E NE RG Y MAT TER S FACT SHEET 14: PASSIVE SOLAR HEATING & COOLING There are two aspects to passive solar design for buildings: heating and cooling. The basic principle for heating is to let the heat from the sun warm the building in winter, and keep the sun and hot winds out in summer, while maintaining access to cooling breezes. EXERCISE 1 If you have access to the video Passive solar energy design of buildings, watch it and answer the following questions. EXERCISE 1 (CONT’D) (c) Why are eaves important? (d) What does the term “thermal mass “ mean? Give some of examples of material with high and low thermal mass. Otherwise, you can find the answers to these questions on the Internet by searching for the phrase “passive solar design”. (a) What are the key principles of passive solar design for buildings? (e) Why is window placement important? (b) What is the ideal orientation and shape of a building in Australia? (f) Why does insulation work? What is the R-rating in insulation? EXERCISE 1 (CONT’D) (g) How do gardens help in passive solar design? The key components that constitute a complete passive solar design are shown in Figure 1. Each performs a separate function, but all five must work together for the system to be successful. Aperture (collector) – the large glass (window) area through which sunlight enters the building. Typically, the aperture(s) should face within 30 degrees of true north (in sthn hemisphere) and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season. Absorber – the hard, darkened surface of the storage element. This surface—which could be that of a masonry wall, floor, or partition (phase change material), or that of a water container—sits in the direct path of sunlight. Sunlight hits the surface and is absorbed as heat. Thermal mass – the materials that retain or store the heat produced by sunlight. The difference between the absorber and thermal mass, although they often form the same wall or floor, is that the absorber is an exposed surface whereas storage is the material below or behind that surface. Distribution – the method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use the three natural heat transfer modes– conduction, convection, and radiation– exclusively. In some applications, however, fans, ducts, and blowers may help with the distribution of heat through the house. Control – roof overhangs can be used to shade the aperture area during summer months. Other elements that control under- and/or overheating include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; and awnings. FIGURE 1 Elements of passive solar design (from Passive Solar Design for the Home, US Department of Energy) Energy Matters 14. Passive Solar Design Page 2 of 4 10:00 am to 5:00 pm Sunlight enters south-facing windows and strikes the thermal mass inside the home. The sunlight is converted to heat energy, which heats both the air and thermal mass materials. On most sunny days, solar heat maintains comfort during the midmorning to late afternoon periods. 5:00 pm to 11:00 pm As the sun sets, it stops supplying heat to the home. However, a substantial amount of heat has been stored in the thermal mass. These materials release the heat slowly into the passive solar rooms, keeping them comfortable on most winter evenings. If temperatures fall below the comfort level, supplemental heat is needed. 11:00 pm to 6:30 am The home owner sets the thermostat back at night, so only minimal backup heating is needed. Energyefficient features in the home minimize heat losses to the outside. 6:30 am to 10:00 am The cool early morning hours are the toughest for passive solar heating systems to provide comfort. The thermal mass has usually given up most of its heat, and the sun has not risen enough to begin heating the home. During this period, the home owner may have to rely on supplemental heat. Energy-efficient features in the home minimize the need for supplemental heating. FIGURE 2 How passive solar heating works in winter (from US Dept of Energy fact sheet) Energy Matters 14. Passive Solar Design Page 3 of 4 Basic tips for Australian houses Cooling Provide an appropriate level of shade and locate openings in the direction of cooling breezes. Shade the entire building in hot humid climates and hot dry climates with warm winters. Design narrow, elongated building forms for best performance, with the long elevations opening up to cooling breezes. Elevating the house so that air can circulate beneath it will also assist performance. Use landscape and building form to deflect cooling breezes into the interior and to exclude undesirable hot winds. Make use of shade or windbreaks provided by adjacent buildings or existing landscape. Design extensions to open to cooling breezes, particularly if they are living areas. Avoid large areas of exposed west facing wall. East and west facing openings receive the strongest sun and are the most difficult to shade. Keep their size to a minimum if this does not compromise cooling by ventilation. Alternatively, ensure they are well shaded. Ensure adequate north eaves overhangs, plus south eaves overhangs above the Tropic of Capricorn. Design open plan interiors to facilitate ventilation. Homes of one-room depth with openings either side are ideal. Design and position openings to control air flow. Use clerestory windows, roof ventilators, and vents in ridges, eaves and ceilings to create convection currents to cool the house in the absence of breezes. Install windows that can be opened for maximum ventilation. When renovating, replace fixed windows with systems like casement windows or louvres. Add additional small windows to rooms with only one window to improve ventilation. Use vents above or in internal doors to facilitate cross ventilation. Energy Matters 14. Passive Solar Design Ensure outdoor living areas are shaded. Covered balconies and verandahs can be useful additions, providing shaded outdoor living space. Use landscape to provide additional shade. Heating Maximise the amount of daytime living space that faces north, whether designing a new home or configuring renovations. Provide passive solar shading to east, west and north facing elevations, particularly glass areas. Correctly designed eaves are generally all that is required to shade the northern elevations of single storey houses. Place a suitable amount of glazing in north facing walls with solar access. The glazing area should be between 10 to 25 percent of the floor area of the room, depending on climate and mass. Glazing on other facades should ideally be less to prevent unwanted heat loss and gain. South facing glass facilitates winter heat loss, while east and particularly west facing glass encourages summer heat gain if not properly shaded. Smaller, well shaded windows are desirable for cross ventilation. Avoid west facing bedrooms where possible. East facing bedrooms are acceptable as they capture morning sun but remain cool on summer evenings. Locate utility areas such as laundries, bathrooms, garages and sheds to the south, west and east to protect living areas from summer sun and winter winds. Maximise the distance between the house and any building development to the north. Avoid placing obstructions such as carports or sheds to the north. Building on the south boundary (if permitted by your local council) can be useful to increase the amount of north facing outdoor space. Avoid compromising the solar access of neighbours by overshadowing. Plant shade trees in the appropriate locations. Landscaping can also be used to block or filter harsh winds. Prune vegetation that blocks winter sun. Page 4 of 4