2. LITERATURE REVIEW
2.6 Classification of Solar Dryers
Solar dryers transfer the radiation, either directly or indirectly, to the product. This section reviews the two different classes of solar dryers, based on how the produce receives radiation.
2.6.1 Open-air uncontrolled solar drying
Open-air uncontrolled solar drying is a common preservation method used for agricultural products in tropical and subtropical countries. It is usually applied where outdoor temperatures are usually 30℃ or higher (Akarslan, 2012; Paul and Singh, 2013). The crop is spread out in open sunlight on the ground, on floors or roofs and is usually turned once or twice daily (Jairaj et al., 2009; Paul and Singh, 2013). The solar radiation that is absorbed in the crop converts it, to thermal energy. The increase in the temperature results in moisture evaporation from the crop, which causes the drying, as shown Figure 2.4.
According to Hossain et al. (2007), the drying rate in open-air uncontrolled solar drying is a relatively low and this increases the drying time. Pangavhane and Sawhney (2002) showed
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that pre-treated grapes require nine to ten days, while, Mercer (2012) took five days to dry 5 mm thick mango fruit. Open-air uncontrolled solar drying has relatively low running costs, because it only requires labour. Product quality is compromised and does not meet either national or international standards (Mustayen et al., 2014). This is because the produce is susceptible to infestation by foreign materials, such as dust, insects and micro-organisms. In addition, there is discolouration from ultraviolet radiation and there is the chance of insufficient drying, or over-drying (Falgari et al., 2008; Sharma et al., 2009; Mustayen et al, 2014). Because of its popularity and the lack of control of the drying parameters, this study will consider open air uncontrolled solar drying as a control experiment.
Figure 2.4 Illustration of the working principle of open-air uncontrolled solar drying (after Tiwari et al., 2016)
2.6.2 Direct solar dryers
In direct type solar dryers, the produce is exposed to the sun’s rays. The produce is placed in a chamber that used as a collector (Ogunkoya et al., 2011; Eswara and Ramkrishnarao, 2013). Akarslan (2012) indicated that it differs from open-air uncontrolled solar drying, because a transparent material covers the produce. This reduces the direct convective losses to the surrounding and increases the drying temperature (Jairaj et al., 2009; Akarslan, 2012).
Direct-type solar dryers typically consist of a drying chamber that is made of glass or plastic, as shown in Figure 2.6. The produce is placed in a perforated tray that allows air to flow through the fresh produce (Wakjira, 2010; Eswara and Ramkrishnarao, 2013; Toshniwal and Karale, 2013). The different types of direct solar dryers include solar cabinet dryers,
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greenhouse dryers, staircase dryers and glass roof dryers. These dryers are successful for drying small amounts of high moisture produce, such as mangoes, pineapples, bananas and carrots (Schiavone et al., 2013). In addition, a greenhouse solar dryer is an attractive solution, for drying large quantities of produce of up to 1000 kg (Tiwari et al., 2016).
According to Ramana (2009) and Belessiotis and Delyannis (2010), the initial cost is relatively low, therefore more than 80% of smallholder farmers use this type of dryer.
However, it requires the frequent turning, of the produce for uniform drying, because the essential component of the product is affected by radiation (Schiavone et al., 2013).
This study considered modifying a greenhouse dryer for drying large quantities of produce, to promote the upscaling of production by smallholder farmers. Experiments were carried out and successful implementation was achieved in India, Thailand, Turkey, Uganda and Australia, for drying fruit, vegetables and herbs. It took four to seven days to dry 1000 kg grapes in a solar greenhouse dryer to a moisture content of about 9% (Tiwari et al., 2016).
Schirmer et al. (1996) evaluated a greenhouse solar dryer and found that it took three to five days to dry banana in temperatures ranging from 40-65˚C, and it took five to seven days when drying under open-air conditions. In addition, the essential changes required for improving a greenhouse solar dryer are simple and economical. This study intends to evaluate a modified greenhouse solar dryer under South African conditions.
Figure 2.5 Illustration of the working principle of a direct solar dryer (after Tiwari et al., 2016)
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In direct dryers, the sun does not act directly on the material that is to be dried. According to Akarslan (2012), the crop is placed in trays or shelves inside an opaque drying chamber and heated by circulating air, which is warmed during its flow through a solar collector, as shown in Figure 2.6. They are used for some perishables and fruits. The vitamin content of the product is reduced considerably by the direct exposure to sunlight (Belessiotis and Delyannis, 2010). Indirect solar dryers have a higher drying rate than direct solar dryers and open sun dryers, because of the higher operating temperatures. Nahar (2009) tested the direct and the indirect solar dryers on onions, tomatoes, turmeric, coriander, okra and mints. It was found that it took 20% more time to dry, when using a direct dryer, compared to an indirect dryer. The limitation, however, is that indirect solar dryers require a large capital investment and higher maintenance costs than direct dryers (Belessiotis and Delyannis, 2010). A typical indirect solar dryer consists, of insulated ducting, a drying chamber and a solar collector.
This study did not consider an indirect type solar dryer because of the increased capital costs and relatively low drying capacity associated with implementing the dryer.