Anyway, something I've wanted to do is to experiment with solar thermal system and geothermal. I've done both now. I wanted to build a "solar furnace" or heater for the house, just to see if it actually would work. While this is still the middle of summer, the test results on the unit I built are nothing short of amazing. Essentially the heater is nothing more than a wooden box, mine is 7ft x 3ft x 5 inches deep, made from a single sheet of 5/8" CDX. Inside the box is a single sheet of polyiso, foil backed rigid insulation, covering all wood areas on the inside. All joints are sealed with high temperature silicone and then painted with high temp flat black paint. Here comes the trick and what makes this type of unit so effective and efficient, the interior has a double layer of black fiberglas (or aluminum) screening, going from bottom to top, at an angle.
Cold air enters the box at the bottom. It travels up through the box and must pass through both layers of screen on the journey to the top where the output vent is located. It is picking up heat from the screens. The face of the box is sealed with very thin, corrugated SunTuf clear plastic roofing. The creates the enclosed heat chamber. Trick to building one of these is to make sure the box is sealed, you want to keep as much air contained as possible.
I'm using a 200 CFM, 6" duct fan that runs from a 50 watt solar panel I have and an old 400 watt inverted I have laying around. Actuation is controlled by a simple, adjustable snap switch placed near the ouput side of the collector. At 110F the switch turns on the fan, at 90 it shuts it down. The actual output of the fan, after going through the collector is about 150 CFM.
Anyway, finished the system today and fired it up for a test. Yes, it is summer, but I'm not concerned about the ambient temperature, I'm concerned with the differential between the input temperature and the output temp from the collector. It was a clear day. Collector was put at a 30 degree angle to the sun. Input temp to the fan was 80F. Output temp, @ 150 CFM, was 133f. That is over a 50 degree differential. Using the standard calculation for BTUs, the collector is generating, at full sun, 7,500 BTU per hour. During the winter months, historically, this area gets about 6 hours of clear sun, 10 days per month. The collector may well generate about 40,000 BTU per day and 400,000 per month. I'll track it during the winter to see what it really does. Total build cost was $119. I had the fan, the solar panel, and inverter. It doesn't absolutely need a fan system, as natural convection puts out a fair amount of heated air, but it is really, really hot air without the boost of the fan.
So what does this have to do with small campers? I realized during the build that you don't have to use plywood for the base, you could build it from rigid foam with poly iso foam inside for the heat sink. It definitely doesn't need to be as big as mine, a small 2'x3' unit can do a heck of a lot in a small space, like a trailer. Yes, it only works when the sun shines, but boy does it work.
I'll put some photos up when I get it installed in the house. It took me about 2 days to build it, but in reality it can be built in about 4-5 hours pretty easily.
dave
