Subcooling systems in transcritical CO2 refrigeration cycles. Experimental evaluation of energy improvement

Résumé

This thesis aims to offer a competitive and free technology solution to the current problems related to refrigeration systems and their efficiency. The main objective of the work is to study and evaluate different mechanical subcooling systems that imply an improvement in the energy performance of transcritical CO2 refrigeration facilities. Global warming is one of the main problems in today's society and in order to reduce the CO2 emissions responsible for global warming, different regulations and directives have been established both at the global and the European level. Specifically, regulations that directly affect refrigeration, responsible for almost 8% of these emissions, regulate or prohibit the use of certain refrigerants in these facilities. This leaves CO2 as the best solution that can be implemented in centralized commercial refrigeration. The problem arises especially in hot climates, where simple CO2 cycles are not very efficient and therefore, although we face the problems derived from direct emissions, indirect emissions are greater than those of the systems used to date. In this thesis, therefore, the focus is on improving the efficiency of these systems thanks to the use of subcooling systems. Specifically, it focuses on two systems, called the dedicated mechanical subcooling (DMS) and the integrated mechanical subcooling (IMS). The first, the dedicated mechanical subcooling, has great potential for improvement although it uses a refrigerant other than CO2 in the auxiliary cycle. On the other hand, the much less studied integrated mechanical subcooling only works with CO2, which can be an important advantage. The thermodynamic study of these systems is fundamental to establish which are the applications to which they can give service and to determine their limits of operation. For this reason, different thermodynamic simulations of both systems and also of the reference system have been developed to be able to know the behavior of these new systems and also their behavior against the different operating parameters. From these studies it has been deduced that they are systems that must be optimized both in terms of gas-cooler pressure and of subcooling degree. As a continuation of this first analysis, an experimental laboratory plant has been designed and set up, which integrates all the aforementioned technologies and in which all the systems have been studied. As a result, the optimal pressures and the subcooling degree of each one of the systems have been experimentally determined and the main energy parameters of the systems have been obtained under different operation conditions: evaporation levels close to -5ºC, -10ºC and -15ºC and temperatures of the hot sink of 25ºC, 30ºC and 35ºC. In turn, the use of zeotropic mixtures in the dedicated mechanical subcooling system has also been studied and different configurations of the integrated system have been analyzed. Finally, the two systems have been compared experimentally against the reference system, the parallel compressor, corroborating the positive effects of both systems and quantifying the improvements achieved. The main results show experimentally that the IMS provides increments in COP of 4.1% at 25.0ºC, 7.2% at 30.4ºC and 9.5% at 35.1ºC and the DMS of 7.8%, 13.7% and 17.5% respectively when comparing them to the use of the parallel compressor for an evaporating level near -10ºC. From these results it can be concluded that the best system, from an energy point of view, is the DMS since it achieves more significant increases.