A 5E framework for hybrid solar dryers: Constraint relocation, transferred burdens, and boundary-sensitive assessment with application to Sub-Saharan Africa

Hybrid solar dryers improve process continuity but transfer requirements to auxiliary energy, exergy destruction, lifecycle cost, maintenance, and deployment support. Existing assessments remain fragmented across technical, thermodynamic, economic, environmental, and enabling dimensions. Such fragmentation limits whole-system comparison. This PRISMA-guided thematic review synthesises the global literature and examines its relevance to Sub-Saharan Africa. The review evaluates Energy, Exergy, Economics, Environment, and Enabling conditions across process, system, and deployment boundaries. Reported performance varied substantially across configurations and assessment boundaries. One study reported 62.02% collector efficiency and 23.87% dryer efficiency. Stronger solar-energy capture did not translate proportionally into useful moisture removal. Across four configurations, extended resource accounting reduced Exergy Return on Investment by 6.87% to 91.61% relative to conventional exergy efficiency. Reported advantages therefore remain sensitive to functional basis and assessment boundary. The proposed 5E framework traces local gains and transferred burdens through sequential boundary expansion. The framework distinguishes process improvement from boundary-robust system advantage. In SSA, energy reliability, financing, maintenance capacity, technical skills, spare-part access, and utilisation shape the persistence of reported 4E gains. Robust comparison requires harmonised functional units and wider system and deployment boundaries.