Passive solar

Before the introduction of boilers and heat distribution systems like radiators or warm air flow, the primary form of controlling the climate inside a building was by means of passive solar architecture. The basic natural processes used in passive solar architecture are the thermal energy flows associated with radiation, conduction, and natural convection. When sunlight shines on a building, the building materials can reflect, transmit, or absorb the solar radiation. Additionally, the heat produced by the sun causes air movement that can be predicted.

These basic responses to solar heat have lead to design elements, material choices and placements that can provide heating and cooling effects in a home. These can often be seen in older buildings and particularly in southern Europe where for example, windows have external shutters which can be used to limit the solar gain during the summer and retain heat in the winter.

Passive architecture has the great advantage in that it requires no external energy source and therefore has neither a running cost nor does it contribute to environmental pollution. Such features can enhance the visual appearance of a building and will help to preserve its fabric. Whilst it is best considered when designing a new building, many of the techniques can be retrofitted to existing buildings. The potential of any building will depend upon the age, orientation and type.

When sunlight falls on a building some of it will be reflected depending upon the colour of the wall –

• white walls will reflect the greatest amount of heat and so traditional buildings in southern Europe generally have white walls to reduce the solar gain in summer
• darker colours reflect less heat but absorb more heat so traditional buildings in northern Europe are often painted red to capture solar radiation.

Heat will flow from the hotter to the colder parts of the building through one of three processes –

• conduction through the external to the internal wall of the building
• convection through the movement of air inside a room from the hotter to colder par of the room or by air movement through open windows or doors
• radiation from walls and roofs when air temperature falls below that of building element

Traditional forms of architecture either enhance or reduce heat flows by altering one or more of these flow processes.

Advantages

• reduction in solar gain in summer so decreasing the need for cooling
• increase in solar gain in winter so decreasing the need for heating
• as only passive elements are used no energy is consumed and no pollution created
• cost effectiveness as passive elements have similar life to the building itself
• enhances appearance of buildings as traditional architectural elements used
• reduced use of fossil fuels
• reduced rate of climate change
• reduced noise levels from air conditioning systems

Disadvantages

• they are best undertaken when a building is being designed
• the building may not be suitably orientated to enhance solar gain in winter
• in some protected areas it may not be possible to change the external appearance of buildings
• for some forms of building construction it may be difficult to attach passive solar elements

Thermal inertia is an important characteristic for the thermal comfort in the home. It is the resistance of a body to a change in temperature when the ambient temperature changes: the greater the mass of a body the greater its thermal inertia. Low inertia buildings are quickly heated by the sun and so quickly cool at night. High inertia buildings keep a more constant temperature as the building acts as a thermal store storing energy in its walls during the day and then gives out this stored heat once the sun goes down and the air cools during the night.

Double wall with cavity for high thermal inertia

Solar shading

Many cultures have learnt how to avoid this undesirable heating by shading the sunny side of the building during the summer. Suitable shading can provide good indoor climate control thereby avoiding air conditioning during the summer whilst helping with heating during the winter. To design good shading, it is necessary to know how the solar radiation reaching the building throughout the day during the different seasons.

Shading can be accomplished by many different means, depending on the location, the type and geometry of the building and the preferences of the designer. The basic principle is to place the shading so as to reduce the solar radiation during summer and to facilitate the solar gain during the winter.

The following are the most common options.

• deciduous trees – leaves provide shade during the summer but fall down in the autumn
• shutters which are preferably mounted outside the window; these can have the sunlight falling on the window during the summer during the night in winter
• blinds – comprise slats which can be inclined to control light (and heat): may be mounted either horizontally (Venetian blinds) or vertically
• external horizontal surface – mounted above the window to cut off direct solar rays when the sun is high in the sky (summer, middle of the day); however, when the sun is low in the sky (winter and early morning and late evening during the summer) the rays can fall upon the window and enter the room
• awning – an external blind that can be extended or retracted depending upon the strength of the sunlight during the summer
• solar panels, flat or tubular, can be used to shade facades or terraces

Examples of solar shading are illustrated.

Solar heating

The basic characteristics of heat can be utilised to provide solar heating during the winter. The simplest method is by absorption of solar rays by an external, south-facing wall which enables heat to be conducted through to the inner wall of the dwelling. To be most effective, walls should not be shaded by trees or lie in the shadow of adjacent buildings for any length of time.

Transmission of light through windows allows infra-red rays to heat the air in the room by convection. If the external window pane is coated with a suitable reflective layer on the inside, then the infra-red rays are reflected back into the room so retaining the heat.

The larger the thermal inertia of a building, the more heat can be stored during the daytime thus reducing the need for heating during the night.