Category: Environment

Post #1858: Indirect solar food dryer, Part 2: Building a roll-up solar air heater.

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.

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.

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.

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.

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..

Post #1852: The USDA says to #leavetheleaves.

 

No less an authority than the USDA is now on the bandwagon for #leavetheleaves.  That is, the idea that gathering and disposing of fallen autumn leaves is foolish from an environmental standpoint.

The conspiracy-minded among you may view this as just another facet of the Deep State, an evil cabal within the U.S. Civil Service determined to disrupt every facet of the American Way.  Yes, stooping so low as to attack that most harmless of small-town fall rituals … 

… requesting that citizens rake/blow leaves to the curb, so the Town can repeatedly drive its high-decibel fleet of dedicated leaf-vacuuming equipment through town, and so spend hundreds of thousands of dollars to suck up those leaves, then trucking hundreds of tons of leaves down the interstate so that they can be sterilized via hot composting at some remote location, ensuring that no offspring of this year’s crop of butterflies and similar insects survive.

Well, at least, that’s the tradition in my small town.  It’s an industrial-scale process that’s a far cry from Normal Rockwell, if you get my drift.

Source:  Pinterest.

The USDA is just the most recent in a long line of organizations that have gotten behind the idea that leaf collection and disposal of this type is a relic of the past.  Historically, in this area, it’s the immediate successor to the era in which suburbanites routinely raked up and burned fall leaves.  Before that was banned owing to the resulting air pollution.

Locally, even the surrounding county (Fairfax County, VA) has proposed to stop doing vacuum leaf collection (see Post #1821).  In part, because that turned out to be a real hassle for county staff this past year.  But also for all the good reasons outlined on the USDA web page.

But in Vienna, VA, traditions die hard, unless there’s some profit to be made in killing them.  And new learning percolates excruciatingly slowly.  Town-wide, this is mostly about doing our bit to slow the insect apocalypse (reference National Academies of Science).  Not sure that matters to most residents, even though it should, from a survival-of-our-species standpoint.  All said and done, it’s still an open question as to whether we can break ourselves of this 40-year-old tradition.  Just to benefit a bunch of butterflies and such.

My prior screeds on this subject include:

  • Post 1822, on the fuel used in this process.
  • Post 1821, on Fairfax County staff recommending no leaf vacuuming.
  • Post 1612, on the emissions from gas versus electric leaf blowers.
  • Post G22-034, on vacuum leaf collection being a relic of the past.
  • Post 1463, on putting the environment first in the Town’s decision-making.

This, in addition to several posts on the economics of the Town of Vienna’s centralized leaf collection and disposal process.

 

Pictures in this post are mainly from Gencraft.com and Freepik AI

Post #1851: Who put the weather on fast-forward? Tropical storm Ophelia.

 

Tonight and tomorrow, it’s going to rain like crazy around here.  In the DC area, we’re expecting to get about three inches, with some significant probability of tropic storm force winds (sustained winds in excess of 39 MPH).  My brother lives in the Tidewater area, and he’s right on the edge of the region where four to six inches of rain are expected.  Along with maybe a four foot tidal surge.  All from tropical storm Ophelia.

What I find odd about is that the very first mention of this disturbance was late yesterday morning.  That’s based on a check of the archives at the the National Hurricane Center.  The very first mention of Ophelia — the point at which it became a named tropical storm — was the forecast update as of late afternoon today.

Source:  National Hurricane Center.

In short, the interval between this thing becoming a name tropical storm, and the start of the deluge, here in NoVA, is maybe eight hours.  Plus or minus. Even worse, my brother in the Tidewater area got about 6 hours’ warning between the time Ophelia became a named storm, and the time that tropical-storm-force winds are expected to hit his area.

I don’t think the weather used to work this way.  Either that, or my memory has become so clouded that I don’t recall the times this has occurred in the past.

I used to have a sailboat, down in Tidewater Virginia, a couple of hours’ drive from where I live.  I spent many autumns tuned into the hurricane forecast for this area, because a major storm meant that I had to move the boat off its dock and moor it in the middle of the river.  (Because, if you don’t, the storm surge floats your boat up and over the dock, at which point wave action destroys both the boat and the dock to which it is tied.  So it’s not optional.  It was written into the contract allowing me to use the dock.)  This, along with everybody else who kept a boat at the same dock.  It was quite a fire drill every time.

My recollection is that major tropical cyclones were well-anticipated events, and that you’re read about them for days before they made landfall.  I’d have plenty of time to gear up, arrange time off work, and get the boat prepped for the oncoming storm.

I further recall that when a few such storms “popped up” in the Gulf of Mexico two years ago, without crossing the Atlantic first, that was news, meteorologically-speaking.

But maybe the short-cycling of tropical storms is now the new normal.  Plausibly, this is brought about by elevated Atlantic Coast sea surface temperatures.  Warm ocean temperatures in the Gulf of Mexico got all the headlines with the most recent hurricane.  But in fact, ocean water temperatures are a few degrees above historical averages all up and down the U.S. East Coast.  And, apparently, a few degrees is all it takes to whirl up a tropical storm in a couple of days, flat.

And so, this went from literally nothing on the weather map, to landfall of a tropical storm, in just about exactly two days.

And, unlike those pop-up tropical depressions of a couple of years back, that doesn’t even seem to be triggering comment this time around.

It’s just the way the world works now.  Having days of warning for a tropical storm landfall?  That’s so last-century.

Get used to it.

All pictures here are from Gencraft.com AI.  We never really did have a meeting of the minds over what I meant by “hurricane”.

Post G23-058: Solar tomato drying fail.

 

A few days back I set up a batch of tomato slices to dry in my tote-based solar food dehydrator.  Without perfect weather, it was a race between sunlight and mold.

Mold won, as shown above.

At the minimum, this convinces me that I need an indirect solar dryer, as described in the just-prior post.  My little plastic-tote dryer just doesn’t have enough power to dry tomatoes in less-than-perfect weather.

The interior of the tote seemed to get pretty hot, in full sunlight.  As in 130F, loaded with just two small trays of tomato slices.  So I’m not quite sure why this failed so badly.

One possibility is lack of direct ventilation of the tomato slices.  I had a computer fan pulling air through this tote.  While that did in fact exhaust the humid air in the tote, there was nothing blowing on the tomatoes to disrupt the  “boundary layer” of air directly adjacent to each tomato slice.  I would then guess that the air directly adjacent to each slice stayed quite humid, thus encouraging mold growth.

A second possibility is the lack of sterilizing UV radiation inside the tote.  I believe the clear Sterilite tote is made of polyethylene, which is a reasonably good absorber of UV radiation.  UV strongly inhibits mold growth, so the presence of warmth without UV was less than ideal.

Yet a third is the level of cloud cover.  Depending on the day and the hour, the summer sky in Virginia can be quite cloudy.  This power-ventilated box is going to cool off pretty rapidly in any extended period of cloud cover.

My bottom line is that if the weather is good enough to use this tote-based direct solar dehydrator, I’d be better off just sun-drying my tomatoes the traditional way.  Lay them on a screen, cover them with netting, and expose them to the breeze and the sunlight.

 

Illustrations in this post are from Gencraft.com and Freepik AI.  The only real picture is the first one, of blackened tomato slices sitting on drying trays.

Post G23-057: Solar food drying, a better understanding

 

Upshot:  Direct solar food drying — putting your food out in the sun (with or without some clear cover) — is an inherently low-powered and slow way to dry food.

By contrast, indirect solar food drying — connecting a solar heat collector to a box full of food to be dried — can be much, much faster.  That’s because you can increase the power of the device.  Mostly, you can greatly increase the efficiency of the solar collector, relative to direct solar drying.  Secondarily, you can also make it larger, if you choose — there’s no necessary relationship between solar collection area and the area covered by food.

And faster drying means lower taxes!!!  Uh, no, I meant, faster drying means fewer days-in-a-row at the mercy of the weather.

The key, to all this new-found wisdom?  Figuring out that a “box-with-clear-lid” food dryer is, technically speaking, a flat-plate solar collector.  Then realizing that flat-plate is really inefficient, relative to other things I could make.

I need to make an indirect solar food dehydrator.  And it only took me two or three years to figure this out.

In the interest of reducing TL;DR, I’m breaking this into two posts. This post is just the setup.  Next post should be the actual construction and use, if any. Continue reading Post G23-057: Solar food drying, a better understanding

Post G23-053: It’s a beautiful day in the neighborhood.

 

Today feels like a fine autumn day, here in Vienna, VA.  So I thought I’d write something unalloyedly nice.  On the screen porch, feet up, sipping iced tea.

Listening to the katydids sing.  As I type.

Or whatever those damned bugs are.  Maybe the right name is locusts, but locusts get such a bad rap that I’ll cut them some slack and call them something nicer.  You might find that loud chirruping annoying, where you live.  But where I live, the katydids are Nature’s white noise machine.  They mask what would otherwise be unrelenting traffic noise, lawn mowers, leaf blowers, and so on.

I say autumn day, because the air is cool and the humidity is low.  After the heat and humidity of summer, this is a welcome change.  So it feels like fall, even if the autumnal equinox is still almost a month away.

Here are my observations on three nice things that are happening in my garden. Continue reading Post G23-053: It’s a beautiful day in the neighborhood.

Post G23-052: I dried my underwear in my food dehydrator.

 

I know what you’re thinking.  You’re thinking “Hey, dummy, that’s what the microwave is for.”

And while I acknowledge the truthiness of that statement, my excuse is that I baked my briefs in this manner for Science.

This post focuses on a seemingly simple question:  Why is an electric food dehydrator such an incredibly energy-intensive way to preserve food?

The answer is unsatisfying.  In roughly equal parts:

  • Evaporating water is energy-intensive
  • Evaporation water out of food is even more so.
  • My particular dehydrator is somewhat inefficient.

My bottom line is that drying a pound of wet produce, in my electric dryer, under optimal conditions, takes about 2 kilowatt-hours of electricity.  And there ain’t much I can do about that, other than coming up with a solar dryer that will function in my climate.  My first attempt at that turned out mediocre (Post G22-015).

Continue reading Post G23-052: I dried my underwear in my food dehydrator.