1. output power and turbulence in a tidal estuary
Assessing tidal turbine performance and the relationship between the turbine
output power and turbulence in a tidal estuary
Alexei Sentchev1, Maxime Thiébaut1, François G. Schmitt1
Le Kien Trung1,2
(1) Lab. Oceanography & Geosciences, UMR LOG Univ. Littoral – Univ. Lille1 – CNRS, FR (2) Univ. Sci. Technology of Hanoi, VN
FINDING POWERFUL SOLUTIONS

2. Motivation
Leo

3. In situ measurements in the Sea Scheldt
Motivation
In situ measurements in the Sea Scheldt
Analysis of flow variability and power generated by W2E turbine
Aquascrew
Blue Energy Canada (BEC)
Water2Energy (W2E)

4. Experimental site – Sea Schelde, Temse (BE)
Mean Tidal difference: 5,15 meter
ebb speed/flood speed: max 1,0 – 1,8m/s Depth: ~6 meter at low tide
Middle of river
North Sea
Exp site
Caracteristics (in Antwerp)
Daily: 2x 70 million m³
Mean high tide: 5,15 mTAW Mean low tide: 0 mTAW
Width: 500 m

5. BILAN Survey II - experimental settings Survey IV
ADCP
ADV
Survey III – vel. profiling by towed ADCP

6. Streamwise velocity (m/s)
BILAN
Current profiling, mapping, resource assessment
Velocity profiling by towed ADCP across the Sea Scheldt on flood flow (18/03/2015)
Streamwise velocity (m/s)
Power density (W/m2)
Current speed > 1m/s at the pontoon location (red). Velocity is higher (>1.3m/s) 50 m northward
P  W/m2 on flood flow on both sides of the pontoon
Patchiness in power distribution

7. Space-time variability of the flow
Current ellipses at different depth levels
Streamwise velocity (Nov 2014)
Streamwise velocity (Nov 2014)
Time series of along-stream (upper panel) and cross-stream (lower panel) velocity during the second survey (velocities are 10 min averaged)

8. Mean flow properties Distribution of V: ebb/flood flow, surf/depth
Depth averaged (dashed line) and surface layer (1.5 m thick) velocity (grey line). Red/blue lines – EbbFlood flow mid-time
Distribution of V: ebb/flood flow, surf/depth
Cumulative distribution of V
Ebb flow velocity is 30% higher than flood flow velocity in the surface layer. The choice of tidal period for power production is not justified (for W2E turbine).

9. Assessment of the W2E turbine performance
Power: example of the raw data
Power recording unit
P after spike removal (<5%)
Power curve
Power coefficient Cp
0.32
0.30
Vel. range (m/s)
0.6 – 0.8
1.1 – 1.3

10. Intermittency in a tidal river turbulent flow
Methodology
Obukhov’41 law: Energy in turbulent flow, cascade, dissipation
E(k)=C ε 2/3 k -5/3
With k = 2πf /U, C = 1.5, U mean velocity of the flow
C0 is derived from the best fit of E(f)=C0 f -5/3
Dissipation rate ε = (C0/C)3/2(2π/U) 5/2
Energy containing scale (integral scale) L = σu3/ε ,
Kolmogorov scale (dissipation scale) η = (ν3/ ε)1/4

11. Scaling properties and turbulence intensity
Velocity monitoring by ADCP (1Hz) & ADV (16/32 Hz)
Experimental settings
Intensity Iu
w/o turbine
with turbine
Td stage
flood
ebb
ADV
4.5
5.5
5.0
16.0
ADCP
6.0
7.0
6.5
12.0
Scaling properties of the flow
ε (m²s-3)
L (m)
η (mm)
w/o turbine
2.10-4
1.7
0.3
with turbine
5.10-3
0.8
0.2
Larger is the sampling interval – higher is the intensity (effect of vel evolution)
ADCP provides good estimates for low Iu
ADCP underestimates Iu for high int level
Iu level is very high at 18D !
What could be an instrument of choice for mapping the turbulent intensity ?
Concl/observation

12. Turbulence and power PSD of output Power Spectra for current velocity
Coherency spectrum

13. Conclusion
Field studies are necessary for assessing the flow potential and finding the best location for energy conversion devices
Measurements provide estimates of the mean and turbulent flow properties in real conditions – of primary importance for turbine test
Trials in the Sea Scheldt allowed estimating the tidal turbine performance with high degree of confidence (Cp up to 0.40)
Scaling properties allow a detailed description of turbulence in the tidal flow. They show the effect of turbine on the background turbulence level, provide estimates of production and dissipation, size of turbulent eddies
Extremely high level of turbulence intensity (15%) is found at large distance (18D) downstream the turbine
In the inertial sub-range, the output power fluctuations are found driven by the turbulence in the tidal flow.

14. BILAN