Passive solar

Passive solar is the use of architectural elements in buildings to take advantage of natural cycles of sunlight and other elements in order to reduce the costs of building heating and cooling, without using mechanical elements. If mechanical elements are used, then the system is termed active solar.

In the strictest sense, passive solar has these main elements:

Building orientation: The building is oriented as close to a north-south-east-west axis as possible, usually with the long axis of the building running east-west for maximum exposure towards the south (in the northern hemisphere; towards the north in the southern hemisphere).
Window placement: Windows are minimized on the north side of the building, which in a northern temperate zone receives no direct sunlight from the fall equinox to the spring equinox. Windows on the south side (in the northern hemisphere) are made large in order to receive maximal sunlight during the winter.
Overhangs: Overhang projections are provided over southern, and sometimes eastern and western, windows to shade them during summer months. Because the sun is at a lower angle in the sky during winter days than summer days, such structures can provide shade during the summer yet allow the sun to shine in through windows in the winter. Landscaping may also play a part in this: deciduous trees may be planted on the window side. In summer, these trees will shade the house, cooling it, while in winter they will not greatly obscure the influx of sunlight.
Thermal mass: Dense building materials are used where the winter sun will shine on them inside the windows, to capture the sun's heat and re-radiate it through the night. A trombe wall is one such structure. Another more efficient form places concrete blocks under the main slab, using the holes in the blocks as ducts to treble the heat exchange area (the bwlow-grade ducts must be drained). Passive solar buildings often use massive materials such as stone, concrete, and adobe.
Daylighting: Windows and internal reflecting areas are placed so that they can maximize the sun's light for interior lighting during the day.