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Transcript
Passive Heating
Passive heating uses the energy of the sun to keep occupants comfortable without the use
of mechanical systems.
[Direct Solar Gain]
Direct gain is the heat from the sun being collected and contained in an occupied space. This
heat can be retained by the building’s thermal mass, or can be avoided with reflective
materials.
Direct solar gain is important for any site that needs heating, because it is the simplest and
least costly way of passively heating a building with solar heat gain. Avoiding direct solar
gain is also important in hot sunny climates.
Good direct gain is measured or predicted by determining how much heat energy the sun
delivers to the interior space throughout the day and year. In many climates, more heat
gain is desired in the winter, when the sun is low, while less or none is desired in the
summer. Likewise, it is usually desired more in the morning, but less or none in the late
afternoon. Direct gain is not the total amount of incident solar radiation, since not all of
that will enter and be retained in the space.
Sunlight can heat a space through the solid walls or roofs of the envelope. Sunlight also
enters the space through windows, and heats interior surfaces.
Some of the sun's light is long-wavelength radiation, which is heat; in addition, the light of
any wavelength absorbed by surfaces turns into heat in those materials. These materials
then warm people in the room by conducting heat to them directly, by warming air which
carries heat by convection, and by reradiating their heat.
Glazing for Solar Gain:
Aperture placement and area is an enormous factor in the amount of heat that is gained, at
what times of day, and at what seasons of year. For most latitudes, it is optimized by large
areas of equator-facing glazing, with the major living spaces exposed directly behind. Shades
block excess sun at warmer times and allow the sun's heat in at cooler times.
Smart orientation, sizing, and shading must be combined with smart selection of glazing
properties. Different kinds of glazing can pull the sun's heat into the interior, or reject it, or
let interior heat escape. Several glazing types may be appropriate on different faces of the
building.
Avoiding Losses:
The same large areas of glazing that let heat in during the day can also readily let heat out at
night. Heat can escape through windows both through direct heat transfer, or by allowing
long-wave radiation to pass through as interior materials re-radiate their heat. Thus, some
form of night-time protection should be incorporated to minimize any conduction and
convection losses through windows. Thick drawn curtains with a closed cornice board or
"pelmet" to seal the top can be used, or insulated internal/external roller shutters.
Thermal Mass for Solar Gain:
Thermal mass is crucial to comfort with solar heat gain. It absorbs and retains heat, slowing
the rate at which the sun heats the space and the rate at which the space loses heat when
the sun is gone. Without thermal mass, heat that has entered a space will simply re-radiate
back out quickly, making the space overly hot with sunlight and overly cold without.
Surface Colour and Cool Roofs:
The amount of sunlight absorbed by a material (and thus converted to heat) depends on its
colour. Light coloured surfaces will bounce light around within the space, distributing it over
a greater number of surfaces. Dark coloured materials will absorb most of the incident
energy as soon as it strikes. Both of these can be useful, depending on the situation.
Roofs often need to avoid direct solar gain in hot climates. "Cool roofs" use light colours to
reflect most of the sun's heat. Cool roof surfaces are often far more effective than simply
adding roof insulation. For example, the surface of a black roof can easily get 40°C (75°F)
hotter than the surface of a white roof on a sunny day.
[Massing & Orientation for Heating]
Massing and orientation are important design factors to consider for passive heating.
Massing and orientation for passive heating is often helped by extending the east-west axis
of buildings to take advantage of the consistent sun on the northern and southern
exposures.
Many strategies that are appropriate for daylighting are also effecting for passive heating
massing and orientation. However, thinner buildings may not be better. It depends on the
climate and the program. Some distinctions of massing and orientation for passive heating
are:
First, the amount of sunlight that is optimal for daylighting is often not optimal for solar
heat gain.
Second, since the sun's heat does not come from all directions like the sun's light can, walls
facing away from the sun's path get no heat gain, even though they can still get large
amounts of diffuse light.
Third, the sun's heat can be stored by thermal mass, which the sun's light cannot. This can
be useful for west-facing walls to store heat for the night.
Massing Strategies for Passive Heating:
In cold climates, massing that minimizes the ratio of surface area to volume (approaching a
cube or hemisphere) can avoid unwanted heat loss. The sun's heat is advantageous,
though, and more surface area facing it can help passively heat the building. The side of the
building exposed to the sun's path can be increased while reducing the exposed areas of the
other sides of the building.
In hot climates, thin buildings with their biggest face exposed to the sun can cause
unwanted solar heat gain. Shading devices and good windows can be used to reduce this
while still allowing natural ventilation. Taller buildings can also reduce unwanted gains in
hot climates, as the sun's heat strikes more strongly on roofs than on walls in warm
latitudes, and tall buildings have less roof area per unit volume.
It’s important to remember that the sun's heat does not come from all directions like the
sun's light can. Walls facing the sun's path get the most light and the most heat. Windows
facing away from the sun's path can still get large amounts of diffuse light, but without heat
gain (and can have excessive heat loss). Windows facing east are warmed in the morning
when it is often welcome to break the chill of night, but windows facing west are heated in
the afternoon when spaces are generally already warm.
Orientation Strategies for Passive Heating:
Buildings that are longer than they are wide should usually be oriented east-west rather
than north-south. This orientation lets you consistently harness thermal gain, or
consistently avoid it, along the long face of the building. It also lets you minimize the area
that’s subject to faster energy swings from the rising or setting sun. Solar heat gain on the
east side can be acceptable or even useful, because it happens in the morning after the
cooler night; but solar heat gain on the west side is rarely desirable at the end of an already
warm day.
Before determining the materials for the façade, it’s important to understand the patterns
of solar radiation that affect the building. The following image shows solar incident radiation
throughout the day and throughout the year on the five exposed faces of a cube-shaped
building. The vertical axis shows times of day while the horizontal axis shows times of year,
and the colour shows the amount of incident heat.
Material Choices for Orientation:
Material choices and glazing are part of a building's orientation for passive. They can avoid
unwanted solar heat gain, or unlike daylighting, they can store the sun's heat with thermal
mass. An orientation that supplies just enough daylight may supply too much heat, or viceversa.
Equator-facing sides of the building are well suited to capture and store the sun's heat via
large windows and materials with high thermal mass, while sides facing away from the sun's
path are not.
To even out temperature swings at sunrise and sunset, east sides may benefit from more
window area for direct solar heat gain, while west sides may benefit from smaller window
areas and high thermal mass to absorb the heat and release it through the night. The right
strategy depends on the climate.
In cold climates, sides facing away from the sun's path will usually benefit from more
insulation than sides facing the sun (which means less glazing or higher-insulation glazing),
while in hot climates the opposite is true.
Advanced glazing can separate the harvesting of the sun's light from the sun's heat. It can
also pull in daylight from sides facing away from the sun, without losing too much heat
through lack of insulation (low U-value).
[Thermal Mass]
Thermal mass is a material's resistance to change in temperature. Objects with high
thermal mass absorb and retain heat. Thermal mass is crucial to good passive solar heating
design, especially in locations that have large swings of temperature from day to night.
Thermal mass is crucial to good passive solar heating design. Objects with high thermal
mass absorb and retain heat, slowing the rate at which the sun heats a space and the rate at
which a space loses heat when the sun is gone. Without thermal mass, heat that has
entered a space will simply re-radiate back out quickly, making the space overly hot with
sunlight and overly cold without.
Thermal mass has virtually no effect in steady-state heat flow, which is when temperatures
are relatively constant on each side of a material.
Designing with Thermal Mass:
Common architectural implementations of thermal mass storage are concrete floor slabs,
water containers, and interior masonry walls such as the back of a chimney. However, many
materials can be used.
Climates and Thermal Mass:
Thermal mass is most useful in locations that have large swings of temperature from day to
night, such as desert climates. Even if the thermal mass does not prevent heat energy from
flowing into or out of occupied spaces, like insulation would, it can slow the heat flow so
much that it helps people's comfort rather than causing discomfort.
In climates that are constantly hot or constantly cold, the thermal mass effect can actually
be detrimental. This is because all surfaces of the mass will tend towards the average daily
temperature; if this temperature is above or below the comfortable range, it will result in
even more occupant discomfort due to unwanted radiant gains or losses. Thus, in warm
tropical and equatorial climates, buildings tend to be very open and lightweight. In very cold
and sub-polar regions, buildings are usually highly insulated with very little exposed thermal
mass, even if it is used for structural reasons.
Thermal Mass for Solar Gain:
Thermal mass is often critical to direct solar gain passive design.
High thermal mass materials conduct a significant proportion of incoming thermal energy
deep into the material. This means that instead of the first couple of millimetres of a wall
heating up 5–10 degrees, the entire wall heats up only 1–2 degrees. The material then reradiates heat at a lower temperature, but re-radiates it for a longer period of time.
This helps occupants stay more comfortable, longer. When the internal temperature of the
space falls at night, there is more energy still stored within the walls to be re-radiated back
out.
The larger the area of thermal mass receiving direct sunlight, the more heat it receives, so
the faster it can heat up, and the more heat it can store.
Thermal Conductivity with Thermal Mass:
Insulation can be extremely valuable in preventing direct gain from being conducted to the
ground or outside air, where it is lost. In hot climates where direct gain is not desirable, it
can even be beneficial for external finishes to have low thermal mass, as well as low
conductivity, to increase the effectiveness of insulation.
Thermal lag from mass can greatly reduce the need for insulation in the building envelope,
especially in climates with large temperature swings from day to night. Combining thermal
mass with insulation can avoid such unwanted temperature swings indoors, while still
allowing solar heat gain or radiative cooling.
However, thermally conductive materials can be highly desirable inside a space. They will
quickly transfer any heat build-up away from a surface struck by sunlight, deeper into the
material, which both stores and evenly distributes the heat within the space. Whereas in
less conductive materials, the surface will heat up more where the light strikes, creating a
hotspot there while other parts of the space may be cold.
For example, a thick concrete floor will conduct heat and store it relatively evenly
throughout the floor (though areas directly struck by the sun will be warmer). However, a
wood floor will not distribute heat well, because although wood has high thermal mass, it
does not conduct heat well.
Be careful when covering thermal mass with materials such as carpet, cork, wallboard, or
other insulating materials. These coverings will isolate the mass from the solar energy you
may be trying to collect. Materials such as ceramic floor tiles or brick make better choices
for covering a direct gain slab. Tiles should be attached to the slab with a mortar adhesive
and grouted with complete contact to the slab--this ensures good heat conduction.
Rules of Thumb for Designing with Thermal Mass:
Choose the right amount of mass. This is determined by how much heat energy the space
requires (based on the climate, massing, and program), and the solar income (based on
climate, orientation, and surroundings). In general, comfort and performance increase with
increase of thermal mass, and there is no upper limit for the amount of well-designed
thermal mass.
Large surface areas of thermal mass, with sufficient solar exposure. A rule of thumb is a
mass surface-to-glass area ratio is 6:1.
In direct gain storage, thin mass is more effective than thick mass. The most effective
thickness in masonry materials is the first 100mm. Thicknesses beyond 150mm are usually
unhelpful as the heat is simply carried away from the surface and lost. The most effective
thickness in wood is the first 25mm.
Insulating the thermal storage from exterior climate conditions, so that they do not add or
remove too much heat. In some climates, however, direct heat gain from sunlight on the
envelope and/or direct heat loss to the ground are beneficial.
It is important to locate as much thermal mass in direct sunlight (heated by radiation) as
possible. However, the mass that is located out of the direct sunlight (heated by air
convection) is also important for overall performance.
Thermal mass storage is as much as four times more effective when the mass is both heated
directly by the sun and is subject to convective heating from warmed air, compared to being
only heated by convection.
Locating thermal mass in interior partitions is more effective than external walls. Assuming
they both have equal solar access, the internal wall heat will transfer heat out of both
surfaces whereas the external wall will often lose half to the outside.
The most effective internal storage wall masses are those located between two direct gain
spaces.
Thermal mass can be combined with glazing to form "Trombe walls".
[Trombe Wall and Attached Sunspace]
A Trombe wall is a system for indirect solar heat gain and, although not extremely common,
is a good example of thermal mass, solar gain, and glazing properties used together to
achieve human comfort goals passively.
It consists of a dark colored wall of high thermal mass facing the sun, with glazing spaced in
front to leave a small air space. The glazing traps solar radiation like a small greenhouse. An
attached sunspace is essentially a Trombe wall where the air space is so big it is habitable.
Trombe walls are a very useful passive heating system. They require little or no effort to
operate, and are ideal for spaces where silence and privacy are desirable. Sunspaces are
equally simple and silent, and can allow views. Rooms heated by a Trombe wall or sunspace
often feel more comfortable than those heated by forced-air systems, even at lower air
temperatures, because of the radiantly warm surface of the wall.
A successful Trombe wall or attached sunspace optimizes heat gain and minimizes heat loss
during cold times, and avoids excess heat gain in hot times.
Trombe Walls:
Trombe walls are thermal storage walls, named after the French inventor Felix Trombe. A
typical Trombe wall consists of a 20 - 40cm (8" - 16") thick masonry wall painted a dark,
heat-absorbing colour and faced with a single or double layer of glass. The glass is placed
between 2 - 15cm (1" - 6") away from the masonry wall to create a small airspace. Heat
from sunlight passing through the glass is absorbed by the dark surface, stored in the wall,
and conducted slowly inward through the masonry.
The glass prevents the escape of radiant heat from the warm surface of the storage wall.
The heat radiated by the wall is therefore trapped within the air gap, further heating the
wall surface. For a 40cm (16") thick Trombe wall, heat will take about 8 to 10 hours to reach
the interior of the building. This means that the room behind remains comfortable through
the day and receives slow, even heating for many hours after the sun sets. Such designs are
ideal for use in residential living areas and bedrooms.
In addition to radiant heat, you can also configure Trombe walls to heat air within the
internal space. Including upper and lower air vents in the wall allows convection currents, as
cooler air from the room enters at the bottom and air heated in the Trombe wall escapes
into the room at the top. These vents must be operable to prevent reverse convention
currents occurring at night, which would cool the occupied space. Operable vents also allow
the occupants control over instantaneous heating.
Avoiding Losses:
Using low-E glazing can prevent heat from re-radiating out through the glass of a Trombe
wall and greatly reduce the amount of heat lost. Applying a spectrally selective surface or
low-E coating to the wall itself can also improve performance by reducing the amount of
infrared energy radiated towards the glass.
Adapting to Day & Season:
To avoid overheating at hot times of day or hot seasons of the year, architects can use
Trombe walls in conjunction with overhangs, eaves, and other building design elements to
evenly balance solar heat delivery.
Ideally, the glazing should have exterior insulating shutters for night-time use in order to
prevent the heat gained from being returned back to the outside.
While even seasonally-adapting Trombe walls can have no moving parts, you should provide
for some method of cleaning the internal area between the glazing and the storage portion
of a Trombe wall.
Sunspaces:
Attached sunspaces (also called "conservatories") work much like vented Trombe walls.
They can heat spaces both through radiation and convection. The difference is that the
space between the glass and the thermal mass creates a habitable space.
Sunspaces are primarily used for indirect solar heat gain and generally have more glazing
area than floor area. Night-time heat loss is not as critical in a sunspace as in direct gain
systems, since the room can be closed off from the rest of the building. However, night
insulation or double-glazing is recommended if the sunspace serves as living space after
sundown.
Designing Sunspaces:
Important considerations for sunspace design are:
In very cold climates, double glazing reduces conductive losses through the glass to the
outside.
Insulated panels, shades, or blinds are more important for sunspaces than for Trombe walls,
as sunspaces are sometimes occupied.
As with Trombe walls, the darker the internal surfaces of the sunspace, the more effectively
the thermal mass can store heat during the day.
Do not overpopulate conservatories with vegetation, as foliage can reduce the system's
heat capture by significantly shading the floor and wall.
For all climates except those with very cool summers, operable or mechanized windows
should be considered at top and bottom. These allow the sun space to avoid overheating by
passively venting hot air out the top of the glazing and pulling cool air in through the bottom
of the glazing.
Water Walls:
Instead of using masonry, water can be used as the thermal mass of a Trombe wall or
sunspace. Due to convection currents within the water itself, heat is transferred through
the entire thermal mass much quicker than a masonry wall that relies solely on conduction.
This can be useful when a shorter delay in heat delivery is required.
Such systems can not only bring heat into a space, they can be translucent to bring light in
as well. When using a water Trombe wall, it is better to seal the air between the glass and
wall, to further increase the surface temperature of the wall.
Solar Chimney:
During hot seasons, a Trombe wall or sunspace with vents through it can be used as a
thermosiphon. If vents are placed at the top of the glazing, then air from the room will be
pulled out by convection in the air gap between glazing and mass wall. This form of passive
ventilation is called a solar chimney.
[Apertures for Heating]
Windows and other apertures bring in heat from sunshine, but can also lose heat by radiant
cooling and by conducting heat better than most wall or roof constructions. Apertures and
shading must be intelligently placed to take advantage of the sun's heat in cold locations
and seasons, while not overheating in hot seasons.
Placement matters for heating and cooling in much the same way it matters for daylighting.
Openings facing towards the sun's path or on top of the building can pull in large amounts of
heat - usually more than east or west apertures.
As with daylighting, glazing with the right properties must be chosen for the right
orientations. For example, a window that lets in plenty of useful heat when located in one
side of the building might allow terrible heat loss if located in another side.
However, there are some differences to consider when designing apertures for passive
heating. Apertures facing away from the sun's path usually lose heat to the outside. In
many climates, east windows can be desirable to warm spaces early in the day after a cold
night. In warmer climates, west-facing windows can be especially prone to overheating.
Passive solar heating usually combines solar heat gain with thermal mass inside the building,
to capture and store the sun's heat for slow release through the night.
[Shading for Solar Heat Gain]
Shades can keep the heat and glare of direct sun from coming through windows. They can
also keep direct sunlight off of walls or roofs, to reduce cooling loads.
The most common form of shade is an exterior fixed horizontal overhang. These are used
on the side of the building facing the sun's path, sometimes including east and west faces.
However, east and west faces often have more need of vertical fins to avoid low-angled sun.
The side of the building facing away from the equator needs no shading, except near the
equator where the sun may be on the north or south side depending on the season.
There are many variations on fixed external shades, to reduce the profile and/or let more
diffuse light in.
In hot climates, it can be especially useful to shade the building's roof to avoid solar heat
gain. Rooftop solar panels, if placed right, can act as shades and thus perform double duty
as energy generators and energy load reducers.
Adaptive Shades:
Shading can be designed to allow the sun's light and heat into the building at some times of
day or year, while rejecting it at other times. The simplest method for this is to use a fixed
horizontal overhang whose width is calculated to shade during summer months when the
sun is high, and allow the sunlight in during winter months when the sun is at a lower angle.
You can visualize the sizes for such overhangs for your location with this shading angle tool.
Shading can also be adapted by making it movable--either manually operated by occupants
or automatically controlled. Such systems can be much more responsive and finely tuned,
but they are also more expensive, and require more maintenance and repair over the years.
User-operated systems may require occupant training, and are often not properly used.
(http://sustainabilityworkshop.autodesk.com/buildings/passive-heating)