Lowering the supply temperature of a District Heating Network through optimal sizing and positioning of Thermal Energy Storages

One amongst the numerous challenges in the field of District Heating (DH) consists of reducing the supply temperature of the water pumped from the central plant into the network. This would be beneficial for several reasons, one of which is the fact that the amount of thermal losses along the pipelines would decrease, thus enhancing the energy efficiency of the network itself. A proper rise in the mass-flow rate injected from the plant should be taken into consideration though, to compensate for the aforementioned temperature reduction, as stated by the First Law of Thermodynamics (FLT). The introduction of Thermal Energy Storages (TES) in specific points of the network – to be filled with hot water during night hours, i.e. when there is little or even no thermal request – would serve the purpose of partially relieving the increased pumping load when the heating systems are switched on, especially in correspondence of the morning peak of demand. This work aims at finding the best combination of the size and position of the storages in an existing DHN, in such a way that it would be possible to inject hot water into the pipelines at a temperature lower than design conditions, still managing to deliver the required amount of heat to the end-users or, if this is not feasible, to minimize the gap between the design and off-design configurations. To do so, a numerical optimization of the network has been performed, after having developed a model of the storages that has been integrated with the transient fluid-dynamic simulation of the network itself. The optimization has been carried out by means of a genetic algorithm.