Unassisted natural ventilation

Pioneers of natural ventilation are Alan Short and Brian Ford in association with Max Fordham. Their first groundbreaking building in the UK was the Queen's Engineering Building at Leicester de Montfort University (Short Ford and Partners). This building has been well documented and a particularly useful reference is Thomas, R. (ed.) (1996) Environmental Design, E & FN Spon.

Maintaining the principle of pure natural ventilation without mechanical assistance is the Coventry University Library, the Lanchester Building, by architects Short and Associates. The environmental strategy was developed in association with Brian Ford. This is a deep plan building making it impossible to employ cross flow ventilation from perimeter windows. There is also the problem of a raised ring road close to the site generating noise and pollution. Accordingly perimeter windows are sealed (Figure 12.2).

The solution was to provide each quadrant of the floor plan with large lightwells doubling up as air delivery shafts. The buoyancy of rising warm air draws fresh air into plenums below floor level to the base of each light tower. From here the air is drawn upwards through preheating coils to be released to rooms at floor level. By now the air has reached 18°C. Additional warmth is provided by perimeter radiators. The air is then drawn into the exit stacks spaced around the external walls. 'Termination' devices at the top of the stacks ensure that prevailing winds will not push air back down the stacks (Figures 12.3 and 12.4).

In a building relying solely on the buoyancy of natural ventilation, control is critical. The building energy management system (BEMS)

Figure 12.2

Coventry University library (courtesy of Marshalls plc)

Figure 12.2

Coventry University library (courtesy of Marshalls plc)

Coventry University LibraryCoventry University Library Lightwell

VENTILATION STACK ROOF PLAN

ÈBiobdb^tiiAcba

Figure 12.3

Plans, Coventry University library

Lightwell providing ventilation and daylight

External solar shading

Lightwell providing ventilation and daylight

External solar shading

Lanchester Library Ventilation

Low emmisivity, argon filled, double glazing

Perimeter

CC>2 and temperature sensors provide BEMS input

Low emmisivity, argon filled, double glazing

Perimeter

- Castellated radiators with beams thermostats Thermally massive (concrete)

ceilings, painted white to assist daylight penetration

Section through central atrium (air outlet) ■ Warm Exhaust air out

CC>2 and temperature sensors provide BEMS input

BEMS Wel1 controlled insulated |0Uvres roof

V"

- 7 ;

Moving Vented translucent ,-void

Daytime Shaded Inlet Penetration

Heater

Fresh air inlet

Trench heating

Heater ! 1 Li9htwells hottpHpc provide ventilation batteries batteries and day|ight

Section through perimeter lightwell (air inlet) ■ Fresh Air intake

Heater

Fresh air supply plenum

Figure 12.4

Air circulation paths adjusts the outlet opening sizes according to outside temperature and the CO2 and temperature readings in each zone of the building. It is tuned to meet the optimum fresh air requirement compatible with the minimum ventilation rate (Figure 12.4).

The BEMS controls dampers which allow night air to flow through the building, cooling the exposed thermal mass during the summer. This is a BEMS which is driven by a self-learning algorithm, meaning that it should progressively optimise the system, learning by its mistakes.

Heat losses through the fabric of the building are minimised by good insulations standards: U = 0.26W/m2K for walls and less than 2.0 W/m2K for windows. The latter comprise Low E double glazing with an argon filled cavity.

The result of avoiding mechanical ventilation and maximising natural light is that the estimated energy demand is 64 kWh/m2 per year which represents CO2 emissions of 20 kg/m2. This is around 85 per cent less than the standard air conditioned building.

The building type which presents the most formidable challenge to anyone committed to natural ventilation is a theatre. Short Ford Associates have risen to the challenge in a spectacular fashion. There is a considerable heat load from stage lighting as well as the audience yet the Contact Theatre at Manchester University achieves comfort conditions without help from air conditioning. This is another building by which Alan Short, Brian Ford and Max Fordham have navigated uncharted waters (Figure 12.5).

Figure 12.5

Contact Theatre, Manchester University

The outstanding feature is the cluster of H-pot stacks over the auditorium reaching a height of 40 metres. The H-pot design lifts them above neighbouring buildings to exclude downdraughts from the prevailing south-west winds. Their volume is calculated to accelerate the buoyancy effect and draw out sufficient hot air whilst excluding rain. Things were made more complicated by the fact that this is a refurbishment of a 1963 auditorium, which has been largely preserved. In a theatre ventilation and cooling are the major energy sinks. Consequently the energy load of this building should be a fraction of the norm (Figure 12.6).

Naturally Ventilated Theater

In circumstances like this theatre it may be necessary to incorporate attenuators in the system to minimise external noise.

The stack effect or gravity displacement is dependent on the difference in temperature between the outside and inside air and the height of the air column. There is considerable variation in the relative temperatures over the diurnal and seasonal cycle. During the summer, night-time cooling can be achieved by passing large quantities of fresh air over the structure. Night-time cooling works when the external temperature is lower than the internal one and gravity drives the cooler air down into the building. In the daytime in summer when the internal temperature has become lower than the outside temperature, it is necessary to cool the incoming air, perhaps by evaporative cooling or a heat pump. If heat is transferred from the input duct to the exhaust duct, this further assists buoyancy.

In the UK this system can work economically up to six storeys. Above this duct sizes may become excessively large to cope with the volume of air.

One objection to naturally ventilated buildings is that they draw polluted air into a building. To reduce the chance of this happening in highly polluted areas, fresh air should be drawn into the building at high level, above the diesel particulate matter zone. At the same time, exhaust air which has risen through the stack effect also needs to be expelled at high level, so a means has to be found of ensuring the exhaust air does not contaminate the fresh air.

One way is to employ a terminal design which rotates according to the direction of the wind. In Figure 12.7 a design of terminal is shown which ensures that fresh air is always drawn in from the windward side and exhaust air to the leeward side. A wind vane ensures that the terminal always faces the correct direction. The aerofoil shape of the wind direction terminal produces negative pressure on the leeward side, assisting the expulsion of exhaust air.

In the section, Figure 12.8, the fresh air is delivered through perimeter ducts to provide displacement ventilation. The exhaust air can either exit through perimeter ducts or a climate facade.

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