Identification of the optimal operational strategy of a large district heating network through POD modeling

District heating is expected to play a major role in the supply of low-carbon heating in urban areas. This result is obtained through heat generation from CHP systems, residual heat from industries or waste-toenergy plants, and is, whenever possible, integrated by additional recourse to renewable energy sources. The constrained minimization of pumping losses is an important aspect of district heating systems design, especially in large networks where it is mandatory to achieve high efficiency and security of supply. In this paper a large district heating system, which supplies heating to a total volume of buildings of about 50 million of cubic meters, is considered. The aim of the work is to identify an optimal operational strategy for the network. An optimization entirely based on fluid-dynamic model requires large computational resources. Therefore, the use of a reduced model based on proper orthogonal decomposition (POD) is investigated. Various operating conditions corresponding to partial load operation are analyzed using a fluid-dynamic model of the network. The results are used to obtain the characteristic eigenvalues and eigenvectors that constitute the basis for the POD model. The latter significantly reduces the simulation time, and therefore it can be conveniently applied to the optimization of the set points of the pumping system. The results show that significant reductions in primary energy consumption can be achieved with respect to a conventional control strategy.