This book discusses the optimal design and operation of multi-carrier energy systems, providing a comprehensive review of existing systems as well as proposing new models. Chapters cover the theoretical background and application examples of interconnecting energy technologies such as combined heat and power plants, natural gas-fired power plants, power to gas technology, hydropower plants, and water desalination systems, taking into account the operational and technical constraints of each interconnecting element and the network constraint of each energy system. This book will be a valuable reference for power network and mechanical system professionals and engineers, electrical power engineering researchers and developers, and professionals from affiliated power system planning communities.