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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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