UWE Bristol Engineering showcase 2015

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