Master's Thesis Friederike Dickel


Development of scalable heat exchanger simulation models for the evaluation of low GWP refrigerants in compression heat pumps

Scalable heat exchanger Copyright: EBC

The compression heat pump is a key technology to meet the environmental objective set by the German ’Energiewende’. Continues improvement is necessary to ensure future operation of heat pumps with alternative refrigerants and thereby realize the targeted decrease in primary energy demand. During the preliminary design simulations of yearly energy demand are required and therefore, a simulation model is neccessary, which allows the assessment of operational strategies. Furthermore, it can be used as a digital mirror in dynamic operation for functionality proof purposes. In this paper this simulation model is refined utilizing the modeling language Modelica respecting the ISO 9126 criteria, namely accuracy, speed and stability.

Regarding accuracy, the condenser modelling is crucial for the whole system performance. Initially, literature research is done to compare different approaches to the model before the physical finite-volume method (FVM) approach is chosen. This method divides the heat exchanger in discrete elements. Similar to the Moving-Boundary-Method (MBM), FVM proves to be a reasonable compromise between the model’s calculation speed and accuracy.

Modelling heat transfer coefficients is challenging. This FVM-model enables the calculation in relation to phase and flow type in order to physically map the effect the multi-phase flow has on the heat transfer. However, instabilities occur during the simulation process due to the implementation of discrete elements in the heat exchanger model and the phase change. Therefore, an analysis on the occurring system status as a result of those instabilities has to be conducted. In this paper, we present a model to analyze the stability of subsystems. In addition we examin transient system behavior to given variations in the thermodynamical and fluid mechanical state variables. Lastly, the behavioral alteration caused by the phase change is analyzed as well. The FVM Model specifically maps the shift in phases that occur in the condenser, namely the change from super heated vapor to the diphase region and finally to sub cooled liquid.

Applying the new condenser model, we evaluate the effect of the vapor injection (VI) on the entire system in the stationary preliminary assessment. It can be shown that at the evaporator’s outlet only a certain temperature range is feasible when applying the VI method.

To obtain a stable working heat pump model a transfer of the robustness tests to the aggregated overall system is essential. Further research is needed in regards to the heat pump’s preliminary design. It is recommended to focus on the dynamical application of the overall system performance in relation to VI as well as sub cooling at the condenser’s outlet.