Impact of the greenhouse drying modes of two-phase olive pomace on the energy, exergy, economic and environmental (4E) performance indicators

This research aims to valorize olive pomace waste as bio-combustible. Therefore, the drying of two-phase olive pomace in a solar greenhouse dryer was studied under the weather conditions of Marrakech, Morocco. The experimental results of natural, hybrid, and forced hybrid drying modes of two-phase olive pomace were used to determine the energy and exergy efficiencies of the greenhouse dryer and to analyze the kinetics of drying. Moreover, the economic viability and environmental suitability of the three drying modes were also investigated. The results indicated that the humidity level of the product dropped from 54% to 4% (wt%) in 28, 19 and 14 h for the natural, hybrid, and hybrid forced drying modes, respectively. The effective moisture diffusivity varied significantly between the drying modes with a lowest average value of 1.28 × 10−09 m2/s and a highest average value of 2.71 × 10−09 m2/s associated to the natural and hybrid forced drying modes, respectively. The drying and exergy efficiency of the hybrid forced greenhouse dryer was improved by 31.61%, 51.95%, and 36.91%, 21.05% compared to the natural and hybrid greenhouse drying modes, respectively. The environmental analysis showed that the carbon emissions, and carbon mitigation depend strongly on the greenhouse drying mode, and the material used for the construction of the drying system. The hybrid forced greenhouse dryer ensured the mitigation of 1.29 and 1.15 times more CO2, compared to the natural, and hybrid greenhouse dryers. The economic analysis revealed a payback period of 1.27 years, 1.03 years, and 0.67 for the natural, hybrid, and hybrid forced modes for a selling price of 3 (MAD), which are very small compared to the lifespan of the greenhouse dryer (35 years).