A review of transcritical carbon dioxide heat pump and refrigeration cycles

Air dielectric coaxial cables as cryogenic transfer lines

Transcritical carbon dioxide heat pump systems: A review

The (H)CFC-phase out and the fear for future problems for other synthetic working fluids, because of their known and unknown impact on the environment, have introduced a rising interest in environmentally safe natural working fluids. CO2is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Because the critical temperature of CO2is only 31.1°C, the transcritical cycle can be used to improve the coefficient of performance of the system. The experimental investigation and theoretical analysis on transcritical carbon dioxide heat pump system are carried out in this paper. It points out that there is an optimum operational pressure on transcritical carbon dioxide heat pump cycle, when the outlet temperature of gas cooler is constant, the coefficient of performance increases with increasing evaporating temperature at the same conditions, and the operational efficiency increased with decrease of gas cooler exit temperature. So in order to obtain the optimum performance, the influence of evaporating temperature, gas cooler exit temperature, and the operational pressure should be considered during the designing and operating transcritical carbon dioxide heat pump system.

Synthetic refrigerants such as CFCs and HCFCs deplete ozone and cause greenhouse effect. CO2 as a natural working fluid has zero Ozone Depletion Potential and its Global Warming Potential is equal to 1, is receiving more and more attention in the refrigeration field. Because the critical temperature of CO2 is only 31.1°c, the trans-critical cycle can be used to improve the coefficient of performance of the system. The thermodynamic analysis and experimental investigation on trans-critical carbon dioxide heat pump system are carried out in this paper. It points out that there is an optimum operational pressure on trans-critical carbon dioxide heat pump cycle, when the outlet temperature of gas cooler is constant, the coefficient of performance increases with increasing evaporating temperature at the same conditions, and the operational efficiency increased with decrease of gas cooler exit temperature. So in order to obtain the optimum performance, the influence of evaporating temperature, gas cooler exit temperature, and the operational pressure should be considered during the designing and operating transcritical carbon dioxide heat pump system.

The (H)CFC-phase out and the fear for future problems for other synthetic working fluids, because of their known and unknown impact on the environment, have introduced a rising interest in environmentally safe natural working fluids. CO2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Because the critical temperature of CO2 is only 31.1°C, the transcritical cycle can be used to improve the coefficient of performance of the system. The experimental investigation and theoretical analysis on transcritical carbon dioxide heat pump system are carried out in this paper. It points out that there is an optimum operational pressure on transcritical carbon dioxide heat pump cycle, when the outlet temperature of gas cooler is constant, the coefficient of performance increases with increasing evaporating temperature at the same conditions, and the operational efficiency increased with decrease of gas cooler exit temperature. So in order to obtain the optimum performance, the influence of evaporating temperature, gas cooler exit temperature, and the operational pressure should be considered during the designing and operating transcritical carbon dioxide heat pump system.

A capillary tube-based CO2 heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve-based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 4°C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two-phase flow. Subcritical and supercritical thermodynamic and transport properties of CO2 are calculated employing a precision in-house property code. Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve-based system. Capillary tube-based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO2 systems. Copyright © 2009 John Wiley & Sons, Ltd.

As one of the core devices of the carbon dioxide heat pump system, the level of heat transfer coefficient of gas coolers has direct impact on the performance and efficiency of the heat pump system. By selecting different experimental conditions, the thesis makes experimental studies about how parameters influence the coefficient of heat transfer on the shell and tube gas cooler when the system is running. Studies have shown that under the same pressure, with the increase of carbon dioxide and water flow, the average heat transfer coefficient of gas cooler also increases, but in the same state inlet temperatures of carbon dioxide almost have no effect on the average heat transfer coefficient and the maximum of heat transfer coefficient decreases gradually with the increase of pressure.

SUMMARY In this paper, a transcritical carbon dioxide heat pump system driven by solar-owered CO2 Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar-powered heat pump cycle using CO2 as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO2 heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO2 heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO2 flow rate of two sub-cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.

Application of the Peltier sub-cooled trans-critical carbon dioxide heat pump system for water heating – Modelling and performance analysis

Optimization of a transcritical CO 2 heat pump cycle for simultaneous cooling and heating applications

Carbon dioxide heat pump is a very promising energy‐saving technology. Based on the system parameters of the transcritical carbon dioxide heat pump cycle, this article has carried out 1D and 3D aerodynamic design of the compressor, and optimized the two‐stage parameters respectively. The reliability of the model is verified by comparing the calculated results with the published experimental data. At the same time, the grid independence of the compressor calculation model is verified. By optimizing the blade parameters, the first‐stage design efficiency is 78.8%, the second‐stage design efficiency is 79.7%, and the final 3D calculation efficiency of the compressor is 79%. By comparing the 1D prediction results with the 3D numerical calculation results, the 1D prediction results of the total pressure ratio at different speeds are in good agreement with the 3D simulation values. 3D calculation results show that there is no obvious airflow separation in the compressor passage, the fluid flow is subsonic, and no obvious high Mach number flow is found in the impeller or diffuser, indicating that the throat will not be blocked. This study provides a reference for the design and optimization of a carbon dioxide compressor.

Chapter 7 - Exergy analyses of refrigeration and heat pump systems

Two-phase flow distribution in parallel flow mini/micro-channel heat exchangers for refrigeration and heat pump systems: A comprehensive review

Today, it is seen that increasing environmental pollution is getting ahead of the increasing energy need. Therefore, more environmentally friendly and more economical refrigerants are needed. In this context, carbon dioxide appears as a natural refrigerant in cooling systems and heat pump (HP) systems, and it has been widely used in recent years. In this study, a single-stage heat pump system with a CO2 refrigerant, with a transcritical cycle, has been experimentally studied. The system is designed as a water-to-water heat pump. The performance of the system has been determined experimentally. In the system, capillary pipes with a diameter of 2.00 mm and two different lengths are used. It is aimed to create different evaporation pressures with two capillary tubes. The first capillary tube is 2.40 m long and the second is 1.20 m long. Gas cooler pressures, gas cooler and evaporator cooling water mass flow rates were kept the same for both cases. A certain gas charge was made and measurements were made for both cases. Thermodynamic analysis and comparison of the system were made. In the short capillary tube system, it was observed that the COPHP value was 7.2% higher, the CO2 mass flow rate increased by 9.1% to achieve the same gas refrigerant pressure value, and the power consumption in the compressor decreased by 1.8%. In addition, the gas cooler outlet temperature, the evaporator outlet temperature and the change in ambient temperatures, as well as the exergetic destruction and exergetic efficiencies in the system and system components are presented in figures with EES.

Revolutionizing 300 °C industrial steam with a novel transcritical carbon dioxide heat pump system: Thermodynamic and configuration analyses