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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Sören Wellenberg RWTH Aachen University, Germany
Co-authors:
Sören Wellenberg (1) F P Markus Marnett (1) Wolfgang Schröder (1)
(1) RWTH Aachen University, Aachen, Germany

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Abstract

Construction of a full-scale vertical axis research wind turbine

Introduction

Unlike regionally quite saturated markets for onshore wind turbines, offshore applications possess a high potential for market growth. Recent research points to several promising attributes of vertical axis wind turbines (VAWT) for this specific market. Although scientific discussion is very diverse, potential advantages of VAWT compared to horizontal axis wind turbines (HAWT) should be fundamentally analysed. Therefore, RWTH Aachen University conducted numerical and experimental studies to establish substantiated knowledge for VAWT. To investigate advantages and disadvantages, a scientific prototype of critical size was designed using unique development tools. The operation has been completed.

Approach

The Institute of Aerodynamics at RWTH Aachen University (AIA) conducted numerous studies to optimize the performance of VAWT. Such fundamental studies are necessary since due to the lack of a new full-scale prototype VAWT advanced findings of HAWT are often simply transferred to studies of VAWT. New codes and designs are developed even for multi-megawatt-offshore turbines without referring to reliable data. It goes without saying that due to the completely different flow physics such a direct knowledge transfer is questionable. The discussion in the literature on this straightforward knowledge transfer shows the inadequacy of this approach.

Although the necessity of a VAWT prototype is without any question its suitability has to be shown. Therefore, a first presentation of its structural parameters is given, which indicates that the prototype overcomes the main problems of the past and fulfils the essential requirements. The choice of an H-rotor concept represents a much more general reference case than other concepts for VAWTs. Which is why, some information will be given on its design procedure. In combination with details on the prototype’s location and the site specifications the expected operational parameters are presented covering the overall performance and the operational envelope, which have been derived by previous studies.

The measurement system will be discussed. The sensors considering the aerodynamic, structural, and electrical subsystem will be described. Advantages and disadvantages of the present equipment are discussed and requirements for additional equipment determined. First measurements have been made, compared to design parameters and the results will be reported on. Finally, an overview of already running short-term, mid-term and long-term future work will be presented.


Main body of abstract

Quite an effort has been made in the past to establish a technical level of VAWT, which makes it comparable to that of HAWT. Despite a lot of problems especially with the Darrieus-concept (see Sutherland [5]) a qualitative discussion still promotes the VAWT’s suitability for other fields like urban and offshore application. These arguments are often based on some kind of generic or model-like investigations. The findings of those studies are often fundamental. However, they lack important influences of the whole system. In other words, little or no reliable data exist that can be used to start a prospective optimization process.

Experimental setups suffer from the model scale which makes transferability of flow and load measurements questionable (see Danao [1]). Furthermore, models mean small Reynolds numbers resulting in subcritical flow around the airfoil. This is of high influence on the dynamic behavior of the air flow and on the blade loads when stall occurs at high angles of attack. High rotational speeds and dominating centrifugal forces restrict data acquisition of loads. Hence, the performance evaluation is generally limited in most cases to simple power measurements.

Despite the lack of high quality experimental data numerical analyses address the problem of aeroelasticity and unsteady flow in the VAWT context (see McIntosh [3], Scheurich [4]). To support such studies and to fill the gap of highly resolved data for VAWT under open field conditions, the AIA constructed a 20 kW VAWT prototype shown in figure 1.

Figure 1: VAWT prototype

As mentioned above there are various VAWTs already in operation which provide only little information for scientific research due to limited time availability, restrictions for necessary modifications or detailed information on their components.

In spite of the additional effort for the design and construction of the support structure of the blades the H-rotor concept of the VAWT represents a well-defined configuration, which avoids blade twist and can be regarded as a general reference setup. The VAWT prototype’s parameters are listed in table 1. Its unique size overcomes the initially explained problems .The operational Reynolds number is high enough for turbulent flow conditions and the aspect ratio is also high enough so that the aerodynamic performance does not suffer from significant losses due to spanwise induced velocities (see Marnett [2]).


Table 1: VAWT prototype configuration data and operational parameters

The design of the prototype is based on steady flow simulation, which have been validated by additional experiments (See Marnett [2]). These calculations were used as input data to determine an envelope in a highly iterative procedure. The electrical system has been chosen so that the turbine can run at variable speed at high performance even under low-speed conditions. It is connected to the low-voltage power grid.

To provide useful and reliable data a bus-system has been chosen with CAN open standard which is linked to all sensor devices listed in table 2. The use of a commutator ring enables data acquisition in the rotational system. In addition, a tilt-up tower will be installed and connected via Modbus to the wind turbine’s microcontroller. For its installation first wind measurements have been evaluated. All data will be available in real-time to satisfy the requirements of a closed-loop control system.


Table 2: Sensor equipment of VAWT prototype and tilt-up tower

It goes without saying that the equipment meets all the requirements. Wind speed and power performance can be measured in compliance with technical regulations. Therefore, on a short-term horizon reliable data will be provided.

Power density spectra of tower acceleration measurements have already confirmed the approximated first tower eigenfrequency. To avoid fatigue mechanical measurements will be extended to blade and strut accelerations. Future work will make use of the active individual blade pitch control by hydraulic actuators, which is indicated in figure 2.


Figure 2: VAWT prototype configuration data and operational parameters


Conclusion

Development of the VAWT often suffers from its high complexity caused by strong interactions between aerodynamics, structure dynamics, and control under stochastic wind speed conditions. Although research has given new insight by generic analyses, problems exist when it comes to suitable and reliable data for the particular case studies which take the aforementioned interaction into account. Providing long-term, high quality and reliable data of open-field operations, the construction of a new variable-speed variable-pitch H-rotor VAWT will help to overcome this drawback. Its critical size and structural parameters are such that it ensures high transferability for future studies. The permanent availability facilitates modifications and enables investigations in a wide parameter rage. New concepts in power transmission, blade and material technologies, data acquisition, and control design can be tested without restrictions.

The environmental conditions needed for future evaluations have been defined by results of site assessment and by first wind speed measurements. In connection with the presentation of the main operational parameters an overview of the different operational conditions is given. These parameters will be complemented by new information derived by the sensor equipment, which can be easily expanded. The expandability is not only granted in the non-rotational system but also in the rotational system offering new opportunities. Besides, power performance and extensive wind-speed measurements load and acceleration measurements will be conducted. Apart from its use for future studies acquired data will be provided to conditional monitoring, since its availability for research states the most important aspect for the construction of this VAWT prototype.



Learning objectives
The main objectives to construct the VAWT prototype are to provide measurement data to validate numerical and model-based findings, to gain operational experience, and to the knowledge on advanced control strategies.


References
[1] Danao, Louis Angelo M.: The influence of unsteady wind on the performance and
aerodynamics of vertical axis wind turbines, University of Sheffield, PhD thesis, 2012

[2] Marnett, Markus: Multiobjective numerical design of vertical axis wind turbine
components, RWTH Aachen, PhD thesis, 2012

[3] McIntosh, SC ; Babinsky, Holger ; Bertényi, Tamás: Optimizing the energy output
of vertical axis wind turbines for fluctuating wind conditions AIAA Paper 2007-
1368. – 45th AIAA Aerospace Sciences Meeting and Exhibit, 8 – 11 January 2007, Reno, Nevada

[4] Scheurich, Frank ; Brown, Richard E.: Modelling the aerodynamics of vertical-axis
wind turbines in unsteady wind conditions. In: Wind Energy 16 (2013), pp. 91–107

[5] Sutherland, Herbert J. ; Berg, Dale E. ; Ashwill, Thomas D.: A retrospective
of VAWT technology / Sandia National Laboratories. 2012. – Tech. Rep.