Carbon Dioxide Heat Pump System Research Articles

Design and analysis of centrifugal compressor in carbon dioxide heat pump system

Based on the advantages of energy saving, environmental protection and high efficiency, carbon dioxide heat pump system has great application prospects. However, there are still many technical problems to be solved, especially the design and optimization of carbon dioxide centrifugal compressor. In this paper, a centrifugal compressor in carbon dioxide heat pump system is designed. The compressor is directly driven by a high-speed permanent magnet synchronous motor. Two-stage impellers are installed on both sides of the motor, and the bearings are active magnetic bearings. The influences of inlet pressure and temperature on compressor performance are analyzed. In the range of inlet temperature from 35 to 55 °C, with the decrease of inlet temperature, the compressor pressure ratio increases by 12–29.8%, the power increases by 2.7–8.6%. In the range of inlet pressure from 4 to 6 MPa, with the increase of inlet pressure, the compressor pressure ratio increases by 12.3–38.6%, and the power increases by 8.7–17.8%. In addition, the calculation method of compressor axial force is introduced, the axial force is calculated, analyzed and optimized. Furthermore, the rotor dynamics of compressor rotor and the influences of bearing stiffness and diameter of motor rotor on rotor dynamics are studied. With the increase of bearing stiffness, the first-order critical speed and maximum displacement of the rotor increase. The research provides a theoretical reference for the design and optimization of centrifugal compressor in carbon dioxide heat pump system.

Research on fault detection and diagnosis of carbon dioxide heat pump systems in buildings based on transfer learning

Nowadays, carbon dioxide (CO2) heat pump systems are widely used in fields such as refrigeration, heating, and hot water. However, the faulty operation of heat pump equipment may lead to higher energy consumption and even problems such as damaged components and systems that do not work properly. It is necessary to establish a fault detection and diagnosis (FDD) model for CO2 heat pumps to reduce energy waste caused by faults and restore the system to the expected performance level. Therefore, this study proposes four transfer learning methods based on feature transformations to establish an FDD model for CO2 heat pump systems. The models are also optimized by feature parameter screening, classifier tuning and training set occupancy. The results show that the accuracy of the methods based on transfer component analysis (TCA), joint distribution adaptation (JDA) and balanced distribution adaptation (BDA) increased from 97.79% to 98.89%, and the accuracy of the method based on correlation alignment (CORAL) increased from 15.67% to 74.23%. The runtime of the four methods TCA, JDA, BDA and CORAL are 0.07s,5.37s, 3.30s and 0.03s respectively. It is demonstrated that the transfer learning-based method proposed in this study can effectively diagnose common faults in CO2 heat pump systems.

Dynamic Modeling and Validation of a Carbon Dioxide Heat Pump System

Dynamic Modeling and Validation of a Carbon Dioxide Heat Pump System

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

Although the Peltier sub-cooled trans-critical CO2 cycle concept has been applied for refrigeration, which typically involves discharging the heat into ambient air, this system is rarely considered for heat pumping purposes. Therefore, this research aims to expand the scope of the Peltier sub-cooled trans-critical CO2 cycle into heat pump water heating where the generated heat is uniquely discharged into water at temperatures progressively higher than ambient. The heat flows between the CO2 and flowing water are modelled as Nusselt based convective heat transfers where a 1D model is imposed to the direct gas cooler to improve simulation accuracy. Moreover, important but often neglected characteristics such as Peltier device size and Peltier heating factor (PHF) will also be analyzed. Results indicate that the PHF has an extremely strong influence on the overall system’s coefficient of performance (COP). Specifically, an optimal PHF value exists as a trade-off between the benefit of sub-cooling and the losses due to reduced CO2 mass flow rate, the latter of which caused reductions in the convective heat transfer coefficient and the direct gas cooler’s heating capacity. In the meantime, although larger Peltier device sizes improves the system COP, the improvement will converge towards a specific maximum.

Turning industrial aerobic fermentation plants into thermal power stations

The industrial aerobic bioprocess of baker's yeast production requires large amounts of water to cool down large bubble column bioreactors. As result, an appreciable quantity of low-grade heat is generated. Because of the low temperature level of the water (~25°C) exiting the bioreactors cooling system, very little attention has been dedicated to heat recovery and conversion from this stream, which is usually released in rivers, streams, and canals. In this work, we simulated the generation of low-grade heat (up to 14.4 MW) from an industrial baker's yeast production plant consisting of seven 150-m3 bioreactors. Subsequently, a dedicated transcritical carbon dioxide heat pump system for the conversion of this low-grade heat into fourth generation district heat (~16.2 MW) was successfully designed. Fourth generation district heat employs low-temperature water (30-70°C) as heat carrier and is expected to play a major role in future sustainable energy system. Finally, an economic study confirmed the feasibility and the applicability of our approach and a concept for long-term energy storage including state-of-the-art phase change material (PCM)–units was discussed.

Preliminary investigation of a transcritical CO2heat pump driven by a solar-powered CO2Rankine cycle

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.

The Experimental Investigation and Theoretical Analysis on Transcritical Carbon Dioxide Heat Pump Cycle

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.

The Experimental Investigation and Theoretical Analysis on Transcritical Carbon Dioxide Heat Pump Cycle

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.

Supercritical Carbon Dioxide Heat Exchanger Tube Numerical Simulation and Analysis

In this paper, the three straight casing tubes and the double helix casing tubes were modeled using the Gambit software. And the fluid flowing in tubes was simulated by Fluent software. According to the simulation results, with the field synergy principle, two casing tubes’ flow and temperature distribution of carbon dioxide and water have been analyzed. Simulation data show that the double helix casing tubes is better than the three straight casing tubes on heat transfer and synergy. Lay the good foundation for design optimization and efficiency of heat exchanger of carbon dioxide heat pump system.

Exergy assessment of an optimized capillary tube-based transcritical CO2heat 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.

Modeling and Performance Simulation of a Gas Cooler for a CO2 Heat Pump System

The outlet temperature of a gas cooler has a great effect on the efficiency of carbon dioxide heat pump systems. In order to obtain a small approach temperature difference at the gas cooler, a near-counterflow type heat exchanger has been proposed, and a larger heat transfer area is demanded. Therefore, the optimum design for gas coolers involving the analysis of trade-offs between heat transfer performance and cost is desirable. For this study, a simulation model has been developed for fin-and-tube type gas coolers, and an air-side heat transfer correlation is proposed. The developed model was confirmed by experimental results. The effects of geometric parameters, such as transverse tube spacing, longitudinal tube spacing, number of tube rows, and fin spacing, on the performance of heat exchangers were investigated using the developed model. This study suggests various simulation results for optimum design of gas coolers.

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

Energetic and exergetic analyses as well as optimization studies of a transcritical carbon dioxide heat pump system are presented in this article. A computer code has been developed to obtain sub-critical and super-critical thermodynamic and transport properties of carbon dioxide. Estimates for irreversibilities of individual components of the system lead to possible measures for performance improvement. The COP trends indicate that such a system is more suitable for high heating end temperatures and modest cold end temperatures. Expressions for optimum cycle parameters have been developed and these correlations offer useful guidelines for optimal system design and for selecting appropriate operating conditions.