Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Riblet4Wind (Riblet-Surfaces for Improvement of Efficiency of Wind Turbines)

Teaser

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. For...

Summary

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. 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 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

Riblet4Wind aims at demonstrating the successful application of the riblet-coating technology on wind turbine rotor blades, investigating the possibilities to coat rotor blades at industrial scale and assessing direct and indirect effects in a semi-quantitative manner.

Work performed

At project start the requirements for the different technological aspects of Riblet4Wind were defined (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 for verification of the numerical algorithm and numerical simulations of rotor blade aerodynamics. Thus, optimum riblet sizes and positions on the blade were identified in order to achieve maximum benefit in terms of power generation.
In parallel, the two technology streams, namely the riblet coating and the cold gas spray technology, were further developed along with their appropriate application technologies. For the painted riblets the coating material is applied to a UV-transparent silicone mould carrying the negative of the required structure. It passes under a pressure roller and is cured by UV radiation. The robot-controlled device thus generates a strip of the cured micro-structured coating. Work covered i) procurement of the master mould with the desired riblet geometry, ii) investigations on the best material selection for the silicone mould; iii) 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 iv) adaptation of the automated riblet applicator and the handling device.
The principle of the cold-gas spray technology is as follows: a compressed and heated gas 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 the particles form a dense and solid adhered coating upon impact. Intensive investigations were performed on 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, the 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 and noise emission measurements will be carried out. Preparations of these demo activities are almost completed and the measurement campaign of the standard rotor blades is about to start.

Final results

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:

Socio-economics:
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 extra weight, 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”.

Environment:
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. Therefore, the envisaged noise reduction by 2 dB achieved by implementing the riblet technology is both an environmentally as well as socially important impact.

Replicability:
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 in terms of 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”.

Policy:
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.

Website & more info

More info: http://www.riblet4wind.eu.