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Hii and Law - Solar Drying of Major Commodity Products 0.48 kg or 22.2 MJ per kg of cured leaves was achieved during drying. The payback period for the dryer was reported at 7 years. 3.2.8. Timber and Wood Timber drying is a highly energy intensive process and this operation consumes about 70% of the total energy used in the production of most wood products (Taylor et al., 1996). Solar drying research on timber was first initiated in 1961 in India as cited by Satar (1994). The following are some aspects that determine the technical feasibility and economic viability of solar timber drying (Satar, 1994): • Temperature and humidity: proper regulation of temperature and humidity to prevent damaging effects to the timber • Drying time: increased in drying rate justified for the use of solar dryer • Drying quality: as above, the better product quality justified the application of solar drying • Thermal energy: this relates to the efficiency of the dryer, which is largely attributed to insulation of the kiln Satar (1994) investigated the drying of several species of hardwood planks and the findings of the studies are shown in Table 3.7. Solar drying can dry timber to a much lower moisture content which cannot be achieved by air drying. Table 3.7. Results from drying of several species of hardwood planks Drying period Drying time (days) from green to 12% moisture content Solar Air Kiln Winter period (Nov. – March) 10 - 26 22 - 68 6 - 12 Post winter period (April – May) 12 - 30 32 - 79 6 – 14 Monsoon period* (June – August) Post monsoon period (Sept. – Oct.) 18 - 40 58 - 123 6 – 14 12 - 32 36 - 84 6 – 14 *18 – 20% mc Studies using solar dryer, electric resistance dryer and dehumidifier were carried out on sawn timber planks by Ong (1999). The solar dryer measured 3 m long x 1.22 m wide with sides and bottom constructed from 50 mm thick polyurethane slabs laminated with iron sheets. The capacity of the dryer was 3.1 m3 and covered with 4 mm glass window inclined at 7°. Side vents allowed air to ventilate and two swing doors at the front of the dryer were used for loading purpose. A black painted corrugated galvanized iron sheet was placed below the glass roof to act as solar collector while a 40 mm diameter axial fan was used to distribute 3000 m3/hr of drying air. Comparison shows that electric drying was fastest with constant energy input throughout the day (Table 3.8). Maximum temperature was recorded inside the solar dryer at mid-day at 50°C. The following results were obtained from the studies. Ong (1999) reported that solar drying 88 Drying of Foods, Vegetables and Fruits
Hii and Law - Solar Drying of Major Commodity Products achieved a faster drying rate compared to natural drying but the difference was marginal. Table 3.8. Comparison of drying durations from different drying methods Electric dryer Solar dryer Dehumidifier No of days 8 12 12 Moisture content 44% to 15% 42% to 15% 84% to 15% Luna et al. (2009) analyzed the evolution of solar timber kilns and recommended the various components of design for future adaptation. The analysis was carried out based on organic approach and the degree of development was later analyzed using the laws of technological system evolution derived from the TRIZ theory. Based on the analyses, three arrangements of solar kiln can be classified in the existing design: • Arrangement 1: solar kiln with integrated collector in which all units are constructed into one • Arrangement 2: solar kiln with lateral semi-integrated collector where the units are linked only by the interaction components, the air ducts and the fan. • Arrangement 3: Solar kiln with thermal energy storage By applying the TRIZ theory and taking into consideration the degree of evolution that has been taking place in existing kiln design, a new design was proposed that consist of the following: • Type 2 arrangement (Figure 3.11) • Use of storage with independent heating • Integration of an heater in the storage system • Management of different drying cycles according to product quality Solar collector (water) Air in Fan Drying chamber Storage Solar collector (air) Air out Figure 3.11. The proposed solar dryer with energy storage Based on this design, drying model and simulation was carried out with and without energy storage (Luna et al., 2010). The energy storage enables drying to be continued at nighttimes. The storage unit used water as the storage fluid in a reservoir and air as the Drying of Foods, Vegetables and Fruits 89
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Solar Drying:Fundamentals,Applicati
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PREFACE Drying using solar radiatio
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Visavale - Principles, Classificati
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