Thermal Analysis of a Passive Solar Dryer for Paddy Rice Drying

Abstract

This paper presents the thermal analysis of a passive solar dryer for paddy rice drying. The dryer, which was designed with a capacity of two tonnes of paddy rice per batch, was deployed and test run in a rural agrarian community. The thermal analysis of the dryer was carried out using a computational fluid dynamic (CFD) model of the system. The system’s two-dimensional continuity, momentum, and energy equations were developed subject to suitable boundary and initial conditions. The CFD model was executed for a day for which the available experimental data was 11 MJ/day of solar irradiance, mean wind velocity of 0.0186 m/s, and mean ambient temperature of 20°C. The simulation was carried out for nine different mesh sizes ranging from extremely coarse (5378 domain elements with 378 boundary nodes) to extremely fine (56153 domain elements with 1385 boundary nodes). The simulation time for the extremely coarse mesh size was 52.32 minutes, while that for the extremely fine was 180.95 minutes. The temperature, velocity, and pressure distributions of the drying air within the drying chamber were determined for each mesh size. From these, their mean values at given times and the day were calculated. It was found that the finer mesh sizes (33134 domain elements with 1004 boundary nodes to 56153 domain elements with 1385 boundary nodes) gave the same results which agreed with experimental data. The results show that the drying process is effective in harnessing solar energy to heat the chamber with the chamber temperature reaching a maximum temperature of approximately 336 K and an average drying chamber temperature of 315.6K. Possible design improvements to the system are suggested, including the incorporation of forced air circulation and phase change material energy storage.