2016 AT (appropriate technologies) course solar dehydrator

At the AT course we badgered, well mostly me, Tim Barker  to let us build a solar dehydrator.  Of course Tim being the good mate  he is, he let us build one. In the next couple of posts when I get around to all of them I will have hopefully walk you  through the basic steps we did.

There was some old I think three eights inch double pane glass around the lab. Most of it has lost its seal so we separated the panes  and cut them into the biggest rectangle we could. I have never had much luck cutting glass and I think I jinxed Tim as the first piece we cut failed miserably.   Then Tim came up from somewhere the idea of putting motor oil on the glass before we cut it and snap it.   That works much better but we had to scrounge up another piece of glass.

I made two wooden boxes with the inside dimension just large enough for the glass to fit in. They had pointed me to some crappy wood pile in the back of the lab where I got the wood. The wood was warped  some so it made it more difficult to square up the boxes. We also scrounged up some hinges and hinged the two boxes together.   I then put sheet metal on the bottom of the bottom box where the trays full set. Note the direction the metal ridges is run as  they serve as air channels to draw heated air through the trays. I also put sheet metal on the bottom of the top box which will be part of the solar chimney.

The top solar chimney  has two panes of glass separated and sealed with foam weatherstripping and set even  with the top of the box.  The inside of the tin  was blackened  by using an oxygen deprived acetylene torch.    The theory is the double pane lens heats up the black tin  which radiates that heat into the bottom box where the drying occurs.  And because of the incline and the ridges on the tin air should move up the tin and across what's been dried.

Setting the angle to the sun was interesting, you want a good Sun angle but it can't be so steep that round fruit  would roll down the trays. I think it was Erica who said that when Tim and I were trying to figure out that angle using our arms  it look like we were doing some hand jive safety dance. You know: "you can dance if you want to .... " safety dance.

You can see the trays we made stacked up next to the hydrator. I think somebody was ordering stainless steel screen to put in them.  I don't know if that ever happened though. We dried some banana chips in it and they  did fairly well. Lesson learned: at that time of year leaving stuff in the dehydrator overnight actually rehydrated them somewhat because that glass mass was a  serious dew machine.  Like a solar well you could get almost 1/4 cup of water off it without trying.  But I think it be better just to take  what ever  in at night and set out again the next day.

In order to try to make the warm air move more across the food  I added a solar chimney to the front made out of sheet metal we took off of the water heater tank used to make the shower rocket stove water heater.  It  made a noticeable improvement in air flow.

How does the solar chimney work?   I grok that it's for creating air flow, and I see that it is black, but it's not shaped like a chimney so I'm confused.

It's hard to see because I painted it black with chalkboard paint. But it's actually a duct,  with a front and a back and maybe a 2 inch gap between the two sheets of metal and open at the bottom.  As the sun hits it, it heats up the air between the two plates and that warm air rises.   That rising warm air is channeled into the bottom box and across the drying trays  as it runs up the ridges of the tin.

OK, so sheet metal is attached to both sides of the wooden legs, then painted flat black.  The legs create the 2" gap.  That makes good sense!  I have a solar dehydrator with a similar design, but no chimney. . .

I wonder if I could improve the functionality with a chimney?

Julia,
Adding a chimney to that one made a noticeable improvement in the airflow across the tray box, I think it would help yours too.   Using solar chimneys to heat and move air around was one of Tim's ( and Howard's) big design features to move air around.  Whether it was for a dehydrator like this or for passive solar heating and cooling of structures.

Looks like you copied the design we came up with back in the mid-80's. It's found at http://geopathfinder.com/Solar-Food-Drying.html It's a good idea to put the whole collector, tray, and base assembly at a slight angle to allow passive airflow under the food tray, allowing moisture out.

My photo skills did not show it...but this one is angled up to allow the air to flow up and out over the food trays.