Thermal-hydraulic modelling of a flexible substation layout for low-temperature waste heat recovery into district heating

This study investigates a thermal substation designed for low-temperature waste heat recovery in district heating networks, where the served user has both heating and cooling demands. The proposed bidirectional substation, equipped with heat pumps and thermal storage units, enables the recovery of excess waste heat in the district heating network and compensates local heating needs when waste heat availability is insufficient. A flexible and detailed TRNSYS-based model is developed to simulate various hydraulic configurations of the substation, tailored for both low- and high-temperature district heating networks, ensuring adaptability to different temperature requirements in diverse applications. The model is applied to a demonstration site in Ospitaletto, Italy, where waste heat is recovered from a steel mill and used to replace gas boilers for space heating and hot water in the factory's canteen and showers. The model's accuracy is validated through a comparison with monitored data, ensuring reliable performance predictions. Performance estimation error is under 5%, demonstrating the model's high reliability. The simulation results show that the system can achieve up to 75% reduction in non-renewable primary energy consumption and carbon emissions, while also allowing the recovery of about 90% of unused waste heat for supply to other connected users.