Design, construction, and performance evaluation of a solar tunnel dryer with an auxiliary flat-plate solar air heater for bitter gourd drying (<em>Momordica charantia</em>)

A solar tunnel dryer and an auxiliary solar air heating unit compatible with the tunnel dryer were designed and constructed. The solar tunnel dryer has a collector chamber and a drying chamber, which are separated by a board with holes in top for air circulation. The drying experiments were conducted for sliced bitter gourd (Momordica charantia) samples in the solar drying system with and without using the auxiliary air heating unit for eight hours in the premises of Eastern University. In order to compare and analyze the performance of the solar drying system, controlled drying experiments were also carried out in open sun on equal mass samples of bitter gourd kept adjacent to the solar drying system. The temperature profiles of the inlet, interface, and outlet of the solar tunnel dryer were compared with ambient temperature, and the wet basis moisture content obtained during the system drying were compared with the open sun drying. The performance of the dryer system has been investigated in terms of temperature profile in the drying area; moisture contents, moisture ratio and effective moisture diffusivity of bitter gourd samples with time of the day. Analysis on bitter gourd samples reveal that the system with the auxiliary air heating unit exhibits enhanced performance compared to the dryer without the auxiliary unit. The moisture ratio decreases exponentially as predicted by logarithmic model. The effective moisture diffusivity values of bitter gourd drying were determined based on Fick’s Second Diffusion model.