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Passive solar heating is just one strategy in a group of design approaches collectively called passive solar design. When combined properly, these strategies can contribute to the heating, cooling, and daylighting of nearly any building.

Passive solar heating in particular makes use of the building components to collect, store, and distribute solar heat gains to reduce the demand for space heating. It does not require the use of mechanical equipment because the heat flow is by natural means (radiation, convection, and conductance) and the thermal storage is in the structure itself. Also, passive solar heating strategies provide opportunities for daylighting and views to the outdoor through well-positioned windows.

It is best to incorporate passive solar heating into a building during the initial design. The whole building approach evaluates it in the context of building envelope design (particularly for windows), daylighting, and heating and cooling systems. Window design, especially glazing choices, is a critical factor for determining the effectiveness of passive solar heating. Passive solar systems do not have a high initial cost or long-term payback period, both of which are common with many active solar heating systems.

In heating climates, large south-facing windows are used, as these have the most exposure to the sun in all seasons. Although passive solar heating systems do not require mechanical equipment for operation, this does not mean that fans or blowers may not, or should not, be used to assist the natural flow of thermal energy. The passive systems assisted by mechanical devices are referred to as "hybrid" heating systems.

Passive solar systems utilize basic concepts incorporated into the architectural design of the building. They usually consist of: buildings with rectangular floor plans, elongated on an east-west axis; a glazed south-facing wall; a thermal storage media exposed to the solar radiation which penetrates the south-facing glazing; overhangs or other shading devices which sufficiently shade the south-facing glazing from the summer sun; and windows on the east and west walls, and preferably none on the north walls.

Design of Passive Solar Heated Buildings

The following are general recommendations that should be followed in the design of passive solar heated buildings.

Passive solar heating will tend to work best, and be most economical, in climates with clear skies during the winter heating season and where alternative heating sources are relatively expensive.
Use passive solar heating strategies only when they are appropriate. Passive solar heating works better in smaller buildings where the envelope design controls the energy demand.
Careful attention should be paid to constructing a durable, energy-conserving building envelope.
Address orientation issues during site planning. To the maximum extent possible, reduce east and west glass and protect openings from prevailing winter winds.
Specify an air-tight seal around windows, doors, and electrical outlets on exterior walls. Employ entry vestibules; and keep any ductwork within the insulated envelope of the house to ensure thermal integrity. Consider requiring blower-door tests of model homes to demonstration air-tightness and minimal duct losses.
Specify windows and glazing that have low thermal transmittance values (U values) while admitting adequate levels of incoming solar radiation (higher Solar Heat Gain Coefficient). Data sources such as the National Fenestration Rating Council "Certified Products Directory" should be consulted for tested performance values. The amount of glazing will depend on building type and climate.

Ensure that the south glass in a passive solar building does not contribute to increased summer cooling. In many areas, shading in summer is just as critical as admitting solar gain in winter. Use your summer (B) and winter (A) sun angles to calculate optimum overhang design.
Avoid overheating. In hot climates, buildings with large glass areas can overheat. Be sure to minimize east- and west-facing windows and size shading devices properly. For large buildings with high internal heat gains, passive solar heat gain is a liability, because it increases cooling costs more than the amount saved in space heating.
Design for natural ventilation in summer with operable windows designed for cross ventilation. Ceiling fans or heat recovery ventilators offer additional air movement. In climates with large diurnal temperature swings, opening windows at night will release heat to the cool night air and closing the windows on hot days will keep the building cool naturally.
Provide natural light to every room. Some of the most attractive passive solar heated buildings incorporate elements of both direct and indirect gain. This can provide each space a quality of light suitable to its function.
If possible, elongate the building along the east-west axis to maximize the south-facing elevation and the number of south-facing windows that can be incorporated.
Plan active living or working areas on the south and less frequently used spaces, such as storage and bathrooms, on the north. Keep south-facing windows to within 20° of either side of true south.
Improve building performance by employing either high-performance, low-e glazing or nighttime, moveable insulation to reduce heat loss from glass at night.
Locate obstructions, such as landscaping or fences, so that full exposure to the sun is available to south windows from 9 A.M. to 3 P.M. for maximum solar gain in winter.
Include overhangs or other devices, such as trellises or deciduous trees, for shading in summer.
Reduce air infiltration and provide adequate insulation levels in walls, roofs, and floors. As a starting point for determining appropriate insulation levels, check minimum levels in the CABO Model Energy Code.
Select an auxiliary HVAC system that complements the passive solar heating effect. Resist the urge to oversize the system by applying "rules of thumb."
Make sure there is adequate quantity of thermal mass. In passive solar heated buildings with high solar contributions, it can be difficult to provide adequate quantities of effective thermal mass.
Design to avoid sun glare. Room and furniture layouts need to be planned to avoid glare from the sun on equipment such as computers and televisions.

Five Elements of Passive Solar Home Design
The following five elements constitute a complete passive solar home design. Each performs a separate function, but all five must work together for the design 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 south and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season.
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 thermal mass is the material below or behind that surface.
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.
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.

Passive solar design is highly energy efficient, reducing a building's energy demands for lighting, winter heating, and summer cooling. Energy from the sun is free. Strictly passive designs capture it without additional investments in mechanical and electrical "active solar" devices such as pumps, fans and electrical controls.

Passive solar design also helps conserve valuable fossil fuel resources so that they can be directed toward other uses. And it saves money. Incorporating passive solar design elements into buildings and homes can reduce heating bills by as much as 50%. Daylighting, a component of many passive solar designs, is one of the most cost-effective means of reducing energy usage in buildings.

A well-designed and built passive solar building does not have to sacrifice aesthetics either. It can be as attractive as conventionally designed buildings and still save energy and money.

Passive solar design also reduces greenhouse gases that contribute to global warming because it relies on solar energy, a renewable, nonpolluting resource.

There are few disadvantages to passive solar design and daylighting. With the help of experienced passive solar designer architects and builders, passive solar design costs little more than conventional building design and saves money over the long run.

But in areas where experienced solar architects and builders are not available, construction costs can run higher than for conventional homes, and mistakes can be made in the choice of building materials, especially window glass. Passive solar homes are often built using glass that, unfortunately, rejects solar energy. Such a mistake can be costly. Choosing glass for passive solar designs isn't easy. The right glass choice depends on which side of the building (east, west, north, or south) the glass is installed and the climate in which you are building (PDF 216 KB). Download Acrobat Reader.

In addition, room and furniture layouts need to be planned carefully to avoid glare on equipment such as computers and televisions.

And along with daylighting comes heat. During the summer or in consistently warm climates, daylighting could actually increase energy use in a building by adding to its air-conditioning load.

Passive solar design and daylighting principles can be applied to buildings in almost any part of the United States, with the exception of Alaska. In hot climates, the design mitigates the sun's heat; in cold climates the design takes advantage of it. However, passive solar heating tends to work best and be most economical in climates with clear skies during the winter heating season and where conventional heating sources are relatively expensive.

Passive solar buildings have been constructed as far north as Maine and as far south as Florida. In the United States, the design principles are most commonly used in residences because passive solar design works best in smaller buildings. But some passive solar design and daylighting applications also are used in commercial construction. For example, the Solar Energy Research Facility at the National Renewable Energy Laboratory, a Department of Energy national laboratory located in Golden, Colorado, has incorporated a sunspace for passive solar heating into its building

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