Experimental and numerical study on the refrigerant charge in chiller and heat pump systems

The current climate change concerns are forcing international organizations and governments to issue protocols and regulations for environmental protection. Regulation No 517 of the European Union, in 2014, has introduced a significant reduction of the number of hydrofluorocarbons (HFCs) to be placed in the European market for the following years. From a global perspective, the Kigali Amendment to the Montreal protocol, in 2016, has asked for the phase-out of the high-GWP refrigerants due to their significant environmental impact. In this context, it is fundamental to identify new alternative fluids and reduce the refrigerant charge of refrigeration and air-conditioning systems. Many low-GWP refrigerants present a lower environmental impact, but have problems with flammability and toxicity. The refrigerant charge reduction is thus essential to attenuate the risks associated with using these fluids. The main goal of this research work is to perform a numerical and experimental study on the refrigerant charge in air-conditioning and heat pumps systems. For this purpose, simulation software has been developed; the model can simulate chiller and heat pump systems and calculate the amount of refrigerant in the various components. As a first step, the heat transfer coefficient of two zeotropic mixtures (R452B and R455A, blends of R32 and R1234yf) has been measured during condensation and vaporization; the experimental data have been used to assess the prediction of heat transfer models. A comparative analysis of the heat transfer performance of refrigerant R410A and the alternative fluids R32, R452B and R455A is then carried out; the study has indicated R32 as an effective alternative to R410A for comfort applications. For this reason, a new system working with refrigerant R32 has been tested; the system is an invertible air-to-water heat pump in replacement of an R410A unit. The unit's performance has been first measured with a finned coil heat exchanger and then a microchannel heat exchanger on the airside. Moreover, measurements of refrigerant mass distribution among the system components have been carried out. The experimental data have been used to validate the software developed. The heat transfer coefficient measurements have been carried out at the two-phase laboratory at the University of Padua. The performance and mass distribution tests have been performed in the test room of Clivet S.p.A, financier of the present work. The manuscript is organized into eight chapters: • Chapter 1 presents a literature review on the available void fraction correlations and lists the previous research on refrigerant mass distribution in air conditioning and heat pump systems. • Chapter 2 describes Charge Calculator, a calculation tool developed to simulate heat pumps and air conditioning systems. • Chapter 3 presents the apparatus and the procedures used in the experimental activities. Measurements of heat transfer coefficient, performance and mass distribution have been performed in this work. • Chapter 4 presents the experimental heat transfer coefficient measured during the condensation and vaporization of the two zeotropic blends R452B and R455A. The heat transfer models' assessment is also reported in this chapter. • Chapter 5 shows the experimental results of the performance tests. • Chapter 6 reports the experimental results of the refrigerant mass distribution measurements among the system components. • Chapter 7 presents the validation of Charge Calculator. • Chapter 8 reports and discusses the numerical results obtained with Charge Calculator. The effect of the refrigerant charge and airflow velocity distribution on the system performance is studied. Finally, a TEWI-based analysis attempts to assess the substitution of refrigerant R32 with two low-GWP alternatives.