Just built a solar heater, wow!

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Just built a solar heater, wow!

Postby daveesl77 » Wed Jul 18, 2018 2:34 pm

Ok, so we moved up to the beautiful State of West Virginia and we are loving it! Since we no longer live in the sauna of Florida, I am actually able to go outside and do stuff in the 10 months that would normally be the Florida summer. (And don't anyone call me a reverse carpet bagger, my family lived in Florida since 1807, so I can complain about it) 8)

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
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Re: Just built a solar heater, wow!

Postby daveesl77 » Wed Jul 18, 2018 3:11 pm

Oh, and this isn't for main heat, it is just to experiment with. I'm going to use it to add heat to my unfinished basement to see if it can help with heating the house this winter.

I've been using it as a geothermal cooling system this summer and it is an unqualified success in that. I'll write about that in another post, but let's just say with the outside temp at 90, the "BCS" or "Basement Cooling System" has kept the house at about 74 by simply running the central air fan (not compressor) and pulling the return air from the basement instead of the house.

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Re: Just built a solar heater, wow!

Postby Bigwoods » Wed Jul 18, 2018 7:42 pm

I built one many years ago. 4' X 8' and 5" deep. Glass was an old patio door. I has Styrofoam inside and pop cans cut in half and painted black. Started it on a 45° day. Had a house thermometer inside. Also a cylinder fan moving air.

After 2 hours the thermometer explodes. It had a plastic back and it melted.. It always pumped warm air during the day and we burned wood at night. Unfortunatly a storm took it out.
Greg in Northern Minnesota

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Re: Just built a solar heater, wow!

Postby daveesl77 » Fri Jul 20, 2018 8:21 am

The pop can designs do work very well, just a lot of work. Many use the aluminum downspouts. However, on numerous tests done on "builditsolar.com" they have found that the layer(s) of aluminum or fiberglas screen works as well or better than any other method. The screening cost is very low and installation is a breeze.

You can't use styrofoam, as it will melt, even the xps as the interior insulation. But, using xps as the frame and foil backed PolyIso for the sink does work well.

Doors work extremely well, but they can be heavy, if that is a concern.

The temps in the unit can get really high, like upwards to 200 F if air stagnates. You pretty much have to "burn in " these units for a few days after construction to get rid of all the smells and fumes that the interior will initially generate.

dave
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