Experimental thermodynamic assessment of an LPG-Assisted hybrid dryer for Night-Time podded bean Drying: Energy and exergy analyses

Hybrid dryers extend drying beyond sunshine hours, but the off-sunshine LPG behavior of practical tri-mode dryers remains less well documented than their daytime solar mode, especially when the thermodynamic boundary conditions, rate notation, and product-state limitations are rigorously stated. This study assessed a field-fabricated tri-mode hybrid dryer under LPG-only night-time operation for podded-bean drying and resolved its air-side energy utilization, useful exergy distribution, and overall irreversibility level. Three independent night-time runs were conducted from 19:00 to 06:00 on 20 kg batches. The key parameters, including temperature, relative humidity, and air velocity were measured using K-type thermocouples, digital thermo-hygrometers, and a BENETECH GM8903 hot-wire anemometer, respectively. The LPG consumption was determined gravimetrically from cylinder mass loss, and batch mass change was measured using a hanging scale; temperature control was maintained using an STC-3028 temperature controller. The energy and exergy balances were formulated on a quasi-steady basis with solar exergy excluded from the LPG boundary and fan electrical power treated as an exergy rate. The results reveal that the mean air-side energy utilization efficiency was 76.5% and the mean exergy efficiency was 72.8%. Of the supplied exergy, 54.5% supported moisture evaporation, 18.3% supported sensible heating of the drying air, and 27.2% was destroyed by overall system irreversibility. The chamber temperature was intentionally maintained below 30 °C to sustain gentle overnight drying rather than aggressive high-temperature heating. These findings establish a thermodynamic baseline for LPG-only operation, highlighting opportunities to improve airflow control, heat transfer, and insulation, and supporting future analysis of drying kinetics.