by James Dulley
Engineering Consultant
Q: The sun shines directly in a living room window, and it’s very warm. I am on a tight budget, so I want to make or install something to capture that heat to lower my heating bills. What do you suggest?
A: Actually, all the heat energy we use in our homes comes directly or indirectly (gas, oil, coal, wood, etc.) 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.
You must be realistic in your expectations for using free solar energy to heat your home. 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.
Because 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. This thermal mass captures the sun’s heat so the room does not overheat or lose as much of the heat back outdoors. Once the thermal mass warms up, it slowly dissipates the stored solar back out into the room once the sun is no longer shining in.
If you are not really into Flintstone-esque décor with a pile of rocks in the center of each room, there are other methods to increase the thermal mass in a room. 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.
You can make 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 the 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 4 feet by 8 feet is good because it makes the most efficient use of inexpensive standard lumber. The box has to be only the depth of standard 2-by-4 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.
James Dulley is a nationally syndicated engineering consultant based in Cincinnati.
If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.