The Development of Commercial Wind Towers for Natural Ventilation: a Review

Applied Energy 92 (2012) 606–627

Contents lists available at SciVerse ScienceDirect

Applied Energy

journal homepage: www.elsevier.com/locate/apenergy

The development of commercial wind towers for natural ventilation: A review ⇑ Ben Richard Hughes a, , John Kaiser Calautit a,1, Saud Abdul Ghani b,2 a School of Civil Engineering, University of Leeds, Leeds LS2 9JT, UK b Mechanical and Industrial Engineering, Qatar University, Doha, Qatar article info abstract

Article history: Wind towers have been in existence in various forms for centuries as a non mechanical means of provid- Received 13 October 2011 ing indoor ventilation, energy prices and climate change agendas have refocused engineers and research- Received in revised form 23 November 2011 ers on the low carbon credentials of modern equivalents. The purpose of this study is to evaluate the Accepted 25 November 2011 development of wind tower device and their integration into buildings, thus providing a comprehensive Available online 23 December 2011 review of current and potential wind tower development. Previous works have investigated wind and buoyancy factors for induced natural ventilation to drive air flow through the wind tower. Numerous Keywords: studies have investigated the effects of different configurations and components on the performance of Wind towers wind towers. Studies include the use of evaporative cooling device inside the tower to improve its ther- Natural ventilation Thermal comfort mal performance, the use of solar chimneys, courtyards and curved roofs to enhance the air movement Evaporative cooling inside the structure, and the use of volume control dampers and ceiling diffuser to optimize the fresh Wind vent air flow rate and indoor conditions. The review further highlights the different cooling techniques which can be integrated with wind tower systems to improve ventilation and thermal performance. The basic principles of each technique along with its corresponding capabilities are summarized along with their advantages, limitations, and applications. Ó 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 607 2. Natural ventilation using a wind tower ...... 607 3. Micro climatic variation...... 608 4. Design of wind tower systems ...... 608 4.1. Sizing and positioning of openings ...... 608 4.2. Wind tower height ...... 609 4.3. Uni-directional and multi-directional wind towers ...... 609 4.3.1. One-sided wind tower ...... 610 4.3.2. Two-sided wind tower ...... 611 4.3.3. Four-sided and hexahedral wind towers...... 611 4.4. Rectangular and circular wind towers ...... 611 5. Wind tower cooling techniques ...... 612 5.1. Evaporative cooling ...... 613 5.1.1. Wind tower with wetted column and wetted surface ...... 613 5.1.2. Wind towers integrated with ground cooling...... 614 5.2. Wind towers with solar chimney...... 615 5.3. Structural night ventilation ...... 616 5.4. Wind towers integrated with courtyards...... 616 5.5. Wind towers integrated with curved roofs ...... 617

⇑ Corresponding author. Tel.: +44 7518715360. E-mail addresses: [email protected] (B.R. Hughes), [email protected] (J.K. Calautit), [email protected] (S.A. Ghani). 1 Tel.: +44 7544158981. 2 Tel.: +974 4852112.

6. Traditional wind tower systems ...... 617 6.1. Wind escapes and air vents ...... 618 6.2. The malkaf ...... 618 6.3. Badgir wind tower...... 619 7. Commercial wind towers...... 620 7.1. Wind vent ...... 620 7.1.1. Damper control system ...... 621 7.1.2. Louver configuration ...... 621 7.2. Solar fan-assisted wind vent...... 622 8. Development potential of future systems...... 623 9. Results summary ...... 623 10. Discussion...... 623 11. Conclusions...... 626 Acknowledgments ...... 626 References ...... 626

1. Introduction comfort without the aid of mechanical systems, thus enables fresh air delivery to occupants using sustainable and energy efficient Increasing awareness of the need for energy efficient and envi- methods [4]. Allard [5] defined optimum air quality as air free of ronmentally friendly approach for building design has renewed contaminants or harmful materials that can present a health risk emphasis on the integration of natural ventilation devices in build- to the occupants, potentially causing irritation and discomfort. It ings. Heating, Ventilation and Air Conditioning (HVAC) accounts for is known that the pollution level decreases exponentially with a substantial amount of the energy use in building and represents a the airflow rate. Hence, optimizing the supply of air is essential significant opportunity for energy savings. Ventilative methods to ensure adequate indoor air quality while maintaining the venti- which do not use mechanical intervention and thus energy free lation rates within a certain range [5]. are termed natural ventilation. Sustainable ventilation technologies Natural ventilation is dependent on three climatic factors: wind have been proposed to reduce the buildings energy consumption velocity, wind direction and temperature difference. The speed and and carbon footprint. An example of one such innovative ventilation direction of the wind over a structure generates a pressure field device is the wind tower. Montazeri and Azizian [1] defined the wind around the building as shown in Fig. 1. tower as a device which facilitates the effective use of natural venti- The wind tower provides natural ventilation by taking advan- lation in a wide range of buildings in order to increase the ventilation tage of the pressure differences surrounding the building. There- rates. Wind towers have been used in the hot and arid regions of the fore, it is essential that the device is positioned to maximize the Middle East for many centuries to provide passive cooling and pressure differential between the inlet and outlet. The primary achieve thermal comfort. Fathy [2] suggested that traditional solu- driving force for the wind tower is the external driving wind, positions in vernacular architecture can be adopted or integrated with tive pressure on the wind ward side drives the fresh air into the new technology to make them compatible with modern require- room and the negative pressure on the leeward side extracts the ments. Conventional and modern wind towers are increasingly stale and warm air. Hughes and Ghani [6] confirmed that a slight being used in modern buildings to minimize the consumption of change in air pressure can create sufficient airflow to improve non renewable energy. Modern design of wind towers combines the thermal comfort of the inhabitants. The wind changes direction the ventilation principles and passive stack in one design. Wind over a certain range on an hourly, daily and seasonal basis, thus the tower architecture can be integrated into the designs of new pressure field surrounding a building will also change accordingly. buildings, to replace or assist mechanical ventilation systems. A wind tower inlet opening can change from positive pressure to a However before adapting new technologies, it is necessary to negative pressure from one day to the next. In this case the open- research into the function and design parameters of traditional ing will function as an exhaust vent. and modern wind catchers and to demonstrate how it can be applied The wind induced forces will always be the main factor influenc- and improved in order to provide a simple and effective means of ing natural ventilation. In the absence of wind, the driving force for natural ventilation. the wind tower is buoyancy or stack effect which is a result of the air temperature difference between the micro and macro climate. The 2. Natural ventilation using a wind tower subsequent variation of air density and pressure gradient of the

Architectural design of buildings highlighted the potential advantage of natural ventilation systems for occupancy comfort. Natural ventilation has become an attractive solution for not only reducing the energy usage and cost but also for providing good indoor air environment while sustaining a comfortable, healthy, and productive internal climate. Natural ventilation systems use the natural pressure differences surrounding a structure, caused by wind and temperature driven forces to direct the flow through buildings. An example of one such low carbon ventilation system is the wind tower. Elmualim [3] stated that naturally ventilating buildings by means of wind towers provided increased control and reliability compared to cross-flow ventilation. Natural ventilation plays a significant role in providing Fig. 1. Wind creates a positive pressure on the windward side of a structure and a optimum indoor air quality and maintaining acceptable thermal negative pressure on the leeward side. 608 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 indoor and outdoor air masses causes the warm air (less dense) to rise up and escape through the wind tower’s exhaust. Consequently, new air is drawn into replace the air that escaped. Hughes and Cheuk-Ming [7] used experimental and computational fluid dynamics (CFD) modeling to investigate the wind pressure and buoyancy driven flows through a natural ventilation system. The work examined the relationship between the two driving forces for the passive wind tower system. The experimental and numerical results showed that wind driven force is the primary driving force for the wind tower device, providing 76% more indoor ventilation than buoyancy driven forces. The study also confirmed that the effect of buoyancy force is insignificant in wind tower systems without exterior airflow openings. However, the effective addition of external windows and vents in combination with buoyancy driven flows have the potential of increasing the internal ventilation and overcoming the unavailability of wind driven forces in dense and urban areas. Jones and Kirby [8] used a Fig. 3. Function of a wind tower system during daytime and nighttime. semi-empirical approach to predict the performance of a similar wind tower device. The author also concluded that the buoyancy Asfour and Gadi [11] described the operation and function of a effect is significant only at relatively low wind velocities. ventilation device which varies continuously throughout the day. The natural wind and buoyancy induced air flows are low com- During daytime, the operation of a wind tower is dependent upon pared to driving pressures produced by mechanical ventilation sys- the wind movement due to the pressure difference across the inlet tems. Kleiven [9] has highlighted the importance of minimizing the and outlet as shown in Fig. 3. The wind tower catches the prevail- resistance in the airflow path through the structure. Thus, the ing wind and directs the air downwards at higher velocity. During building itself, with its interior and exterior interface, rooms, hall- night time, the lower external air temperature cools the building ways and staircase are used as air path rather than the ducts used mass, which provides additional cooling the next day. by mechanical ventilation systems. Naturally induced airflow is an Elizalde and Mumovic [12] highlighted the effect of urban struc- important parameter among all the other parameters influencing tures on the micro–macro climatic conditions. The wind condition the design and architecture of a building. in an urban environment determines the potential for natural ventilation for buildings. Therefore, defining and controlling the wind flow in built-up areas are critical factors for the design of a climat- 3. Micro climatic variation ically responsive structure. Generally, wind speed is reduced by 10–20% in urban regions, although high wind speeds can occur Roaf [10] investigated different types and functions of wind tow- due to urban canyons which channels and accelerates the wind ers which varied significantly from one region to another, depending flow. The main urban design elements which can modify the wind on factors such as micro–macro climate conditions and occupant’s conditions are: the overall density of the urban area, size and comfort expectations. Microclimate refers to the specific climatic height of the individual buildings, existence of high-rise buildings, condition in a relatively small area, such as a room or an enclosed orientation of the streets, availability, size distribution, and design space which can be controlled and modified. The atmospheric con- details of open spaces [13]. ditions in a microclimate; wind, temperature, humidity may differ from the conditions existing over the areas that surround it. Macro- climate is the climatic characteristic of a large region which sur- 4. Design of wind tower systems rounds the microclimate as shown in Fig. 2. It is essential to understand the local weather patterns when designing a wind tower The design of wind tower system has been traditionally based on system in order for it to relate to and gain from its environment. the topography, climatic conditions, personal experience of archi- There are several factors that can influence the micro and macro tects, social position of the occupants and variation in height, climate conditions such as land topography, time of operation, cross-section of air channel, number of openings, size and position- built environment and surface of the earth. In general, wind speeds ing of openings, form, construction materials, and placement of the increases with altitude as surface friction is diminished. Therefore, tower with respect to the building. Montazeri et al. [14] stated that taller wind towers will have stronger wind passing over it, hence a the efficiency of wind towers is reliant upon creating the maximum greater negative pressure. The topography of the earth’s surface pressure difference between the air inlet openings and exhaust of can greatly influence the micro climate, altering the direction of the passive device. The air movement around the structure will the winds. determine the size, location and form of the wind tower and its openings, so as to maximize the pressure differential. The following section reviews the application and design parameters of wind tower systems and demonstrates how these Macro climate devices can be adapted and improved in order to provide a simple and effective means of ventilation. Different methods of assessing Wind Tower the detailed design of wind tower systems have been used such as experimental wind tunnel and smoke visualization testing, ana- Micro climate lytical, and numerical modeling.

4.1. Sizing and positioning of openings

Fig. 2. CFD ﬂow domain representation of the micro and macro climate The accurate sizing and positioning of the openings of wind environment. tower contributes to its cooling operation. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 609

Fig. 4. CFD analysis showing the positive pressure on wind ward façade and negative pressure on leeward.

Wind tower inlet openings are provided in the direction of the 5–33 meters. Wind towers in the hot and dry areas were essen- prevailing wind and the outlet on the opposite side, to take advan- tially tall because in these areas, the air at higher altitudes contains tage of the pressure difference created by wind speed and direc- less dust and pollution. In dense urban areas where surrounding tion. The wind tower is most effective when located at the wind buildings obstruct free stream air flow, wind towers have to be ward edge of the roof where the positive pressure is greatest and very high to be able to catch enough air. However, taller towers less effective at the leeward side as shown in Fig. 4. However, are expensive to build and maintain. the ideal position for a multi-directional wind tower is therefore Fig. 5a shows the pressure difference across a building. In this at the centre of the structure. The opening size depends on factors case, the ideal position for inlet openings is along the wind ward such as location, topography and required air flow rate. The system side where positive pressure is greatest. Fig. 5b demonstrates the is based on Bernoulli effect, the air flow increases as it flows positive pressure against the vertical facade increases with height. through a smaller cross section. For indoor air movement caused by a pressure differential, the buoyancy induced flow is steadier resulting from low air pressure 4.3. Uni-directional and multi-directional wind towers than on the high air pressure caused by wind induced flows. An inlet opening will not produce the desired air movement in an enclosed Wind towers can be divided into one-sided, two-sided, four space unless an air outlet is also present. Previous research has sided, hexahedral, and octahedral both of the shaping and regional shown that movement of indoor air is faster and steadier when the point of view. It is essential that a natural ventilation system will area of outlet openings of the structure is larger than the inlet. perform well for all wind conditions. A’zami [17] suggested that structures with multi-directional wind towers are often built in conditions where there is no predictable prevailing wind direction. 4.2. Wind tower height Multi-directional wind towers are divided by partitions to create different shafts. One of the shafts functions as inlet to supply the The extrusion of the wind tower creates the same effect as the wind and the other shafts works as outlet to extract the warm structure form by obstructing the wind current and generating a and stale air out of the living space. The temperature difference be- lower pressure over the opening. The wind tower must be high en- tween the micro and macro climate creates different pressures and ough above the structure to prevent roof top turbulence. The result in air currents. height of the wind tower (distance from air entrance to discharge In general, the induced air flow rate decreases by increasing the point) also affects the ventilation rate. Taller wind towers will have number of opening. A one or two-sided wind catcher will induce stronger wind passing over it, hence a greater negative pressure. more air into the room at zero wind angles. On the other hand, However, the elevation of the tower must be weighted up against the sensitivity of wind tower against the incident wind angle de- aesthetic concerns and planning limitations. Gage and Graham [16] creases by increasing the number of openings. The optimum angle described wind towers as tall structures which generally rise up to in which every wind tower model induces the highest volume of

Fig. 5. CFD analysis showing (a) negative pressure over the roof (b) positive pressure on the wind ward side of the wind tower and negative on the leeward. 610 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627

Fig. 6. Traditional wind towers with different number of openings (a) one-sided, (b) two-sided, (c) four-sided, (d) octahedral.

wind [18]. The uni-directional device will not function when the incident wind blows from a direction other than its design range unless the air inlet is at a sufficiently high level for the buoyancy forces to overcome the unfavorable wind conditions. In this case, the tower will function as a solar chimney and the airflow will be reversed. Detailed metrological data is available for most areas and this will give wind speed and direction. However, the wind blows from directions other than the prevailing directions for a significant amount of time. In urban areas, local airflows can be significantly different from prevailing directions because it is dependent on the surrounding buildings which may be erected or demolished. Fig. 7 shows a structure with a uni-directional tower using cross-flow ventilation for cooling. The air enters from its single opening and passes through the living space and exits the window, Fig. 7. A flow diagram representing ventilation through a room with a one-sided door and exhaust vents. The uni-directional device can provide wind tower. wind assisted displacement ventilation when the wind is blowing from the design direction. air is the angle in which there is the most effective area exposing to Montazeri and Azizian [18] evaluated the performance of a one- the wind current [1]. Fig. 6 illustrates different configurations of sided wind catcher using experimental wind tunnel and smoke wind tower systems in Yazd, Iran. visualization testing. A 0.7 Â 0.4 Â 1.4 m scale model of an ancient Montazeri and Azizian [1] investigated the natural ventilation wind tower was mounted to a test room which was installed below performance of various multi-opening wind towers using experi- the base of the wind tunnel device. The induced airflow rate into mental wind tunnel and smoke visualization testing as well as the test room and the pressure coefficients around all surfaces of computational fluid dynamics (CFD) modeling. The study demon- its channel were measured for different values of approaching air strated the effect of the number of openings on the hydrodynamic incident angles. Neglecting the dependence of discharge coefficient performance of wind tower systems. Five cylindrical scale models on the flow direction, natural ventilation performance of one-sided with similar cross-sectional areas and vertical height were divided wind catcher was estimated using a non-dimensional analytical internally into several sections; two-sided, three-sided, four-sided model. The result showed that the existence of separation zones and twelve sided configurations. The achieved experimental and and wake regions near the lower edge of the opening causes the numerical results showed that increasing the number of the wind variation of pressure coefficient at the tower’s entrance. As a result, tower openings reduces the induced internal air flow rate. How- the wind tower could not achieve its maximum efficiency. ever, increasing the number of openings can reduce the sensitivity The work also investigated the effect of locating the wind tower of the natural ventilation device against the wind angle. It was in the wake of an upstream object which simulates the condition in found that the two-sided wind catcher delivers the most amount urban areas where the air flow is obstructed by neighboring build- of air supply into the room at zero air incidence angle. ings. For an isolated wind tower model, it was found that the highest efficiency is attained at zero air incident angles. Placing a 4.3.1. One-sided wind tower shorter upstream object before the wind tower reduces the circu- One-sided wind towers are built in many regions of the Middle lation region at the entrance opening and the area of lower edges East, predominantly on houses in areas where there is a prevailing as shown in Fig. 8. This increases the effective inlet area of the

Fig. 8. Smoke visualization testing, for (a) isolated wind catcher model and (b) placing under the short upstream object [19]. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 611 wind tower and signiﬁcantly increases the ventilation capacity of the wind tower. For a taller upstream object, the opening of the tower lies in the wake region of the upwind model and the uni- directional tower acts as a suction device. The work concluded that the one-sided wind catcher has the potential to be an effective ventilation design for urban setting.

4.3.2. Two-sided wind tower A two-sided wind tower has two opening and two separate underneath channels and is often used in areas where there is a strong prevailing wind. The wind catcher is divided into two halves for the purpose of air supply and extraction as shown in Fig. 9. The main advantage of a two-sided wind catcher over the uni-directional wind tower is related to the angle in which the uni-directional tower opening exposed under the transition angle and the airflow rate Fig. 10. A flow diagram representing ventilation through a structure with a four- sided wind tower. through it tends to zero. For the same wind incident angles, maximum air flow rate occurs at the transition angle of the leeward side more flexibility against the wind [17]. The top part of the tower of a two-sided wind tower. A disadvantage of multi-directional wind may have vents on one, two or four sides that face the prevailing towers is air short circuiting. Short circuiting is a harmful phenom- wind direction to capture the wind from different direction and a enon in wind tower systems which causes the air to enter through pair of partition is placed diagonally across each other down its the supply opening and leaves through another without flowing in- length. It catches the predominant wind and directs the fresh air side the enclosed space. For a two-sided wind tower, there is no air flow down to the enclosed space. Concurrently, the warm and stale short-circuiting for lower air incident angles. However for higher air inside the building rises up and out the exhaust of the tower as incident angles, short-circuiting appears into the wind tower system shown in Fig. 10. and reaches its maximum value at 60° incident angle. Fig. 11 shows a four-sided wind tower oriented at 0° and 45° into Montazeri et al. [14] evaluated the natural ventilation perfor- the prevailing wind. A smaller area is available to the incoming wind mance of a similar passive ventilation system but for a two-sided when the tower is oriented at 0°. Turbulence is created by the air wind tower device. Experimental wind tunnel testing smoke visual- flow moving down and striking the dividers at 45°. This slows down ization was used to analyze the effect of the pressure coefficient dis- the speed of the air moving down to the enclosed space below it. tributions at the wind catcher openings on the level of performance Three of the four quadrants are available to the stale air moving of the ventilation device. The work also developed numerical CFD out of the building due to negative pressure and stack effect. While and analytical models to validate the accuracy of the experimental the tower oriented at 45° into the prevailing wind has a larger area results. Good correlation between the different methods of analysis available to capture the wind. The air moves more directly down was observed. The achieved results showed that the pressure coef- the tower with less turbulence at its opening. In this case, two wind- ficient varies accordingly with the wind incidence angle. For a two- ward quadrants are available for air flowing into the tower and two sided wind catcher model, the maximum efficiency is achieved at leeward quadrants for the air flowing out of the tower. the air incidence angle of 90°. At this air incident angle the wind Gage and Graham [16] carried out experimental wind tunnel catcher efficiency increases approximately 20% more than the one testing to compare the performance of a four and six-sided wind at zero angle. The experimental investigations demonstrated the towers and analyze the effect of wind speed and direction on the de- potential of two-sided wind catcher for enhancing the natural ven- vice. The wind tower scale models were connected to a test room tilation inside residential buildings. The author also concluded that which is located below the wind tunnel. Three different configura- the one-sided wind tower is more effective for areas with a prevail- tions were used to study the air flow rate and pressure differences ing wind from economic and design point of view. inside the device. The first two experiments were a set of artificially controlled test; pump and blower devices were placed underneath 4.3.3. Four-sided and hexahedral wind towers the floors to allow the flow rate to be set independently of the exter- Studies have indicated that four-sided wind tower is the most nal wind. The final experiment was carried out with a direct flow used wind tower in the Middle East [20]. While the hexahedral through the device to simulate the conditions during normal opera- wind towers are limited and can be found in the water-reservoirs tion. The results confirmed that driving pressure for the wind tower of Iran. The six-sided wind tower is higher than any other wind is reduced as the flow rate flowing through the device is increased. towers and was designed in the form of a hexagon, so it will have The work concluded that in variable wind conditions, multi- directional wind towers with more than four openings have a more consistent and predictable and reliable performance. However, in areas with a predominant prevailing wind, the four sided wind tower will generate the highest pressure difference between the inlet and outlet openings when oriented at 45° to the wind direction. It was also established that the airflow speed is significantly reduced as it enters the towers channel, approximately 80 to 88% lower than the external wind speed.

4.4. Rectangular and circular wind towers

Traditional wind towers in the Middle East are usually in the form of quadrilateral and regular polygons. The cross-section of wind Fig. 9. A ﬂow diagram representing ventilation through a room with a two-sided towers may be of any shape, although it is important to maximize wind tower. the pressure drops on the leeward side (exhaust) and so existing 612 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627

(a) (b)

Fig. 11. Four-sided wind tower (a) with the incident angle at 0° (b) with the incident angle at 45°. commercial designs are either circular or rectangular (Fig. 12). How- ventilation rate decreases with the increase in the wind direction ever, experimental studies have shown that a wind tower of a rect- angles from 0° to 45°, measured from the normal to the face of angular cross-section surpasses the performance of other designs. the device. The achieved results showed that the square two-sided Elmualim and Awbi [23] compared the natural ventilation per- wind tower provided a higher efficiency (13%) compared to a tower formance of wind towers with square and circular cross-sections with a circular cross-section for the same external wind speed. The using experimental and numerical analysis. Wind tunnel testing author claimed that this was a result of the aerodynamic shape of was carried out on a full size scale model based on the design of the square wind tower. The sharp edges of the square wind tower a commercial wind tower device. The study analyzes the pressure create a large region of flow separation and a higher pressure dif- distribution, internal wind velocity and volumetric air flow rate ference across the device as shown in Fig. 13. achieve by the device for varying wind speeds and directions. The wind tower device was also subject to numerical CFD analysis 5. Wind tower cooling techniques to validate the accuracy of the experimental results. Good correlation between both methods of analysis was observed. Naturally ventilated buildings do not require additional energy to The performance of both devices was found to be primarily move the airflow within a structure. However, the cooling capabilities dependent on the speed and direction of the prevailing wind. The of conventional wind towers which depend on the structure design

Fig. 12. Roof mounted circular and square wind tower systems [21,22].

Fig. 13. CFD analysis showing the pressure difference across a (a) circular wind tower and (b) square wind tower. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 613 itself are limited. Therefore it is essential to cool the air in order to im- The ambient air is passed through wetted columns or over prove the thermal comfort of its occupants [24]. This section looks at underground water streams before entering the structure. The cooling techniques which can be incorporated to a standard wind evaporative cooling capability of a wind tower combined with tower design to improve its ventilation and thermal performance. the high airflow rates can be fully utilized in the summer to reduce Fig. 14 shows a concept design of a wind tower system integrated the internal temperature load and provide a greater thermal com- with cooling devices. Evaporative cooling pads sit at the top of a wind fort for the inhabitants. tower with pump re-circulating water over them. Hot air is passed through these pads and cooled by the water evaporation. Cool moist 5.1.1. Wind tower with wetted column and wetted surface air is denser than ambient air and sinks down the tower and into the Wind towers equipped with wetted columns or wetted surfaces enclosed space. In order for the cool air to flow in, hot air must be re- improves the ventilation and thermal performance of the passive leased. Solar chimney is located directly opposite the wind tower to device and overcomes the limitations of the conventional design. establish effective cross-flow ventilation inside the structure and ex- These towers can be employed in the hot arid regions and provide haust the stored hot air using buoyancy-driven forces. great saving in the electrical energy consumed for the summer cooling of buildings [27]. The evaporative cooling systems pre-cool the 5.1. Evaporative cooling external air before admitting it into the structure. The cooled air be- comes denser than ambient air and sinks down the tower. Hence, air Passive evaporative cooling is a traditional method used in old loss through other tower openings will be reduced. Wind towers Middle Eastern buildings to improve its natural ventilation and incorporating wetted columns are equipped with cloth curtains or thermal performance. This type of cooling is particularly effective clay conduits, spaced 5–10 cm from each other and hanging verti- in relatively dry and humid climates [26]. External air is cooled cally inside the column as shown in Fig. 16. The curtains are wetted down to its dew point temperature by saturating it with moisture. by spraying drops of water through a nozzle system at the top of the

Fig. 14. Concept design of a passive wind tower integrated with different cooling devices [25].

34°C 344°C

222°C 220°C 223°C

Fig. 15. Thermal performance of a wind tower incorporating evaporative cooling devices. 614 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 tower. Wind towers with wetted surfaces are equipped with evapo- number of conduits (small sized partitions) was found to be more rative cooling pads at the entrance of the wind tower. Similarly the efficient than increasing the height of the wetted column of the cooling pads are wetted by spraying water on top of the device. cooling tower. The cooling tower integrated with wetted interior The cooling system is particularly suitable in arid regions with good surfaces was able to reduce the indoor air temperature by up to winds. 17.6 K depending on the height, diameter of the conduit partitions Bahadori [24] evaluated the thermal performance of two novel and climatic conditions. Bouchahm’s research concluded that wind designs of cooling tower systems using experimental testing. The towers can provide a fresh supply of air and improve the thermal two designs were one with wetted columns, equipped with cloth comfort of the inhabitants regardless of the extreme external con- curtains suspended in the tower and one with wetted surfaces was ditions. The work demonstrated the significance of passive cooling equipped with evaporative cooling pads at the entrance. The exper- towers and its potential as an alternative to the more prevalent imental results showed that the air entering the living space through mechanical ventilation systems. the wind tower with evaporative cooling systems were much lower Badran [29] also investigated the performance of an evaporative than the air temperature exiting the conventional wind tower. The cooling wind tower system but chose to measure the air flow rates results established that the tower with wetted column was more and internal temperature for a multi-directional tower. A mathe- effective during high wind conditions while the tower with wetted matical model was developed to analyze the condition of air passing surfaces was more effective during low wind conditions. The work through the evaporative cooling column of the tower for different concluded that integrating cooling device to the conventional wind external conditions. Similarly, clay conduits were installed inside tower system has proved successful, with the air exiting the towers the tower’s channel to cool the passing airflow before inducing it in- at a significantly lower temperature than the external air. However, side the structure as shown in Fig. 16. During the night, the ambient small reduction in airflow movement was observed inside the cool- air coolness is stored in the conduits mass to let it function during ing tower. Fig. 15 shows the evaporative cooling methods analyzed the day. The results showed that a 0.57 Â 0.57 m evaporative cooling in the experiment. It was observed that the highest temperature tower with a vertical height of 4 m can generate an airflow of reduction was achieved by employing evaporative cooling pads in 0.3 m3/s and reduce the internal temperature by 11 K which is the wind tower system. equivalent to the capacity of a 1 ton refrigeration system. Therefore, da Silva [28] evaluated the level of performance of passive evap- the author suggested that reducing the height of the wind tower orative cooling systems integrated to an auditorium building. A which generally reached up to 15 m can decrease the construction theoretical model was developed to predict the temperature and cost without having a noticeable decrease in performance. relative humidity of the air entering and leaving the structure Furthermore, Safari and Hosseinnia [15] used analytical and through the cooling towers. The model was also used to demon- numerical CFD modeling to investigate the thermal performance strate the influence of the physical parameters of the cooling de- of new designs of wind towers under different structural parameters vice on the thermal environment within the building. The results and external conditions. The novel wind tower design is equipped showed that the performance of the passive cooling system is with wetted curtains suspended inside the column of the cooling de- mainly dependent upon the evaporative cooling efficiency and vice, which are formed as surfaces that inject droplets of water at ex- number of cooling degree hours. The author concluded that the tremely low speeds. The multi-phase CFD model, based on the use of passive cooling methods in the summer is suitable in the Lagrangian–Eulerian approach was used to study the effect of the viewpoint of improving the internal thermal comfort and reducing diameter and temperature of the injected water droplets on the level the usage of conventional air-conditioning. of performance of the device. The achieved numerical results Bouchahm et al. [26] evaluated the ventilation and thermal per- showed that the wetted columns with the height of 10 m was able formance of a uni-directional wind tower system incorporated to a to reduce the internal air temperature by 12 °C and increase the rel- climatically adaptable house using experimental and theoretical ative humidity of air by 22%. The study also revealed that the de- methods of analysis. The purpose of this investigation was to as- crease in diameter of the injected water droplets results in the sess the potential of the evaporative cooling devices integrated to reduction of temperature of air leaving the wetted columns. The the passive ventilation system. Clay conduits were mounted inside small diameter droplets will form larger evaporation surface area the shaft of the uni-directional tower to improve the mass and heat which leads to better heat and mass transfer. The author also con- transfer and a water pool is situated at the bottom of the device to cluded that the change in the air temperature at the inlet of the wind increase the humidification process. The analytical model was val- tower has more significant influence in comparison with the varia- idated against the experimental measurements and a good agree- tion in the relative humidity. ment between the results was observed. The results confirmed that the airflow induced by the 0.75 Â 0.70 m tower has a direct 5.1.2. Wind towers integrated with ground cooling effect on the reduction of internal temperature. Increasing the Passive evaporative cooling is a traditional method used in old Middle Eastern buildings to improve its ventilation and thermal performance. An example of this is the Iranian traditional wind tower which is used in conjunction with a qanat or underground water system. The wind tower is positioned above the house with its opening facing away from the direction of the prevailing wind. The airflow across the vertical shaft generates a lower pressure at the leeward side of the structure. As a result, cool air is drawn from the qanat tunnel to replace the released warm and stale air. The cool air from the qanat is drawn into the tunnel at some distance away from the structure as shown in Fig. 17. The hot air is passed through the cooled tunnel wall (several meters beneath the ground, the earth stays continuously cool) and water stream running through the qanat, giving up its latent heat of evaporation as water evaporates into the air [32]. Hence, the air is relatively cooler when it reaches the rooms with the water vapor from the Fig. 16. Wind tower integrated with wetted columns or clay conduits. qanat having an added cooling effect. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 615

Early studies of cooling system were carried out for traditional houses built in Iran. Boustani [30] described in details the qanat integrated with wind towers which dates from the tenth century. In dry desert climates the combined system can result in a greater than 15 K reduction in the air temperature coming from the qanat. The mixture of air from the underground cooling system and the tower is circulated throughout the basement and into the rooms, enhancing the occupant’s thermal comfort.

5.2. Wind towers with solar chimney

Solar or thermal chimneys are used to enhance natural ventilation through stack effects for exhausts at purposefully designed exits with the effect of thermally induced ventilation in buildings. The air flow rate through the solar chimney is influenced mainly by a pressure differential between inlet and outlet, caused by thermal gradients (naturally-driven convection) and the incident wind (forced convection). Forced convection takes place when the flow is induced by an external force such as the negative pressure gener- Fig. 18. Solar-wind tower system integrated to a naturally ventilated building. ated by the wind current at the outlet of the tower (venture effect). Natural or buoyancy flow convection occurs when the air is driven by the presence of a temperature gradient. Solar energy heats the Reynolds k–w turbulence model proved to be the most effective wall of the solar chimney and warms the air within it. Consequently, model for this type of analysis. warm air rises and exits at the top of the tower (air updraft) and The numerical results confirmed that for a positive wind veloc- draws cool air in through the openings or vents [33]. ity (windward side exposed to the prevailing wind) of 2–3 m/s, the The basic design of a solar chimney is composed of three main wind driven force was the primary driving force for the device, components: the solar collector area, ventilation shaft and the inlet obtaining significantly higher values of induced mass flow rate and outlet openings. The solar collector is located in the top part of compared to buoyancy induced flow. However, for lower values the solar chimney or can include the entire shaft as shown in of wind velocity, a combined buoyancy-wind induced flow was ob- Fig. 18. The orientation, type of glazing, insulation and thermal served inside the thermal chimney. For a negative wind velocity properties of the collector are essential for harnessing, storing (leeward side facing the prevailing wind), the mass flow rate be- and utilizing the solar gains. The vertical shaft connects the interior comes negative through the solar chimney. As a result, external and exterior of the building. The height, cross-sectional area and air comes in through the upper opening of the chimney and gener- thermal properties of the ventilation shaft can also affects the per- ates pockets of reverse air flow at the top. The pressure analysis formance of the passive device. showed that for positive values of wind velocity, outlet suction of Different methods have been used for assessing the ventilation the chimney generates substantial suction relative to the velocity and thermal performance of a solar chimney such as experimental pressure of wind and overpressure was observed for negative val- and field testing, analytical, and numerical modeling. Zamora and ues of wind velocity. The results obtained for the average Nusselt Kaiser [34] studied the effect of wind and buoyancy induced air number also confirmed that the wind driving force was dominant flows on the ventilation performance of a building incorporating for wind velocities of 2–3 m/s and wind suction effects were signif- a solar chimney. The work developed a mathematical and CFD icant even at low values of wind velocity. model to analyze the induced mass flow rate, pressure coefficients The use of solar chimney and its potential benefits regarding and average Nusselt number of the air flow within the updraft natural ventilation are widely investigated over the recent years. tower for varying wind velocities (0–10 m/s). Different values of However, majority of the studies have dealt with situations based Rayleigh number were applied to simulate the heating conditions on the buoyancy induced flow of air. The concept of a solar chim- of the channel wall (isothermal and uniform heat flux). The ney coupled with a wind tower to induce natural ventilation has

Fig. 17. Wind tower systems integrated with underground cooling. 616 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 been studied analytically and numerically in several works. Several In most conditions, the ambient air is only adequately cold dur- works have reported on the feasibility of solar chimneys integrated ing night-time. Hollmuller et al. [39] recommended that a storage with wind towers for enhancing the ventilation rates. Wind tower medium may be incorporated to allow the ventilation system to be systems integrated with solar chimneys are capable of enhancing use during the day and the coolness created during the previous the cross flow ventilation and providing natural ventilation on night. The coolness which is stored for use to equalize heat loads hot windless days. is relative to the storage capacity of the structure. The amount of Nouanégué et al. [35] carried out a numerical investigation of thermal mass as tower has controls how much coolness it can solar-wind tower systems integrated to a passive house. The work store, and how rapidly it heats up the next day. Towers with low focused on establishing the governing parameters (constant thermal mass cannot store much coolness and heat up very Prandtl, Reynolds and Rayleigh number) and geometrical parame- quickly, so stop cooling morning air quickly. The major disadvan- ters (dimensional shape, outlet size, wall thickness and conductiv- tage of structural night ventilation is the limited controllability ity) influencing the ventilation and thermal performance of the and slowness of the charge/discharge process as surface heat trans- solar chimney. Simplified algorithm and control volume methods fer relies on natural convection. were applied to solve the equations for the conservation of mass, Wang et al. [40] investigated the feasibility of night ventilation momentum and energy. The effect of wall thickness and outlet size control strategy in office buildings. EnergyPlus software was used was found to be significant on the system’s ventilation perfor- to simulate the internal thermal conditions and energy consumption mance for high Reynolds and Rayleigh numbers. The numerical in a office buildings integrated with night ventilation. The study also study indicated that the solar-wind tower system achieved maxi- analyzed the factors influencing the performance of night ventila- mum performance for forced convection heat transfers, which is tion such as ventilation rates, thermal mass and weather conditions. a result of negative air pressure generated at the outlet. Strong con- The study concluded that the efficiency of night ventilation strategy vection with some reverse flow at the outlet was observed and the is higher when the active cooling time is nearly equivalent to the cooling of the heated surface is effective with high temperature night ventilation operation time. It was found that the mean radiant gradients on the surface. However, this would reduce the heat temperature of the interiors was reduced up to 3.9 °C with the night transfer at the outlet. The work concluded that Richardson and ventilation rate of 10 ACH. Rayleigh numbers are major parameters influencing the performance of the solar-wind tower system. The author also suggested 5.4. Wind towers integrated with courtyards that geometrical parameters can be optimized to obtain the highest ventilation performance. The traditional vernacular style of architecture in the Middle Furthermore, Bansal et al. [32] used a mathematical model to East is influenced mainly by the local climate, culture and the calculate the performance of a wind tower system integrated with availability of the building materials. Houses are constructed close a solar chimney. The work aimed to predict the results based on to each other, with high walls, creating narrow alleys in between the proposed energy balance equation of the solar chimney and the structures which provided shading for the inhabitants through- air flow rate equations. The study also confirmed that the thermal out the day. The rooms with large windows are built around an performance of the solar chimney was comparatively higher for open courtyard, allowing the wind to circulate freely throughout lower incident winds. The result showed that the solar chimney the room and provide daylight. The courtyard is exposed to the so- can increase the mass flow rate of induced air by up to 50% for lar radiation for long hours. Hence, the air in the courtyard be- the case of high incident solar radiation and low wind speeds. comes warmer and rises up due to buoyancy forces. To replace it, The solar chimney integrated with a wind tower was able to gen- cool make up air from the ground level flows through the openings erate airflow up to 1.4 kg/s which doubles that of a single wind of the living spaces, thus creating the indoor air flow as shown in tower producing only up to 0.75 kg/s. The author concluded that Fig. 19. the effect of the solar system is more significant than that of the During the night, the cooling process is reversed. The cooled wind tower and combining both systems will enhance the ventila- surface air sinks down to the courtyard and enter the rooms tion rates by increasing the mass flow rate of induced air. through the low level openings and leaves through higher level openings. This system can work effectively in hot and dry climates, where day time ventilation is undesirable. However, when the 5.3. Structural night ventilation courtyard receives extreme solar radiation, much heat will be conducted and radiated into the living spaces as against the induced Hughes et al. [36] defined structural night ventilation as a pas- draft of air which will significantly reduce the efficiency of the de- sive cooling strategy that relies solely on wind-driven or buoyancy sign configuration [41]. forces. The thermal comfort during the day is provided by cooling Traditional courtyards are also integrated with evaporative the internal surface of the wind tower channel during the night cooling towers which contributed significantly in enhancing the time, which results in heat absorption during the day time. Night thermal conditions within the courtyard and enclosed spaces. This time coolness stored in walls and partitions of the wind tower proved to be an effective method in creating an enclosed space that cools the air induced during the day, making it denser so it sinks has unique environmental qualities within extreme climatic condi- down through the base of the tower and into the structure. Natu- tions. The courtyard is turned into a thermal sink that provides rally, the tower heats up itself as the tower cools the air passing coolness to the rooms around it with less humidity and suitable through it. The cooling effect of the tower is lost when the temper- place for creating a comfortable environment [42]. ature of the thermal mass reaches the same temperature as the Sharples and Bensalem [43] investigated the air flow pattern ambient air. Bahadori [37] suggested that wind tower systems through a courtyard and atrium buildings located within an urban may use shaft dividers which are arranged to provide more sur- setting. The study used experimental wind tunnel testing to ana- faces in contact with the flowing air, so that the air can interact lyze different ventilation strategies resulting from the use of differ- thermally with the heat stored in the mass of the shaft dividers. ent courtyard and atrium pressure systems (positive pressure and Ghaemmaghami and Mahmoudi [38] described the shaft dividers suction). The model structures were observes both in isolation and used in the traditional wind towers as thermal sinks made of in simulated urban settings of different group layout densities. The mud bricks which functioned like radiator fins, absorbing the heat effect of wind direction on the ventilation performance was also during the day and releasing the stored heat during the night. observed. The research found that the open courtyard in an urban B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 617

Fig. 19. vernacular structures integrated with a central courtyard and wind towers. environment had a poor ventilation performance while an atrium Asfour and Gadi [11] carried out a numerical CFD investigation roof with openings operating under a negative pressure was more of the effect of integrating wind towers with curved roofs. A scale effective. model was developed and simulated for varying wind incident angle (0°,45°, and 90°) and different wind speeds. The study focused on improving the ventilation rates and air flow distribution inside 5.5. Wind towers integrated with curved roofs building (upstream and downstream zones) with curved roofs by introducing a passive ventilation device. The results showed that In hot and arid region, curved or domed roofs are usually chosen locating the wind tower in the middle of the building windward over flat roofs. Curved roofs have a much greater surface area and façade improves the ventilation performance of the curved roofs thus a larger surface to release heat from. Therefore, curved roof by inducing air by suction. Deep-plan buildings incorporating suc- cools much faster than flat roofs. In general, the passive roof is tion wind towers also allow the air to effectively penetrate the positioned centrally at the top of the structure. Warm air inside downstream area of the structure. The study confirmed that the the building rises up to the dome (buoyancy) and exits through proposed configuration was successful in increasing the air flow the curved roof vents. The increase in velocity of the air flowing rates and improving the air flow distribution inside the building. over the curved roof lowers the external pressure and draws the hot air out from the dome, then the cooler air enters the windows or wind tower openings. The design of the curved roof or dome is 6. Traditional wind tower systems usually defined by the outdoor conditions it is subjected to such as wind patterns. A dome roof with opening at the top is more effec- The traditional architecture of Central Asia and the Middle East tive when the wind is blowing from different direction [44,45]. The is the product of the land topography, the climatic conditions, so- vault and curved roof are more effective in areas with a single pre- cial and culture. The vernacular architecture has brought many vailing wind during hot season. practical solutions and strategies to solve the human’s needs and Curved and domed roofs have been used in many Middle East- local environmental problems such as the wind tower, which is a ern houses for many centuries. Integrating the dome with other key element in the architectural design of traditional structures. natural ventilation device such as windows and wind towers could The wind tower design is based on the natural cooling systems increase the effectiveness of its suction ventilation. In most cases, in Iranian traditional architecture [47]. Almost all historic build- wind towers have been incorporated with these passive ventilation ings were naturally ventilated. Domed roof and air vent systems roofs to improve the ventilation rates. Fathy [2] has highlighted the were incorporated in building systems as early as 3000 BC in Iran. function of the vernacular architecture elements of roofs and tow- Some of the most significant, examples of traditional building sys- ers found in many types of buildings situated in hot and arid re- tems utilizing both wind and the stack effect for natural ventilation gions. Wind towers were positioned on top of room located on can be found in the hot regions of the Middle East. the northern façade of the structure. Incident air enters the open- Traditional wind towers are essentially tall structure with a ing of the tower and slowly passes through the room, before it height between 5 and 33 m mounted on top a building. It has ver- accelerates towards the central part of the building. The central tical openings on top which captures wind at high elevations and hall is enclosed by a curved or domed roof with opening vents at directs the air flow into the interior living spaces of the building. its base. The warmer air expands and rises up to the curved roof This structure extracts and supplies air into the buildings using due to buoyancy as shown in Fig. 20. The low pressure air current ventilation principles of wind tower and the stack effect. Tradi- outside the dome pulls out the stale air collected inside the dome tional wind towers are integrated with courtyards, domed roofs through the external vents (high pressure to low pressure zone) and evaporative cooling systems to reduce the heat during the [46]. hot summer months. There are many different types of wind 618 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627

Fig. 20. Vernacular architecture integrated with a domed roof and multi-sided wind tower. towers, whose function and form are based on the climatic conditions of the region it is situated in. The most common traditional wind towers are the air escape, malkaf or uni-directional wind tower and the badgir or multi-directional wind tower.

6.1. Wind escapes and air vents

The technique of using the suction caused by low air-pressure zones to generate steady air movement indoors is used in the design of the wind escape. Fig. 21 shows the funnel and side tube illustrating the Bernoulli or Venturi effect which is used accelerate Fig. 21. Funnel with a side tube to illustrate the Bernoulli effect. air movement and to create drafts in areas with no exposure to the exterior, such as the basements. The wind escape system is based on the Bernoulli effect, which increases the air velocity as it is forced through a reduced area [2]. This concept can be applied more advantageously in designs for use above ground as shown in Fig. 22. The wind escape can accelerate effective natural ventilation and air circulation when integrated with other devices for air movement such as windows, doors, and malkaf wind tower.

6.2. The malkaf

The malkaf or uni-directional wind tower is a shaft mounted on top of the building with an opening facing the prevailing wind. The passive device traps the wind from high above the building and channels it down into the interior rooms. The malkaf tower is more effective when employed in dense cities with warm and humid cli- Fig. 22. Flow diagram representing ventilation through a structure with a wind mates, where thermal comfort relies mostly on the movement of escape and air vent. air. Dense development in urban area reduces the wind speed at street-level, making windows insufficient for ventilation. The mal- wall to opening ratio of 0.6 and external wind speed of 2 m/s. kaf can then be used to ensure indoor air ventilation without While the combined system obtained up to 5.6 ACH with the same depending on ordinary windows [48]. opening ratio and external conditions. The results established that Attia and Herde [48] investigated the ventilation performance maximum indoor air flow pattern can be achieved by sizing and of malkaf wind towers used in low rise buildings for summer positioning the inlet and outlet of the malkaf as large and high as cooling. Experimental wind tunnel and smoke visualization testing possible. were carried out to evaluate the air flow rate and distribution in a The malkaf wind tower can be combined with an air escape to scale model test room integrated with a malkaf wind tower. Two increase the cross-flow ventilation. The warm air stored inside different design configurations was used for the experimental the building is drawn out through the wind escape by suction analysis, one with a single uni-directional malkaf tower facing and the indoor airflow movement is accelerated. The airflow rate the prevailing wind and the other with two malkaf towers placed is directly proportional to the size and elevation of the inlet and at both end of the structure (inlet and outlet). Both device outlet openings. The external conditions determine the size of configurations proved to be effective in improving the air flow the malkaf tower. Larger malkafs are used in areas with low ambi- distribution inside the building. The first configuration with a ent air temperature while smaller malkafs were employed in areas single uni-directional tower obtained up to 4 ACH with an outlet with high external temperatures [48]. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 619

Fig. 23 illustrates the malkaf wind tower placed directly over a windows openings and doors with negative or lower values of roof opening of a vernacular architectural design. The malkaf traps pressure coefficients. Multi-directional wind towers can also lower the cool breeze and channels it down into the lower floor which is the indoor temperature by removing the hot air stored inside the a result of the increased air pressure at the entrance of the tower. structure through its exhaust openings [49]. The wind at higher elevations contains less dust particles and The vernacular architecture of Iran is defined by its specific cli- much of the sand that enters the structures is accumulated at matic condition. Climate is the most effective factor that influences the base of the tower. Within the structure, the air slows down the design of architectural elements such as the badgir. The wind and flows through the central area, with some of the air escaping tower functions according to the condition of the wind and solar through the domed roof and some flowing directly to the outflow radiation in the region. Due to the geographical coordinates of re- vents of the air escape which has a lower air pressure. Maximum gion, varying wind speed and direction, traditional wind towers temperature difference between the lower and upper floors and were built with different height and level, cross-sectional area of strategic arrangement of the openings ensures air circulation even air passage, orientation, and positioning of inlet and outlet open- when the external air is at very low speeds. ings [50]. A’zami [17] investigated the designs of badgir wind towers in 6.3. Badgir wind tower traditional Iranian structures. The work examined and classified different types of wind towers based on its climatic function, loca- The badgir wind tower has been traditionally used in Iran and tion, size and construction materials used. It was found that each the countries of the Gulf to provide natural ventilation and passive region had a unique type of its own wind tower with a unique cooling in buildings. The badgir has a shaft with the top opening on range of tower height, dimension and orientation resulting from two or four sides, and with dividers placed diagonally across each local geomorphology, macro and micro climatic conditions, social other down its length to catch the wind coming from any direction. and traditions of a particular settlement. The study further high- The shaft extends down to a level that allows the air flow to reach lights the capability of the wind tower to create a temperature bal- the lower floors of the structure as shown in Fig. 24. The passive ance in the building during night and daytime. The walls exposed device captures the prevailing wind at higher elevations and in- to sun heats up the indoor air and rises up during the day. The air duces it into the building to maintain natural ventilation through released from the exhaust side of the wind tower is compensated the living spaces and provide cool air supply directly to the occu- by replacing it with cool air. While, cool night air sinks down to pants. This would allow the stale and warm air to exit through the structure and cools the internal walls which provides

Fig. 23. Vernacular architecture integrated with malkaf wind catchers and a domed roof.

Fig. 24. Flow diagram representing ventilation through a traditional structure incorporating badgir towers. 620 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 additional cooling the next day. The author concluded that strate- was achieved by running the numerical model with varying wind gic arrangement of the wind tower allows it to provide thermal velocity and two wind directions, concurrent and counter-current. comfort in hot regions by relying only on natural wind energy The achieved results demonstrated the potential of the modern and temperature differences. wind tower in delivering the required fresh air supply and provid- Kalantar [49] attempted to evaluate the ventilation and thermal ing a sustainable alternative ventilation system. However, at low performance of a badgir wind tower in the hot and arid region of external wind of 1 m/s the device barely supplies the minimum Yazd. The work developed a numerical CFD model to simulate ventilation rate, providing only 4 L/s per occupant which is lower and analyze the airflow pattern inside the wind tower in three- than the recommended minimum of 5 L/s per occupant. However dimensional and steady state conditions. The study also presents as the external velocity is increased the recommended rates are ex- a numerical technique to simulate the effect of integrating evapo- ceeded, reaching up to 23 L/s for an external wind velocity of 4 m/s. rative cooling systems to the wind towers performance. The effect It was also observed that the flow short-circuiting was reduced for of several design parameters such as wind speed, temperature, the counter-current flow model. This is a result of the increase in humidity and density were also considered. The result yielded a the delivery performance of the counter-current flow model by good correlation between the numerical simulations and experi- 5% compared to that of the concurrent flow model. mental data obtained from literature. It was found that the badgir Jones et al. [53] compared the performance of two natural ven- wind tower was able to reduce the air temperatures by 10 to 15 °C tilation devices employed in classrooms in the UK, wind towers at its optimum performance. and conventional windows. The study focused on assessing the potential of the commercial wind tower to replace the ventilation provided by windows. The performance of the device will be mea- 7. Commercial wind towers sured in terms of air quality supplied in the living space. The work further highlights the capability of the wind tower to provide night Modern architects and engineers integrated the principles of cooling without affecting the occupant’s security. The results traditional wind tower with modern technology as helpful devices showed that the classroom equipped with wind towers was able to increase the quality and efficiency of the supplied air. Modern to meet the UK Government requirements for carbon dioxide levels wind towers provide natural ventilation and light to any space in and thermal comfort, while the classroom relying only on windows a building. Hughes and Ghani [22] described a commercial wind failed to provide the fresh air delivery rates. Also, it was found that tower as a top-down roof mounted, multi directional device used night-time ventilation can reduce indoor temperatures by up to for naturally ventilating buildings. Modern wind towers are usu- 2.8 K. ally compact and smaller in size compared to the traditional wind Su et al. [54] used CFD modeling, experimental and far-field towers. The device extends out from the top of a structure to catch testing to evaluate the ventilation rate of a commercial wind tower the wind at roof level and channels fresh air through a series of device for different external wind speeds. A cone flow meter and a louvers into the enclosed space under the action of air pressure, blower fan was use to simulate the outdoor conditions and mea- and simultaneously the negative pressure extracts stale air out of sure the ventilation rates. CFD modeling of the windcatcher is car- the room. Unlike the traditional wind tower systems, air is sup- ried out to create the conditions similar to the situation of outdoor plied to the enclosed space through the diffusers located at ceiling far field wind. It was found that the calculated extract flow rate of level. Hence, more free space is available for ventilation on the ceil- the wind tower in a far field wind is approximately double that for ing than on the corresponding floor of equal area. the situation using a blower fan. The effect of the direction of the wind on the extract flow rate was found to be insignificant. The results also confirmed that the effect of buoyancy driven flows has a 7.1. Wind vent negligible effect on the flow rates of the wind tower at higher wind speed. The windvent is a commercially available natural ventilation Furthermore Jones and Kirby [8] used a semi-empirical model device. The device is constructed from sheet metal and works on to investigate the effect of the change of external wind speed the principle of pressure differential. Whereby warm air rises, cre- and direction on the performance of a similar wind tower device. ating a low pressure in the receiving room, which then draws in the fresh air. The windvent is divided into four quadrants, which allow fresh air to enter as well as stale air to escape irrespective of the direction of the wind. The device is equipped with volume control dampers and ceiling diffusers which allows the occupants to regulate the air flow depending on temperature requirement,

C02 level and air distribution inside the building. The windvent louver protects the interior from rainwater or snow, direct sunlight and noise entering the device while allowing the external air into the living space. Fig. 25 shows a windvent device incorporating solar powered internal fans. The solar fan can be used to overcome excessive heat gains and boost the air movement through the wind tower when extra ventilation is required. Hughes and Ghani [52] investigated the CFD modeling of a commercial wind tower device. The work focused on the effects of the varying external wind speed and direction on the capability of the passive ventilation device in providing fresh air at the recommended air delivery rates. The 1x1 m square wind tower was connected to a small room with a recommended occupancy figure of 20 occupants. The study further highlights the effect of the dampers on the airflow and its potential to limit the short-circuiting of the air movement which decreases the efficiency of the device. This Fig. 25. Commercial wind tower device integrated with PV-panels [52]. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 621

The approach combined a detailed analytical model with experi- reduced by 20 to 50% and the exhaust air flow rate by 29 to 33% mental data obtained from published works. at an external wind speed of 1 to 3 m/s. The study further high- The predicted results were validated against the measured and lights the effect of installing heat source inside the enclosed area, CFD data and a good correlation between the different methods of increasing the intake air flow rate by 7 to 54% and reducing the in- analysis were observed. The work concluded that the proposed door temperature by 6–8 K Therefore, it was concluded that the semi-empirical model was a simple and quicker method for esti- thermal performance of a wind tower is greatly dependent on mating the performance of a wind tower device. The study also the velocity and direction of the wind. highlighted the relationship between the buoyancy effects and Furthermore, Hughes and Ghani [56] investigated the perfor- the indoor ventilation rate. It was found that the effect of buoyancy mance of a similar wind catcher device integrated with a con- forces is insignificant at wind speeds higher than 1 m/s and inci- trolled damper system using computational fluid dynamics dence angle of 0°. However, the buoyancy effects became signifi- (CFD). The work demonstrated the effect of different damper an- cant when the incidence angle was increased to 45°. gles on the achieved velocity and pressure distributions inside the test model. The work simulated 19 different numerical models with the dampers rotated 5° for each test, for a range of 0 to 90°. 7.1.1. Damper control system The achieved numerical results confirmed that optimum oper- The wind catcher performance depends greatly on the direction ating range for the windvent dampers is of the range 45 to 55°.It and speed of the wind in respect to the wind tower’s quadrant. The was observed that increasing the angle of the dampers decreases air circulation and air change rate are determined by controlling the air velocity and increases pressure drop as shown in Fig. 27. the amount of air that passes through the wind tower channel. This is a result of the flow path restriction caused by the dampers Damper control system allows the occupants to reduce and regu- oriented at a certain angle. The author concluded that the inte- late the air flow depending on indoor and outdoor temperature, grated damper control system was able to control the air move- C02 level and air distribution inside the building. The volume con- ment effectively, thus sustaining a constant fresh air delivery trol dampers are either manually or automatically operated rate. The computational results showed a good correlation with through actuators, and provides the basis of the control mecha- the experiments conducted by Elmualim [55]. nisms for the natural ventilation device. Fig. 26 illustrates a series of adjustable dampers installed inside the wind tower at room ceiling level. 7.1.2. Louver configuration Modern wind catchers with automated volume control damper Wind tower louvers are horizontal blades mounted in air inlets systems, constantly control the airflow into and out of the occupied and exhausts of the ventilation device. The louver blades are angled space. This would allow the ventilation device to serve different against the prevailing wind to allow the external air into the en- conditions such as summer or winter and day-time or night-time closed space and prevent rainwater or snow, direct sunlight and cooling. In the hot summer months the control dampers are pro- noise from entering the device. The louver blade angle has an impor- grammed to fully open at night to maximize night-time cooling, tant role in the movement of air at the inlet and outlet of the venti- allowing the fresh and cool air to enter the building and extract lation device. A properly designed louver will minimize the wind the stale air out. Occupants may override the control settings at resistance and maximize the internal airflow rate. The number of any time using a wall-mounted override switch that will fully open layers and length of louvers are other design aspects that affect the or close the dampers. performance of the wind tower. Proper louver length should both Elmualim [55] studied the effect of volume control dampers on improve performance and reduce the manufacturing costs. Modern the performance of wind towers using experimental wind tunnel wind towers incorporate adjustable louver system which allows the testing and numerical CFD model. For the experimental analysis, louver blades to be raised and lowered to alter the size of the opening a 0.5 m x 0.5 m x 1.5 m four-sided wind tower scale model was according to the control strategy. Maximum air flow rate is achieved connected to a test room mounted below the wind tunnel. The when they are fully open. The system is able to modulate their posi- experimental results were then compared with that of a simulated tion to increase the resistance of the louver, or they can be fully shut wind tunnel configuration subject to CFD analysis. The approach to prevent the ingress of rain and snow, which any open louver sys- showed a good correlation between the two methods of analysis. tem is susceptible to [57]. The indoor air speed, pressure difference and air flow distribution Hughes and Ghani [6] used CFD to analyze the effect of varying was investigated using a model simulated for varying wind speeds the angle of the louvers on the velocity and pressure distributions and a single direction. The results showed that the integration of inside the wind tower’s channel and the space below it. For the CFD the dampers and diffusers was able to successfully reduce the air analysis, eight numerical models were developed with the louver flow inside the enclosed space. The supply air flow rate was angles rotated in 5° increments for a range of 10 to 45°. The study

Fig. 26. Modern wind tower devices with damper control system. 622 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627

Fig. 27. Effects on air velocity and pressure by damper angle variations [55]. also highlighted the effect of the louver stall angle on the performance of the wind tower. The louver was compared to a thin airfoil which was subjected to similar aerodynamic forces. The results demonstrated that for an external velocity of 4.5 m/s, the blade an- Fig. 29. Reference length of the windcatcher louver, Lreference = Lgap A–B/sina. gle of 35° provided the maximum air velocity inside the microclimate, as shown in Fig. 28. The research has also established the stall angle of the wind tower as 35–40°, at which the angle of at- 7.2. Solar fan-assisted wind vent tack goes beyond the critical value and the separation of flow occurs above the surface of the blades. Performance comparison of Passive natural ventilation devices supply fresh air into the results of the 35° louver with the standard 45° louver demon- interior of a building and exhaust stale air out without requiring strated a 45% increase in internal comfort level and 42% reduction mechanical or electrically driven components. However, airflow in trailing edge stall. rates achieved through standard wind towers often cannot meet Liu et al. [57] also used CFD to investigate different louver con- the required indoor ventilation rates due its sole dependence on figurations for a similar wind tower system. The worked focused the speed and direction of the wind. Hughes and Ghani [22] sug- on evaluating the ventilation performance of the device using dif- gested that a passive-assisted natural ventilation system may be ferent number of layers and length of louvers. The study showed employed to provide continuous supply of fresh air without affect- that increasing the number of louver layers increased the air flow ing the energy requirements of the wind tower. The hybrid system induced into the wind tower. The addition of 2 to 3 layers of lou- incorporates a low-powered fan installed inside the wind tower vers improved the airflow rate by 12.7%. However, less improve- with the fan functioning when required to assist the flow of air be- ment in air flow was observed after 6 or more layers, improving tween the building exterior and interior via the ventilator. The so- the airflow below 1.5%. This is a result of the flow short circuiting lar driven fan can also function as an exhaust device for extracting on the top layers. It was also observed that maximum indoor ven- stale air out of the building. It provides a constant supply of tilation rate was achieved once the length of louver equates with ventilation air, even when there is no wind. Zero energy solution the reference length as shown in Fig. 29. However, increasing it is guaranteed, allowing the building natural ventilation design to further than the reference length will reduce the airflow signifi- be optimized and ensuring a low energy natural ventilation strat- cantly and long louvers would increase the complexity of the de- egy to be maintained (see Fig. 30). sign and manufacturing cost. The study concluded that the Priyadarsini et al. [58] used experimental and CFD techniques to proposed louver configuration is effectively stimulating airflow in- investigate the feasibility of the application of an active (fan-as- side the structure. sisted) and passive stack device to improve the natural ventilation inside the enclosed space. The air movement along the passive L is the nominal length of louver, while Lreference is the louver length when the dimension of louver projection on the correspond- stacks ventilation system is achieved just by the temperature dif- ing quadrant equates with the gap between two adjacent louvers, ference or moisture in the air. The results showed that the increase in the internal air velocity of the active stack is proportional to the Lgap [57]. increase of the speed of the extract fan and size of stack. The integration of the active stack in the ventilation system resulted with the internal air velocities ranging from 0.26 to 0.69 m/s. A similar device was used for the testing of the passive stack but with the extract fan removed. It was found that the temperature difference inside the passive stack device was only 10 °C and low indoor air velocity was observed. Similarly, increasing the size of the passive stack enhances the air flow. However, larger stacks won’t be eco- nomical as it reduces the internal space in the building. The study concluded that the active stack system was significantly more effective in providing indoor ventilation than the passive stack. The study further highlights the remarkable energy efficiency of an active stack system with solar powered internal fans. Moreover, Hughes and Ghani [22] studied the feasibility of a passive-assisted natural ventilation stack device using computational fluid dynamics (CFD). A standard wind tower model combined with a simulated low powered axial flow fan was used for the numerical analysis. The CFD results demonstrated the effect Fig. 28. Effect of external louver angles on the wind tower performance [6]. of the fan in top, middle and bottom positions. It was observed that B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 623

Fig. 30. Schematic of a wind tower system integrated with a solar powered fan. the fan mounted on top, effectively draws fresh air flow and directs Although numerous studies have looked into the effects of dif- it down centrally through the wind tower channel and the space ferent configurations and components on the performance of wind beneath while allowing the stale air to exit on the sides of the towers, knowledge of the system remains incomplete due to its fan controlled air flow. While fans positioned in the middle and complexity and the lack of real world model development, study bottom position does not have a clear path for the exhaust air to of wind tower performance has generally been limited to con- exit the wind tower. Hence, fresh air supply rates were reduced. trolled experiments. It is important for the reliability of the tech- The numerical investigation confirmed that external wind speeds nology to be thoroughly evaluated in order to improve its of 1 m/s combined with the induced fan pressure of 20 Pa satisfied performance in the future. This then requires further research the minimum recommended fresh air requirements for residential and development works on innovative solutions, recognized test- buildings in the UK. ing procedures and standards.

8. Development potential of future systems 9. Results summary

In recent years, there has been an increasing awareness of the From the reviewed wind tower systems and cooling methods, it need for energy efficient and environmentally friendly approach is considered that each system has its own advantages and for future building design which has renewed emphasis on the limitations. Table 1 summarizes an outline of the traditional and application of wind tower devices. Experimental theoretical and modern wind tower systems under review along with the features, numerical investigations studies have been conducted to investi- application, limitations and typical flow rates obtained from previ- gate the performance of wind towers. The results proved that pas- ous related works. It is observed that the commercial wind vent sive air movement inside the building improves the air quality and device generated the highest air flow rates, supplying up to reduces the internal temperatures. This concept has been applied 650 l/s while the solar-wind tower which rely on buoyancy forces, commercially in the UK for at least 30 years. Over 5500 wind tower generated the lowest airflow rate. The application of a passive-as- systems have been installed in the UK, about 70% of the ventilation sisted ventilation device increases the wind vent airflow by up to units are being used to ventilate school classrooms and building four times. offices [8]. Recent advancements in the development of wind Table 2 summarized the different types of cooling methods un- tower system include integrated building principles and technol- der review along with the features, application, limitations and ogy such as the utilization of evaporative cooling columns to in- typical temperature reductions obtained from various case-stud- crease the cooling potential of the air stream, fully automatic and ies. The temperature reduction with the application of evaporative programmable dampers and diffusers to control the volumetric cooling methods such as wetter columns, wetted surfaces and flow rate, solar driven fans to assist the wind tower in times or underground cooling are found to be in the range of 11–15 K. areas of little wind speed, and different types of sensors to adjust air temperature, wind movement, noise, humidity depending upon 10. Discussion the specific requirement of the space. At present, unresolved areas in the application of wind tower Experimental, theoretical and numerical investigations of the systems include the need for extensive data on the application of performance of different types of wind tower systems have been re- the subject in extreme temperature conditions including informa- ported in the literature. However, the study of wind tower perfor- tion regarding humidity control measures using wind tower sys- mance has generally been limited to controlled experiments. tems. Moreover, data available on the effectiveness of installing a Hence, it is difficult to compare the performance of the wind tower commercial wind tower system involving cooling devices within systems. Montazeri [1] and Gage and Graham [16] used wind the terminal of the device to improve its thermal performance is tunnel and smoke visualization analysis to analyze the ventilation minimal. There is a requirement for complete and definitive infor- performance of multi-directional wind towers. Elmualim and Awbi mation on the application of solar wind tower, night ventilation [23] performed a similar experimental analysis but chose to com- systems and omni-directional wind scoops. Furthermore, the effec- pare the ventilation performance of square and circular wind tow- tiveness of using wind tower systems for heat recovery and pre- ers. Later, Montazeri used theoretical models to validate the heating in cooler climates requires investigation. accuracy of the experimental results. Furthermore, Safari and 624

Table 1 Summary of the wind driven and passive ventilation systems under review.

Type Passive System Features/application Limitations Case-study Typical ﬂow rate Refs. Traditional method Wind escapes Accelerate internal air movement when used with Unpredictability of the driving forces. Completely Yazd, Iran No data available [2,51] other devices such as windows and doors. Create reliant on the direction of the wind. Dust, insects drafts in areas of the structure which are not and small birds can enter the building through the exposed to external conditions openings

Traditional method Malkaf wind Wind pressure/stack effect. Traps the wind from The malkaf has to be very high to be able to catch Cairo, Egypt Depending on wind [50] 606–627 (2012) 92 Energy Applied / al. et Hughes B.R. tower high above the building and channels it down into enough air in a dense urban area. Design speed, and outlet/inlet (uni-directional) the room. The increase in airflow rate is directly restrictions such as height or location of openings ratio: up to 5.6 ac/h at proportional to the size and elevation of the (can only catch the prevailing wind). Relies 2 m/s openings. Effective when used in dense cities with completely on the rate of air flow to provide a warm climate cooling effect Traditional method Badgir wind tower Wind pressure/stack effect. No moving parts Taller towers are required to keep out the dust from Amman, Depending on wind [52,53] (multi-directional) required. Incorporates evaporative cooling systems. entering the device. Taller towers are expensive to Jordan speed, direction: can Typically used in hot and arid regions with build and maintain. Conflict between wind and produce up to 300 l/s constantly varying wind directional patterns buoyancy driving forces can adversely affect its for 4 m tower height performance Modern system Solar-wind tower Enhance natural ventilation through buoyancy and The circulating air heated up by the tower structure Al Ain, UAE Supply of 105 l/s and [34,36,37] wind driven flows. Very effective when used in may cause discomfort to occupants. It does not find extract rate of 92 l/s warmer climates any application in areas with a high wind speed and low solar radiation Modern system Wind vent Wind tower and passive stack are combined in one Reliable wind force is required for consistent Midtherm Reported supply of [6,7,54,56] ventilation system. Air passage is divided into two performance. Does not contain any thermal mass Eng. Ltd. 85 l/s and 650 l/s at or four quadrants. Volume control dampers regulate (unlike its vernacular counterparts). Draughts Sheffield, UK 1 m/s and 5 m/s the induced air. Louvers prevent rainwater, direct through the element may cause discomfort to external wind speeds sunlight and noise from entering the device. occupants during the winter. Air admitted in the (1 Â 1 m wind vent Installed in classrooms, offices, residential and tower is lost through the other tower openings (Air device) industrial buildings short-circuiting) Modern system Solar fan-assisted Provides extra ventilation during hot sunny days. Increased installation and maintenance cost. Has to Midtherm Increases the wind [58,22] wind vent Provides steady supply of fresh air into the be controlled well to maintain a comfortable indoor Eng. Ltd. vent airflow by up to buildings, even when there is no wind. Utilizes solar climate. Draughts through the device may cause four times. Reported power technology for energy free operation discomfort to occupants during the winter supply of 370 l/s at 1 m/s external wind and 20 Pa fan pressure Table 2 Summary of the wind tower cooling methods under review.

Cooling Techniques Features/application Limitations Case-study Typical temperature reduction Refs. Wetted columns Cloth curtains or clay conduits spaced 10 cm from The tower air ﬂow rate is slightly reduced prior to Yazd, Iran Reduction of internal temperature by 11 K and 15 K [27,29] (curtains and clay each other and suspended vertically in the wind the integration of the cooling device. Height of the for a 0.4 m Â 0.4 m and 1 m Â 1 m cooling tower conduits) tower channel. The curtains are wetted by spraying tower limits the length of wet surface water on top. Performs better with low wind speeds

Wetted surface Evaporative cooling pads sit at the top of a wind The addition of the cooling device may reduce the Constantine, A1mÂ 1 m wind tower integrated with wetted [27,28] 606–627 (2012) 92 Energy Applied / al. et Hughes B.R. (cooling pads) tower with pump re-circulating water over them. air flow rate inside the channel. Higher Algeria interior surfaces decreased the indoor air The pads are wetted by spraying water on top of maintenance cost. Not effective in regions with high temperature by 7.8–12.9 K them. More effective during high wind conditions relative humidity Qanat (underground The air from the qanat is drawn into the tunnel at High construction and maintenance cost and lack of Marvdasht, Iran In dry desert climates the integrated cooling system [30] cooling) some distance away from the structure and is flexibility. The waterways of the qanat must be can provide up to 15 K indoor air temperature cooled by contact with the tunnel wall/water. Wind periodically inspected for corrosion and cleaned of reduction tower and qanat cooling have been used in the accumulated sand and mud Middle East since early times Solar chimneys Enhance natural ventilation through stack effects Lower magnitude compared to wind ventilation. Morelos, México The temperature reduction with application of solar [32–35] for exhausts at purposefully designed exits the Require properly insulated structures. The system chimney solely are found to be in the range of 1– effect of thermally induced ventilation in buildings. relies on internal and external temperature 3.5 K. Lack of available experimental data Simple design and low maintenance cost. Effective differences in warmer climates Structural night Thermal mass material. Warm structures lose heat High sensitivity to annual climatic variability and Athens, Greece Reduces the interior temperature by up to 3.9 K [36,39,40] ventilation to the atmosphere on cool nights. Ambient air the climatic limitations of building cooling by with the night ventilation rate of 10 air changes per coolness is stored in the tower mass and cools the night-time ventilation. Limited controllability and hour morning air. Typically employed in hot desert slow charge/discharge process summer regions with dry air and cool clear nights Wind towers The courtyard generated wind movement in the During periods of intense solar radiation, much heat Sharjah, UAE Maximum temperature reduction of 11 K inside the [41–43] integrated with house by allowing warm air to rise up and exit the will be conducted and radiated into the living courtyard and 7 K in the living spaces courtyards wind tower. The courtyard serves as a thermal sink spaces as against the induced current of air providing extra coolness to the surrounding rooms Wind towers Curved roofs have a much greater surface area and Higher build and maintenance cost compared to Nottingham, UK No data available [11,44–45] integrated with thus a larger surface to release heat from. Curved conventional flat roofs curved/domed roofs and domed roofs have been used in many Middle Eastern houses for many centuries to enhance the ventilation rates 625 626 B.R. Hughes et al. / Applied Energy 92 (2012) 606–627

Hosseinnia [15] and Jones and Kirby [8] also developed mathemati- 11. Conclusions cal models to calculate the performance of wind tower. Few of the papers reviewed, investigated the detailed thermal conditions and This paper reviewed wind tower designs with respect to tradi- indoor air quality of rooms employing the wind tower system. Bou- tional techniques such as the badgir wind tower and modern sys- chahm [26], Correia da Silva [28], and Badran [29] performed theo- tems including the wind vent and wind scoop. This study retical analysis of the thermal performance of a wind tower system emphasized the importance of wind towers and gives insight into incorporating evaporative cooling methods. Bahadori [31] used far- the application of passive cooling systems as an alternative to the field experimental testing to evaluate the thermal performance of a high-energy consuming mechanical ventilation systems. Tradi- similar cooling tower. Later on, Safari and Hosseinnia [15] used mul- tional wind towers have been designed and built using old meth- ti-phase CFD model, based on the Lagrangian–Eulerian approach to odologies; therefore they have design limitations which can be confirm the theoretical results and good correlation between both eliminated with advances in technology. The successful wind methods of analysis was observed. Liu et al. [59] used numerical tower technologies existing today can provide a starting point for models to analyze the ventilation performance of a roof mounted the research required to develop practical guidelines for the design square wind tower device and established good agreement with of future wind towers. the data obtained from literature. Hughes and Ghani [55] used CFD The review further highlighted the different cooling techniques to study the effect of various damper and lover angles on the venti- which can be integrated with wind tower systems to improve its lation rates of a commercial wind tower. Liu and Mak [58] also inves- ventilation and thermal performance. The basic principles of each tigated different louver configurations for a similar wind tower technique along with its corresponding capabilities were summa- device but chose to modify the length and number of layers of the rized along with their advantages, limitations, and applications. louvers. Recently, Hughes and Ghani [6] and Priyadarsini et al. [58] The study’s conclusion is based on the research of various case- studied the feasibility of a passive-assisted natural ventilation stack studies utilizing the cooling techniques for their operations. Key device using CFD and experimental methods. Although numerous parameters including the ventilation rates and temperature were studies have looked into the effects of different configurations and evaluated in order to determine the viability of implementing the components on the performance of wind towers, knowledge of the devices for their respective use. system remains incomplete due to its complexity and the lack of real world model development. Acknowledgments Table 1 summarizes an outline of the traditional and modern wind tower systems under review along with the features and This publication was made possible by a NPRP Grant from the typical flow rates obtained from related literature and case-studies. Qatar National Research Fund (A member of the Qatar Foundation). It is observed that the commercial wind vent device generated the The statements made herein are solely the responsibility of the highest air flow rates, supplying up to 650 l/s for a 5 m/s external authors. NPRP 09-138-2-059. wind. However, airflow rates achieved through standard wind towers often cannot meet the required indoor ventilation rates due its sole dependence on the speed and direction of the wind. A passive- References assisted natural ventilation system can be used to assist the pas- [1] Montazeri H, Azizian Experimental R. Experimental and numerical study on sive stack without affecting the energy requirements of the wind natural ventilation performance of various multi-opening wind catchers. Build tower. The hybrid system enhanced the systems ventilation rate, Environ 2011;46:370–8. producing up to four times as much air flow as the standard [2] Fathy H. Natural energy and vernacular architecture: principles and examples with reference to hot arid climates. University of Chicago Press; 1986. configuration. On the contrary, the lowest temperature range is [3] Elmualim A. Dynamic modeling of a wind catcher/tower turret for natural produced by the solar-wind tower with a supply rate of 105 l/s ventilation. Build Services Eng Res Technol 2006;27:165–82. and extract rate of 92 l/s. This is due to the sole reliance of the sys- [4] McCarthy Battle. Consulting engineers. Wind towers: detail in tem on internal and external temperature differences (buoyancy building. London: Academy Press; 1999. [5] Allard F. Natural ventilation in buildings: a design handbook. European induced flows) which is only effective in areas with low wind Commission. Directorate General for Energy Altener Program; 1998. speeds and very high solar radiation. [6] Hughes BR, Ghani AS. A numerical investigation into the effect of Windvent Table 2 summarized the different types of cooling methods em- louvre external angle. Build Environ 2010;45:1025–36. [7] Hughes BR, Cheuk-Ming M. A study of wind and buoyancy driven flows ployed in wind tower systems which were obtained based on pre- through commercial wind towers. Build Environ 2011;43:1784–91. vious related works. It is observed that the highest temperature [8] Jones B, Kirby R. Quantifying the performance of a top–down natural reductions are achieved from incorporating evaporative cooling ventilation Windcatcher™. Build Environ 2009;44:1925–34. [9] Kleiven T. Natural ventilation in buildings: architectural concepts, techniques into the wind tower such as wetter columns (clay con- consequences and possibilities. Norwegian University of Science and duits), surfaces(cooling pads) and underground cooling. The tem- Technology; 2003. perature reduction with the application of wetted columns and [10] Roaf S. The traditional technology trap: more lessons from the windcatchers of yazd. In: Conference on passive and low energy architecture, Dublin; 2008. surfaces are found to be in the range of 12 –15 K. However, the [11] Asfour O, Gadi M. The operation and function of a ventilation device such can addition of the cooling devices may reduce the air flow rate inside vary depending on the time of the day. Architect Eng Des Manage the channel and reduce the overall efficiency of the wind tower. 2006;2:289–304. [12] Elizalde T, Mumovic D. Simulated performance of windcatchers in an urban Another disadvantage of this configuration is the requirement of environment. In: Conference on passive and low energy architecture, Dublin; taller towers to have sufficient contact time between the air flow 2008. and cooling surface. The underground integrated cooling system [13] Tabhaz M. The estimation of the wind speed in urban areas. School of provided up to 15 K reduction in air temperature. However, it Architecture and Urban Planning, Iran; 2005. [14] Montazeri H, Montazeri F, Azizian R, Mostafavi S. Two-sided wind catcher may not be economically feasible, the high construction and main- performance evaluation using experimental, numerical and analytical tenance cost are the main reason why these underground systems modeling. Renew Energy 2010;35:1424–35. are no longer built in Iran. On the other hand, the solar chimney [15] Saffari H, Hosseinnia SM. Two-phase Euler–Lagrange CFD simulation of evaporative cooling in a wind tower. Energy Build 2009;41:991–1000. produced the lowest temperature range, reducing the indoor tem- [16] Gage SA, Graham JM. Static split duct roof ventilators. Build Res Informat peratures only up to 3.5 K. Other disadvantages of the system is the 2000;4:234–44. limited controllability and slow charge/discharge process which [17] A’zami A. Badgir in traditional Iranian architecture. In: International conference for the built environment, Greece; 2005. are very important parameters for achieving internal thermal [18] Montazeri F, Azizian R. Experimental study on natural ventilation performance comfort. of one-sided wind catcher. Build Environ 2008;43:2193–202. B.R. Hughes et al. / Applied Energy 92 (2012) 606–627 627

[19] Soheila Aghlmand. Sustainable perspective in Iranian vernacular architecture [40] Wang Z, Yi L, Gao F. Night ventilation control strategies in office buildings. of wind tower. Int J Acad 2011;33:778–80. Solar Energy 2009;83:1902–13. [20] Mahnaz M, Shemirani M. Analysis on typology and architecture of wind [41] Safarzadeh H, Bahadori MN. Passive cooling effects of courtyards. Build catcher and finding the best type, vol. 33. University of Tehran Electronic Environ 2005;40:89–104. Journals Database; 2009. p. 27–36 [42] Edwards B, Sibley M, Hakmi M, Land P. Courtyard housing: past, present and [21] Windcatchers: A cost effective solution for Walworth academy sports hall; future. Taylor and Francis; 2005. p. 168–9. 2011. http://www.ascinfo.co.uk> [access 14.08.11]. [43] Sharples S, Bensalem R. Airflow in courtyard and atrium buildings in the urban [22] Hughes BR, Ghani SA. A numerical investigation into the feasibility of a environment: a wind tunnel study. Solar Energy 2001;70:237–44. passive-assisted natural ventilation stack device. Int J Sustain Energy [44] Bahadori MN. Natural cooling in hot arid regions, in solar energy application in 2010;30:193–211. buildings. Academic Press; 1979. p. 195–225. [23] Elmualim AA, Awbi HB. Wind tunnel and CFD investigation of the performance [45] Bahadori MN, Haghighat F. Passive cooling in hot, arid regions in developing of ‘‘wind catcher’’ ventilation systems. Int J Ventilat 2002;1:53–64. countries by employing domed roofs and reducing the temperature of internal [24] Bahadori M. Viability of wind towers in achieving summer comfort in the hot surfaces. Build Environ 1985;20:103–13. arid regions of the Middle East. Renew Energy 1994;5:879–92. [46] Soflaee F, Shokouhian M. Natural cooling systems in sustainable traditional [25] Passive Air Conditioning 2011. [access 10.08.11]. architecture of Iran international conference ‘‘passive and low energy cooling [26] Bouchahm Y, Bourbia F, Belhamri A. Performance analysis and improvement of for the built environment’’, Greece; 2005, p. 715–9. the use of wind tower in hot dry climate. Renew Energy 2011;36:898–906. [47] El-Shorbagy A. Design with nature: windcatcher as a paradigm of natural [27] Bahadori MN, Mazidi M, Dehghani AR. Experimental investigation of new ventilation device in buildings. Int J Civil Environ Eng 2010;10:26–31. designs of wind towers. Renew Energy 2008;33:2273–81. [48] Attia S, Herde A. Designing the Malqaf for summer cooling in low-rise housing, [28] da Silva C. Passive downdraught evaporative cooling applied to an auditorium an experimental study conference on passive and low energy architecture, international conference ‘‘passive and low energy cooling for the built Canada; 2009. environment’’, Greece; 2005. p. 555–60. [49] Kalantar V. Numerical simulation of cooling performance of wind tower [29] Badran AA. Performance of cool towers under various climates in Jordan. (Baud-Geer) in hot and arid region. Renew Energy 2009;34:246–54. Energy Build 2003;35:1031–5. [50] Zarandi MM. Analysis on Iranian wind catcher and its effect on natural [30] Boustani F. Sustainable water utilization in arid region of Iran by Qanats. Int J ventilation as a solution towards sustainable architecture (case study: Yazd). Human Soc Sci 2009;4:505–8. Eng Technol 2009;54:574–9. [31] Bahadori MN. Passive cooling systems in Iranian architecture. Sci Am [51] Khan N, Su Y, Saffa, Riffat B. A review on wind driven ventilation techniques. 1978;238:144–54. Energy Build 2008;40:1586–604. [32] Bansal NK, Mathur R. Solar chimney for enhanced stack ventilation. Build [52] Hughes BR, Ghani SA. Investigation of a windvent passive ventilation Environ 1989;28:373–7. device against current fresh air supply recommendations. Energy Build [33] Arce J, Jiménez MJ, Guzmán JD, Heras MR, Alvarez G, Xamán J, et al. 2008;40:1651–9. Experimental study for natural ventilation on a solar chimney. Renew [53] Jones BM, Kirby R, Kolokotroni M, Payne T. Air quality measured in a classroom Energy 2009;29:2928–34. served by roof mounted natural ventilation windcatchers. In: Conference for [34] Zamora B, Kaiser AS. Numerical study on mixed buoyancy-wind driving the engineering doctorate in environmental technology; 2008. induced flow in a solar chimney for building ventilation. Renew Energy [54] Su Y, Riffat S, Lin Y, Khan N. Experimental and CFD study of ventilation flow 2010;35:2080–8. rate of a Monodraught™ windcatcher. Energy Build 2008;40:1110–6. [35] Nouanégué HF, Alandji LR, Bilgen E. Numerical study of solar-wind tower [55] Elmualim AA. Effect of damper and heat source on wind catcher natural systems for ventilation of dwellings. Renew Energy 2008;33:434–43. ventilation performance. Energy Build 2006;38:939–48. [36] Hughes BR, Chaudhry HN, Ghani SA. A review of sustainable cooling [56] Hughes BR, Ghani SA. A numerical investigation into the effect of windvent technologies in buildings. Renew Sustain Energy Rev 2011;15:3112–20. dampers on operating conditions. Build Environ 2009;44:237–48. [37] Bahadori MN. An improved design of wind towers for natural ventilation and [57] Liu L, Mak CM. The assessment of the performance of a windcatcher system passive cooling. Solar Energy 1985;35:119–29. using computational fluid dynamics. Build Environ 2007;42:1135–41. [38] Ghaemmaghami PS, Mahmoudi M. Wind tower a natural cooling system in [58] Priyadarsini R, Cheong KW, Wong Enhancement NH. Enhancement of natural Iranian traditional architecture international conference ‘‘passive and low ventilation in high-rise residential buildings using stack system. Energy Build energy cooling for the built environment’’, Greece; 2005. p. 71–6. 2004;36:61–71. [39] Hollmuller P, Lachal B, Zgraggen J. A new heat exchange technique for [59] Liu S, Mak CM, Niu J. Numerical evaluation of louver configuration and ventilation controlled thermal phase-shifting heat transfer in components and ventilation strategies for the windcatcher system. Build Environ systems for sustainable energy technologies; 2007. 2011;46:1600–16.