The term solar architecture refers to an approach to building design that is sensitive to Nature and takes advantage of climatic conditions to achieve human comfort rather than depending on artificial energy that is both costly and environmentally damaging. Unlike the conventional design approach that treats climate as the enemy which has to be kept out of the built environment, solar architecture endeavours to build as part of the environment using climatic factors to our advantage and utilising the energy of Nature itself to attain required comfort levels. Nature’s energies can be utilised in two ways - passiveand active and consequently solar architecture is classified as passive solar and active solar architecture.
It relies upon the design or architecture of the building itself to ensure climate control by way of natural thermal conduction, convection and radiation. The rudiments of solar passive design were developed and used through the centuries by many civilisations across the globe; in fact, many of these early civilisations built dwellings that were better suited to their climatic surroundings than those built today in most developed and developing countries. This has been largely due to the advent of cheap fossil fuels that allowed for artificial temperature and light control at the cost of natural light and cooling. A substantial share of world energy resources is therefore being spent in heating, cooling and lighting of such buildings. The use of solar passive measures such as natural cross ventilation, sufficient day-lighting, proper insulation, use of adequate shading devices coupled with auxiliary energy systems that are renewable and environment friendly can considerably bring down the costs as well as the energy needs of the building.
Passive solar systems
The term passive solar refers to systems that absorb, store and distribute
the sun’s energy without relying on mechanical devices like pumps and fans,
which require additional energy. Passive solar design reduces the energy
requirements of the building by meeting either part or all of its daily
cooling, heating and lighting needs through the use of solar
energy.
Passive heating
Heating the building through the use of solar energy involves the absorption
and storage of incoming solar radiation, which is then used to meet the
heating requirements of the space. Incoming solar radiation is typically
stored in thermal mass such as concrete, brick, rock, water or a material
that changes phase according to temperature. Incoming sunlight is regulated
by the use of overhangs, awnings and shades while insulating materials
can help to reduce heat loss during the night or in the cold season. Vents
and dampers are typically used to distribute warm or cool air from the
system to the areas where it is needed. The three most common solar passive
systems are direct gain, indirect gain and isolated gain. A direct gain
system allows sunlight to windows into on occupied space where it is absorbed
by the floor and walls. In the indirect gain system, a medium of heat storage
such as wall, in one part of the building absorbs and stores heat, which
is then transferred to the rest of the building by conduction, convection
or radiation. In an isolated gain system, solar energy is absorbed in a
separate area such as greenhouse or solarium, and distributed to the living
space by ducts. The incorporation of insulation in passive systems can
be effective in conserving additional energy.
Passive cooling
Passive solar technology can also be used for cooling purposes. These
systems function by either shielding buildings from direct heat gain or
by transferring excess heat outside. Carefully designed elements such as
overhangs, awnings and eaves shade from high angle summer sun while allowing
winter sun to enter the building. Excess heat transfer can be achieved
through ventilation or conduction, where heat is lost to the floor and
walls. A radiant heat barrier, such as aluminium foil, installed under
a roof is able to block upto 95% of radiant heat transfer through the roof.
Water evaporation is also an effective method of cooling buildings,
since water absorbs a large quantity of heat as it evaporates. Fountains,
sprays and ponds provide substantial cooling to the surrounding areas.
The use of sprinkler systems to continually wet the roof during the hot
season can reduce the cooling requirements by 25%. Trees can induce cooling
by transpiration, reducing the surrounding temperature by 4 to 14 degrees
F.
Active cooling systems of solar cooling such as evaporative
cooling through roof spray and roof pond and desiccant cooling systems
have been developed alongwith experimental stratergies like earth-cooling
tubes and earth-sheltered buildings.
Desiccant cooling systems are designed to dehumidify and cool air. These
are particularly suited to hot humid climates where air-conditioning accounts
for a major portion of the energy costs. Desiccant materials such as silica
gels and certain salt compounds naturally absorb moisture from humid air
and release the moisture when heated, a feature that makes them re-useable.
In a solar desiccant system, the sun provides the energy to recharge the
desiccants. Once the air has been dehumidified, it can be chilled by evaporative
cooling or other methods to provide relatively cool, dry air. This can
greatly reduce cooling requirements
Evaporative cooling
Evaporation occurs whenever the vapour pressure of water is lesser
than the water vapour in the surrounding atmosphere. The phase change of
water from liquid to the vapour state is accompanied by the release of
a large quantity of sensible heat from the air that lowers the temperature
of air while its moisture content increases. The provision of shading and
the supply of cool, dry air will enhance the process of evaporative cooling.
Evaporative cooling techniques can be broadly classified as passive
and hybrid.
Passive direct systems include the use of vegetation for evapotranspiration, as well as the use of fountains, pools and ponds where the evaporation of water results in lower temperature in the room. An important technique known as ‘Volume cooler’ is used in traditional architecture. The system is based on the use of a tower where water contained in a jar or spray is precipitated. External air introduced into the tower is cooled by evaporation and then transferred into the building. A contemporary version of this technique uses a wet cellulose pad installed at the top of a downdraft tower, which cools the incoming air.
Passive indirect evaporative cooling techniques include roof spray and roof pond systems.
Roof spray
The exterior surface of the roof is kept wet using sprayers. The sensible
heat of the roof surface is converted into latent heat of vaporisation
as the water evaporates. This cools the roof surface and a temperature
gradient is created between the inside and outside surfaces causing cooling
of the building. A reduction in cooling load of about 25% has been observed.
A threshold condition for the system is that the temperature of the roof
should be greater than that of air.
There are, however, a number of problems associated with this system,
not least of which is the adequate availability of water. Also it might
not be cost effective, as a result of high maintenance costs and also problems
due to inadequate water proofing of the roof.
Roof pond
The roof pond consists of a shaded water pond over an non-insulated
concrete roof. Evaporation of water to the dry atmosphere occurs during
day and nighttime. The temperature within the space falls as the ceiling
acts as a radiant cooling panel for the space, without increasing indoor
humidity levels. The limitation of this technique is that it is confined
only to single storey structure with flat, concrete roof and also the capital
cost is quite high.
Earth cooling tubes
These are long pipes buried underground with one end connected to the
house and the other end to the outside. Hot exterior air is drawn through
these pipes where tit gives up some of its heat to the soil, which is at
a much lower temperature at a depth of 3m to 4m below the surface. This
cool air is then introduced into the house.
Special problems associated with these systems are possible condensation
of water within the pipes or evaporation of accumulated water and control
of the system. The lack of detailed data about the performance of such
systems hinders the large-scale use of such systems.
Earth-sheltered buildings
During the summer, soil temperatures at certain depths are considerably
lower than ambient air temperature, thus providing an important source
for dissipation of a building’s excess heat. Conduction or convection can
achieve heat dissipation to the ground. Earth sheltering achieves cooling
by conduction where part of the building envelope is in direct contact
with the soil. Totally underground buildings offer many additional advantages
including protection from noise, dust, radiation and storms, limited air
infiltration and potentially safety from fires. They provide benefits under
both cooling and heating conditions, however the potential for large scale
application of the technology are limited; high cost and poor day-lighting
conditions being frequent problems.
On the other hand, building in partial contact with earth offer interesting cooling possibilities. Sod roofs can considerably reduce heat gain from the roof. Earth berming can considerably reduce solar heat gain and also increase heat loss to the surrounding soil, resulting in increase in comfort.
It involves the use of solar collectors and other renewable energy systems like biomass to support the solar passive features as they allow a greater degree of control over the internal climate and make the whole system more precise. Active solar systems use solar panels for heat collection and electrically driven pumps or fans to transport the heat or cold to the required spaces. Electronic devices are used to regulate the collection, storage and distribution of heat within the system. Hybrid systems using a balanced combination of active and passive features provide the best performance.
Active solar systems
Active heating
In active systems, solar collectors are used to convert sun’s energy
into useful heat for hot water, space heating or industrial processes.
Flat-plate
collectors are typically used for this purpose. These most often use
light-absorbing plates made of dark coloured material such as metal, rubber
or plastic that are covered with glass. The plates transfer the heat to
a fluid, usually air or water flowing below them and the fluid is used
for immediate heating or stored for later use. There are two basic types
of liquid based active systems- open loop and closed loop. An open loop
system circulates potable water itself, through the collector. In closed
loop systems, the circulating fluid is kept separate from the system used
for potable water supply. This system is mainly used to prevent the freezing
of water within the collector system. However, there is no need to go in
for such a system in India, as freezing of water is not a possibility.
Also closed loop systems are less efficient as the heat exchanger used
in the system causes a loss of upto 10 degrees in the temperature of water,
at the same time, one has to reckon with the extra cost of the heat exchanger
as well as the circulating pumps. Compared to these, thermosiphon systems
are more convenient and simple.
In Thermosiphon systems, the water circulates from the collector to
the storage tank by natural convection and gravity. As long as the absorber
keeps collecting heat, water keeps being heated in the collector and rises
into the storage tank, placed slightly above (at least 50 cm). The cold
water in the tank runs into the collector to replace the water discharged
into the tank. The circulation stops when there is no incident radiation.
Thermosyphon systems are simple, relatively inexpensive and require little
maintenance and can be used for domestic applications.
Solar
ponds have been developed ,which harness the sun's energy that can
be used for various purposes including production of electricity.
Other devices such as solar cookers, water distillation systems, solar
dryers, etc. have been developed which can be used to reduce energy requirements
in domestic households and in industrial applications.
Active cooling
Absorption cooling systems transfer a heated liquid from the solar
collector to run a generator or a boiler activating the refrigeration loop
which cools a storage reservoir from which cool air is drawn into the space.
Rankine steam turbine can also be powered by solar energy to run a compressed
air-conditioner or water cooler.
Solar refrigeration is independent of electric supply and without any
moving parts, for example, Zeolite refrigerator.