Simulative comparison of concepts for simultaneous control of heat flow and outlet temperature of heat exchangers for highly flexible use in the test facility "District LAB"

An important measure for decarbonising the heating sector is the transformation of existing district heating (DH) systems into low-emission heating grids based on renewable heat sources. The test facility for DH applications "District LAB (D-LAB)" is currently being set up in order to support the transition by enabling the experimental investigation of various transformation measures. It consists mainly of a Flexible Heating Grid (FHG), which connects several decentralised Hardware-in-the-Loop (HIL) and is fed by a central generation plant. The HIL units in the FHG should be able to map almost any heat producer or consumer, so that a wide range of investigation scenarios is possible. In order to fulfil this requirement, the Heat Exchanger (HEx) that connects the HIL units with the FHG must extract or feed a defined heat flow from the grid and at the same time ensure a defined secondary-side outlet temperature. In a literature research conducted to the best of our knowledge, it turned out that in all technical applications only one outlet temperature is controlled. Therefore a suitable control concept had to be developed. This has been done by extensive simulative investigations, which are presented in this paper. Two control concepts, (I) "decoupled PID controller", (II) "Characteristic Field (CF)-Based Control", were self-developed and compared with each other. In order to represent a large spectrum of possible operating conditions with regard to the transformation of DH systems, 48 investigation scenarios based on target value steps were defined and used for the comparison. Subsequently, the average performance using the Root Mean Square Error (RMSE) as well as the time dependant step responses were examined. It turned out that, the CF-based concept appears to be more suitable for use in the D-LAB. It is conceivable that these results can also be transferred to other test facilities in which both, the heat flow and one outlet temperature need to be controlled.