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The Basic Elements of Passive Solar

"Passive" Solar and "Active" Solar are frequently confused in Energy discussions. The typical registered image is PV Solar Panels (Active) on the roof. Passive Solar is more fundamental and important than just the hardware up there!
by: Art Smith
Posted: September 17, 2013

The Basic Elements of Passive Solar

 With the general public now aware of the importance of Green Building concepts, an old related friend has reappeared from its 1970/80’s Heyday: Passive Solar. In fact the most respected technical reference on this energy topic was amply named by Ed Mazria way back in 1979 as simply “The Passive Solar Book”. Passive Solar is as important now as when the 1970’s Oil Crisis’s initially thrust the topic into the public realm.

 A typical scenario about “Solar Energy” discussions is that the two distinct types are frequently muddled: Passive Solar & Active Solar. Ironically the “Active” version, with the symbolic PV (Photo-Voltaic) panels generating electricity directly on a roof is usually one’s first visual image when the Solar Energy phrase is overheard. But we will save this Active type discussion for a later session and focus on the more subtle partner: “Passive”. Eventually, we shall also see how the two halves of this solar equation actually work best in tandem!

 The Basic’s of Passive solar essentially all focus on the design and construction of a home in relation to the Sun’s natural path. The essence is that the designer should utilize as many of, and to the most extent as practical, the following Elements:

  • South facing location for home (major wall)
  • Rectangular shape along East-West axis
  • Majority of windows along the long South wall
  • Minimal windows on North & West sides, very few on East side
  • South roof overhangs lengths set by latitude ratio over medium, not tall windows (less than 60”)
  • Significant “Thermal Mass” on South side of home near large South windows

This list has more technical details to be applied with the aid of the noted reference, but a brief expansion here on each of these elements may assist the curious.

 First, the South-Facing orientation is by far the most significant issue since the Sun won’t move for us…so we have to locate our house for the Sun. Even one reasonable portion of the house facing South can help if a suburban development location minimizes the ideal spot. But with the preferred rectangular, rather than a square-shaped, structure’s long side facing South the potential Sun energy is higher. Now really utilizing this long South wall, the designer can place the largest and most windows there. Obviously, planning and forethought of the owner’s desired room locations must be factored in. By clustering key rooms like the Great Room, Master and Guest Bedrooms, and maybe even the Dining room along this longer South wall, they also serve a “View” purpose as well (Particularly in our beautiful North Georgia Lake and Mountain settings!). The other rooms, like bathrooms, closets, etc. then take secondary importance on the North side of the home. If the lot is sloped southward also, the potential daylight basement space can even benefit from this natural sun positioning.

 With our “Passive” Solar house now bringing in this energy ("harvesting"), we need to properly regulate those South windows when we want it (winter) and keep it out when our Georgia summer is peaking. The Sun’s path due to the Earth’s natural changing axis angle, resulting in our changing “seasons” for our latitude location works for our desired goal. That is, as we all intuitively sense, the Sun is higher in the sky in summer (Peak height is June 21 at, say, 79 degrees). We also know it is at a lower angle in the winter (December 21 around 32 degrees). This angle change between seasons enables the proper roof overhang to shade the Sun out of the window in the summer and let it enter the widows in the colder months. A quick, rough, North Georgia example may help visualize the proper roof overhang. Let’s assume our South window bottom is 60” down from the roof (54" tall windows is excellent choice!) as shown in the included sketch. Our region’s roof overhang “shading factor” is about 1/3. Thus, our roof overhang on the South roof is about 20-22” long past the vertical window wall. Thus, in mid-summer, near mid-day, no direct sun is coming in the windows. But in winter, the lower sun angle is supplying warmth into the home since the sun angle falls below this roof overhang. This also means that the lower rated SHGC (Solar Heat Gain) typically specified for southern climates is not preferred here (since house design is proper...not like “typical” house window locations...as the full sun blazing in on July afternoons!) Note that this shading concept only works effectively on the South side, thus minimizing windows on the other sides of the home is also important.

 Now that we have all this “free” energy coming in when we desire it, we need to control its impact on the home’s air temperature. Our assistant, “Thermal Mass” is the answer. To quantify this energy potential, let’s use a quick example of say, a well-built, 1500 square foot, home with an Energy Star or LEED crafted insulation package. Likely a 2-ton or so efficient heat pump is all that is needed to heat and cool this concept home. With about 150 s.f. of window area on this South wall, somewhere close to ¾ ton of BTU’s of equivalent energy is coming in the home in winter days. This energy boost must be tempered by thermal mass to store the energy and release it. Good thermal mass examples are fireplace stone or brick, some internal brick or stone walls, and ideally, a concrete floor with chosen finished pavers. A very rough estimate of the thermal mass area is four to eight times the area of the south windows. Essentially, this mass absorbs the energy during the warmer day and releases it back to the house at night when the air cools. Thus, stabilizing the temperature swings within the home. And finally…We may not quite need that full 2-ton heat pump…maybe only 1-1/2 tons!

 Now with these few design concepts working in our home, our inside environment’s temperature is more comfortable (by Passive means), and we will simultaneously save energy costs.  Additionally, our home is better prepared to add the “Active Solar” portion when we desire to make the investment. Utilizing both Solar types will get us closer to Net Zero!

Art Smith  -- LEED AP