Coupling Energy Efficiency in Industry and District Heating: Modelling of Low-Temperature Waste Heat Recovery Substations and a Case Study Analysis

The substantial amounts of low-temperature waste heat generated by industrial processes frequently remain an underutilized energy resource. In the LIFE4HeatRecovery project, various solutions have been explored to harness this energy source in the context of district heating. While local reuse was always prioritized, the connection with existing nearby thermal networks was exploited to recover the remaining excess heat and at the same time to satisfy local needs during periods of insufficient waste heat availability. In order to achieve this objective, bi-directional thermal substations were designed, including heat pumps and storages to meet the desired temperature levels and to enhance source/load matching. This contribution focuses on the modelling of these advanced substations, with tailored hydraulic connections. A TRNSYS model was developed, general enough to adapt to the different configurations of the LIFE4HeatRecovery project, including both low- and high-temperature networks. In spite of its flexibility, the model includes detailed aspects such pipe and storage thermal losses, as well as pumping electricity consumptions. Particular attention was devoted to the modelling of heat pumps, a crucial component in this type of applications: the full compressor operating cycle was implemented, with performance maps calibrated on real manufacturer data. The model is here applied to one of the project demonstration sites, namely the waste heat recovery system installed at a steel mill located in Ospitaletto (Italy). The case is particularly interesting as it is connected to a cold network (15-25 °C), partly supplied by the foundry cooling towers and partly by aquifer wells. The heat recovery was used to replace the gas boiler system supplying heating and sanitary hot water (for canteen and showers) to the whole factory. The model is used to estimate the corresponding energy benefits on an entire reference year. The results show that non-renewable primary energy savings (as well as carbon emission savings) of up to 70% can be achieved for the local heat demand. As the waste heat reused locally is less than 10% of the potentially recoverable amount, this case highlights the important role of district heating for this type of applications in order to exploit waste heat in its entire availability.