When most folks hear about solar power, the first thing that comes to mind is solar electricity. Solar electricity is amazing stuff, but for households on a budget or for individuals who love to tinker in the shop, it is not the most accessible or affordable solar technology.
Besides needing electricity, most homes, shops, or offices also need to be heated. Capturing solar energy for space heating is a straightforward and efficient way of bringing renewable energy into our lives. One of the most direct, easiest, and economical ways of doing this is with a solar air heater, used for supplemental heating.
More than any other solar technology, solar heaters are DIY-friendly, since they require only a basic knowledge of carpentry and electrical skills, can be made of easy-to-find materials, and can be installed on a south-facing wall rather than on a potentially dangerous roof. Solar air heaters are tolerant of less-than-exact construction details. A small air leak will only reduce the heater’s overall efficiency, not leak fluid or potentially overheat or shock you if installed improperly. While care should be taken with any project, the consequences of potential mistakes are much less dire. For homeowners interested in the basics of renewable energy, building a solar air heater can be a great project.
Before beginning, it’s important to have access to the sun where and when you need it—or all of your hard work will be for naught. Make sure there is full sunlight on the south-facing wall in the winter months from about 10 a.m. to 2 p.m. The sun is at a low angle during the winter, so the number of potential obstructions increases. If you can’t accurately assess the winter solar window within a few weeks of the winter solstice, use a solar analysis instrument such as a Solar Pathfinder or Solmetric’s SunEye.
DIY solar heaters should be installed on south-facing walls and never on roofs. Homemade heaters tend to fare poorly in the extreme weather conditions that exist on roofs, and the produced heat tends to stay at the ceiling level. Hot air is likely to stratify into layers and stay stuck away from where residents want it—near the floor. Using a stronger blower might seem like it would solve this problem and help circulate the air, but it risks moving the air through the heater too quickly before it has time to heat up, and quickly moving air, regardless of temperature, has the effect of cooling the skin and making a room feel drafty.
A solar air heater is basically a glazed, insulated black box with two vents. This simplicity allows a great variety of potential designs. Primary concerns are the design’s efficiency, construction ease, and cost—and there’s some trade-off between these goals. I decided on a simple design that blows air between a black metal absorber plate and polycarbonate glazing. A fan moves the air from the bottom of the collector to the top, transferring heat from the absorber to the air.
Because this design uses a fan, it is an active solar air heater—there are moving parts that require electricity. A passive solar air heater moves air by convection only—as the air inside a passive heater warms, it expands and becomes lighter, moving upward until an unassisted flow called a thermosiphon develops. Active heaters are roughly 200% more efficient than passive heaters, but with additional complexity and cost. Passive heaters tend to become overheated and reradiate much of the heat trapped in the collector back to the outside before it warms the air flowing through into the building.
Besides the difference between active and passive, there are four heater design considerations based on time, skill level, and available materials. The one we built is an empty box collector—air is heated by passing through an empty glazed box facing the sun.