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AdVanced Aerodynamic Tools for lArge Rotors

Final Report Summary - AVATAR (AdVanced Aerodynamic Tools for lArge Rotors)

Executive Summary:
This report describes the final results of the EU FP7 project AVATAR (AdVanced Aerodynamic Tools of lArge Rotors). AVATAR was initiated by EERA (European Energy Research Alliance). The project started on November 1st 2013 and lasted until December 31st 2017.
AVATAR was carried out in a consortium with 11 research institutes and two industry partners.
• Energy Research Centre of the Netherlands, ECN (Netherlands, coordinator)
• Delft University of Technology, TU Delft (Netherlands)
• Technical University of Denmark, DTU (Denmark)
• Fraunhofer IWES (Germany)
• University of Oldenburg, ForWind (Germany)
• University of Stuttgart (Germany)
• National Renewable Energy Centre, CENER (Spain)
• University of Glasgow (UK) (until September 1st 2015: University of Liverpool)
• Centre for Renewable Energy Sources and Saving, CRES (Greece)
• National Technical University of Athens, NTUA (Greece)
• Politecnico di Milano, Polimi (Italy)
• General Electric, GE (Germany)
• LM Wind Power (Denmark)

The focus of AVATAR was on aerodynamics of large wind turbines, i.e. larger than 10 MW, indicated as 10MW+. The specific aim was to improve and validate aerodynamic models, and to ensure applicability of these models for such 10MW+ turbines with and without flow devices and with and without aero-elastic implications.
A wide variety of aerodynamic models was considered, ranging from low complexity / computationally efficient models (i.e. Blade Element Momentum - BEM) to high fidelity / computationally demanding models (e.g. Computational Fluid Dynamics - CFD), with intermediate models (e.g. free vortex wake models-FVW) also included. This enabled an improvement of the fast, low complexity tools via calibration by results from high fidelity models. The model assessment was carried out on two 10 MW reference wind turbines (RWT’s), one originating from the INNWIND.EU project, and another one designed in AVATAR. The improvement and validation of models was also based on suitable experimental data, mainly wind tunnel measurements and a selected number of field measurements.

AVATAR resulted in a long list of model improvements and lessons learned on the use of models. Also several recommendations have been defined, one of the most important is that more databases of validation material are needed, preferably experimental but also databases of results from high/intermediate confidence codes which can serve as validation material for low fidelity codes.

Project Context and Objectives:
The AVATAR project was realized in response to the European Commission’s FP7 Topic: ENERGY.2013.2.3.1 Advanced aerodynamic modelling, Design and testing for large rotor blades

The main goal of this call was:

To develop advanced rotor design models, using integral design tools in order to enable new and optimised designs for the next generation of large scale wind turbines (up to 20MW).
– Definition on large-scale rotor blades and aero-tools for turbines to be developed and tested. The project should focus on turbines in the 8 to 12 MW range but may as well pave the way for larger turbines up to 20 MW;
– Development of advanced aerodynamic modelling for selected
elements, including flow devices for distributed aerodynamic control;
– Design and demonstration of new large-scale rotor blades and
aero-tools.”
The AVATAR consortium, with 13 partners from across Europe, successfully applied to this call description and obtained funding from November 1st 2013 to October 31st 2017 (later extended to December 31st 2017), i.e. 38 months duration. The budget was 9.2 million euro, of which 6.68 million euro was contributed by the EC.
3. Main objectives of AVATAR
AVATAR focused on the aerodynamic and aero-elastic modelling of large wind turbines with a rated power of 10 MW or more (denoted as 10MW+ turbines). The application for such large-scale turbines is mainly thought to be off-shore. This is due to the fact that the costs for installation, support structure and grid connection are much more significant for off-shore application. This makes the cost share of the turbine hardware as percentage of the total investment roughly half the value that applies to an on-shore turbine, which then pleads for large turbines. Moreover, increasing the ratio between rotor diameter and installed generator power, i.e. a lower specific power, corresponds to a higher capacity factor leading to more operating hours in full power. This reduces the variability in wind power and allows more effective use of the power transport cables, which is a major advantage for utilities. This again pleads for very large wind turbines. Besides these there are other Cost of Energy () drivers to grow the rotor area like the plain economies of scale and increasing energy capture per foundation.
So although 10MW+ turbines are a way to reduce the overall CoE for off-shore wind energy, the design of the resulting very large rotor blades falls outside the validated range of current state-of-the-art aerodynamic and aero-elastic tools in various aspects: Very large blades operating at high tip speeds mean high Reynolds and Mach numbers for which the effects are uncertain and not enough validated; thick(er) airfoils need to be assessed in terms of aerodynamic performance; increased flexibility will lead to larger deflections and more pronounced non-linear aero-elastic behavior with unknown aerodynamic implications, etc. Further complications enter by the desired implementation of active and/or passive flow devices.
The aim of AVATAR was then ‘to deliver aerodynamic and aero-elastic models and tools for a more validated and higher fidelity design modelling of 10MW+ scale wind turbines’.

Project Results:
In line with the target from section 3, the AVATAR project validated, improved and calibrated aerodynamic models for 10MW+ turbines also including an assessment of the aero-elastic consequences. Thereto a wide variety of aerodynamic models was considered, ranging from low complexity / computationally efficient models (e.g. BEM) to high fidelity / computational demanding models (e.g. CFD), with intermediate models (e.g. free vortex wake models, FVW) in between. In this regard, it is important to realize the crucial role of calculation time for wind energy design calculations [44] by which, even in modern times, it is still imperative to use engineering aerodynamic models based on the Blade Element Momentum Theory (BEM theory)

The improvement and validation of aerodynamic models was partly based on suitable experimental data but as 10MW+ turbines do not exist, experimental data are gained from a range of sub-model tests or tests at a smaller scale. Amongst others 2D airfoil measurements at high Reynolds numbers (up to 15 Million) were taken in the pressurized DNW HDG wind tunnel in Göttingen. Moreover, LM has provided airfoil measurements taken in their tunnel and ForWind provided wind tunnel data under controlled turbulent conditions. Also several wind tunnel measurements on flow devices are offered, either in-kind or from the project framework.

In addition to the experimental validation, AVATAR benefitted from the wide variety of computational tools which are available within the project. This enabled a calibration from low complexity / fast tools with results from the high fidelity models.
The model assessment was carried out on two 10 MW reference turbines, one from the INNWIND.EU project and one designed in AVATAR with or without aero-elastic coupling. The latter used the INNWIND.EU reference turbine as a basis, but it was intended to be more challenging in terms of aerodynamic modelling, i.e. aspects like airfoil thicknesses, Reynolds and Mach numbers etc. are pushed toward the limit of what is still feasible to expect in future commercial applications.

The AVATAR project was organized in different Work Packages (WP’s). Work Package 1 was an integrating Work Package, in which the reference turbines were designed and evaluated. (WP2) dealt with the advanced aerodynamic modelling of all aspects, which were expected to play a role in the design of large 10MW+ wind turbine blades. The modelling of flow devices was included in a separate Work Package (WP3). Also, the modelling of aero-elastic effects on large and flexible rotor blades was carried out in a separate Work Package (WP4).
In the next section the S&T results are reported along these four work packages.

Potential Impact:
The most important impact of the AVATAR results is that they help to accelerate the implementation rate of wind energy in Europe by which EU targets for climate and renewable energy can be met. These targets are set for the relatively near future (in 2030 Europe's greenhouse emissions should be 40% below 1990 levels and in 2040 they should be 60% below 1990 levels) and for the long term (in 2050 the greenhouse gas emissions should be reduced with 80–95% relative to 1990 levels). The Energy Roadmap 2050 [45] defines the transition of the energy system to support these greenhouse gas reductions targets. In this Roadmap renewables are placed in the centre of the future energy mix in Europe with large scale implementation of wind energy being a very significant contributor in all scenarios. As an example: [47] explains that Europe has the potential to realise 3500 TWh of offshore wind energy in its waters by 2030, which corresponds to 78% of the projected electricity demand in Europe by that time, with little emission of greenhouse gases. Wind turbines offset all construction emissions already within 6 months; afterwards they perform virtually carbon free for their remaining 20-year lifetime [46]. Additional advantages of large scale wind energy lay in the reduction of other environmental pollutions, in economic benefits due to employment at a strong European industry, less protection measures and avoidance of imported fuel costs. Other advantages of wind energy lie in the fact that it is a safe energy source; it reduces dependency on finite energy sources and non-EU energy suppliers.
One of the most important requirements to make these scenarios and their positive impact on the society reality is that the costs for wind energy are reduced further. Since the cost of energy is driven by loads and performance predicted by the design tools considered in AVATAR, the uncertainties in these tools are crucial for the design of cost-effective and reliable wind turbines: The uncertainties cause that turbines behave unexpectedly, experiencing instabilities, or higher loads than expected leading to the risk of failure which can only be overcome with high (costly) safety margins. Alternatively, the loads may be lower than expected which implies an over-dimensioned (and costly) design. Hence, the knowledge on model uncertainty and the reduction in model uncertainty from section 5 translates straightforwardly into more cost-effective wind turbines. This benefit is, strictly speaking, valid for both on-shore and off-shore turbines, including small-scale wind power. It is however most relevant for large scale wind turbines applied off-shore with which the costs of off-shore wind energy can be reduced significantly.The drive for such large turbines is illustrated by the recent contract won by EnBW, which offers subsidy-free wind energy for a project in the year 2024. In its offer, EnBW assumes that 12 MW turbines or larger have entered the market by that time to make this project subsidy-free. Similar assumptions are made by Orsted in their offer for another subsidy-free wind energy project. Thereto it should be realised that for offshore applications, the rotor cost as percentage of the total cost of energy (CoE) is much less compared to onshore applications. Since the rotor remains the only energy-producing component it is possible to increase the energy production by increasing the rotor diameter at a favourable overall cost balance. Moreover, increasing the rotor diameter for a constant rated power improves the capacity factor leading to less variability in wind power, which is a major advantage for utilities.
So although the use of 10MW+ turbines constitutes an important instrument to reduce the overall costs of off-shore wind energy the design of blades for such turbines largely fell outside the validated range of application of wind turbine design codes. These blades are expected to be much longer and slenderer with thicker airfoil profils and they might run at higher tips speeds than conventional blades. Moreover, active and passive flow and load control devices in combination with aero-elastic tailoring offer interesting design options to reduce load levels, to enhance power production and so decrease the cost of energy. All these aspects were up to now insufficiently validated and it is on this field where AVATAR achieved major steps: the model improvements and the lessons learned with respect to the model performance for these large wind turbines reduces the uncertainties in the response calculations. As such aerodynamic and aero-elastic models and tools of a more validated character and of higher fidelity are provided for the 10MW+ wind turbine design space with and without flow devices and with and without aero-elastic tailoring. This makes AVATAR an important and vital stepping stone for the high-fidelity design of 10MW+ turbines, which eventually reduces the cost of off-shore wind energy.
However, also the data sharing and knowledge exchange amongst scientific and industrial partners is crucial to make this impact come true. The open access publication in the project allows all interested parties to absorb the results easily. An important example in this respect is a public blind test on measurements at high Reynolds numbers (up to 15 Million) in which multiple parties (also outside the project) provided calculation results without knowledge of the measurements after which results were compared with these measurements. Moreover, the deliverables from the technical work packages are all stored on the public AVATAR web site. This includes the designs of the reference wind turbines as carried out in AVATAR and which can be used as testbed to validate newly developed tools. Almost all wind tunnel measurements are uploaded to a web-based validation platform, i.e. the WindBench platform http://windbench.net/avatar which is also used by other EU projects, like the IRPWIND project.

List of Websites:
http://www.eera-avatar.eu/
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