This book provides a concise and technical overview of energy technology: the sources of energy, energy systems and frontier conversion. As well as serving as a basic reference book for professional scientists and students of energy, it is intended for scientists and policy makers in other disciplines (including practising engineers, biologists, physicists, economists and managers in energy related industries) who need an up-to-date and authoritative guide to the field of energy technology.Energy systems and their elemental technologies are introduced and evaluated from the view points of efficiency and the global environment. Principles of effective conversion are explained with the use of irreversible thermodynamics and exergy. Advanced converters, catalysts, fuel cells, membranes, metalhydrides, refrigerators, M.H.D., solar cells, superconductivity and water decompositers are among the main subjects introduced.