The decline of fossil fuels and the search for viable alternatives: A critical review of biodiesel, electric, and biogas energy systems under global socio-political and technical constraints

The global energy landscape is undergoing a profound transformation as long-standing dependence on fossil fuels becomes increasingly unsustainable from environmental, economic and geopolitical perspectives. This review examines the growing body of literature on renewable alternatives with a focus on biodiesel, electric mobility and biogas systems, highlighting how each technology responds to the multifaceted pressures of climate change, resource depletion and energy insecurity. The review synthesizes research across engineering, environmental science, economics and policy to clarify how these energy pathways operate within real-world constraints rather than idealized laboratory conditions. Biodiesel systems offer immediate compatibility with existing engines and can support rural development, they are shaped by feedstock availability, land use dynamics and production costs. Electric energy systems have expanded rapidly due to advances in battery chemistry, digital control and charging infrastructure, though their progress is influenced by material supply risks, grid readiness and large disparities between regions. Biogas systems provide circular solutions that link waste management, nutrient recovery and decentralized energy access, but face operational, logistical and regulatory barriers that limit widespread adoption. Across all technologies, the literature reveals strong interconnections between technical performance and broader social, political and economic conditions. Trends indicate that successful energy transitions require not only technological innovation but also coherent policy frameworks, resilient supply chains and strategies that support equity between regions with different economic capacities. By comparing these systems through a unified perspective, this review clarifies their complementary roles and identifies opportunities for comparative solutions that enhance energy security, lower emissions and strengthen community resilience. The synthesis underscores that the decline of fossil fuels should be understood not only as a global challenge but also as an opportunity to design cleaner and more equitable energy futures. • No single pathway dominates; viability depends on sector, region, and system constraints • Waste-based biodiesel outperforms crop-based routes under harmonized LCA boundaries • EV climate benefits remain strongly dependent on grid mix and charging infrastructure • Biogas performance is governed by methane leakage control and upgrading requirements. • LMIC evidence is limited, especially for harmonized LCA and techno-economic assessments