Análisis y optimización de un sistema de refrigeración mediante compresión por eyector

Abstract

Since mid-nineteenth century until today, the world energy consumption has risen notably. Being the fossil fuels the main contributors to the energy production, the CO2 emissions have increased in the same proportion boosting therefore the green house effect. In particular, the air conditioning and heating sectors represent more than 20% of the global electricity consumption, widely exceeding this level if the industrial refrigeration installations are considered. Nevertheless, a reduction of this demand is not expected but an increment of 70% in the following eighty years or even triple of the current demand according to the scenario foreseen by the International Energy Agency (IEA). Due to that situation governments, companies, institutions and researchers are making countless efforts to reduce the dependency of the fossil fuels by means of the use of renewable energies or by means of the energy efficiency improvement of the thermal equipments. The application field of the present thesis is the industrial, commercial or domestic refrigeration. One possibility to improve the efficiency of the refrigeration units is by means of the heat recovery in co-generation units. The cooling circuits can use the wasted heat from industrial processes, or received from renewable energies, by means of absortion systems or by the use of ejectors. In the case of ejectors, its smaller cost of installation, operation and maintenance justifies its use even with a lower performance level compared with the absortion cycles. It is worth mentioning that the ejectors as an expansion devices can be used in refrigeration units without any external heat source. As highlighted by several authors the performance shown by the ejector cycles is very poor. As a consequence, this thesis is focused on the analysis and optimisation of the ejector cooling systems. The problem has been addressed in three different phases, being the main the energy optimisation of the cooling units. Firstly, as shown by the literature review, previous studies to this thesis proposed several configurations of ejector cooling cycles. However, the comparison between them is not possible because of the different working conditions applied or because of the use of different refrigerants. Thus, with the purpose of knowing the feasibility of the ejector cooling cycles, a systematic comparison of the most representative cycle typologies has been carried out against the basic vapour compression cycle and by using simplified models of the ejector and the cycle. A revision of the most suitable refrigerants for the ejector cooling cycles applications has been done. Those should have a low Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) as well as being neither toxic nor flammable. In addition, the performance should be optimal for this particular application. Thus, the refrigerants R134a, R1234yf and R600a have been chosen for the subsequent study. The results showed that, depending on the working conditions and the working fluid, the performance of the ejector cooling cycles can improve up to 27% with respect to the basic vapour compression cycle. However, there are many situations in which the ejector cooling cycles shows worse performance than the basic cycle. The first results showed that the ejector provides a certain potential for improving the energy efficiency of the cooling units. However, the prediction of the ejector behaviour is rather complex. Thus, the detailed design of the ejector geometry require of a very deep understanding of the fluid physics. This is the main issue of the second stage in which a Multi-Objective optimisation methodology, based on Computational Fluid Dynamic (CFD) simulations, has been developed. This methodology is able to provide the set of geometries which maximise the ejector performance. Furthermore, a sensitivity analysis of the geometrical variables has been performed showing that the most sensitive variables are the diameter of the primary nozzle throat and the diameter of the constant section zone in the secondary nozzle. For the validation of the optimisation methodology, CO2 and air have been used as working fluids. Finally, the problem of optimising the seasonal efficiency of the cooling units by means of the \textit{Seasonal Performance Energy Ratio} (SEPR) has been addressed. To this end, both former methodologies were coupled by applying the thermodynamic cycle models with the CFD simulations of the ejector to thermally driven cycles using R134a as working fluid. The results showed that the ejector is able to increase the rated cooling capacity of the cycle up to 13% with respect to the basic vapour compression cycle but with a modest increase in the seasonal efficiency of 2%. However, by replacing the compressor for a smaller one, the ejector allows the achievement of the same rated cooling capacity with an increment in the seasonal efficiency of 23%. On the whole, based on the results obtained, three main points are to be highlighted. First, there are several combinations of ejector and cooling cycles which are capable of increasing its cooling capacity. Second, the ejector geometry has been proved to be a key factor in the performance of both the device itself and the cycle. Finally, there is a significant increase of the seasonal efficiency of the cooling cycle by choosing a suitable set of ejector-compressor, which will contribute to the reduction of both, the CO2 emissions and the long term energy demand.