The aim of this work, included in the “THERMENERGY” project of the French energy transition institute INEF4, is to propose a tool to help the design of district heating networks (DHN) at the beginning of an urban project. The configuration (network layout, choice between the different production technologies, existence or not of such utilities) and the design (mass flow rate, temperature, heat capacity to install and area of the heat exchanger in sub-station) are optimized simultaneously. The tool works as well for energy supply systems to be created as for the expansion of existing sites.

The originalities of this work are to: 1 - avoid any constraint on pipe forcing them to be both supply and return, 2 - permit centralized - decentralized – or individual heat productions, 3 - permit cascade connections between consumers. Such a formulation enables to work on the 4th generation of DHN and also on branched or looped networks.

The formulation of the problem leads to a mixed integer non-linear programming (MINLP) problem. That means the optimization problem has a single nonlinear objective function (global cost) subjected to numerous linear constraints (e.g. mass flow rate balance) or nonlinear constraints (e.g. energy conservation) with both continuous (mass flow rate, temperature distribution, area of the heat exchanger, heat capacity to install...) and discrete variables (logical existence of the pipe or existence of the technology of production).

A study case based on an existing DHN is discussed. First some optimal design parameters (thermal generating capacity, temperature and network layout) are compared to the existing ones. Then, a potential expansion of the DHN to the neighbourhood and the introduction of renewable energy are studied.