Master's Thesis Mirko Engelpracht
Development of modular and scalable simulation models for heat pumps and chillers considering various refrigerantsCopyright: EBC
Heat pumps are a key technology in order to achieve the German energy transition as well as energetic and environmental objectives related to this. Therefore, there is both an urgent need to optimise heat pumps with respect to energy efficiency and control systems must be developed that allow efficient application of heat pumps in complex energy systems. Increasing system complexity leads generally to simulative studies and developments and, for this purpose, simulation models required must be both transient and component-based. For that reason, this thesis broaches the issue of developing modular and scalable heat pump simulation models using Modelica.
Considering the overall simulation speed and model accuracy, the refrigerant's properties model is the most important one. Therefore, a fluid property library is developed that is based on a simplified equation of state using reduced Helmholtz energy. The refrigerants R134a and R410A are implemented in this library. Apart from that, an architectural approach is developed in order to flexibly build up various heat pump configurations. Thus, the approach focuses on the modularity as well as on the scalability of the simulation models. Simulation models required for this approach are developed for compressors, expansion valves and heat exchangers.
Based on a literature review, the compressor is an efficiency model. The authors implement the efficiencies using the Buckingham PI- theorem and, thus, the compressor model is appreciated to compute impacts of refrigerants that are not measured yet. We model the expansion valve using Bernoulli's principle and implement the flow coefficient required using the Buckingham PI- theorem again. Both the compressor model and the expansion valve model are scalable and, hence, an adaption of the components is possible to different power classes. A moving boundary approach is a basis to model the heat exchangers and the authors prepare a switching approach between different flow states. As a consequence, the heat exchanger models offer a compromise between model accuracy and simulation speed.
Finally, the simulation model of the heat pump is evaluated analysing the compressor and expansion valve models. These analyses illustrate that the compressor model can easily be calibrated using experimental data. Average deviations of the predicted mass flow rates and the electrical power consumptions are 1.5 % and 3.0 % respectively. Furthermore, we show that the fluid property models developed in this thesis have errors that are almost zero compared to fluid property models developed by NIST. The simulation speed is improved by a factor of 30. A final investigation shows the possibility to simulate closed-loop heat pump cycles using transient boundary conditions.