DEVELOPMENT AND OPTIMIZATION OF AN AUTOMATED SOLAR POWERED HOT-AIR SUPPLEMENTED DRYER FOR WHITE YAM SLICES

Abstract

Inability to stabilize and regulate temperature level inside solar dryer has been major challenge in achieving uniformly dried biological materials. This study developed solar powered hot-air supplemented dryer (SPHSD) with controlled logging system for 20 kg of yam slices. The SPHSD has 3 sections: solar collector chamber, drying chamber and hot-air supplement chamber powered with 300 watt solar panel and 200 amps battery for continuous operation during bad weather. For drying experiment, sun drying (SD), indirect solar drying (ISD) and solar powered hot-air supplemented drying (SPHSD) were used. Yam slice thickness (3 and 5 mm), air velocities (0.8 and 1.2 m/s), pretreatment methods (blanching, adding sodium metabisulfite (Na2S2O5), soaking in Neem bark extract) and control, drying temperatures (50, 60 and 70 °C) were used for system and process analysis. Experimental data were converted to dimensionless moisture ratio and fitted into fifteen thin layer drying models. The results showed temperature stability inside the drying chamber during peak and off-peak insolation period with ability to attain 70 °C for SPHSD while drying temperature fluctuates in ISD. Results showed that moisture content of white yam slices under blanching, adding Na2S2O5, soaking in Neem bark extract and control pretreatment methods were 73.043, 69.089, 70.354 and 67.867 % w.b. respectively. The SPHSD has a capacity of 20 kg/batch, drying efficiency of 87.020 %, specific energy consumption 4620.571 J/kg.K and thermal efficiency of 52.539 % at optimal condition. Drying curve was found in the falling rate period as there was no constant drying rate for all drying process and Midili et al. model best described drying kinetics of white yam slices under different drying systems and pretreatments. Moisture diffusivity value using Fourier approach increases with increase in drying temperature and air velocity for SPHSD while there was fluctuation in the Deff of white yam slices for ISD and SD. The proximate analysis of 2 thicknesses of dried yam slices under different parameters revealed that slices pretreated with Na2S2O5 has the lowest moisture content (3.079 ± 0.001 - 3.374 ± 0.006), highest carbohydrate content (88.918 ± 0.009 - 89.975 ± 0.017). Neem extract pretreated yam has highest crude fibre content and energy content at (2.205 ± 0.008 - 2.534 ± 0.003) and (363.196 ± 0.051 - 371.398 ± 0.031) respectively. Conclusively, at desirability value of 0.650 the optimal results was obtained using SPHSD to dry 3 mm yam slices giving Deff of 3.38 × (10-10 m2/s), drying time of 309.15 minutes, specific energy consumption of 4746.88 J/kg.K, solar collector efficiency of 91.29 %, thermal efficiency of 51.20 % and drying efficiency of 83.77 %. Likewise, at temperature of 67.73 °C and air velocity of 1.20 m/s, Na2S2O5 pretreated yam has ash content of 1.72, carbohydrate value of 90.01, crude fibre content of 2.25, 3.04 %M.C, protein content of 3.00 and energy value of 327.05 to give yam colour of 1.69 %T. The results therefore showed that SPHSD gave stable and controlled temperature level in the drying chamber which enhanced uniformity and quality of yam slice product.