Showing papers on "Subcooling published in 2015"

Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

Abstract: 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.

Recent advances in vapor compression cycle technologies.

Abstract: This paper comprehensively reviews the recent studies on advanced vapor compression cycle technologies. These technologies are categorized in three groups: subcooling cycles, expansion loss recovery cycles, and multi-stage cycles. The subcooling cycle research is focused on a suction-line heat exchanger, thermoelectric subcooler and mechanical subcooler. The expansion loss recovery cycles are mainly focused on utilizing an expander and ejector. The multi-stage cycle research includes a vapor or liquid refrigerant injection cycle, two-phase refrigerant injection cycle. All these advanced vapor compression cycle technology options are reviewed, and their effects are discussed. In recent years, the research and development have been made to improve the performance of the VCC. This paper presents the improved cycle options and their comprehensive review. From the review results, several future research needs were suggested.

Experimental investigation of the effect of condenser subcooling in R134a and R1234yf air-conditioning systems with and without internal heat exchanger

Abstract: This paper presents an experimental study about the effect of condenser subcooling on the performance of an air conditioning system operating with R134a and R1234yf, under the same operating conditions For both refrigerants, it has been shown that the COP undergoes a maximum as a consequence of the trade-off between increasing refrigerating effect and increasing specific compression work At a given operating condition, the system COP increased up to 18% for R1234yf and 9% for R134a These results confirmed the trends obtained from a previous theoretical analysis, demonstrating that a system operating with R1234yf can benefit more from the condenser subcooling than that with R134a The experimental results also showed that the presence of an internal heat exchanger significantly reduces the COP increase due to condenser subcooling, since both improvements compete towards reducing the throttling losses Besides the interference between IHX and condenser subcooling, the use of both simultaneously still yields a more efficient air conditioning system, especially for R1234yf

Effect of the condenser subcooling on the performance of vapor compression systems

Abstract: This paper presents a theoretical study about the effect of condenser subcooling on the performance of vapor-compression systems. It is shown that, as condenser subcooling increases, the COP reaches a maximum as a result of a trade-off between increasing refrigerating effect and specific compression work. The thermodynamic properties associated with the relative increase in refrigerating effect, i.e. liquid specific heat and latent heat of vaporization, are dominant to determine the maximum COP improvement with condenser subcooling. Refrigerants with large latent heat of vaporization tend to benefit less from condenser subcooling. For an air conditioning system, results indicate that the R1234yf (+8.4%) would benefit the most from condenser subcooling in comparison to R410A (7.0%), R134a (5.9%) and R717 (2.7%) due to its smaller latent heat of vaporization. On the other hand, the value of COP maximizing subcooling does not seem to be a strong function of thermodynamic properties.

Performance improvement potentials of R1234yf mobile air conditioning system

Abstract: In this paper, the performance improvement potentials of R1234yf mobile air conditioning (MAC) system under various operation conditions were studied based on thermodynamic cycle analysis. The effects of superheat at evaporator outlet, subcooling at condenser outlet and compressor efficiencies on system performance were analyzed. It was found that superheat was few benefits for both system coefficient of performance (COP) and cooling capacity. Increasing subcooling from 1K to 10K at condenser outlet, system cooling capacity and COP could be improved by 15% if compressor consumption power was fixed. The isentropic efficiency of the compressor was a key factor in system COP improvement. It was concluded that adding an internal heat exchanger and improving compressor efficiencies would be good options for the future R1234yf MAC system enhancement.

Statistical Study of the Memory Effect in Model Natural Gas Hydrate Systems

Abstract: A high pressure automated lag time apparatus (HP-ALTA) was used for the investigation of the controversial memory effect in methane-propane mixed gas hydrates. The instrument can apply a large number of linear cooling ramps to a small volume of sample water under an isobaric condition of up to 15 MPa and record the maximum achievable subcooling for each cooling ramp. Over a hundred nucleation events were recorded for each of the several superheating temperatures used for the dissociation of the gas hydrate in a sample. In total, four different sample cells were used, and the effect of heating time was also studied for two of the four sample cells. A difference between two stochastic nucleation probability distributions was systematically and unambiguously quantified in terms of the most probable difference in the maximum achievable subcoolings. The protocol offers by far the most statistically robust method of quantification of the magnitude of the memory effect in each sample. From the analysis of several thousands of nucleation events, the following conclusions were made: (1) Even though the nucleation phenomena were intrinsically stochastic, a clear bias was observed which supported the existence of the memory effect. In particular, a reduction in the most probable subcooling of at least 4 K was required for positive identification of the memory effect for one of the sample cells. (2) The reduction increased as the superheating temperature was lowered. (3) The magnitude of the memory effect varied substantially among the sample cells used. (4) No significant effect of the heating time was observed in the range studied.

A general model for two-stage vapor compression heat pump systems

Abstract: Two-stage vapor compression technology has high potential of performance improvement for cold climate heat pumps, and there are several types of inter-stage configurations that need to be evaluated before making a choice. A general model of these configurations is first derived from a subcooler cycle and then is extended to be capable of evaluating many other inter-stage configurations by employing an “input domain”. The model is solved with a sequential algorithm and an analytical initial solution of the intermediate pressure is presented. After an experimentally validation with additional calculations of the subcooling parameter, the evaporating and condensing pressure, this general model is then used in the performance comparison and analysis of eight different inter-stage configurations. At last, case studies show that, this general model is capable of performing performance comparison among cycles with different types of inter-stage configurations, as well as refrigerant selection and operational analysis.