Solar Drying

Solar Drying

Solar drying can be used to dry crops, food, wood and many other products. Solar food drying preserves foods by removing enough moisture from food to prevent decay and spoilage. Water content of properly dried food varies from 5 to 25 percent depending on the food. Successful drying depends on;

1) Obtaining sufficient heat to draw out moisture, without cooking the food,

2) Dry air to absorb the released moisture, and

3) Adequate air circulation to carry off the moisture.

When drying foods, the key is to remove moisture as quickly as possible at a temperature that does not seriously affect the flavour, texture and colour of the food.  If the temperature is too low in the beginning, microorganisms may grow before the food is adequately dried.  If the temperature is too high and the humidity too low, the food may harden on the surface. This makes it more difficult for moisture to escape and the food does not dry properly.  Although drying is a relatively simple method of food preservation, the procedure is not exact (Kendall & Allen, 1998).

Solar food drying requires a space to spread the food where dry air in large quantities can pass over the food. “Draping food over branches or spreading it on wide shallow baskets on the roof is an old widespread tradition still in use around the world. Many other arrangements have been used to support a thin spread of food pieces. Some options that have been used are to thread the pieces on a cord or a stick and hang it over a fire, wood stove or from the rafters. Or one can bundle herbs or strawflowers and suspend them from bushes or a door knob or nails in rooms with good ventilation. Screen doors placed across chairs or sheets hung between clotheslines or possibly on a quilting frame have also been used. Vans, clean garages, or backseats of cars can be safe places to spread trays of drying food just as well as specially constructed cabinets. In the pioneer tradition food might be spread in the attic or in an upstairs room with screened windows wide open” (Kerr, 1998).
Variations on these traditional techniques are specially built enclosed drying racks or cabinets designed to regulate and control the products exposure to heat and humidity (see Figure 1). The reason for these new technologies is to produce a consistent and uniform product throughout the dryer, and in the course of the year, as the energy available from the process changes with the seasons. While any source of heat may be used, solar energy is free and usually plentiful in season. A glass panel (or similar material) screened on both ends with air intake on one end and opening to the product at the other is universally used to solar heat air. In addition, hot dry air may be moved over the product by use of natural convection or a solar chimney or a fan run on solar electricity” (Kerr, 1998).

Low Temperature Solar Thermal Technologies in Australia

For some of the research in solar thermal technologies in Australia click here

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

The Energy Resources Institute – Solar Thermal

The Solarserver – Solar Thermal Technologies in the United States

Australian National University – Solar Thermal Energy Research

Wikipedia – Solar Energy

Renewable Energy Commercialisation in Australia – Solar Thermal

References

Kendall, P. & Allen, L., 1998. DRYING VEGETABLES; Food and Nutrition Series – Preparation. Colorado State University Cooperative Extension Service Publication.

Kerr, B., 1998. A Review of Solar Food Drying. (Online) http://solarcooking.org/dryingreview.htm (Accessed 23 February 2007).