I've been thinking about installing a heater in my garage some time, as a lot of my hobbies involve being there and I get discouraged when the prospect of working on something in -10 Celcius comes up. In some years I've stopped working on many hobbies in the winter time, but then summer comes along and I want to spend it outdoors doing things like hiking and camping. I ended up putting a couple electric space heaters in there which help quite a bit, but electric heating is expensive. One day I got thinking about solar heating, after watching the snow melt on a black car despite the -15 C air temperature. My first thought was to build a solar water heater, but after doing a bit of reading I decided to go with a solar air heater for simplicity. A good resource I found for solar heating information is
builditsolar.com.
At first I settled on a design made of recycled pop cans similar to
this one. After collecting a large number of cans and starting the build, I discovered
this post describing the performance of a few different solar collector designs, and decided I wanted to build a screen collector. Since I had already started the pop can collector, I decided to go ahead and just build both.
Each collector is built on a 4'x8' piece of 3/8" OSB, with 2x6 sides and a 3/4" thick piece of insulation on the inside. For glazing I decided to go with a cheap 6 mil polyethylene sheet, which probably won't last very long but is easy to replace and a fraction of the price of polycarbonate. The collectors are connected to my garage with a 4" diameter insulated flex hose (which, in hindsight, is much too small). In the garage I have a 12v blower out of a pick up truck to move the air. Currently it is powered with a battery charger, and on medium setting it puts out about 50 CFM.
Being an engineer, I also wanted a way to measure the output and monitor the performance. So, I grabbed a Raspberry Pi that I had left over from another project, hooked it up to a serial ADC chip reading an analog temperature sensor, and had it record the temperature of the outlet every 5 seconds. It's currently running a web server which shows the outlet temperature and fan status (currently at
http://70.72.253.183/ but that may change). The raspberry pi also has a relay driver which I use to turn the fan on and off.
The performance is OK, though not as good as some of the designs on the above mentioned site. I think the biggest issue is that I'm not getting enough airflow with my fan and hose combination. In direct sunlight I've been getting about 1.25 kW. Surprisingly to me, even on a cloudy day I can get a 200-300 W heat out of it, with a solar insolation of about 200-250 W/m^2. The actual collector area is about 4.5 m^2 so in diffuse light it is about 25% efficient, and a bit more in direct light. Total build cost was about $260 for both collectors, including the insulated hoses.
