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Bayside Park Farm solar dehydrator
- 1 Background
- 2 Problem Statement
- 3 Project Criteria
- 4 Timeline
- 5 Budget and Costs
- 6 Literature Review
- 7 Construction
- 8 Operation
- 9 Conclusion
- 10 Team
- 11 References
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.
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.
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.
|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|
|2/02/14||Began setting up Appropedia page.|
|2/08/14||Chose two designs to begin prototyping.|
|2/14/14||Met with client in order to determine specific requirements for dehydrator.|
|2/20-2/22/14||Constructed two prototypes from cardboard and cellophane wrap; conducted 1 day of testing.|
|3/02/14||Began accumulating materials for full-size construction and possible wood prototypes.|
|3/14/14||Finish prototype construction|
|3/16-3/22/14||Prototype testing in Southern California or Hawaii (spring break destinations) in order to determine effectiveness. Source materials for final design.|
|3/23/14||Determine dimensions and parts necessary for final design.|
|3/26/14||Finish sourcing materials for final design. Begin construction of final design.|
|4/18/14||Finish construction of final design.|
|4/10-4/19/14||Trial runs of design in order to determine flaws and necessary improvements|
|4/20-4/30/14||Fix flaws, final trial run. Record results.|
|5/01/14||Finish testing effectiveness of various solar absorbers|
|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.
|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|
This is a review of the available literature pertinent to the proposed solar dehydrator at Bayside Park Farm.
The dehydration of food is the oldest method of preservation, dating back as far as 12,000 B.C., with proponents and users including the Roman empire, ancient Middle Eastern and Asian cultures, and the Europeans of the Middle Ages.  Solar dehydration was likely the first (and least resource intensive) method of food drying, and is very simple to do in a sunny, warm environment. Food is placed in a container designed to trap heat, and the air is then heated to a temperature high enough to prevent microbial growth, but not high enough to actively cook the food (roughly 130 degrees Fahrenheit for produce and 150F for meat).  One of the most important aspects of a dehydrator is the airflow: the air should be able to flow in one side, heat up and absorb water from the food, and escape from the other side.
One of the biggest issues with solar dehydration, particularly in Humboldt County, is its reliance on the ambient climate. Arcata falls and winters are typically characterized by cool temperatures with intermittent sun. Alongside the cool, cloudy days, an average relative daytime humidity of roughly 60% (with nighttime humidities commonly reaching 90%) will pose a challenge to achieving satisfactory food dehydration. 
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).
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.
Components of solar dehydrators
There are a multitude of designs, shapes, and plans for solar dehydrators. However, all of them share a few similar components: a heat chamber, a method of keeping it off the ground, and a removable tray that contains the food.
The heat chamber is most often a solid-sided box with a clear polycarbonate top; this acts as a greenhouse and traps incoming solar heat in order to facilitate the drying process. It is almost always backed by a dark material in order to promote heat absorption. The heat chamber can either be used as a simple heat collector (which acts as a way to foster convective air currents in a system) or as the drying chamber itself. 
In order to prevent the intrusion of insects and foreign contaminants, the dehydrator is commonly placed on legs or elevated in some way. This prevents animals from approaching the dried goods from the ground, and keeps dirt from contaminating the food as well. 
The trays must be permeable in order to allow for both good airflow and to allow any excess moisture to easily escape from the materials being dried. Some sources advocate the use of readymade plastic trays , others suggest custom-building the trays in order to allow for optimal circulation and permeability. 
Phase 1: Design
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 instructables.com 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.
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 dehydrator is simple to operate, and requires minimal human intervention in order to run:
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
During testing, our dehydrator consistently temperatures between 40 and 55 degrees Fahrenheit above ambient:
|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 thin-sliced bananas, apples, and berries.
Things we learned here.
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.
Project completed during Spring semester, 2014.
- "Historical Origins of Food Preservation." National Center for Home Food Preservation. http://nchfp.uga.edu/publications/nchfp/factsheets/food_pres_hist.html (accessed February 7, 2014).
- Torrey, M. Dehydration of Fruits and Vegetables. Park Ridge, N.J.: Noyes Data Corp, 1974.
- "Dehydration." Pickyourown.org. http://www.pickyourown.org/dryingfoods.htm (accessed February 9, 2014).
- "DIY Solar Dehydrator." TheHomesteadingBoards.com. http://thehomesteadingboards.com/2012/03/diy-solar-dehydrator/ (accessed February 9, 2014).
- "WeatherSpark Beta." Average Weather For Arcata, California, USA. https://weatherspark.com/averages/29570/Arcata-California-United-States (accessed February 9, 2014).
- Hui, Y.H., Clary, C., Farid, M.M., et. al. Food Drying Science and Technology. Lancaster, Penn.: [DEStech Publishing], 2008
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- "How To Build a Solar Food Dehydrator." Off The Grid News. http://www.offthegridnews.com/2012/04/16/how-to-build-a-solar-food-dehydrator/ (accessed February 9, 2014).
- Russon, Jonathan K., Michael L. Dunn, and Frost M. Steele. "Optimization of a Convective Air Flow Solar Food Dryer." International Journal of Food Engineering 5, no. 1 (2009). http://contentdm.lib.byu.edu/cdm/ref/collection/IR/id/67 (accessed February 9, 2014).
- Karla, S. K., and K. C. Bhardwaj. "Use of simple solar dehydrator for drying fruit and vegetable products." Journal of Food Science and Technology (1981). http://agris.fao.org/agris-search/search.do?f=2013/US/US2013019795410012440.xml;US201301979541
- Wheeler, E. Home Food Dehydration: The Hows, What and Why. Seattle, Wash.: [Wheeler Enterprises], 1974.