Energy and exergy analyses of a mixed-mode solar dryer of pear slices (Pyrus communis L)

A mixed-type solar dryer operating with three different modes of operation (mixed-mode natural convection (MM-NC); mixed-mode forced convection (MM-FC) and indirect-mode natural convection (IM-NC)) were evaluated to reduce the moisture content of pear of 82.15 +/- 0.13% up to 8.86 +/- 1.3% (wet basis). The drying times were 5.25, 4.5, and 6.6 h; the drying efficiencies were 13.6, 11.2, and 26.6%, and the average instantaneous thermal efficiencies of the solar air heater were 33.0%, 56.3%, and 30.9% for the MM-NC, MM-FC, and IM-NC modes, respectively. The drying chamber's exergy efficiency values in the MM-NC mode was 65.2%, while the lowest values were in the MM-FC mode (54.4%). The MM-FC mode had a more significant improvement potential (308.76 kJ) than the other modes. The highest average value of the energy utilization ratio was 29.1% in the IM-NC mode. The experimental data shows that the MM-NC mode had a higher exergy efficiency than the other two operation modes. The IM-NC mode shows a lower improvement potential, taking better advantage of the supplied energy. Having these results would be expected that by increasing the load, the solar dryer's thermal and exergy efficiency in the MM-NC mode and MM-FC mode could be improved. (c) 2020 Elsevier Ltd. All rights reserved.