Análisis y comparativa de sistemas de subenfriamiento en sistemas de refrigeración de CO2 en climas cálidos

Treball Final de Màster en Enginyeria Industrial. Codi: SJA020. Curs acadèmic 2016/2017La refrigeración con ciclos de CO 2 simple está experimentando un fuerte cambio debido a las nuevas directivas europeas y los bajos rendimientos de estos sistemas en climas cálidos. Estudios teóricos y algunos ensayos experimentales apuntan al uso de ciclos de subenfriamiento mecánico para aumentar el rendimiento y la capacidad frigorífica de los ciclos base. En este trabajo se analizan el sistema de subenfriamiento mecánico dedicado y el sistema de subenfriamiento mecánico integrado desde una perspectiva teórica y experimental con el fin de evaluar sus prestaciones y cuantificar las mejoras energéticas que aportan. Primeramente ambos ciclos son estudiados teóricamente para luego dimensionar las instalaciones que permitan realizar los ensayos experimentales. Las simulaciones se realizan para el nivel de evaporación de 0ºC y para el de -10ºC; cubriendo un abanico de temperaturas ambiente comprendidas entre los 15ºC y los 40ºC. Además se analiza el comportamiento de ambos ciclos a lo largo de un año tipo en diferentes zonas climáticas y diferentes aplicaciones. Finalmente, el estudio de viabilidad muestra el interés de estas aplicaciones en climas cálidos y tamaños de planta elevados.The refrigeration with simple CO 2 cycles is undergoing a strong change due to the new European directives and the low performance of these systems in hot climates. Theoretical studies and some experimental trials point to the use of mechanical subcooling cycles to increase the performance and cooling capacity of the base cycles. In this paper, the dedicated mechanical subcooling cycle and the integrated mechanical subcooling cycle are analyzed from a theoretical and experimental view, in order to evaluate their performance and quantify the energy improvements they provide. Firstly, both cycles are studied theoretically, for then sizing and designing the plants in order to perform the experimental tests. The simulations are carried out for the evaporation levels of 0ºC and -10ºC, covering a range of ambient temperatures between 15ºC and 40ºC. In addition, the behavior of both cycles is analyzed over a year in different climatic zones and for different applications. Finally, the feasibility study shows the interest of these applications in hot climates and high plant sizes

Disseny i instal·lació d'un cicle de subrefredament mecànic per a la millora de l'eficiència energètica d'un clicle de CO2 transcrític

Treball Final de Grau en Enginyeria en Tecnologies Industrials. Codi: ET1040. Curs: 2014/2015Els cicles de refrigeració de CO2 transcrític no resulten completament eficients a altes temperatures. Estudis teòrics han demostrat que un cicle de subrefredament mecànic combinat al cicle transcrític pot augmentar el rendiment i la capacitat frigorífica. En aquest treball s’analitzen de manera experimental les millores energètiques per a un cicle de CO2 transcrític causades per un sistema de subrefredament mecànic. Aquestes millores són estudiades d’una manera teòrica, simulant el comportament del cicle, per després dimensionar i dissenyar la instal·lació que permeti realitzar els assajos adients. A més de l’estudi teòric, les possibles millores son analitzades per a un nivell d’evaporació de 0ºC i una temperatura ambient de 30ºC utilitzant R1234yf com refrigerant en el cicle de subrefredament. Els assajos experimentals mostren un increment del COP d’un 22,8% i de la capacitat frigorífica d’un 34,9%. Els resultats presenten també una reducció de la pressió òptima de treball, coincidint amb les simulacions teòriques prèvies. Finalment, l’estudi de viabilitat mostra l’interès d’aquesta aplicació per a temperatures ambient elevades, majors a 25 ºCTranscritical CO2 refrigeration cycles are not efficient at high temperatures. Theoretical studies have shown that a mechanical subcooling cycle combined with the transcritical cycle can improve performance and refrigeration capacity. In this paper we analyze experimentally the energy improvements to the CO2 transcritical cycle caused by a mechanical subcooling system. These improvements are studied from a theoretical way, simulating the behavior of the cycle, for then sizing and design the cycle to realize the experimental evaluation. Besides the theoretical study, the improvements have been evaluated for one level of evaporation (0ºC) for environment temperature of 30ºC using R1234yf as refrigerant in the subcooling cycle. The results indicate an increase of COP of 22.8% and 34.9% of cooling capacity. It can also be observed a reduction in the optimal working pressure. Finally, the feasibility study confirms the interest of this application at high ambient temperatures, over 25 ° C

Évaluation expérimentale des frigorigènes zéotropes dans un système à sous-refroidissement mécanique dédié dans un cycle au CO2

Use of zeotropic blends in the dedicated mechanical subcooling system of a CO2 refrigeration system was suggested as a possible improvement due to matching of evaporating temperature with CO2 temperature profile during subcooling. This work has verified this possibility and has determined theoretically the best performing compositions of R-600, R-32 and CO2 with the base fluid R-152a. Then, the mixtures have been tested experimentally in a lab-test bench for constant heat load temperature for three heat rejection temperatures (25.1, 30.3 and 35.1°C). Optimum conditions are measured (subcooling degree and heat rejection) and a COP increase of 1.4% has been obtained. The work, for the optimum conditions, analyses the operating parameters of the cycles and focus specially on the thermal parameters of the subcooler. It has been verified that the use of zeotropic mixtures allows to reduce irreversibilities in the cycle, as pointed out theoretically by Dai et al. (2018).Funding for open access charge: CRUE-Universitat Jaume

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

Subcooling methods for transcritical CO2 plants are being studied in order to improve their behavior. Among them, the Integrated Mechanical Subcooling system is one of the most promising owing that performs with high efficiency and it is a total-CO2 system. This work presents the experimental determination of the optimum working conditions of a transcritical CO2 plant working with an integrated mechanical subcooling system. The plant was tested at different pressure and subcooling conditions in order to optimize the COP of the plant and determine the optimal conditions for three ambient temperatures 25.0 °C, 30.4 °C and 35.1 °C and evaporation levels between −15.6 °C and −4.1 °C. Optimum operating conditions were determined and two correlations are proposed to determine the optimal pressure and subcooling as function the gas-cooler outlet temperature and the evaporation level

Improvements in CO2 Booster Architectures with Different Economizer Arrangements.

CO2 transcritical booster architectures are widely analyzed to be applied in centralized commercial refrigeration plants in consonance with the irrevocable phase-out of HFCs. Most of these analyses show the limitations of CO2 cycles in terms of energy e ciency, especially in warm countries. From the literature, several improvements have been proposed to raise the booster e ciency in high ambient temperatures. The use of economizers is an interesting technique to reduce the temperature after the gas cooler and to improve the energy e ciency of transcritical CO2 cycles. The economizer cools down the high pressure’s line of CO2 by evaporating the same refrigerant extracted from another point of the facility. Depending on the extraction point, some configurations are possible. In this work, di erent booster architectures with economizers have been analyzed and compared. From the results, the combination of the economizer with the additional compressor allows obtaining energy savings of up to 8.5% in warm countries and up to 4% in cold countries with regard to the flash-by-pass arrangement and reduce the volumetric displacement required of the MT compressors by up to 37%

CO2 with Mechanical Subcooling vs. CO2 Cascade Cycles for Medium Temperature Commercial Refrigeration Applications Thermodynamic Analysis

A recent trend to spread the use of CO2 refrigeration cycles in warm regions of the world is to combine a CO2 cycle with another one using a high performance refrigerant. Two alternatives are being considered: cascade and mechanical subcooling systems. Both respond to a similar configuration of the refrigeration cycle, they being based on the use of two compressors and same number of heat exchangers. However, the compressor, heat exchanger sizes and energy performance differ a lot between them. This work, using experimental relations for CO2 and R1234yf semi-hermetic compressors analyzes in depth both alternatives under the warm climate of Spain. In general, it was concluded that the CO2 refrigeration solution with mechanical subcooling would cover all the conditions with high overall energy efficiency, thus it being recommended for further extension of the CO2 refrigeration applications

Experimental analysis of R-450A and R-513A as replacements of R-134a and R-507A in a medium temperature commercial refrigeration system

This work presents the experimental evaluation of R-513A (GWP = 573) and R-450A (GWP = 547) as R-134a (GWP = 1301) drop-in replacements and as R-507A (GWP = 3987) retrofits in a commercial direct expansion refrigeration system for medium temperature applications (2 °C). The evaluation covered 24-hour tests using a single-stage cycle with semi-hermetic compressor, an electronic expansion valve customized for each refrigerant and a commercial vertical cabinet with doors placed inside a climatic chamber. The tests were performed at three water dissipation temperatures (23.3, 32.8 and 43.6 °C). Experimental results indicate that R-513A and R-450A can operate with R-134a plants, with increments in energy consumption between −1.6 to +1.2% for R-513A and from +1.3 to +6.8% for R-450A, whereas in comparison with R-507A, R-513A offered reductions in energy consumption between 4.4 to 8.2% and R-450A between 0 to 3.3%. The paper analyzes the modification of the operating pressures/temperatures and the energy indicators using the four refrigerants

Energy assessment of an R134a refrigeration plant upgraded to an indirect system using R152a and R1234ze(E) as refrigerants

In this work the experimental results obtained from a direct expansion refrigerating plant converted into an indirect plant, are presented and discussed in terms of energy consumption for a commercial refrigerating unit plugged to a medium temperature cabinet. Refrigerants used in the analysis are R134a for direct system, and R134a, R152a and R1234ze(E) for indirect system. The evaluation has been performed at laboratory conditions using three different heat rejection levels: 43.6, 32.8 and 23.3 °C keeping constant the environmental conditions in the cabinet (25 °C; 55%). From the experimental tests performed during 24 h, it has been demonstrated that the adoption of an indirect configuration results in an increment of the total energy consumption of the refrigerating plant regardless the refrigerant adopted. The average increment registered was 21.8% for R134a, 18.7% for R152a and finally 27.2% for R1234ze(E). The adoption of an indirect system also reduces the refrigerant mass charge of the facility up to 62% depending on the refrigerant used

Energy evaluation of a Low Temperature commercial refrigeration plant working with the new low-GWP blend R468A as drop-in of R404A

This work analyses experimentally the energy behaviour of a centralized commercial refrigeration plant designed to be used with R404A (GWP100=3,943) at low temperatures, when it is replaced with the new low-GWP refrigerant blend R468A (GPW100=148). The tests have been done at a selected product temperature of -20°C, Class III indoor conditions according to ISO 23953-2:2015 and 20, 30 and 40°C of heat sink temperature. The product has been kept at the requested temperature in whichever conditions tested. Respect to R404A, running with R468A presents an increase in compressor discharge temperature, greater compressión operation time, and a reduction in compressor electric comssumption that yields in a very similar total energy consumption of the refrigeration plant. So it has been proved that, from an operational point of view, R468A can be a R404A drop in replacement fluid. Also, it has been measured energy savings when an internal heat exchanger is placed

Alternative blends of CO2 for transcritical refrigeration systems. Experimental approach and energy analysis

As a natural refrigerant, Carbon Dioxide (CO2) has been extended in almost all refrigeration fields due to its environmental friendliness, high availability, and high technological development in the main components of the cycle. However, despite the benefits, CO2 is limited to high-capacity applications due to the cost of components and the complex cycle arrangements to overcome the low coefficient of performance (COP) at high ambient temperatures. A simple but effective method to solve this issue involves mixing CO2 with other refrigerants obtaining a new refrigerant mixture with higher critical points, lower working pressures and low global warming potential (GWP). Depending on the mixture percentage, the mixture flammability can be suppressed, and the performance of the refrigeration plant can be increased. Building on this, this work determines theoretically different CO2­based zeotropic blends as an alternative to pure CO2 with the restrictions of non-flammability, GWP below 150 and higher COP than pure CO2. The results suggest two blends of CO2/R1270 and CO2/R32 that have been prepared and energetically tested in a vertical display cooler using pure CO2 as a reference. The results revealed that CO2-blends reduce energy consumption by up to 17.2 % at the ambient temperature of 25 °C and up to 12.2 % at 30 °C. Moreover, the results with CO2-blends were closest to those obtained with pure R1270 and better than R134a under the same operating conditions