A R717/R744 cascade model was designed to be able to simulate the freezing process of pelagic fish. The modelling language Modelica was applied and it was implemented in the simulation environment Dymola Control. Various simulations were performed for an example design of the freezing tunnel. The total energy consumption of the R717/R744 cascade unit was compared to a state of the art two-stage ammonia system. In addition, the temperature reduction due to pressure drop in an ammonia evaporator at low temperatures was investigated and compared with CO2.
The results show that the R717/R744 cascade system architecture, when properly designed, offers higher overall system performance expressed by energy consumption lower by 3.6% and reduction of the freezing time by 4% at the same time, compared to the conventional ammonia system. What is more, by reduction of evaporation temperature in the R744 cycle, the facility operator is able to boost the system refrigeration capacity by 36%, maintaining the same freezing time.