Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

Cavallini, Alberto; Cecchinato, Luca; Corradi, Marco; Fornasieri, Ezio; Zilio, Claudio

doi:10.1016/j.ijrefrig.2005.09.004

Cited by 122 publications

(54 citation statements)

References 1 publication

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“…Aprea & Maiorino (2008), Torrella et al (2011) and Sánchez et al (2014) studied the improvements due to the use of internal heat exchangers (IHX) in singlestage plants and Cavallini et al (2005; in two-stage systems. All the experimental analysis demonstrated that the IHX can improve the COP of the cycle up to a 10% approximately, because it increments the specific cooling capacity and reduces the optimum high pressure.…”

Section: Introductionmentioning

confidence: 99%

Energy improvements of CO 2 transcritical refrigeration cycles using dedicated mechanical subcooling

Llopis

Sánchez

Torrella

2015

International Journal of Refrigeration

165

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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 ºC) for environment temperatures from 20 to 35 ºC 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 ºC. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed. KEYWORDS

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Section: Introductionmentioning

confidence: 99%

Energy improvements of CO 2 transcritical refrigeration cycles using dedicated mechanical subcooling

Llopis

Sánchez

Torrella

2015

International Journal of Refrigeration

165

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“…The base cycle is composed of four basic processes: compression (1-2), heat rejection (2-3), expansion (3)(4) and heat absorption (4-1). This cycle is similar to the DCFI cycle except for substituting the throttle valve B with an expander and using the recovery work to drive the compressor.…”

Section: System Description and Analysismentioning

confidence: 99%

“…However, due to the high throttling loss, the energy efficiency of the basic transcritical CO2 cycle is lower than that of the conventional low pressure refrigeration cycle. In order to improve the performance of the transcritical CO2 cycle, some modifications of the basic cycle have been tried, such as the introduction of internal heat exchangers [1][2][3], the use of two-stage compression systems to enhance the compression process [4,5], or an expander or an ejector to recover the expansion work [6,7]. Cecchinato et al [5] discovered that thermodynamically the double-throttling, double-compression split cycle presents the greatest COP improvement among the different two-stage CO2 cycles.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamic Analysis of Double-Stage Compression Transcritical CO2 Refrigeration Cycles with an Expander

Zhang¹,

Tong²,

Wang³

2015

Entropy

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Four different double-compression CO2 transcritical refrigeration cycles are studied: double-compression external intercooler cycle (DCEI), double-compression external intercooler cycle with an expander (DCEIE), double-compression flash intercooler cycle (DCFI), double-compression flash intercooler cycle with an expander (DCFIE). The results showed that the optimum gas cooler pressure and optimum intermediate pressure of the flash intercooler cycles are lower than that of the external intercooler cycle. The use of an expander in the DCEI cycle leads to a decrease of the optimum gas cooler pressure and little variation of the optimum intermediate pressure. However, the replacement of the throttle valve with an expander in the DCFI cycle results in little variation of the optimal gas cooler pressure and an increase of the optimum intermediate pressure. The DCFI cycle outperforms the DCEI cycle under all the chosen operating conditions. The DCEIE cycle outperforms the DCFIE cycle when the evaporating temperature exceeds 0 °C or the gas cooler outlet temperature surpasses 35 °C. When the gas cooler exit temperature varies from 32 °C to 48 °C, the DCEI cycle, DCEIE cycle, DCFI cycle and DCFIE cycle yield averaged 4.6%, 29.2%, 12.9% and 22.3% COP improvement, respectively, over the basic cycle.

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“…On the other hand, simulation models for CO2 heat pump transcritical cycles have been developed for [11]. Some studies for performance enhancement of CO2 heat pump transcritical cycles have been conducted: Cavallini et al have conducted a theoretical and experimental analysis to optimize a two-stage cycle [12]; Aprea and Maiorino have conducted an experimental investigation to optimize heat rejection pressure for a split system [13]; Xu et al have conducted an experimental study on performance of a system with ejector under optimum high-side pressure [14]; Qi et al have conducted an experimental investigation to optimize rejection pressure for a water heater [15]. Some studies for applying CO2 heat pump systems to simultaneous cooling and heating have been conducted: Sarkar et al have conducted studies on optimization and simulation of heat pump systems in this application [16,17]; They have also investigated the performance of a system in this application by experiment [18].…”

Section: Introductionmentioning

confidence: 99%

Numerical analysis on performance enhancement of a CO2 heat pump water heating system by extracting tepid water

Ohkura

Yokoyama

Nakamata

et al. 2015

Energy

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Water heating systems each of which is composed of an air-to-water heat pump using CO2 as a natural refrigerant and a hot water storage tank have been developed, and are expected to contribute to energy saving in residential hot water supply.However, the area of tepid water in the storage tank expands because of heat conduction during long time storage, which leads to a degradation of the system performance.One of the ways to reduce the area of tepid water and enhance the system performance is to extract tepid water from the side of the storage tank. In this paper, the performance enhancement of a CO 2 heat pump water heating system with such a revised storage tank is analyzed by numerical simulation. The simulation model developed previously for a conventional system is extended by modifying the model for the storage tank and verifying its validity through experiments. Through a performance analysis using a standardized hot water demand, it is clarified how system performance 2 values of the revised system are enhanced, and especially how the trade-off relationship between the system efficiency and the volume of unused hot water is improved, in comparison with those of the conventional system.

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Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

Cited by 122 publications

References 1 publication

Energy improvements of CO 2 transcritical refrigeration cycles using dedicated mechanical subcooling

Energy improvements of CO 2 transcritical refrigeration cycles using dedicated mechanical subcooling

Thermodynamic Analysis of Double-Stage Compression Transcritical CO2 Refrigeration Cycles with an Expander

Numerical analysis on performance enhancement of a CO2 heat pump water heating system by extracting tepid water

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