When it comes to solar, are you passive?

A passive solar home means there are no moving parts or equipment that need to be maintained. Being “passive” means just that – it’s all about how your house “lays” there and absorbs or reflects heat.

Heat naturally moves from a warmer material to a cooler material until there is no temperature difference between the two materials. Passive solar homes take advantage of this property to distribute heat in the winter and to deflect heat away from surfaces in the summertime.

Direct heat gain is the simplest passive type of solar system, as it relies only on sun entering through a collector of some sort (such as a large south-facing window with a glazing material of transparent or translucent glass), and being absorbed by a darker material, such as a masonry floor. The floor acts as the “thermal mass” and absorbs the heat during the day. At night, as the floor cools, the heat is radiated back into the room as heat.

Some people use water-filled containers to absorb the heat, as water stores twice as much heat as masonry material. However, these types of systems need to be carefully designed and placed, as the structural support required is high.

In the summertime, if the home has been designed and oriented properly, with appropriate roof overhangs, awnings, or shutters, the sun does not enter through the windows and no heat is absorbed. Think of how the sun is aligned in the winter and summer and this will make sense. In the winter, the sun hits us from a much sharper southern angle. That allows more of the sun into our windows in the winter. In the summer, the sun is higher in the sky and shines down more directly. The sun can therefore be blocked with larger roof overhangs, window awnings, shutters, and even trellises with blooming vines.

An indirect heat gain system is more complex, as it involves installing the thermal storage medium (the thermal mass) in between the south-facing windows and the living areas. In this type of setup, a Trombe wall is installed which consists of an 8-16 inch-thick masonary wall on the south side of the house. A single or double layer of glass is mounted about 1 inch or less in front of the wall’s surface. Solar heat is then absorbed by the wall’s dark-colored surface and stored in the wall’s thermal mass, where it begins to radiate into the house. Because the transfer of heat takes a lot longer, the heat doesn’t begin to radiate into the living spaces until the sun has set. It typically will take about 8 hours for the heat to radiate into the home, so this system is very affective in winter-time heating. And as with the direct heat gain method, proper use of roof overhangs, shutters, and trellises will eliminate the summer-time heat gain.

For more information on designing a passive solar heat system for your home, see the Energy Savers web site, part of the U.S. Department of Energy’s web site.

If you live in the Massachusetts area, be sure to check my web site at CyberGreenRealty.com for local solar vendors and others who can help you get as “active” or “passive” as you want!

Until next time. Peace!

-TMC

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