Thursday, July 12, 2012

WHAT IS A PASSIVE SOLAR HOUSE?

Passive Solar House? People to whom the thought of “solar energy” is uncommon to immediately think of a big array of solar collectors on the roof, or stuck outside in the yard by the house. But solar collectors for producing electricity are actually a hybrid system between passive solar and active solar. So then what IS the difference between the two?

ACTIVE solar implies using solar energy in a system with mechanical pumps and fans that will inevitably break down.

PASSIVE solar is a system using solar energy where there are no moving parts whatsoever; in fact, you probably couldn’t tell a passive solar house was even “solar’s orientated unless you had a good idea of window placement and where solar south is.

Let me start by saying one thing: EVERY HOUSE IS A SOLAR HOUSE!

The difference being that a properly designed house will be warmer in the winter and cooler in the summer than those built by “standard construction.” Why? Does it have to do with having more insulation? It could, but no. Better, more efficient heaters and air conditioners? No. More cost into building the home than a standard house? No, not necessarily that either. Then what? The key is knowing how to let the sun IN to help warm the house in the wintertime and keep the sun OUT in the summer.

How is this little trick done? Well, before I tell you, let me give ask you something... Do you have one of those windows in the home you live in now that faces west? Do you find that in the summer, alter an already hot day, that the sun is shining straight through that window making your air conditioner work even harder because it is heating up your house even more? If so, I can almost bet that you live in a building built under “standard construction” methods. The key to letting the sun in a passive solar house is WHERE THE HOUSE BECOMES THE SOLAR COLLECTOR in an efficient way.

In the northern hemisphere, the majority of the sun’s energy comes from the south. Since between 60 and 80 percent of direct sunlight is available to use as solar energy, large windows facing solar south can provide up to 60 percent off a building’s heating needs when the windows are insulated during the night with tight-fitting shutters, insulated curtains, or some other exterior device that closes to trap the heat in. Clerestory windows can be used for additional light to upper levels within the structure or to back rooms that would not normally receive direct sunlight.

Also, it is important to note that a passive solar house should have about 1 square foot of window area for every 3-5 square feet of floor area. Otherwise, you will be looking at either a cold house or a very overheated one.

In order to know which way to face your windows, you must be able to find solar south (not magnetic south-since the sun does not rise and set exactly due east and west). Finding the sun in the sky at any given time of the year is easy.

Basically, there are three different ways to find solar south:

1. Place a stick in the ground and draw a line on the shadow cast at sunrise. Then draw another line on the shadow cast at sunset. Directly in between the two shadow lines draw a third line (on the south side). This indicates the direction that the sun sill be coming from exactly in the middle of the day (any time of the year), and which way your windows (double-paned) should face to receive the most ample amount of sunlight for your structure. This should be south-southeast for most areas of the U.S.

2. Watch the news on television, or check your Almanac and note the times of sunrise and sunset. At exactly in between these two times place a stick in the ground and draw a line where the shadow is cast. Facing towards south (where the sun is) is where the line should point. This is solar south.

3. Find your location and see what the compass deviation (or angle of declination) is by looking on a topographical map. Examples: For Lincoln, Nebraska solar south is 9 degrees east of magnetic south according to the deviation. Because of the latitude being at 41 degrees, the main solar heating hours for the Lincoln area are between 8AM and 2PM each day. For El Paso, Texas solar south is about 11 degrees east of magnetic south, and the peak solar hours are between 9AM and 3PM.

Since so many of the hours to receive direct gain are in the morning and noon hours, many people choose to have some windows on the east side as well as on the south. But beware, any windows facing more than 40 degrees off solar south (either way) start to lose efficiency at a rapid rate.

Another few pointers includes proper use of THERMAL MASS (acts as a heat sink), the glass-to-mass ratio, insulating on the exterior of that mass rather than the interior (standard stick-and-brick construction, where the brick (thermal mass) is on the outside and the insulation is on the inside), and properly designed overhangs.

See my other blog post on "House Design Blunders" for details: http://sunstarangel.blogspot.com/2012/07/house-design-blunders.html

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