  With the need for optimisation based supervisory controllers for complex
energy systems, comes the need for reduced order system models representing not
only the non-linear characteristics of the components, but also certain unknown
process dynamics like their internal control logic. We present in this paper an
extensive literature study of existing methods and a rational modelling
procedure based on the grey-box methodology that satisfies the necessary
characteristics for models to be applied in an economic-MPC of a real-world
polygeneration system at the Offenburg University of Applied Sciences. The
engineering application of the models and their fitting coefficients are shared
in this paper. Finally, the models are evaluated against experimental data and
the efficacy of the methodology is discussed based on quantitative and
qualitative arguments.
