The difference between active and passive applications of solar energy is that passive technologies have no components that consume energy. Active solar heating systems, for instance, have a pump to circulate the hot water, so mechanical energy is involved. Photovoltaic cells actively transform solar energy into electricity. Passive systems rely more on the natural behaviours and characteristics of different substances, such as the tendency of hot air to rise, and can provide cooling as well as heating.
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Building-In Passive Solar Power
It is possible to retrofit features to existing premises, but for maximum effect, as many passive solar features as possible should be incorporated into the building at the planning stage.
One of the first things to consider when designing a building is its orientation in relation to the sun, as correct alignment will vastly enhance its ability to exploit passive solar heating and lighting. The important part of the building is the south elevation, because this side receives most sunlight.
Passive Solar Lighting
The less sunlight penetrating a building, the more artificial lighting will be needed. This makes the building more energy-hungry, and it has also been shown that an environment with natural sunlight is healthier for the occupants than constant artificial light. ‘Daylighting’ requires large, south-facing windows that are not overshadowed by another nearby structure. North-facing rooms, too, can benefit from natural light if a skylight or a clerestory can be included; a clerestory is a row of windows at high level. Passive solar lighting is most effective where there is an open floor plan, so that daylight can enter at roof level and filter through the building.
Passive Solar Heating
Like passive solar lighting, passive solar heating can reduce electricity consumption significantly; but, again, it is most effective if designed into the building. If the walls are built from materials that absorb and store thermal energy, at the end of a hot day they will have collected a lot of heat, and when the air inside the building becomes cool, they will transmit heat into the atmosphere. This is termed direct solar gain.
Indirect solar gain comes from features outside the living space, such as sunspaces or Trombe walls. Sunspaces or solariums are glass structures built onto the sunny side of the building; the air inside becomes very hot, like a greenhouse, and the hot air is circulated round the building by convection. A Trombe wall is a thick, dark-coloured wall with vents at top and bottom, constructed a few inches from a glazed panel or window. The air in the channel between wall and window is heated and convection currents carry the heat through the vents and around the building.
Passive Solar Cooling and Ventilation
Features that promote solar heating must be balanced by cooling and ventilation features, to prevent excessive heat during periods of strong sunlight. Effective passive solar ventilation can be provided by a thermal chimney, which uses passive solar energy to create air movement , and enhances natural ventilation throughout the building.
It is also necessary to provide an efficient means of shading windows, clerestories and skylights and closing off sunspaces. There should be plenty of fresh-air ventilation, and there should be a minimum of physical obstructions that completely block the passage of air, as this prevents convection from taking place and creates ‘hot spots’ in the building.
At its most basic level, the application of passive solar energy simply means taking a little trouble to ensure that a few fundamental principles are implemented effectively. Simple passive solar features such as double glazing are now standard in most modern buildings, and we have developed many innovative methods of using and combining active and passive solar power, that can be installed into premises with little or no disruption.
Some are widely used and quite inexpensive, such as solar lights in parks and gardens, that and charge up while the sun is shining and emit light at night, requiring no electricity supply. Some are more complex; for example, Hybrid Solar Lighting systems (HLS) use focusing mirrors to track the sun, collect direct sunlight and transmit it inside the building along optical fibres.
The savings on energy consumption through passive solar power can be very considerable, and are greater still when active and passive solar features complement each other; for instance, electricity produced by photovoltaic panels can operate fans to circulate thermal energy collected by passive solar means, or pumps to circulate water heated by passive solar heat. The buildings that make the best use of passive solar energy, and therefore have the lowest carbon footprint – and the lowest energy bills – will always be those that were designed with passive solar power as a priority.
