One of the benefits of living in the Wild West is our abundant sunshine. An advantage for home builders in Denver is that Colorado averages over 300 sunny days per year; Arizona and New Mexico have even more.
My home here in Colorado is located on the north facing side of a mountain. We have some pretty sweet views of more mountains and the Continental Divide to the west and north of us, but not much potential for passive solar heating. It is not uncommon to have snow in my yard well into May. My neighbors across the valley, however, bask on the sunny side of the mountain. The snow in their yards often melts only a few days after it falls. When I designed and built our small cabin in New Mexico, one of my primary design considerations was to provide some passive solar heating.
What is passive solar?
Passive solar is pretty simple: The solar part is obvious ... it refers to the sun. The passive part means it works without any mechanical devices, added energy inputs or efforts from the occupants. It basically heats itself by smart design. Anyone who has ever left his car sitting in the hot summer sun knows all too well that windows allow sunshine to enter and warm the inside. In a vehicle, these temperatures can reach 150 degrees F or more very quickly. Concentrated versions of this same phenomenon allow solar ovens to bake and cook foods at 350 degrees F to 400 degrees F. When it comes to using this system to heat a home some considerations need to be addressed.
-- Site location: For starters, you're going to need some sun. Having a home located deep in a dense pine forest is not going to work to well. Similarly with deep canyons and hillsides, the quantity of sun that "lands" on the property will have a direct effect as to how effectively it can be harnessed. In urban areas, large buildings may provide shade that can restrict or prevent good passive solar designs.
-- Orientation: Another design constraint is to point your solar collectors (aka "windows") towards the sun. In the northern hemisphere this direction is south. A home with a large glazed southern exposure and minimal northern glazing is the basis for practical design.
-- Thermal Mass: All of this sun energy needs to be stored or moderated; this is best done with heat-and energy-absorbing components.
-- Seasonal Shading: This is the trickiest part. You want the Sun to work for you in the cool months when if provides needed warmth, but not in summer when you do not need it.
When solar designs were first being developed, there was a common idea that the more windows you had the better off you were. This design led to homes that would get smoking hot when the Sun was shining but then get wickedly cool once the Sun went down. Others would just be baking hot all summer long. These homes lacked the critical thermal mass component to moderate day to night swings and often lacked the seasonal shading part to prevent overheating in the summer.
Thermal mass
In a passive solar design, the thermal mass element is what keeps things from swinging widely from hot to cold, during a typical day and night cycle. An example of how it works: Thermal mass is like cooking a very large pot of water. When you start, the water is cold and the stove adds heat. The pot of water "collects" the heat until it reaches a boil, and then when the heat is turned off the pot of water will remain hot for a long time. The thermal mass acts a bit like a mechanical flywheel to even out the highs and lows.
