Solar drying

Solar drying refers to using the power of the sun to extract moisture from substances. It most often refers to drying fruit and vegetables, but solar drying techniques can be used on other organic and inorganic materials as well. The power of the sun can be used to dry materials either directly or indirectly; however, in the twenty-first century, there is growing interest in indirect methods that increase the efficiency and sanitation of drying food items.

Background

All forms of solar drying are based on the idea that the sun’s radiation warms the water that is naturally in fruits, vegetables, leaves, and other plant parts used as crops. When water is warm, it evaporates more quickly. This means it turns from liquid to vapor and moves into the air. As a result, the water vaporizes, leaving behind the solid material, so crops that are exposed to the sun will dry out relatively quickly. Reducing the moisture level protects food from spoiling and helps it to remain edible longer.

Solar drying was the only form of drying food or other items that was available to people until making fire was mastered. Archeological evidence indicates that drying was indeed one of the earliest forms of food preservation. There are indications that cultures in the Middle East used the sun to dry food as early as 12,000 BCE. Dehydrating, or removing the moisture, from fruits, vegetables, and even meat helped it last longer. This allowed people to have food even in seasons in which little could be grown. It also provided a source of portable food for travel. The use of solar drying for food helped early cultures survive and expand into new areas.

Eventually, people began using drying techniques simply because they liked the resulting products. Tea leaves were dried in the sun to provide a beverage that became popular in Asia and spread to other parts of the world. Early Romans were especially fond of dried fruits. As food became a cash crop, the ability to dry fruits and vegetables provided a way to use crops that might otherwise go to waste before they could be eaten. Drying other plants, such as spices and herbs, as well as products like tobacco leaves, provided totally new crops for sale.

Many of these crops continue to be important in contemporary times. Natural products such as teas, herbs, and spices continue to be popular, as do dried fruits such as raisins, figs, dates, and cranberries. Solar drying continues to be an important method of food preservation.

Overview

In many areas, the techniques for drying food products remain much the same as they have for centuries. They often involve simply spreading the crop out on the ground or on some form of covering on the ground to allow the sun to dry them. This method, which is called direct solar drying or passive solar drying, presents a number of challenges. The sun only shines for part of the day, allowing the moisture to remain unabated during the overnight hours. In some places, there is little sun during part of the year, making solar drying difficult or impossible. Direct exposure to the sun can also affect the quality of the resulting food product or its nutrients, making it a less desirable way to dry in some instances.

In addition, food laid out on the ground or on a covering is subject to getting wet when it rains. Animals, birds, and insects are likely to attempt to eat their fill, affecting both the amount and the sanitation of the crop. This can lead to mold and diseases, some of which can make people sick. To prevent this, people who use simple direct solar drying will cover the crops when it rains and often watch the crop while it dries to keep away insects and animals.

To lessen these challenges, people began developing alternative ways of drying food. One way was to build dehydrators over fire pits. Vents at the top of the dehydrators exhausted excess heat so the food was dried instead of baked as it would be in an oven. However, this method increases the cost of drying; not only does it require a device to be constructed, but it also requires a constant supply of fuel during the drying process.

Direct solar drying is often preferable because it lowers the cost of drying. Solar energy is free, and it is also an earth-friendly technique. However, since direct solar drying is not always practical or even possible, additional new methods have been developed. These include indirect solar drying using a specially designed cabinet or other device; mixed-mode drying that uses a fan or other method to help speed evaporation and drying; and hybrid drying, which combines solar with some other fuel source.

Indirect solar drying is the second-most common form and usually the second-least expensive after direct solar. Indirect solar drying uses a box or some device that allows the sun’s rays to penetrate. The sun’s radiation increases the heat in the box; the increase is not enough to bake the food items but is enough to speed the evaporation process. The sides and top of the box are usually transparent but colored, or in some way altered, so that the items inside are not subject to direct sunlight. As a result, indirect solar drying provides all the benefits of solar drying with few of the downfalls. The drying happens in hours instead of days, and the crop inside is protected from rainfall, animals, and most dangerous microorganisms such as mold. The only disadvantage is the initial cost of the drying devices.

Mixed-mode drying combines an indirect style cabinet or box with a fan. This adds the drying effects of moving air to the radiation of the sun to further speed the drying process. The mixed-mode method has all the advantages of indirect drying and a similar disadvantage in the cost. The final method, hybrid drying, combines another form of heat besides the sun with solar power. For example, a gas generator may be used to add heat overnight, so the drying process can be continuous. This adds the disadvantage of the cost and the carbon footprint of the other fuel source.

Solar drying could be an important way to improve food availability in areas where food insecurity is a problem. It also helps extend the value of crops by providing an alternate product. Developing lower cost dryers and sharing information on how to use them could benefit many people in countries where food availability is a challenge.

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