The hierarchical structure of energy carriers is reviewed, and their interf
aces analyzed physically. The value of each interface is due to the interfa
ce potential. The typical examples are clusters and built-in-fields. The co
hesive energy confined in the energy medium is reviewed. It is shown that e
nergy can be effectively released by the interfaces. Examples of frontier e
nergy converters are introduced. (C) 2000 Elsevier Science Ltd. All rights
reserved.