Basics of Solar Dyers

Drying on the ground by the sun or sun drying is one application of solar energy with a long history. It has traditionally been by open-air drying where the product is exposed directly to the sun allowing the solar radiation to be absorbed by the material. The term “solar dryer” is applied to a structure made for the deliberate use of solar energy to heat air and/or the products and so achieve dehydration, or drying, of the products.

Solar drying has been used in many places for many kinds of agricultural products. Drying rice using solar dryers has been well known for many years especially in the countries producing rice such as Thailand and the other Asian countries. For other products, many types of fruits (including apples, bananas, grapes, mangoes, and papaya), vegetables (including beans, carrots, cassava, taro, tomatoes, and yams), beverage crops (cocoa, tea, coffee) and peanuts are in the list of agricultural products that have been dried by solar dryers. In addition, flowers, herbs and medicinal plants have also been used in solar dryers. However, improvement on quality and quantity of the products by solar dryer systems have been obviously attempted continuously by researchers, even for the same products.

Solar dryers usually consist of a transparent cover above a chamber or collector that is blackened to absorb the solar radiation. The construction of solar dryers varies from a simple tent solar dryer to a really complicated one. In practice, the construction is determined by quality requirements, product characteristics, and economic factors (Leon et al., 2002).

Solar dryers system can be classified into several categories:

i. Based on the mode of drying, they are classified into direct and indirect dryers. Direct dryers are the dryers in which the solar radiation is absorbed directly by the product to be dried, while indirect dryers are those in which solar radiation is used to heat the air which then flows through the space containing the product to be dried. Indirect solar dryers employ a separate solar collector which absorbs solar radiation, convert it into thermal energy which in turn heats up the flowing air, and then supply the heated air to a chamber containing trays of product to be dried. Dryers of this type have several advantages over direct dryers including:

· The airflow rate can be easily controlled according to the drying characteristics of product to be dried.

· The food is not directly exposed to the sunlight which usually reduces the quality of the products. Indirect dryers are, however, more expensive and complex to construct.

Combination of direct and indirect method are applied in a hybrid system.

ii. Based on the auxiliary energy used to operate the system, dryers are classified into active and passive dryers. Active dryers employ an external means, such as fans or blowers, for moving the heated air from the solar collector to the drying beds, while passive dryers use only the natural movement of heated air.

iii. Based on the type of product to be dried, they are classified into single and multi-product dryers.

Example of Solar Dryer Designs : (from many sources)

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