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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.

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