  The fundamental theory of energy networks in different energy forms is
established following an in-depth analysis of the nature of energy for
comprehensive energy utilization. The definition of an energy network is given.
Combining the generalized balance equation of energy in space and the Pfaffian
equation, the generalized transfer equations of energy in lines (pipes) are
proposed. The energy variation laws in the transfer processes are investigated.
To establish the equations of energy networks, the Kirchhoff's Law in electric
networks is extended to energy networks, which is called the Generalized
Kirchhoff"s Law. According to the linear phenomenological law, the generalized
equivalent energy transfer equations with lumped parameters are derived in
terms of the characteristic equations of energy transfer in lines(pipes).The
equations are finally unified into a complete energy network equation system
and its solvability is further discussed. Experiments are carried out on a
combined cooling, heating and power(CCHP) system in engineering, the energy
network theory proposed in this paper is used to model and analyze this system.
By comparing the theoretical results obtained by our modeling approach and the
data measured in experiments, the energy equations are validated.
