DEVELOPMENT AND PERFORMANCE EVALUATION OF AN INDIRECT ACTIVE SOLAR DRYER FOR COWPEA (Vigna unguiculata) AND CONTROL OF THE STORAGE PESTS

Abstract

An active solar dryer for cowpea (Vigna unguiculata) was designed, fabricated and assembled. The major components were: the solar collector, thermal storage device, moist air exhaust duct, air-ducts, and tray. The materials of construction were basically plywood, mild steel sheets and white mica; which were selected based on cost, efficiency and ease with which the material could be worked upon. The dryer was evaluated using cowpea as test material. The evaluation focused on the technical performance criteria of the dryer including the equilibrium moisture content attained as drying load varied from 10 – 50% of the designed capacity, the thermal energy storage performance based on the temperature difference between the heat storage chamber air temperature at the onset of the rest period (5pm local time) and the temperature at the beginning of the active period (8am) of the dryer; and the quality of the cowpea dried in the dryer based on the level of storage stability after a 6-month storage period. The effect of thermal processing temperature and exposure time on storage stability of cowpea at interval of 10 o C from 40 o C to 80 o C and samples drawn at 30 minutes interval for 150 minutes using thermostatically controlled cabinet dryer. Results showed the mean values for the three technical criteria namely: solar collector efficiency, system drying efficiency, and pick-up efficiency were respectively 25.5%, 21% and 30.2%. Observations showed that the system drying efficiency was marginally affected by the drying load (depth) with 18% minimum and 26% maximum; while the pick-up efficiency was significantly affected by drying load, moisture content, and drying period at both 1% and 5% levels of significance with 6% minimum and 53% maximum at 10% and 40% of the designed load capacity, respectively. Mathematical equations derived in this study for predicting these three echnical criteria did not differ significantly from the results of the existing models at either 5% or 1% level of significance, since F-calculated was consistently lower than F- tabulated. The thermal elevation (efficiency of 90.8% maximum and 26.4% minimum) for solar absorber recorded was quite typical. The equilibrium moisture content ranged from 6.1 to 13.4% (wb) as loading depth varied from 60 mm to 300 mm. The granite chips as heat storage device stabilized air temperature inside the dryer to a range of 52 – 63 o C, irrespective of the solar collector and ambient air conditions; and the range of nocturnal drying of 14-32% is quite typical. The ANOVA means showed that thermal treatment temperature and exposure time, and the interactive effect of the two factors respectively, affects the storage stability of the cowpea significantly after 6-month storage period. The ANOVA showed that while the cowpea storage stability improved from 0 to 99.402% as the treatment temperature moved from 34 o C to 80 o C, the time factor was 55 – 71% as the exposure time was increased from 30 – 150 minutes. The temperature time ANOVA means showed that the storage stability varied from 0% to 100% as the two factors increased. The dryer effectively achieved drying of cowpea and pre-storage thermal checking of the bruchid in one operation, thereby reducing cost and making the cowpea free from synthetic chemicals.