Review of experimental research on supercritical and transcritical thermodynamic cycles designed for heat recovery application

Supercritical operation is considered a main technique to achieve higher cycle efficiency in various thermodynamic systems. The present paper is a review of experimental investigations on supercritical operation considering both heat-to-upgraded heat and heat-to-power systems. Experimental works are reported and subsequently analyzed. Main findings can be summarized as: steam Rankine cycles does not show much studies in the literature, transcritical organic Rankine cycles are intensely investigated and few plants are already online, carbon dioxide is considered as a promising fluid for closed Brayton and Rankine cycles but its unique properties call for a new thinking in designing cycle components. Transcritical heat pumps are extensively used in domestic and industrial applications, but supercritical heat pumps with a working fluid other than CO2 are scarce. To increase the adoption rate of supercritical thermodynamic systems further research is needed on the heat transfer behavior and the optimal design of compressors and expanders with special attention to the mechanical integrity

Modeling of S-RAM Energy Recover Compressor Integration in a Transcritical Carbon Dioxide Cycle for Application in Electronics Cooling in Varying Gravity

As electronics in military aircraft become increasingly complicated, additional cooling is necessary to enable efficient and high computing performance. Additionally, the varying forces that a military aircraft endure during maneuvering and inverted flight introduce unique design constraints to the electronics cooling systems. Because this cooling system will be in an aircraft, the capacity and unique design constraints must all be met with a design that is as lightweight as possible. This paper presents a study comparing the coefficient of performance (COP) of several cycle architectures with both R134a and carbon dioxide ( ). Cycles with single-stage and two-stage compression with intercooling are compared, and both are modeled with suction-to-liquid line heat exchangers. The cycles utilizing are transcritical in order to reach the required temperatures for heat rejection from the gas cooler. Additionally, cycles with expansion work recovery and an ejector are compared. The cooling requirements are up to 150 kW with a heat source temperature as low as and a heat sink temperature of up to . The purpose of this analysis is to understand which of the above cycles performs with the highest efficiency for the given electronics cooling application

Sistemi di refrigerazione innovativi a CO2 per climi caldi

Soluzioni per migliorare le prestazioni di cicli frigoriferi a CO2, per applicazioni in climi caldiope

Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

[EN] In this work the possibilities of enhancing the energy performance of CO2 transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO2 transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, 5 and 30 C) for environment temperatures from 20 to 35 C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed.The authors gratefully acknowledge Jaume I University of Spain, who financed the present study through the research project P1.B2013-10.Llopis Doménech, R.; Cabello, R.; Sanchez, D.; Torrella Alcaraz, E. (2015). Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling. International Journal of Refrigeration. 55:129-141. https://doi.org/10.1016/j.ijrefrig.2015.03.016S1291415

Ejector refrigeration: A comprehensive review

The increasing need for thermal comfort has led to a rapid increase in the use of cooling systems and, consequently, electricity demand for air-conditioning systems in buildings. Heat-driven ejector refrigeration systems appear to be a promising alternative to the traditional compressor-based refrigeration technologies for energy consumption reduction. This paper presents a comprehensive literature review on ejector refrigeration systems and working fluids. It deeply analyzes ejector technology and behavior, refrigerant properties and their influence over ejector performance and all of the ejector refrigeration technologies, with a focus on past, present and future trends. The review is structured in four parts. In the first part, ejector technology is described. In the second part, a detailed description of the refrigerant properties and their influence over ejector performance is presented. In the third part, a review focused on the main jet refrigeration cycles is proposed, and the ejector refrigeration systems are reported and categorized. Finally, an overview over all ejector technologies, the relationship among the working fluids and the ejector performance, with a focus on past, present and future trends, is presented. (C) 2015 Elsevier Ltd. All rights reserved

An experimental study of an ejector-boosted transcritical R744 refrigeration system including an exergy analysis

The field of refrigeration witness a massive transition in the supermarket with a strong focus reflected on energy consumption. The use of ejector allows for overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. In this study, a detailed experimental work and exergy analysis on the R744 transcritical ejector cooling system was investigated. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The impact of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature and receiver pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destruction were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provide a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by rising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the exergy consumed and destruction were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. © 2021 Elsevier LtdacceptedVersio

Experimental assessment of dedicated and integrated mechanical subcooling systems vs parallel compression in transcritical CO2 refrigeration plants

Mechanical subcooling systems, both dedicated and integrated, have attracted lot of attention in the recent years due to their great potential to improve transcritical CO2 refrigeration systems. Numerous studies have theoretically determined the COP increments that these systems can offer compared to classic systems and experimental works have evaluated the optimum working conditions for each individual system. However, they have not been contrasted experimentally. In this work, the dedicated and integrated mechanical subcooling systems are experimentally contrasted to the parallel compression one, which is considered as base system. The optimum energy performance of the three systems is contrasted for three heat rejection levels: 25.0 ◦C, 30.4 ◦C and 35.1 ◦C. The experimental tests show increments in COP of 4.1% at 25.0 ◦C, 7.2% at 30.4 ◦C and 9.5% at 35.1 ◦C thanks to the use of the integrated mechanical subcooling and of 7.8%, 13.7% and 17.5% respectively when using the dedicated. It is concluded that the dedicated mechanical subcooling system is the best system, however the integrated mechanical subcooling also performed better than the reference system.Funding for open access charge: CRUE-Universitat Jaume

Performance analysis on Free-piston linear expander

The growing global demand for energy and environmental implications have created a need to further develop the current energy generation technologies (solar, wind, geothermal, etc.). Recovering energy from low grade energy sources such as waste heat is one of the methods for improving the performance of thermodynamic cycles. The objective of this work was to achieve long-term steady state operation of a Free-Piston Linear Expander (FPLE) and to compare the FPLE with the currently existing expander types for use in low temperature energy recovery systems. A previously designed FPLE with a single piston, two chambers, and linear alternator was studied and several modifications were applied on the sealing and over expansion. An experimental test bench was developed to measure the inlet and outlet temperatures, inlet and outlet pressures, flow rate, and voltage output. A method of thermodynamic analysis was developed by using the first and second law of thermodynamics with air as the working fluid. The experimental tests were designed to evaluate the performance of the FPLE with varying parameters of inlet air pressure, inlet air temperature, and electrical resistance. The initial and steady-state operation of the FPLE were successfully achieved. An uncertainty analysis was conducted on the measured values to determine the accuracies of the calculated parameters. The trends of several output parameters such as frequency, average root mean square (RMS) voltage, volumetric efficiency, electrical-mechanical conversion efficiency, isentropic efficiency, irreversibility, actual expander work, and electrical power were presented. Results showed that the maximum expander frequency was found to be 44.01 Hz and the frequency tended to increase as the inlet air pressure increased. The FPLE achieved the maximum isentropic efficiency of 21.5%, and produced maximum actual expander work and electrical work of 75.13 W and 3.302 W, respectively