Hybrid Solar Dryer
Why should we dehydrate fruit?
Fresh fruit is delicious (we love breadfruit, mangos, bananas you name it) but they don’t last forever. Leave a fruit on your kitchen counter for a few days and surely it will spoil. Furthermore fruit comes in seasons. Sometimes too much to eat, other times almost none! Can we preserve the excess for later?
Drying, or dehydrating fruit, removes most of the moisture without cooking. The fruit still has all its nutrients, but now has a long shelf life. Ideally, we take the fruit from 70 – 75 percent moisture content down to 10 – 12 percent moisture, in less than 24 hours for best quality.
But drying can be expensive. Fuel is expensive and the cost of electricity in many countries is prohibitive. And yet sunshine is in abundance where tropical fruit grow. Solar drying makes so much sense, both economic and environmentally.
Solar drying by the way is NOT photovoltaic. We don’t try to convert the sun’s heat into electricity, store it and then convert it back to heat! That would be very inefficient! We trap the sun’s heat and funnel it over the fruit directly in a safe, hygenic, inexpensive way.
You could just spread fruit out in the sun to dry. But better, more hygenic, TTFF has designed and tested a hybrid solar dryer, and it works well! Here’s a picture of an early prototype …
This dryer uses solar collectors feeding hot air into a cabinet. The lower half of the cabinet optionally holds a propane or kerosene heater to supplement solar power when needed, like a cloudy day, hence the Hybrid designation. The upper half of the cabinet holds 4 or 5 mesh wire shelves which in turn hold breadfruit shreds or mango slices placed on trays to be dried. The rooftop shelters the interior from rain and dust, and also holds an exhaust fan and a vent or chimney. It’s essential to remove the moist hot air from the interior of the cabinet.
We’ve worked with local organizations to build more of these solar dryers in Jamaica and Haiti. Already we have sixteen in use in Haiti and Jamaica, with several more planned. Our users are ecstatic! One person could hardly believe the effectiveness–seemed like it was heated with fuel or electricity–but no. Only the sun. Approximately 100 pounds of fresh fruit can be dried within 4 to 8 hours!
Build Your Own!
Update: in response to users’ feedback we recommend a few modifications to the original design:
- Paint all exterior metal surfaces black, to increase solar insolation
- Don’t paint the collectors’ clear plastic cover. Use rigid plastic not thin, flexible plastic.
- Build the shelves from galvanized open mesh wire–and place the food to be dried on removable perforated metal trays (picture below)
- We now recommend 5 or 6 shelves not 4.
- If possible use a turbine vent instead of a chimney, in addition to the solar fan, to boost air flow (picture below)
- Some people suggest using thermal ballast in the lower cabinet. It doesn’t make much difference but you’re welcome to try it. But don’t use water!
Read on below for details on how to build your own.
Start with this video animation. It’s a short, clear summary. Thanks go to Jose Urena.
Third, a complete parts list.
Fourth, here’s a link to our technical paper (also see abstract, below)
Fifth, as to costs, this is very variable depending on sources but the materials costs will run roughly $1,000. The recommended material is galvanized sheet metal on a metal frame, for food safety and durability. Exterior parts of the collector modules may be constructed out of weatherproof plywood to reduce cost.
Labor costs vary but figure 80 hours of skilled work such as metalwork, welding, and carpentry. Many groups use volunteer labor to keep costs low.
Sixth, as to energy savings: in developing countries electricity typically costs $0.40 per kilowatt-hour. The equivalent of 6 hours of solar drying (6 kW), 5 days a week, 26 weeks a year, would cost roughly $1,500 to $2,000. This solar dryer will pay for itself in less than a year! Not a bad return on investment!
This is the solar fan we use.
Also a turbine vent is used to increase air flow:
The fruit to be dried is placed on perforated metal trays (stainless steel or aluminum):
Trees That Feed can usually help to source these items. Contact us for assistance.
This design is based on experience in Asia where fruit are regularly preserved by drying, along with a project through our partnership with Northwestern University. TTFF developed a first prototype for testing at our Chicago area headquarters.
The key to this solar dryer is the solar collector module. This is made from a 4′ x 8′ corrugated metal sheet, painted black, within an enclosed frame, covered by a clear plastic top. The metal sheet gets hot in direct sunlight. Outside air passes above and below the corrugated metal and warms up, typically 30 – 35 degrees Fahrenheit above ambient temperature. The warm air has low relative humidity and readily absorbs moisture. The plastic cover provides a greenhouse effect to capture the heat and to feed warm air up into the cabinet. Solar energy at the earth’s surface is approximately 1 kW per sq. meter. This collector is roughly two square meters. With three collectors, up to 6 kW of heat energy enters the drying cabinet.
The solar dryer comprises six modules: three collectors, lower cabinet, upper cabinet and roof. The collectors funnel warmed air into the cabinet. The lower cabinet optionally holds a fueled heater, such as a kerosene stove, in case of a cloudy or rainy day. The upper cabinet holds 4 or 5 shelves which carry the fruit. As warm, dry air rises through the cabinet, it absorbs moisture and dehydrates the fruit.
The last module is the roof which protects against rain and augments natural convection. The roof will hold a solar exhaust fan. The cabinets are not sealed off! We’ve seen that mistake made. Moist air has to be pulled out, and so transporting moisture away and leaving the fruit drier. This solar dryer will work for breadfruit, mango, paw paw and many other fruit, depending on what is in season.
Your humble designer Mike McLaughlin was a Physics major from way back. His calculations have optimized the dimensions, cost and performance of this design. Change the design at your own risk!
Download and read the published paper here:
By Michael McLaughlin
Co-founder, Trees That Feed Foundation
Fresh breadfruit is a healthful, widely accepted food in the Caribbean. During the main bearing seasons there is a glut of fruit, much of which is wasted, while outside of the bearing seasons there is little breadfruit in the local diet. Preserving the excess fruit will increase the food supply overall and make breadfruit products available year round. Fresh breadfruit contains 70 percent by weight of moisture. Dried breadfruit has a shelf life of one year or more, and has great food potential, for example by being ground into flour. Currently, equipment for peeling, chopping, shredding and grinding breadfruit is readily available. Drying the fruit tends to be the bottleneck to increased production capacity.
Given the cost and availability of electricity and fuel, solar drying has the greatest potential to increase the food supply of breadfruit, mango and other fruit. In this paper we discuss the importance, design, and usefulness of a cost effective hybrid solar dryer. Design criteria included low initial cost, simple construction based on locally available materials, rapid drying (within 24 hours), and the optional utilization of fuel. A modular design was developed based on detachable solar heat collectors, a cabinet with a lower section for collection, and upper section to hold food-safe stainless steel shelves, and a rooftop with a solar-powered exhaust fan and turbine vent to augment convection-based air movement. Construction materials include mainly plywood (preferably pressure treated or otherwise waterproofed), galvanized metal sheets, and clear plastic sheets (preferably UV stabilized). Construction skills needed are basic. The cabinet is 4 feet square, 10 feet maximum height, and the collectors’ dimensions are 4 x 8 feet. The upper cabinet holds four shelves, each holding up to 25 pounds of fresh, shredded breadfruit. Total costs of construction fall below US$500.00, depending on local prices.
One, two or three solar collectors may be built, depending on available space and desired capacity. It is recommended that three similar collectors be built and installed on the west, south and east facing sides of the cabinet. This delivers maximum volume of solar heated air flow hence greatest drying capacity.
Solar energy is of course free, but not always available. An important design criterion was the optional use of fuel. The lower cabinet section is large enough to accommodate solar collectors on three sides, but also optionally a small kerosene or propane fueled heater. This increases operating costs, but avoids the potential spoilage of a crop of fruit that needs to be dried on a cloudy or rainy day.
Design calculations indicate a drying capacity of 100 pounds of fresh shredded breadfruit in 8 hours or less, the equivalent of over 600 pounds of dried breadfruit per month. Field testing is underway, as of this writing.