Transcritical CO2 Heat Pump System Research Articles

Subcritical recuperative air-to-water heat pump with zeotropic mixtures: Comparative and performance assessment

For high temperature heating, traditional single-stage air-source heat pumps suffer severe challenges in low efficiency and poor operating conditions. Heat recuperation is a simple and effective method to expand the temperature lift. In this paper, two kinds of recuperative air-to-water heat pump (RAWHP) systems were investigated for heating water from 15 °C to 90 °C at an ambient temperature of 15 °C. The energy, exergy, and economy models were established to assess the performance of RAWHP systems and compare with that of transcritical CO2 heat pump (TCHP) system in detail. The results show that the three-stream RAWHP system is superior to the two-stream RAWHP system from the perspective of energy and exergetic performance. For the three-stream RAWHP system, the highest COP of 4.08 and exergy efficiency of 45.4% were obtained by propylene/n-hexane with the optimal mole fractions of 0.9/0.1. In this case, the overall temperature matching indicator is 2.87, indicating a smaller heat transfer irreversible loss. Compared with the TCHP system, RAWHP systems have more advantages in higher COP and lower payback period at the ambient temperature ranges from −25 °C to 25 °C. The three-stream RAWHP system by using propylene/n-hexane (0.9/0.1) is recommended due to its outstanding energetic and economic performance.

Experimental study on energy, exergy, and exergoeconomic analyses of a novel compression/ejector transcritical CO2 heat pump system with dual heat sources

A novel compression/ejection transcritical CO2 heat pump system with dual heat sources (CHP) was proposed in our previous research. This paper experimentally investigates the feasibility and performance of the novel system and compares it with the conventional transcritical CO2 heat pump system (THP). The experimental results show that the maximum heating capacity and outlet water temperature can reach 50.49 kW and 85.8℃; The maximum exergy destruction of the novel system at 20 Hz to 30 Hz occurs in the gas cooler, which varies from 701.05 W to 1335.47 W, but the maximum exergy destruction at 35 Hz is the compressor, which is 1824.21 W; The total exergy cost rate of the throttle valve plays a dominant role in the system operation. In addition, the coefficient of performance (COP) and the exergy efficiency (ηEX) of the CHP are higher than that of the THP; The COP, ηEX, and IREX improvements of the CHP over the THP at the optimum discharge pressure, and the maximum improvements accordingly are 14.83 %, 38.14 %, 10.26 %, respectively. It can be concluded that the novel system is feasible and superior to the THP system. The research results will help to further promote the research and application of air source heat pumps integrating solar energy or geothermal energy due to the addition of the water source evaporator.

Numerical investigation on the performance characteristics of a novel biomimetic honeycomb fractal gas cooler of transcritical CO2 heat pump

In order to improve the heat transfer performance of the gas cooler in transcritical CO2 heat pump system, a novel biomimetic honeycomb fractal heat exchanger, with the advantages of great heat transfer efficiency and high compactness, is proposed for the transcritical CO2 heat pump in this study. Also, the conjugate heat transfer performance between supercritical CO2 and water in the novel gas cooler is investigated by the computational fluid dynamics method. The results firstly indicate that the difference of peak heat transfer coefficient in various flow channels reaches to 20.4% when the inlet flow flux of CO2 is 30 kg/(m2·s), significantly influenced by the uneven distribution of density and the secondary flow caused by buoyancy force. Secondly, when the flow flux of CO2 is 30 kg/(m2·s), compared with the horizontal flow, the heat transfer coefficients decrease by 11.4% and 40.7%, respectively, as CO2 flows vertically upward and downward. Finally, the heat transfer coefficient and comprehensive performance of the biomimetic honeycomb fractal heat exchanger are 144.6% and 20.4% higher than those of the printed circuit heat exchanger. The comprehensive performance of the 2nd and 3rd level biomimetic honeycomb fractal heat exchangers are 59.7% and 92.4% higher than that of the 1st level biomimetic honeycomb fractal heat exchanger.

Optimal compressor discharge pressure and performance characteristics of transcritical CO2 heat pump for crude oil heating

The compressor discharge pressure is an important parameter for optimizing the performance of a transcritical CO2 heat pump system. In this study, the factors influencing the optimal compressor discharge pressure of an air source transcritical CO2 heat pump system were examined experimentally. The results showed that the optimal compressor discharge pressure is affected by the gas cooler water inlet temperature and gas cooler CO2 outlet temperature. Consequently, a correlation formula for the optimal compressor discharge pressure appropriate for a more comprehensive temperature range and higher prediction accuracy was determined based on the experimental data. Compared with the experimental results, the maximum error was less than 10%, which is minor considering the lower gas cooler CO2 outlet temperature. In addition, a control strategy for the optimal compressor discharge pressure based on the degree of superheat correction was proposed. Subsequently, under variable operating conditions, the air source transcritical CO2 heat pump crude oil heating system was continuously operated based on this control strategy. The results showed that the compressor discharge pressure agreed with the optimal compressor discharge pressure calculated using the new correlation formula. Compared with the control strategy of constant opening under a variable frequency mode, the results showed that the system could operate under the control strategy proposed in this paper, the thermodynamic perfection of the system being higher.

Performance study of transcritical CO2 heat pump integrated with ejector and latent thermal energy storage for space heating

This study proposes a new transcritical CO2 heat pump integrated with ejector and latent heat energy storage system, and compares the performance of the system with and without energy storage equipment based on TRNSYS and experimental results. The results show that the new system operates more stably and is more conducive to space heating. Compared with the transcritical CO2 heat pump system with only ejector, the new system is more affected by the water flow rate and its performance can be improved by 18.53% when the water flow rate is 0.103 kg·s−1. The new system has more energy saving potential in low temperature environment, for which the utilization rate of primary energy and the primary energy ratio could reach 15.33 kgce·m−2 and 1.24, respectively. After thermo-economic optimization, it can be found that the minimum life-cycle cost of the new system is about 51.91 thousand CNY, which is reduced by 11.46% compared with the only ejector system. Therefore, the new system is superior to the previously transcritical CO2 heat pump systems both in terms of thermodynamic performance and economic performance. The research results are helpful to further promote the research and application of air source heat pump for space heating in low temperature area.

Experimental and theoretical analysis of the optimal high pressure and peak performance coefficient in transcritical CO2 heat pump

Based on the high performance and the environmentally friendly characteristics, the transcritical CO2 heat pump has attracted much attention in last few decades. The high-pressure control is one of the essential factors determining the coefficient of performance due to the large pressure changes in the transcritical CO2 heat pump system. In this work, the contribution rates of different factors to the optimal high pressure and the peak performance coefficient were analyzed by the method of the analysis of variance. Then, the influences of compressor frequency, inlet water flow rate and inlet water temperature on the optimal high pressure and the peak performance coefficient were experimentally investigated on the basis of theoretical analysis. Finally, a novel discharge pressure deviation was proposed to calculate the optimal high pressure, and the occurrence regularity of optimal high pressure was investigated. The results indicated that the gas cooler outlet temperature was the main parameter influencing on the optimal high pressure, peak performance coefficient and discharge pressure deviation. The calculated discharge pressure deviation can seek the optimal high pressure with the average error of 0.57% and the performance coefficient loss of 0.34%. Additionally, the optimal discharge pressure of the experiment was likely to appear in the descending stage of the gas cooler outlet temperature.

Experimental Study on Heating Performance and a Novel Calculation Method of Water Outlet Temperature Based on Air Source Transcritical CO2 Heat Pump System

As a natural refrigerant, CO2 can greatly improve the environmental protection of the heat pump system. Since there is no perfectly suitable heat transfer correlation for supercritical CO2 at present, the water outlet temperature of the transcritical CO2 heat pump system has not been predicted. To study the applications of transcritical CO2 heat pump systems in heating performance and hot water supply, a series of experiments are carried out by an air source transcritical CO2 heat pump test rig. The experimental results show that the main factors affecting outlet water temperature are the system COP, the discharge pressure, the compressor frequency, and the ambient temperature. Based on the experimental results, a dimensionless correlation equation on outlet water temperature is proposed by the Buckingham PI theorem. This equation can be used to calculate the outlet water temperature of the air source transcritical CO2 heat pump systems with different sizes, and the calculation accuracy can be maintained at 13% with experimental results. Finally, the influence factors of the gas cooler water outlet temperature are analyzed based on the novel calculation method. Therefore, this study provides a reference for the prediction of the water outlet temperature of the transcritical CO2 heat pump system.

System development and simulation investigation on a novel compression/ejection transcritical CO2 heat pump system for simultaneous cooling and heating

The problems that the expansion work cannot be recovered and the heat source of the evaporator is insufficient have been found in the previous experimental research, and thus this paper proposes a novel compression/ejection transcritical CO2 heat pump system for simultaneous cooling and heating. An ejector and a water source evaporator are added to the novel system, which realizes the improvement of the compressor inlet grade and the utilization of multiple heat sources. The feasibility of the novel system is verified at first, by numerically analyzing the system operating characteristics variation tendency. The simulation results show that COPtotal of the novel system can reach 11.7 in the compressor frequency of 30 Hz and the discharge pressure of 8000 kPa. In addition, the novel system is compared with the basic transcritical CO2 heat pump system and the transcritical CO2 ejector heat pump system in the exergy, energy and economic aspects. The dual-evaporator design helps the novel system achieve the highest total COP. The simulation results show that the novel system owns 12% higher exergy efficiency over the ejector system, and yields the shortest payback period of 1.10 years among the three considered systems.

Bi-objective optimization of transcritical CO2 heat pump systems

For the transcritical CO2 heat pump cycle, the gas cooler (GC) pressure can be controlled independently and it can be optimized for optimum COP. Control correlations can be developed offline either based on simulations or experiments or optimization can be done online in real time with continuous pressure and temperature measurements at different locations. The online methods provide more accurate results than the offline correlations, however, they have a long convergence time to the optimum value, especially if the initial GC pressure condition is far away from the optimum. In this paper, we investigate the transcritical cycle as a bi-objective optimization problem where COP and cooling/heating capacity are conflicting objectives with primarily optimizing the GC pressure for maximum cycle COP and maximum cooling or heating capacity which can be useful for transient conditions. The Non-Dominated Sorting Genetic Algorithm (NSGA-II) is used, which generates for different operating conditions the best non-dominated solutions i.e. the Pareto Front that includes the maximum COP, max cooling capacity, and also the intermediate optimum trade-off solutions. The effect on the Pareto Front of each optimization variable; GC outlet temperature, evaporation temperature, evaporator useful superheat, and compressor speed is shown separately, where all except superheat have a significant effect on the Pareto Front. A control methodology is proposed where according to a pre-defined preference, steady-state or transient operation, an optimization parameter is set to either maximize cooling or heating capacity (e.g. obtaining comfort as soon as possible in transient operation), maximize COP (for minimum energy consumption) or operate at a trade-off point as desired. The proposed approach can be applied as an offline control method and it can also be integrated as a hybrid solution with any online optimization method. In both, whenever a new condition is imposed, the offline based bi-objective algorithm will provide a very close estimate of the optimum GC pressure. In a hybrid solution, the online optimizer will then start searching for the true optimum from this very close value and hence reach much faster to the true optimum value. Thus, yielding enhanced capabilities (bi-objective optimization of COP and Q˙c), and in a hybrid solution, additionally further enhanced accuracy (vs offline) with reduced convergence time to the optimum COP and Q˙c (vs online only). A gain to loss ratio in terms of the conflicting objectives (COP and capacity) is presented that can be used as a criteria for deciding whether to move from one solution to another on the Pareto Front.

An optimisation study on a real-world transcritical CO2 heat pump system with a flash gas bypass

The configuration of a recently installed CO2 heat pump system with a cooling capacity of 33.5 kW and heating capacity of 35.8 kW used on a dairy farm is presented. This system has been studied in order to determine the optimal performance when used for combined water heating and refrigeration. Furthermore, the effect of the interstage pressure on the CO2 heat pump system has been investigated. This required the development of a thermodynamic cycle based on the configuration of the actual constructed system and data acquired from monitoring. A compressor model with a compression ratio over three was developed and validated against experimental data with an acceptable agreement. The numerical model of the counter-flow heat exchanger between water and sCO2 has been developed assuming that the water outlet temperature is variable (just over the set water temperature), which is more consistent with actual practice. In the desktop study, the optimal COPs for heating, cooling and combined performance have been simulated considering the interstage pressure, discharge pressure and gas cooler outlet temperature. It was found that the optimal combined COPs generally decrease from 3.84 to 3.68 with the interstage pressure from 29.65 bar to 53.60 bar. Through optimisation of discharge pressure, it was found that the optimal combined COP of 4.27 occurs at the same pressure where the optimal heating COP of 2.63 is achieved (at a discharge pressure of 94 bar), while the optimal cooling COP of 1.66 occurs when the discharge pressure is around 88 bar. A further optimisation study was conducted based on a fixed cooling capacity, and it was found that the optimal cooling COP of 1.78 can be achieved when the discharge pressure is 80 bar, while the optimal heating and combined COPs are still achieved at the same discharge pressure of 93 bar (with the heating COP of 2.59 and combined COP of 4.18). Based on the simulation results, it can be summarised that the overall system thermal performance designed for heating and refrigeration is dominated by the heating performance.

Experimental investigation and comparison on heating performance characteristics of two transcritical CO2 heat pump systems

An experimental investigation and comparison of the heating performance of the transcritical CO2 single-stage and vapor injection cycles were conducted. The effects of operating parameters on the heating performance of the two cycles were investigated. The experimental results showed that the heating capacity of the vapor injection cycle was 7.0% higher and the discharge temperature of the refrigerant was 7.7–11.8 ℃ lower than that of the single-stage cycle under the same condition. Besides, both the compressor input power and heating capacity were increased with the increasing temperature of the glycol aqueous solution. However, the growth rate of heating capacity was greater than that of the compressor input power. Therefore, the coefficient of performance of the vapor injection cycle was 4.4% greater than that of the single-stage cycle. So the vapor injection cycle has a wider range of applications in cold conditions. When the vapor injection pressure increased, the coefficient of performance of the vapor injection cycle increased then decreased, so there was a peak of the coefficient of performance. Therefore, a new correlation of the optimal vapor injection pressure was proposed. The new correlation has a significant meaning for designing a transcritical CO2 vapor injection cycle.

Experimental studies on transcritical CO2 heat pump system for simultaneous water cooling and heating application

ABSTRACT An in-house transcritical CO2 heat pump experimental facility was developed for simultaneous cooling and heating of water. Hand-operated (Needle) valve is used as an expansion device to control the gas cooler pressure and mass flow rate of refrigerant. The tests were conducted to study the performance of the system and components at various operating conditions. Four independent input operating variables and four output variables are considered for investigation. The factorial experimental design was employed to achieve the optimum set of experiments. Taguchi L9 array is used to design experiments for chosen variables and results were analyzed by using response surface methodology (RSM). It is observed that system charging pressure and degree of expansion valve opening affect the system performance greatly. Maximum cooling and heating output from the system was recorded as 3.8 kW and 6.6 kW, respectively, and the corresponding system COP obtained was 3.1. Empirical correlations are developed employing RSM technique to predict the system performance.

A comprehensive investigation on the effect of internal heat exchanger based on a novel evaluation method in the transcritical CO2 heat pump system

With the increasing demand for environmental protection, CO2, as a natural refrigerant, has always been environmentally friendly and safe, which makes transcritical CO2 heat pump system attract more attention. The internal heat exchanger is employed to improve the performance of transcritical CO2 heat pump system to match the traditional heat pump circulation system. In this work, two evaluation methods are proposed, one is the actual operation thermal effectiveness of internal heat exchanger, the other is the actual increase rate of COP. Compared with the experimental results, the conclusions can be extracted that the actual increase rate of COP can effectively evaluate the impact of the internal heat exchanger on system performance. In addition, this research deeply analyzes the influence of the internal heat exchanger on exergy efficiency. The results show that the internal heat exchanger greatly reduces the exergy efficiency under the discharge pressure of 7310 kPa and the ambient temperature of 15 °C, and the difference value between the system with and without internal heat exchanger reaches 39%. However, the influence of the internal heat exchanger on exergy efficiency decreases with the increase of discharge pressure and ambient temperature, the minimum difference value is only 0.1% at 11000 kPa and 30 °C.

A comprehensive study on a novel transcritical CO2 heat pump for simultaneous space heating and cooling – Concepts and initial performance

Carbon dioxide (CO2) is a natural substance and an environmentally benign, safe and economical refrigerant that can be used for cooling and heating systems. Unlike a traditional reversible heat pump system that only works either in heating or cooling, the air-to-air transcritical CO2 heat pump system reported in this paper is capable of providing simultaneous space heating and cooling through an Air Handling Unit (AHU) at any time. The system with cooling and heating capacities of 210 kW and 110 kW, respectively, was installed in a cinema complex in South Australia, and continuously operated from September to November in 2020. In this paper, the detailed configuration of the CO2 heat pump system with parallel compression is described. A complete system layout, comprising of cooling, heating and shared circuits, is revealed for the first time. A mathematical model based on highly efficient Bitzer compressors has also been developed in this study. The experimental data recorded in an entire heating period of approximately one hour has been utilised for model validation and analysis. Through the validation, the simulated power consumption for the compressors with Variable Speed Drive (VSD) and Fixed Speed Drive (FSD) has been compared to the experimental data with acceptable agreements. The simulated discharge temperatures agree with the actual temperatures measured within 10%. Moreover, the latest system performance from September to November has been evaluated based on the heating and cooling loads and combined COPs. It was found that the combined COP was relatively stable at around 3 regardless of climate conditions.

Investigation of the performance of a transcritical CO2 heat pump system subject to heated water conditions: Perspective from the second law

• A rational model of CO2 heat pump cycle is studied based on second-law perspective. • The effect heat source on the transcritical CO2 heat pump is analyzed in details. • The effects of irreversibility for every component on optimal discharge pressure are discussed. • Optimal discharge pressure is independent of compressor, and evaporator. • Optimal discharge pressure is mainly control by the thermal characteristics of gas coolers. For the past two decades, appreciable efforts are made to predict the optimal discharge pressure ( P opt ) of carbon dioxide (CO 2 ) transcritical system. These approaches are normally empirically derived and were limited to relevant conditions. The deviations may be also related to the inlet or outlet temperatures of the secondary fluid in the heat pump gas cooler. Thus, this study aims to develop a rational method to predict optimum discharge pressure based on the simulated data through exergy analysis. To simplify the simulation, the model assumes the constant heat transfer capacity ( UA) within the evaporator, and the model is verified with a 70 kW trans-critical CO 2 heat pump system. The influence of inlet (10–45 °C) and outlet (40–90 °C) water temperature on the system performance are examined from the perspective of the first and the second law of thermodynamics. The detailed exergy analysis for the cycle and each component is elaborated in detail subject to discharge pressure. It is found that the maximum COP declines but the P opt rises upon variable outlet temperature and inlet temperature of heated water. The P opt is mainly controlled by a gas cooler when compared to other components. The irreversibility of the gas cooler accounts for 3% to 3.6% of total heating capacity when P opt occurs regardless of the inlet or outlet water temperature. The corresponding maximum COP is about 4 times larger than the reciprocal of the irreversibility rate of the compressor. Yet, a new strategy is proposed to predict the P opt in terms of the irreversibility of the gas cooler. The proposed method is rationally based and can quickly diagnose the P opt through the four terminal thermal states instead of continuous monitoring the local maximum COP . Compared with the eight existing correlations listed in Appendix, the proposed novel method yields the least average and standard deviation of 2.6% and 3.1%, respectively when compared to the existing correlations.

Energetic, environmental and economic comparative analyses of modified transcritical CO2 heat pump system to replace R134a system for home heating

Due to low efficiency of a basic CO2 transcritical cycle, the feasible system scheme of CO2 is studied to replace R134a for heat pump heating. Three modified CO2 systems are discussed for home heating, in which a transcritical CO2 cycle system combining technologies of two-stage compression, vapor injection and inter-stage cooling (CO2 HPTSC, VI, ISC) is proposed based on the other two modified CO2 systems. Meanwhile, they are evaluated with two basic systems of CO2 and R134a in energy, environment and economy. All modified CO2 systems in terms of energy efficiency are better than R134a base heat pump system (R134a HPbase). In particular, CO2 HPTSC, VI, ISC is the optimal system. By combining typical meteorological parameters of different regions in China, the seasonal performance factor (SPF), gases emissions and costs are studied. The annual gas emissions of modified CO2 heat pump systems are all lower than those of R134a HPbase. If the compressor cost could be decreased by 67.66%, the life cycle cost (LCC) of CO2 HPTSC, VI, ISC will rival that of R134a HPbase in Beijing area. It can be predicted that the modified CO2 heat pump systems will be more competitive in the future.

Simulation and Experimental Study on the Characteristics of the Gas Cooler in the Simulink CO2 Heat Pump

So as to research impact of inlet temperature of gas cooler on the expression of carbon dioxide hot water unit, the ambient temperature was controlled at 15 °C, and the inlet temperature was adjusted. The exhaust temperature, heating capacity, and system heat exchange capability were calculated. Testing and measuring the change of exhaust temperature, outlet temperature of gas cooler, exhaust pressure, heat exchange capability. Based on simulink simulation tools, a calculation simulation platform of CO2 heat pump water heaters was established, and a test bench of transcritical CO2 heat pumps was built. In the experimental simulation, the inlet water temperature was increased from 15 °C to 40 °C, the simulation result of the exhaust temperature increased by 14.2 °C, the heating capacity decreased by 4.69 kW, the heat pump coefficient decreased by 0.66. In the experimental result, the exhaust temperature increased by 18.7 °C, the heating capacity decreased by 4.31 kW, heat pump coefficient decreased by 0.63. The research results can provide some guidances for the practical application and optimization of transcritical CO2 heat pump system.

Thermodynamic analysis with energy recovery comparison of transcritical CO2 heat pump system using various expansion devices.

The high irreversibility caused by the expansion valve in the conventional transcritical CO2 heat pump cycle has been reported as the major drawback on the overall system performance. To overcome this problem and recover some of the energy lost, different isentropic expansion devices such as turbine expander and two phase ejector have been proposed. This study aims to numerically compare the performance of the transcrtical CO2 heat pump in terms of first and second law of thermodynamics. In addition, the energy recovered by the two phase ejector and the turbine expander cycles have been evaluated. The pressure recovery and entrainment ratio in the ejector device were investigated comprehensively. Two numerical models using MATLAB and ASPEN PLUS software have been developed, and REFPROP database was used to estimate the refrigerant thermophysical properties. The results showed that the heating coefficient of performance (COPh) of the ejector cycle is higher than that of the turbine and valve cycles by 10.15 % and 20.84 % respectively. In addition, the ejector cycle has the highest second law efficiency (0.1) and the recovered energy is (0.63 kW) compared to (0.107 kW) gained by the turbine cycle. The ejector device has the least exergy destruction (0.2 kW) and can recover 0.7 Mpa of the pressure losses.

Modified transcritical CO2 heat pump system with new water flow configuration for residential space heating

Utilizing air-source heat pumps for residential space heating in cold climates is an important measure for environmental protection and energy conservation. In this paper, a new water flow configuration for water-heating process is proposed for transcritical CO2 heat pump combined with a dedicated mechanical subcooling (DMS) subsystem. In the modified system, the conventional parallel water flow configuration is changed to a new series water flow configuration, in which the CO2 gas cooler is separated into two parts with the condenser of DMS subsystem in the middle. The water is sequentially heated by CO2 gas cooler 1, condenser of DMS subsystem and CO2 gas cooler 2. Through three-stage heating, the uniformity of refrigerant-water temperature difference field is remarkably improved. The evaluations are conducted from energetic, exergetic and economic perspectives. Firstly, the impact of heat transfer area allocation among the three water-heating heat exchangers on system performance is numerically investigated. The results show that the COP of modified system is 1.6%–7.6% higher than that of the existing system. The greater the temperature difference between supply and return water and the higher the ambient temperature are, the more significantly the COP increases. By evaluating heating seasonal performance in three typical cities, energy efficiency of the modified system is increased by 2.4%–6.5%. Besides, the exergy efficiency is also improved by 2.1%–6.6%. As for life cycle cost, the proposed system can save up to 3.7% investment in terms of annual operation.

Bi-Objective Energy Efficiency and Capacity Optimization for Transcritical Co2 Heat Pump Systems

Bi-Objective Energy Efficiency and Capacity Optimization for Transcritical Co2 Heat Pump Systems