Experimental analysis of the scaled DTU10MW TLP floating wind turbine with different control strategies
Introduction
European Union (EU) made a plan to meet 20% energy consumption from renewable energy source by 2020 in 2006 [1]. A new target of at least 27% of EU’s energy consumption from renewable energy by 2030 has been agreed in 2014. In order to meet the target the share of renewable energy in the electricity would increase from 21 persent to 45 persent by 2030 [2]. The considered renewable sources are wind, solar, wave, tidal, hydro, geothermal, biomass, etc. Among them wind energy has become the most promising renewable energy source. The global wind energy installed capacity reached 591 GW in 2018 and in total 51.3 GW of new wind farm capacity was installed in 2018, where 4.49 GW, which is approximately 20% of growth rate, was covered by offshore wind energy. Especially in Europe in total 409 new offshore wind turbines were connected to the grid in 2018 and has approximately 18.5 GW installed capacity in the offshore sector [3]. The next step of offshore wind energy will be DEEP WATER, i.e. water depths greater than 50 m, meaning that floating wind turbine concepts must be developed. In response to this challenge the first commercial floating wind farm, Hywind Scotland Pilot Park, has been successfully installed and operated since 2017. Here the Hywind type spar buoy floater is applied with a 6 MW SWT-6.0-154 wind turbine. However, the technology is still at early stage to achieve commercial and large-scale deployment. It requires the technological breakthroughs from wind turbines, control system, foundations, mooring and anchors, electrical infrastructures, installations, O&M, design standard, design tools, etc. [4,5].
There are large efforts to reduce risks of technology challenges and to identify innovations through the Floating Wind Joint Industry Project which is a collaborative R&D project. From this project, the knowledge of the key technology challenges and innovation needs for electrical systems, mooring systems, infrastructure and logistics are investigated [6]. Additionally, the second phase of the project, which aims to build knowledge on wind turbine requirements, foundation scaling, installation and maintenance, monitoring and inspection, is on-going. Furthermore, research on developing floating wind turbine system design tools has been conducted and the developed numerical design tools are verified against other numerical results [7,8]. Since floating wind turbine concepts are in the prototype stage there are not enough experimental data to use for code validations. Recently, few experimental studies with small scale floating wind turbine concepts have been performed [[9], [10], [11]]. Only few experimental tests considered matching the aerodynamic thrust and controlling blade pitch on the rotor. From Ref. [9,11] it is clearly seen how different controller settings affect the floating wind turbine responses. DTU also demonstrated small scaled floating wind turbine basin tests with different types of floater designs and blade pitch control [[12], [13], [14], [15]].
This paper extended the one of previous DTU experimental test performed by Hansen and Laugesen [16] by including a newly designed TLP floater and active blade pitch control. The small scale experimental tests are challenging, because of contrasting physical scaling laws for aerodynamic and hydrodynamic loads. In addition, the reproduced environmental conditions will have an impact on the relevant dynamics and loading captured in the laboratory tests. In this paper a 1:60 scaled DTU10MW Reference Wind Turbine model, where the rotor was redesigned to deliver the right Froude scaled thrust at the low Reynolds number [17], is applied with the basic DTU Wind Energy controller [18]. The main purpose of this test campaign is to understand dynamic responses governed by wave, aerodynamics, and control system. Moreover, the test results are able to be used to validate numerical tools for a floating wind turbine system design. Therefore the tests are conducted to establish a data set in a number of wave climates such as regular, irregular, and focused wave groups, both with and without wind. Additionally, emergency shutdown events with extreme wave condition, focused wave, were tested to analyse large surge effects condition driven by controller.
In the following the experimental setup during the test campaign will be described followed by a section on how the DTU Wind Energy controller was implemented. Next a validation of wind and waves, together with test and corrections on the performance of rotor thrust and decay tests are presented. Selected results highlighting the difference in behaviour of the floating wind turbine is shown based on three controller versions. This will be treated in both regular, irregular and focused waves, where the latter includes shutdown cases of the turbine.
Section snippets
Model concept
The TLP concept, developed by Korea Institute of Energy Research (KIER), consists of a floater body with a slender transition piece connecting the tower base to the floater body. Three outgoing spokes are mounted on the floater body, where the taunt mooring lines are attached. The stability of the concept is achieved through the excess buoyancy of the platform, thus the tension in the mooring lines. Fig. 1 shows the model-scaled floating wind turbine.
Experimental setup
The experimental campaigns were carried out at DHI in a large basin with a water depth of 3 m. Fig. 2 shows a top view of the basin, which also illustrates the experimental setup. For consistency the setup was chosen to follow the setup of previous test campaigns ([14,21]). The wave basin consists of 60 individual wave paddles, a, installed on the front side of the basin which enable creation of unidirectional waves, misaligned waves and directionally spread waves. In the opposite end
Control
The implementation of control systems to the experimental model required the development of an interface between the two servo motors on the model turbine and the dynamic-link library (DLL) of the controller itself. The control implementation is a further development from the one utilized in Ref. [14]. The interface consisted of (1) a PC, hosting the user-interface which allowed for real time interaction with the controller, i.e. shutdown and logging data, (2) a National Instruments (NI) MyRIO
Climate validation
In this section we investigate to what extent the environmental conditions produced by the experimental setups such as wind generator, wave maker, wind turbine etc. resemble the target climates.
Effect of wind and wave misalignment
The irregular sea states consist of several wave groups with different heights and lengths. Hence they are best suited to represent real sea states. However, due to the random nature of irregular waves, the comparison of responses in time series becomes impractical. Therefore the exceedance probability P is introduced.
For each signal and for each environmental state, the peaks were identified and stored based on a wave elevation zero down-crossing [24]. The time series of the incident wave
Conclusion
The present work detailed the experimental testing of a Tension Leg Platform (TLP) substructure designed by the Korea Institute of Energy Research (KIER) and the pitch-regulated scaled 1/60 DTU 10 MW reference wind turbine. The investigation included the response in regular waves, irregular waves, focused waves and a low turbulent wind field tested with different turbine controllers.
Before the wave tests, the aerodynamic performance of the fixed model scaled turbine was successfully tuned to
Author contribution statement
1. Freddy J. Madsen: Methodology, Writing original draft preparation, visualization, Investigation, Formal analysis, Data Curation.
2. Thomas R. L. Nielsen: Methodology, Writing original draft preparation, visualization, Investigation, Formal analysis, Data Curation.
3. Taeseong Kim: Supervision, Methodology, Conceptualization, Investigation, Writing, Reviewing and Editing, Project administration, Funding acquisition.
4. Henrik Bredmose: Supervision, Methodology, Investigation, Writing, Reviewing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was conducted under framework of the research and development program of the Korea Institute of Energy Research (B6-2498) and supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (no.20168520021200). Also the authors would like to thank Bjarne Jensen and Jesper Fuchs of DHI Denmark for the assistance provided for the duration of the experiments conducted in the DHI deep-water
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