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Title:
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Turbine Interaction in Large Offshore Wind Farms; Wind Tunnel Measurements
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Author(s):
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Published by:
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Publication date:
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ECN
Wind Energy
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1-8-2004
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ECN report number:
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Document type:
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ECN-C--04-048
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ECN publication
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Number of pages:
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Full text:
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140
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Download PDF
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Abstract:
Background and Aims
Large wind
farms are to be expected in the coming decades. We are talking about
farms of for example 20 times 20 turbines or larger. Of course it is
of utmost importance to be able to predict the production of such farms
accurately. However, little is known on cumulative effects of wind turbine
wakes. Therefore this project studies the wind turbine interference
and especially wake losses for larger offshore farms.
Model
The decrease
of the wind speed in the wake of wind turbines can be compared to the
frictional drag exerted to the planetary boundary layer by surface roughness.
We calculated the effect to the boundary layer when the wind encounters
a wind farm. The wind farm was implemented as a sudden change of the
surface roughness. Our calculations showed that after an increase of
the surface roughness, the wind speed was decreasing up to large distances
downstream. Several wind farm models assume that equilibrium is reached
after 5 rows of wind turbines, our model showed that it will take at
least 10 rows before equilibrium is reached to some extent. This means
that our classical models predict a too high power output for really
large wind farms.
Experiment
In order to validate the results of the model we set up an experiment
in the boundary layer tunnel of TNO, Apeldoorn. We designed and manufactured
30 wind turbines on a scale of 1:400. The turbines are 25 cm diameter
and have a hub height of 25 cm. Our design philosophy has delivered
rotors which are very similar to full scale rotors regarding the axial
force and wake properties, however regarding the efficiency their performance
is less. The power coefficient is about 0.30, while is can be between
0.45 and 0.50 for commercial rotors. In the wind tunnel the surface
roughness (excluding that due to the wind turbines) was set to sea conditions:
after scaling this was approximately 0.2 mm / 400. Sometimes the roughness
was adapted to onshore circumstances as well. We installed several wind
farms layouts and measured the wake losses and the effects on the boundary
layer.
Results and Discussion
The validation suggests that the wind velocity has not reached
its equilibrium value after 5 rows, which indicates that the numerical
model can be used for global studies of atmospheric flows above (and
behind) large wind farms. The words 'suggests' and 'indicates' were
used since the situation in the tunnel was different from the real situation
in several aspects. For several reasons the interpretation of the results
was difficult. Especially the accurate measurement of the wind speed
was a difficulty. A way out for this accuracy problem was the application
of differential measurements: we finally set up two farms next to each
other in the tunnel.
Conclusion
We can not rely on wind farm models that assume that the wind
flow in a farm is stabilized after five rows of turbines. Many models
will over predict production.
Recommendation
We recommend that much attention is to be paid to improvement
of the accuracy in the wind tunnel and to the organization of good experiments
with actual farms in the field. Both suggestions have been implemented
in successive projects, which already have been started.
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