The worldwide scarcity of fossil fuels regarding primary energy demand together with growing environmental concerns have raised new challenges to the world economy, and led to changes in the energy paradigm. Industry, services, researchers, and the Academy are challenged to envisage new solutions through setting up new conversion processes, designing new power systems, and investigating and developing new energy sources and vectors. A broad and up-to-date views on flow systems, energy vectors, together with fuel cells, biomass, and energy recovery issues are covered in this collection. The Guest Editors are grateful to the authors who contributed to this special issue and to all colleagues who participated in the review process.