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

<br>Afree electricity tandem-twin-hybrid-solar-biomass dryer comprised oftwo drying rooms and operated with solar and biomass energy combustion of10kg rubber wood per hour separately todry Robusta coffee cherries with 3, 6, 9, and 12cmbed thicknesses were studied with the drying completion time (<i>tc</i>), number ofdefects (<i>ND</i>), and colour parameters, i.e., lightness (<i>L</i>*), hue angle [<i>H</i>(^o)], and chroma (<i>C</i>), used asthe performance indicators. The experimental results indicated that the drying room, bed thickness, and drying room-bed thickness interaction significantly affected the <i>tc</i> and <i>ND</i> and bed thickness only significantly affected <i>C</i> for both the solar energy drying and the biomass energy drying. The solar energy drying generated adrying air temperature of44.6± 3.5C with a<i>tc</i> of70.9–90.2h for the front drying room and 40.1± 2.8C with a<i>tc</i> of77.2–116.5h for the rear drying room, whereas the biomass energy drying produced adrying air temperature of57.2± 3.6C with a<i>tc</i> of34.1–44.9h for the front drying room and 45.6±6.0C with a<i>tc</i> of56.3–96.6h for the rear drying room. Both drying processes produced coffee beans with the <i>ND</i>s less than11qualified for Grade1 with similar colour characteristics.