Help with dehydrator

So I built this dehydrator and fired it up for some tests. It doesnt have a bottom yet, it just sits on a board I lined with foil, and just sits on the grass. I put some black objects in the solar collector area, but I just can't seem to get it hot enough inside. I was thinking that the solar collector has too much head space, but Im just not sure since this is my first. The glazing is plexiglass, but I dont think that should matter. It was low to mid 80's today but the hottest it got inside the riser portion was 94F. Any ideas?

Maybe a taller chimney to provide more 'suck' factor.  I've never used one, but I spent far too long watching Paul's new video yesterday, and then a load of fluid dynamic videos, so my head is full images of air moving through things. 

I think the key is to get the air moving as much as to get it hot, and it seems to me that the easiest adaptation to try is extending the chimney.

http://youtu.be/oVTcnCuX2Qc

Not enough energy gain.

Solutions:
-Paint the entire thing with flat black paint.
-raise it off the ground
-insulate the device
-cap the vent until temperatures rise to the desired level
-add ducting, a dryer duct will work, paint it black, cut a hole near the bottom of the box to let the warmed air in.
-add mirrors to increase gain
-rebuild with more glass/plexiglass

Burra Maluca wrote:
Maybe a taller chimney to provide more 'suck' factor.  I've never used one, but I spent far too long watching Paul's new video yesterday, and then a load of fluid dynamic videos, so my head is full images of air moving through things. 

I think the key is to get the air moving as much as to get it hot, and it seems to me that the easiest adaptation to try is extending the chimney.

http://youtu.be/oVTcnCuX2Qc

Yeah his video sure was timely for me; I will see if the taller riser will work.

Ken Peavey wrote:
Not enough energy gain.

Solutions:
-Paint the entire thing with flat black paint.
-raise it off the ground
-insulate the device
-cap the vent until temperatures rise to the desired level
-add ducting, a dryer duct will work, paint it black, cut a hole near the bottom of the box to let the warmed air in.
-add mirrors to increase gain
-rebuild with more glass/plexiglass

Do you have any ideas on what to paint it with? Ive gone to great lengths not to use treated wood or anything else that would have chemicals in it for off gassing. Its whitewood, plexi(should have used glass), screws and weatherstripping.

Are you saying that I should add ducting to the exterior of the box and piple that air into the collector? Again I would have a paint issue. I like the mirror option, but I wasn't sure what you meant in your last point. Thanks everyone for your help.

i would have made the solar gain part at least 2x as big as it is.

I spraypainted the bottom part all black and by noon it was up to 102F so a big improvement. Ill see how much hotter it gets. I basically need a safe paint at this point.

The solar window is too small! See

http://www.builditsolar.com/Projects/Cooking/msfooddry.htm

I've built a couple of these things, used flat exterior latex on each unit.  There is a light odor when the things are first exposed, but I have worked around paint a great deal, it does not bother me in the least.
The outside should be painted at the very least, to protect the unit from the weather as well as help warm the box.  You don't have to paint the inside.

The 2 units were built with glass from sliding glass doors, 34"x72", double pane.  In north Florida, the thing regularly reached 175 degrees.  Got so hot one day the urethane softened, allowing the top pane to slide off, then came the weedeater incident, but that's a story for another day.  In NY 130 was typical.

130 degrees is the magic number.  Maintaining that temperature for a half hour will pasteurize the food.  If the vent is open at all times, getting the box up to temperature will take a hot summer day.  Having the vent closed allows the box to warm up.  Once warmed, the duct can be opened for air flow to carry away humidity.  For your design, a towel stuffed in the vent will do the job.

A vent with a variable vent opening may be something to consider.  Cooler/less sunny days, open the vent less.  On those hot summer days when the thing is really cranking, you can open the duct full bore.

The angle of the glass is important.  Directly facing the sun gets the best energy gain.  I built mine so the angle has the best gain in the fall, when more goods are ready to be processed. 

You may consider adding an absorber to the inside.  Black window screen is available at hardware stores.  The idea is to add a few layers in a manner such that the sun enters the box, hits the black screen.  The screen absorbs the sunlight.  As air moves through the screen, it heats up.  Aluminum foil on the bottom of the collector reflects whatever sunlight made it through, giving it another chance to be absorbed by the screen.  The need is gone for paint on the inside of the collector.  Give the screen a cleaning and rinse to remove factory odors.  This is referred to as Absorption Pass Through design.  I read a study by Appalachian State College that tested different designs.  4 layers of screen offered the best energy gain.

There are others substitutes for paint on the inside.  Black construction paper would do the job.

Let me try to explain the ducting on the outside:
For the air to flow through the dehydrator, there is an opening at the top to let out the humid air.  There would also be an opening at the bottom to let air in.  It would be possible to connect ducting to this inlet.  Standard ducting for a clothes dryer would probably fit, is readily available, and fairly simple to work with.  With the dryer ducting laid out on the ground and connected to the inlet, the ducting would also be exposed to the sun.  Painting the exterior of the ducting black would increase gain.  The air going into the box would be preheated, giving you higher temps.  If you have seen the aluminum can heaters, this is the same idea.  You can add ducting indefinitely, saving you from scrapping the job and starting over.

Thanks Ken, thats very helpful

I just took a look at the link provided by mekennedy, it is the design upon which I based my construction.

When vents are open, air flows through, it is a dryer.
When vents are closed, air does not flow through, it is an oven.

PAINT

It's expensive, but it is the best, IMHO.

Benjamin Moore's new paint, I don't recall the name.  YOu can drink it.  I don't know how it will handle the heat.  Have to ask them.

It is a far superior product, in durability, ease of application.  Way better than the 100% acrylic alternatives.

Probably don't have spray version though.

Are you talking about the Aurora low VOC? We used that on our walls, im not sure you can drink that though lol.

Im thinking this is what I need....

http://www.realmilkpaint.com/products.html

thoughts? experiences?

Never used it, sounds like whitewash or something similar.  Made with milk, lime, and pigments.  Sounds safe enough, no fumes.  Will it hold up to the heat?  Being a water based coating, will being left in the rain affect performance?  Would humidity from the food drying be an issue?  What about bugs? 

With the lime content, I'm thinking it might be best if you did not drink the stuff.
I suspect it would dry out your hands for a couple of days.

...read up a little on it, its also known as Casein Paint.  This I've heard of, seen the results of its use, was impressed.  Try adding borax if you intend to use it outdoors, it will help dissolve the casein and serve as a preservative.

Its cheap enough to try out, I dont see much wear happening on the inside...the outside could be an issue. Dont worry I dont plan on drinking it!