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Riblet4Wind Report Summary

Project ID: 657652

Periodic Reporting for period 1 - Riblet4Wind (Riblet-Surfaces for Improvement of Efficiency of Wind Turbines)

Reporting period: 2015-06-01 to 2016-07-31

Summary of the context and overall objectives of the project

Energy efficiency remains a major challenge for many industrial sectors. Micro-structured surfaces (so-called riblet surfaces or ‘shark-skin’) reduce the frictional resistance of flat surfaces by up to 10%. Applied on airfoils they also improve the drag to lift ratio. The riblet technology has already demonstrated its capacity for increasing the energy efficiency of aircraft. For wind energy applications the direct aerodynamic effect will allow gaining the same amount of electrical energy with smaller rotor blades. Indirect effects will increase the benefit to approximately more than 8%:
• Operation at lower wind speeds and improved balance in the electrical grid system
• Improved stall and turbulence behaviour of the rotor blades thus allowing also operation at higher wind speeds and/or operation in less optimum conditions
• Novel design options due to changes of the mechanical loads
• Substantial reduction of noise emission
• Applicable for OEM and retrofit
In total Riblet4Wind aims at demonstrating the successful application of the riblet-coating technology on wind turbine rotor blades, investigating the possibilities to coat rotor blades on a series production-like basis and assessing direct and indirect effects in a semi-quantitative manner.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The initial work during the first 14 project month work focused on defining the requirements for the different technological aspects of Riblet4Wind (i.e. performance of materials and processes, production, riblet tolerances, maintenance, and means of compliance).
The work on rotor blade aerodynamics comprised two major aspects: extensive wind tunnel experiments of airfoils with and without riblets of different sizes for verification of the numerical algorithm used in the project on the one hand and numerical simulations of rotor blade aerodynamics on the other hand, with the aim of identifying optimum riblet sizes and positions on the blade in order to achieve maximum benefit in terms of power generation. The models used show very good agreement with the experimental results from the wind tunnel tests.
In parallel, the two technology streams, namely the riblet coating technology and the cold gas spray technology, were further developed along with their appropriate application technologies. The painted riblets technology is based on a coating process with a roller applicator: the coating material is applied to a UV-transparent silicone mould carrying the negative of the required structure. It passes under the pressure roller and is cured by the UV radiation produced by the UV curing unit. The robot-controlled device generates a strip of the cured micro-structured coating. Work during the project period covered i) investigations on the best material selection for the silicone mould; ii) optimization of the coating material itself in order to comply with the mould material for excellent reproduction of the riblet structure, with the automated application process (i.e. adaptation of viscosity and curing speed) as well as with the general requirements for rotor blade coatings; and iii) adaptation of the automated riblet applicator in order to meet the requirements for rotor blade coating.
The principle of the cold-gas spray technology is as follows: a compressed and heated gas, generally nitrogen, helium or their mixtures, is accelerated to supersonic speed into a nozzle. The feedstock material is injected into the gas jet in powder form in front of the nozzle, preheated and then propelled onto the substrate. Above a certain particle velocity, which is characteristic of the respective coating material, the particles form a dense and solid adhered coating upon impact. The performed work in the past project period involved intensive investigations of the best cold spraying parameters for reproducibly generating high-quality riblets of the required dimensions. Several techniques were tested and it was shown that micro-sized riblet structures could be generated on different substrate materials.
One important objective of Riblet4Wind is the full-scale demonstration of the developed technologies. To this aim, all three rotor blades of our demo wind turbine will be equipped with the riblet coating and tested in field in comparison to its twin turbine without our prototype coating. Power generation measurements will be carried out in order to demonstrate the efficiency increase that can be achieved by the riblet technology. Furthermore, noise emission shall be monitored since it represents another essential aspect governing wind turbine operation. These demo activities are already being prepared involving necessary maintenance and optimization measures of the demonstration turbines.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project results will bring about a completely new technology to the wind energy sector, increasing the efficiency of wind turbines and reducing their noise emissions. The socio-economic impact and the wider societal implications are outlined below:

The successful project will impact socio-economics in two major aspects: i) reduced noise emissions and ii) increased energy efficiency of the wind plants. Numerical simulations, carried out in advance to the project, indicated a significant reduction of noise emission. In wind channel experiments a considerably lower noise emission was observed corroborating the numerical simulations. In areas that are densely populated, noise emission is one of the central concerns that negatively influence public acceptance of wind farms also leading to the fact that wind plants often have to be switched off at night. If this could be avoided, the project contributes to the goal “Making variable renewable electricity more predictable and grid friendly” as well as to the goal “Improving EU energy security”.
“Bringing cost of renewable energy down by increasing technology performance” will be realized by enabling to produce a new generation of rotor blades with higher energy efficiency without more weight or material consumption, leading to a new generation of high-performance rotor blades. Since the riblet technology is protected in Europe, the implementation of the technology would increase the competitiveness of the European wind industry and thus contribute to “Strengthening the European industrial technology base, thereby creating growth and jobs in Europe”.

The most important impact of the project lies in the increase of annual power generation of wind plants by at least 6%. This performance increase will be achieved only by application of a functional coating, without the need to increase weight or to reinforce the structure of the blade. Additionally the blade sizes remain unchanged, which means that regarding transport or handling no additional effort is necessary. Thus, green energy production will be increased on a completely material- and emission-neutral basis.
Noise pollution is an important issue particularly in densely populated regions of Europe and is. Therefore, the envisaged noise reduction by 2 dB achieved by implementing the riblet technology is both of environmental as well as socially important impact.

The target TRL of 7 of the Riblet4Wind technologies ensures that replicable results are achieved. Development of automated manufacturing technologies and application devices will enable stable and repeatable processes.

Market Transformation:
A completely new business opportunity for blade maintenance companies (mainly SMEs) will be opened up by the possible retrofit of existing plants with the new coating technology. The size of this possible market is huge. In this sense the project gives a significant contribution to the goal “Nurturing the development of the industrial capacity to produce components and systems and opening of new opportunities”.

On the whole, the achievements of Riblet4Wind significantly help to increase attractiveness of wind power on multiple levels. The various individual impacts outlined above contribute to the objective of “Solving the global climate and energy challenges” thus paving the way for policy makers to moving forward along the transition of primarily fossil- and nuclear-based to entirely renewable ways of energy generation.

Related information

Record Number: 196221 / Last updated on: 2017-03-28
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