An improvement on the empty box collector design is the screen collector, which uses either a dark metal screen, lathe, soffit vent, or some other metal material with lots of holes, placed between the metal absorber plate and the glazing to cause air disruption. Creating turbulence in the air inside the heater allows more air to come in contact with the absorber plate to increase heat transfer.
One problem with empty box and screen collectors is that dust and dirt accumulating in the heater and on the interior of the glazing can reduce efficiency. Other designs can eliminate the problem by isolating the air from the glazing. A can collector consists of aluminum cans (painted black) with their tops and bottoms removed. Stacked end-to-end, they form channels within the collector through which air passes. Other metal tubing, such as gutter downspouts, can also be used. The dead air between the glazing and the metal channels creates a layer of insulation, reducing heat loss through the glazing and raising the collector’s overall efficiency.
A backpass collector uses the same principle as the can collector, but instead of air traveling through channels, it travels behind the black metal absorber plate. This is common with pre-manufactured solar air heaters. For information on a variety of solar heater designs, see Gary Reysa’s website, www.builditsolar.com.
Using an electric fan adds complexity to the collector design, but, in addition to increasing efficiency, allows the possibility of automation. Automating the fan with a thermostat (essentially a programmable switch) and temperature sensors (called thermistors) make the heater come on only when there is useful heat to be gained. But a simpler method would be to switch on the fan manually. I was interested in maximizing understanding of renewable energy, while at the same time keeping the design most useful and the construction as easy as possible. It made sense to use a small PV module to directly power a DC fan. A PV module placed where it gets the same solar exposure as the heater adds operational elegance: The fan only works when the sun shines on the collector and PV module, and the stronger the sun is, the faster the fan blows air through the heater. A manual DC switch allows users to turn off the heater when it’s not needed.
Using an infrared temperature gun, we were able to determine that the air in the heater was heated by about 60°F at midday (from 60°F to 120°F on the day of the installation). Earlier in the morning and later in the afternoon, the added heat was around 40°F. After two winters, the heater is consistently capable of raising the temperature inside the building 15°F on a sunny winter day.
Some rough calculations of the amount of heat captured, based on a conservative efficiency of 50%, show that the 18-square-foot heater generates about 2,800 Btu per hour when in full sun (solar energy is about 317 Btu/ft.2). In the six hours the fan runs in the middle of winter (from about 9:30 a.m. to 3:30 p.m.), this adds up to 17,000 Btu, comparable to the heat energy of a pound of coal burned every three hours or a gallon of propane burned every five and a half days. Keep in mind that the size of our heater was confined by space and it would not have been much more money or trouble to make a 4- by 8-foot collector and almost double the heat output, assuming the fan and PV module were likewise upgraded.
Payback for this heater will take longer as well, because of its smaller size and the relatively short winters here in North Carolina. Even so, it will save about a gallon of propane each week over our four-month heating season, saving roughly $60 each year and paying back its $600 price (relatively high because we used new materials) in about 10 years. Since it is well-protected from the rain and out of the sun during the summer, we expect at least a 20-year lifespan of producing clean renewable, homemade energy.
The box is constructed using 1/2-inch plywood, and 1 by 4s are attached to the perimeter using wood glue and exterior-grade screws. Weight is a concern, especially for full-sized 4- by 8-foot heaters. This heater is only 3 by 6 feet. Most of the materials came in dimensions that would have worked for a 4-by-8 heater, so the expense for the larger heater would not be much more. Airtight construction is important; wood glue helps greatly.
Before insulating, the perimeter is lined with 1 by 2s to support the sheet metal. Across the back, 1-by-2 crosspieces are placed approximately one-third of the way in from both of the ends to support the baffles. Gluing and then screwing into the crosspieces from the back of the plywood adds strength to the box.
Fiberglass batts are placed in the box. Planning ahead will save you some cutting.
Once the fiberglass insulation is installed, cut a hole for the 4-inch vent pipe. The exact positioning will depend on the room where you are installing the heater. Make sure it can go through the wall between the wall studs.