| dc.description.abstract | The increasing global average ambient temperatures coupled with stringent environmental regulations has turned the focus on natural working fluids such as Carbon dioxide (CO2), and hydrocarbons for use in the refrigeration industry. CO2 being a natural, non-flammable and non-toxic fluid with a triple point temperature of -56.6°C at pressure of 5.2 bar makes it a good choice for wide range of refrigeration applications including deep freezing. The use of CO2 for refrigeration applications is not new, it was perhaps the only refrigerant in the early 20th century for deep freezing applications. However, the advent of efficient synthetic refrigerants with lower operating pressures led to a gradual decline of CO2 for refrigeration applications. CO2 has a low critical temperature of 31.1°C at pressure of 73.8 bar, making it unsuitable for operating in the subcritical mode for cooling at high ambient conditions. Therefore, CO2 refrigeration systems work in transcritical mode for ambient conditions greater than 30°C. Consequently, the standstill pressures of transcritical CO2 (T-CO2) systems are not only higher, but also require substantial energy for compressing CO2 to higher pressures. Compressing CO2 to high pressures yields high gas cooler temperatures with significant amount of heat rejection. As a result, T-CO2 systems suffer from lower COP’s compared to conventional subcritical refrigeration systems.
There has been considerable work reported in the literature for improving the COP of T-CO2 systems. Among the many methods that have been proposed, meaningful utilization of gas cooler heat seems to be the most preferred choice. The thesis proposes a host of novel concepts like cascaded gas cooler heat-driven vapor ejector refrigeration systems (VERS) to CO2 propane mixtures to reduce compression work. The first part of the thesis presents a comprehensive analysis of variable geometry ejector systems for reducing the compressor work in standard T-CO2 systems. The developed ejector model is verified by experimental measurements obtained from IIT-Madras T-CO2 refrigeration test system. The proposed T-CO2 systems can be simultaneously used for different applications: deep freezing (-40 to -25°C), refrigeration (-8 to 2°C), air-conditioning (5 to 9°C) and heating applications till 80°C. These systems can be used in supermarkets, hospitals, hotels, dairy industries or shipboard cooling. Subsequently, a comprehensive analysis of a novel T-CO2-VERS hybrid system is presented which is entirely driven by the heat rejected in the gas cooler. The performance and cooling capacity of the VERS not only depends on the compressor discharge temperature of CO2 entering the generator but also on the quantity of heat available at this temperature. Both factors limit the choice of working fluids for the ejector refrigeration system. The hybrid system shows a significant improvement in cooling capacity over the baseline T-CO2 system in the range of 10% -50% for the range of evaporator temperatures. The system provides the highest COP at an evaporating temperature of 12.5°C, which is ideal for chilled water-based data center cooling or centralized air conditioning applications. Finally, a zeotropic mixture of CO2+Propane (C3H8) is proposed for reducing the operating pressure of the system. The presence of CO2 in the mixture suppresses the flammability of C3H8, while the presence of C3H8 reduces the critical pressure of the CO2+C3H8 mixture. Amidst the growing environmental concerns, improving the system performance of zeotropic hydrocarbon mixture has been the biggest challenge in recent years. Addressing this concern, this work aims to arrive at an optimum CO2+C3H8 mixture composition with significantly reduced working pressures to allow subcritical operation without adversely affecting system performance. In this case, two evaporation models are proposed based on constant pressure operation and constant temperature operation, both of which utilize the temperature glide to provide cooling. The performance of the mixture is analyzed for the various mass compositions of CO2 permissible within the flammability suppression envelope. The analysis shows that 15% CO2 in the mixture is ideal for most refrigeration and air-conditioning applications. | en_US |
