the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
19 Aug 2021
19 Aug 2021
Vertical wake deflection for floating wind turbines by differential ballast control
- 1Wind Energy Institute, Technische Universität München, 85748 Garching b. München, Germany
- 2School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece
- 1Wind Energy Institute, Technische Universität München, 85748 Garching b. München, Germany
- 2School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece
Abstract. This paper presents a feasibility analysis of vertical wake steering for floating turbines by differential ballast control. This new concept is based on the idea of pitching the floater with respect to the water surface, thereby achieving a desired tilt of the turbine rotor disk. The pitch attitude is controlled by moving water ballast among the columns of the floater.
This study considers the application of differential ballast control to a conceptual 10 MW wind turbine installed on two platforms, differing in size, weight and geometry. The analysis considers: a) the aerodynamic effects caused by rotor tilt on the power capture of the wake-steering turbine and at various downstream distances in its wake; b) the effects of tilting on fatigue and ultimate loads, limitedly to one of the two turbine-platform layouts; and c) for both configurations, the necessary amount of water movement, the time to achieve a desired attitude and the associated energy expenditure.
Results indicate that – in accordance with previous research – steering the wake towards the sea surface leads to larger power gains than steering it towards the sky. Limitedly to the structural analysis conducted on one of the turbine-platform configurations, it appears that these gains can be obtained with only minor effects on loads, assuming a cautious application of vertical steering only in benign ambient conditions. Additionally, it is found that rotor tilt can be achieved in the order of minutes for the lighter of the two configurations, with reasonable water ballast movements.
Although the analysis is preliminary and limited to the specific cases considered here, results seem to suggest that the concept is not unrealistic, and should be further investigated as a possible means to achieve variable tilt control for vertical wake steering in floating turbines.
Emmanouil M. Nanos et al.
Status: open (until 04 Oct 2021)
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RC1: 'Review of WES paper.', Anonymous Referee #1, 14 Sep 2021
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----------------------------------General Comments:
Overall a very interesting and in-depth paper on the topic vertical wake deflection using the extra capabilities of a floating wind turbine. The necessity for the research is well explained and a clear literature research is conducted. The research is very thorough and provides a good insight in all the aspects related to the vertical wake deflection. I think it is also a nice showcase of the extra possibilities for floating wind farms. The paper as is I would accept, I just have a few comments/questions.
----------------------------------Specific Comments:
- In figure 4 the percent error is shown. Is this a time average error or is the data from a single time shot. It might also be nice to use a different scale, the main percentual range looks between 0-10%. That way the differences might become more apparent.
- For equation 2, I find it difficult to understand the "rate" aspect of this equation. As it is defined it shows how much "more" wind there is at each distance downstream, but I wouldn't say it is a measure of how quickly the wake is recovering, only by how much it has recovered. As it is now the word "rate" throws me of a bit.
- At line 232 it is noted that as the power decreases due to the tilt, the wake intensity also decreases which in it of itself already provides higher windspeeds. Would it be possible to discern how much of the power gain for the second turbine comes from the wake redirection and how much from the fact less power is extracted from the incoming wind?
- A (maybe naive) question I have in general is how far (in terms of angle) is the turbine platform combination from tipping over. Could such a scenario exist with a sudden drop in wind speed/thurst force or an onset of large waves, especially if the variation in wind or waves is faster than the pumping system.
----------------------------------Technical Comments:
I only found 3 small technical points:
- On page 16, line 279 it says: ...rotor to a slightly faser of slower wind speed. I think of should be or?
- On the same page, I get the impression that the labels in figure 13b are mixed up (or the bars). The percentages mention in the text for the cluster power match the bars with the Upstream xlabel and vice versa for cluster.
All in all a very interesting paper.
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RC2: 'Comment on wes-2021-79', Anonymous Referee #2, 20 Sep 2021
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Report on the manuscript submitted to Wind Energy Science
“Vertical wake deflection for floating wind turbines by differential ballast control”
by Nanos, Bottasso, Manolas and RiziotisThe paper investigates the feasibility of tilting floating wind turbines to steer the wake in the vertical direction by pitching the floater with differential ballast control. The work continues and extends the one presented by Nanos et al. at Torque 2020 by considering a 10MW turbine model instead of a 5MW one, and considering two specific semi-submersible platform designs. The paper clearly deserves to be published in Wind Energy Science: it is interesting, well written, it clearly defines the problem at hand, provides a good panorama of previous work and defines specific objectives. The findings are discussed in a convincing way highlighting the original results of the study. There are, however, a few issues that should be considered before publication.
- I am concerned with the ΔP(tilt angle) curve reported in Figure 8a. As mentioned in the manuscript, previous work on wake steering where the rotor hub is tilted or yawed shows curves of the type P=P0 (cos(angle))p which, as such, are very “flat” near the reference position with no effective yaw or tilt (see e.g. Fig. 10 in the Nanos et al. Torque 2020 paper). This is not what is observed in Fig.8a where the slope of the ΔP(tilt angle) curve is large and strongly discontinuous through the zero-tilt. The effect of the vertical displacement of the hub in a sheared mean wind, mentioned in the discussion of Fig. 8a, does not explain the behavior of the curve as it would be associated to a non-zero but continuous slope of the curve through zero-tilt; indeed, in the chosen configuration, the effect of the vertical displacement of the hub is to induce a negative ΔP for a decreased hub height (negative tilt, negative floater pitch) but a positive ΔP for an increased hub height (positive tilt, positive floater pitch) which is not observed in Fig.8a. Furthermore, with this type of curve, the cosinus-power-law fit is highly questionable (as would be probably apparent by comparing the fit to the actual curve in Fig 8a) ans so is the fitted exponent p=3.5.
The authors should clarify this issue which is of primary importance for the subsequent discussion of tilt-induced power change of the cluster. - The study includes a detailed analysis of the loads experienced by the unwaked tilted turbine (that would be the most upwind one in a wind plant). It would be interesting to know also the loads experienced by the downwind fully waked turbine in the case where it is not tilted, but possibly also in a tilted case that would be expected if additional turbines were added in the column. It would be great if the authors could add this analysis to the revised paper but if the can't, they should at least emphasize in the conclusions that additional work is needed to estimate the loads experienced by the waked floating turbine.
- The stability bounds on the tilt angle that can be accessed with the proposed technique should be clearly mentioned/discussed, possibly by showing/discussing at least the hydrostatic restoring moment curve as a function of the tilt (or pitch) angle. Also, the approximate position of the center of gravity should be reported in figures 11 and 12, where the center of flotation is shown.
- I am concerned with the ΔP(tilt angle) curve reported in Figure 8a. As mentioned in the manuscript, previous work on wake steering where the rotor hub is tilted or yawed shows curves of the type P=P0 (cos(angle))p which, as such, are very “flat” near the reference position with no effective yaw or tilt (see e.g. Fig. 10 in the Nanos et al. Torque 2020 paper). This is not what is observed in Fig.8a where the slope of the ΔP(tilt angle) curve is large and strongly discontinuous through the zero-tilt. The effect of the vertical displacement of the hub in a sheared mean wind, mentioned in the discussion of Fig. 8a, does not explain the behavior of the curve as it would be associated to a non-zero but continuous slope of the curve through zero-tilt; indeed, in the chosen configuration, the effect of the vertical displacement of the hub is to induce a negative ΔP for a decreased hub height (negative tilt, negative floater pitch) but a positive ΔP for an increased hub height (positive tilt, positive floater pitch) which is not observed in Fig.8a. Furthermore, with this type of curve, the cosinus-power-law fit is highly questionable (as would be probably apparent by comparing the fit to the actual curve in Fig 8a) ans so is the fitted exponent p=3.5.
Emmanouil M. Nanos et al.
Emmanouil M. Nanos et al.
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