Control strategies for constant supply temperature of a high-temperature Brayton heat pump
High-temperature heat pumps are the most efficient and potentially emission free option to supply process heat at temperature levels up to 300 °C. Many industrial processes require high-temperature heat but at the same time reliable, and controllable process heat parameters even under varying load conditions such as mass flow or inlet temperature variations. The Brayton heat pump facility “CoBra” is a demonstrator for a closed-loop high-temperature heat pump using dry air as working fluid. It can supply sensible heat to process air streams at temperatures above 200 °C. In order to be a viable heat supply option for industrial processes, a suitable control method to ensure precise temperature control must be identified and evaluated. In this work, an experimental campaign with two different control strategies for process temperature control is conducted and the results subsequently compared. A variation of the fluid inventory and compressor speed are used to control the process temperature. Both control methods are capable of maintaining a constant process temperature of e.g. 200 °C ± 2 °C in case of occurring perturbations. Variations of the set point temperature are also achievable with both controllers. Overshoots can be eliminated by choosing appropriate controller parameters. The results show that compressor speed control is faster in achieving a new set point process temperature compared to the fluid inventory change. A constant process temperature can be maintained despite a reduction in fluid inventory with an appropriate increase in compressor speed. This indicates that the two controllers can compensate for each other’s effect on the process temperature. Controller performance could be further improved by a rigorous and systematic parameter tuning.