Current practices and studies regarding wind catcher system

Recently, several buildings designed to incorporate wind catchers have attracted a lot of attention; The Jubilee Campus at Nottingham University has a revolving cowl that removes hot, stale air from internal spaces of the building; this is similar to a mushroom wind catcher system used at the Inland Revenue building in Nottingham (Hurdle, 2001).

Most research studies on the use of wind catcher systems have been conducted in relation to hot and arid climatic regions like the Middle East; less has been done on the application of wind catchers in temperate climatic regions like the UK. Bahadori (1994) highlights that wind catcher systems are beneficial because they do not consume energy. Wind catchers using passive cooling techniques to naturally cool buildings; cooling occurs due to buoyancy or wind effect and traditionally due to evaporative effects when air passes over a wet surface.

Wind catchers have some disadvantages as well; for instance, dust, sand, tiny birds, insects and other airborne pollution get into the building. Also, some amount of air drawn into the wind catcher system escapes through the opening of the system and does not reach the space requiring ventilation. For a wind catcher system with a single vent that faces the direction of prevailing wind, air that is drawn into the wind catcher also gets into the building. The wind catcher system can only store a small amount of cool air. Wind catchers were found to be ineffective in regions with low prevailing wind speeds while conventional wind catcher designs

that employ evaporative cooling had limited application (Karakatsanis, Bahadori and Vickery, 1986). Bahadori (1994) conducted a wind tunnel test on a small scale. He indicated it is important to understand the air-flow surrounding a building and the building's envelope pressure distribution in order to approximate natural ventilation potential in the building's internal space (Karakatsanis, Bahadori and Vickery, 1986) .

Karakatsanis et al., (1986) also conducted a wind tunnel test on a limited scope for a wind tower linked to a building with a courtyard and examined the coefficients of pressure distribution. The researchers established that the rate at which air flows into the building from the system is dependent on the coefficients of pressure at the building's openings. Covering the opening reduces pressure coefficient resulting in an increased rate of air flow into the building.

The damping technique employed in wind towers reduces the rate of air flow. Sometimes the wind catcher functioned like a suction device, but this was dependent on the angle of the prevailing wind and the design of the courtyard. The researchers further indicated that the rate of air flow could be augmented by stopping air from escaping through the openings on the leeward side of the system which is essentially is short-circuiting.

Nevertheless, the studies based on the wind tunnel tests could not be corroborated in real buildings with wind catcher systems. It is important to carry out post-occupancy assessment studies where it is possible to accurately measure indoor air parameters and evaluate the human comfort.

Sensitivity analysis indicated that wind direction has minimal effect on the performance of wind catchers compared to base models. Nevertheless, different wind catcher configurations behaved differently depending on the height of the system, where the system is located in a building and also the characteristics of the buildings adjacent to the building requiring ventilation. This finding to some extent differs with other publications (Elmualim and Awbi, 2001; Kolokotroni, Ayiomamitis and Ge, 2002) that clearly indicated that the performance of a wind catcher system is dependent on the direction and speed of the wind. To achieve a maximum rate of air flow, the focus should be on the cowl design and the connection between the openings. The wind catcher design that has only one opening that faces the direction of the wind was found to have the best performance. Including filters and moist pads to cool the spaces increased obstructed air flow into the indoor space and hence decreased indoor air flow rates.

In situations like this, it would be best use higher wind catcher systems facing away and this produces upward flow due to negative pressure from the direction of the wind (Sharag-Eldin, 1994). Hines et. al., (1994) analysed thermal storage in square shaped wind catchers. The researchers found out that increasing mass inside the wall of the tower had minimal effect on the performance of the wind tower. However, it is effective and cheap to use the traditional cooling walls technique inside the tower walls.

Battle et. al., (2000) conducted several small-scale tests in a boundary layer wind tunnel laboratory using different wind catcher configurations. The researchers found that when the wind blows at 0° relative to inlet, it creates higher wind pressure at inlet segments and lower wind pressure at outlet segments. The opposite happens when the wind blows at 45°. The inlet segment is slightly effective whilst the extract segment operates more efficiently. Wind catcher system's performance is therefore affected more by the angle at which the wind is blowing; its effectiveness increases as the system moves towards the direct wind direction (Battle, Zanchetta and Heath, 2000; Elmualim and Awbi, 2001).

Gage and Graham (2000) carried out a study to investigate the performance of a four and six segment wind catcher systems in a small scope wind tunnel model. The study revealed that a 4-segment wind catcher system having a 45° orientation generated the highest differences in pressure between the supply segments and extract segments, therefore producing the highest air velocities in the duct.

Bansal et al (1994) carried out a study to investigate induced natural ventilation a solar chimney- wind tower combination. The researchers indicated that a solar chimney- wind tower combination created a remarkable system that can be integrated into buildings to deliver natural ventilation.

Two researchers carried out a CFD simulation to investigate how wind catcher systems performed in hot climatic regions. The study revealed that wind catchers can be employed in such regions to facilitate ventilation during the night (Aboulnaga, 1998). However, the researchers did not provide information regarding the CFD codes used, boundary conditions or computer requirements. Nevertheless, CFD is a popular tool used in the design and analysis of air movement in indoor spaces (Anderson, 1995; Al., 2003). Harris and Webb (1996) carried out a small scope wind tunnel test to investigate the application of wind catcher systems; the researchers measured air movement using an anemometer, used coloured dyes for flume tests and the sulphur hexafluoride (𝑆𝐹₆) tracer gas method to measure ventilation rates. Wind

velocities of 5 𝑚/𝑠 produced ventilation rates of over 6 𝑎𝑐/ℎ while wind velocities of 3 𝑚/𝑠 produced ventilation rates of 2.5 𝑎𝑐/ℎ. The test room had an area of 15 𝑚²and the wind catcher had an area of 0.0729 𝑚² (Harris and Webb, 1996). The findings indicate inconsistencies between the approaches the researchers employed.

A number of studies to investigate the performance of wind catchers in actual buildings was have been conducted for commercial use. Researchers conducted a tracer gas analysis test on the Building Research Establishment (BRE) office of the Future (Riain et al., 1999). A study was conducted in hot climatic regions to investigate thermal performance in three public establishments that are naturally ventilated using wind towers. The study established the effectiveness of wind towers in improving air circulation inside building spaces (Yaghoubi, Sabzevari and Golneshan, 1991). Farija (1997) carried out studies to investigate the performance of wind catchers in houses in hot and arid areas. Nevertheless, their findings were inadequate and could not justify the potential and applicability of wind catchers in temperate regions like the UK.