Operational Behaviour of a Solar-Fed Bidirectional Substation for 4GDH Networks

Decentralized feed-in of solar surplus heat from prosumers into a District Heating Network is here addressed. A specified architecture combining a Return/Supply connection, a set of valves, a feed-in pump and a primary side connection of the solar field led to the conception and fabrication of a bidirectional substation prototype. The latter is tested in a hardware-in-the-loop configuration regarding the consumer and solar field sides while the DHN side of the substation prototype is connected to a real DHN. A 12-days sequence, representative of typical days, with varying solar irradiation, DHN thermo-hydraulic conditions and consumer demands is performed with the prototype. A set of indicators showed i) an inappropriate behavior of the controller during the coldest days, ii) various improvements of the solar fraction during the hottest days (with up to 52% of total solar fraction) and iii) rather good performances of the controller (about 90% effectiveness). The analysis was completed by the observation of detailed dynamic result for one day. Different operating regimes (full DHN consumption, solar self-consumption complemented by the DHN, solar self-consumption and feed-in) could be observed during a single day highlighting the modularity of the BSST to switch automatically (by hydraulic balancing) from one mode to the other.