Hybrid Solar Dryer

Processing fresh breadfruit into flour takes several steps.  The fruit must be picked at the right stage, cleaned, shredded, dried and ground.  Each stage requires care.  TTFF seeks to aid with practical solutions at each stage.  One process bottleneck is speedy, hygenic drying.  We have prototyped a hybrid solar dryer, and here we show our steps and the results of initial testing.

This dryer uses one, two or three 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, hence the Hybrid designation.  The upper half of the cabinet holds shelves which in turn hold breadfruit shreds to be dried.  The rooftop shelters from rain and dust, and also holds exhaust fans, to remove the moist hot air.

Currently we are partnering with local organizations to build additional prototypes in Jamaica and Haiti.

Solar Collector for Hybrid Solar Dryer

Based on some great preliminary work through our partnership with Northwestern, we developed a first prototype for testing at our Chicago area headquarters.

This is the first solar dryer component, the solar collector. This is a 4′ x 8′ corrugated metal sheet, painted black, within a wood frame, covered by an acrylic top. Solar energy at the earth’s surface is approximately 1 kW per sq. meter. This collector is roughly two square meters and the idea is to feed warm air into a cabinet dryer. The warm air will rise through the cabinet and dry out the shredded breadfruit, which sits on stainless steel mesh shelves. Dried breadfruit has a long shelf life (one year or more), so this process will help supply food year-round to people in Haiti, Jamaica and elsewhere. 

Solar collector attached to lower cabinet

The solar dryer comprises four modules: the collector(s), lower cabinet, upper cabinet and roof. The collectors get warm in the sun, and warm air flows into the cabinet. The cabinet (lower portion) gathers the warm air from the collectors and directs it upwards. So, each of the heat collectors feeds into this lower portion, which is now mostly complete. The upper portion of the cabinet holds the shelves which carry the fruit. As warm air rises through the cabinet, it dries the fruit.

The fourth module is the roof which protects against rain and augments natural convection. The roof will hold a solar exhaust fan to transport moist air out, and so drying out the fruit. This solar dryer will work for breadfruit, mango, paw paw and other fruit, depending on the season. Stay tuned for our next dryer update!

Roof and upper cabinet unit
Rear view of roof unit

The rooftop module for the hybrid solar dryer is now finished. The sides are marine plywood and the top is sheet metal painted black. (Cutting out the holes for ventilation was the most difficult part!) On top is a solar fan and a turbine vent for sunny and/or windy days.

These pictures show front and back views of the roof module mounted on the upper cabinet. The solar exhaust fan is white and faces south; the turbine vent is a bronze color on the other side. These two units will ultimately mount atop the lower cabinet.

The next step is stainless steel mesh shelf and runners, and then we will just need a few nice hot days for testing purposes!

Assembled Dryer

Here is the assembled dryer.  All units are present.  Currently the prototype has only a single collector, but in practical use there would be two or three.  In the Northern hemisphere the unit should be aligned to face south.

The photo shows sweet potato shreds (orange) used for testing (breadfruit was not available on the days of testing).   Results were very good, as summarized in the published paper. The ladder was used in collecting temperature data.

For full information, download and read the published paper here:

A Hybrid Solar Dryer for Processing Breadfruit

By Michael McLaughlin

Co-founder, Trees That Feed Foundation

ABSTRACT

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 24 hours or less, the equivalent of 600 pounds of dried breadfruit per month.   Field testing is underway, as of this writing.