Session: 06-02 Power to Heat Solutions

Paper Number: 79399

79399 - 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 an urgent topic to achieve significant greenhouse gas emission reduction. Heat pumps using renewable electricity can potentially cover the heat demand below 500 °C. Commercially available heat pumps can only deliver heat below 150 °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. The system is able to utilize a variety of heat sources at temperatures between -60 to 80 °C.

Steady-state operation with different settings as well as technical details of the prototype have been described in previous work of the authors. To achieve safe operation, transient operation of the heat pump - including start-up and shutdown - must also be considered. The large volumes and heat capacity of heat exchangers and tubing lead to a deviation from the steady-state working line as long as these volumes are being filled or heated. Temperature gradients must be kept below a limit, defined mostly by thermal tensions 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 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, different control strategies are analyzed in order to minimize operational risks. Available control parameters are compressor shaft speed, heat sink (cooling) cycle mass flow and the turbine bypass. The main focus lies on accommodating the conflicting requirements of crossing eigenfrequencies quickly while ensuring tolerable temperature gradients in the heat exchangers and avoiding compressor surge at the same time.

The work showed that a turbine bypass is necessary to avoid compressor surge during start-up and shut-down. Temperature and pressure changes in the regime where resonant frequencies must be crossed can be kept in the prescribed limits, if eigenfrequency bands are determined precisely. Our first experimental study confirms the calculated eigenfrequencies with an error of less than 1 %. The results also show that pressure rise delay through volume dynamics is in the order of seconds.  Further pressure changes stem from the thermal inertia of the heat exchangers that produces a larger delaying effect. As a result, transients with durations of up to one hour until steady-state operation are estimated. In conclusion, safe operation of the heat pump is possible, especially transition from standstill to normal operation and back.

Presenting Author: Johannes Oehler Deutsches Zentrum für Luft- und Raumfahrt (DLR)

Presenting Author Biography: Johannes (he/him), is an aerospace engineer with a passion for researching technologies that aim to replace fossil fuel usage. His current project is the development of a turbomachine-driven high temperature heat pump for industrial heat requirements at the german aerospace center DLR.

Authors:

Johannes Oehler Deutsches Zentrum für Luft- und Raumfahrt (DLR)
A. Phong Tran Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Panagiotis Stathopoulos Deutsches Zentrum für Luft- und Raumfahrt (DLR)