Free electricity tandem-twin-hybrid solar-biomass dryer increased the performance of coffee cherry drying

A free electricity tandem-twin-hybrid-solar-biomass dryer comprised of two drying rooms and operated with solar and biomass energy combustion of 10 kg rubber wood per hour separately to dry Robusta coffee cherries with 3, 6, 9, and 12 cm bed thicknesses were studied with the drying completion time (tc), number of defects (ND), and colour parameters, i.e., lightness (L*), hue angle [H(o)], and chroma (C), used as the performance indicators. The experimental results indicated that the drying room, bed thickness, and drying room-bed thickness interaction significantly affected the tc and ND and bed thickness only significantly affected C for both the solar energy drying and the biomass energy drying. The solar energy drying generated a drying air temperature of 44.6 ± 3.5 °C with a tc of 70.9-90.2 h for the front drying room and 40.1 ± 2.8 °C with a tc of 77.2-116.5 h for the rear drying room, whereas the biomass energy drying produced a drying air temperature of 57.2 ± 3.6 °C with a tc of 34.1-44.9 h for the front drying room and 45.6 ± 6.0 °C with a tc of 56.3-96.6 h for the rear drying room. Both drying processes produced coffee beans with the NDs less than 11 qualified for Grade 1 with similar colour characteristics.