Dynamic models for the analysis of vapour-compression refrigerating machines

Vapour-compression refrigeration systems represent one of the most relevant fields in terms of energy consumption. Among the research works aimed at reducing their environmental impact, those related to control systems are appealing, since they usually involve lower costs than interventions at system level and directly affects the energy consumption. In this work, a library of models able to reproduce the dynamics of the main components of a vapour-compression refrigeration machine has been developed in Matlab/SIMULINK to supply the building blocks to investigate the effects of different control strategies on the energy performance and on the control quality of the system. To obtain models with low computational cost, the switched moving-boundary approach was adopted for the mathematical model of the heat exchangers. Firstly, the models were numerically verified, using the Matlab toolbox Thermosys as a benchmark. A good agreement between the predictions of the two models was highlighted in the simulation of an air-to-air refrigerating machine; moreover the in-house models showed a better repeatability in the prediction of important quantities like refrigerant superheating, subcooling and charge. The dynamic models were also experimentally validated, comparing their predictions with the experimental data acquired on a water-to-water refrigerating machine equipped with brazed-plate heat exchangers. Results showed a very good agreement between numerical and experimental data, also in terms of electric power absorption and COP, thus proving the model usefulness in transient energy analysis. The effects of the suction accumulator and liquid receiver on the dynamics of the machine were also investigated. A novel lumped-parameter model combining the dynamics of the condenser and of the receiver was developed, allowing the coexistence of a non-zero subcooling at the condenser outlet and a partial filling condition of the receiver. An improvement in the prediction of the refrigerant pressure and temperature at the compressor outlet was highlighted.