A simulation model of hybrid solar drying as alternative sustainable drying system for herbal and medicinal plants was developed. Heat absorbed from the solar radiation, heat gained by the collector and supplemented by the burner, heat gained or lost by the product, heat gained or lost through the drying bin wall, heat gained or lost with the drying air and the latent heat of the moisture evaporation from the product were the main components of the equations describing the drying system. The model was able to predict the moisture loss from the product at a wide range of temperatures and air recirculation percentages (70, 80 and 90%). The
energy consumption for drying herbs was investigated by using of different energy sources including solar energy, diesel and propane fuels. Experiments were conducted to validate the simulated model results. The
results indicated that the solar collector could raise the temperature by 5-13 oC. The mint moisture content was dried from 80% to MC ranged from 10.99 to 13.09% when it was dried at 60oC and the drying air was recirculated at 70 to 90% during the month of January, meanwhile, it reached from 5.27-8.6% at the same percentages for air re-circulation during July. The energy consumption was 31.81 MJ/kg dried mint at 90% of
fresh air recirculation while it was 42.91 and 53.95 MJ/kg for 80 and 70% of air recirculation percentages, respectively during January. During July, the energy consumption for water evaporation was 24.18 MJ/kg dried mint at 90% of fresh air recirculation while it was 30.56 and 36.89 MJ/kg for 80 and 70% of air recirculation
percentages, respectively. On the other hand, using the solar collector contributes by 45% of the total energy required for drying the mint at 60oC and 90% air recirculation percentage during January, while it was 52% contribution during July at the same conditions. One kg dried mint costs LE 1.32 and 1.00 under the hybrid system during Jan. and July months, while it was LE 2.39/kg and 2.08 under the artificial drying system during the same months using propane fuel. In case of using diesel fuel; it costs 2.43 and 4.4 LE/kg and 1.84 and 3.82 LE/kg for the hybrid and the artificial systems, during January and July, respectively, when 90% of the
drying air was re-circulated. The model results were validated at 90% of air recirculation and showed reasonable agreement with the experimental ones in temperature, moisture content of mint and energy consumption.