Development of a System Model for an Indirect Passive Solar Dryer with Experimental Validation

Solar crop drying is a cheap and effective way to preserve food material, especially in developing countries where fuel and electricity are expensive or unavailable. Some tropical fruits are difficult to transport and store leading to significant spoilage. Without access to fuel and large drying systems, preserving fruits for later use is challenging or not possible for the rural farmer. Developing low-cost, easily assembled locally and low maintenance fruit drying systems would improve access to off-season and distant markets. A mathematical model of an indirect passive solar drying system was developed to design and optimize drying systems for use in developing countries and was validated through experimental testing. The prototype drying system consisted of a transpired solar absorber, drying chamber, and chimney. The transpired solar collector allows for indirect heat gain using cheap materials, specifically landscape fabric. The drying chamber houses fruit on eight screen trays and the chimney induces airflow through the system without a power source. The novel collector efficiency regularly exceeded 50% with an average temperature rise over 20oC. Bananas were dried over a two day period from an average moisture content of 73% to 8%. A total of 4kg of banana slices per square meter of absorber area were dried over the testing period. The mathematical model uses solar irradiance, ambient relative humidity, ambient temperature, and initial fruit moisture content to predict the fruit drying curves. The predicted average final moisture content of the bananas starting at 73% was 9% over the two day test period, indicating the model predicts performance reasonably well. Results from the system model highlight the need for additional experimentation to determine parameters such as the diffusivity of bananas and the mass transfer coefficient independently of experimental setup before it can be used as a tool to simulate drying performance for different environmental conditions and dryer system configurations