Bayside Park Farm solar dehydrator

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Engr305 Appropriate Technology page in progress
This page is a project in progress by students in Engr305 Appropriate Technology. Please do not make edits unless you are a member of the team working on this page, but feel free to make comments on the discussion page. Check back for the finished version on May 15, 2014.


Bayside Park Farm (formerly known as the Arcata Educational Farm) has been in operation since 1993, and was Arcata's first CSA (community supported agriculture) farm. In order to allow for the preservation of the farm's excess crops, a solar dehydrator was implemented in 2009 by a previous Engr305 Appropriate Technology class. Dehydrating food is an especially important process for farms because it allows them to maintain their livelihood (food) for the long term. However, after problems with adequately drying produce, the dehydrator fell into disuse and was ultimately removed from the property. We were asked to build an effective, educational follow-up to the initial dehydrator.

Problem Statement

The objective of this project is twofold:

  • To produce a durable and effective solar dehydrator to aid in the preservation of Bayside Park Farm's excess produce.
  • To design the solar dehydrator simply and accessibly enough to foster adoption and replication of the solar dehydration process.

In summary, it is our ultimate goal to provide the Bayside Park Farm with a solar powered food dehydrator that will meet their produce needs, is easy to maintain, and will last much longer than the one before it.

Project Criteria

This criteria list, while not final, represents what we feel are the most important considerations concerning the scope and objective of this project. The scale is weighted from "1" being of the least significance to "10" being of the most significance.

Criteria Constraints Weight
Functionality Device working as intended
Maintainability Has to be maintenance friendly to the user, little knowledge required for maintenance and repairs
Safety Device must pose no outstanding operational dangers to user
Usability Ease of regular operation is key, intuitive handling
Budget Must not exceed budget
Reproducibility Design can be easily reproduced by others
Aesthetics Must look like a natural extension of the farm



Completion Date

Completion Date Task
2/02/14 2/02/14 Began setting up Appropedia page.
2/08/14 2/08/14 Chose two designs to begin prototyping.
2/14/14 2/14/14 Met with client in order to determine specific requirements for dehydrator.
2/20-2/22/14 2/20-2/22/14 Constructed two prototypes from cardboard and cellophane wrap; conducted 1 day of testing.
3/02/14 3/02/14 Began accumulating materials for full-size construction and possible wood prototypes.
3/14/14 3/09/14 Finished prototype testing
3/16-3/22/14 N/A Prototype testing in Southern California or Hawaii (spring break destinations) in order to determine effectiveness. Source materials for final design.
3/23/14 3/24/14 Determine dimensions and parts necessary for final design.
3/26/14 3/28/14 Finish sourcing materials for final design. Begin construction of final design.
4/18/14 4/20/14 Finish construction of final design.
4/10-4/19/14 4/22-4/30/14 Trial runs of design in order to determine flaws and necessary improvements
4/20-4/30/14 5/05/14 Fix flaws, final trial run. Record results.
5/01/14 5/05/14 Finish testing effectiveness of various solar absorbers
5/11/14 5/11/14 Deliver dehydrator to Bayside Park Farm.

Budget and Costs

Below is a proposed budget for the Bayside Park Farm solar dehydrator. Costs are estimates and thus may be inaccurate, however a finalalized budget is underway.

Materials Source Quantity Cost($) Total($)
1 3/4" 12mm Wood Screws McKenny's Do It Best Hardware 60 0.13 7.80
1 1/2" 10mm Wood Screws McKenny's Do It Best Hardware 30 .08 2.40
3/4" Machine Screws and Nuts McKenny's Do It Best Hardware 6 0.25 1.50
Lumber: 2"x4'x20' McKenny's Do It Best Hardware 1 17.00 17.00
Lumber: 16'x1'x3/4" Pine Plank McKenny's Do It Best Hardware 2 28.00 56.00
Short Wood Tacks McKenny's Do It Best Hardware 30 1.50 1.50
Plexiglass Sheet: 30"x60" Ace Hardware 1 35.00 35.00
Stainless Steel Hinges McKenny's Do It Best Hardware 1 pack of 2 8.00 8.00
Wire Racks Dollar Tree 4 1.00 4.00
Heat Absorber: 30 Gallon Trash bag Dollar Tree 1 1.00 1.00
Velcro Pads Michael's Craft Supply 1 2.50 2.50
Metal L-braces McKenny's Do It Best Hardware 1 pack of 4 3.50 3.50
Total Cost $97.30

Types of solar dehydrators

Three main types of solar dehydrators exist: natural-convection dryers (which do not require electricity or mechanical energy for operation), forced-convection dryers (which commonly use an electric fan to maintain a constant airflow), and solar-assisted dryers (which uses an auxiliary heating source as its primary dryer).[1] Because of the energy-intensive nature of solar-assisted dryers, we limited our literature review to the first two categories:

Natural-convection dryers

Two main types of natural-convection dryers exist: the simplest, a box dryer, is essentially just that: a large box with one face angled to catch the maximum amount of sun. The box serves as both the solar collector and the drying chamber, with air intake holes drilled low on the box and exhaust holes drilled high to allow moisture-laden, warm air to escape. The box dryer's simplicity allows it to be easily reproduced, and the single-chamber design keeps the heat where it's needed, reducing heat loss via convection. However, the single-box design dramatically reduces the dehydrator's working capacity, and the solar angle cannot be adjusted to maximize seasonal sunlight.

The other type, called a tower dryer, relies on an attached solar collector to heat air, which then rises, is directed into a tall drying chamber, then is released from ventilation holes at the top of the tower. The tower dryer's design is more complicated than that of the box dryer, requiring fabrication of a secure, flexible duct to move air from the solar box to the dehydration chamber. This allows for increased heat loss; however, the increased shelf space in the dehydrator tower and the ability to change the angle of the solar collector make it a more adaptable design for certain climates.

Forced-convection dryers

Forced-convection dryers require external power to operate. Household power (120V AC in the United States) is commonly used to power fans, which provide a mechanical assist to pull in and expel air, facilitating a faster drying speed. Forced convection dryers are commonly tunnel-shaped, or constructed to maximize surface area and air contact with the material to be dehydrated, but any dehydrator can be turned into a forced-convection dryer with the addition of an electric fan.


Phase 1: Design

Our initial, simple schematic for the dehydrator using plywood and 2x4s.

We started with a very general idea of what we wanted to do: we envisioned a simple, effective dryer that would be able to raise the internal temperature to roughly 130F under full sun; the accepted ideal temperature for drying produce. During our meeting with Jayme Buckley at Bayside Park, we were informed that the dryer would likely be used to dry herbs. In light of this, we settled on using a box dryer, as the design would heat more quickly and wouldn't lose as much heat in the transition from solar collector to drying tower. We used a design from as a reference, but as our materials differed, we extensively modified the proportions and overall structure in order to optimize the dryer for our climate and make the most of the materials we had.

Phase 2: Materials Gathering

Initially, we wanted to use a combination of plywood and 2x4 lumber due to their inexpensiveness, durability, and ease of working with. However, concerns were raised about the possibility of formaldehyde offgassing from the plywood, as the chemical is present in the glues used to hold the thin sheets of wood together. After some deliberation and consulting with McKenny's Do It Best in Arcata, we decided to forego plywood and instead use multiple lengths of 1" thick x 12" wide pine planks for our walls, top, and base. 2x4s remained our choice for the legs in order to maximize the sturdiness of the structure while minimizing cost. Our solar collector would be lit via a large sheet of Plexiglass, which we chose for its light weight, durability, and ease of mounting due to its ability to be drilled. Stainless steel hinges were sourced due to its resistance to rust, as the dehydrator would be dealing with large quantities of moisture, both from the food and from the humid climate. Small metal L-braces were also procured in order to reinforce the legs.

Phase 3: Construction

Armed with 8 4-foot long pieces of 1" by 12" pine, 6 3-foot long sections of 2x4, 70 1 3/4" wood screws, two sets of bits and drives, sandpaper, and two power drills, we set to work. We built half of the bottom and the back wall first, sinking screws through the pine board into the tops of two 2x4s in order to form the back legs of the dehydrator. Another pine board was attached to the bottom of the base in order to provide the attachment for the front half of the dehydrator floor, which was then attached using six wood screws. 2x4s were attached to the bottom of the front half of the dehydrator floor in the same method as before.

A very simplified version of our dehydrator design.

In order to maximize solar insolation, we found estimates online suggesting that a 40-degree angle would be ideal for Arcata, providing the most heat during the winter and early spring. Leaving a 1" lip on the bottom to account for where we would be joining the sides to the baseboard, we used a protractor to measure out the angle, then cut both side pieces using a circular saw. The side pieces were joined to the baseboard by putting several wood screws through the bottom and back edge of each side piece and into the baseboard and back wall.

The top piece of the dehydrator box was simple to attach: after some minor trimming in order to account for a small amount of warp in one of the side panels, we used one of our 4-foot pieces of pine and secured it at the top of the side and back pieces using wood screws. However, due to the necessary 40-degree angle of the side pieces, we had a small "lip" that jutted out past the sloped front face of our dehydrator, which needed to be eliminated if our Plexiglass were to lay flush with the box edges. We were able to borrow a hand planer from one of Justin's awesome neighbors, and quickly managed to whittle the lip down to be flush with the sloped front edge.

With the box complete, we needed to attach our transparent medium. We'd decided on acrylic (Plexiglass) instead of pane glass because of its light weight and durability; it needed to be able to stand up to the occasional bump or over-eager closing without shattering. Additonally, using acrylic allowed us to mount hinges directly to the transparent surface: however, this opened the door to a lengthy debate over where and how the hinges should be mounted. After some time, we decided that in order to keep the window as close to the frame as possible to minimize gaps, we'd mount one side the hinge to the top of the bottom piece, then "wrap" the hinge around the window and fasten the other side of the hinge to the outside of the pane. This can be seen here.


The dehydrator is simple to operate, and requires minimal human intervention in order to run:

Dehydrator Operation
Caption Step 1 : Flatten solar absorber and arrange dehydrator shelves. Absorber can also be wrinkled/ fluffed to increase surface area if an increase in temperature is desired.
Caption Step 2 : Clean any debris or moisture off Plexiglass screen.
Caption Step 3 : Rotate dehydrator to face the sun. Simply rotate the dehydrator until its shadow is cast directly behind itself.
Caption Step 4 : Place material to be dried on racks, ensuring that light can still get through to the heat absorber.
Caption Step 5 : Occasionally change dehydrator's position to optimize heat intake, if needed.
Caption Step 6 : Monitor materials until satisfactorily dried.

For a video guide on how to use the dehydrator, see the clip below:


The box dehydrator design is low-maintenance by nature, as there is very little wear and tear associated with the operation of the device. With some very simple, easy routine tasks (each taking less than 5 minutes), the dehydrator should sustain many years of continuous usage.



  • Cover dehydrator at sundown to keep the wood as dry as possible. It is extremely necessary to keep the unfinished wood away from poor weather, as it will quickly deteriorate the device. Keeping the device dry and shaded when not in use will prevent rot, sun damage, and other weathering effects.
  • Re-position dehydrator to take advantage of changing sun angles


  • Check wood joints for splitting
  • Ensure that ventilation holes are clear
  • It is highly recommended that the exterior of the device is covered in some type mineral oil (sold commonly at hardware stores as 'Lemon Oil') to keep the wood hydrated and protected. An application of this once a week or every other week is recommended. Alternatively, for a long term solution a permanent wood finish may be applied to increase the dehydrator's lifespan.


  • Examine Plexiglass for signs of yellowing or cracking
  • Check wood joints for splitting
  • Check screening to ensure it's intact
  • Examine hinges for rust


  • When not in use, cover and store dehydrator on dry ground


Testing Results

During testing, our dehydrator consistently temperatures between 40 and 55 degrees Fahrenheit above ambient:



Weather External

Temperature (F)


Temperature (F)

1 Partly Cloudy 64 115 No load. Achieved this temperature after 1.5 hours in intermittent full sun.
2 Partly Cloudy 60 102 No load. Moderate winds blew open our hinged Plexiglass door.
3 Sunny 60 120 Ideal conditions, no load.
4 Sunny 65 112 Light breeze. No load. 2 dark aluminum pizza pans and a cookie sheet were present to act as thermal mass.
5 Sunny 60 110 Moderate wind, loaded with parsley. Temperature was enough to fully dry parsley in 40 minutes; to the point of disintegrating upon touch.
6 Sunny 60 120 Light to moderate wind, loaded with one slice of tomato cut to be thin on one end and thick on the other. Tomato was dried in under one hour.


Overall, we were extremely happy with the results of the dehydrator: the humid climate was a large concern for us, so we concentrated first on raising the internal operating temperature of the dehydrator, then focused on airflow that would provide adequate moisture transport out of the drying chamber. Since the dehydrator will primarily be used for drying herbs, we believe our design will work well to dry moderate quantities of tobacco, leeks, basil, and most other leafy greens. As the moisture content of the material being dried increases, we expect the dehydrator to take longer, but still to be able to accomplish the task of drying small quantities of high-moisture foods such as tomatoes, as well as small quantities of moderate-moisture foods, such as thinly sliced bananas, apples, and whole berries.

Lessons Learned


  • When cutting the lumber materials, avoid cutting through knots and other weak points in the lumber to avoid cracking and splitting.
  • Plan ahead where each screw will go to avoid competition with space for other screws that will be placed in the future
  • Avoid putting in any hardware where easily removable/ disposable hardware can go instead (wood tacks vs. construction staples). This allows easy replacement of components, for example the screening is held in place with a couple dozen wood tacks and replacing the screening would require the removal and replacement of each wood tack.
  • Reinforcement of legs is a must, large metal braces for leg-body joints and a horizontal brace on the legs will prevent harmful torque on the body when being moved.

Solar Absorber

  • Finding a correctly sized solar absorber was difficult because we were forced to search for specific dimensions. using an amorphous absorber such as a garbage bag alleviated this problem, but may be aesthetically unpleasant to some.

Next Steps

We recommend that the exterior of the dehydrator be sealed, then treated with deck or siding stain in order to protect the wood from external moisture and preserve its structural integrity. Due to our limited budget, the dehydrator had to be constructed from pine; an excellent indoor wood, but due to its limited resistance to insects and decay, it does not perform well outdoors.


Problem Possible Fix
Dehydrator is not drying produce Make sure that there is as much surface area of the solar absorber being exposed to the sun as possible. Do not clump the produce or layer on top of each other as this will trap moisture. Make sure that all ventilation points are unobstructed.
Pests are entering drying chamber Check to see that all screens are intact. Examine all points where the chamber walls or plexi meet to ensure there is no un-screened gap. If unprotected gaps are found apply screen immediately.
Dehydrator body/ legs are deteriorating Apply mineral oil to exterior of dehydrator thoroughly and evenly (short term) or apply a stain or finish to protect the wood (long term protection).
Cracking and or splitting in wood components Inject high strength epoxy wood glue or other wood adhesive to fill cracks. Take measures to ensure injection points are free of debris, dust, and other foreign matter so the adhesive can have full contact with the wood.
Scratches on Plexiglass Avoid cleaning the plexi with any rough or coarse materials. Try to clean using only soft cloth or microfiber cleaning cloths and plexi-safe cleaning chemicals.
Yellowing or other discoloration, stiffness, warping of Plexiglass Do not leave dehydrator in full sun when not in use, this will greatly reduce the amount of sun damage on the device. It is highly recommended to cover the device with a tarp to prevent such sun damage.


Project completed during Spring semester, 2014.


  1. Hui, Y.H., Clary, C., Farid, M.M., et. al. Food Drying Science and Technology. Lancaster, Penn.: [DEStech Publishing], 2008