I read about this design when I was prob. 14 in boys life magazine. so now with a sawmill and materials, I decided I would make one, and made it a project for a scout meeting. used ERC milled at camp Alaska last year. I let some VIPs hold the lever forward, as I controlled the speed so they got to feel like they were running the sawmill. One of these was the 84 y/o mother of our past scoutmaster. so now the boys know they were makin this with wood milled by Mr Fees Mom. She still hold swimming records at KU. She has been active with scouts for most of her adult life.
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side boards
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outer frame
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bottom plywood, with polycarbonate laying on top
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meatal to divide the column in half, top and bottom
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innards, and bracing to hold the middle metal divider. the far end is the cold air intake (back) and the hot air will go out the far end (top), that will be elevated at a 45° angle. air movement is by hot air rising in the front and pulling cold air in the back.
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here it is with the metal divider/collector installed, and beginning to paint flat black. there is a gap at the bottom (near side). so when set up, cold air comes in the back near the top (far end). goes down the back column and makes a U turn and up the front and out the top.
Doc, you could increase thermal efficiency by adding more heat absorbing mass to the hot side. Black painted of course to also absorb sun rays.
Gary. this will be used as a demo at winter campouts. I plan to put casters on it and handles so one or two people can set it up, and load and unload. we do trappers in January, and what we call camp Alaska in February. it will have legs on the sides so it can stand up at 45°. we also want to heat up quickly. increased mass would make it heavier, and take longer to heat up, although the heat would be more even. thanks for the idea.
I was thinking of thermal mass rather than weight. Something like copper tubing.
One of the limiting factors in that design is the incoming cold air is in contact with your metal divider plate at the top which will limit the out going air plate temp on the hot side exit, also at the top. One way to help would be to insulate the bottom of the metal plate on the upper half or to install some half length pieces of copper tubing on the upper half of the hot side, not directly in contact with the metal plate.
Or you could also challenge your scouts to come up with ideas to improve the design. Good project for your next meeting.
the wood that makes the back is insulated. I see your point, and the idea is that of a counter currant style the begins to warm the air on the backside. we may look at a 2 D parabolic reflector 4 feet long to heat water running through a copper tube at the focal point. most things that increase thermal mass are heavy like water and concrete. it flattens the highs and lows. for our purpose we like that the heat will spike soon after sunlight is passing onto the collector.
the original plans did have a bunch of pop cans on the back.
Yes in a solar kiln, things like water, rocks and concrete add to the thermal storage but that is a closed loop system where heat storage capacity is essential and weight is not an issue. What you have is a portable continuous flow system where heat transfer efficiency along with thermal storage is the key. Materials like copper and aluminum will add heat transfer capacity and thermal capacity without adding too much weight. Kind of like fins on a heat exchanger.
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SO? How does it work?
eghh. as my son would say. need to seal up the edges. not much sun by the time we sat it up. i was afraid it might burn my hand :D a few more things to tidy up as they say. I will try to be objective and take some measurements. the metal was hot. not much passive air movement. but in theory need a sealed unit, and the sides had foam along the edge but no pressure down on it.
There's really no passive air movement in those, they stall because the heat really wants to rise on both sides of the plate. A small fan pushing cold into the back will get it moving.
We tried something like that when I worked for the low income 35 years ago. Seem like we had a hole in the top and bottom. The "cold" air would come in the bottom and than "hot" air out at the top. Seem like we would do it in a mobile home. We would cut a hole into the side and put that unit in.
I have 7 of the old solar air heating panels that were on roofs near here heating my solar wood drier. They have aluminum skin with insulation, some have screen painted black for the absorber, and 6 had glass, one broke and it has a poly carbonate twinwall panel, and one other had a single wall fiberglass panel, which was so cloudy I replaced it with a twinwall panel. The things work very well especially in August and September. I have the air powered with a furnace blower running on low, with a furnace thermostat inside, which turns the blower on when it gets hot enough.
I had a commercial unit very similar to what you and the boy scouts made. It had a small solar panel on one end that ran a small blower. The brighter the sun the faster the fan. On a sunny day it used to fully heat an 18x22' room with 4 slider windows (3 were 36"H x 60" w) the 4th was 3x3' and it had a 6' atrium door). It heated so well in good sun that I mounted a small doorway fan to move some of the heat into the kitchen. On cloudy days I depended on a small gas fired free standing fireplace with a 4" exhaust vent out the roof.
I did order a small solar powered fan to make the heat more obvious.
I had a commercial unit very similar to what you and the boy scouts made in my previous house. It had a small solar panel on one end that ran a small blower. The brighter the sun the faster the fan. On a sunny day it used to fully heat an 18x22' room with 4 slider windows (3 were 36"H x 60" w) the 4th was 3x3' and it had a 6' atrium door). It heated so well in good sun that I mounted a small doorway fan to move some of the heat into the kitchen. On cloudy days I depended on a small gas fired free standing fireplace with a 4" exhaust vent out the roof. The unit was mounted on the south facing wall and there were 2 vents, on one end was a 4" vent from the room, to the heater, on the other end was a 4" duct the blew the warm air into the room. It was controlled by a thermostat, and I'm guessing, but it was about 36-40" wide and 6-7' long counting the solar panel. It was mounted horizontally under a window because there was not enough wall space to fit vertically.