RENEWABLE ENERGY TECHNOLOGIES IN ASIA: A Regional Research and Dissemination Programme Phase III SOLAR BOX DRYER DESIGN, CONSTRUCTION AND OPERATION MANUAL Energy Field of Study School of Environment, Resources and Development Asian Institute of Technology P.O. Box 4, Klong Luang Pathumthani 12120, Thailand February 2003
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Abstract A direct-solar box-type solar dryer suitable for household drying of agricultural products has been developed at AIT. The dryer can dry 4-5 kg of fruits and vegetables in a single batch, at a temperature of about 50-60 C. The performance of the box dryer was evaluated as per an evaluation procedure for solar dryers, which was also developed at AIT. A comparison of the test results with a solar tunnel dryer indicate superior performance of the dryer, considering not only the thermal performance but also factors such as loading/unloading convenience, operation and maintenance, quality of dried products, floor area requirement for dryer installation and cost of dryer. 3
Contents 1. 2. 3. 4. 5. 6. Introduction Design Details and Drawings Construction Dryer Performance 4.1 Drying Curves 4.2 Temperature Profile in the Dryer 4.3 Drying Efficiency Results and Discussion Performance comparison with Solar Tunnel Dryer 5 5 7 10 11 12 13 13 14 4
1. Introduction A direct-solar box-type dryer has been developed after optimising the design parameters through a detailed study. This natural convection type dryer was aimed at rural households in countries like Cambodia and Lao PDR, as an alternate to the expensive tunnel dryer. Generally, direct solar dryers are used to dry small quantities of food at a time, and provide moderate drying temperatures (40-60 C) and airflow rates. They are easily and inexpensively built out of locally available materials, and operation is straightforward. Crops such as coffee, paddy, grains, and fruits and vegetables can be dried successfully and economically in this kind of dryer. 2. Design Details and Drawings The design of this dryer was based on thermal performance and product quality optimization. The box dryer consisted of a rectangular box made of GI sheet, with a open top. The box was insulated at the outside with glass wool, and clad with GI sheet. A 3mm window glass covered the top of the box, and was hinged to the box at the left edge. This facilitated opening and closing of the cover glass, for access into the box. Figures 1 and 2 illustrate the dimensional and construction details of the dryer. 700 620 135 1520 Outlet opening Inlet opening (wire mesh) 590 x 80 40 Aluminium 'L' angle frame Not to scale G I sheet box All dimensions are in millimetres Figure 1: Isometric view of inner box 5
Glass cover A - A 40 105 35 Glass wool insulation 620 700 G.I. sheet cladding Section A - A Aluminium channel Rubber beading Glass cover Aluminium hinge Rubber sheet insulation Aluminium angle frame Glass wool insulation Not to scale All dimensions are in millimeters Figure 2: Sectional views of the box dryer 6
The optimised dryer operated in natural draft principle. While the hot and humid air escaped through the opening at the top, fresh ambient air entered through the bottom opening. The physical parameters of the dryer are presented in Table 1. Table 1: Physical parameters of the box dryer Type of dryer Direct solar Overall size - length x width (cm) 152 x 70.5 Height (cm) 15.5 Aperture area 0.91 Loading (tray) area (cm 2 ) 0.84 Air inlet/outlet areas 472 cm 2 each (59 cm x 8 cm) Box material GI sheet, 22 SWG Tray materials Aluminium frame; SS wire mesh Glazing 3 mm window glass Gap: Tray surface-cover glass (cm) 10.0 Gap: Tray surface - Dryer bottom 2.5 surface (cm) The dryer has two trays kept inside the box, for loading the products to be dried. The trays were made of aluminium angle frames and stainless steel wire mesh. The critical parameters set inside the dryer box are shown in the last two rows of Table 1. The distance between the tray surface, cover glass and dryer bottom surface is kept such that air flows both above and below the tray, for maximum drying effect. 3. Construction The dryer construction is started with the fabrication of the inner box. Aluminium L angles are used to construct the box frame, which is then covered with a GI sheet at the back and two sides. Figure 3: Fabrication of dryer frame 7
The outer box is made of GI sheet, but without a frame. As in the inner box, two sides of the outer box are also kept open. The outer box is then filled with a fibreglass wool mat of 50 mm thickness, and the inner box is placed over it. The sides are then packed, screwed and sealed with silicone sealant to make the joints watertight. The step-by-step construction of the dryer is illustrated in the photographs in Figures 3-6. Figure 4: Completed Dryer Figure 5: Fixing of Cover Glass 8
Figure 6: Insulation with fibreglass wool The dryer box is placed on a mild steel stand, which provides a tilt of 15 to the dryer (Figure 7). The dryer is placed south, to maximise the solar radiation. The fully assembled dryer is shown in Figure 8. Figure 7: Stand for the Dryer (15 slope) 9
Figure 8: Photograph of the fully assembled box dryer 4. Dryer Performance The dryer was tested with mushroom as the product. Four kilograms of ear-lobe mushroom was loaded in the dryer (Fig. 9). Control samples were dried in the open sun also, in another wire-mesh tray, of 20cm x 45cm size. Figure 9: Mushroom loaded in the dryer 10
The loading density in the dryer sample trays were kept the same as that in the main trays and in the open sun drying tray for consistency. Figure 10: Open sun-drying sample of mushroom Results of the experiments were analysed by drawing the drying curves and by estimating the drying efficiency. The results of three days of drying is presented in Table 2. 4.1 Drying curves: Drying curves were drawn for the samples taken from two sample trays in the dryer. The sample from open sun drying was also included, for comparison (Fig. 11). 1.00 0.90 0.80 0.70 Moisture content 0.60 0.50 0.40 0.30 0.20 0.10 m.c.-tray top m.c.-tray bottom m.c. open 0.00 0 2 4 6 8 10 12 14 Drying time, hours Figure 11: Drying curves for ear-lobe mushroom 11
4.2 Temperature profiles in the dryer: The following graphs provide the detailed temperature data at the top, middle and bottom trays. Global solar radiation on the collector/dryer plane (i.e., at 15 slope, facing south) is also plotted in the graphs. Day 1: Temperature, 0 C 120 110 100 90 80 70 60 50 40 30 20 Tray temp in top section Tray I Tray II Tray III Glo (15deg) 1400 1200 1000 800 600 400 200 0 Insolation, W/m 2 845 945 1045 1145 1245 Time, hours 1345 1445 1545 1645 Figure 12: Radiation and temperature profile for day 1 (9 November, 1999) Day 2: Temperature, o C Tray temp. in top-section 120 110 100 90 80 70 60 50 40 30 20 Tray I Tray II Tray III Glo (15deg) 1200 1000 800 600 400 200 0 Insolation, W/m 2 845 945 1045 1145 1245 Time, hours 1345 1445 1545 1645 Figure 13: Radiation and temperature profile for day 2 (10 November, 1999) 12
The experiment was continued for the third day until 11:00 a.m. because of insufficient solar radiation to complete the drying in two days. 4.3 Drying Efficiency: The drying efficiency values for the three days of the experiment are presented in Table 3. Table 3: Daily drying efficiencies during the three days of drying (9-11 November 1999) Insolation at 15 slope (MJ/m 2 ) Day 1, 9 Nov.99 Insolation (MJ/dryer area) Useful energy (MJ) Efficiency Solar 13.47 12.02 6.08 50.5 Open sun 13.47 0.67 55.0 Insolation at 15 slope (MJ/m 2 ) Day 2, 10 Nov.99 Insolation (MJ/dryer area) Useful energy (MJ) Efficiency Solar 11.52 10.28 1.97 19.2 Open sun 11.52 0.24 23.2 Insolation at 15 slope (MJ/m 2 ) Day 3, 11 Nov.99 Insolation (MJ/dryer area) Useful energy (MJ) Efficiency Solar 5.66 5.05 1.73 0.087 Open sun 5.66 0 0.0 The results indicate drying to be the most efficient during the first day of drying, when the process is mostly in the constant-rate drying period. Products were dried to their final moisture content of average 8% in 50 hours. Quality of the product was found generally good. Drying is expected to be completed in less than 32 hours during clear sunny days. 5. Results and Discussion The results of the drying experiments conducted on the above dryer are compiled in Table 4. The first day drying efficiency of 50.5% was recorded. The study on the effect of airflow rate across the dryer (by adjusting the area of inlet and outlet openings) reveal that drying is considerably affected by restricted airflow. An optimum inlet/outlet area of 472 cm 2 (59 x 8 cm) was noted to give the maximum drying efficiency. It was found that allowing air to flow partly above and partly below the tray/product offered the best performance results. 13
Solar radiation (MJ/m 2 ) Average tray temperature ( C) Table 4: Experimental results Maximum tray temperature ( C) Daily drying efficiency (%) 13.47 43.77 64.0 50.5 11.52 46.90 68.5 19.2 5.66 55.10 72.2 0.09 Product quality was evaluated throughout the experiments. It was seen that the dried mushrooms were of good quality. Open sun dried products, although of acceptable quality, were slightly inferior due to contamination by dust. Quality difference may be substantial for products such as fruits and certain other vegetables. 6. Performance comparison with Solar Tunnel Dryer As regards the thermal performance, the first day drying efficiency is likely to give a reasonably accurate evaluation on the thermal performance of a solar dryer. The results of the previous experiment have been compared with the performance of a solar tunnel dryer. The data for tunnel dryer have been taken for a day which had similar radiation levels as that during the box-type dryer experiment. Table 5 presents the comparison. 14
Table 5: Comparative evaluation of direct solar box-type dryer and solar tunnel dryer Product: Ear-lobe Mushroom; Initial Moisture Content: 92% (w.b) Global radiation on 15 sloped surface for day1/day2/day3 (for box dryer): 13.47/11.52/5.66 MJ/m 2 Average ambient relative humidity for day1/day2/day3 (for box dryer): 65/62/74 % Global radiation on horizontal surface (for tunnel dryer) for day 1/day2: 17.3/14.6 MJ/m 2 Average ambient relative humidity (for tunnel dryer) for day1/day2: 60/78 % Parameter Solar tunnel dryer 1 Direct solar box type dryer Dryer Type Direct, forced convection, tunnel type Direct, natural convection, box type Airflow rate 360 m 3 /hr Not available Solar aperture area 14.85 m 2 0.91 m 2 Dryer (tray) area 6.80 m 2 0.84 m 2 Solar insolation during the day 13.13 MJ/m 2 13.47 MJ/m 2 Load (Ear-lobe mushroom) 21 kg 4 kg Loading density 3.09 kg/ m 2 4.76 kg/ m 2 Drying time 35 hours 50 hours Max. drying temperature - with load 64 C 69 C Max. drying temp. at no-load 69 C 76 C Duration of drying air temperature 10 C above ambient (with load) 4.5 hours 5.5 hours [day 2: cloudy; 30 C<Tamb<33 C] Full load capacity 32.4 kg 4 kg Average first day drying efficiency 17.2% 50.5% Loading time 1 min/kg 1 min/kg Unloading time 0.5 min/kg 0.5 min/kg Loading/unloading convenience Dryer operation and maintenance Quality of dried product Requires electricity/battery to run the fans; more no. of components that require maintenance Very good taste; excellent colour no electricity/battery required; maintenance practically limited to glass only Generally good; slight variation in taste & colour due to higher temperature drying (compared to tunnel dryer) Floor area requirement for dryer 16 m 2 1.2 m 2 installation Cost of dryer US$ 1,100 US$ 90 General comments Suitable for professional large-scale drying; very good product quality Suitable for households and single farmers; ideal for small quantity highvalue products Overall evaluation 1 Data from previous experiments conducted for the RETs in Asia Programme : poor; : best The table above clearly illustrates the superior thermal performance (in terms of drying or system efficiency) of the direct solar box-type dryer over the solar tunnel dryer. This is largely due to the absence of a separate absorber in the box dryer, where the product itself acts as solar absorber. 15
In the above comparison, however, loading density in the tunnel dryer was considerably lower than that in the box-type dryer. In other words, the tunnel dryer was not loaded to its full capacity (which was estimated to be about 32.4 kg, for the same loading density) unlike the box-type dryer. This is likely to be the reason for the slightly shorter drying time in the tunnel dryer. The efficiency of the tunnel dryer (as well as the drying time) is expected to increase moderately when it is loaded fully. In the comparative evaluation, it can be seen that both dryers have advantages as well as disadvantages on different aspects. In any case, the applications of the two dryers are different. While the tunnel dryer suits well for the requirements of small farmers and cooperatives for probably commercial drying, the box-type dryer is likely to be preferred for household drying. The overall evaluation shows that the box-type dryer scores better in important user considerations such as cost, floor area requirement, loading/unloading convenience, and maintenance requirements. The investment requirements for the two dryers differ widely, and so is the drying capacity. Specific advantages in one type of dryer may be important for one user, while they may be of little consideration for another. Therefore, ultimately, it is the user who can select the right type of dryer for his/her unique drying requirements. 16