Category: Garden

In a nutshell:  In an afternoon, I made a roll-up solar air heater using plastic sheeting, a pile of green mesh vegetable sacks, some tape, and a fan.  At solar 3PM, that’s now putting out a nice stream of air at just under 130F.  That should be adequate to serve as the hot air source for drying food.

See the just-prior post for the theory.  In particular, why a mesh-filled tube is a pretty good choice for a solar collector.

When I’m done with it, I can just roll the whole thing up and store it in a nice, compact package.

Background

I want to make a solar air heater, to use for drying my garden produce.  Mainly, for making dried tomatoes.  Solar-powered, because otherwise, in the humid climate of Virginia, my only reliable option is to use an ungodly amount of electricity to make those dried tomatoes..

Source: Post G22-010.

At this point, I’ve exhausted all of the simplest solar-drying options.

Even in Virginia, if you get perfect drying weather for four days in a row, you can dry tomatoes using old-fashioned open-air drying (Post G23-056).  The problem is, you can rarely count on a stretch of weather like that, around here, when you need it.

I also tried making a simple power-ventilated direct solar food dryer.  (That is, a clear-topped, ventilated box in which sunlight shines directly onto the food to be dried.)  My conclusion is that direct solar dryers just don’t have enough power to dry tomatoes reliably in my humid climate (Post G23-058, Post G23-057).

My aha! moment came when I realized that direct solar food dryers are simple flat-plate solar air heaters.  The food sits on or above that flat plate.  Simple flat-plate solar collectors are the least efficient way to convert sunlight into heat energy.

So here I am.  My late-season tomatoes are (finally!) ripening, so it’s time to get this done.  I’m upping my game by making an indirect solar food dryer.   This is a dedicated solar air heater, hooked up to a box that contains the food to be dried.  That arrangement allows you to increase the power input, both by increasing the efficiency of the solar energy capture, and increasing the ratio of solar energy capture area to area of food to be dried.

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Celebrating the cheap and flimsy design.

Source:  Government of New Zealand.

Funny thing about the word “cheap”.  Once upon a time, it had no negative connotations.  It was used as we might use “inexpensive” today.  Goods were advertised for their exceptional cheapness.  Which, back in (say) Colonial American times, meant low price, not shoddy construction.

My point being that as long as I’m making a cheap and flimsy solar air heater, I might as well celebrate that.  No sense in trying to make a high-quality cheap and flimsy device.  Might as well make it as cheap and flimsy as possible.

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Construction overview.

Solar air heater.

• Make a big plastic tube out of a single sheet of clear plastic.  When flattened, roughly 4′ across by 16’+ feet long.
• Place a 4′ wide piece of black or reflective plastic inside the tube, to form the inside bottom of the collector.
• Stuff the tube, between the black/reflective plastic bottom and the clear plastic top, to a few inches depth, with a loose layer of dark, porous material.  I’m using mesh produce sacks, see below for other suggestions.
• Leave 2′ empty, at either end of the tube, for attaching it to fan and duct.
• “Quilt” the stuffed portion of the tube.  That is,  stitch through it with twine, or otherwise tack top to bottom, just enough that the top of the tube cannot “balloon” when the fan is turned on.  (The point of this is to force the air through the porous filler, not above it.)
• OR, simply weigh down the top of the tube, with pieces of wood, to achieve the same end of keeping the top of the tube sitting firmly on the stuffing.
• Tape the fan to one end, oriented to blow air into the tube.
• Tape a piece of flexible dryer duct to the other end.

In pictures:

Plastic, about 8′ wide by about 20′ long.

Plastic sheet folded in half to make a long 4′ wide tube, then taped (see tape seam at left), with a 4′ wide piece of radiant barrier inside the tube to serve as the bottom.  I’m not even sure that radiant barrier (or equivalent piece of black plastic) is necessary.  FWIW I used Gorilla (r) duct tape, and that seems to be sticking well to the plastic sheet.

A bunch of mesh vegetable sacks drying in the sun, after being hosed off.  Why do I own these?  Long story.  But because I already owned them, I’m using this as my solar collector material, rather than black screening.

The plastic tube, now stuffed with those mesh sacks, balled up.  Try to pack it loosely, but with no voids that would let air flow around the mesh, rather than through the mesh.  You want to force the air to flow through the mesh so that it will pick up heat from the sun-exposed mesh.

Ready to run.  Fan is clipped into the near end, a piece of flex duct is clipped into the far end, and chunks of wood weigh down the top.  You can see that the top still balloons up a bit, between the pieces of wood, from the force of the fan.

The fan is an ancient twin-bladed window fan.  I’m guessing that with no resistance, it moves 250 CFM on low, and draws maybe 30 watts.  With the resistance imposed by passing through the mesh, I have no idea how many CFM it moves.

Same, side view.

Same, end view.

A nice stream of 129F air, at solar 3 PM, on an 80F day, with no adjustments?  That’ll do.

Total assembly time was around two hours.  And I now have a roll-up solar air heater that is adequate for the task of drying tomatoes.

Food dryer (TBD).

I am greatly simplifying my task by using Nesco food dryer trays.  With the addition of a bit of tape, these can be stacked to form an air-tight cylinder, with the food to be dried neatly laid out within that cylinder.  All I need to do is place that cylinder above an appropriately-sized hole in a cardboard box.  Run the flexible duct from the solar air heater into that box, and that will serve as the food dryer unit.

Note that with this design, the cardboard box itself doesn’t get wet.  All the humid air from the food goes up the stack of trays, and out.

 

Materials/tools list for the solar collector:

• Pair of scissors.
• Clear plastic sheet, approximately 8′ x 16′, or longer as desired.
• Black or reflective plastic sheet, 4′ x 12′, or ditto.
• Optional:  Additional clear plastic sheet 4′ x 16′, or ditto.
• Dark, porous material to fill the tube (e.g., plastic screening, see below).
• Tape (packing tape, duct tape, or whatnot).
• Twine and something to use as a needle for “sewing” with that twine,
  • OR, double-stick tape.
  • OR four 8′ 2x4s.
• Window fan or standard 20″ box fan.
  • Cardboard (e.g., 20″x20″ square) for modulating air flow from box fan.
• Small length of flexible dryer duct.

Materials for the food dryer unit

• Cardboard box
• Nesco round drying trays (or substitute what you have).
• Tape.
• Cardboard sheet to cover the top of the stack of trays.
• Instant-read thermometer.
  

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Details

Step 0:  How big?

I’d like this to to produce around 900 watts of heat, on average, over an eight-hour sunny summer day, at 40 degrees north latitude.  Assuming this is 30% efficient at capturing sunlight, then, based on my prior calculations, this should capture an average of 18 watts per square foot.  So I’m shooting for about 50 square feet of collector.

I have no intuition as to the right shape.  I’m guessing that depends on a lot of factors.  The material I’m starting from is almost 20′ wide, so I’m tentatively planning on a tube about 4′ wide and 20′ long.  That’s a bit larger than necessary, but it matches what I have on hand.  Of that 20′, a couple of feet on either end will be used to connect to fan and duct, and so will not contribute much, if anything, to solar energy collection.

I’m guessing that one 8′ x 16′ piece of clear plastic sheet should be adequate to form the tube.  I’ll need a further 4′ x 16′ piece of black or reflective plastic to line the bottom of the tube.  And, optionally, one more piece of clear plastic, 4′ x 16′, to add to the top for “double glazing” of the finished, quilted tube.

Step 1:  Obtain a large amount of dark, porous, lightweight material.

In my case, that’s a box of mesh produce sacks that I’ve had on hand for years.  (These were part of a failed attempt to simplify the handling of my firewood.)

Plastic window screening should work fine, but is an expensive solution if you are using new materials, due to the amount of material required.

Or, you might try doing this with no filler.  Just blow air down a hollow clear-topped tube.  That should make this much less efficient at capturing sunlight.  So make the tube bigger than you would otherwise.

Beyond that, if you use something that isn’t compressible, you lose the ability to roll this up when you are done with it.  If you don’t value that, you could consider:

• Styrofoam packing peanuts, painted black.
• Coarse, dark, shredded wood mulch.
• Possibly, lava rock.

Step 2:  Make a large plastic tube with a clear top and a black or reflective bottom.

This couldn’t be easier.  Get some clear plastic sheeting, e.g., the stuff they sell as dropcloths at the hardware store.  (In my case, I’m using greenhouse plastic, which is more UV-resistant than garden-variety hardware-store plastic sheeting.clear-topped plastic tube.)  Fold it in half, and tape the edges together.

I’m going for a reflective bottom because I own a roll of house-construction radiant barrier material.  The idea is that any light penetrating the layer of loose fill will get reflected back up into that loose fill.  And, where the fill is at least a half-inch away from the radiant barrier, the barrier will act as insulation against radiation heat loss through the back of the tube.

Step 3:  Stuff the tube — but NOT the last 2′ on either end — with a few inches’ thickness of dark, porous material.

For me, this was as simple as temporarily closing off one end of the tube, scrunching up the mesh sacks, chucking them inside, and using a stick to arrange them into a single, packed mass.

This isn’t precision work.  The air is going to flow through all 16′ of the tube.  As long as there’s no continuous channel through which the air can flow from end to end and bypass your porous material, you should be fine.

Step 4:  Keep the top of the tube from ballooning up.

You want air to pass through the porous filling, not above it.  So you want to keep the plastic top sheet right down on top of the filling, in some fashion.

I was originally going to “sew” or tape the top and bottom together in places, to do this.  But on reflection, the easiest thing to do is weight the top down, while obstructing as little light as possible.

I’m just going to toss some 2×4’s onto the top of the sheet, and, if necessary, weigh them down with (e.g.) bricks.

Optionally, add a second layer of “glazing” by tossing another clear plastic sheet on top of this.  That will trap insulating air where the top of the tube is depressed by the quilting or the 2x4s.  I’m guessing this isn’t worth it, but I make try adding it and taking it off to see what happens to the resulting air temperature.

Step 5:  Attach fan and duct.

Attach a window fan or 20″ box fan to one end, and a short length of flexible dryer vent to the other.

I want to be able to take this apart at the end of the season, to store it, so I’m doing these attachments with lengths of bungee cord.  You could just as easily make the attachments with tape, and peel back the tape at the end of the season.

Step 6:  Make and attach air distributor for food drying (TBD).

Because I’m using round Nesco trays, my air distributor will just be a box, somewhat larger than the trays, with holes for the dryer duct and the trays.  Run the dryer duct into the box.  Place a piece of cardboard on top of the uppermost tray, to make sure the hot air hits all the food as evenly as possible.  That’s it.

Step 7:  Operation.

Place the tube on the ground, in the sun.  Place weighs on top of the flat solar-collector tube, to keep the top from ballooning up.  Attach duct, fan, and (eventually) food drying box.  Turn on the fan.

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Summary

It works.  And it’ll roll up at the end of the season.  So that’s a success.

This could use a bit of tweaking.

I probably used way more mesh “stuffing” than I really needed.  I can’t even see the reflective bottom of the solar air heater, through the green mesh bags stuffed inside.

I’m sure this would get hotter if I could tilt it so that it was perpendicular to the sun’s rays.  As one would do with a solar panel.

But … it works well enough as-is.  So I don’t see any need to modify it.  I can unroll it in the sun, attach small fan and duct, and produce a nice stream of hot air as long as the sun shines.  That’s really all I need it to do.

Arguably the most mickey-mouse aspect of this right now is the weights for the top.  Without those, the plastic sheeting simply balloons up, and the air passes over the mesh, not through the mesh.  Tossing some sticks on top makes it work, for now, but I’d like to get a more elegant solution at some point.

I’m now going to roll this up and put it away until my final crop of late-season tomatoes starts ripening in earnest.  Then I’m going to use this for a last batch or two of dried tomatoes.  Weather permitting.

Addendum:  Oh for duh!

Turn the fan around and stick it in the other end of the tube.

After I put this together, I went looking for a better fan.  Window fans of the sort I’m using really shouldn’t be used to push against considerable resistance. 

That’s when I realized that if I sucked air out of the tube, instead of blowing air into the tube, the entire problem of having the surface of the tube balloon up simply goes away.  All the wood and metal pieces on top of the solar air heater are unnecessary.  And I end up with a simpler and more elegant design.  If such a word can be applied to this cheap and flimsy roll-up solar air heater..