What is Passive Solar?

The latest issue of Home Power magazine (April-May 2010) just arrived. As usual, I devoured nearly every article. Since 2001, I have been a devotee of Home Power, fascinated by their variety of technical information about home energy: solar, wind, hydro, design, and build. I am most intrigued by the ability to heat, cool, and ventilate a home naturally–that is the definition of passive solar. Here’s the definition from Wikipedia:

Passive solar is using sunlight for useful energy without use of active mechanical systems (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air, thermal mass), cause air-movement for ventilating, or future use, with little use of other energy sources. A common example is a solarium on the equator-side of a building. Passive cooling is the use of the same design principles to reduce summer cooling requirements.[1]

The Home Power article on “Designing a Passive Solar Slab,” notes that a truly passive solar house has these elements operating without mechanical power:

  • summer solar gain handled by properly designed overhangs, and
  • the distribution system—natural convection within an open floor plan, with storage and release handled by concrete.

Further in the article, the author, Robert Riversong, writes that “south facing glazing design standard for passive solar homes is a window area between 7% and 12% of the floor area. (1000 sq ft space would have between 70 and 120 sq ft of south glazing–that’s between 8’x8′ to 11’x11′.) Beyond 12%, direct-gain thermal mass is not sufficient to maintain a uniform and comfortable indoor temperature without fans or pumps to move the heat.” What does that mean to the uninitiated?

It means that the ratio of windows on the south side of a house should not be more than 7%-12% of the floor area when using passive solar because the room will become overheated in the summer. Case in point: when we built our home office in 1991, I insisted that it be full of windows because I wanted to see the great views. So we put two large windows on three sides of the building facing south. The result? Massive overheat in the summer and very cold in the winter. My contractor neighbor, Joe Petrello, says, “Every window is an open door.” During the summer when we realtors go on tour to visit homes for sale, it is easy to pick out the homes with too many windows that overheat easily—they’re HOT!

The author states that the “goal in designing a passive solar home’s thermal mass is to be able to store midday solar heat until the early evening, when it will passively return to the living space. The most cost-effective thermal mass floor is concrete slab-on-grade (formed by pouring concrete within forms.)

“Dense materials, like concrete, tend to allow heat diffusion at a rate of about 1 inch per hour. So the heat of the noontime sun will penetrate to the bottom of a 4-in-think slab by about 4 pm and all that heat will have returned to the interior by about 8 pm.” In other words, if the sun hits a dense object (4 inch concrete slab) that is unobstructed by rugs and furniture, you’ll notice that not only do the cats love the heat, but by that evening you will feel some of the heat return through the floor.

How can you avoid over-heating in the summer? Because the summer sun is higher in the sky, having wider overhangs that are calculated for the latitude will keep the sun from entering, and therefore, overheating. The key to controlling the sun for warmth, coolness and ventilation is by not allowing it in during the summer (use wider overhangs, trees, and outside shades). Winter sun is lower in the sky, so it will enter, hit the concrete slab, and help to keep your house warmer through the late afternoon. For more info, check out http://www.homepower.com  where you’ll find online subscriptions for $10 per year.

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