Optimisation and 4E analysis of hybrid indirect solar electric dryer (HISED) for bitter gourd drying

• Hybrid Indirect Solar Electric Dryer (HISED) with ETSC optimized for bitter gourd. • RSM-BBD used to analyse effects of air velocity, mass, and slice geometry. • Achieved 13.9% final moisture, 3.98 kWh energy consumption, and 76.4% shrinkage factor. • System reached 70.25% thermal and 6% exergy efficiency with strong model fit. • Economic analysis showed 85-month EPBP and 1,126 kg CO 2 reduction per year. This study evaluates a Hybrid Indirect Solar Electric Dryer (HISED) integrated with an evacuated tube solar collector (ETSC) for optimising the drying of bitter gourd (Momordica charantia) slices. A Box-Behnken Design (BBD) combined with Response Surface Methodology (RSM) is applied to analyse the effects of air velocity, product mass and slice geometry on key drying parameters: moisture content (MC), energy consumption (EC) and shrinkage factor (SF). Optimal conditions are achieved at an air velocity of 0.286 m/s, product mass of 2 kg, and circular geometry, resulting in 13.9% final moisture (dry basis), 3.98 kWh energy consumption and a shrinkage factor of 76.4%. Model predictions closely matched experimental values, with errors below 10% and R 2 values exceeding 0.97. The HISED-ETSC system produced outlet air temperatures of 59–63 °C and drying chamber temperatures of 85–97 °C. Across 15 trials, blower power ranged from 2.1–3.1 kW, while the air preheater required 12–20 kW. The system achieved a thermal efficiency of 70.25%, with electrical efficiency between 5.9 and 12.5% and exergy efficiency of 6%. Among tested drying models, the Midilli–Kucuk model provided the best fit, yielding R 2 = 0.9965, RMSE = 0.449, χ 2 = 0.302 and MAE = 0.372. Economic and environmental analysis showed an Energy Payback Period (EPBP) of 85 months, annual savings of $203.76 and CO 2 reduction of 1,126.08 kg/year, with a total embodied energy of 972 kWh.