Experimental characterization of a prototype of bidirectional substation for district heating with thermal prosumers

The prosumer, already widespread in the electrical sector, is still uncommon in the district heating (DH) sector. Nevertheless, this figure can potentially provide a relevant contribution to increase the renewable fraction of heat and to decrease the fossil fuel consumption, hence enhancing sustainable and efficient district heating. Moreover, prosumers are more informed and responsible towards energy production and energy savings. In order to enable the two-way heat exchange, the thermal substation at the interface between the prosumer and the DH network must be properly upgraded. The present paper aims at providing a comprehensive contribution to the design and testing of an innovative bidirectional substation for active district heating, focusing on the hydraulic configuration and on the control strategies. The realized substation primarily fulfils the prosumer's heat demand and supplies the excess heat to the DH network only if it is available at the temperature contractually defined with the DH operator, while it uses the network as a source if the local production is not sufficient to cover the user's heat demand. The prototype, with a technology readiness level TRL 4, can be connected to a generic micro-generation system, e.g. solar thermal or heat recovery units. An extensive test campaign has been carried out in order to evaluate its dynamic behavior both from energetic and hydraulic points of view. Test confirmed the reliable operation of the substation, which is able to handle simultaneous heat exchanges with the user's heating system and the DH network at pressure loss lower than 0.2 bar and at temperatures within ±0.6 °C of the prescribed setpoint after transients shorter than 5 min, even with ±10 °C step changes in the inlet fluid temperatures.