Framework for design and performance evaluation of natural convection mixed mode solar dryer

Natural convection mixed mode solar dryer of tunnel type adopted by Tanzania Industrial Research and Development organization (TIRDO) from university in the design of Hohenheim University in Germany for curbing postharvest losses is involved for this research work. The research targeted the development of a framework for performance improvement and evaluation of natural convection mixed mode solar drier. Therefore, experimental testing and evaluating the scaled down existing TIRDO tunnel dryer, computer simulation for prediction of the scaled down dryer, and redesigned full scaled dryer operating parameters were done. Research results reveal uniform variation of temperature inside the dryer; inverse uniform variation of relative humidity and wind speed to temperature, and unrelated variation of ambient temperature and temperature inside the dryer. Also the platform part carrying the air heater retained higher temperature (35°C) than that inside the dryer (29°C). The full scale redesigned dryer has the following expected improved parameters: the air flow rate, of 4500 m3/h, drying time of 9 •hour (including nights) for up to 11% moisture content, heat collection efficiency of 27%, pick-up efficiency, up to 11% moisture content of 26.5%, and cost of drying of TZS 85/kg fresh. The redesigned natural convection mixed mode solar dryer and schematic framework of performance improvement and evaluation of natural convection mixed mode solar dryer developed are recommended for further investigation and application for solar dryer improvement respectively.