Physics-based dynamic model for reversible liquid-to-liquid heat pump systems. Development, validation and simulation with different systems, time-scales, operation conditions and working mode switch

Abstract

In order to develop new HPs, there are two important aspects. The improvement of the components and the improvement of the joint system. The components would improve by optimizing geometrical parameters and materials, among others. Regarding the joint system, the main goal is to improve the control loop of the system. Nowadays, the best way to address these aspects is by the dynamic modeling of components and systems that could simulate accurately their behavior. During this thesis a model for reversible liquid-to-liquid HP has been developed.

The physics-based dynamic model to simulate the behavior of reversible liquid-to-liquid HPs is presented. The model of each component of the HP is developed separately and then they are joined together. The model is implemented in Matlab/Simulink environment.

A refrigerant-to-liquid plate heat exchanger (PHEX) model was developed using the Finite Control Volume method. The behavior of PHEXs either if they are working as condensers or as evaporators can be simulated. Moreover, they allow the user to choose the configuration of the PHEX between parallel-flow and counter-flow. The equations and their implementation have been presented.

After that, the model is validated in micro-scale situations, understanding that the micro-scale is the time scale at which fast transient-states are produced. Examples include operation condition changes, working mode switches or system start-ups.

Different tests under different transient situations were developed. Measurements have been used to validate the model under heating, cooling and start-up situations. Simulation results present good agreement with test data.

Then, the reversible behavior of the system was simulated by starting in cooling mode, switching to heating mode and going back to cooling mode. Since experimental data of working mode switches was not available, it was not possible to compare the simulation results with test data. However, the obtained results could have a good alignment to the actual behavior of the system during a working mode switch.

Finally, the model was used to simulate the behavior of other HP under macro-scale situations, understanding that the macro-scale is the time scale at which the dynamics of the system are studied during long time periods. With it, the utility of the model to obtain the performance of the system is studied.

A good agreement is observed between test data and simulation results in water temperatures but the compressor power consumption is underestimated. in order to clarify the divergences in the compressor power consumption, a system energy performance study was conducted concluding in the improvement of the compressor model and therefore improving the simulation results.