Passive cooling
Encyclopedia
Passive cooling refers to technologies or design features used to cool building
s without power consumption, such as those technologies discussed in the Passive house
project.
Passive cooling building design
attempts to integrate principles of physics into the building exterior envelope to:
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 vertical windows (South side in the Northern hemisphere
, North side in the Southern hemisphere
).
Passive solar buildings should not allow direct sunlight through, use large glass areas directly into the living space in the summer. 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 - 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.
Awning
s, 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), radiation is one of the most significant in most climates, 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 air rises by natural convection. An inlet for cool air at the bottom of this space creates an upward-moving air current.
Allergens such as pollen can be an issue when windows are used for fresh air ventilation. Anything that creates an air pressure difference (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. As an alternative, air can be filtered through a Minimum Efficiency Reporting Value
MERV 8+ air filter to remove allergens.
An energy audit uses a calibrated exhaust fan to measure and locate poor-weatherization air-infiltration leaks cause by careless conventional construction.
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, an international society of HVAC engineers, recommends 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.
Building
In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
s without power consumption, such as those technologies discussed in the Passive house
Passive house
The term passive house refers to the rigorous, voluntary, Passivhaus standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling. A similar standard, MINERGIE-P, is used in...
project.
Passive cooling
The term "passive" implies that energy-consuming mechanical components like pumps and fans are not used.Passive cooling building design
Building design
Building design refers to the broadly based architectural, engineering and technical applications to the design of buildings. All building projects require the services of a building designer, typically a licensed architect or structural engineer...
attempts to integrate principles of physics into the building exterior envelope to:
- Slow heat transferHeat transferHeat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
into a building. This involves an understanding of the mechanisms of heat transfer: heat conductionHeat conductionIn heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
, convective heat transferConvective heat transferConvective heat transfer, often referred to as convection, is the transfer of heat from one place to another by the movement of fluids. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid. Convection is usually the dominant form of heat...
, and thermal radiationThermal radiationThermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
(primarily from the sunSunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
). - Remove unwanted heat from a building. In mild climates with cool dry nights this can be done with ventilatingVentilation (architecture)Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...
. In hot humid climates with uncomfortable warm / humid nights, ventilation is counterproductive, and some type of solar air conditioningSolar air conditioningSolar air conditioning refers to any air conditioning system that uses solar power.This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion . The U.S...
may be cost effective.
Shading
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
Eaves
The eaves of a roof are its lower edges. They usually project beyond the walls of the building to carry rain water away.-Etymology:"Eaves" is derived from Old English and is both the singular and plural form of the word.- Function :...
or overhangs above the Equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
-side vertical windows (South side in the Northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
, North side in the Southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
).
Passive solar buildings should not allow direct sunlight through, use large glass areas directly into the living space in the summer. 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 - 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
Energy Star
Energy Star is an international standard for energy efficient consumer products originated in the United States of America. It was first created as a United States government program during the early 1990s, but Australia, Canada, Japan, New Zealand, Taiwan and the European Union have also adopted...
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.
Awning
Awning
An awning or overhang is a secondary covering attached to the exterior wall of a building. It is typically composed of canvas woven of acrylic, cotton or polyester yarn, or vinyl laminated to polyester fabric that is stretched tightly over a light structure of aluminium, iron or steel, possibly...
s, 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
Deciduous
Deciduous means "falling off at maturity" or "tending to fall off", and is typically used in reference to trees or shrubs that lose their leaves seasonally, and to the shedding of other plant structures such as petals after flowering or fruit when ripe...
planting and vertical shutters
Window shutter
A window shutter is a solid and stable window covering usually consisting of a frame of vertical stiles and horizontal rails...
or blinds
Window blind
A window blind is a type of window coverings. There are many different kinds of window blinds, using different systems and materials. A typical window blind is made with slats of fabric, wood, plastic or metal that adjust by rotating from an open position to a closed position by allowing slats to...
. West-facing windows should be minimized or eliminated in passive solar design.
Solar heat also enters a building through its walls and roof
Roof
A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....
. In temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
climates, a poorly insulated building
Building insulation
building insulation refers broadly to any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation...
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
Cool roof
Cool roofs are the roofs that can deliver high solar reflectance and high thermal emittance...
or a green roof
Green roof
A green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems...
, 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
Radiant barrier
Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection....
and an air gap, which blocks 97% of downward radiation from the sun.
Of the three mechanisms of heat transfer (conduction, convection and radiation), radiation is one of the most significant in most climates, 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 air rises by natural convection. An inlet for cool air at the bottom of this space creates an upward-moving air current.
Allergens such as pollen can be an issue when windows are used for fresh air ventilation. Anything that creates an air pressure difference (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. As an alternative, air can be filtered through a Minimum Efficiency Reporting Value
Minimum Efficiency Reporting Value
Minimum efficiency reporting value, commonly known as MERV rating is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers to rate the effectiveness of air filters...
MERV 8+ air filter to remove allergens.
An energy audit uses a calibrated exhaust fan to measure and locate poor-weatherization air-infiltration leaks cause by careless conventional construction.
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, an international society of HVAC engineers, recommends 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
Thermal mass
Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect...
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.
Examples
- Druk White Lotus School in LadakhLadakhLadakh is a region of Jammu and Kashmir, the northernmost state of the Republic of India. It lies between the Kunlun mountain range in the north and the main Great Himalayas to the south, inhabited by people of Indo-Aryan and Tibetan descent...
, IndiaIndiaIndia , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
makes using of both passive heating and cooling systems.