Comparative study of a cascade cycle for simultaneous refrigeration and heating operating with ammonia, R134a, butane, propane, and CO2 as working fluids

In this paper, a cascade system for simultaneous refrigeration and heating is simulated with different working fluids. Ammonia, R134a, butane and propane are evaluated in the low-temperature (LT) cycle and carbon dioxide (CO2) is used in the high cycle. The effects of the thermodynamic parameters on the cascade system are evaluated with the aim of finding the best working fluid performance and optimum design parameters. Coefficients of performance (COP) and exergetic efficiencies were estimated for each one of the cycles and for the entire system. The behaviour of these parameters is presented as a function of the internal heat exchanger effectiveness and main operating system temperatures. The results showed that the cascade system using butane in the LT cycle increased the COP up to 7.3% in comparison with those obtained with NH3-CO2. On the other hand, the cascade systems operating with the mixtures R134a-CO2 and propane-CO2 presented similar results reaching COPs up to 5% higher than those obtained with the NH3-CO2 system.