  Large Combined Heat and Power (CHP) plants are often employed in order to
feed district heating networks, in Europe, in post soviet countries and China.
Traditionally they have been operated following the thermal load with the
electric energy considered as a by-product, while the modern trend includes
them in the electric market to take advantage of the flexibility they could
provide. This implies the necessity to consider the impact on the electric grid
while filling the thermal load requests. A detailed Mixed Integer Linear
Programming (MILP) optimization model for the solution of the
network-constrained CHP unit commitment of the day-ahead operation is
introduced. The developed model accounts for lossless DC network approximation
of the electric power flow constraints, as well as a detailed characterization
of the CHP units with useful effect, heat and power, function of one and two
independent variables ("degrees-of-freedom"), and thermal energy storage. A
computational validation of the outlined model on a CHP test system with
multiple heating zones is presented in the form of computational test cases.
The test cases illustrate the impact on the flexibility of the implementation
of the energy storage, network constraints and joint multi-system operation.
The conducted studies have highlighted the importance of a comprehensive and
integrated analysis of multi-energy systems to exploit the operational
flexibility provided by the cogeneration units. The joint operation of the
thermal and electric system allows to reap economic, operational efficiency,
and environmental benefits. The developed model can be easily extended to
include diverse multi-energy systems and technologies, as well as more complex
representations of the energy transmission networks, and the modeling of
renewable energy resources dependent of one or more independent,
weather-related, variables.
