Design and performance analysis of PID controlled geothermal and solar hybrid rack type dryers

In this study, the drying performance of three different food products of a dryer, in which solar and geothermal energy are used together, was investigated in order to ensure an uninterrupted operation in drying processes. This study compares the performance of a geothermal and solar hybrid indirect cabinet dryer with only solar indirect cabinet dryer and traditional shade drying method. In experiments conducted in three groups, the performance of the hybrid dryer was investigated by drying three different tomato sizes in the first group and half size tomatoes in the second group under natural and forced convection conditions. In the third group, local okra samples were studied under forced convection in completely closed, partly cloudy and completely open weather conditions. The drying curves of the dried samples in these three groups of experiments were calculated according to the drying methods, tray level and position on the tray. In all experiments, all of the samples in the hybrid dryer dried earlier than the other drying methods. In all experiments, the drying rate of the hybrid dryer is higher than the other drying method. Forced convection conditions and decreasing tomato size increased the drying rate. In this study, energy and exergy analysis were applied to hybrid and solar dryers and compared. The energy and exergy efficiency of the geothermal heat exchanger unit gave close results in all experimental groups. While the air heated solar collector had an exergy efficiency of around 3% in natural convection experiments, it increased to a maximum of 7% under forced convection conditions. In addition, the SMER, SEC, and energy fractions of the hybrid dryer were determined in the experiments performed in three groups. Temperature and humidity distributions in all components of both dryers were analyzed. In the economic analysis of the hybrid dryer, the payback period varied between 0.17-0.63 years, depending on the type of product dried, product size and the circulation of the drying air in the dryer. Color analysis was performed on the dried samples. In the color analysis, samples at tray-2 level generally gave results close to the color criteria determined in the fresh sample. The color quality of the dried tomato samples gave values above the acceptable level. Finally, aflatoxin analysis was performed on the dried samples. The presence of aflatoxin was not found in the analyzed samples.