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Abstract— A numerical study has been conducted particularly graphical analysis to investigate wind flow above roof of the constructed structure of various shapes from side views such as the conventional structure, fillet, dome shaped, half wedges. The velocity contours and velocity profiles have been the points of observation to find out the shape having the efficacy to initiate the turbine rotation. In addition, the suitable location for mounting wind turbines has been illustrated and the desired roof shapes for future buildings are proposed to be constructed in Bangladesh. Computational result shows that dome shaped roof delivers the maximum velocity among all the other shapes taken into consideration here. On the other hand, when corners are filleted the velocity has a relative increase than the conventional one. Case study shows that among all the cities Chittagong, Comilla and some certain places in Dhaka have the minimal wind speed to initiate the turbine to generate energy.
Keywords— Wind Turbine, Energy, Fillet, Wedge, Dome, Velocity profiles
Bangladesh is a developing country in South Asia. Dhaka is the capital and largest city, followed by Chittagong. Bangladesh is the world’s eighth most populous country. The rate of population here is growing day by day. Also, there is lack of accommodations as well as lack of lands. Therefore, many buildings without any proper design are constructed in an unplanned manner. Despite this unavoidable condition these buildings can be utilized as the source of power generation. The power generated in our country is not sufficient to cover all the regions. In addition, the pollution is growing sharply with the increase of industrial advancement. This requires to reduce the environmental impact as much as possible. Demand is always higher than the production. To meet up the demand, building-integrated micro-wind turbines are suggested to install on the roof of commercial as well as residence structures to generate energy. Wind turbine power of a wind generator  can be expressed as follows:
Where, is the wind turbine power, is the air density , is the co-efficient of performance, A is the swept area of the blades and V is free wind speed (m/s). From the above equation, it is showed that the power of the wind increases with the cube of the wind speed.
On the other hand, the rate of carbon emission is rising day by day. An expansion in the degree of public awareness regarding the effect of Green House Effect on the nature and the detrimental effect of our dependency on fossil fuels has acquired a developing enthusiasm on site-generated electricity from sustainable energy sources. Therefore, use of fossil fuels should be reduced to lessen environmental damage. Hence, the focus is on alternative energy solutions, which will reduce our carbon footprints.
The Global Institute of Sustainability building located in Arizona State University has mounted 6 wind turbines and under average condition, 36 computers can run with the generated electricity. The wind turbines accommodated there can produce electricity with winds down to 5 mph . Numerous studies regarding utilization of urban wind energy have been done beforehand and a few has given hints of further work.
Durson (2012) has introduced the concept of Building Augmented Wind Turbines (BAWT). The advantages of roof mounted wind turbine was represented by him. The idea was to utilize the buildings as an accelerator of wind . Study of Islam Abohela (2013) shows that as the building height increases, the wind velocity increases and accordingly the energy yield of a roof mounted wind turbine increases. He also found, barrel-vaulted roof is the desired roof shape for mounting wind turbines on the roof in order to accelerate wind above the roof . In another study of Islam  different designed roofs were compared and stated that mounting wind turbines on barrel vaulted roof would deliver 56% more electricity than a free standing wind turbine at the same location. Although wedge shaped roof will deliver only 9% more electricity than a free standing wind turbine, it is more feasible to take the advantages of the building’s acceleration effect . On the other hand, Sari (2012) found through numerical analysis and wind tunnel test that the wind speed raises at the center of rounder towers . For the detail setup in wind flow simulations, S. Murakami (1993) have analyzed the distinctions of turbulence models and finished up with a decision that no model can simulate wind flow over buildings splendidly, although standard turbulence is a standout amongst the most sensible models and is broadly utilized .
CASE STUDY ON WIND FLOW
A case study is important to identify suitable locations to mount wind turbines where maximum efficiency can be utilized. Nandi (2012), in his study uncovers that Kutubdia and Kuakata have potential for wind power. His result shows that the average wind speed is around 4.17 m/s . The residential areas where the alignment of the buildings is similar and gap between buildings are standard are eclectic for the construction of wind turbines. Besides, the environment should be such that under average condition, the wind speed is at least 3-4 meters per second (m/s). Most wind turbines start generating electricity at wind speeds of around 3-4 m/s (8 miles per hour) . Bangladesh Meteorological Department has done statistics on different places monthly and given annual results on that basis . Statistics show, on the month of June and July, Chittagong has the highest wind speed of around 8.7 m/s. On the other hand, Jessore possesses the wind speed of 6 m/s on April and May. Similarly, Cox’s Bazar has a relatively higher wind speed on June and July. Dhaka, Sayedpur and Comilla have the same kind of wind speed on April, May, June and July which is around 4m/s. The annual wind speed of four different months in those cities of Bangladesh is charted below:
The idea here is to see the possibility of this plan through computational approach to see how the wind speed can be put to profitable use to get the desired power generated from wind turbines. Moreover, different roof top designs are investigated in numerical approach to find out a suitable one for future construction. The side views of these future buildings are observed in computational methods individually. Side views include normal structure, fillet, dome shaped and half wedge. From the numerical results the possible combinations are observed.
COMPUTATIONAL SETUP & DATA ACQUISITION
According to Bangladesh National Building Code (BNBC) , A 10 storied building should be at least 32 meters in height [e.g. h=32 meter]. According to the ratio, width was given to be 21 meters [e.g. w=21 meter]. This is taken as the reference for computational domain dimensions. The domain is 4.57w in length and 1.52w in height. Line of the roof is taken as the datum line. The rooftop is focused in these investigations and parts lower in the ground are not included as the mounting of wind turbines are suggested for rooftop.
The domain consists of triangular elements and number of elements are around 9000. Dhaka, Chittagong and Cox’s bazar had a velocity of 4.21, 8.77 and 6.15 m/s respectively in some selected months. Most of the velocity fluctuates between 4 and 8 in different cities. Hence, the former was taken as input to find out the results. The temperature is defined as 303 k. Computations are done in realizable k-ε model as it contains a modified transport equation for the dissipation rate. Also this model is efficient for and validated for boundary layer flows. The numerical results obtained particularly the Velocity contours and Velocity profiles over the building at different heights were the issues of observation.
All of the aforementioned shapes were 21 meters in length. As the rooftop is taken as preferred location to install the turbines the complete height of the tower was not taken into consideration. In addition, the wind velocity near to the ground is not going to be good enough provided that the vast population as well as densely constructed buildings. The wind velocity taken as input was 4.21 m/s which is highest for Dhaka recorded in the chart given earlier. For this specified input computations were done for all of the aforementioned shapes. There were some wakes formed behind the conventional structure. However, when the corners were fillet it had an increase in velocity than the conventional one. In the conventional one maximum velocity was 4.54 m/s whereas the latter one had a maximum velocity of 6.51 m/s, enough to initiate the rotation of the turbine blades. The fillet structure had this maximum value near the corners. Furthermore, dome shaped structure which had a radius of 12 meters was observed to have the highest of velocity among all four. The maximum velocity which occurs at the vicinity of the top of the dome is 7.42 m/s. The wedge was given an angle of 10o and had a low increment of velocity.
RESULT ANALYSIS AND COMPARISON
The velocity profiles were plotted at different l/w ratios. Where l is the height from datum line and w is the width of the building.
The computational domain extends upto 96 meters where the building width is from 35 to 56 meters. The velocity profiles particularly focuses on that part of the solution. At four different l/w ratios the profiles are plotted below.
The velocity profiles shown above are for four different ratios and the lines are along the datum level. It is observed from the figure that the dome shaped roof reached the highest velocity at different ratios. Whereas the fillet has relatively improved profile than the conventional one. At l/w = 0.14 the dome shaped has a velocity above 6 m/s. The conventional one has a relatively constant velocity along the datum level whereas the filleted rooftops have a slight increase in velocity. Wedge shaped rooftops had a relatively low initial velocity at the contact level of first edge. Although it had a steeper slope compared to conventional and the filleted one the maximum velocity reached by this 10 degree half wedge shape was not above than that of those. As a whole all the curves at different ratios maintain a similar pattern.
The four shapes that were investigated here are proposed as the side views of the structures for future buildings. The results suggest that dome shape should be preferred for maximum utilization of turbines. However, when the corners are filleted the resulting velocity improves a lot better than the conventional structures. A slight amount of increment in velocity will result in a higher amount of power generation. Wedge having a 10 degree did not have an effective increment in velocity to produce a desired effect for renewable application. Further studies can be conducted by varying the wedge angle as well as the radius of the dome shaped structure.
However, there are some limitations which needs to be considered. The wind speed throughout Bangladesh is not the same over the years. The months with relatively higher velocities are charted earlier. The Turbine selection is of paramount importance as the noise and vibration levels come into account. Encraft Windcrift Wind Trials Project (2009) and Wineur Project (2007) suggest that the rotor of the wind turbine must be mounted at least 30% above the building height from the flat roof level. However, there is advancement in technology in turbines and it is suggested here that parapet wind turbines are the solution of this problem. Arizona State University used this type of turbines. It is only 6.5 feet tall and weight around 60 pounds. It can produce electricity even at 5mph wind speed. The production rate is 55kWh per month. Moreover, it reduces noises as well as vibration which is also major concern.
In addition, temperature and wind speed are dependent parameters on each other. The study conducted here had a constant temperature although the practical scenario may differ. Also, an experimental investigation is necessary for further evaluation of real life applications. The cost analysis as well as the power generation should be investigated. The wind velocity will be non-uniform in different locations which also comes into account. Sometimes it is even difficult to get wind speed of 5mph in Bangladesh specially in Dhaka. New types of turbines are in progression for increased efficacy to spin the turbines at wind speed low as 4mph (2m/s).
The vision 2041 of “Digital Bangladesh” is to bring the whole country under the radar of renewable energy for the presevement of Green Environment. Utilizing the wind flow to generate energy is a promising prospect to mitigate the usage of fossil fuels and ensure a pollution free environment in the future.
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