Design, commissioning and closed-loop operation of a high-temperature Brayton heat pump — CoBra

This work presents the design, commissioning, and closed-loop operation of the high-temperature heat pump (HTHP) “CoBra”, developed by the Institute of Low-Carbon Industrial Processes at the German Aerospace Center (DLR). CoBra is the first HTHP of its kind to implement a closed-loop reverse Brayton cycle using dry air as the working fluid. The system consists of a two-stage centrifugal compression unit, an axial turbine, and three shell-and-tube heat exchangers, with recuperation enabled, via a switchable 3/2-way valve. The compression and turbine systems are mounted on separate shafts, allowing for independent speed control and modular component replacement. The demonstrator was successfully operated in a closed-loop configuration for the first time in May 2024 and is now fully operational. Compressor outlet temperatures of up to 230 °C were reached. On the heat source side, turbine outlet temperatures of roughly -30 °C were feasible. However, during the commissioning experiments, presented in this current work, the cooling cycle was operated mostly at temperatures above 0 °C, due to a lack of insulation, which has been subsequently added to the system. The data presented in this work serves as proof-of-concept and shows that a closed-loop Brayton HTHP can deliver process heat and process cooling at industry-relevant temperature levels.