Simulation of a Safe Start-Up Maneuver for a Brayton Heat Pump

With about 50 % of the final energy used as heat in Europe, reducing fossil fuel consumption in this sector is crucial to achieve significant greenhouse gas emission reduction. Heat pumps using renewable electricity can potentially cover the heat demand below 500 °C. The DLR’s prototype CoBra (Cottbus Brayton cycle heat pump) aims at demonstrating the feasibility of a turbomachine driven closed-loop Brayton cycle heat pump with a thermal output of 200 kW and a heat sink temperature of up to 350 °C. In order to achieve safe operation, transient operation of the heat pump, such as start-up, must be analyzed. Temperature gradients must be kept below a limit, defined mostly by thermal stresses in the heat exchangers. At the same time, compressor surge and resonant frequencies of rotating components must be avoided during transient maneuvers of the system. In the current work, the heat pump prototype has been modeled with Modelica based on the component geometries and with the use of compressor and turbine maps obtained from 3D CFD simulations. For the start-up of the prototype, a suitable control strategy is developed and analyzed in order to minimize operational risks. Control parameters are compressor shaft speed, secondary mass flows and the turbine bypass. It is shown that a turbine bypass is necessary to avoid compressor surge during start-up. The conflicting requirements of crossing natural frequencies quickly while ensuring tolerable temperature gradients in the heat exchangers can be met. The results also show that pressure rise delay through volume dynamics is in the order of seconds. Slow transient effects in the evolution of fluid and heat exchanger temperatures arise from the thermal inertia of the heat exchangers