UWE Bristol Engineering showcase 2015
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Peter Lawson<br />
Project Supervisor<br />
Chris Toomer<br />
Meng Mechanical <strong>Engineering</strong><br />
Development of a Shrouded Roof Top Turbine<br />
for Frenchay Campus<br />
Wind turbines are a source of renewable energy, through converting wind<br />
energy to electrical energy. Before implementing a new turbine, the wind<br />
resources at the desired location should be modelled and the turbine design<br />
optimised to maximise energy yields from the available wind. This<br />
investigation examines the feasibility of locating a wind farm on the Frenchay<br />
Campus of the University of the West of England. The previous study<br />
modelled the wind speeds across the campus and explored the costs and<br />
feasibilities of implementing various turbine designs, concluding rooftop<br />
turbines sited on the high-rise student accommodation to be the most<br />
feasible option. This investigation examines how these designs can be<br />
optimised to maximise electrical energy outputs. Firstly, the wind flow over<br />
the chosen buildings was modelled through computational fluid dynamics<br />
(CFD) to identify optimal rooftop sites for turbine installation and the optimal<br />
elevation above which the wind speeds are highest. Secondly, a shroud,<br />
which encloses a turbine to passively encourage higher effective wind speeds<br />
to result in higher electrical outputs, was designed and computationally<br />
modelled by CFD. The CFD predictions were subsequently used to predict<br />
electrical energy outputs in relation to the installation feasibilities of various<br />
turbines described within the previous study. Wind speeds were shown to<br />
increase across the majority of the roofs studies at elevations relevant to<br />
wind turbines, and designs encompassing shrouds displayed increases in<br />
effective wind speeds over the turbines, allowing higher energy productions.<br />
Future work should validate these results through testing shroud prototypes<br />
in wind tunnels.<br />
The building analysis results show that siting a wind turbine at any position<br />
on the rooftop offers higher effective wind speeds than if the same wind<br />
turbine was sited on the ground; in most cases wind speeds could be<br />
increased by 1.157 times. When these increases in velocity are substituted<br />
into kinetic power calculations for the air, it actually causes a rise of 55%.<br />
Very small differences in wind speeds were shown between positions<br />
offering the highest and lowest rooftop velocities.<br />
Wind turbines with different incorporated shrouds were shown to have a<br />
wide variety of effective wind velocities, however many of the designs<br />
showed a large flow separation area off the shroud which gave rise to large<br />
amounts of turbulence and eddies; these effects are associated with loud<br />
wind noise and are therefore very undesirable for roof top wind turbines.<br />
The study firstly investigated the addition of straight angled shrouds,<br />
showing high peak velocities, however these shrouds also experienced large<br />
amounts of flow separation. These flow separations would cause eddies and<br />
turbulence to form behind the shroud, which reduces the cross sectional<br />
size of the desired high velocity stream. In order to reduce the flow<br />
separation and maintain a larger cross sectional high velocity stream, the<br />
radial model was designed and modelled. This design also showed high peak<br />
velocities but still suffered from large flow separations. Cosine modelled<br />
shrouds were subsequently modelled which offered a good solution since<br />
they maintained a high velocity stream with very limited flow separation.<br />
The half cosine model was eventually selected as the most suitable design,<br />
offering the second highest kinetic power air flow but with the advantage of<br />
having greatly reduced flow separation and thus a much smaller turbulent<br />
wake.<br />
Project summary<br />
This investigation was undertaken to improve<br />
the output of a roof top mounted wind<br />
turbine by passively increasing the air velocity<br />
around the turbine through the use of a<br />
shroud. This project also modelled the roof<br />
top of building on Frenchay campus to<br />
highlight the best possible installation sites<br />
for a roof top turbine<br />
Project Objectives<br />
This project aimed to test a variety of<br />
different shroud shapes in order to find the<br />
most suitable shape. This involved the careful<br />
balance of maximizing the low pressure area<br />
behind the shroud in order to encourage<br />
increased velocity flow through the shroud.<br />
However with large increases in the cross<br />
sectional area flow separation can cause large<br />
turbulence regions.<br />
Project Conclusion<br />
The project highlighted that a shroud based<br />
on a half cosine line offered the highest<br />
velocity while minimizing flow separation and<br />
turbulence. The results showed that with the<br />
increase in velocity causes by the roof top<br />
placement and the shroud electrical output<br />
could be increased by 2.7 times for an<br />
example wind turbine.