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A hybrid cooling system is a type of cooling system that combines two or more different cooling technologies to achieve optimal cooling efficiency. The two main types of cooling technologies used in hybrid cooling systems are air cooling and liquid cooling.

Hybrid cooling systems are whole house cooling solutions that employ a variety of cooling options (including air conditioning) in the most efficient and effective way. They take maximum advantage of passive cooling when available and make efficient use of mechanical cooling systems during extreme periods.

1) Fans

  • Fans provide reliable air movement for cooling people and supplementing breezes during still periods.
  • At 50% relative humidity, air movement of 0.5m/s creates maximum cooling effect; faster speeds can be unsettling. As noted above, air speeds up to 1.0m/s can be useful in higher relative humidity, but prolonged air speeds above 1.0m/s cause discomfort.
  • Standard ceiling fans can create a comfortable environment when temperature and relative humidity levels are within acceptable ranges. In a lightweight building in a warm temperate climate, the installation of fans in bedrooms and all living areas (including kitchens and undercover outdoor areas) significantly reduces cooling energy use.

Air movement relative to fan position.

  • Fans should be located centrally in each space, one for each grouping of furniture. An extended lounge/dining area needs two fans.
  • In bedrooms, locate the fan close to the centre of the bed. Because air speed decreases with distance from the fan, position fans over the places where people spend the most time.

2) Whole of house fans

  • Whole of house or roof fans are ideal for cooling buildings, particularly where cross-ventilation design is inadequate. However, they do not create sufficient air speed to cool occupants.
  • Whole of house fans should be positioned centrally, e.g. in the roof, stairwell or hallways.
  • Typically, a single fan unit is installed in a circulation space in the centre of the house (hallway or stairwell) to draw cooler outside air into the building through open windows in selected rooms, when conditions are suitable. It then exhausts the warm air through eaves, ceiling or gable vents via the roof space. This also cools the roof space and reduces any temperature differential across ceiling insulation.
  • Control systems should prevent the fan operating when external air temperatures are higher than internal.
  • Drawing large volumes of humid air through the roof space can increase condensation. A dew-point forms when this humid air comes in contact with roof elements (e.g. reflective insulation) that have been cooled by radiation to night skies (see Insulation and Sealing your home for ways to mitigate this).
  • Whole of house fans can be noisy at full speed but are generally operated in the early evening when cooling needs peak and households are most active. If run at a lower speed throughout the night, they can draw cool night air across beds that are near open windows, provided doors are left open for circulation. On still nights this can be more effective than air conditioning for night-time sleeping comfort.

3) Air conditioning

  • Refrigerated air conditioning lowers both air temperature and humidity and provides thermal comfort during periods of high temperature and humidity. However, it is expensive to install, operate and maintain, and has a high economic and environmental cost because it consumes significant amounts of electricity unless high efficiency equipment is used in a very high performance building envelope. As it also requires the home to be sealed off from the outside environment, occupants are often unaware of improvements in the weather.
  • Air conditioning is commonly used to create comfortable sleeping conditions. The number of operating hours required to achieve thermal comfort can be substantially reduced or eliminated by careful design of new homes, as well as alterations and additions to existing homes.
  • Running a refrigerated air conditioner in a closed room for about an hour at bedtime often lowers humidity levels to the point where air movement from ceiling fans can provide sufficient evaporative cooling to achieve and maintain sleeping comfort. Some air conditioning units simply operate as fans when outdoor ambient temperature drops below the thermostat setting, so they can replace a ceiling fan.
  • Efficient air conditioning requires more than simply installing an efficient air conditioner.
  • Hybrid cooling solutions require a decision early in the design stages about whether air conditioning is to be used and how many rooms require it. Many inefficient air conditioning installations occur when they are added to a home designed for natural cooling as an afterthought to improve comfort.

4) Design of air conditioned spaces

  • There is usually no need to air condition all rooms. Decide which rooms will receive most benefit, depending on their use, and try to reduce the total volume of air conditioned air space (room size, ceiling height).
  • Often one or two rooms are sufficient to provide comfort during periods of high humidity and high temperatures.
  • Design for night-time sleeping comfort by conditioning rooms commonly used in the early evening with bedrooms adjoining.
  • A conditioned, masonry-wall television room in the centre of a free running (passively cooled) home with sleeping spaces adjoining it provides both direct and indirect cooling benefits.
  • Efficient (low heat output) lighting and appliances are important in such an application.
  • A cool masonry wall in a bedroom gives both psychological and physiological comfort through combined radiant heat loss and reliable air movement from fans.

A well-ventilated tropical house.

  • Design conditioned rooms with high levels of insulation and lowest exposure to external temperature influences, usually found in the centre of the house. Adjoining living spaces should be well ventilated, free running (passively cooled), with fans to encourage acclimatisation, and provide a thermal buffer to conditioned spaces.
  • Address condensation in externally ventilated rooms surrounding conditioned rooms. Walls with high thermal mass have fewer dew-point problems than lightweight insulated walls and can store ‘coolth’.
  • When insulated walls surround an air conditioned space, a vapour barrier should be installed between the warm humid air and the insulation material to prevent the insulation being saturated by condensation.
  • Choose materials and finishes that are resistant to damage from condensation for any linings placed over the vapour barrier: placing reflective foil insulation under a plasterboard wall lining, for example, causes the dew-point to form under the plasterboard.
  • Avoid conditioning rooms that have high level indoor−outdoor traffic.
  • Alternatively, use airlocks to minimise hot air infiltration or install an automatic switching device (such as a reed switch or other micro switch) to the doors leading to the air conditioned room that allows operation only when the door is closed.

5) Operation

  • Identify the months and times of day when mechanical cooling will be required and use control systems, sensors and timers to reduce total operating hours. Turn air conditioners off when you go out.
  • Set thermostats to the warmest setting that still achieves comfort. Experiment — you may find 26°C quite comfortable when you thought you needed 21°C.
  • Adapt your lifestyle where possible to take advantage of comfortable external conditions when they exist, to minimise operating periods for mechanical cooling systems.

However, they can also be more complex and expensive to install and maintain than traditional cooling systems.

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