Exergetic analysis and optimization of a hybrid solar-biomass greenhouse dryer for drying banana slices

Abstract

Bananas have high moisture content, making them highly susceptible to microbial damage and as such reducing their shelf life. Thus, the need to lowering the product moisture content is paramount. This study aimed at optimizing a hybrid solar-biomass greenhouse dryer for drying banana slices based on exergy analysis. The main components of the dryer were the greenhouse enclosure, biomass stove, double duct heat exchanger, drying trays and ventilation fans. Models were developed to simulate energy supply, drying time and dryer design parameters for the system based on dryer capacity of 240 kg banana slices. The study treatments comprised four banana varieties, peeled and unpeeled and three slice thicknesses. Three drying methods (solar, biomass and solar-biomass) were used to dry fresh banana slices in the greenhouse dryer with the open sun as the control. All measurements for the required parameters were monitored and recorded accordingly. Evaluation of the developed dryer was based on energy and exergy analyses. Optimization of the developed dryer was achieved by the use of genetic algorithm. The objective functions of optimization were to maximize exergy efficiency and drying air temperature. A comparison of colour, vitamin C content and firmness of the dried banana slices to that of fresh banana slices was made. The coefficient of determination (R2) between the simulated and measured values for the developed model were 0.8099, 0.5393 and 0.8845 for solar, biomass and solar-biomass modes, respectively. Under no load condition, the dryer had a temperature and relative humidity difference of 16.61±6.81oC and 9.77±17.89%, 8.20±1.42oC and 20.60±7.44% and 16.77±5.77oC and 21.48±13.94% for solar, biomass and solar-biomass modes, respectively, compared to ambient air. These conditions in the dryer indicated the potential of the drying air in holding more moisture as compared to open sun drying. During the drying period, the dryer had a temperature and relative humidity difference of 12.96±5.25oC and 8.76±8.28%, 8.88±1.38oC and 24.26±8.83% and 13.21±6.21oC and 27.51±10.24% for solar, biomass and solar-biomass modes, respectively, compared to ambient air. The average energy utilization ratio in the greenhouse dryer was noted as 35.58±24.78, 40.60±10.52 and 33.46±13.45% for solar, biomass and solar-biomass modes, respectively. The corresponding average hourly exergy efficiency of drying air was found to be 64.60±24.78, 59.37±10.52 and 66.50±13.47%, respectively. The results of optimization showed that solar and solar-biomass modes required the same airflow rate (0.05 kg/s) indicating that at these combinations the dryer received almost equal energy input. Biomass mode trade-offs indicated that a lower flow rate (0.01 kg/s) was required for drying. Further, both solar-biomass and biomass modes required a fuel feed rate of 0.001 kg/s for maximum exergy efficiency (solar-biomass: 80.21±14.26%; biomass: 79.28±10.38%) and drying air temperature of 333 K. Performance evaluation of the optimised system demonstrated its superiority compared to the un-optimised system. Regardless of the drying method, results obtained showed that the dried products were darker, firmer and had lower vitamin C content than the corresponding fresh products. Open sun drying had the highest colour difference of 39.14±7.63 compared to 24.27±2.52, 16.53±12.81 and 21.71±2.28 for solar, biomass and solar-biomass, respectively. The highest percentage increase in firmness of banana slices after drying was found to be 317.50±8.92%, 153.19±60.54%, 233.33±26.07% and 262.50±12.62% for solar, biomass, solar-biomass and open sun drying, respectively. Biomass mode drying had the highest retention of vitamin C content of 87.19±13.97% compared to 86.90±2.84%, 85.87±2.64%, and 77.14±6.38% for solar-biomass, solar and open sun drying, respectively. However, drying of banana slices in the greenhouse dryer resulted in better conservation of quality as compared to open sun drying. Generally, drying of products in the optimized hybrid greenhouse dryer has advantages of continuous drying despite changes in weather and time, as well as protecting the product from contamination. Further research should be conducted on optimization based on extensive thermodynamic properties of the system.