ADDING CHEAP SOLAR POWER TO YOUR HOME

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ENERGY TIPS

Two weekend project ideas for slashing fuel bills.

THERMAL MASS

When I was around ten, my mother decided to build herself a modest 3' x 7' step-in greenhouse as a small addition to the family room. She and my father sat around the kitchen table for weeks, poring over plans and deciding how they could piece it together without going broke. And each day I came from school to find something different; a large hole in the southfacing wall, sheets of plastic as temporary windows, piles of bricks, concrete. For a while it was better than television. In a month, construction was finished, and we had a handsome little topiary bubble sticking out of the house like a turret. The first hot weekend in May, however, brought a shock for my Mom. That bubble, while we were away for just one warm, sunny weekend, had managed to kill all but the hardiest of Mom's plants and turn the family room into a convection oven. It was my first lesson in just how much heat a few windows can muster. Mom and Dad restocked the plants, invested in some reflective shades and monitored the weather a little more closely after that.

Nearly all homes have some southern facing glass, but most homes, especially older ones, were not built to take advantage of the heat energy those windows can transmit. One low-cost and simple way of taking advantage of southern exposure is through the addition of thermal mass material, which absorbs the heat energy and stores it, radiating the stored heat during the cool fall and winter evenings. You can do this by placing a substantial amount of heat-holding material such as masonry, concrete or containers of water where the sun can shine on them. This thermal mass will help to prevent the sunny room from being overheated and will reduce the need for opening windows or shading out the sun's free energy just to maintain comfort. At night, this mass will radiate its stored solar heat and reduce the need for conventional heat.

There are many ways to add thermal mass to a room. The challenge is to add enough. Try to provide 2 to 4 gallons (1/4 to 1/2 cubic feet) of water per square foot of south window, or 1/2 to 1 cubic foot. (75 to 150 lbs.) of concrete or masonry per square foot. If the mass is not in direct sunlight, double or triple these amounts. If you have a concrete slab, you already have plenty of thermal mass — just allow the sun to reach it. The mass should be dark-colored to absorb radiation and should be placed as close to the window as possible so that the sunshine hits it all day. It's important, though, that the windows should normally overheat the space where the thermal mass material is placed. If the windows don't overheat the space, the mass won't attain the higher temperatures needed to help with heating, except by moderating uncomfortable temperature change.

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