The thermal and drying efficiency of three operating configurations for a hybrid solar-gas dryer were calculated in transitory state. The hybrid dryer, comprising a solar collector, auxiliary LPG (liquid propane gas) combustion heater, and a drying chamber, can be operated through an LPG (GHS) heating system, a hybrid solar-gas (HHS) heating system, or a solar (SHS) heating system. Global efficiency was calculated considering the energetic contributions of the solar collector and/or the auxiliary heating system, in accordance with the mode of operation being evaluated. Losses resulting from reflection and absorption were considered in the analysis of the solar collector. The thermal efficiency of the collector was principally affected by air mass flow, collector angle of inclination, and the difference between ambient temperature and the collector's internal temperature. A simulation varying air velocity parameters inside the solar collector was utilized to estimate the air mass flow needed to produce a thermal efficiency greater than the efficiency calculated under current design and operational conditions (26%). Maximum drying efficiencies were 86%, 71%, and 24% for GHS, HHS, and SHS, respectively. HHS and GHS exhibited similar drying rates in the constant period of the curve (similar to 0.030 kg H2O/kg d.s. min). The efficiency of the hybrid drying system was similar to the LPG drying system, with the advantage of consuming 20% less fuel without sacrificing quality in the dried product. (C) 2013 Elsevier Ltd. All rights reserved.