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Structural Design Analysis of a novel Tidal Turbine

Authors:
  • Delft University of Technology faculy Aerospace Engineering

Abstract and Figures

Despite the large resource of tidal and wave energy, the marine energy industry is still lag-ging far behind the wind industry. Much of the knowledge required to develop reliable and prof-itable marine energy systems is available in the wind energy sector. A novel marine energy technology developed by Ecofys Netherlands BV, dubbed the C-Energy project, uses the knowledge of both vertical and horizontal axis wind turbines by combining a Darrieus and a Wells type rotor. The use of these two unidirectional rotor types enables the system to extract energy from both tidal currents and waves. During the summer of 2009 the 30kWp turbine was installed as a demonstration project in the Westerschelde River, The Netherlands. With use of blade element momentum theories for horizontal and vertical axis turbines, a hydrodynamic model has been developed for this turbine to predict its performance and the loads on the turbine blades. Measurements from the C-Energy demonstration project have been used to validate this computational model. A finite element model provides the strain data re-quired for the validation process and is also used to estimate the fatigue life. The combination of the hydrodynamic model and the finite element model gives reliable es-timates for the occurring stresses. With this result, the structural design of the turbine blades can be optimized for any site condition and expected life time.
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1
Structural Design Analysis of a novel Tidal Turbine
Stefan Mieras
1
s.mieras@ecofys.com
Turaj
Ashuri
2
t.ashuri@tudelft.nl
Peter
Scheijgrond
1
p.scheijgrond@ecofys.com
Gerard van Bussel
2
G.J.W.vanBussel@tudelft.nl
Abstract
Despite the large resource of tidal and wave energy, the marine energy industry is still lag-
ging far behind the wind industry. Much of the knowledge required to develop reliable and prof-
itable marine energy systems is available in the wind energy sector. A novel marine energy
technology developed by Ecofys Netherlands BV, dubbed the C-Energy project, uses the
knowledge of both vertical and horizontal axis wind turbines by combining a Darrieus and a
Wells type rotor. The use of these two unidirectional rotor types enables the system to extract
energy from both tidal currents and waves. During the summer of 2009 the 30kWp turbine was
installed as a demonstration project in the Westerschelde River, The Netherlands.
With use of blade element momentum theories for horizontal and vertical axis turbines, a
hydrodynamic model has been developed for this turbine to predict its performance and the
loads on the turbine blades. Measurements from the C-Energy demonstration project have been
used to validate this computational model. A finite element model provides the strain data re-
quired for the validation process and is also used to estimate the fatigue life.
The combination of the hydrodynamic model and the finite element model gives reliable es-
timates for the occurring stresses. With this result, the structural design of the turbine blades
can be optimized for any site condition and expected life time.
Keywords: Marine Energy, Tidal turbine, Wells blade, Darrieus blade, Structural design, Renew-
able energy, C-Energy project
1 Introduction
12
In Europe, the total long-term potential of
tidal and wave energy is equal to the long-
term potential of onshore wind energy [1].
Therefore, there is increasing interest in the
development of tidal and wave energy con-
version systems, but still the marine renew-
able energy industry is lagging far behind the
wind industry. Most of the marine energy
technologies are at the proof-of-concept or
part-system R&D stage [2]. Where for the
wind industry the horizontal axis turbine is by
far the most commonly used system, many
different techniques are still being developed
1
Ecofys Netherlands BV, PO BOX 8408, 3503 RK
Utrecht, T. +31 30 662 34 47
2
Delft University of Technology, Faculty of Aero-
space Engineering, Department of Aerodynamics
& Wind Energy, Kluyverweg 1, 2629 HS Delft, T.
+31 15 278 98 04
for wave and tidal energy conversion. Until
now it is not clear which of these techniques
are most reliable and profitable. There is a
number of advances required to develop
economic and reliable marine energy tech-
nologies by 2020 [3] and many of these ad-
vances can be achieved by working more
closely together with the offshore and wind
industry.
One of only a few grid-connected demo
scale marine energy projects world wide was
installed in 2009, in a free tidal flow (Figure
1). This project, dubbed the C_Energy pro-
ject, uses the principles of both vertical and
horizontal axis wind turbines by combining a
Darrieus and a Wells type rotor. The blades
of the Darrieus rotor extract energy from
horizontal moving tidal currents that, in differ-
ent geometrical set-up, also have the ability
to use energy from moving wave particles.
The Wells rotor blades are functioning as
spokes in a tidal application and can be used
2
Figure 1: C-Energy Demonstration Project
to convert energy from the vertical moving
water particles in waves. The Darrieus rotor
has been used before in several demonstra-
tion projects for tidal energy production [4]
and the Wells rotor has been applied in wave
energy as part of oscillating water column
systems [5, 6]. The C-Energy turbine inte-
grates both rotor types creating a marine
energy converter that is rotating independent
of the direction of the waves and water cur-
rents. The 5 by 5 meters turbine is installed in
open sea conditions for continuous operation.
Earlier scale models up to 2.4m diameter
have proven their performance in a labora-
tory set-up [7]. Based on these laboratory
test results and theories from horizontal and
vertical axis wind turbines, a hydrodynamic
model was developed to calculate the forces
acting on the rotor blades. This model can
now be validated with strain measurements
from the C-Energy project. After validation of
the theoretical models, a load distribution will
be generated to estimate the fatigue life of
the rotor blades. The results of this research
will be used for new turbines to be designed
at lower structural costs, in order to be com-
petitive in the emerging marine energy mar-
ket..
2. Validation of the hydrodynamic model
The C-Energy demonstration project is
equipped with strain gauges located in all
three Darrieus blades, between the two
spokes, and in the Wells blades close to the
rotor axis. The 50Hz strain signal, which was
calibrated before attaching the blades to the
rotor axis, is sent via a wireless connection
from the rotor to a Programmable Logic Con-
troller.
For the validation process, a finite ele-
ment model is developed to calculate the
strain in the rotor blades subjected to the
theoretical loads. The quasi static hydrody-
namic model is based on aerodynamic mod-
els for wind turbines and uses the classical
blade element momentum (BEM) theory, the
double multiple streamtube theory and the
Strickland modification for dynamic stall
[9,10]. The finite element model is used to
model the structural behaviour of the turbine
blades based on the loads that follow from
the hydrodynamic model. The 3D solid finite
element model geometry is simplified and the
mesh is optimized in order to quickly give
strain results at the strain gauge locations in
the Darrieus and Wells blades (Figure 2).
Figure 2: Finite Element Model
The validation method is depicted in
Figure 3, where the values of the water cur-
rent velocity (U), the rotational speed (ω), the
submergence (s) and the strain (ε) are most
important. During the measurements for this
validation, the turbine operates at a constant
tip speed ratio.
3
Figure 3: Validation Method
The strain measurement data (blue line)
and computed (red line) values of the Darri-
eus and Wells blades are shown in Figures 4
and 5.
The Darrieus strain has more or less the
same pattern and magnitude as the theoreti-
cal (ANSYS) results predict. This is a first
indication that the theoretical models used in
this model give reliable estimates of the
stresses that occur in the Darrieus blade. The
Wells strain fluctuates a bit more compared
to the output of the finite element analysis,
even though the order of magnitude seems to
fit as well. The irregular disturbance is most
likely caused by the impact of waves.
Figure 4: Darrieus Strain Validation (ANSYS results versus real data measurements from the C-
Energy installation)
4
Figure 5: Wells Strain Validation (ANSYS results versus real data measurements from the C-Energy
installation)
Figures 6 and 7 show the strain measu-
rement data for in total 15 minutes of operati-
on. The red line indicates the value of the
peaks of the theoretical model; the green
lines are +/- 5% values. Based on these vali-
dation cases, there is high confidence in the
accuracy of the model. Small deviation and
differences from the model predictions may
be caused by local turbulence effects, influ-
ence of surface waves and non-uniformity of
the water current velocity.
The next step in the research is to apply
the model to estimate the fatigue life for diffe-
rent load cases.
Figure 6: Darrieus Strain Validation
5
Figure 7: Wells Strain Validation
3. Fatigue life estimation
The fatigue life of the C-Energy turbine
blades is estimated by forecasting the stress
cycles in certain water conditions. Figure 8
shows the histogram of 10 minute averaged
velocities for 2 lunar periods (~60 days). Like
it is done for wind speed distributions [11],
the water current velocity distribution is ap-
proximated with a Weibull curve:
( )
1
0
k
kU
k U
W e U
λ
λ λ
 
= ⋅
 
 
where W is the number of occurrences for a
given water current velocity U, λ is the scale
parameter or average water current velocity
and k shape parameter, in this case k = 3.
From the Weibull curve, the number of ro-
tor cycles and the corresponding fatigue
damage can be estimated (Figure 9). The
fatigue life is than calculated using the Miner
rule [12].
For the C-Energy demonstration project,
the allowable average water current velocity
for an estimated fatigue life of 25 years is
found to be 1.4 meters per second. Since the
average water current velocity in the Wester-
schelde River is 0.6 m/s, this shows that the
installation will not fail due to fatigue damage.
With this result, the structural design of future
turbines can be further optimized for any load
spectrum.
Figure 8: Water current velocity histogram with Weibull fit.
6
Figure 9: Fatigue Damage Estimation
4. Conclusion
With use of theories from the wind indus-
try a hydrodynamic BEM model has been
developed and validated for a novel tidal
turbine,. The combination of this hydrody-
namic BEM model and a finite element model
of the turbine blades gives reliable estimates
for the occurring stresses. With this result,
the structural design of the blades can be
optimized for any site condition and expected
life time.
The maximum allowable site conditions
for the current C-Energy turbine have been
estimated using the Weibull distribution of the
water current velocity. The major part of the
fatigue damage is caused by high water cur-
rent velocities that occur only a few times a
year.
Based on constraints for the total power
output, design costs and expected life time of
the rotor, one can optimize the rotor geome-
try for each specific site, using the developed
model. The model is currently being used as
a design tool for further upscaling of the tur-
bine.
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Design of offshore wind turbine structures, Appendix K: Calculations by finite element method
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Wind turbine design: with emphasis on Darrieus concept. Presses intl Polytechnique
  • I Paraschivoiu
Paraschivoiu I. Wind turbine design: with emphasis on Darrieus concept. Presses intl Polytechnique, 2002.