Passive cooling

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Passive cooling refers to technologies or design features used to cool buildings without power consumption, such as those technologies discussed in the Passive house project.

The term "passive" implies that energy-consuming mechanical components like pumps and fans are not used.

Passive cooling building design attempts to integrate principles of Physics into the building exterior envelope to:

Slow heat transfer into a building. This involves an understanding or the mechanisms of heat transfer: heat conduction, convective heat transfer, and thermal radiation (primarily from the sun).
Remove unwanted heat from a building. In mild climates with cool dry nights this can be done with ventilating. In hot humid climates with uncomfortable warm / humid nights, ventilation is counterproductive, and some type of solar air conditioning may be cost effective.

Shading a building from solar radiation can be achieved in many ways.

Buildings can be orientated to take advantage of winter sun (longer in the East / West dimension), while shading walls and windows from direct hot summer sun. This can be achieved by designing location-specific wide eaves or overhangs above the Equator-side verticle windows (South side in the Northern hemisphere, North side in the Southern hemisphere.

Passive solar buildings should not use large glass areas directly into the living space. A greenhouse / solarium is usually integrated into the equator side of the building. It captures low winter sun, and blocks direct sunlight in the summer, when the sun's altitude is 47 degrees higher. The outer glass of the solarium, plus interior glass between the solarium and the interior living quarters acts like a Thermal Buffer Zone^[1] - Two smaller temperature differentials produce much lower heat transfer than one large temperature differential.

The quality of window-and-door fenestration can have a significant impact on heat transfer rate (and therefore on heating and cooling requirement). A solid wood door with no windows conducts heat about twelve times faster than a foam-filled Energy Star door. Older fenestration, and lower-quality doors and windows can leak a lot of outside air infiltration, conduct and radiate a lot of undesirable heat transfer through the exterior envelope of a building, which can account for a major portion of heating and cooling energy bills.

For many good thermal reasons, roof-angled glass is not a great option in any building in any climate. In the summer, it creates a solar furnace, with the sun nearly perpendicular to it. On cold winter days, the low angle of the sun mostly reflects off of roof-angled glass. Warm air rises by natural convection, touches the roof angled glass, and then conducts and radiates heat outside. Vertical equator-facing glass is far superior for solar gain, blocking summer heat, and daylighting throughout a well-designed passive solar building.

Awnings, shade screen, trellises or climbing plants can be fitted to existing buildings for a similar effect. West-facing rooms are especially prone to overheating because the low afternoon sun penetrates deeper into rooms during the hottest part of the day. Methods of shading against low East and West sun are deciduous planting and vertical shutters or blinds. West-facing windows should be minimized or eliminated in passive solar design.

Solar heat also enters a building through its walls and roof. In temperate climates, a poorly insulated building can overheat in summer and will require more heating in winter.

One sign of poor thermal design is an attic that gets hotter than the peak outside summer air temperature. This can be significantly reduced or eliminated with a cool roof or a green roof, which can reduce the roof surface temperature by 70 degrees F (21 degrees C) in the summer. Below the roof there should be a radiant barrier and an air gap, which blocks 97% of downward radiation from the sun.

Of the three mechanisms of heat transfer (conduction, convection and radiation), radiationis one of the most significant in most climates is, and is the least easy to model. There is a linear relationship between temperature differential and conductive / convective heat transfer rate. But, radiation is an exponential relationship, which is much more significant when the temperature differential is large (summer or winter).

The rate of heat transfer (which is related to heating-and-cooling requirement) is determined in part by the surface area of the building. Decorative corners can double or triple the exterior envelope surface area, and also create more opportunities for air infiltration leaks.

In mild arid climates with comfortable cool dry nights, two types of natural ventilation can be achieved through careful design: cross ventilation and passive-stack ventilation.

Cross ventilation requires openings on two sides of a room.

Passive-stack ventilation uses a vertical space, like a tower, that creates a vacuum as it rises by natural convection. An inlet for cool air at the bottom of this space creates an upward-moving air current.

An issue arises when windows are used for fresh air ventilation. Window screens don't filter small bugs, dust, dust mites, mold, pollen, and other pollutants, air-borne toxins, and allergens. The need to open and close windows is non-ideal operation. Anything that creates an air pressure diference (like an externally vented clothes dryer, fireplace, kitchen and bathroom vents) will draw unfiltered outside air in through every small air leak in a building.

An energy audit uses a calibrated exhaust fan to measure and locate poor-weatherization air-infiltration leaks cause by careless conventional construction.

Fresh air ventilation can be filtered through a Minimum Efficiency Reporting Value MERV 8+ air filter, avoiding the use of window screens for direct unfiltered ventilation, which can allow allergens to enter.

In hot humid climates with uncomfortable nights, fresh air ventilation can be controlled, filtered, dehumidified, and cooled (possibly using an air exchanger). A solar air conditioner can be used to cool and dehumidify hot humid air. ASHRAE requires a minimum 0.35 air changes / hour AND 15 CFM of fresh air for each person in a room (year round regardless of outside conditions). Carbon dioxide monitors can be used to increase fresh air intake in high-occupancy rooms when the air becomes unhealthy.

In a climate that is cool at night and too warm in the day, thermal mass can be strategically placed and insulated to slow the heating of the building when the sun is hot. Phase change materials can be designed to extract unwanted heat during the day, and release it at night.