Energy- and exergy-based optimal designs of a low-temperature industrial waste heat recovery system in district heating

Abstract This paper illustrates how the choice of indicators changes the design of a waste heat recovery system in district heating. A prospective system in Grenoble (France) aims to valorize waste heat from the French National Laboratory of Intense Magnetic Fields (LNCMI) by injecting it at 85 °C to the nearby district heating network. We optimize its design for three possible waste heat temperatures: 35 °C (current), 50 °C (viable) and 85 °C (innovative). As major components, the system includes a thermal storage (ranging from 10 MWh to 40 MWh) and may include a heat pump depending on the waste heat’s temperature. Different optimizations are guided by two energetic indicators (one source-oriented, the other demand-oriented) and by the overall exergy efficiency. The system’s annual performance is assessed through the Sankey and Grassman diagrams and compared between optimal designs. Yearly simulation included optimal management of the thermal storage, through mixed-integer linear programming. The demand-oriented optimal design suggests recovering waste heat at 35 °C with a heat pump and a 40-MWh storage, granting the highest coverage of residential needs (49%). On the other hand, the source-oriented optimal design suggests recovering waste heat at 85 °C without heat pump and with a 40-MWh storage, reaching the highest recovery of waste heat (55%). Exergy analysis supports the source-oriented design, as it reaches the highest global exergy efficiency (27%). Our prospective techno-economic and exergo-economic analyses should complement these results and may change some conclusions, especially regarding the storage capacity.