Passive Solar Energy use.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
Comments by Energy Saving Now.
The following sample house, in article about Passive Solar design, do not comply with our understanding of sustainability or efficiency. It represents popular opinions within the Architectural community and from this point of view it is very interesting. It also promotes an understanding of thermal mass and wider temperature bandwidth, that are essential issues for energy saving and as such very positive. About the sample (not the principles), we have the following to clarify,
- The displayed building is very nice to look at and we find it attractive from a design perspective.
- Many of the principles mentioned are important and complaint with what we are promoting on Energy Saving Now.
- The sample building with large glass areas and the functionality is only valid for very limited geographical areas.
- The savings are expected to be 15% and we assume that this is in comparison to normal buildings in USA and the area where it is built. If it is compared to a building according to the Swedish Building Codes, the energy consumption will be 2 to 3 times higher than the Swedish house. If it is located in a cold or warm climate, the surface temperature on the glassed areas will be very uncomfortable.
- A 15% energy saving can be achieved in many easier ways and even in existing buildings. If you use paint for the building, that are Aluminium based instead of normally Titan, the saving will be around 15% and work in all geographical areas. It would also provide better comfort.
- With the extended allowed temperature variation and operation policies, nearly all buildings will give 15% or better performance.
- Sustainable buildings, based on long experience, was standard until a couple of hundred years ago. They used the location, thermal mass and/or insulation. A very common feature was the small window openings and covering of the windows to regulate the indoor temperatures depending on time of day and seasonal changes.
The values of the sample are the emphasises on thermal mass and natural regulation of the environment. This is opposing the current energy wasting design principles in the building and HVAC industry and as such very refreshing. The sample can be discussed and there are much more efficient ways to design for energy efficiency. We suggest that you carefully assimilate the information on "Energy Saving Now", before you go ahead and build. But if you absolutely want to build a glass house, read this article carefully.
Article about passive solar design.
Passive solar design, or climate
responsive buildings use existing technologies and materials to heat, cool and
light buildings. They integrate traditional building elements like insulation,
south-facing glass, and massive floors with the climate to achieve sustainable
results. These living spaces can be built for no extra cost while increasing
affordability through lower energy payments. In many countries they also keep
investment in the local building industry rather than transferring them to short
term energy imports. Passive solar buildings are better for the environment
while contributing to an energy independent, sustainable energy future.
Passive solar system uses the building structure as a collector, storage
and transfer mechanical equipment. This definition fits most of the more simple
systems where heat is stored in the basic structure: walls, ceiling or floor.
There are also systems that have heat storage as a permanent element within the
building structure, such as bins of rocks, or water-filled drums or bottles.
These are also classified as passive solar energy systems. Passive solar homes
are ideal places in which to live. They provide beautiful connections to the
outdoors, give plenty of natural light, and save energy throughout the year.
HISTORY
Building design has
historically borrowed its inspiration from the local environment and available
building materials. More recently, humankind has designed itself out of nature,
taking a path of dominance and control which led to one style of building for
nearly any situation. In 100 A.D., Pliny the Younger, a historical writer, built
a summer home in Northern Italy featuring thin sheets of mica windows on one
room. The room got hotter than the others and saved on short supplies of wood.
The famous Roman bath houses in the first to fourth centuries A.D. had large
south facing windows to let in the sun’s warmth. By the sixth century, sunrooms
on houses and public buildings were so common that the Justinian Code initiated
“sun rights” to ensure individual access to the sun. Conservatories were very
popular in the 1800’s creating spaces for guests to walk through warm
greenhouses with lush foliage.
Passive solar buildings in the United States were in such demand by 1947,
as a result of scarce energy during the prolonged World War 2, that
Libbey-Owens-Ford Glass Company published a book entitled Your Solar House,
which profiled forty-nine of the nations greatest solar architects.
In the mid-1950’s, architect Frank Bridgers designed the world’s first
commercial office building using solar water heating and passive design. This
solar system has been continuously operating since that time and the
Bridgers-Paxton Building is now in the National Historic Register as the world’s
first solar heated office building.
Low oil prices following World
War 2 helped keep attention away from solar designs and efficiency. Beginning in
the mid-1990’s, market pressures are driving a movement to redesign our building
systems to more in line with nature.
Passive Solar Space Heating
There are few basic architectural modes for the utilisation of
passive solar utilisation in architecture. But these modes, as presented
below, can be developed into many different scheme, and enrich the
design.
The essential elements of a passive solar home are:
good siting of the house, many south-facing windows (in Northern
Hemisphere) to admit solar energy in winter (and, conversely, few east or
west facing windows, to limit the collection of unwanted summer sunshine),
sufficient interior mass (thermal mass) to smooth out undesirable
temperature swings and to store heat for night time and a well-insulated
building envelope.
Siting, insulation, windows orientation and
mass must be used together. For least variation of indoor temperature the
insulation should be placed on the outside of the mass. However where
rapid indoor heating is required some insulation or low heat capacity
material should be placed at the inside surface. There will be an optimum
design for each micro-climate and indications are that a careful balance
between mass and insulation in a structure will result not only in energy
savings but in initial material cost saving as
well. |
Site
Landscaping and Trees
According to the U.S.
Department of Energy report, “Landscaping for Energy Efficiency”
(DOE/GO-10095-046), careful landscaping can save up to 25% of a household’s
energy consumption for heating and cooling. Trees are very effective means of
shading in the summer months as well as providing breaks for the cool winter
winds. In addition to contributing shade, landscape features combined with a
lawn or other ground cover can reduce air temperatures as much as 5 degrees
Celsius in the surrounding area when water evaporates from vegetation and cools
the surrounding air. Trees are wonderful for natural shading and cooling, but
they must be located appropriately so as to provide shade in summer and not
block the winter sun. Even deciduous trees that lose their leaves during cold
weather block some winter sunlight - a few bare trees can block over 50 percent
of the available solar energy.
Windows
All effective passive systems depend on windows. Glass or other
translucent materials allow short-wave, solar radiation to enter a
building and prohibit the long-wave, heat radiation, from escaping.
Windows control the energy flow in two principle ways: they admit solar
energy in winter, so warming the house above the otherwise cool to cold
internal conditions; and by excluding sun from the windows (by orientation
and shading) there exist the opportunity to use ventilation to cool the
otherwise warm hot house in summer. If use is to be made of the sun’s
heat, then it has to reach buildings when it is useful. Generally, the sun
should be able to reach the collection area between 9 a.m. and 3
p.m. in winter with as little obstruction and interference as
possible.Trees on the site or the neighbours’ site might shade the vital
areas of the building. This need to be checked and the building located to
minimise any such interference. It is possible to plan a house to have its
main outlook in any direction and still be an efficient low energy
building. The building envelope, i.e. the walls, floor and roof are the
important elements in design, rather than the location of internal spaces.
If a window needs to face west it requires correct shading and its size
restricted. | Glass permits sun radiation of
wavelengths 0.4 to 2.5 microns to pass through it. As this radiant energy
collides with opaque objects on the other side of the glass, it’s wavelength
increases to 11 microns. Glass acts as an opaque barrier to light of this
wavelength thereby trapping the sun’s energy. The amount of light penetrating a
glass is dependent on the angle of incidence. The optimum angle of incidence is
90o. When sunlight strikes the glass at 30o or less, the most radiation is
reflected.
Understanding the Solar Spectrum and Heat Transfer
To make good choices on glazing, it is needed to understand a bit about
light and heat. The sunlight that strikes the Earth is comprised of a variety of
wavelengths and different glazing will selectively transmit, absorb, and reflect
the various components of the solar spectrum. Likewise, reducing glare (via
reflection or tinting) is helpful in the workplace by allowing the transmission
of visible, or natural, light it is possible to save energy for artificial
light. But perhaps the greatest effect on human comfort levels is determined by
infrared heat transfer. By specifying the right type of glass, it is possible to
trap the infrared heat for warmth, or reflect the infrared heat to prevent
warming.
There are three ways that heat moves through a glazing material. The first
is conduction. Conductive heat is transferred through the glazing by direct
contact. Heat can be felt by touching the glazing material. The second form of
heat transfer is radiation; electromagnetic waves carry heat through a glazing.
This produces the feeling of heat radiating from the surface of the glazing. The
third method of heat transfer is convection. Convection transfers heat by
motion, in this case, air flow. The natural flow of warm air toward colder air
allows heat to be lost or gained.
The R-value of a glazing - its
insulating capabilities or resistance to the flow of heat - is determined by the
degree of conduction, radiation, and convection through the glazing material.
However, air infiltration will also determine the overall R-value of a glazing
system. The amount of heat that travels around a glazing is as important as the
heat transfer through a glazing. Air can leak in or out of a building around the
glazing via the framing. The quality, workmanship, and the installation of the
entire glazing system, including the framing, affects air infiltration.
Advances in glass technology have perhaps been the single largest
contributor to building efficiency since the 1970s and they play an important
roll in solar design. Some window advances include:
- Double and triple pane windows with much higher insulating values.
- Low emissivity or Low-E glass employing a coating which lets heat
in but not out.
- Argon (and other) gas filled windows that increase insulating
values above windows with just air.
- Phase-change technologies that can switch from
opaque to translucent when a voltage is applied to them.
Basic Glass Types
Glazing materials include glass, acrylics, fibreglass, and other
materials. Although different glazing materials have very specific applications,
the use of glass has proven the most diverse. The various types of glass allow
the passive solar designer to fine-tune a structure to meet client needs. The
single pane is the simplest of glass types, and the building block for higher
performance glass. Single panes have a high solar transmission, but have poor
insulation - the R-value is about 1,0. Single pane glass can be effective when
used as storm windows, in warm climate construction (unless air conditioning is
being used), for certain solar collectors, and in seasonal greenhouses.
Structures using single pane glass will typically experience large temperature
swings, drafts, increased condensation, and provide a minimal buffer from the
outdoors.
Perhaps the most common glass product used today is the double pane unit.
Double pane glass is just that: two panes manufactured into one unit. Isolated
glass (thermopane) incorporate a spacer bar (filled with a moisture absorbing
material called a desiccant) between the panes and are typically sealed with
silicone. The spacer creates a dead air space between the panes. This air space
increases the resistance to heat transfer; the R-value for double pane is about
1,8-2,1. Huge air spaces will not drastically increase R-value. In fact, a large
air space can actually encourage convective heat transfer within the unit and
produce a heat loss. A rule of thumb for air space is between 1 and 2
centimetres. It is also possible to go as large as 10-12 centimetres without
creating convective flow, but at that point you are dealing with a very large
and awkward unit. The demand for greater energy efficiency in building and
retrofitting homes has made insulated glass units the standard. With good solar
transmission and fair insulation, such unit is a large improvement over the
single pane. Windows, doors, skylights, sunrooms, and many other areas utilize
double pane glass.
HIGH PERFORMANCE GLASS
High performance or
enhanced glass offers even better R-value and solar energy control. By further
improving the insulating capability of glass, it is possible dramatically
increase also design options. What were once insulated walls may become
sunrooms. Solid roofs and ceilings become windows to the sky. Dark rooms can
“wake up” to natural light, solar heat gain, and wonderful views. For a
relatively small increase in cost it is possible to improve efficiency, provide
better moisture and UV protection, and gain design flexibility. A variety of
high performance glass is now available.
What are the advantages of this glass? Low emissivity (Low-E) glass is
succeeding double pane glass in energy efficient buildings. Emissivity is the
measure of infrared (heat) transfer through a material. The higher the
emissivity, the more heat is radiated through the material. Conversely, the
lower the emissivity, the more heat is reflected by the material. Low-E coatings
will reflect, or re-radiate, the infrared heat back into a room, making the
space warmer. This translates into R-values from 2.6 to 3.2. In warmer climates
it is possible to reverse the unit and re-radiate infrared heat back to the
outside, keeping the space cooler. Low-E glass improves the R-value, UV
protection, and moisture control.Gas-filled windows increase R-value. Properly
done, gas-filling will increase the overall R-value of a glass unit by about
1,0. The air within an insulated glass unit is displaced with an inert, harmless
gas with better insulation properties. Typical gases used are Krypton and
Argon.
Window curtains
In addition to decorative functions, curtains can be used to reduce the
heat losses that occur during the cold months as well as the heat gains during
the warmer months. The plywood box over the curtain top prevents warm ceiling
air from moving between the glass and curtain. The curtain should drop at least
30 cm below the window for it to be effective. The optimum condition would be
for it to drop to the floor.
Thermal mass
Solar
radiation hitting walls, windows, roofs and other surfaces is adsorbed by the
building and is stored in thermal mass. This stored heat is then radiated to the
interior of the building. Thermal mass in a solar heating system performs the
same function as batteries in a solar electric system (see chapter on
photovoltaics). Both store solar energy, when available, for later use.
Thermal mass can be incorporated into a passive solar room in many ways,
from tile-covered floors to water-filled drums. Thermal mass materials, which
include slab floors, masonry walls, and other heavy building materials, absorb
and store heat. They are a key element in passive solar homes. Homes with
substantial south-facing glass areas and no thermal storage mass do not perform
well.
It is important to know that dark surfaces reflect less, therefore, absorb
more heat. In case of a dark tiled floor, the floor will be able to absorb heat
all day and radiate heat into the room at night. The rate of heat flow is based
on the temperature difference between heat source and the object to which the
heat flows. As described above heat flows in three ways - conduction (heat
transfer through solid materials), convection (heat transfer through the
movement of liquids or gasses), and radiation. All surfaces of a building lose
heat via these three modes. Good solar design works to minimize heat loss and
maximize efficient heat distribution. The need for thermal mass (heat-storage
materials) inside a building is very climate-dependent. Heavy buildings of high
thermal mass are consistently more comfortable during hot weather in hot-arid
and cool-temperate climates, while in hot-humid climates there is little
benefit. In cool-temperature climates the thermal mass acts as a cold-weather
heat store thus improving overall comfort and reducing the need for auxiliary
heating, except on overcast or very cold days. In intermittently heated
buildings, however, it tends to increase the heat needed to maintain the chosen
conditions.
Providing adequate thermal mass is usually the greatest challenge to the
passive solar designer. The amount of mass needed is determined by the area of
south-facing glazing and the location of the mass. In order to ensure an
effective design it is important to follow these guidelines:
 Locate the thermal
mass in direct sunlight. Thermal mass installed where the sun can reach it
directly is more effective than indirect mass placed where the sun’s rays do not
penetrate. Houses that rely on indirect storage need three to four times more
thermal mass than those using direct storage.
Distribute the thermal mass. Passive
solar homes work better if the thermal mass is relatively thin and spread over a
wide area. The surface area of the thermal mass should be at least 3 times, and
preferably 6 times, greater than the area of the south windows. Slab floors that
are 8 to 10 centimetres thick are more cost effective and work better than
floors 16 to 20 inches thick.
Do not cover the thermal mass.
Carpeting virtually eliminates savings from the passive solar elements. Masonry
walls can have drywall finishes, but should not be covered by large wall
hangings or lightweight panelling. The drywall should be attached directly to
the mass wall, not to covers fastened to the wall that create an undesirable
insulating airspace between the drywall and the mass.
Select an appropriate mass
colour. For best performance, finish mass floors with a dark colour. A medium
colour can store 70 percent as much solar heat as a dark colour, and may be
appropriate in some designs. A matte finish for the floor reduces reflected
sunlight, thus increasing the amount of heat captured by the mass and having the
additional advantage of reducing glare. The colour of interior mass walls does
not significantly affect passive solar performance.
Insulate the thermal mass
surfaces. There are several techniques for insulating slab floors and masonry
exterior walls. These measures should introduced to achieve the energy
savings. Unfortunately, problems in some case can arise like with termite
infestations in foam insulation for perimeter slabs. This can complicate the
issue of whether and how to insulate slab-on-grade floors.
Make thermal mass
multipurpose. For maximum cost effectiveness, thermal mass elements should serve
other purposes as well. Masonry thermal storage walls are one example of a
passive solar design that is often cost prohibitive because the mass wall is
only needed as thermal mass. On the other hand, tile-covered slab floors store
heat, serve as structural elements, and provide a finished floor surface.
Masonry interior walls provide structural support, divide rooms, and store
heat.
When developing a thermal storage system or simply comparing materials it
is useful to look at the storage capacity of the proposed building materials
which is referred to as the volumetric heat capacity (J/m3. Deg. Celsius) or
more commonly the specific heat and the rate at which the material can take up
and store heat. Some examples of common storage materials are given in the
following table:
|
Material |
Density
(kg/m3) |
Volumetric heat
capacity (J/m3. Deg. C) |
|
Water |
1000 |
4186 |
|
Concrete |
2100 |
1764 |
|
Brick |
1700 |
1360 |
|
Stone: marble |
2500 |
2250 |
| |
Materials not suitable for
thermal storage |
|
|
Plasterboard |
950 |
798 |
|
Timber |
610 |
866 |
|
Glass fibre matt |
25 |
25 | Early solar
designers used water (stored in large containers) as the heat storage medium.
Although water is cheap, the containers and the space they take are not. Some
solar designers turned to rock storage bins as reservoirs for thermal mass. It
took three times as much rock to store the same amount of heat as an equivalent
volume of water and the moist warm environment of the bins became breeding
grounds for odor producing fungi and bacteria. The high cost and the foul odors
started to give solar design a bad name. Both water and rock heat storage
require complicated control systems, pumps, and blowers. Heat storage is not
common in today‘s solar energy utilisation. Main reason for this is that all of
these systems rely on electricity, require maintenance, and are subject to
periodic breakdown.
Thermal insulation
Materials generally available for building purposes can be
classified into two generic groups - bulk materials and reflective foil
laminates (RFL). The first of these relies on the resistance of air
trapped in pockets between the fibres of the blanket type materials
(mineral fibre materials) or the cells formed in the foamed structure of
board or slab type materials (usually made from plastics such as
polystyrene and polyurethane foams). The second reflects radiant energy
away from the object or surface being protected. Thermal insulation in the
outer fabric of a building is a vital component of an energy-efficient
design strategy. The key to successful energy-efficient design is the
control of heat flow through the external fabric. All the solar energy
gained could be easily lost from an inadequately insulated building before
it is able to be of benefit. It will have been noted that some materials
have a very much higher thermal resistance per unit thickness than others
irrespective of their density. The fact that air is a good insulator
especially if it is bounded by a bright foil surface to limit radiation
transfer can be very useful as well. |
Cooling
In many
parts of the world a passive solar building needs cooling as much as heating.
One of the best, time proven methods of cooling is thermal coupling with the
earth’s constant temperature. Dropping the ground floor at least one meter into
the earth provides a more even exterior temperature which aids cooling as well
as heating. Adequate structural engineering, drainage, and damp proofing are
essential in below ground areas. Thermal isolation is the best and most
economical way to temper the building’s environment. Using the earth’s thermal
mass keeps the house at a reasonable temperature, and so does good insulation.
Shades located outside and inside the windows, ventilation and reflective films
on the windows are also very important in order to control temperature inside
the building.
External Shades and Shutters
Exterior window
shading treatments are effective cooling measures because they block both direct
and indirect sunlight outside of the home. Solar shade screens are an excellent
exterior shading product with a thick weave that blocks up to 70 percent of all
incoming sunlight. The screens absorb sunlight so they should be used on the
exterior of the windows. From outside, they look slightly darker than regular
screening, but from the inside many people do not detect a difference. Most
products also serve as insect screening. They should be removed in winter to
allow full sunlight through the windows. A more expensive alternative to the
fibreglass product is a thin, metal screen that blocks sunlight, but still
allows a view from inside to outside. Hinged decorative exterior shutters which
close over the windows are also excellent shading options. However, they obscure
the view, block daylight completely, may be expensive and may be difficult for
many households to operate on a daily basis.
Interior Shades and Shutters
Shutters and shades located inside the house
include curtains, roll-down shades, and Venetian blinds. Interior shutters and
shades are generally the least effective shading measures because they try to
block sunlight that has already entered the room. However, if passive solar
windows do not have exterior shading, interior measures are needed. The most
effective interior treatments are solid shades with a reflective surface facing
outside. In fact, simple white roller blinds keep the house cooler than more
expensive louvered blinds, which do not provide a solid surface and allow
trapped heat to migrate between the blinds into the house.
Reflective Films and Tints
Reflective film, which adheres to glass and is
found often in commercial buildings, can block up to 85% of incoming sunlight.
The film blocks sunlight all year, so it is inappropriate on south windows in
passive solar homes. However, it may be practical for unshaded east and west
windows. These films are not recommended for windows that experience partial
shading because they absorb sunlight and heat the glass unevenly. The uneven
heating of windows may break the glass or ruin the seal between double-glazed
units.
Ventilation
Ventilation is the changing of air in buildings to control oxygen, heat
and contaminants. Ventilation may occur in few forms. Building orientation,
form, plan and user actions also alter air flow paths. Natural ventilation
consumes no energy and has few if any running costs, but depends on weather
conditions and can be difficult to control. Mechanical and air-conditioned
ventilation are energy-driven alternatives to natural ventilation, normally
dictated by building type, site and function. They can be particularly efficient
as supplements to natural ventilation. Mechanical ventilation uses fans and
ducts to supply and extract air in localised areas such as a kitchen. Air
conditioning both treats and supplies air. It is particularly useful to cool air
below ambient temperatures.
SOLAR ARCHITECTURE & ACTIVE SYSTEMS
It is important to design the house with the
aim to incorporate active solar systems (see below) like collectors or
photovoltaic modules as well. The building should orient these appliances
due south. Tilt of the solar collectors should be in Europe and North America
more than 50° (from horizontal) to maximize winter heat collection. Solar
collectors should be thermally locked with the roof. Non-tracking photovoltaics
receive the most yearly insolation (exposure to the sun’s rays) when tilted at
an angle, from horizontal, equal to the building’s latitude. Design of the
building’s roof should be done to such angles and southern orientation as
integral aspects of the building. Hot water collectors and photovoltaic panels
should be located as close as possible to their main areas of use. It is
important to concentrate these areas of use. For example, putting the bathrooms
and kitchen close together economizes on their installation and minimizes energy
loss. All appliances should be selected with efficiency as the prime
criterion.
SUMMARY
 |
Passive use of sunlight contributes around 15% of space heating
needs in typical building. It is important source of energy savings which
can be utilised everywhere and almost at no extra cost. There are some
principles which can help a designer to harness solar energy through
thermally efficient buildings. |
SITE
It is
important to become familiar with the energy flows of house surroundings. The
nature and relationship of the lay of the land, water courses, vegetation, soil
types, wind directions, and exposure to the sun should be investigated. A site
suitable for solar design should balance and complement these elements. It must
have unobstructed exposure to the sun from 9 am to 3 pm during the heating
season.
HEATING
In
Northern hemisphere orientation due south of the main solar insolating spaces,
i.e. greenhouse, and/or main daytime activity areas is important. Glass should
be open to the sun patterns during the winter. By facing of the windows to the
south, and virtually none to the north maximaze solar gain. Multiple pane glass
in all windows is recommended.
THERMAL MASS
Thermal mass including masonry floors, walls and
water storage is important to absorb ambient heat during the day and release it
at night. Insulation of the building further minimize heat loss through
windows, walls and roof.
NATURAL HEAT FLOW
It is useful to design the house with the natural heat flow in mind. Hot
air rises, so placing some activity areas on a second floor to draw heat
up from a lower collector area and across other areas can save a lot of energy.
Buffer areas of the building (unheated rooms, or partially heated spaces such as
utility rooms, vestibules and storage areas) should be oriented due to the north
to lessen the impact of the winter’s cold. Using a vestibule on doors to the
exterior can lead to energy savings. Vestibules cut heat loss and provide a
buffer zone between the exterior and the interior.
© Copyright energysavingnow.com 2000.
© Copyrights to Software @ this site
|
