Night and natural cooling is normally only an option for new buildings. However, an intelligent fan convector now being tested could herald a new era of low energy cooling in existing offices.
Natural ventilation and night cooling have a reputation for low operating costs and energy use. However, fears about limited control and performance stop some building owners from going the natural route.

Some owners are also sceptical about natural ventilation's ability to cope in high ambient temperatures. A common shortfall in modelling and analysis does nothing to subdue these fears.

On top of this, natural ventilation and night cooling regimes must normally be established for the whole building, using a building energy management system.

Research by Biddle Air Systems and Coventry University suggests that a fan-assisted ventilation system provides a simpler, more effective method of night cooling. Fan assistance is perceived to offer better control and more security against high external temperatures, and so helps to reassure building owners and occupiers.

Biddle and Coventry University have developed a prototype standalone ventilation and heating unit with local controls. They designed the unit to be fitted at floor level on the external wall of an office or in a ceiling void. It has a damper that can cycle between open and closed positions. The damper is intended to control incoming fresh air accurately, and without draughts.

It is controlled to:

  • meet minimum ventilation requirements;

  • provide 'free' cooling if needed during occupied hours;

  • implement night cooling.

    The control strategy is rule-based, and includes BSRIA's 'previous day setpoint' night cooling algorithm1. People using the office are provided with a wall-mounted control pad, allowing them to adjust how much fresh air enters the room.

    They may use the pad to open the damper continuously for a default period to remove internal pollutants like cigarette smoke. If they wish, they can set the damper manually to full recirculation – to keep out external pollutants like car exhaust fumes.

    Thermal modelling using Facet's Apache simulation software and Manchester weather data found that the dry resultant temperature lay between 21ºC and 24ºC for up to 88% of occupied hours. The simulated temperature rose above the 24ºC threshold for as little as 3% of occupied hours.

    The simulation also revealed a total annual primary energy consumption of 316 kWh/m2 – between the 'typical' figure quoted in Econ 192 for a naturally ventilated office and that quoted for a standard air conditioned building.

    Testing for real

    Biddle and Coventry University built two identical rooms to compare the unit against a standard recirculation fan convector with no night cooling. Both rooms had bare masonry walls, a precast concrete ceiling and a carpeted concrete floor. They were set up with internal heat loads of 35 W/m2, and sensors measuring temperature, humidity and air velocity.

    Night cooling in the room with the fan unit started when the outside temperature had fallen to 3ºC lower than room temperature – at 21.00 h. It ran until the outside temperature had dropped to 12ºC – the lower limit, chosen to prevent over-cooling – occurring at 06.00 h.

    As expected, the temperature of both rooms rose steeply as occupancy started at 09.00 h. By midday the room temperature reached 23ºC, triggering an increase in fan speed in the night-cooled room.

    Overall, the night-cooled room always had lower air and radiant temperatures, making it more comfortable as a working environment. The small price to pay for lower peak temperatures is a chilly (19-20ºC) period for the first hour of occupation. This should not pose a problem for people inside the office, though, because they will be acclimatised to outside temperatures early in the morning.

    One surprising outcome of the research so far is that the external wall seems to affect room temperatures as much as the exposed ceiling. This is a challenge to the accepted wisdom that exposed ceiling slabs are most significant.

    Taken together, standalone ventilation units and the knowledge that exposed walls are as important for night cooling as exposed ceilings, could make retrofit night cooling a distinct possibility.