The price of conventional energy is on the rise, due to the ever-widening gap between demands and supply. The main reason for such shortages is the depletion in natural resources, such as coal, which is the main fuel used for electrical energy generation. Since these fuels are made up of carbon compounds, burning them has rapidly increased the amount of carbon dioxide in the atmosphere over the last 100 years. This has brought about a chain reaction of hazards such as global warming, climate change, destruction of ecosystems, etc with predictions for adverse outcomes in the future. In response to this threat and to initiate an end to such processes, the UN agreed the Kyoto Protocol in Japan in 1997. This requires industrialised nations to reduce greenhouse gas emissions by 5% of 1990 levels by 2008-2012.
The UK has agreed to meet this target and furthered its promise by setting a goal of 50% reduction in carbon emissions by 2050[ ]. Part of its government energy policy is to increase the contribution of electricity supplied by renewable energy to 10% by 2010 (Blackmore P, 2004). A similar promise has been undertaken by many world nations, which has led to a plethora of new and innovative methods for power generation.
Renewable is the key to climate friendly forms of energy, due to the absence of emissions detrimental to the environment (Stiebler M, 2008). It includes energy derived from sunlight, wind, wave, tides and geothermal heat. Out of the afore mentioned resources, geothermal heat is restricted to only limited locations on the globe while wave and tidal power is still in its research stage. Thus sunlight and wind are the key elements that can be tapped for energy generation. However, on comparison between the two systems, wind energy systems are more advantageous both in availability of resources and cost of generation.
This report mainly focuses on wind energy, with a keen interest on harvesting it for ventilation and power generation purposes in high-rise buildings. Plan forms that aid this purpose will be studied using Computational Fluid Dynamics to understand the flow of wind in and around a thirty-storey structure and the building configuration well suited for natural ventilation and wind turbine integration would be identified at the end of the test. To obtain a complete picture of wind flow patterns and to closely mimic real life situations, the wind will be simulated from different directions at different wind speeds.
Wind is the term used for air in motion and is usually applied to the natural horizontal motion of the atmosphere (Taranath Bungale S, 2005). It is brought about by the movement of atmospheric air masses that occur due to variations in atmospheric pressure, which in turn are the results of differences in the solar heating of different parts of the earth’s surface (Boyle G, 2004). At a macro level wind profile differs from place to place depending on geographic location and climatic conditions while in a microstate the immediate physical environment of a particular place modifies the nature of the winds. For example, the velocity of the wind recorded in the countryside which has acres of unobstructed grassland would be greater than that recorded in a city dominated by skyscrapers.
Hence to obtain a clear idea of the wind characteristic corresponding to a particular area the wind rose is utilized. They are based on metrological observations and depict the varying wind speeds experienced by a site at different times of the year together with the frequency of different wind directions [ ]. It is the first tool consulted to judge the wind resources of a site and its ability to support power generation.
The winds have been tapped from ancient times by means of ship sails, windmills, wind catchers, etc. The history of windmills goes back more than 2000 years (Stiebler M, 2008) when they were predominantly used for grinding grain and pumping water. However, the breakthrough occurred when Charles.F.Brush erected the first automatically operating wind turbine at Ohio in 1888 [ ]. It was fabricated using wood and had a rotor diameter of 17m with 144 blades. The system recorded very low efficiency and was mainly used to charge batteries. The reason behind the poor efficiency was due to the large number of blades, which was later discovered by Poul la Cour who introduced fewer blades into his wind turbine. Though such developments were achieved at an early stage in innovation, it was not until 1980 that the prominent application of renewable energies was sought after (Boyle G, 2004).
Wind energy is the harnessing of the kinetic energy prevalent in moving air masses. This kinetic energy for any particular mass of moving air (Boyle G, 2004) is given by the formula:
K.E = 0.5mV²
m – mass of the air (kg) and
V – wind velocity (m/s).
However this mass of moving air per second is:
m = air density x volume of air flowing per second
m = air density x area x velocity
m = rAV
r – density of air at sea level = 1.2256 kg/m³ and
A – area covered by the flowing air (m²)
Substituting this value of m in the former equation,
K.E. = 0.5rAV³ (J/s)
But energy per unit of time is power and hence the above equation is the power available from the wind. It is also evident that the power is directly proportional to thrice the wind velocity. In other words even a marginal increase in wind speed would yield three folds of the nominal power. This is the critical fact based on which the whole energy process is evolved. However not all of this power can be exhausted since it would lead to nil outflow through the wind turbine, that is no flow of air behind the rotor. This would lead to no flow of air over the turbine causing total failure of the system. According to Albert Betz the maximum amount of power that can be harnessed from the wind is 59.3%. This is often referred to as the Betz limit and has been proven by modern experiments.
Some of the advantages of wind energy include:
- It is based on a non-exhaustive resource and hence can be harnessed for generations.
- It is a clean and eco friendly way of producing energy.
- In its working lifetime, the wind turbine produces eighty times the amount of energy that goes into its manufacturing and thus has diminishable net impact on the environment.
- It does not require any additional resources such as water supply unlike conventional power generation.
- It can boost the economy of the region (wind farms).
Wind turbines are the modern day adaptations of the yesteryear windmills but unlike their counterparts they are mainly used for power generation. These new age systems come in different shapes and have various configurations, the well established of them all are the Horizontal axis wind turbine and the Vertical axis wind turbine.
Write a brief about horizontal wind turbines and vertical wind turbines.
BUilding integrated Wind Turbines (BUWT):
Building integrated wind turbines are associated with buildings designed and shaped with wind energy in mind (Stankovic S et al, 2009). They are relatively a new way of harnessing energy that is gaining popularity at a quick pace. Small scale wind turbines on house roofs and retrofitting also fall under this category.
The design of BUWTs is a complicated affair and involves the careful consideration of various factors. Since turbines are fixed into the building’s fabric its impact on the environment, building’s response and needs of its owners and occupants need to be weighed equally. Also numerous design decisions such as planning, structure, services, construction and maintenance depend on this single process (Stankovic S et al, 2009). With the increase in the scale of the proposal the importance of these factors increases simultaneously. The proposal generally spans from the number, scale, type and location of the turbines together with its annual energy yield and design life. A good BUWT based building should be a wholesome design that does not prejudice the buildings efficient functioning for energy generation.
Generic options for BUWTs:
Stankovic S et al (2009) explains that the wind turbines can be fixed on to a building in enumerable ways. Each method can accomplish a different level of power depending on the type of turbine used and the form of the building it is mounted upon.
- On top of a square/ rectangular building:
This configuration is on the principle that the wind velocity increases with height and hence the amount of energy generated would be of a higher order (10% increase with wind acceleration). An added advantage is that the turbine would experience relatively little turbulence. But access to the turbine for maintenance and decommissioning works may be difficult. If mounted on tall buildings the turbines may threaten the visual quality of the skyline.
- On top of a rounded building:
This case is very similar to the previous configuration except that with the use of rounded façade the mean tower height can be considerably diminished. Also the rounded profile influences the local acceleration (15% increase in energy). The low tower height favors easy access to the turbine but leads to blade flicker and noise issues.
- Concentrator on top of a rounded building:
This case is well suited to areas with bi-directional winds (20% energy increase over a free standing equivalent due to local acceleration). Vertical axis wind turbines are better suited for this feature while Horizontal axis wind turbines need to be suitably altered to achieve the same status. The building spaces that act as concentrators may be inhabited with suitable acoustical treatment. This case also encounters the same drawbacks as listed in the previous case.
- Square concentrator within a building façade:
As before, this configuration takes advantage of the higher quality winds at higher altitudes and local acceleration thereby achieving 25% increase in energy and 40% increase for bi-directional winds. This option is best suited for buildings with narrower profiles. There may be a loss in the saleable area of the building but the aperture can be converted into an exclusive feature such as a sky garden. The opening also relieves the wind loading on the building’s facade leading to simpler structural solutions. Vertical axis wind turbine is the only choice for integration due to its square swept area.
- Circular concentrator within a building façade:
This is very similar to the square concentrator except the opening is accustomed to hold pitch controlled horizontal axis wind turbines with fixed yaw. Also, a 35% increase for uniform wind and 50% increase in energy for bi-directional winds are achievable in this method. But on the down side, this technique is more expensive due to the cylindrical shroud.
- On the side of a building:
In this technique, an increase in 80-90% in energy than the freestanding equivalents is achievable only if the building form is optimized to the local wind character. Only reliable vertical axis wind turbines can be used for power generation due to access issues. For higher swept area, more number of turbines should be used.
- Between multiple building forms:
This type of an option opens out many doors for a range of architectural forms. Unlike the previous cases, the buildings orientation, form, shape and spacing play key roles in the performance of the turbines. Vertical axis wind turbines are better suited for this purpose.
Guidelines for BUWT’s:
The following are some guidelines outlined by Stankovic S et al (2009) for incorporating wind turbines into a structure:
- BUWTs should be tailored to the specific site for good results.
- Adequate wind resources should be available on site. If however if the site is under resourced steps are to be adopted to deliberately elevate the quality of the wind through the buildings form or turbine. The impact of its surroundings should also be considered before commissioning such a project.
- The dominating wind direction and its intensity should be observed from meteorological data. This would help in determining the form and orientation of the building together with finalizing the position of the wind turbine to make the most out of the available resource.
- Environmental impact assessment corresponding to the site should be carried out to foresee the adverse effects the turbines may create.
- Acoustic isolation may be sought for in some areas within the building if it lies at close proximity to the rotor.
- Natural ventilation and day lighting qualities of the building may be challenged and forced to settle for artificial means.
- The type and position of openings, external shading devices, smoke extracts etc should be handled with appropriate care to avoid draught winds.
- Access to the wind turbines for maintenance and decommissioning must be provided suitably.
- The aesthetic quality of the mounted turbines must harmonize with its surroundings and should not over power the pedestrians at ground level. To this end well suited screening devices such as canopies, screens and landscape may be utilized as per the necessity.
The overall success of BUWT project depends on its ability to deliver the expected power. Inability to comply with this effect would result in the failure of its intended purpose from both an environmental and design point of view. Thus the electricity demand of the building and the level to which this would be met with should be estimated prior to turbine design to secure maximum benefits.
Wind flow prediction and energy yields:
For any project to be successful,
Wind flow and building design
(Taranath Bungale S, 2005) When the air moves in a vertical direction it is referred to as a current. These currents play a major role in meteorology whereas the gradual decrease in wind speed and high turbulence of the horizontal motion of air, at the ground level, are vital in building engineering. In urban areas, this zone of turbulence extends to a height of approximately one quarter of a mile aboveground and is called the surface boundary layer. Above this layer, the horizontal airflow is no longer influenced by the ground effect. The wind speed at this height is known as the gradient wind speed, and it is precisely in this boundary layer where most human activity is conducted.
Characteristics of wind:
The flow of wind is complex because many flow situations arise from the interaction of wind with structures. A few characteristics of wind include:
- Variation of wind velocity with height:
The viscosity of air reduces its velocity adjacent to the earth’s surface to almost zero. A retarding effect occurs in the wind layers near the ground, and these layers in turn successively slow the outer layers. The slowing down is reduced at each layer as the height increases, and eventually becomes negligibly small. The height at which velocity ceases to increase is called the gradient height, and the corresponding velocity, the gradient velocity. At heights of approximately 366m aboveground, the wind speed is virtually unaffected by surface friction, and its movement is solely dependant on prevailing seasonal and local wind effects the height through which the wind speed is affected by topography is called the atmospheric boundary layer.
- Wind turbulence:
Motion of wind is turbulent and it occurs in wind flow because air has a very low viscosity-about one-sixteenth that of water. Any movement of air at speeds greater than 0.9 to 1.3 m/s is turbulent, causing air particles to move randomly in all directions.
- Vortex shedding:
In general, wind buffering against a bluff body such as a rectangular building gets diverted in three mutually perpendicular directions. However, only the longitudinal winds and the transverse winds or crosswinds are considered in civil engineering. When a free flowing mass of air encounters a building along its path, the originally parallel upwind streamlines are displaced on either side of the building. This results in spiral vortices being shed periodically from the sides into the downstream flow of the wind, called the wake. At relatively low wind speeds the vortices are shed, that is, break away from the surface of the building and an impulse is applied in the transverse direction.
Distribution of pressures and suctions:
When air flows around the edges of a structure, the resulting pressures at the corners are much in excess of the pressures on the center of elevation. This has been evident by the damages caused to corner windows, eave and ridge tiles, etc in windstorms. Wind tunnel studies conducted on scale models of buildings indicate that three distinct pressure areas develop around the building. They are:
- Positive pressure zone on the upstream face (Region 1)
- Negative pressure zone at the upstream corners (Region 2)
- Negative pressure zone on the downstream face (Region 3)
The highest negative pressures are created in the upstream corners designated as Region 2. Wind pressures on a buildings surface are not constant, but fluctuate continuously. The positive pressure on the upstream or the windward face fluctuates more than the negative pressure on the downstream or the leeward face. The negative pressure region remains relatively steady as compared to the positive pressure zone. The fluctuation of pressure is random and varies from point to point on the building surface.
Nearby buildings can have a significant influence on wind forces. If they are the same height as the structure being considered then they will mostly provide shelter, although local wind loads can be increased in some situations. Where surrounding buildings are significantly taller they will often generate increased wind loading (negative shelter) on nearby lower structures. Shelter can result from either from the general built-environment upwind of the site or from the direct shielding from specific individual upwind buildings (Blackmore P, 2004).
The three natural ventilation airflow paths in buildings are (Pennycook, 2009):
- Cross ventilation
- Single-sided ventilation
- Passive stack ventilation
Advantages of cross ventilation:
- Greater rates of ventilation can be achieved under amicable weather conditions.
- Can be utilized for deep-plan spaces with operable windows on the external wall.
- Incumbents have control over ventilation.
- Relatively cost free.
- Can be incorporated with thermal masses.
However, it has certain limitations such as:
- Internal space layout must be hindrance free for easy, clear flow of air.
- Internal partitions must be within 1.2m height and tall cupboards must be placed alongside the windows.
- Natural ventilation can occur only under the presence of suitable winds.
- Poor planning and positioning of windows may cause disruptive draughts and gusts.
- Winter ventilation is problematic.
- Unsuitable for buildings located in noisy and pollution prone environments.
The requirements of fresh air supply are governed by the type of occupancy, number and activity of the occupants and by the nature of any processes carried out in the space (Koenigsberger et al, 2001). When natural ventilation is stipulated for good indoor air quality, the amount and nature of the dominant pollutant source in the space should be identified. Based on this data the ventilation rate for the space can be calculated such that the pollution level does not cross a preset specific mark. Generally the concentration of the pollutants decreases with the increase in airflow rate (Figure –1).
However, in terms of thermal comfort especially during winter the heating requirement of the building will increase with the ventilation rate. This demand varies with time, wind characteristics of the place, opening and closing of windows and doors by its occupants and the thermal state of the building. In summer, cooling is ideal for both the building and its occupants to prevent internal heat gains. By directing the high velocity wind around the human body the evaporative rate at the skins surface can be increased thereby achieving a cooling sensation. The recommended upper limit of indoor air movement is 0.8 m/sec, which permits the inhabitants to occupy a space about 2°C warmer and 60% relative humidity with optimum comfort. The traditional way to cool buildings is to provide large openings along the exterior wall with the principle that higher the ventilation rate greater the loss of heat to the external environment. But such an arrangement would work only when the outdoor temperature is in the range of comfort zone. When controlled indoor environments are desired especially during the occupancy period’s night ventilation is recommended. In this technique the building is cooled at night so that it can absorb the heat generated during the day (Allard F, 1998).
Based on wind tunnel experimental observations, the factors that affect the indoor airflow are:
- External features:
- Position of openings:
- Size of openings:
- Control of openings:
The following are studies that have been made of different aspects of wind using Computational Fluid dynamics.
- CFD evaluation of wind speed conditions in passages between parallel buildings:
This analysis undertaken by Blocken B et al (2007) mainly focuses on the wind speed conditions in passages between parallel buildings in combination with the accuracy of the commercial CFD code Fluent 6.1.22 when the wall-function roughness modifications are applied to them. The Venturi effect is also studied to determine the amount of increase in wind speed in the passage due to the decrease in flow section. The results obtained were compared with various previously proven experiments carried out by experts in the field.
As the title indicated the case undertaken involves a pair of rectangular buildings measuring 40m x 20m x 20m, placed adjacent to each other and separated by a narrow passage. The width of the passage is widened (for example, 2, 4, 6, 8, 10, 15, 20, 30, 40, 60, 80, 100 m) with every case to clearly understand the Venturi effect. The dimension of the computational domain is 20.5x14x18m3; the whole setup is placed at a distance of 5m from the inlet and simulated with a wind speed of 6.8m/sec based on initial results.
The results recorded at the end of the simulation process are discussed as follows. They are based on the amplification factor, which is defined as the ratio of the mean wind speed at a certain location to the mean wind speed at the same location without the buildings present. As such it is a direct indication of the effect of the buildings on the wind speed (Blocken B et al, 2007).
- Pedestrian level wind profile: In context to this research, for narrow passages (example w=2m) this amplification factor occurs maximum at the centerline immediately behind the entrance. When the distance between the buildings are slightly increased (example w=10m), the flow streams deflecting off the inner edges of the buildings combine into a large jet stream and records an increase in the amplification factor. However this property is lost when the width of the passage is of a high order (example w=30m).
- Overall wind profile: To understand the overall wind profile, six vertical lines were identified along the passage’s center plane for the case of w=6m. The lines depicted the fact that there was an increase in the wind speed at the ground level due to the downdraft of the wind along the front façade of the building and a decrease in wind speed at the end of the passage due to the exit of flow from the passage. Also for these cases, there was no significant increase in the wind speed with the increase in height.
- Flow rates at different points in the passage: To evaluate the Venturi-effect three fluxes were defined, one along the vertical plane, another along the horizontal plan and the final being similar to the former one but in the absence of the buildings. When the flow rate was calculated for narrow passages, it stated an increase in wind speed by only 8% due to the Venturi effect. However for larger widths the flow rate was lower than the free-field flux. This shows that the wind has a tendency to flow over and around the building rather than be forced through the passage as previously believed. Thus there is a lack of strong Venturi effect and the flow in the passage can be attributed as the channeling effect for these cases.
The research also concluded that there were discrepancies in the CFD results due to the use of the roughness factor and advised future users to simulate an empty field before positioning the buildings to clearly identify the difference in results. Further research into the Venturi effect was also implied.
- Computational analysis of wind driven natural ventilation in buildings:
Evola G and Popov V (2006) research focuses on the application of three-dimensional Reynolds Averaged Navier-Strokes (RANS) modeling on wind driven natural ventilation with specific detail to the pressure distribution and flow pattern within the building. The various cases would be simulated with the standard k-e model and the Renormalization Group theory (RNG). Within the framework of natural ventilation both single sided ventilation and cross ventilation would be studied and the results obtained using CFD will be compared with LES models and empirical methods for its reliability.
The building undertaken consists of a 250mm x 250mm x 250mm cube punctured with a centrally located 84mm x 125 mm opening on the wind ward side (Case 1). In Case 2 the door like opening is placed on the leeward side and in Case 3 both the openings are retained to test the cross ventilation principle.
On comparison between the CFD results obtained for Case 1 and 2, Case 2 portrays a better flow pattern especially at the mouth of the opening. This leads to a better ventilation rate than Case 1 though in contrast to the theoretical data that good ventilation rate and flow patterns are achievable only when the opening faces the incoming winds. To establish the phenomenon further experimentation into the field was suggested. Between Cases 1, 2 and 3, cross ventilation clearly stands out as the best option of them all, both in terms of velocity and distribution.
Also the study concluded that the measured RNG results matched approximately to the theoretical results of Cases 1 and 2. But a significant amount of deviation was observed in Case 3. The RNG model was only slightly intense than the k-e model generally used.
The research also concluded that there were discrepancies in the CFD results due to the use of the roughness factor and advised future users to simulate an empty field before positioning the buildings to clearly identify the difference in results. Further research into the Venturi effect was also implied.
- CFD modeling of unsteady cross-ventilation flows using LES:
This research undertaken by Cheng-Hu Hu et al (2008) employs the LES method to investigate the fluctuating ventilation flow rate induced by the wind for a cross-ventilated building. The results from CFD were compared with those previously acquired from wind tunnel tests.
The building proposed for the study consists of a rectangular box with two openings of equal size located opposite to each other. The wind is simulated from 0°(Case 1) and 90°(Case 2) to the building at a rate of 1m/sec, to study the flow pattern in and around it.
When the air approaches the building the ventilation rate is unsteady at the mouth of the openings due to turbulence and in the flow separation layer due to shear. In Case 1 the wind is accelerated through the opening and directed downwards inside the building. This phenomenon brings about a circulation of the internal air before guiding the wind upwards and out through the window on the leeward side of the building. The air exchange occurs due to the mean flows through the opening. In Case 2 where the wind is parallel to the windows, the air moves in and exits rapidly causing fluctuating flows thereby leading to air exchange. In this case turbulence prone areas are formed at the rear of the building.
When these results were compared with the wind tunnel data, Case 1 portrayed similarities while Case 2 had major deviations. Further study was proposed for understanding the reason behind such deviations.
The Bahrain world trade centre was the world’s first building to ‘aesthetically incorporate commercial wind turbines into the fabric of the building’ [ ].
The complex consists of a three-storied sculpted podium and basement from where the 240m high towers rise up into the sky. The two towers comprise of 51 floors each and are connected by means of three, 31.5m span bridges at 60m, 96m and 132m levels [ ]. They are oval in section for aerodynamic reasons and follow a shallow V-shape in plan for adequate blade clearance. Sitting on each of this 70 ton spandrel is an 11-ton nacelle to which the industry approved horizontal axis wind turbines are fixed by special means. The turbine has a rotor diameter of 29m and is stall controlled with centrifugally activated feathering tips for air brakes (Killa S & Smith Richard F, 2008). The turbines are oriented facing the Arabian Gulf intercepting the path of the dominant winds.
The decision to harness the prevailing wind was thought of from the initial stage drawing inspiration from ‘the regional wind towers and the vast sails of the traditional Arabian Dhow which utilise the wind to drive them forward’. Numerous Computational fluid dynamics models and wind tunnel tests were carried out to determine the final building form. The result was a skyward tapering, elliptical structure, carved out by the wind that functions as aerofoil sections (Wood A, 2008). The shape and spatial relationship of the towers aid in adhering the wind in a “S’ flow whereby the center of the wind stream remains nearly perpendicular to the turbine within a 45° wind azimuth, either side of the central axis (Killa S & Smith Richard F, 2008). This increases the turbine efficiency, number of working hours and minimizes the stress on the blade caused by yawing [ ].
Furthermore, the two towers were placed such that they create a ‘V’ shaped space in between them, as well as a negative pressure behind the blocks, thus creating an opportunity for the Venturi effect to accelerate wind velocity onto the turbines (Binder G, 2006) by as much as 30% more than the source wind (Killa S & Smith Richard F, 2008). The tapering profile combined with the increased onshore wind velocity at higher altitudes creates a near equal regime of wind speed on each of the three turbines, irrespective of its location, allowing them to rotate at the same speed and generate approximately the same amount of energy (Wood A, 2008).
Table – 1: Annual energy output