Thermodynamic Analysis of a CO2 Refrigeration Cycle with Integrated Mechanical Subcooling

Abstract

Different alternatives are being studied nowadays in order to enhance the behavior of transcritical CO 2 refrigeration plants. Among the most studied options, subcooling is one of the most analyzed methods in the last years, increasing cooling capacity and Coefficient Of Performance (COP), especially at high hot sink temperatures. A new cycle, called integrated mechanical subcooling cycle, has been developed, as a total-CO 2 solution, to provide the subcooling in CO 2 transcritical refrigeration cycles. It corresponds to a promising solution from the point of view of energy efficiency. The purpose of this work is to present, for the first time, thermodynamic analysis of a CO 2 refrigeration cycle with integrated mechanical subcooling cycle from first and second law approaches. Using simplified models of the components, the optimum operating conditions, optimum gas-cooler pressure, and subcooling degree are determined in order to obtain the maximum COP. The main energy parameters of the system were analyzed for different evaporation levels and heat rejection temperatures. The exergy destruction was analyzed for each component, identifying the elements of the system that introduce more irreversibilities. It has been concluded that the new cycle could offer COP improvements from 11.7% to 15.9% in relation to single-stage cycles with internal heat exchanger (IHX) at 35 • C ambient temperature.Energies 2020, 13, 4 2 of 17 the parallel compressor in a real plant, which lead to increments in COP not as promising as the theoretical results. Also, the use of ejectors is widely studied as a way to improve CO 2 installations either using multi-ejectors [6] or adjustable ejectors [7]. Even with the promising results of this solution, the operation and control remain complex.The other great research line is focused on subcooling methods [8]. The purpose of subcooling methods is to subcool the CO 2 at the exit of the gas-cooler, which increases the COP of the plant due to the increment on the specific cooling capacity, the reduction of the optimum working pressure, and the reduction of the specific compression work [9]. First, studies show that when the subcooling is higher, the increments are higher. However, not all the subcooling systems have the same performance, nor the same range of application. The improvements they can produce depend on the cost of the subcooling and on the working conditions. To obtain the greatest benefits of this type of system, they must be optimized in terms of pressure and subcooling degree to achieve maximum COP. In addition to the benefits that contribute to the energy efficiency of the plant, these systems also have benefits from an exergy analysis. The reduction of the optimum pressure and the subcooling allow for reducing the exergy destruction that takes place in the expansion process, leading to configurations with greater exergy performance.The main subcooling methods are classified as internal methods and dedicated subcooling methods. The first studied method and widely applied in applic...

Experimental determination of the optimum working conditions of a transcritical CO2 refrigeration plant with integrated mechanical subcooling

“…This contribution is greater the higher the water temperature and the lower the evaporation level are (when further the heat source and hot sink are). The effect of the subcooling cycle is higher at high rejection temperatures and low evaporation levels because these are the conditions where the behaviour of the plant needs to be more improved due to the reduction of the COP, as it was presented by Nebot-Andrés et al (2019b). The contributions represent between 25.6% and 21.5% at 25.0ºC, between 31.5% and 24.1% at 30.4ºC and between 33.2 and 24.9 at 35ºC.…”

Experimental Investigation of the Effect of a Recuperative Heat Exchanger and Throttles Opening on a CO2/Isobutane Autocascade Refrigeration System

“…Basic flash gas bypass transcritical systems lack efficiency at high heat sink temperatures mainly because of high throttling irreversibilities [5]. Several research works have been carried out to make these systems suitable for high ambient temperature conditions, including ejectors [6] and mechanical subcooling [7].…”

Desarrollo de una instalación frigorífica para la implementación de sistemas Booster CO2. Análisis teórico y experimental de diferentes configuraciones

“…Utilizando esta mejora para sistemas con CO2 de simple etapa, Cechinato et al 2009 [11] realizó un análisis termodinámico de este sistema para un ciclo con un nivel de evaporación, realizando el subenfriamiento con CO2, pero con un ciclo de compresión de vapor independiente del ciclo principal, teniendo, a términos prácticos, el mismo principio de funcionamiento. Así mismo, Nebot-Andrés et al 2019 [58] analizó termodinámicamente el uso del subenfriamiento integrado en un ciclo de simple etapa con CO2, obteniendo incrementos del COP de hasta el 15.9% para temperaturas de evaporación de -10ºC.…”

“…The minimum pressure limitation is needed to keep a minimum pressure differential at the expansion valves. Although a minimum pressure differential around 3.5 bar is needed in all the valves for proper operation [54], in a transcritical booster, the minimum flash tank pressure considered is approximately 5 bar higher than the pressure of the medium temperature loads [58]. Thus, for the evaporation temperature of -6ºC (≈30 bar), the minimum flash tank pressure considered will be 35 bar.…”