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1.
The main purpose of this study is to investigate the performance of a transcritical CO2 cycle with an internal heat exchanger for hot water heating. Performance test and simulation have been carried out for a transcritical CO2 cycle by varying secondary heat transfer fluid temperatures at evaporator and gas-cooler inlets as well as the discharge pressure. Variations of mass flow rate of refrigerant, compressor power, heating capacity, and co-efficient of performance (COP) with respect to the length of an internal heat exchanger are presented at various operating conditions. Good quantitative agreement between model predictions and experimental results has been found; most parameters have absolute average deviations of less than 4%. As the length of the internal heat exchanger increases, COP is enhanced but heating capacity tends to decrease due to the trade-offs between the effectiveness and pressure drop in the internal heat exchanger.  相似文献   

2.
Thermodynamic (energy and exergy) analyses and optimization studies of two-stage transcritical N2O and CO2 cycles, incorporating compressor intercooling, are presented based on cycle simulation employing simultaneous optimization of intercooler pressure and gas cooler pressure. Further, performance comparisons with the basic single-stage cycles are also presented. The N2O cycle exhibits higher cooling COP, lower optimum gas cooler pressure and discharge temperature and higher second law efficiency as compared to an equivalent CO2 cycle. However, two-stage compression with intercooling yields lesser COP improvement for N2O compared to CO2. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler exit temperature and evaporator temperature are obtained. This is expected to be of help as a guideline in optimal design and operation of such systems.  相似文献   

3.
The cooling performance and reliability of a transcritical CO2 cycle can be significantly improved by using a multi-stage compressor with gas injection because the CO2 cycle has a large pressure difference across a compressor. The objective of this study is to investigate the performance and operating characteristics of a two-stage CO2 cycle with gas injection. In this study, the performances of a two-stage CO2 cycle with gas injection (called as “two-stage gas injection cycle”) were measured as the amount of refrigerant charge, first- and second-stage compressor frequencies, and first- and second-stage EEV openings were varied in the cooling mode operation. The cooling COP of the two-stage gas injection cycle was maximally enhanced by 16.5% over that of the two-stage non-injection cycle in the experiments. In addition, when the first- and second-stage EEV openings were increased, the compression ratio decreased and this in turn, improved the cooling COP of the two-stage gas injection cycle. However, when the first-stage EEV opening was increased, the mass flow rate through the evaporator decreased, and this in turn, decreased the cooling capacity of the two-stage gas injection cycle. Therefore, in the two-stage gas injection cycle, an optimum control of both EEV openings is required.  相似文献   

4.
This paper advances a study of the transcritical expansion of carbon dioxide (R-744, CO2) through adiabatic capillary tubes. The influence of both operating conditions (inlet and exit pressures, inlet temperature) and tube geometry (capillary diameter and tube length) on the CO2 mass flow rate was experimentally evaluated using a purpose-built testing facility with a strict control of the measured variables. A dimensionless correlation to predict the refrigerant mass flow rate as a function of tube geometry and operating conditions was developed. In addition, a theoretical model was put forward based on the mass, energy and momentum conservation principles. The model results were compared with experimental data, when it was found that the model predicts 95% of the measured refrigerant mass flow rate within an error band of ±10%. The model was also employed to advance the knowledge about the transcritical carbon dioxide flow through adiabatic capillary tubes.  相似文献   

5.
The cooling performance of a CO2 cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO2 cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO2 system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO2 cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO2 cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.  相似文献   

6.
An ejector expansion transcritical CO2 refrigeration cycle is proposed to improve the COP of the basic transcritical CO2 cycle by reducing the expansion process losses. A constant pressure mixing model for the ejector was established to perform the thermodynamic analysis of the ejector expansion transcritical CO2 cycle. The effect of the entrainment ratio and the pressure drop in the receiving section of the ejector on the relative performance of the ejector expansion transcritical CO2 cycle was investigated for typical air conditioning operation conditions. The effect of different operating conditions on the relative performance of the ejector expansion transcritical CO2 cycle was also investigated using assumed values for the entrainment ratio and pressure drop in the receiving section of the ejector. It was found that the COP of the ejector expansion transcritical CO2 cycle can be improved by more than 16% over the basic transcritical CO2 cycle for typical air conditioning operation conditions.  相似文献   

7.
This paper describes a detailed procedure into the investigation of optimised control strategies for CO2 cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO2 system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.  相似文献   

8.
In order to research the adaptivity of the capillary tube in the transcritical CO2 refrigeration system, a separated flow model of a CO2 capillary in which the metastable phenomenon was taken into consideration was built in the simulation of transcritical CO2 refrigeration system, and related experiments were carried out by a plant under different gas-cooler outlet temperatures. The performances of the capillary based system were further compared with that of the EEV (electronic expansion valve) based system. The results showed that the relative COP ranged from 82% to 98% when there was a 10 °C deviation between the real gas-cooler outlet temperature and the design value. Thus the capillary based transcritical CO2 system is promising and could achieve performance close to that of EEV based system in a wide range of gas-cooler outlet temperature.  相似文献   

9.
An integrated system for simultaneous production of triple-effect cooling and single stage heating is proposed in this paper to harness low grade solar energy. The proposed system combines the heliostat field with a central receiver and the ejector-absorption cycle with the shaft power driven transcritical CO2 cycle. A parametric study based on first and second laws of thermodynamics is carried out to ascertain the effect of varying the exit temperature of duratherm oil, turbine inlet pressure, and evaporators temperature on the energy and exergy output as well as on the energy and exergy efficiencies of the system. The results obtained indicate that major source of exergy destruction is the central receiver where 52.5% of the inlet solar heat exergy is lost followed by the heliostat where 25% of the inlet exergy is destroyed. The energy and exergy efficiencies of the integrated system vary from 32% to 39% and 2.5%–4.0%, respectively, with a rise in the hot oil outlet temperature from 160 °C–180 °C. It is further shown that increase in evaporator temperature of transcritical CO2 cycle from −20 °C to 0 °C increases the energy efficiency from 27.45% to 43.27% and exergy efficiency from 2.51% to 2.97%, respectively. The results clearly show how the variation in the values of hot oil outlet temperature, turbine inlet pressure, and the evaporator temperature of transcritical CO2 cycle strongly influences the attainable performance of the integrated system.  相似文献   

10.
Blends of CO2 with ten low-global warming potential (GWP) working fluids are evaluated for use in a heat pump water heater. The effects that the discharge pressure, component ratio, hot-water outlet temperature and chilled water inlet temperature have on the coefficient of performance (COP) of heat pump are analyzed when the pinch point of the heat exchange is considered. It is found that temperature glide of zeotropic mixture has a good thermal match with the temperature change of water as two pinch points appear in the gas cooler/condenser or evaporator. The good thermal match in the heat exchangers promotes the system COP. Addition of low-GWP working fluids to pure CO2 can reduce the high-side pressure. The results show that CO2/R41 and CO2/R32 are suitable candidates for heat pump water heaters because of their high COP and low high-side pressure in comparison with those of a pure CO2 cycle.  相似文献   

11.
Simulation analyses for a vapour compression heat pump cycle using nonazeotropic refrigerant mixtures (NARMs) of R22 and R114 are conducted under the condition that the heat pump thermal output and the flow rate and inlet temperatures of the heat sink and source water are given. The heat transfer coefficients of the condensation and evaporation are calculated with empirical correlations proposed by the authors. The validity of the evaluation method and the correlations is demonstrated by comparison with experimental data. The relations between the coefficient of performance (COP) and composition are shown under two conditions: (1) the constant heat transfer length of the condenser and evaporator; and (2) the constant temperature of refrigerant at the heat exchanger inlet. The COP of the NARMs is higher than that of pure refrigerant when the heat transfer lengths of the condenser and evaporator are sufficiently long.  相似文献   

12.
CO2 transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO2 has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO2 transcritical refrigeration plant, experimentally. It tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO2 transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO2 cycle (0 and −10 °C) and three heat rejection temperatures (24, 30 and 40 °C). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7% and 30.3% on COP.  相似文献   

13.
Using economizer in R-744 heat pump cycle is an effective way to improve the heating capacity in cold climates. In this paper, a modification construction of reciprocating compressor with economizer port, a Voorhees compressor was introduced and the heat pump cycle with Voorhees economizer was compared with the traditional screw or scroll economizer cycles. Both the R-744 transcritical heat pumps with and without Voorhees economizer were tested at the same conditions with different air mass flow rates and different evaporating temperatures. The results show that the heating capacity of the heat pump with Voorhees economizer can be two times higher than the transcritical heat pump without economizer at low evaporating temperature conditions. At the same capacity operation conditions, the efficiency of the heat pump with Voorhees economizer is higher at high refrigerant mass flow rate conditions. The optimum discharge pressure of the heat pump with Voorhees economizer is found to be higher than the heat pump without economizer at the same ambient conditions. For mobile heat pump application, CO2 transcritical heat pump with Voorhees economizer demonstrates better performance comparing to the conventional transcritical CO2 heat pump without economizer when the evaporating temperature is lower than −20 °C, or when the mobile is idling with low compressor RPM.  相似文献   

14.
This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.  相似文献   

15.
介绍以毛细管为节流装置跨临界CO2热泵热水试验系统。实验研究了不同气冷器进水温度下系统的COP及其变化、气冷器沿管长水温温升梯度变化。分析了不同环境温度下制冷剂充注量对系统高压侧压力的影响。可以得出:以毛细管做节流装置也可以得到较高的COP;气冷器水温在高温CO2进口段温升幅度最大;系统对环境温度的变化很敏感,环境温度较低时要想得到合适的高压侧压力,制冷剂的充注量要比环境温度高时多。  相似文献   

16.
This paper evaluates performance merits of CO2 and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO2 system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO2 with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO2 was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO2.  相似文献   

17.
The performance of transcritical R744 systems with direct expansion (DX) can be significantly improved by implementing a Flash Gas Bypass (FGB). The idea behind the concept is to bypass refrigerant vapor, created during the isenthalpic expansion process, around the evaporator. By feeding the evaporator with liquid refrigerant, pressure drop is reduced and refrigerant distribution is improved. With R744 as the working fluid, increased refrigerant-side heat transfer coefficients are expected as well. In addition, the FGB concept proves to be beneficial in terms of system design, in particular for combined air-conditioning and heat pumping applications. An experimental comparison to a conventional DX-system reveals that FGB increases the cooling capacity and COP at the same time by up to 9 and 7%, respectively. Even larger improvements are possible in case a variable speed compressor is utilized to match the performance of the conventional DX-system. A simulation model helps to separate the individual improvement mechanisms. It was found that the reduction of refrigerant-side pressure drop is the dominant improvement mechanism of FGB.  相似文献   

18.
The objective of this paper is to develop a new energy transport system for district cooling application by using type 2 absorption cycle. Cold energy from the LNG storage system is utilized as the cooling source of the condenser and the rectifier. The pressures of the system, UAs of the evaporator and the desorber, the inlet temperatures of the refrigerant for each component, transportation distance and the pumping power per unit length are considered as the key parameters. The results show that UA of the evaporator has more dominant effect on COP than that of the desorber. The optimum system pressure for the demand side is also determined. For the present system, it is recommended that the refrigerant inlet temperature of the evaporator be lower than 4.3 °C for long distance transportation. It is concluded that the cold energy from the LNG storage system can be effectively applied to the long distance transportation system for district cooling application with the type 2 absorption cycle. The maximum transportation distance and the pumping power per unit length are calculated. The optimum operation conditions are also predicted from the parametric analysis.  相似文献   

19.
Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO2 transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO2 flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5 MPa to 11 MPa and inlet temperature from 20 °C to 40 °C. Outlet pressure varied from 1.5 MPa to 3 MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness.  相似文献   

20.
A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.  相似文献   

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