Innovative approach and numerical modeling to retrofit existing substations for bidirectional operation: Enabling thermal prosumer participation in District Heating Network

The heating and cooling sector accounts for nearly half of Europe's total energy consumption, highlighting the urgency of its decarbonization. District heating networks, which serve approximately 100 million people across Europe, are recognized by the recent European Energy Efficiency Directives as key infrastructures for integrating renewable energy and waste heat. In Italy, while energy efficiency improvements have primarily focused on thermal production plants, decentralized substations still require upgrades to enhance their performance and support the emergence of thermal prosumers. This study presents a non-invasive retrofit design for an existing substation in a district heating network in Italy, aimed at preserving the integrity of the original infrastructure while implementing a new control system to recover unused solar energy in a supply-to-return configuration. Modelica language and Dymola software are used to develop the numerical model of the bidirectional substation, and multi-domain simulations based on real thermal load and solar production data from a multifamily building are performed with a one-second time step to dynamically test the entire system. Results show that the control strategy effectively manages thermal exchange dynamics even under fluctuating loads. Despite a limited match between solar production and hot water demand, resulting in low self-consumption rates (10.6 % in mid-season and 6.2 % in summer), the bidirectional setup enabled 94 % of the solar energy to be effectively utilized during the summer day. In line with the results obtained for the representative days, the annual extrapolation shows that the proposed configuration enables 78.4 % of the solar energy to be effectively utilized, with a self-sufficiency rate of 32.5 % and a self-consumption rate of 14.9 %. The proposed solution offers a replicable and efficient approach to support the integration of distributed renewable heat sources in existing district heating systems.