Drying behaviour of lemon balm leaves in an indirect double-pass packed bed forced convection solar dryer system

Abstract

The thin-layer drying behaviour of lemon balm leaves in an indirect-mode solar dryer with forced convection was investigated. The solar air heater employed a novel counter flow double-pass packed-bed wire mesh layer configuration to enhance the energy gain of the heated air. The thermal performance of the solar dryer was experimentally evaluated under Babol (36.5387°N, 52.6765°E) prevailing weather conditions. The system was comprised of a double-pass packed bed of wire mesh air heater attached to a dryer cabinet. The collector thermal efficiency was determined under realistic conditions wherein solar irradiance changed from 600 to 900 W/m2, atmospheric temperature varied from 22 °C to 25 °C and the collector outlet temperature ranged from 38 °C to 68 °C. It was found that while increasing the flow rate from 0.006125 m3/s to 0.01734 m3/s improved the collector thermal efficiency by ~ 20%; further increasing the flow rate to 0.034378 m3/s had an adverse effect on the collector efficiency. Drying experiments were performed for lemon balm leaves with initial moisture content of 80% on wet basis to the final moisture content of 10%. Mathematical models were also tested to find the best thin layer model for describing the drying behaviour of lemon balm leaves.