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The complete DIY solar food dryer. A
passive dryer is underneath the solar panel.
Source: All photos by Casey Bahr
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Solar
Food Dryer Designs
A "quick and dirty" way to
make a solar food dryer is to build a screen tray into a box and top the box
with a piece of metal roofing painted black. Tilt the box slightly to increase
airflow and you're done. Another design often seen is a set of shelves for the
food racks with the frame covered by clear panels, such as polycarbonate
roofing, which is resistant to UV damage.
The project detailed here takes an
unconventional (some might say geeky!) approach. Though the first prototype was a failure, a
quick modification created a working food dryer. This dryer is both solar
active and passive. That is, it generates heat passively, but uses solar generated electricity
to circulate the hot air, which improves its efficiency markedly.
It's also a
great way to recycle any old PC cases, which are easy to locate. Also, note
that the food itself is not exposed directly to any damaging sun rays.
Old PC case to be gutted.
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| Original PC case |
Creating
the Dryer Box
This project is one of those using
materials on-hand, which is one of the author's favorite methods for
do-it-yourself inventions. An old desktop PC case holds the drying racks. The
natural heat-collecting property of the black metal case and a fan to blow air
across the racks from the outside was the plan.
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| PC case stripped down |
The first step was to strip out any
unnecessary components from the inside of the case, so that you have an empty
metal box. If there is still a case fan or power supply fan in the
"guts", save that aside. It should be marked as running on 12 volts,
which most PC fans do.
A door was cut on one end into which
the drying racks will slide.Great time to get out your reciprocating saw. Attach the hinges first to the top and mark the
hinge holes on the door panel before cutting
Door
and Rack Supports
This particular PC case is a
clam-shell variety, which gives solid walls on three sides. Using a small
handheld grinder with a cutting disc, a door was cut on one end into which the
racks slide. Before cutting the side door, the hinges were screwed onto the top
panel with self-tapping truss head screws.
Fold the hinges down over the door side, and mark the
panel through the hinge holes. This step will ensure good alignment of the
door. Cut the top of the side panel where the hinges are and then re-attach the
panel via the hinges with the same kind of truss head screws.
Rack supports are made from pieces
of 1" by 2" wood; held in place with screws through the flanges on
the bottom part of the case. If there are a large number of holes in the bottom
or side panels, cover them with thin foam, plastic sheets, or even cardboard.
You don't want it to be airtight, however, since air needs to escape to
keep an even air flow.
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| Air distribution tube, later abandoned |
Now
for the Ugly Part
Improvisation is a valuable virtue
when approaching any DIY project such as this one. In this case, an adapter was
needed from the fan to the smaller hole for the air spreader.
Using three
different sizes of PVC tubing and a heat gun, a solution of sorts was found.
The first piece of tubing, which entered the rack case, was 1-1/4"
electrical conduit, connected to a 2" piece of PVC waste pipe, which in
turn connected to a piece of 3" PVC pipe to match the fan diameter.
This tube from the failed prototype
lay on the bottom of the case. The numerous holes help spread the air from the
fan evenly. An adapter was made for attaching the PC fan to the distribution tube by using a heat gun to melt a 3" PVC scrap tube.
Notches were cut into the large end of this home-brewed adapter. These tabs were heated just enough so that they could be easily bent downwards to form mounting tabs to which the fan was attached with screws.
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| Adapters from the fan end to the distribution pipe |
To create the intermediary adapters, heat the end
of the tubing with a heat gun
and wearing heavy gloves twist the end of the pipe into a smaller diameter. The
smallest tube was inserted through the PC case along one side. It had two rows
of 1/4" holes drilled along its length to spread the air out from the fan.
If at First You Don't Succeed
Unfortunately, as sometimes happens
with DIY projects, the initial design failed.
Heat collector with fan covered with
glass. The envelope insulates a small remote thermometer
An
Improved Design with Heat Collector
The first prototype of the solar
food dryer simply didn't generate enough heat. Though the drying box would get
warm, all the heat was lost as the fan pulled in cool air from the outside.
This deficiency was overcome by creating an external heat collector
using a second PC case.
A tower case, which by chance had a fan already mounted
dead-center on the back panel, was employed. The ugly plastic tubing was
happily discarded.
Screw the heat collector directly to
the drying rack case after cutting a hole (with a jigsaw) in the drying case
the same size as the fan. Conveniently, a tower case is enclosed already on
five sides with a removable panel on the sixth side. This removable panel was
set aside and replaced with a piece of glass slightly larger.
To boost the heat generating
capabilities of collector, it was lined on the bottom and sides with
eighth-inch foam with a foil backing on one side. A piece of corrugated roofing
was screwed into the bottom for additional heat mass and everything painted
flat black.
By Golly, It Works!
Hook-up and Testing
