This is a research-based textbook, in the area of sustainable energy systems and aimed to address some key pillars, better efficiency, better cost effectiveness, better use of energy resources, better environment, better energy security, and better sustainable development. It also includes some cutting-edge topics, such hydrogen and fuel cells, renewable, clean combustion technologies, CO2 abatement technologies, and some potential tools (exergy, constructal theory, etc.) for design, analysis and performance improvement.