Malnutrition Matters, in partnership with SolarFlex Inc., is currently developing a passive solar fruit and vegetable dryer that has a flexible design and is inexpensive to manufacture. Drying fruits and vegetables with this system is a cost-effective long-term food storage solution that requires no external electricity to operate, and eliminates the need for chemical preservatives. The ability to dry fruits and vegetables is a crucial element in achieving the long-term food security of many communities, where certain crops may be abundant during harvesting season, but the lack or high cost of canning and refrigeration facilities makes these foods unavailable at other times. The system can work with or without an optional boiler and radiator: this allows for greater flexibility when drying on cloudy days or at night.

Front of dryer with fresh air / humidity vents at top

Additionally, this system will be of interest to entrepreneurs looking to provide dried food products in markets where food security is not an issue. The very low production costs associated with dried foods make this method highly cost-competitive compared with foods preserved through other methods. Dried fruits and vegetables are also regarded as premium specialty foods in many markets. In addition, the system can process many other kinds of food products, such as dried fish and herbs.

The system offers approximately 32 square meters of drying space (approx. 340 square feet). Component setup can be accommodated to specific site requirements or preferences, including the separation of the solar array from the dryer cabinet. This allows for an interior installation of the dryer cabinet, with ducting passing through a wall to an exterior installation of the solar array. Alternatively, the solar array can be installed directly against the dryer cabinet, as pictured below.

Although the solar array produces heat even on cloudy days, an optional biomass boiler and radiator may be desired in certain circumstances. Possible fuels for the boiler include wood, coal, propane, or other biomass such as dried corncobs. The boiler heats the water up to a super-heated steam, which is then injected into the radiator. The condensed water from the radiator is then collected in a bucket, which can be reused in the boiler.

The fan, powered by the solar-charged 12V battery, draws the hot air from the solar array and radiator through ducting tubes into the dryer cabinet. The dryer cabinet contains four compartments, each with its own hot air intake and exhaust. The compartments contain several drying racks stacked vertically, each of which is perforated to allow maximum airflow through the cabinet. A simple humidistat indicates the moisture content of the air inside the compartments: this directs the operator to open the exhaust when the air is very moist, and to close it again when the air is dry, allowing for greater drying efficiency.

Drying racks with various fruits, before drying

Fruits after drying

The SolarFlex “Family” model uses a combination of traditional and innovative passive solar drying in a very cost-effective combination. Ambient air enters the bottom of the solar collector and moves upwards naturally while picking up considerable heat. It then enters the bottom of the drying cabinet, which contains 10 drying racks. At the top of the cabinet, a group of five metal chimneys, heated by the sun, cause additional updraft of hot air which pulls the humid air out of the cabinet at the same time that fresh dry and hot air is entering the bottom. An additional solar collection plate at the front of the dryer cabinet transfers more heat into the cabinet through holes in the steel absorber.

While this system is somewhat slower in drying products than a model with forced air circulation (such as the larger SolarFlex dryer), and some products may take a second day of drying (depending on the fruit, the length of the sunny day and ambient temperatures) the low cost, versus capacity makes this option attractive.

Click here to download SolarFlex "Family" Dryer info sheet (.pdf)