Optimisation of a vertical axis tidal turbine and testing of a prototype in an unblocked environment
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Abstract
Vertical Axis Tidal Turbines (VATTs) have become the subject of increased
interest in recent decades, but the development of this type of hydrokinetic
turbine has faced several challenges that have not yet been overcome.
The influence of rotor parameters on turbine performance is one of these
challenges. No axiom can be found in the literature about the effect of these
parameters on the turbine behaviour, and sometimes research projects even
show contradictory results. As a consequence, parameters that define turbine
rotors may differ substantially from each other but have performed similarly in
terms of efficiency.
In relation to this matter, experimental modelling has been carried out in the
School of Engineering facility at Cardiff University. Using small-scale set-ups,
experimental testing provided useful and reliable information that shed light
into these design uncertainties. Blade roughness has been found to have a great
impact on the turbine behaviour, and the influence of this parameter has been
tested and analysed in depth in a subsequent chapter of the thesis.
Apart from the parametric analysis, a mechanical and an electrical system
were utilised for the turbine set up. Thanks to these different systems of energy
conversion, it was possible to compare the extracted power and to evaluate their
inherent losses. Electricity was generated from the electrical approach, which
was very useful in order to accurately evaluate the turbine efficiency.
Taking into account the results coming from physical testing, an optimised
prototype of a VATT was designed and manufactured; estimated to be a 1:15
scale device. Not just the rotor but the whole super structure was built, in order
to analyse both the efficiency and the performance of the rotor, as well as the
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structural response of the entire device.
Performing experimental testing without removing the effect that a blocked
channel provides to the turbine rotation is no trivial issue, and intermediate
scale tests will become a fundamental step for recognition of the technology.
Aiming to achieve a reliable source of information, the manufactured tidal turbine
prototype has been tested in a water sports centre (White Water Rafting
Centre, Cardiff).
There is a clear lack of information in the literature about testing tidal
turbines on an intermediate scale, and the chance to test a tidal turbine is a
very valuable opportunity. The experiments were accomplished in a very similar
environment to a real tidal stream, but with the related advantages of complete
control over the turbine deployment. Thus, these experiments are considered
to provide very useful data for scientific knowledge and also the tidal stream
energy sector.
Finally, the study of the hydrodynamic turbine wake in small scale was carried
out. Not only velocity measurements were collected at the turbine operating
point, but also water elevations upstream and downstream were measured by
using depth gauges based on water conductivity. At the time of writing and
to the best of the authors’ knowledge, not many research articles have studied
the wake characteristics of vertical axis tidal stream turbines, and none have
used this equipment. The quality of the data is deemed to be excellent and
the following process of the information described perfectly the near and far
wake. This could be extremely useful for a future scale-up of the turbine, and
the development of an array.