PASSIVE SOLAR OPTIONS
Passive solar design strategies vary by building location and regional climate, but the basic techniques remain the same— maximize solar heat gain in winter and minimize it in summer. When designing a passive solar house consider the following tips:
- Start by using energy-efficient design strategies.
- Orient the house with the long axis running east/west.
- Select, orient, and size glass to optimize winter heat gain and minimize summer heat gain for the specific climate.
- Consider selecting different glazings for different sides of the house (exposures).
- Size south-facing overhangs to shade windows in summer and allow solar gain in winter.
- Add thermal mass in walls or floors for heat storage.
- Use natural ventilation to reduce or eliminate cooling needs.Use daylight to provide natural lighting.
These techniques are described in more detail below.
Cutting Losses. A passive solar home should be well sealed and well insulated. By reducing heat loss and gain, remaining energy loads can be effectively met with passive solar techniques. Approaches that contribute to minimizing heating and cooling loads include using advanced framing guidelines, properly installing insulation, using recommended insulation levels, reducing duct losses, and tightening the building envelope.
Site Orientation. The building’s southern exposure must be clear of large obstacles (e.g., tall buildings, tall trees) that block the sunlight. Although a true southern exposure is optimal to maximize solar contribution, it is neither mandatory nor always possible. Provided the building faces within 30° of due south, south-facing glazing will receive about 90 percent of the optimal winter solar heat gain.
Window Selection. Heating with solar energy is easy: just let the sun shine in through the windows. The natural properties of glass let sunlight through but trap long-wave heat radiation, keeping the house warm (the greenhouse effect). The challenge often is to properly size the south-facing glass to balance heat gain and heat loss properties. Increasing the glass area can increase building energy loss. New window technologies, including selective coatings, have lessened such concerns by increasing window insulation properties to help keep heat where it is needed. In climates requiring more heating, reduce the window area on north-, east-, and west-facing walls, while still allowing for adequate daylight. Effective south-facing windows require a high Solar Heat Gain Coefficient (SHGC)—usually 0.60 or higher—to maximize heat gain, a low U-factor (0.35 or less) to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer. SHGC refers to the portion of incident sunlight admitted through a window, and U-factor indicates the heat loss rate for the window assembly. In climates requiring more cooling, particularly effective strategies include preferential use of north-facing windows along with generously shaded south-facing windows. Shading from landscaping, overhangs, shutters, and solar window screens helps lower heat gain on windows that receive full sun. Cost -effective windows for cooling climates have a U-factor and SHGC below 0.4 (a lower SHGC cuts cooling costs). Wherever possible, follow climate-specific window property recommendations from the Efficient Windows Collaborative.
Suntempering. In cold climates, a strategy termed “suntempering” orients most of the home’s glazing toward the south—a glazing area of up to 7 percent of the building floor area. Additional south-facing glazing may be included if more thermal mass is built in. Such a shift in window location is a great strategy for cold climates and costs nothing beyond good planning. Many passive solar homes are merely suntempered.
Shading. The summer sun rises higher overhead than the winter sun. Properly sized window overhangs or awnings are an effective option to optimize southerly solar heat gain and shading. They shade windows from the summer sun and, in the winter when the sun is lower in the sky, permit sunlight to pass through the window to warm the interior. Landscaping helps shade south-, east-, or west-facing windows from summer heat gain. Mature deciduous trees permit most winter sunlight (60 percent or more) to pass through while providing dappled shade throughout summer.
Heat Storage. Thermal mass, or materials used to store heat, is an integral part of most passive solar design. Materials such as concrete, masonry, wallboard, and even water absorb heat during sunlit days and slowly release it as temperatures drop. This lessens the effects of outside air temperature changes and moderates indoor temperatures. Although even overcast skies provide solar heating, long periods of little sunshine often require a backup heat source. Optimum mass-to-glass ratios, depending on climate, may be used to prevent excessive heating and minimize energy consumption. Avoid coverings such as carpet that inhibit thermal mass absorption and transfer. For tropical climates that have hot night-time temperatures, low thermal mass materials such as wood and metal may be appropriate to reduce the absorption of solar heat.
Natural Cooling. Apt use of outdoor air often can cool a home without need for mechanical cooling, especially when effective shading, insulation, window selection, and other means already reduce the cooling load. Cooling loads can be further reduced by using reflective materials on the roof to reduce solar heat gain in the summer. In many climates, opening windows at night to flush the house with cooler outdoor air and then closing windows and shades by day can greatly reduce the need for supplemental cooling. Cross-ventilation techniques capture cooling, flow-through breezes. Exhausting naturally rising warmer air through upper-level openings (stack effect; e.g., clerestory windows) or fans (e.g., whole-house fan) encourages lower-level openings to admit cooler, refreshing, replacement air. Natural cooling with unconditioned outdoor air may not be called for in climates where high humidity levels are present because outdoor humidity can affect occupant comfort. For humid climates, outdoor and indoor humidity should be removed through the use of dessicants or mechanical systems.
Natural Lighting. Sometimes called daylighting, natural lighting refers to reliance on sunlight for daytime interior lighting. Glazing characteristics include high-VT glazing on the east, west, and north facades combined with large, south-facing window areas. A day-lit room requires, as a general rule, that at least 5 percent of the room floor area be in glazing. Low-emissivity (low-E) coatings can help minimize glare while offering appropriate improved climatic heat gain or loss characteristics. Sloped or horizontal glass (e.g., skylights) admit light but are often problematic because of unwanted seasonal overheating, radiant heat loss, and assorted other problems.
Passive Solar Design Tools. One of the best ways to design an energy-efficient house featuring passive solar techniques is to use a computer simulation program. Energy-10™ is a PC-based design tool that helps identify the best combination of energy-efficient strategies, including daylighting, passive solar heating, and high efficiency mechanical systems. Another tool to optimize window area and aid window selection is RESFEN. Access these and other passive solar design tools from Lawrence Berkeley National Laboratory or the National Renewable Energy Laboratory.
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