in a number of disciplines, which necessarily involves a number of technical risks. These technical risks are minimised through the use of high quality test facilities and advanced measurement and analysis techniques. By the definition of milestones between the phases, the financial risks are kept to a minimum as well.