Build a Solar Dehydrator

Build a Solar Dehydrator
Dennis Scanlin
Introduction
The solar dehydrator described in this article is easy to build with mostly locally
available tools and materials and fun to use. It is constructed from ¼” and ¾”
exterior grade plywood, fiberglass reinforced polyester glazing, metal screening,
¾” thick pine boards and a few other miscellaneous parts and pieces. The
materials to construct one cost about $300 and it can be constructed with only a
circular saw with a rip guide, an electric drill and several other commonly used
hand tools. The construction should take between 20 & 40 hours. The dehydrator
has approximately 10.5 SF of glazed area and 4 CF of drying volume and can hold
7-10 pounds of thinly sliced food, which is approximately equal to 35 – 40
medium sized apples. It can dry this much food in two good sunny days or about
½ this much food in 1 sunny day and can reach temperatures of over 140 degrees
F or approximately 60 – 70 degrees F above the ambient air temperature with the
vents mostly closed and down to within 10 or 20 degrees of the ambient with the
vents completely open. This provides a range of operating temperatures suitable
for drying anything one would want to dry.
Principle of Operation
The dehydrator contains both a drying chamber where the food is placed
on screened racks and a solar air heater below the drying chamber. They are
combined into one unit. The sun’s radiation enters the collector through the solar
glazing and is absorbed by the dark metal screen located behind the glazing. Air
around the screen picks up the absorbed solar energy by convection, warms up
and rises into the drying chamber where it picks up moisture from the food and
then exits the dehydrator out the vents at the top of the dehydrator. The rising
warm air creates a negative pressure at the bottom of the dehydrator which
draws in outside air to replace the air rising into the drying chamber. As long as
the sun is shining or another source is providing heat to the dehydrator this
process will continue. (figure 1)
Research Completed
Over the last 15 years we have built quite a few of these dryers and have
conducted a variety of “side by side” experiments to try and figure out what
works best. We have also tried to incorporate the research others have done on
solar air heater design and solar food drying into our work. There are quite a few
variables. Some of the things we have learned include the following:
1)
When comparing the indirect dryer design described in this article to a
single glazed Brace direct dryer and Poisson Drum dryer, this dryer
removed more moisture per day from drying food. It can remove
approximately 4 pounds of water per day.
2)
Food drying effectiveness is affected by temperature, air flow, humidity and
thickness or density of the food one is trying to dry. One needs to find a
balance between these variables to optimize performance. In general
higher temperatures, up to 180 F when food begins to cook, will enable the
air to absorb more moisture from the food and speed up the drying
process. Strategies for increasing temperatures and drying effectiveness
include; using the dryer on hot sunny days, using a reflector, decreasing
area of vent opening, increasing the area of glazing, use of more efficient
mesh type absorbers, using a reflective surface behind a mesh absorber,
and employing efficiency measures such as double glazing and insulation.
3)
Effective reflector strategies include painting the wall of the drying
chamber a light color or gluing a reflective surface (tin foil) to it and/or
adding an adjustable reflector to the bottom of the solar air heater. These
measures can increase temperatures inside the dehydrator by over 20
degrees F and drying effectiveness.
4)
Adding a second layer of glazing with a ¾” air space between the two layers
can increase temperatures inside the dryer by approximately 10 F.
5)
We have found the most effective absorber strategy to be a diagonally
positioned dark colored mesh type absorber. We have found that 6 layers
produces higher temperatures than fewer layers and that steel lathe
painted black produces slightly higher temperatures than charcoal
aluminum window screening, although it costs more ($65 vs $28) and
requires more labor. We have also found that a reflective surface behind or
below the absorber screen will provide higher temperatures and better
drying than a black surface below the screen.
6)
Adding 3 – 5 100 watt light bulbs right underneath the lowest drying shelf
will enable the dehydrator to continue working at night or on cloudy and/or
colder days to finish drying a load of food.
7)
When we increased air flow through the dehydrator by adding a fan (PV
powered small DC computer fan) we found that the temperatures inside
the dehydrator and the food drying effectiveness decreased. But we have
also found that decreasing the total exhaust vent opening below about 15
square inches (3” opening on one set of vent covers) will increase
temperatures in the dehydrator, but actually diminish drying effectiveness.
The dehydrator as depicted in this article will work best when the minimum
total exhaust vent area is approximately 15 square inches.
8)
Food is effectively dried when the dry weight is between 10 & 20% of the
food’s wet weight.
Tools Required
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Circular Saw with rip guide
Electric drill
Pilot hole & countersinking bit for #8 screws
2 saw horses
Long straight edge
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Marking pen/pencil
Protractor
Framing square
Tape measure
Staple gun
Caulk gun
Paint brush
Wrenches
Tin snips
Several clamps
Router with ¾ straight cutting bit and rip guide (optional)
Materials
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1 - 4’ x 8’ x 3/4” exterior Plywood
1 - 4’ x 8’ x ¼” exterior plywood
5 – ¾” x 5 ½” x 8’
Quart of paint
Water proof glue
2 tubes silicone caulk
1 ¼” exterior phillips head flat head screws (200)
1 5/8” exterior phillips head screws (30)
¼” staples for staple gun
2 – 8” diameter wheels
2 – 2x4x8’ Pressure treated
36” long x ½” diameter steel axle
8 – 3/8” x 3” bolts, nuts and washers for attaching legs
4 – 3/8” x 4” bolts, nuts and washers for attaching handles
2 - Heavy duty hinges
2 or 3 cans of High temperature black spray paint if using lathe
6 – 2’ x 6’ lengths of charcoal aluminum window screening or lathe
 2’ x 6’ piece of plastic glazing (.040 Sun-lite HP from Solar Components Inc,
New Hampshire)
 30 SF of screening material (MacManiman Inc, Fall City, WA 98024)
 ¾” wide x 1/8”x 16’ thick aluminum battens
Figure 1: Section Drawing of Solar Dehydrator
Laying out the ¾” Plywood Pieces
Most of the major pieces can be cut from a single 4’ x 8’ sheet of ¾” thick
exterior grade plywood. A layout is included (figure 2) for the dryer depicted in
this article, which is designed for drying food in North Carolina at 36 degree
latitude. This design will work well anywhere, but if you want to customize the
layout for your latitude just measure in along one of the long sides of the plywood
sheet about 1’ from the edge and measure an angle up from that mark equal to
your latitude plus 80 degrees (see table 1 for suggested angles for other
latitudes). This will maximize performance between July 21 and September 21st
and March 21st to May 21st. Draw a line from the mark approximately 1 foot in
from the edge and through the angle measured and up to the other edge of the
plywood. Measure the length of this line. It should be 30” long. If it is not move
the line to the right or left of the original line and parallel to it until you get a 30”
long line at your desired angle.
Latitude
20
30
40
50
60
Suggested Collector
Angle for late summer
early fall drying
10
20
30
40
50
Angle of drying chamber
from long plywood edge
100
110
120
130
140
Table 1: Suggested collector angles for various latitude
Figure 2: Plywood layout of 4’ x 8’ x 3/4" plywood dehydrator pieces for 36 degrees latitude
Cutting Out the Side Pieces
I placed the plywood on two saw horses and laid out the two side pieces for
cutting using a tape measure, straight edge, marker and the information in figure
2. I then proceeded to cut them out with a skill saw. I tried to carefully cut
straight lines so the pieces will fit together tightly. You want the unit to be as
airtight as possible except where the air enters the dryer at the bottom and
where the air and moisture exit at the top. When cutting out the side of the
drying chamber above the collector and the roof area above this side, I try to
carefully make a “plunge” cut, by placing the front edge of the saw’s base on the
plywood. I align the blade with the line and then after turning the saw on I
carefully hinge the saw down into the plywood and along the line. Then proceed
to finish the cut. I try to avoid cutting into the parts of the plywood I am not
cutting out so I can use this portion of the plywood for the other components.
After cutting out the two pieces lay one on top of the other and check to see if
they are the same size and shape. If not, mark the areas that are different and
trim the larger piece with a circular or jig saw so that both pieces are the same
size.
Figure 3: 3/4" exterior grade plywood on saw horses and ready for layout and cutting
Painting the Side Pieces
After cutting out the two side pieces I like to prime and paint them. I
typically put on 1 coat of primer and one top coat on both sides. It is a good idea
to prepaint all the major pieces before installing them. These include the front of
the dryer, door, and vent covers.
Figure 4: Painting the side pieces
Putting the Two Sides Together
Once the paint dries place the two pieces upside down and side by side on
the saw horses and space them about 24” apart. Then cut some ¾” thick pine
boards into strips using a rip guide on a circular saw or a table saw if you have one
available. Most of these strips serve more than 1 function so it is important to get
them located at the right locations and to use the proper size struts at each
location. See figure 6 for proper strut sizing and placement. I use ¾” x ¾” strips,
1 ½” x ¾” strips and one 5 ½” x ¾” strip. They all need to be cut 22 ½” long. You
will need 6 pieces ¾” x ¾” x 22 ½”; 4 pieces ¾” thick x 1 ½” wide and 22 ½” long;
and 1 piece ¾” thick x 5 ½” wide by 22 ½” long. This wider piece needs to have a
bevel cut down one side so that it lines up along the bottom of the collector and
along the door side of the dryer as well (116 degrees in the example in this
article). I made this cut with a circular saw adjusted to the correct angle and with
a rip guide installed to make a straight cut. Begin by installing the two bottom
struts between the two plywood sides. These are the ¾” x 5 ½” piece with the
bevel cut down one side which is installed at the end of the dryer where the
collector and drying chamber come together and a ¾” x ¾” piece installed at the
lowest portion of the dryer where the air enters. I predrilled the holes with a #8
countersinking bit and fastened the struts in place with #8 x 1 5/8” phillip’s head
exterior screws. (see figures 5 & 6).
Figure 5: First 3 struts installed
Figure 6: Strut sizes and locations
Fastening the Collector Bottom to the Dehydrator
Cut a piece of 1/4 “ thick exterior grade plywood to a width of 24”and
whatever the length of the bottom is. It is approximately 6’ – 11” long on the
dehydrator depicted in this article, but it will vary if you changed the angle of the
collector. With the dehydrator still upside down on the saw horses place a healthy
strip of waterproof wood glue down both edges of the ¾ plywood side pieces and
on the 5 ½” wide strut and the ¾” strut just installed. Then lay the 1/4” plywood
piece on top and screw in place using 1 1/4” exterior screws every foot or so. I
used shorter 1” screws at the locations where the plywood was on top of the
struts so the screws would not poke through the pine struts. It is important to get
this plywood piece totally sealed around the complete perimeter so no air can
leak in around the edges and dilute the warmed air that has passed through the
absorber mesh. (see figure 7)
Figure 7: 1/4" plywood bottom installed with glue and screws
Installing the rest of the struts
After gluing and screwing the bottom onto the dryer turn it over on top of
the saw horses and begin to install the rest of the struts as depicted in figure 6. I
try to carefully predrill pilot holes before installing the screws. I put two screws
into each side of the 1 ½” wide struts. (see figures 8, 9 & 10)
Figure 8: Drills with pilot bit and phillips screw bit used to install struts and bottom
Figure 9: Struts installed on top side of dryer
Figure 10: Strut installation
Installing Front of Drying Chamber and Screen over Vents Areas
The front of the food dryer is made of a piece of ¾” exterior grade plywood. I
used some of the material left over after cutting out the two side pieces. The
approximate layout has been shown previously in Figure 2. The size of this piece
is 22 ½” long by 24” wide. One of the 24” sides should be cut to a bevel so that it
fits tightly against the strut at the top of the collector. This piece should be
primed and painted before installing to reduce moisture migration into the wood
which could possibly cause the wood to warp and create an air leak. I didn’t do
that in the dehydrator illustrated here, but wish I would have. After the paint
dries glue and screw this piece to the dehydrator and then run bead of silicone
caulk around the perimeter of the piece from the inside of the dryer. You don’t
want any air leaks around this piece. This would dilute the warm air and diminish
the dehydrators performance. (see figure 11 & 12).
Figure 11: Dehydrator front piece installed with glue and screws
Figure 12: Caulk around perimeter of front piece on inside of dehydrator
Next I stapled aluminum screening over the one intake vent at the bottom of the
collector and on the inside of the two exhaust vents on either side of the top of
the dehydrator. I used ¼” staples and a staple gun for this. For the intake screen
at the bottom of the solar air heater I put a ¾” x ¾” frame around the perimeter
to finish off that section of the dehydrator. (see figure 13)
Figure 13: Frame around intake screened area
Installing the shelf supports
This design can support 10 – 12 ¾” thick drying shelves. The dehydrator
depicted here has 11 supports for holding 11 shelves. These shelf supports are
constructed of ¾” x ¾” pine strips which are screwed to each side of the
dehydrator’s drying chamber with a 1” spacing between the shelf supports. The
extra space between the shelves is nice so that they are easy to put in and take
out without getting stuck. The pine strips were cut from a 1x6 with a circular using
a rip guide. Each support is 16” long except for the lowest support which is 15 ¼”.
The holes are predrilled and the supports are fastened with 1 ¼” exterior grade
phillips head screws. Make sure you don’t put the screws in too deep because
the screws can stick out through the plywood sides, which is probably not
desirable.
Figure 14: Installing the drying shelf supports
Figure 15: Dryer shelf supports and screens installed
Roof
The roof is made of 2 pieces of ¾” plywood. There was not enough ¾”
plywood left from the original sheet for these pieces, so I used some scrap I had
around. You could use some of the leftover ¼” plywood and glue several pieces
together. The pieces are 12” wide x 30” long and have a 30 degree bevel cut
along one of the long edges so the pieces with join at the peak of the roof. These
two pieces are attached to the dehydrator sides and struts with 1 ¼” screws.
Figure 16: Roof Pieces attached
Legs, Wheels and Handles
Building the unit on sawhorses works very well, but eventually you need to
get it off and standing on its’ own legs. There are two front and two rear legs and
they are made from pressure treated 2 x 4s and are attached to the dehydrator
with 2 - 3/8” x 3” bolts and nuts and washers. I put the front legs on first and
typically add wheels so the dehydrator can be more easily moved around. The
front legs are 18” long with a 26 degree angle cut on one end. They are located
approximately 6” from the bottom front edge of the dehydrator. Approximately
2” from the bottom of the legs, I drilled a ½” hole through the center of the face
of both legs for a ½ steel axle that I mounted 2 – 8” diameter wheels on. Make
sure you keep the drill straight when drilling these holes or the axle won’t go
through both of them (see illustration 17).
Figure 17: Bottom legs and wheels
With the front legs attached you can lift up the dryer until the shelf supports are
level and then measure the length of the two rear legs. Place a small level on the
top drying shelf support while you are lifting up the dryer to determine when the
dryer is level. The legs are 76 ½” long in the dryer depicted here. The two rear legs
are made of pressure treated 2 x 4s attached to the dryer with 2 – 3/8” x 3” bolts
and nuts and washers. Make sure to put the nuts and bolts above and below the
drying area to avoid interfering with the operation of the drying shelves. Two 30
degree angles need to be cut from the center of the 2 x 4 out to either edge so
the legs fit right up to the roof of the dryer.
Figure 18: Rear Legs Attached
Installing the vent covers
There are two vent covers above the collector side of the dehydrator and
two vent covers above the door. These are made of ¾” thick plywood. The two
vent covers above the collector side of the unit are approximately 5 ½” wide by
12” long. There is a 30 degree bevel cut along the top long edge of the cover to fit
up against the roof, which helps holds the cover in place along its top edge. The
cover is held in place along the bottom edge by a strip of wood screwed to the
front of the dryer. The strip is 2” wide by 24” long. Depending on how easily the
vent covers slide you may need to add a thin shim between this strip and the
front of the dryer to provide a little more sliding room. I used a piece of bicycle
tire inner tube for this. It takes a little playing around to get the vents in tight but
still able to slide easily.
Figure 19: 3/4" vent covers
Figure 20: 2" x 24" wood strip to hold vent covers in place
Figure 21: Vent cover support with rubber shim
Figure 22: Finished vent covers installed with handles
The vent cover above the door to the dryer are also made of 3/4 “ plywood. The
strut at this location is 1 ½” wide. Half of this strut supports the top edge of the
door to the dehydrator and the top half of the strut supports the vent cover
system. First attach a ¾” x ¾” x 24” strip of wood to the top half of this strut. (see
figure 23) And then screw a 1 ½” wide x ¾” thick x 24” long piece of wood to the
top of the first piece. Predrill pilot holes to avoid cracking the wood. Make sure to
stagger the screws so you don’t run into screws that attached the first strip to the
dehydrator.
Figure 23: Vent support system
Figure 24: Vent covers installed above door
Figure 25: Vent covers above door
Installing the Door
The door is made of 3/4” plywood. It is 24 ½” high and 25 ½” wide. The
door is painted on both sides to try and protect it from the moisture both inside
the dehydrator and on the outside as well. The door is hinged to the bottom of
the dryer and hinges down. A string (1/4 braided nylon) supports the door
horizontally so it can be used as a shelf to assist with loading or unloading the
dehydrator. I used 2 heavy hinges and fastened them to the dryer with nuts and
bolts so they would stay in place. The door is 1 ½” wider than the dryer so it
sticks out past the sides of the dryer ¾” on each side. This enables the secure use
of hook and eye fasteners. Both the hook and eye can be secured into the face
grain of the plywood. 4 hook and eyes are used; 2 on each side. I use 2 on each
side to hopefully prevent the door from warping and letting cold air leak into the
dryer. They need to be carefully installed to get a tight fit, but not too tight that
you can’t fasten the hook into the eye. There is also a piece of weather stripping
around the perimeter of the door frame to seal this door as well as possible.
Figure 26: Door of Dehydrator
Figure 27: One of 2 door hinges
Building the Collector
The collector is built by installing 6 layers of black screening material in
between the two plywood sides of the dryer and then covering the screen with a
glazing material. The screening absorbs the solar energy and transmits the solar
energy in the form of heat to the air around it. Either metal lathe used for plaster
work or charcoal aluminum window screen can be used for the absorber screen.
The screen is easy to work with, readily available, doesn’t have to be painted and
is about ½ the price of the lathe, but in our tests the lathe often produced higher
temperatures so for this dryer I used lathe. The lathe came in 8’ lengths that
were 27” wide. So I first cut the lathe down to size (22 ½” wide by 69” long) and
then spray painted it black with high temperature flat black paint. I then glued
aluminum foil on the bottom of the airflow channel to reflect both light and heat
back onto the absorber screen. I then installed a ¾” x ¾” wooden strip diagonally
up both sides of the air collector with 1 ¼” wood screws. I installed the lathe one
layer at a time and put a few screws into the wooden support strips along the
sides and at the bottom to hold the lathe in place. Along the top I bent up the
end of the lathe so I could fasten it to the edge of the strut at the top of the
collector.
Figure 28: Aluminum foil and absorber mesh support strips
Figure 29: Painting the lathe
Figure 30: First piece of absorber mesh installed
Figure 31: 6 layers of painted lathe installed
Installing the glazing
For this dehydrator I am using .040” Sun- Lite HP glazing from Solar
Components in Manchester, New Hampshire. It is a Fiberglass Reinforced
Polyester (FRP) material and has been used for many solar technologies. It is a
thick, durable, translucent plastic and has a high solar transmittance and a low
heat transmittance. Any plastic glazing will work, however. One problem with this
FRP material is that it is not sold in 24” widths. It comes in 3 and 4’ widths and
some others and can be purchased in any length. It costs about $2.50/SF. I
purchased a 4’ wide roll and cut it in half for the solar dehydrators. It can be cut
with tin snips or with a utility knife. The piece needed for this dehydrator is 24”
wide x about 70” long. Make sure the smoothest side of the material is installed
towards the weather.
Figure 32: 4' wide Sun-Lite HP glazing being cut to 24”
I hold the glazing in place with 1/8” thick by ¾” wide aluminum bars or battens.
The battens are first cut to length and then holes are predrilled in the aluminum
battens for the screws that will hold the battens and glazing to the dehydrator. I
then lay the glazing in place on the dehydrator, put the top aluminum batten in
place and drill through the holes in the batten and through the glazing and
dehydrator. I then remove the glazing from the dehydrator and run a bead of
silicone caulk across the top edge of the air heater frame and carefully put the
glazing back in place. Place the batten on top on the glazing and then install a 1” #8 stainless steel round head screw into the center of the batten, through the
glazing, and into the dehydrator. Make sure the glazing is on straight and install
the other screws across the top. Lift up the glazing and carefully run a bead of
caulk around the sides and bottom of the dehydrator. Then line up one side and
place the long aluminum batten on that side and predrill holes through the holes
in the batten and through glazing and into the dehydrator. Install stainless round
head screws down that side. Repeat the process for the other side and then the
bottom.
Figure 33: Silicone caulk, Sun-Lite HP glazing, aluminum batten and #8 x 1" round head stainless steel screws
Figure 34: Dehydrator with glazing installed
Making the drying shelves
There are 11 shelves required for the dryer described here. They were
made of ¾” pine boards which were 1st cut to the proper lengths. Two lengths
were required; 22 ¼” and 16”. After cutting the pieces to length I used a router
with a ¾” straight cutting bit and a cutting guide to cut a ¾” wide by 3/8” deep
rabbit on the edge of each board. I then ripped up the boards into ¾” strips using
a table saw, but a circular saw with a rip fence could work as well. 22 – 16” strips
and 22 – 22 ¼” strips were needed. I cut some extras in case I needed them. I
then set up a jig using a framing square to get the frames together square and
used glue and 1 - 5/8” long flat head phillips head screw at each corner. After
letting the glue dry, I cut plastic food grade screen to size (16” x 22 ¼”) and
stapled the plastic screening to one side of the frames using ¼” staples and a
staple gun. We purchased the screen from MacManiman Inc, 3023 362nd Ave, Fall
City, Wa. 98024.
Figure 35: Using router to cut lap joints
Figure 36: Cutting rabbits on the edge of boards
Figure 37: Making drying frames
Figure 38: Getting ready to staple screen to frame
Figure 39: Screen staples to frame
Figure 40: Drying frame completed
Figure 41: Drying shelves installed inside dryer
Roof and Handles
The final stages of the construction involved installing some shingles on the
roof of the dryer and some handles to make it easier to move around. The
handles were made out of some scrap 2 x 4’s and were about 24” long. I bolted
the handles to the rear dryer legs with two 4” long x 3/8” bolts with nuts and
washers.
Using the Dryer
I would suggest initially trying to dry no more than 5 pounds of food slices.
The food should be sliced into uniformly thin pieces (around 1/8” thick) for best
drying. Apples are a great fruit to start with. Pick a nice warm, sunny day and get
an early start. Spread the fruit out on the drying trays. Open one set of the set of
exhaust vents (leeward side if windy) to approximately 3”. Fruit is dry when no
wetness comes out when squeezed and it is a leathery consistency. Its weight will
be between 10 & 20% of the starting weight. Store the food in small airtight
containers and keep in a cool, dry location. Well I hope you will find this dryer to
be a pleasure to build and use and an effective tool for preserving food. I have
put together a longer and more detailed instructional manual and am making a
video which describes the construction process as well and I would be happy to
share these with anyone interested.
Figure 42: Completed dehydrator with shingled roof and handles