Browsing by Author "Wentzel, Louis Frans Jacobus"

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    Biogas-fuelled aqua-ammonia absorption refrigeration as a means of reducing post-harvest losses in rural Sub-Saharan Africa
    (Stellenbosch : Stellenbosch University, 2022-04) Wentzel, Louis Frans Jacobus; Van Rensburg, Eugene; Görgens, Johann Ferdinand; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
    ENGLISH SUMMARY: Post-harvest losses are particularly prevalent in the food value chains of Sub-Saharan Africa (SSA), characterised by poverty, food insecurity and poor infrastructure. This project combined anaerobic digestion (AD) and absorption refrigeration (AR) to provide off-grid cold storage in SSA and focussed on the artisanal fisheries of Lake Victoria as an example of the utility of this technology. The main objective of this study was to prototype a biogas-fuelled aqua-ammonia absorption refrigerator to determine its biogas consumption rate and thermodynamic performance. A pipe injection burner was designed to convert a 42 L aqua-ammonia absorption refrigerator to run on a mixture of 59.2% methane and 40.8% carbon dioxide, which simulated a standard biogas mixture. This prototype was used as proof of concept and achieved cold storage temperatures that decreased from 6.4 to -19 ˚C as the normalised biogas flowrate increased from 0.51 to 0.74 m3 day-1. The thermodynamic performance of this prototype informed a novel empirical model that determines the biogas consumption rates of larger aqua-ammonia absorption refrigerators at various temperatures. This model found that a 250 L aqua-ammonia absorption refrigerator operating at sub-zero temperatures consumes 11 to 13 times the amount of biogas consumed by an electric 254 L vapour compression refrigerator connected to a biogas-fuelled combined heat and power (CHP) system. Therefore, whereas vapour compression refrigeration is more energy efficient, AR is more practical in the context of rural SSA, because it has no moving parts and does not require access to electricity. Two biochemical methane potential (BMP) assays with two objectives were performed to determine the potential biogas production rates of biomass available in the Lake Victoria region. The first BMP assay considered the co-digestion of fish effluent, blended with cow manure, maize stover, water hyacinth and reeds, respectively, to investigate whether co-digestion achieves statistically significant synergetic effects that increase the combined BMP and/or digestion rate of the combined biomass due to an improved nutrient balance. It was found that co-digestion did not achieve statistically significant synergetic effects at a 95% confidence level for any of the biomass co-digested with fish effluent. The second BMP assay considered the ensilage of maize stover, water hyacinth and reeds as a biological pre-treatment method that may increase the BMP and/or digestion rate of these lignocellulose because hydrolysis takes place during ensilage. There were discrepancies in literature where some workers reported significant increases in the BMP of ensilaged lignocellulose, while others reported significant decreases. An alternative protocol was developed to correct these overestimations, which showed that silage BMPs were overestimated when the standard BMP protocol was used. The final objective of this study was to combine the results from the biogas-fuelled aqua-ammonia absorption refrigerator and the BMP assays to model large-scale AD-AR dual systems, which demonstrated the potential of this dual system as a robust off-grid cold preservation technology at the nexus of water, energy and food in the context of small holder farming in rural SSA.