AN APPROACH TOWARDS ENERGY
CONSERVATION IN BUILDINGS
1.0 Introduction
The sun, as a
faithful neighbour in the solar system, contributes essentially heat, daylight
and solar radiation to the earth. The rotation and revolution of the earth
around the sun is beyond mere planetic romance but rather gives rise to
multifaceted climatic and environmental phenomena. All solar radiation travels
through space in waves, and it is the length of these waves by which solar
radiation is classified.
The aggregate of
all radiation aspects of the sun is called the solar spectrum.
While the sun
emits radiation in all wavelengths, it is short wavelength radiation that
accounts for the majority of energy in the solar spectrum. For instance, the
portion of the spectrum received, as the visible light is a relatively small
segment compared to the variety of spectrum wavelengths, yet accounts for 46
percent of the energy radiating from the sun. Another 49 percent that is
perceived as heat is derived from the infrared band of the spectrum.
The proportion
of different wavelengths in the solar spectrum does not change and therefore
the energy output of the sun remains constant. A measurement of this is known
as Solar Constant; defined as the amount of heat energy delivered by
solar radiation to a square meter of material set perpendicular to the sun’s rays
for one hour of the outer edge of the earth’s surface..
1.1 Energy Density
Since solar radiation travels in parallel rays, the perpendicular
position identifies the maximum density of rays striking the surface. Any deviation
from the perpendicular reduces radiation density and the amount of energy
intercepted.
1.2 Radiation and Surfaces
When sunlight strikes a surface, it is reflected, transmitted or absorbed
in any combination depending on the texture, colour and clarity of the surface.
The wavelengths of solar radiation that are reflected are determined by the
colour of the surface material. A red stucco surface, for example, will scatter
(diffuse) wavelengths in the red band of the spectrum and absorb all others;
while a white glossy surface will reflect all wavelengths in the visible
spectrum at an angle equal and opposite to the angle of incidence. On the other
hand, a rough black surface absorbs all wavelengths in the visible spectrum,
while the transparent surface of window
glass allows nearly all radiation to pass through it, with comparatively little
reflection or absorption and without deflecting it from its parallel lines of
travel. Translucent materials also transmit radiation but scatter the rays as
they pass through.
1.3 Heat Absorption
Sunlight in the form of short wave solar radiation exhibits a
transformation from solar energy to heat energy when impacting a material
(Absorption). The temperature of a white surface, and a black surface expressed
to the same direct sunlight is a simple demonstration of this conversion. The
temperature of a black surface is higher because it is absorbing more solar
energy. As solar energy is absorbed at the surface of the material, it
stimulates movement of the molecules in the material and heat is absorbed.
1.4 Conduction
As a material absorbs radiation, and
molecular movement accelerates, the heat energy is distributed through the
material due to the natural phenomenon of maintaining equilibrium. This occurs
when stimulated molecules vibrating at a faster rate, impact adjacent molecules
vibrating at a lower rate; thereby dissipating and “spreading” the heat which
is eventually distributed throughout the material. The rate at which energy is
distributed through a material, depends on the density of the material and
conduction and the rate at which molecules are capable of receiving and passing
on energy.
1.5 Heat Transfer
Heat transfer from a solid material to a fluid medium (liquid or air)
occurs by radiation The added dimension of using fluid is that they can move
across hot solid surfaces, allowing molecules of the fluid to become agitated,
and move away from the heat source, and then be replaced by new, unheated
molecules. This process of fluid movement is called Natural Convection
1.6 Emmissivity
Unlike solar energy, radiant energy is limited to infrared radiation
emitted from a material at low temperatures. The extent to which a material
emits energy depends both on the temperature of the material and the nature of
its surface. Polished metal surfaces are poor emitters and poor absorbers of
thermal energy. Glass has the special ability to transmit nearly all solar
radiation it intercepts.
1.7 Heat Storage
All materials can store heat to some degree and the ability of a material
to store heat is called specific heat. A
good storage medium must absorb heat when it is available; and give it up when
it is needed, and it must be a relatively good heat conductor. Figure 1 below,
expresses emittances, and absorptances of selected materials.
Figure 1:
Emittances and Absorptances of Selected
Materials
Item
|
Emittance (at 10-400c)
|
Absorptance (for solar
radiation)
|
Black non-metallic surface such as asphalt, carbon, slate, paint
|
0.90-98
|
0.85-0.98
|
Red brick and tile, concrete and stone, rusty steel and iron, dark
paints(red, brown, green, etc.
|
0.85-0.95
|
0.65-0.80
|
Yellow and buff brick and stone, firebrick, fireclay.
|
0.85-0.95
|
0.50-0.70
|
White or light cream brick, tile, paint or paper, plaster, whitewash
|
0.85-0.95
|
0.30-0.50
|
Bright aluminum paint: gilt or bronze paint
|
0.40-0.60
|
0.30-0.50
|
Polished brass, copper,
Monel metal
|
0.02-0.05
|
0.30-50
|
2.0 Passive Solar Design
Dennis Holloway, Nathan New Mexican architect wrote, “In 19973, during the first Arab Oil Embargo, I began to investigate
alternative energy supports for architecture – ways to heat and cool buildings
that did not rely on fossil fuels, solar energy utilization was the obvious
solution, and was the best not new – having been in use around the world for
more than thirty centuries. I found that bringing solar energy knowledge into a
design process brought a new vitality and contemporary relevance to
architecture. Since then, passive solar architecture has been one of my
passions. I believe that designing architecture that minimizes its dependency on
fossil fuel is the most responsive contribution an architect can make to the
global environmental crisis including the carbon dioxide driven Greenhouse Effect. My own work and
research convinced me that for most of the world, the external energy requirements
for space conditioning of buildings can be drastically reduced by the use of
passive solar technology.
Architecturally, the first approach has been to reduce energy wastage in
buildings. The physical structure of the building has been subjected to rigorous
examinations to find where energy is being unnecessarily lost – through poor insulation,
uncontrolled ventilation or by pouring hot water down the drain. In addition,
energy standards have been assessed to discover where, for example, lightning
levels or room temperatures are unnecessarily high and could be reduced without
loss of comfort.
The other approach has been the utilization of ‘ambient’ and renewable
energy sources, for example sun, wind, and water or biomass. By direct use or
by storage prior to use, these energy sources can reduce the buildings demand
for non-renewable fossil fuel energy. There are two basic approaches to solar
application to buildings, the first is known as ‘active system’ and is
essentially a kind of ‘hardware’, which is applied to a building. This consists
of solar collecting panels, storage tanks, an energy transfer mechanism and an
energy distribution mechanism. The system relies on fans and pumps to circulate
the working fluids. The other approach, ‘passive design’, seeks to reduce the
energy budget of the house by close attention to orientation, insulation, window placement and design and to the subtleties
of the energy transfer properties of the building materials used.
Since the industrial revolution, the availability of inexpensive and
apparently infinite energy has caused the vernacular understanding of climate
sensitive design to fall into disuse. The architecture of technology and
high-energy use, characterized by the International style has encouraged, for
example, the construction of glass walled office blocks throughout the world.
Advances in material science have produced highly efficient building
materials such as glass and thermal insulation; which have combined to make
passive solar design possible. The United States , with the highest per
capita energy consumption in the world has been in the forefront of the recent
passive solar research. The first passive American Solar conference was held in
Albuquerque in
1976. Other passive solar conferences were held in Amherst ,
Massachusetts , and in Europe ,
the interest in passive solar energy has grown rapidly.
A passive solar home, makes use of the materials from which the dwelling
is constructed to capture, store and distribute the solar heat to its
occupants. High heat capacity materials absorb this energy as it enters the
house and stores it in the form of heat. Passive design requires consideration
of solar and heat flow in every detail and component. Floor plan layout,
circulation patterns, window location and the selection of wall and floor
materials, all influence how well a passive design will work. The entire house
is now a solar energy system with many of its components having dual functions;
both the traditional function of an enclosure and the solar functions of
collecting, storing and distributing heat. Windows not only let in light and
allow a view, but collect heat as well. Walls which subdivide the enclosed
space can also store and radiate heat. Components whose functions are primarily
structural, spatial or aesthetic may double as solar heating mechanisms.
This paper will deal almost entirely with space heating and minimally
with cooling as research is less developed in the later requiring further
testing and intensive research. The challenge therefore is to encourage
research in passive solar design with particular reference to the equatorial
climate where cooling is more desirable than heating. The basic solar space
heating equation for a building however is:
|
Solar
Incidental Auxilliary Fabric ventilation
Gains Internal heating loss infiltration
Gains Requirements loss
3.0 Comfort, Health and Building Planning and
Design
Our own bodies are controlled
to maintain a core temperature close to 37oC
We function best at this
temperature and variation on either side are detrimental. A major factor is the
need to get rid of the heat we generate as a by-product of our metabolic
systems. The heat we produce varies from about 100w at rest to about 1000w when
physically very active. A seated adult male indoors in normal conditions
produces about 115w, about 90w of which is Sensible heat and the remaining 25w
is latent heat. Sensible heat is that
which we can “sense” or feel; it is detectable through changes in temperature.
Latent heat is the heat taken up or released at a fixed temperature during a
change of phase, for example from a liquid to a gas.
Sensible heat
loss from the skin or outer clothing surface occurs by convection and
radiation, and there is a sensible heat loss during respiration. Latent heat
loss occurs through the skin, and sweat, and the evaporation of moisture during
respiration. In temperate climates, the atmospheric water vapour pressure has a slight effect on heat flow from the
body; but in hot, humid situations the effects can be much more significant.
The naked body,
if shaded from the sun, can be quite comfortable at around 28-300c
and at moderate relative humilities. As the ambient temperature rises, the
body’s response is to direct more blood to the surface, which increases the
skin temperature and heat loss, and to sweat and to loose heat through
evaporation.
Two points are
relevant to building design, firstly to
make buildings comfortable, they should be kept within a suitable temperature
range, which is not wide as that in an uncontrolled external environment.
Secondly, our bodies are capable of maintaining a very stable core temperature
with a fairly constant metabolic heat output over a wide range of external
temperatures. This is done, with little or no additional energy expenditure, by
a combination of control processes including sweating, altering the blood flow
(and therefore the heat loss to the skin) and changing clothes to suit
conditions.
Modern buildings
have achieved the first objective of maintaining fairly constant internal
conditions to comfort standards with the use of significant amounts of energy
to provide heating or cooling to compensate for the changing external
environment. The amount of energy used could be reduced significantly if
buildings adopted the principle of animal physiological control. Comfort is a
subjective matter and will vary with individuals and it involves a large number
of variables, some of which are physical with a physiological basis for
understanding. Classically, for thermal comfort they include.
- Air temperature and temperature gradients. - Radiant temperature.
- Amount of clothing worn by the occupants. - Ambient water vapour pressure.
- Occupant’s level of activity. -
Air movement.
3.1 Form
The orientation
of a building may be fixed but if choice is possible it should face south to
take advantage of the sun’s energy. Most
often the first major design decisions are allocating volumes to various
activities and developing the form of a building.
Form is governed by a number of functional
considerations such as:
-
The use of the sun’s energy and
daylight - Provision
of views for occupants.
-
Heat loss through the building
envelope - The need for
ventilation
-
Acoustic attenuation if required.
In the recent
past, the glass blocks of Mies Van der Rohe epitomized an architecture that
shut out the natural environment and provided an acceptable internal
environment through the use of considerable energy and sophisticated services.
The Queen
Building
3.2 Building Body
The admittance, Y, of a constructional element, put simply, is the amount of energy entering
the surface of the element for each degree of temperature change just outside
the surface and, as such, has the same units as the u- value (w/m2k).
The admittance of a material depends on its thickness, conductivity, and
density: specific heat and the frequency at which heat is put into it.
As can be seen from figure 2, dense constructions
have higher admittances, which is to say they absorb more energy for a given
change in temperature.
If a building absorbs a great deal of heat and only
experiences a small temperature rise it is said, in no very precise manner, to
be thermally heavyweight. Such buildings tend to have high admittances and a
great deal of thermal mass, usually in the form of exposed masonry. Lightweight
buildings, on the other hand, may have thin-skinned walls, false ceiling with
lightweight panels, metal partitions and so fort.
Figure 2:
Admittance and Density of selected construction
elements
|
Item
|
Admittance (w/m2 k)
|
Density (kg/m2)
|
|
220mm solid brickwork unplastered
|
4.6
|
1700
|
|
335mm solid brickwork unplasterted
|
4.7
|
1700
|
|
220mm solid brickwork 16mm lightweight plaster
|
3.4
|
1700 for brickwork 600 for
plaster
|
|
200mm solid cast concrete
|
5.4
|
2100
|
|
75mm lightweight concrete block with 15mm dense plaster on
both sides
|
1.2
|
600 for concrete
600 for plaster.
|
Normally, the heat flow into a building from the
outside is almost cyclical. On a daily
basis, when the sun rises, the air temperature increases, and as heat is
transferred, the building starts to cool, and the following day the cycle
continues
By using high-admittance elements the building fabric
can store more of the heat that reaches the internal and external surfaces,
thus reducing the peak temperatures. This ‘balancing’ effect can apply both
during the day and at night, because if cool night air is brought into contact
with high-admittance surface their temperatures will drop. The next day, when
warmer daytime air flows over the same elements, they will be cooled thus
improving comfort conditions for the occupants..
3.3 The
Building ‘Skin’
Development of the building envelope, or ‘skin’, is
likely to be on the increase in the next
decade. Technological innovation in
glass will allow window systems to respond to environmental conditions in way
not previously commercially viable for buildings. Sunglasses, which react to different light
conditions, are but a hint of the potential of glass in the coming years.
Building envelopes obviously need to be durable,
economical, aesthetically pleasing, weather tight, structurally sound and
secure. Psychologically, views out are very important. Environmentally, the
questions that need to be addressed are:
how they respond to solar radiation (both for the Sun’s heat and light),
how ventilation is made possible, how heat loss is minimized and how noise is
controlled. The envelope will, to a large extent, determine how the internal
environment is affected by external one. (See figure 3 below)
Figure 3:
Characteristics
of Glazing Systems
|
Type
|
U-value (w/m2K)
|
Light transmittance
|
Solar radiant heat transmittance
|
Mean sound insulation
|
|
|
Direct
|
Total
|
||||
|
Single (4mm clear float glass)
|
5.4
|
0.89
|
0.82
|
0.86
|
28
|
|
Double glazing (6mm clear float inner, 12mm airspace
6mmclear float outer)
|
2.8
|
0.76
|
0.61
|
0.72
|
30
|
|
Double with low emissivity coating (6mm pikington k inner
12 mm airspace, 6mm clear float outer
|
1.9
|
0.73
|
0.54
|
0.69
|
30
|
|
Double with low emissivity coating and cavity
(6mmpikington k inner, 12mm airspace with argon, 6mm clear float outer
|
1.6
|
0.73
|
0.54
|
0.69
|
30
|
4.0 Types
and Systems
Having thoroughly
examined the theoretical principles of passive solar design, we now examine the
different types of systems which have evolved in practice. Most of these
prototypes have been built singly and experimentally and increasingly flexible
designs are now being produced with two or more of the prototype systems, to
fully use the advantages of each and to create functional and comfortable
housing.
4.1 Non-
diffusing
In the simplest
type of passive solar system, sunlight enters the living- space through a
large, south facing window and the radiation falls directly onto primary heat
storage, where it is absorbed and stored in the form of heat, and released to
warm the room.
To perform effectively
as primary thermal mass, the area of mass used must be directly insulated for
most of the typical winter day. The primary storage can have various
configurations: in the floor, in the free standing mass within the room, or in
the external or internal walls. When the thermal mass of external wall is
utilized, it must be externally insulated.
Many modern
houses have large south- facing windows, but it is the lack of thermal storage
which prevents them from benefiting fully from their solar gains and the lack
of adequate insulation standards, without which the solar gains cannot
contribute significantly to the total heating requirement.
4.2
Diffusing
The use of
diffusing glass, blinds, or reflection from a light colored surface behind
clear glass, will all have the effect of spreading the incoming solar radiation
evenly throughout the room.
The even
distribution of the solar energy will result in lower surface temperature of
the thermal mass, and thus, reduced air temperature and fluctuations within the
space. The conflict of space use and energy storage is reduced slightly, but
there is now the added problem of restricted views from the room.
4.3 Mass
Trombe Wall
This system,
named after the pioneering work by Dr Felix Trombe and Jacque Michael at Odeillo
in France ,
consists of a dark-colored, massive wall placed directly behind a glazed, south
facing solar aperture. It can take several hours for the energy falling on the
wall to be transmitted to the room behind and this effect is used for night-time
heating. The time lag depends on the conductivity of the wall material and will
vary with its thickness and construction.
The warmed air,
in the space between the glazing and the wall, rises, drawing cool room air in
through the bottom vent at the top and bottom of the wall. The warmed air, in
the space between the glazing and the bottom vent and thus the warm air is
circulated around the room.
4.4 Water
Trombe Wall
The water Trombe
wall is similar in operating principle to the mass Trombe wall. The thermal
storage is water, which has greater heat capacity per volume than brick or
concrete. Water is almost an isothermal heat store and convection currents spread the absorbed energy
rapidly throughout the store, eliminating the time lag of the mass Trombe
system. This ensures a lower surface temperature of a container and so reduces
both heat losses through the solar aperture, and the risk of overheating within
the room.
Water is a cheap
and effective form of heat store. However, in addition to the problem of
weight, there are the problems of containment of this water and of humidity
control if the water is not completely enclosed.
4.5 Sunspace
A sunspace is
thermally isolated from the living space. Solar energy is collected through the
south facing glazing and it can be transferred to the living space in several
ways. If the sunspace contains little thermal mass the air temperature will
rise quickly and this can be circulated throughout the living- space. The
sunspace can incorporate thermal storage and so act much as an extended Trombe
wall. In this way, the stored heat can be distributed to the living- space by
means of convection, conduction and radiation. The great advantages of the
sunspace are in its seasonal extension of the living- space, and in its
potential for food production.
4.6 Thermosyphon
A thermosyphoned
passive system normally employs collector panels which are isolated from the
living- space. In this system, the heat transfer medium (air or liquid) is
heated in the collector panel and rises to the primary thermal store. Having
released its energy to the store the cooled fluids fall back to the bottom of
the collector and thus continues to circulate while the sun shines.
Apart from its
vertical position relative to the height of the heat store, the collector is
free of the living- space and each can be located in its most suitable position
on the site.. Alternatively, rock- bed or contained water stores below the
living-space can release their energy by radiation and convection.
4.7 Roof
pond
In low
latitudes, the solar altitudes in winter, is high enough to make vertical
collector surfaces almost useless. and a horizontal collector will be more
beneficial. Thermal mass is provided in the form of water filled clear P.V.C.
plastic bag. These are supported by steel decking which is also the ceiling.
The contained water is covered by movable insulation which retracts to allow
solar radiation to be collected during the winter day. At night the insulation
is closed and the stored solar energy is radiated from the ceiling to the
massive structural walls of the space below. These walls perform as secondary
thermal mass. Room- air convection currents are suppressed by having the
primary thermal storage mass in the ceiling.
The roof pond is
one of the few passive systems which can provide both heating and cooling.
Cooling is effected in summer by retracting the movable insulation to allow
radiation to the cold night sky. The insulation is closed during the day to
reduce solar gains and to allow the cooled water to act as a heat sink for the
living- space below. This radiation cooling is particularly suitable where
there is large diurnal temperature swing.
4.8 Underground Building
The temperature
of the ground remains almost stable throughout the year, and varies with latitudes and depth below the
ground surface, and is about 100c at a depth of 1.2m in temperate
climate zones. Burying a building will therefore reduce heat gain or loss, in
climate with severe summer or winter temperatures, and this effectively
ameliorates the outside environment.
4.9 Evaporative
Cooling
When water
evaporates it absorbs energy without a change in temperature. This energy
required for phase change is termed ‘latent heat’.
In hot dry
climates a body of water will be able to reduce air temperatures by
evaporation, by absorbing energy from the air. This process increases comfort
both by reducing air, temperatures and by increasing the relative humidity of
the air.
Increasing the
proportion of surface area to volume of water will improve the performance of
this system and this can be achieved for example; with a fountain.
4.10 Dessicant
Cooling
The human body
loses unwanted heat by evaporation, through sweating. In hot, humid climate,
sweat does evaporate and the body becomes uncomfortably hot.
Dessicants are
porous materials with high affinity for water. If humid air is passed through a
dessicant bed, water vapor is absorbed and the air is dried. A suitable low
temperature heat sink is therefore required to absorb this energy and to ensure
thermal comfort. When the dessicant bed is fully loaded, heated air is passed
through it to drive out the water and so regenerate the bed.
If absorbent
materials were placed in the east and west walls of a building, the morning sun
could heat the east wall, regenerating the material which had absorbed moisture
the previous afternoon. At the same time, the west wall could be dehumidifying
the incoming air. In the afternoon, the processes would reverse as the sun
moves into the west.
4.11 Induced
Ventilation
When solar
heated air is allowed to escape from the living- space at high level, a ‘solar
chimney’ is set up. This provides continuous solar- powered ventilation. The
air drawn in should be from the coolest source available, such as across a shaded
pond or through a buried duct.
This system can
be used in conjunction with a heating system, such as the Trombe wall or with a
separated collector. It is important that the collector be insulated from the
living- space; otherwise it becomes a radiant heat source for the living- space
and defeats the objective of cooling
5.0 Conclusion
and Recommendations
Climate
sensitive design is not a new concept and the principles have been used by
almost all indigenous builders for centuries. The rise of science in the
Renaissance led to the Industrial Revolution which has enabled environmental
engineers to produce reasonable comfortable conditions in almost any building
and in almost any climate. Some of the most powerful architecture of our time
has taken technology and pushed it to the limits of possibilities, but the new
challenge now, is to reduce a building’s reliance on fossil fuel derived high –
grade energy, yet still provide comfort inside for the occupants.
Solar energy
technology is a timely intervention as it utilizes the process we live by every
day – photosynthesis, and carbon dioxide cycle, weather and the water cycle.
Today, these skills have to be re-taught so that designers can use available
materials to full potential and for the comfort of the users as demonstrated in
the various proto-types examined in this paper.
6.0
References
1.
Humphreys, M. A. and Nicol, J.F
(1971). Theoretical and practical aspects of thermal comfort. Current Paper
14/71. BRE, Garston.
2.
Anon, (1988) CIBES Guide, Volume A: Thermal Properties of Building Structure, CIBSE, London .
3.
Anon (1981). Window design and
solar heat gain. BRS Degest 68 (second series) BRS. Garston.
4.
Anon. (1981) Guideline for environmental Design and Fuel Conservation in Educational Buildings ,
Department of Education and Science, London
5.
Taylor , A. (1987) Curing window pains. Energy in Buildings, (6), 21-4
6.
Lebens, Ralph M. (1580). Passive Solar Heating Design Applied Science Publishers Limited.
7.
Mazria, Edward. (1979). The
Passive Solar Energy Book’ Rodale Press, 1979
8.
Mibank, N. O. and Harrington-Lynn,
J (1974). Thermal response and the admittance procedure. Current Paper 61/74.
BRE, Garston.
9.
Loudon, A. G. (1968) Sumertime
temperatures in buildings without air-conditioning Current Paper 47/48. Building Research Station Garston.
Anon. (1992) Glass and solar control Performance of Blinds Pilkington, St Helens.
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