Design and optimization of an integrated vacuum fermentation and sustainable solar energy system for high-performance bioethanol production as a biofuel: Experimental and simulation study

Bioethanol production faces critical challenges under very-high-gravity fermentation conditions due to elevated osmotic stress and bioethanol inhibition. This study evaluates vacuum-assisted fermentation integrated with a solar-based renewable energy system coupled with electrical and thermal storage, enabling continuous energy supply for fermentation, temperature control, and freshwater provision via solar desalination. A hybrid renewable energy system comprising photovoltaic panels, parabolic trough collectors, solar still desalination, battery storage, and phase-change materials was developed to continuously supply renewable energy and freshwater to the fermentation. Experimental batch fermentations under intermittent vacuum conditions were optimized using response surface methodology, and validated through Aspen Plus simulation, showing agreement with experimental results up to 32% Brix. Under optimal conditions (43% Brix, pH 5.3, 71 h), bioethanol concentration reached 26% v/v in bioreactor with 96.3% fermentation efficiency, 2.6 times higher than conventional methods. At lower sugar concentrations (15% Brix), vacuum application reduced fermentation time by 66%, significantly intensifying the process. Intermittent vacuum application proved superior to continuous vacuum by minimizing cellular stress at higher substrate concentrations. Additionally, the system notably reduced water usage and wastewater generation per bioethanol produced. This study confirms the feasibility of integrated vacuum-assisted solar-powered fermentation, recommending research into yeast osmotic tolerance and enhanced bioethanol recovery.