CoBra - A High-Temperature, Load-Flexible Brayton-Cycle Heat Pump Demonstrator

Kurzfassung

Industrial decarbonization requires technologies capable of delivering high-temperature process heat using renewable electricity. High-temperature heat pumps (HTHPs) based on the reverse Brayton cycle are a promising solution for applications above 150 °C, particularly where large temperature glides and simultaneous heating and cooling are required. The CoBra heat pump demonstrator, developed at the German Aerospace Center (DLR), is a closed-loop Brayton HTHP that uses dry air as the working fluid. The system is driven by turbomachinery and realizes part-load operation through two strategies: (i) variable compressor speed, which directly influences pressure ratio and temperature lift, and (ii) fluid inventory control, which adjusts the mass of working fluid to adapt thermal output while maintaining constant system temperatures. Additionally, recuperation can improve both the coefficient of performance (COP) and achievable supply temperature by internally recovering heat between the high- and low-pressure streams. During operation, CoBra delivered up to 55 kW of process heat at delivery temperatures of 170 °C with a COP of 1.39, using 15 °C ambient air as the heat source, while simultaneously providing cooling down to -25 °C. The maximum compressor discharge temperature reached in experiments was 280°C, demonstrating the HTHP’s capacity to deliver heat at temperatures exceeding 250°C. A dynamic simulation of CoBra was developed and experimentally validated against commissioning data, achieving mean-normalized root-mean-square errors below 1.5% for key thermodynamic variables. The model captures the main system dynamics and has been used as a virtual testbed for control strategies. Both traditional and advanced methods, including reinforcement learning (RL), were investigated to optimize heat pump control, ensuring heat delivery at target temperatures with optimal efficiency while maintaining safety margins. Experimental results from three measurement campaigns on CoBra are presented: the first being the commissioning campaign, the second focusing on fluid inventory control, and the third demonstrating the use of RL control algorithms in real-world HTHPs. Measurement data from these campaigns are made available in an open-access public repository. These experiments demonstrate that the Brayton cycle is a viable option for industrial heat electrification and that CoBra serves as a research platform for testing and refining related concepts and technologies.