A large-scale solar greenhouse dryer using polycarbonate cover: Modeling and testing in a tropical environment of Lao People's Democratic Republic

摘要

A large-scale solar greenhouse dryer with a loading capacity of 1000 kg of fruits or vegetables has been developed and tested at field levels. The dryer has a parabolic shape and the dryer is covered with polycarbonate sheets. The base of the dryer is a black concrete floor with an area of 7.5 × 20.0 m~2. Nine DC fans powered by three 50-W solar cell modules are used to ventilate the dryer. The dryer was installed at Champasak (15.13 °N, 105.79 °E) in Lao People's Democratic Republic (Lao PDR). It is routinely used to dry chilli, banana and coffee. To assess the experimental performances of the dryer, air temperature, air relative humidity and product moisture contents were measured. One thousand kilograms of banana with the initial moisture content of 68% (wb) was dried within 5 days, compared to 7 days required for natural sun drying with the same weather conditions. Also three hundred kilograms of chilli with the initial moisture content of 75% (wb) was dried within 3 days while the natural sun drying needed 5 days. Two hundred kilograms of coffee with the initial moisture content of 52% (wb) was dried within 2 days as compared to 4 days required for natural sun drying. The chilli, coffee and banana dried in this dryer were completely protected from insects, animals and rain. Furthermore, good quality of dried products was obtained. The payback period of the dryer is estimated to be 2.5 years. A system of partial differential equations describing heat and moisture transfer during drying of chilli, coffee and banana in the greenhouse dryer was developed. These equations were solved by using the finite different method. The simulated results agree well with the experimental data. This model can be used to provide the design data for this type of dryer in other locations.