Solar Drying: Fundamentals,Applications and Innovations - National ...

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Visavale - Principles, Classification and Selection of Solar Dryers Sharma (1987), described the design details and performance of two types of low cost solar crop dryers, conventional cabinet dryer with direct heating mode and an integrated solar collector-cum-drying system based on the principle of natural convection. Results obtained with several vegetables viz. cauliflower, green peas and potato showed satisfactory overall efficiency and performance of both dryers. Banout et al. (2011) recently had studied the design and performance of a double pass solar dryer for drying of red chilli and Montero et al. (2010) has studied the design and performance for drying of agro-industrial by-products. Several attempts at developing forced convection solar dryers (both of the cabinettype and the tunnel-type) have been investigated and experimented on, over the years, and are described in the literature. The forced convection solar crop dryer design include: direct, indirect and mixed-mode solar-dryers. Comparative studies on these three dryer designs suggested that the performance of the mixed-mode forced convection solar dryer is potentially most effective and it appears to be particularly promising in tropical humid regions. The forced convection type solar cabinet dryer, basically a cabinettype of solar dryer with attached solar air heater and forced airflow, has been considered favorable since such a dryer utilizes solar energy directly, as well as the convective energy of the heated air. Bennamoun (2003), presented the effect of surface of the collector, the air temperature and the product characteristics during solar drying of onion. Nandwani (2007), has discussed the design and development of a multipurpose fruit and vegetable dryer for domestic use. 1.8.2.2. Solar Drying of Marine Products Marine products i.e. fish being highly perishable are processed by freezing, canning, salting and dehydration (Posomboon, 1998). In most of the developing countries, it has been estimated that about 40% of fish landing is still preserved by either sun-drying or salt curing or drying. Prawns are treated with boiling water prior to drying to reduce the microbial flora to an acceptable level and to improve the flavor of dried prawns. Jain (2006) studied the drying of Bombay duck to determine the heat and mass transfer coefficients by solar drying. Bala and Mondol (2001) described the use of solar tunnel drying as an effective method for drying of silver jew fish (Johnius argentatus) along with salting pretreatment and Reza et al. (2009) have optimized marine fish drying using solar tunnel drying. Jain and Pathare (2007) studied the drying behaviour of prawn and chelwa fish (Indian minor carp), by open sun drying. Sankat and Mujaffar (2004) studied the sun and solar cabinet drying of salted shark fillets. Baird et al. (1981) have extensively reviewed the solar drying of seafood by forced air convection and by direct insolation, as well as hot smoking fish with a solar assisted fish smoker. Kittu et al. (2010) had simulated the thin layer drying model of tilapia fish using a solar tunnel dryer. As a number of solar dryers exist for a large variety of products to be dried, the preference of one over the rests depends on a number of factors like ease of material handling, its drying characteristics, final product quality and sometimes the available resources. Table 1.1 (section 1.12) presents a general checklist to be considered in the preliminary selection of a solar dryer, and the steps involved in consideration are described (Perry, 2007). 18 Drying of Foods, Vegetables and Fruits
1.9. WORKING PRINCIPLE Visavale - Principles, Classification and Selection of Solar Dryers Solar energy dryers can broadly be classified into direct, indirect and hybrid solar dryers. The working principle of these dryers mainly depends upon the method of solarenergy collection and its conversion to useful thermal energy for drying. 1.9.1. Open Sun Drying (OSD) Figure 1.10 shows the working principle of open sun drying by using only the solar energy. The crops are generally spread on the ground, mat, cement floor where they receive short wavelength solar energy during a major part of the day and also natural air circulation. A part of the energy is reflected back and the remaining is absorbed by the surface depending upon the colour of the crops. The absorbed radiation is converted into thermal energy and the temperature of the material starts to increase. However there are losses like the long wavelength radiation loss from the surface of crop to ambient air through moist air and also convective heat loss due to the blowing wind through moist air over the crop surface. The process is independent of any other source of energy except sunlight and hence the cheapest method however has a number of limitations as discussed in section 2.1.1. In general, the open sun drying method does not fulfill the required quality standards and sometimes the products cannot be sold in the international market. With the awareness of inadequacies involved in open sun drying, a more scientific method of solar-energy utilization for crop drying has emerged termed as solar drying. Figure 1.10. Working principle of open sun drying (Sharma et al. 2009) 1.9.2. Direct Solar Drying (DSD) The working principle of direct solar crop drying is shown in Figure 1.11, also known as a solar cabinet dryer. Here the moisture is taken away by the air entering into the cabinet from below and escaping through at the top exit as shown in Figure. In the cabinet dryer, of the total solar radiation impinging on the glass cover, a part is reflected back to atmosphere and the remaining is transmitted inside the cabinet. A part of the transmitted radiation is then reflected back from the crop surface and the rest is absorbed by the surface of the crop which causes its temperature to increase and thereby emit long wavelength radiations which are not allowed to escape to atmosphere due to Drying of Foods, Vegetables and Fruits 19
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