DIY projects boost efficiency of solar heat

By James Dulley
DIY projects boost efficiency of solar heat

This shows various applications of vertical solar heaters mounted on a south-facing wall. Notice how the size, shapes and wall locations vary, depending upon the design of the house. Source: U.S. Department of Energy’s National Renewable Energy Laboratory

The heat energy we use in our homes comes directly or indirectly from the sun. Some of it, such as oil, gas or coal, has stored the sun's energy over millions of years. Trees store it for decades until we burn them. Residential solar systems use the sun's heat as it shines on houses each day.

A homeowner should have realistic expectations for using free solar energy. Although it can be accomplished, trying to provide 100 percent of the heating needs of an existing house with solar is very difficult to do while still maintaining acceptable comfort. An initial target of a 10 percent savings is reasonable for a do-it-yourself solar project.

If you are new to solar energy and plan to build a heating system yourself, stick with one or more simple passive systems.

Just having the sun shine in a large window is effective passive solar heating, but it can be made more efficient. This type of solar heating is especially efficient in warm southern climates where the winter days do not become as short as in northern areas. Also, because it is warmer outdoors in mild climates, less heat is lost through the window at night.

To be most effective in every climate, there should be adequate thermal mass in the room with the window. Thermal mass captures the sun's heat so the room does not overheat or lose as much of the heat back to the outdoors. Once the thermal mass warms up, it slowly dissipates the stored solar back into the room once the sun is no longer shining. It is preferable to have the thermal mass in the direct path of the sun's rays, but this is not critical to be effective.

To increase thermal mass in a room, you can add planters with concrete blocks or bricks. You can also pour and make your own concrete planters using tinted concrete similar to contemporary concrete kitchen countertops. A large terrarium with much damp soil has a reasonably high thermal mass, and it adds humidity to the air.

The best solar option, if you do not need a view outdoors from the entire window, is to build a solar Trombe wall. A simple design uses stacked bricks or concrete blocks very close to the window. The vertical stack gets warmed by the sun, which creates an upward warm air current. This circulates the warm air throughout the room while it also stores heat for nighttime. During the summer, just remove the bricks or blocks and store them away.

If you want to keep the view from your window, make a shallow, flat solar heater that rests against the outside wall facing the noon-to-afternoon sun. A size of four feet by eight feet makes the most efficient use of inexpensive standard lumber. The box has to be only the depth of standard two-by-four studs.

Once the plywood box is completed, attach foil-backed rigid foam insulation on the inside of the box with the foil facing inside. Paint the foil surface flat black. Cut one hole in the back at the top and one at the bottom and install duct stubs. Cut holes in your house wall so the duct stubs come through to indoors.

Cover the front of the box with a sheet of clear acrylic plastic and seal it. The solar-heated air will flow up and out into your room. Make airtight indoor covers to seal off the duct stubs at night otherwise the air flow will reverse and actually cool your house.

About the Author

James Dulley is an engineer and syndicated columnist for the National Rural Electric Cooperative Association. www.dulley.com

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