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Passive Design: Roof Pond

A roof pond uses a store of water above the roof to mediate internal temperatures, usually in hot desert environments. At night, the insulation is removed and the water exposed, losing significant amounts of heat by radiation to the night sky. Early in the morning, the insulating panels are replaced to protect the water from the heat of the day and solar radiation.

The water remains relatively cool throughout the day, cooling the ceiling of the space below. A cool ceiling is particularly effective in rooms where heat is being generated as warm air always rises to the top, to lose its heat almost immediately to the ceiling. The water is typically contained in black bags or dark coloured containers to maximise radiant emissions and minimise evaporation.

Figure 1 - The operation of roof pond system for passive cooling.
Figure 1 - The operation of roof pond system for passive cooling.

Application for Heating

A roof pond can also be used to provide heating. In this case, the store of water above the roof is used to absorb solar radiation during the day.

At night, the water is covered with insulating panels and the stored heat warms the ceiling and radiates down into the spaces below. No convection current is created, however, as the warmed air tends to remain beneath the ceiling. The distribution of collected heat is by radiation only, so proximity of the ceiling to the individual being warmed is important as radiation density drops off quickly with distance.

The water in a roof pond is typically surrounded by or contained within black surfaces to maximise solar collection. Since thermal storage is the ceiling of the building, it will radiate uniform low-temperature heat to the entire layout in both sunny and cloudy conditions.

Important Considerations

  • A large volume of exposed water, with the major living and sleeping spaces directly beneath.
  • Moveable insulating panels to protect the water from heat losses when undesirable.
  • A thin conducting ceiling immediately below, preferably with ridges or corrugations to increase its interfacing surface area.
  • Sufficient structure to support a considerable load.
  • Not effective for heating at high latitudes as the Sun is at too low an angle to effectively heat the water.

Summary

Given the angles of incidence of sunlight during the day, roof ponds are only effective for heating at mid-latitudes, in moderate to temperate climates. Roof ponds are usually installed more for their passive cooling effects, with any heating potential simply being a bonus.

Passive Design: Direct Gain
Passive Design: Sun Spaces

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