The Unique Plant Concept of the Linz District Heating and Cooling System
The utilization of district heating heat pumps (DHHP) has become increasingly prevalent due to their energyefficient and sustainable operation. However, the use of DHHPs is mainly focused on the supply of heat energy for district heating (DH), with the usage of the provided cooling energy by the heat source often not being considered. In this paper, we show the unique plant concept of the district heating and cooling (DHC) system of Linz, which is set for a partial start of operation in 2028. The DHC-network (DHCN) stands out from others due to its special heating and cooling operation: In heating operation, the DHHP uses the condensing flue gases from the cogeneration plant “Energiepark Linz-Mitte” as heat source; while during the usual summer DHHPdowntimes, a special hydraulic switching enables the use of the heat source to provide additional cooling energy for the district cooling (DC). In the latter, river water as well as cooling towers are used for re-cooling. Additional DC and DH storage tanks help to avoid operation during unfavorable boundary conditions. We have found that by the serial arrangement of the DHHP in heating operation, exceptionally high system- COPs of 4.4 at full load (instead of 2.8 in parallel arrangement) can be achieved. Facing very different temperature spreads on the heat source in cooling and heating operation, during cooling operation the sourceside serial interconnection is hydraulically switched to parallel. Thereby, excessive variations of the water mass flows in the evaporators are avoided. Furthermore, the necessary maximum installed cooling capacity of the DC supply can be reduced by utilizing the DHHP to provide cooling, resulting in investment savings. These results are of high significance for both existing DHHP systems and those newly installed, showing the advantages of using the DHHP cooling energy through the implementation of appropriate system designs.