Passive
Solar Architecture - Heating
Passive
Solar Heating presents the most cost effective means
of providing heat to buildings.
Generally, the amount of solar energy that falls
on the roof of a house is more than the total energy
consumed within the house. Passive solar applications,
when included in initial building design, adds little or
nothing to the cost of a building, yet has the effect of
realizing a reduction in operational costs and reduced
equipment demand. It is reliable, mechanically simple, and is a viable asset to
a home. The
following are rules of thumb and an explanation of the
essentials of passive solar design.
The
mechanism of heating and cooling equipment is usually
referred to as a system. A building is designed (home,
apartment house, etc.) and a heating/cooling system
using forced air equipment with air ducts; radiant
floors using hot water; etc., is specifically designed
for it. In
passive building designs the system is integrated into
the building elements and materials - the windows, walls, floors, and roof are used as the heat
collecting, storing, releasing, and distributing system.
These very same elements are also a major element in
passive cooling design but in a very different manner.
It should be understood that passive solar design
does not necessarily mean the elimination of standard
mechanical systems, although recent designs coupled high
efficiency back-up heating systems greatly reduce the
size of the traditional heating systems and reduce the
amount of non-renewable fuels needed to maintain
comfortable indoor temperatures, even in the coldest
climates.
The
preceding explanations show that two elements must be
present in all passive solar heating designs:
a south facing exposure of transparent material
(glass, plastic) to allow solar energy to enter; and a
material to absorb and store the heat (or cool) for
later use. With
these two basic elements in mind a number of approaches
to designing a passive solar heated structure are
available. Passive
cooling is discussed elsewhere in this tutorial.
PASSIVE
SOLAR DESIGN
(top)
The
following section contains a brief description of basic
passive solar design approaches. Subsequent
sections contain more detailed information
regarding each design and some advantages and
limitations of each.
Do not assume that because a particular design is
more effective for a particular purpose (i.e. water
walls respond more quickly in the absorption and release
of heat) that it will serve in all climates and in all
designs as the most effective approach to passive solar
heating or cooling.
Conversely, identified limitations do not mean
that the approach is ineffective, only that it is more
appropriate and effective under specific conditions.
In choosing a particular design approach, site
and climate conditions must be evaluated carefully
so that the best approach or combination of approaches
is incorporated. No
one passive design approach is most advantageous in all
climates or on all sites and situations.
In the building design industry there are certain
ways of doing things that have been developed over years
of experience. These
are sometimes called rules of thumb (RT). The same is true of passive solar building design.
In the following sections, rules of thumb are
identified for emphasis.
Keep in mind that these are simply guidelines
that will, if observed, produce favorable passive solar
heating performance.
In some cases tables that give detailed
information based on climate and site variations
supplement the rules of thumb.
DIRECT
GAIN
(top)
The
simplest of approaches is a direct gain design. Sunlight
is admitted to the space (by south facing glass) and
virtually all of it is converted to thermal energy. The
walls and floor are used for solar collection and
thermal storage by intercepting radiation directly,
and/or by absorbing reflected or reradiated energy (Fig.
4). As long as the room temperature remains high in the
interior space storage mass (walls, floors) will conduct
heat to their cores. At night, when outside temperatures
drop and the interior space cools, the heat flow into
the storage masses is reversed and heat is given up to
the interior space in order to reach equilibrium. This
re-radiation of collected daytime heat can maintain a
comfortable temperature during cool/cold nights and can
extend through several cloudy days without
"recharging".
Direct
gain design is simple in concept and can employ a wide
variety of materials and combinations of ideas that will
depend greatly upon the site and topography; building
location and orientation; building shape (depth, length,
and volume); and space use.
|
|
|
|
| Figure 4. Direct
gain design - A direct gain design collects and
stores heat during the day. At night stored
heat is radiated into the living spaces. |
|
Figure 5. Direct
gain interior - A direct gain design with an
interior water wall for heat storage. Heat
stored in the water wall is radiated into the
living space at night. |
A
direct gain design requires about one-half to two-thirds
of the total interior surface area (RT)
to be constructed of thermal storage materials. These
can include floor, ceiling and wall elements, and the
materials can range from masonry (concrete, adobe,
brick, etc.) to water (Fig. 5). Water contained within
plastic or metal containment and placed in the direct
path of the sun's rays has the advantage of heating more
quickly and more evenly than masonry walls during the
convection process. The convection process also prevents
surface temperatures from becoming too extreme as they
sometimes do when dark colored masonry surfaces receive
direct sunlight. The masonry heating problem can
be alleviated by using a glazing material that scatters
sunlight so that it is more evenly distributed over
walls, ceiling, and floor storage masses (Fig. 6). This
decreases the intensity of rays reaching any single
surface but does not reduce the amount of solar energy
entering the space.
 |
| Figure 6.
Diffusing glazing materials. Translucent
glazing scatters sunlight to all storage surfaces. |
INDIRECT
GAIN
(top)
This
passive solar design approach uses the basic elements of
collection and storage of heat in combination with the
convection process. In this approach, thermal storage
materials are placed between the interior
habitable space and the sun so there is no direct
heating. Instead a dark colored thermal storage wall is
placed just behind a south facing glazing (windows).
Sunlight enters through the glass and is immediately
absorbed at the surface of the storage wall where it is
either stored or eventually conducted through the
material mass to the inside space. In most cases the
masonry thermal storage mass cannot absorb solar energy
as fast as it enters the space between the mass and the
window area. Temperatures in this space can easily
exceed 100°F. This build up of heat can be utilized to
warm a space by providing heat-distributing vents the
top of the wall (where the heated air, rising upward due
to less density, can flow into the interior space (Fig.
7). Vents at the bottom of the wall allow cool air to be
drawn into the heating space thereby replacing the
outflowing hot air, and picking up heat itself. The top
and bottom vents continue to circulate air as long as
the air entering the bottom vent is cooler than the air
leaving the top vent. This is known as a natural
convective loop. At night the vents can be closed to
keep cold air out and the interior space is then heated
by the storage mass, which gives up its heat by
radiation as the room cools. A variation of the vented
masonry wall design is one that employs a water wall
between the sun and the interior space (Fig. 8). Water
walls used in this way need not be vented at top and
bottom and can be constructed in many ways - even
55-gallon drums filled with water, or specially
constructed plastic or sealed concrete containers.
Again, as the water is heated, the convection process
quickly distributes the heat throughout the mass and the
interior space is warmed by heat radiated from the wall.
Figure 7. Indirect gain
Trombe wall stores heat during the day. Excess
heat is vented to the interior space. At night
Trombe wall vents are closed and the storage wall
radiates heat into the interior space.
|
|
Figure 8. Indirect gain water
wall collects and stores heat during the day. Heat
stored in indirect gain water wall is radiated
into the living space at night.
|
|
Another
design approach (Fig. 9) takes advantage of the
greenhouse effect as well as the direct gain storage
wall. A south facing "greenhouse space" is
constructed in front of a thermal storage wall exposed
to the direct rays of the sun. This wall would be at the
rear of the greenhouse and the front of the primary
structure. The thermal wall absorbs heat at the same
time the interior space of the greenhouse is being
heated. If a vented masonry wall is used as storage,
heat can also be released into the living space by
convection. This combination also works with an unvented
water wall. The greenhouse, then, is heated by direct
gain while the living space is heated by indirect gain
(Fig. 10). The advantage is that a tempered greenhouse
condition can be maintained through days of no sun, with
heating from both sides of the thermal storage wall.
 |
 |
| Figure 9. Attached
greenhouse with vented storage wall. Heat is
stored in the wall during the day - excess heat is
vented to the interior space. At night the wall
vents are closed and stored heat is radiated to
both the greenhouse and the interior space. |
Figure 10.
Attached greenhouse with water storage wall.
|
An
indirect gain design which provides both heating and
cooling is the thermal pond approach, which uses
water encased in ultraviolet ray inhibiting plastic beds
underlined with a dark color, that are placed on a roof.
In warm and temperate climates with low precipitation,
the flat roof structure also serves directly as a
ceiling for the living spaces below (Fig. 11) thereby
facilitating direct transfer of heating and cooling for
the spaces below. In colder climes, where heating is
more desirable, attic ponds under pitched roof glazing
are effective. Winter heating occurs when sunlight heats
the water, which then radiates energy into the living
space as well as absorbs heat within the water thermal
mass for nighttime distribution. During the summer, a
reverse process, described later, occurs. For best
effect, roof ponds must be insulated (movable) so that
heat will not radiate and be lost to the outside. One of
the major advantages of this approach is that it allows
all rooms to have their own radiant energy source with
little concern about the orientation of the structure or
optimal building form.
 |
Figure 11. Heating
cycle - Roof pond collects and stores heat during
the day. At night roof ponds are covered and
stored heat is radiated into the space below. |
ISOLATED
GAIN
Finally,
the isolated gain design approach uses a fluid (liquid
or air) to collect heat in a flat plate solar
collector attached to the structure. Heat is
transferred through ducts or pipes by natural convection
to a storage area - comprised of a bin (for air) or a
tank (for liquid), where the collected cooler air or
water is displaced and forced back to the collector
(Fig. 12).
 |
Figure 12. Water
or air convection loop. |
If
air is used as the transfer medium in a convection loop,
heated air coming from the collector is usually directed
into a rock (or other masonry mass material) bin where
heat is absorbed by the rocks from the air. As the air
passes its heat to the rocks it cools, falls to the
bottom of the bin and is returned to the collector
completing the cycle. At night the interior space of the
structure is heated by convection of the collected
radiant energy from the rock bin. If water is the
transfer medium, the process works in much the same way
except that heat is stored in a tank, and as hot water
is introduced, cooler water is circulated to the
collector. In naturally occurring convection systems
(non-mechanically assisted) collectors must be lower
than storage units, which must be lower than the spaces
to be heated (RT). Of
course, the addition of distribution assisting equipment
can allow for placement of system elements anywhere, but
that would then be an Active Solar System.
*PASSIVE
SOLAR HEATING DESIGN CONSIDERATIONS:
SITE
CONSIDERATIONS
(top)
The
performance of any solar energy building, especially one
of a passive design, is strongly impacted by the site
and the siting of the building in relation to its
surroundings. During the winter, the north side of a
building receives little direct solar impact due to
shading from the winter sun, which is low in the sky,
while the south side is exposed to the benefits of
winter sun exposure. If site conditions are restricted,
the passive solar heated building location will be the
area that receives the most sunlight between the hours
of 9:00 a.m. and 3:00 p.m. during the winter months. The
building location should be near at northern extremity
of the sunny area so that future development on
properties to the south will not block access to winter
sunlight. This location also allows open space for
winter activities or gardens to be exposed to as much
winter sun as possible.
BUILDING
SHAPE AND ORIENTATION
(top)
Generally,
buildings oriented along an east-west axis are more
efficient for both winter heating and summer cooling (RT).
This orientation allows for maximum solar glazing
(windows) to the south for solar capture for heating.
This orientation is also advantageous for summer cooling
conditions since it minimizes east-west exposure to
morning and afternoon summer sunlight. This does not
mean that all buildings must be rigidly shaped oriented.
Different building shapes and orientations can be
designed to perform efficiently by combining effective
glazing, solar exposure, and shading into the building
form. This efficiency can be enhanced by variations in
the placement of interior spaces and by the use of such
options as clerestories and skylights. Depending upon
the site, topography, and shape of the available space,
orientations other than east and west may be desirable.
However, for most climates, an east-west axis is the
most efficient for both heating and cooling.
NORTH
WALLS
(top)
In
an east-west oriented building, a north facing exterior
wall will receive little sunlight during the winter and
this will be a major source of heat loss since heat
always moves toward cold. Additionally, building shading
of north side open space usually renders it unusable for
outdoor use. To alleviate these situations the building
should be shaped so that the roof slopes downward from
the south to the north wall. This reduces the height of
the north face of the building and therefore the area
through which heat is lost. This also allows sunlight to
reach more area of north side outdoor spaces. Variations
of reducing heat loss conditions manifest in north walls
include backing the building into a sloped hillside or
providing a berm, both of which reduce the exposed north
area. Both of these measures accomplish the purpose of a
south to north downward sloping wall (Fig. 13).
 |
| Figure 13. South
to north downward sloping roof reduces heat loss
from the north wall and allows sunlight to reach
north side open space. |
INDOOR
SPACE PLANNING
(top)
With
location, orientation and shape of the building is the
consideration of interior space distribution. Habitable
spaces that are most occupied and have the greatest
heating and lighting requirement should be arrayed along
the south face of the building. Rooms that are least
used (closets, storage areas, garages) should be placed
along the north wall where they can act as a buffer
between high use living space and the cold north side
(Fig. 14).
 |
| Figure 14.
Interior space should be arranged so that rooms
with high heating and lighting requirements are
arrayed along the south wall. |
ENTRYWAYS
(top)
Entries
account for great deal of heat loss especially in small
structures. Heat is lost during opening and closing of
doors (or windows). Heat can also be lost by seeping
between the doorframe and the door, and at windows. This
kind of window, door or other wall penetration heat loss
(or heat gain in the summer) is called infiltration.
To reduce both direct and infiltration losses, entryways
should be recessed or protected against the direct force
of prevailing winds. Additional loss reduction can be
accomplished by providing an enclosed interior "air
lock" space between an entrance door and the main
building. This double entry, or vestibule, creates a
tempered zone between the outside elements and the
interior living space thereby reducing the amount of
warm air lost (and summer heat gained). It also reduces
the amount of cold or warm air entering the living space
when the interior door is opened.
WINDOWS
(top)
The
major expanse of windows in a passive solar energy
structure will be south facing solar windows. Whole
design planning should include considerations re: the
impact of heat gain in the summer; views; natural
lighting; and privacy requirements in determining the
placement and size of windows in the structure.
For
the most part, window areas on east, west and north
facing walls should be kept as small and as minimal as
is consistent with interior requirements and should be
recessed and all should be double-glazed. Windows are
the least effective heat flow inhibitors of a building's
shell, both in terms of letting heat out in the winter,
and letting heat in the summer. When the outside
temperature is 30°F and the inside temperature is 68°F,
a square foot of single pane glass will lose 20 times as
much heat (about 43 BTU's per hour) as a square foot of
standard wood frame wall with 3'/z inches of insulation.
DIRECT
GAIN DESIGN
(top)
SOLAR
WINDOWS
(top)
For
Direct Gain heating the area of the glazed collecting
surface is determined in response to the duration and
severity of winter temperatures; the building size; and
the amount of interior thermal storage mass. A correct
balance between these factors must be found in order to
avoid large daily temperature fluctuations that could
result in overheating, even in the winter. As a rule
(assuming the correct amount of thermal storage mass),
0.19 to 0.38 square feet of south facing glass for each
square foot of interior floor area will provide enough
sunlight to maintain an average temperature between 65°F
and 70°F during the winter months in cold climates
(average winter temperatures between 20°F and 30°F).
In more moderate climes the same temperatures can be
achieved with 0.11 to 0.25 square feet of south facing
glass (average winter temperatures between 35°F and 40°F).
More precise data is given in Table 3. Location and
sizing of glazing is also dependent upon the building
layout and types of spaces i.e., frequently used spaces
vs. infrequently used spaces. Adjustments in glazing
size and location of solar windows can occur by the use
of reflectors, or when other passive solar design
elements are used in combination with a direct gain
system such as heat loss reduction by the use of movable
insulation, double glazing, and by using wooden sash and
frames. The lower heat conductivity of wood can reduce
the heat loss around windows by as much as 20 percent.
CLERESTORIES
AND SKYLIGHTS
(top)
Earlier
it was mentioned that there may be considerations that
override the need for a large expanse of south facing
windows in passive heating designs. One is that interior
room layout may affect the distance between the
collecting windows and the interior thermal storage mass
wall. Thermal storage masses in direct gain designs must
receive direct sunlight impact, and the farther away
from the collecting surface they are, the taller the
collecting surface (glazing) must be in order to have
sufficient surface solar contact because of the sun's
angle. This situation may produce intolerable glare and
overheating. Usually, storage walls should not be set
back more than 1.25 to 1.5 times the height of the
collecting surface - i.e. an interior thermal storage
wall should be no more than 12' from the 8' tall solar
collecting glazing wall. Also, direct sunlight may
impact materials - i.e. dry out wood furniture; discolor
certain fabrics; etc. so care should be taken re:
interior decorating and maintenance.
Finally, and most important, adjacent structures
and/or vegetation may reduce the amount of direct
sunlight to the point that a south facing collecting
surface at ground level becomes infeasible or seriously
reduced.
Any
one, or combination of these conditions make south wall
heat collection for direct gain problematic, but can be
easily mitigated by the use of clerestories or
skylights. Both of these features admit sunlight at the
roof structure of a building and can be used to direct
sunlight to a specific interior surface. They can also
be used in combination with or as a supplement to a
south facing glazed wall. Additionally, they provide for
natural light applications, which can reduce the need
and cost of artificial lighting.
Clerestories
are vertical south facing windows located at roof level
(Fig. 15). Their advantages are that they allow diffuse
lighting into a room; they provide privacy; and they can
be placed almost anywhere on a roof. In a
compartmentalized building layout, each room can have
its own source of heat and light. They should be located
at a distance from a thermal storage wall that allows
direct sunlight to hit the wall throughout the winter.
This distance is roughly 1.5 times the height of the
wall. Ceilings in rooms containing clerestories should
be light in color to reflect or diffuse sunlight into
the living space. Large
interior spaces may have multiple clerestories arranged
to allow maximum admission of sunlight. Care must be
taken that they do not shade each other, so the
clerestorey roof angle (from horizontal) of each
clerestorey should be roughly the same angle of the sun
at its lowest winter point (noon on December 21).
Skylights are simply openings in a roof, which
admit sunlight -they are either horizontal (a flat roof)
or pitched at the same angle as the roof slope. In most
cases. horizontal skylights are used with reflectors to
increase the intensity of solar radiation (remember the
angle of incidence). Large skylights should be provided
with shading devices to prevent heat loss at night and
heat gain during the summer months (Fig. 16).
|
|
|
| Figure 15.
Clerestory - clerestories can be used to provide
sunshine onto interior walls which would normally
not have a clear view of winter sunlight. |
Figure 16.
Skylights provide an alternative for direct solar
gain, shading devices must be included as an
integral part of the skylight to prevent
overheating the space during mild periods. |
STORING
HEAT – MASONRY
(top)
The
major concern in designing a direct gain passive solar
structure to avoid uncomfortable temperature
fluctuations over the day-night cycle. It is relatively
easy to calculate the amount of heat that will be
admitted to a room by any particular collecting surface.
It is not, however, easy to predict the percent of that
heat which will be stored. Since the difference between
heat gained and heat stored will largely determine the
temperature fluctuation over a 24-hour period, this is
one of the most important design considerations. About
65 percent of all heat gained through solar windows
during a clear winter day can be lost during the night.
This means that during the day, 65 percent of the heat
gained must be stored to offset nighttime losses, and to
maintain a relatively even temperature profile.
Furthermore, efficient storage during the day prevents
the build-up of heat during daylight hours.
In
general terms, the following rules should be observed in
regard to design of heat storage systems:
*
masonry and concrete floors, walls and ceilings to be
used for heat storage should be a minimum of 4 inches
thick.
*
sunlight should be distributed over as much of the
storage mass surface as possible by using translucent
glazing. * a number of small windows to admit sunlight
in patches gives better control re: overheating.
*
use light colored surfaces (non-thermal mass storage
walls, ceilings, floors) to reflect sunlight to thermal
storage mass elements.
*
thermal storage mass elements (floors, walls, ceilings)
should b dark in color.
*
masonry floors used for thermal mass should not be
covered with wall-to-wall carpeting.
*
direct sunlight should not hit dark colored masonry for
long periods of time.
Storing
heat can be accomplished in many ways but the most
favorable storage occurs when each square foot of
sunlight is spread (diffused) over a nine square foot
area of storage surface. With this distribution of
sunlight the storage mass need not be more than 4 inches
thick. If sunlight is distributed over less area,
storage efficiency can be increased somewhat by
increasing the thickness of the storage mass- up to
eight inches in thickness. Increasing thickness beyond 8
inches has no beneficial effect on heat storage
efficiency. Therefore, the most efficient way to
increase heat storage capacity is to increase the
storage surface area and the distribution of sunlight
rather than the thickness of the storage mass. This is
because masonry absorbs heat slowly, and intense
sunlight on a small area will have a negative affect by
increasing room temperature while not significantly
increasing the rate heat is absorbed by the storage
mass, while a system using dispersed, less intense
radiation across a larger surface of thermal mass
storage will moderate room temperature fluctuations and
store most heat at the same time.
The
heat retention efficiency of masonry storage masses is
also influenced by the kind of masonry used. Table 4
shows that magnesium brick has twice the conductivity of
the other materials listed. Higher conductivity means
that a material responds more quickly in both absorbing
and giving up heat - a quality that increases storage
efficiency and decreases temperature fluctuations.
 |
| Table 4. Thermal
storage material properties. |
INTERIOR
WATER WALLS
(top)
Because
of the material's good convective properties, interior
walls of water (water walls) are much more efficient for
thermal collection, storage, and re-radiation than are
masonry walls. Water walls should be a dark color to
increase heat absorption when exposed to direct
sunlight, and will perform well without the problem of
heat build-up in the room. The convection process
carries heat away from the storage surface quickly,
preventing heat build-up, and allows the storage mass to
heat evenly in a relatively short time. Widely
fluctuating interior temperatures are not often a
problem when interior water walls are used for heat
storage.
If
interior water walls receive direct sunlight between the
hours of 10:00 a.m. and 2:00 p.m. and are
a dark color, about one cubic foot of water is
required for each square foot of solar window to
maintain comfortable temperatures during
winter months. Increasing the volume of water in
this ratio to 3 cubic fed of water will decrease daily
temperature fluctuations by as much as 6°F.
INDIRECT
GAIN DESIGN:
(top)
Properly
sized, indirect gain thermal storage elements, usually
walls, can provide a high percentage of the heat
required to keep a space at comfortable temperatures,
even in very cold climates. Thermal storage walls, like
interior storage walls, can be constructed of either
masonry or water. For masonry walls, between 0.43 and
1.0 square feet of south facing double glazed solar
window is needed for each square foot of floor area in
climates with average winter temperatures between 20°F
and 30°F A smaller area can be used (0.31-0.85) if
water is the storage medium. In climates where the
average winter temperature is 35-F to 45°F, the figures
are 0.22 to 0.6 for masonry walls and 0.16 to 0.43 for
water walls. Table 5 gives more precise information.
 |
| Table 5. Sizing a
thermal storage wall for different climatic
conditions. |
THERMAL
STORAGE WALL SIZE
(top)
Correct
wall size varies with local conditions and climate; the
amount of insulation used; and the latitude of the
building site. The first two elements affect the rate of
temperature loss from a space, which is influenced by
the amount of difference between outside and inside
temperatures; and how much insulation is used to slow
that rate. Latitude is important because it affects the amount of solar
radiation received at the collecting surface on any
particular day. For instance, on a clear January day in
Tulsa, Oklahoma, a square foot of collecting surface
will intercept 1883 BTU's while on the same day, a like
amount of surface located in Seattle will intercept only
1537 BTU's (assuming the same slope for both surfaces).
Local conditions aside, it is generally true that wall
size will need to be increased as one moves further
north in latitude.
Other
conditions that bear on wall size include obstructions
to solar radiation exposure, such as trees, structures,
etc.. While unhampered solar access will determine an
optimal wall size, reduced or variable access to the sun
will require a larger wall area to compensate for
reduced solar impact. Sizing guidelines identified
herein are based on providing enough heat on a clear
January day to maintain an average space temperature of
65 to 75°F over a 24-hour period. Since thermal storage
walls are placed between the solar collecting glazing
and the interior space to be heated, overheating is less
problematic than with a direct gain design. Very cold
climates, or where site conditions allow less than
recommended wall size, can benefit from the use of
reflectors, which bounce the sun's rays to the
collection window, thereby increasing the amount of
solar radiation moving through the glazing and
intercepted by the energy absorber wall. Thermal wall
size reduction can occur by as much as 15% if
substantial insulation is used at all other wall and
roof locations to reduce heat loss from the the
building.
WALL
DETAILS
(top)
Aside
from the wall area and amount of exposure, the
thickness, type of material, and exposed surface color
of a wall are main considerations that determine the
effectiveness of a wall in meeting the thermal needs of the structure's
occupants. In general, cold climate design benefits from
dark, exterior surface colors at the thermal storage
wall.
A
variation of the cold climate, indirect gain masonry
wall, which relies on absorption of solar energy at
its face and radiation of converted heat at its back, is
the addition of vents at upper and lower wall locations
to provide convected heat to the interior space. Thermal
storage wall vents take advantage of the excessive heat
build up between the glazing and the surface of the
storage wall (Table 7) which can easily exceed 100°F.
Venting the top and bottom of the wall of the living
space allows trapped heated air to move, rising (hot air
is lighter than cold air) from the space between the
glazing and wall and flowing into the building interior
through upper vents. As warm air moves through the
vents, cooler room air is drawn through the lower vents,
where it is warmed and rises to repeat the action. This
convective cycle directly warms the interior
space at the same time heat is being absorbed by the
thermal mass of the wall, which will give up its heat to
the interior at a later time. This dual action is a
benefit for direct heating as well as delayed heating.
 |
|
| Table 6. Material
and recommended thickness.
|
|
 |
|
| Table 7. Effect
of wall thickness on space air temperature
fluctuations. |
|
Convection
can continue 2 to 3 hours after sunset due to the heated
exterior surface of the thermal storage wall, so it is
important that upper vents are closed when the exterior
surface begins to cool. This action prevents a reverse
convection cycle, which removes heat from the space.
Interior space heating quality can be controlled by
providing a movable reflective screen across part or all
of the exterior surface of the thermal wall. Vent sizing
is about two square feet for each 100 square feet of
thermal wall wall surface, equally distributed between
top and bottom vents (Fig. 17).
 |
Figure 17. Masonry
wall - Vent placement and sizing in mass wall
should be as shown in the illustration. |
Appropriate
wall thickness varies with the material used (Table 6).
Again, the idea is to provide comfortable conditions
while avoiding large and rapid temperature fluctuations.
Table 7 gives an idea of the range of temperature
fluctuations for six different wall thicknesses
constructed of five different materials. Regardless of
the material used, it can be expected that while thick
walls will produce both minimum and maximum temperatures
at different times of day than thin walls,
due to the additional time it takes heat to be
conducted, their increased capacity for assimilating and
radiating heat will be much greater .
Water
walls are slightly more efficient than masonry walls in
collection, retention and re-radiation of heat. They
require different design consideration i.e. containing
the water in an effective, inexpensive and esthetically
pleasing way. There are a variety of water encasing
systems ranging from manufactured, elegant free standing
cylinders and containers to do-it-yourself systems of
used, stacked drums, free-standing plastic cylinders,
and site-built tanks.
ATTACHED
GREENHOUSES
(top)
An
indirect gain heating system, using a green house
structure as a heat collector, is multi-purpose,
practical and efficient. It also requires the most
rigorous design because of its multiple nature, which
affects sizing for space heating, creating ideal
conditions for greenhouse conditions, and accurately
predicting performance for both. The attachment of a
greenhouse to the south side of a building enables the
structure to benefit from the normal, heat collecting
greenhouse operation. The greenhouse collects heat due
to its solar exposure. This heat can be conducted
through a thermal storage wall separating house and
greenhouse, or can be convected to the interior space of
the building. In this way the greenhouse serves both as
a heat collector, and a solarium for people and plants.
Generally, in cold climates, a greenhouse design would
use between 0.65 and 1.5 square feet of south facing
double glass greenhouse collecting surface for each
square foot of floor area in the adjacent living space.
In more moderate climates this can be reduced to 0.33
and 0.9 square feet. This should provide enough heat to
keep the average temperature in the adjacent space
between 60°F and 70°F. More data are given in Table 8.
 |
|
| Table 8. Sizing
the attached greenhouse for different climatic
conditions. |
|
It
is desirable for a solar greenhouse structure to be
recessed into the south facade of the building, thereby
minimizing east and west exposures, which have little
effect on heat collection but can be a great source of
heat loss. Furthermore, heat transfer through the common
wall between the greenhouse and the living space is
increased in this configuration (Fig. 18). Greenhouses
for heating purposes can be added to frame buildings to
provide for direct heat during winter days, but such
buildings, without thermal storage mass, do not have the
ability nor capacity to store heat for use at night.
Some designs integrating greenhouses, adjust for
nighttime heating by transfer of all greenhouse heat to
main building storage mass (floors, walls, etc.) for
deferred use at night. This only works in moderately
cold climates, because very cold climates require the
residual heat collected by the greenhouse to keep it,
and its contents, from freezing at night.
The
common wall between the greenhouse and the building
interior can be constructed of thermal mass materials
(masonry, water, etc.). The greenhouse side of common
walls should be dark in color (better absorber) and
should receive maximum sunlight throughout the day. Wall
vents and/or operable windows can be used to allow
heated air directly into the interior space during the
daytime.
Wall
thickness should be same as that provided for in an
indirect gain (Trombe) wall. If water is used, its
minimum thickness should be 8 inches (or 0.67 cubic feet
for each square foot of south-facing glass).
Masonry
walls cannot absorb and transfer heat as fast as a
greenhouse can collect it. As a result, temperatures in
the greenhouse will fluctuate as much as 60°F on a
clear day. To dampen (level out) these fluctuations,
extra storage mass (such as masonry units or containers
filled with water) can be placed in the greenhouse.
These act as an interior heat dampening water wall (1
cubic foot of water for each square foot of south facing
glass will reduce temperature fluctuations 25°F to
29°F).
If
water is used as the common wall between the greenhouse
and the living space, temperature fluctuations will be
smaller, and if more than the 0.67 cubic feet of water
for each square foot of glass is used in the wall,
temperature fluctuations will be further reduced.
In
cold climates, it may be advantageous to utilize a heat
storage, in the form of a rock bin, under the building
or living space, which would act as thermal storage of
heat collected during the day. Heat transfer can occur
by natural convection if the building is terraced up a
slope, or by use of a fan, which would transfer heat to
a rock bed located in a crawl space under the floor of
the structure (Fig. 19). The rock bed should spread
across 75 to 100% of the floor area of the structure in
cold climates, and 50 to 75% of the floor area in
moderate climates. Heat from the greenhouse should be
directed over the rock bed and a means of returning cold
air from the bottom of the rock bed to the greenhouse
should be provided. If a terraced design is used, colder
air will naturally settle due to the convective loop
cycle. About 1.5 to 3 cubic feet of fist-sized rock is
necessary in cold climates and .5 to 1 cubic feet in
temperate climates. Rock bin storage has also been used
as a part of a cooling system in warmer climes.
 |
|
 |
| Figure 18. By
extending the end walls of an attached greenhouse,
loss to the outside is reduced while the heat
gains to the space are incresed. |
|
Figure 19. Small
fans (less than 0.25 H.P.) can be used to aid in
the transfer of heat from collection space to more
remote parts of the structure. |
ROOF
PONDS
(top)
Roof
ponds can be used both for heating during the winter
months and for cooling during the summer months. In this
section only the heating aspects of roof ponds will be
discussed. Table 9 gives the ratios of roof pond
collecting surface for each square foot of floor area in
the interior space. It should be understood that this
will vary depending on location, exposure and local
conditions. The lower ratio given in the table should be
adequate at lower latitudes while the higher ratio
should be used at higher latitudes (colder climates).
For latitudes higher that 36° north, roof ponds require
greater solar gain exposure as well as greater
protection from loss of gained heat.
 |
Table
9. |
The
system is simple in concept. The roof pond approach
brings the differing building aspects of a building - roof, ceiling, heating (and cooling) system, and heat
distribution (does away with ducts) into one system. The
roof ponds of contained water are the heating (and
cooling) unit. The roof/ceilings of the building act as
the structural support for the roof ponds; the
"radiator" device for evenly distributed
heating of the spaces below; and as a waterproof roof
system providing protection from the elements. The
movable insulation above the ponds is the weather
protection, heating/cooling system "manager"
and additional protection from the elements. Wintertime
heating is comprised of daytime opening the insulating
roof layer to allow solar radiation to heat the water
beds; water bed warming heats the supporting structure
which is also the ceiling for spaces below; heated
support structure radiates heat to the space. At night
the insulated roof panels close to contain heat gathered
by the ponds to continue heating the spaces below.
Cooling strategies, discussed later, are the opposite
operation plus additional elements.
The
sun in northern latitudes is at a lower angle with solar
radiation traveling through a greater mass of
atmosphere, which reduces its energy content by
scattering and reflection. In this situation, increased
area of exposure or use of solar reflection can be used
to increase roof pond effectiveness. Additionally, in
colder climates, the roof pond system benefits from
insulating covers to prevent nighttime losses. The most
beneficial insulation system is one that is
multi-purpose and movable, operating only twice a
day to 1) expose the ponds for heat collection, and 2)
to cover the ponds to prevent heat loss at night. This
insulation system is also beneficial in the summer when
the roof ponds must be insulated to prevent summer heat
gain. The movable insulation structure can operate in a
number of ways - rolling, hinged, etc.. In climates
where snow is likely, ponds can be placed in a solar
attic below the sloping roof with south facing glazing
to allow solar gain, and the attic ceiling can be
painted o a reflective color or sheathed with a
reflective material. To increase system performance,
glazing for cold climate solar ponds can be dual pane,
or the ponds can contain an upper layer inflated air
cell.
Ceiling
structural support for solar ponds (64#/cu. ft.) include
structural metal decking (excellent for thermal transfer
to spaces below) or thin reinforced concrete decks (more
costly, less effective for direct transfer of heat).
In
constructing the support structure for roof ponds, the
clear span can be as much as 16 - 20 feet or more (for
metal decking or reinforced concrete), requiring
intermediate structural beams and supports depending on
the layout of the interior space and the weight of the
ponds and insulating devices. This can be a complicated
matter and it is recommended that assistance be sought
from a structural engineer prior to design.
It
is important to provide a waterproof layer (membrane,
etc.) at the pond support system surface to provide
protections during draining of water for maintenance,
and from water bed material failure and/or weather
impacts. The
capability to drain the ponds in an easy and
non-damaging manner is important. The water should be
enclosed in ultra-violet light inhibiting (prevents
degradation) plastic bags, waterproof structural metal
or fiberglass tanks which form the ceiling below. The
top of the water containment system must be transparent
and the sides/bottom a dark color. Insulation panels
should be constructed so that they can be tightly sealed
when closed to prevent infiltration heat loss. In some
applications insulating panels can also serve as
reflectors when open in order to direct more solar to
the ponds.
Heat
is transferred from the roof ponds through the support
deck to the interior space below. The edges of the deck
should be carefully insulated to reduce heat loss. The
underside of the support deck serves as an interior
ceiling, and all surfaces (including galvanized metal
decking) should be painted. Because the system provides
a "radiant" ceiling it is important that no
insulation is used
between the root pond and the interior space. The one
exception to this rule is at the bathroom, which
generates high humidity from showers and tubs. Here, an
uninsulated ceiling can result in condensation and
drippage, so effective water barriers and insulation are
critical.
Thermal
ponds are typically water filled (6 to 12 inches deep)
clear, ultraviolet inhibiting plastic bags. Care should
be taken to choose materials which do not degrade when
exposed to sunlight and water, nor are easily damaged
from handing and local conditions. Temperature
stratification in the ponds is avoided by using a clear
top and dark bottom. With this configuration, sunlight
will penetrate the water, be absorbed at the black
surface and heat from the bottom will cause a continual
convection cycle effect in the pond.
Finally,
exposed pond tops should be sloped to drain. This avoids
heat loss caused by evaporative cooling of the ponds
during. While not a problem for waterbed
"bags", this is a problem for more fixed types
of installations. (see
Haggard, K., et al., "Research Evaluation of a
System of Natural Air Conditions." National
Technical Information Service, Springfield, VA 22161,
Order Number PB-243498).
Insulating
panels can take many forms and configurations depending
on local conditions and design (flat roof with exposed
ponds in temperate and desert settings; pitched roof
with glazing in snow country), and range from pivoting
panels to horizontal, sliding panels constructed from
standard metal building construction systems -
insulation of polyurethane foam reinforced with
fiberglass strands between aluminum skins placed within
standard or easily formed metal frames. Panel tracks and
supports must be designed so that the panel system
(insulation and frames)
fit as tightly as possible when closed in order
to prevent compromise of the systems effectiveness.
Without an effective seal system a great deal of heat
stored can be lost at night due to infiltration. .
COMBINING
SYSTEMS
(top)
Many
times it is desirable, or even necessary, to use more
than one passive heating design strategy. For instance,
the use of a thermal storage wall may block a beautiful
view while a direct gain design in the same south wall
may create intolerable glare and have a tendency to
overheat. In such cases the two designs can be used side
by side or in any other configuration (a thermal storage
wall on each side of a direct gain window). It is
essential, however, to properly size this combination in
order to maintain quality control and avoid undesirable
temperature fluctuations. About 60 to 75% of the energy
striking the collecting surface of a direct gain window
can be used in space heating. On the other hand, only
about 30 to 45% of the energy striking the collecting
surface of a thermal storage wall is transferred to the
interior space as heat. It can be seen that the
approximate ratio in sizing this combination would be
one square foot of direct gain window equals two square
feet of thermal storage collecting surface. With these
approximations in mind, it should be a relatively easy
matter to size various combinations of passive solar
designs.
Variation
of roof pond designs make it impossible to present a
simple rule of thumb for integrating with other design
approaches. However, by reviewing the sizing techniques
for the other systems and by knowing the specific design
of the roof pond system, an approximate ratio can be
calculated.
CLOUDY
DAY STORAGE
Even
on cloudy days passive solar heating designs continue to
collect energy from diffused sunlight. However, this
greatly reduced and diffuse solar radiation usually does
not provide enough energy to keep interior temperatures
at 70°F. Well designed thermal mass systems are sized
to have carryover capacity and when combined with some
auxiliary heating systems, provide for a comfortable
environment for a number of cloudy days.
As
a rule, direct gain systems can provide comfortable
conditions for 1-2 cloudy days if the collecting area is
increased by 10-20%, and the interior walls and floors
are of solid masonry more than 8 inches thick. If water
walls are used in place of masonry, increase the amount
of water to two or three cubic feet for each square foot
of south facing collecting area.
In
climates where a number of consecutive sunny winter days
are common there is a concern for overheating. For
example, average temperatures with the above sizing may
result in average interior temperatures of 74°F.
However, on cloudy days, if the interior
temperature drops an average of four degrees per day,
comfortable conditions can be maintained for two days
with no additional heating needed.
Oversizing
in very cloudy or foggy climates is not recommended
since thicker masonry takes a few days of sunshine to
become fully charged. In climates such as these, overly
thick storage masses are likely to result in
under-heating problems. Glazed areas with minimum mass
thickness should be used so that the system can respond
quickly when sun is available.
Indirect
gain systems differ slightly when designing for one or
two cloudy days' storage. The collector area should be
increased by 10 to 20%, and thermal storage walls of
greater conductivity be used. If water walls are used,
one or more cubic feet of water should be used for each
square foot of collector area.
In
indirect designs, if standard masonry wall material is
used for cloudy day storage.,
the surface area of the wall should be increased
instead of increasing masonry wall thickness.
This may increase average daily temperatures, but
overheating is easily mitigated by use of insulating
panels or curtains drawn across the interior of the wall
to provide temperature control. Built in ventilation
systems can be used to control overheating for both
indirect gain and direct gain systems..
MOVABLE
INSULATION
In
all structures, the greatest amount of heat gain is lost
through glazing, either by conduction through the glass
or by infiltration around the window frame. All heat
loss during the winter reduces the efficiency of all
heating systems including passive solar design, and
where possible, movable, tightly sealed insulation
should be used to cut losses to a minimum. The extent of
heat loss under varying conditions of protection is
given in Table 10.
 |
| Table 10.
Conduction losses through single and double
glazing with and without shutters for Boston1. |
The
use of movable insulation can simply involve manually
operated panels that slide on a track across the glazed
area, or be motor driven and temperature activated (more
expense but more consistent control). Mechanical systems
can be used to operate insulation difficult to reach
manually. If the building is unattended when insulation
should be moved, automatic timers connected to
thermostats and/or light sensitive devices can operate,
providing appropriate operation of the system. Some
machinations can include automatically operated louvers,
motor driven panels (roof ponds, etc.) and movable
insulation. Whatever the method used, an effective
insulation system will greatly increase the efficiency
of passive solar designs.
REFLECTORS
FOR PASSIVE SOLAR HEATING
(top)
If
partial shading is problematic, collection of solar
radiation can be greatly enhanced by the use of
reflecting devices.
Generally, horizontal reflector equal in width
and one to two times the height of the glazed opening
in. length should be used for vertical (wall) glazing.
South sloping skylights, benefit from reflectors located
above the skylight at a tilt angle of roughly 100° from
the slope of the roof (Fig. 20). The reflector should be
roughly equal to the length and width of the skylight.
Solar collection can be increased 30-40% when reflectors
are used with either vertical, horizontal, or sloping
glazing. For greatest efficiency, the angle of a
reflector in relation to the collecting surface must be
carefully selected. Table 11 gives the proper reflector
angle for skylights with varying degrees of slope.
In
some instances, reflecting surfaces can be used to
direct sunlight to an interior storage surface such as a
water wall. Reflectors should also be constructed so
that they can be used to block heat gain in the summer.
Table 12 gives the reflecting properties of some
surfacing materials.
Table 11. Reflecting tilt-angles for south-facing
skylights. |
|
Table 12. Normal specular solar
reflectance of various surfaces. |
|
SHADING
(top)
South
facing glass can be a source of overheating during
summer months. The potential for overheating can be
controlled by a roof overhang carefully designed to
shade the glass during the summer (sun higher in the
sky) but not block sunlight during the winter (lower in
the sky), and by the use of movable outside shading
devices.
Overhangs
should be equal in length to roughly one fourth the
height of the window opening in southern latitudes (36°
NL) and one-half the height of the opening in northern
latitudes (48° NL).
The
projection of the overhang that will be adequate
(provide 100% shading at noon on June 21) at particular
latitudes can be quickly calculated by using the
following formula:
Projection = window opening (height)/ F (see
Table 13)
A
slightly longer overhang may be desirable at latitudes
where this formula does not provide enough shade during
August.
The
usefulness of overhangs can be increased if they are
constructed so that they are adjustable. Adjustable
overhangs can be rolled back to admit sunlight on cold
spring days. Trellised overhangs that support deciduous
vines (vines that lose their leaves in the winter) are
another way to block sunlight in the summer and admit
sunlight in early spring. Retractable awnings and
adjustable louvers can also be useful shading devices.
Table 13. |
|
OUTSIDE
INSOLATION (OUT-SULATION)
(top)
Masonry
wall exterior surfaces can lose large amounts of heat.
For example - to
achieve an insulation quality equal to 3.5 inches of
fiberglass insulation, a concrete wall would have to be
12 feet thick. To avoid masonry wall heat loss to the
exterior, it can | 