Transcritical CO2 Heat Pump Water Research Articles

Energetic analysis and performance improvement algorithm of transcritical CO2 heat pump water heater system

Transcritical CO2 heat pump water heaters have been widely studied by researchers for its environmentally friendly nature and energy efficiency. To analyze the impact of various operating parameters on the heating performance of transcritical CO2 heat pump water heater systems, simulation models were developed and their accuracy was verified through experiments. Results indicated that the discharge pressure and CO2 temperature at the gas cooler outlet (Tgc,out) had a significant effect on the system performance, the coefficient of performance (COP) increased notably with higher evaporation temperatures, and the optimal discharge pressure varied depending on the evaporation temperature. The system equipped with intermediate heat exchanger demonstrated a lower optimal discharge pressure and a higher COP compared to the basic system. The inclusion of an intermediate heat exchanger in a system, the maximum COP is 2.99 % to 3.83 % higher than the basic system under the same conditions. It has been observed that the introduction of intermediate heat exchanger significantly improved the system performance when Tgc,out were 35 °C, 40 °C and 45 °C, corresponding to discharge pressures lower than 10.75 MPa, 11.167 MPa and 11.625 MPa. According to the research results, the optimal discharge pressure equation was fitted and an improved algorithm was proposed. Compared to the experimental values, the improved algorithm exhibits a maximum error of 8.81 % and 7.55 % for determining the optimal discharge pressure and temperature, and the improved algorithm is effective in engineering applications. Consequently, the simulation data holds great importance in guiding the design of transcritical CO2 heat pump water heater systems.

Self-optimized efficient operation of trans-critical CO2 ejector-expansion heat pump water heater system with two-stage evaporation

In application scenarios with lower ambient temperatures and higher sink temperatures, the trans-critical CO2 heat pump water heater system using an ejector instead of a conventional expansion device offers better performance, and the higher pressure differential allows for partial pressure loss recovery through the ejector. However, the complexity of discharge pressure and ejector matching affects the further application of the system. Therefore an optimization method called Extremum seeking control method is proposed to achieve the lowest power consumption implementation under off-design conditions. The ESC controller applied to the system uses the compressor power consumption as the feedback signal and the discharge pressure is used as operating input set-point. The dynamical model of a trans-critical CO2 heat pump water heater built on Modelica/Dymola is used as the study object and optimized for three scenarios: fixed, sloped step profile, and actual ambient temperature. The simulation results show that the ESC applicable to the proposed system can optimize the matching of discharge pressure and ejector driving nozzle throat area to obtain the best performance. Compared with the benchmark system, the extremum seeking control for actual ambient temperature conditions results in a 5.7% increase in system efficiency.

Real-time optimization of integrated subcooling transcritical CO2 heat pump water heater via phasor extreme seeking control (ESC)

This article presents a novel approach to real-time optimization of integrated subcooling transcritical CO2 heat pump water heater using the phasor ESC. Compared to the traditional dither-demodulation ESC, the phasor ESC shows improvements in optimization efficiency. A dynamic model of the heat pump system with high fidelity was developed in Dymola. Starting from a single phasor ESC for discharge pressure, we proposed practical guidelines for tuning the ESC parameters, such as the dither signal frequency, which significantly shortens the searching process by almost half. The effectiveness and adaptability of the multi-variable phasor ESC were demonstrated under various operating conditions, including time-varying disturbances such as ambient temperature. Simulation results showed that the phasor ESC outperforms both standard strategies and classic ESC in terms of performance.

Comprehensive evaluation of the transcritical CO2 ejector-expansion heat pump water heater

Nowadays, the transcritical CO2 heat pump water heater is widely used for hot water production. However, the larger pressure difference of the basic system leads to the huge throttling loss, which will significantly worsen the system performance. The ejector-expansion method accompanied by the internal heat exchanger is an effective way to reduce the throttling irreversibility. Nevertheless, the CO2 ejector-expansion system is seldom studied with the exergy analysis, including the exergo-economic and exergo-environmental analysis. In this paper, this system is theoretically evaluated from comprehensive perspectives with emphasis on the exergy analysis and the sensitivity analysis of ejector cost and electricity price is also conducted. Results show that with the ejector and internal heat exchanger, the coefficient of performance is improved by 7.38% and pressure ratio will reduce to 2.66 with the ejector efficiencies of 0.90. Furthermore, the exergy destruction of the throttling value could be reduced by 95.38%-98.76%, leading to the increased exergy efficiency. The largest temperature matching factor is achieved by the basic system with the internal heat exchanger. The cost rate of the throttling valve and cost per unit of exergy production is reduced by 85.65% and 0.42 CNY⚫MJ−1, respectively. Moreover, the maximum pollutant emission reduction is 115.41 tons, which results in the lowest specific exergo-environmental cost of 1.95 CNY⋅(kW⋅h)−1 in the proposed system.

The Role of the Compressor Isentropic Efficiency in Non-Intrusive Refrigerant Side Characterization of Transcritical CO2 Heat Pump Water Heaters

Characterizing the refrigerant side of heat pump water heaters (HPWHs) can be intrusive and expensive. On the other hand, direct external measurement techniques can be unfeasible, particularly in commercial HPWHs for residential applications. Non-intrusive in situ characterization methods have already been successfully implemented in subcritical heat pumps. They provide the refrigerant mass flowrate and the equipment energy performance, by using contact temperature sensors and electric power meters. Subcritical suction and discharge-specific enthalpies necessary to apply the method can be obtained from the measured temperatures and their corresponding saturation pressures. Nevertheless, this approach does not apply to the transcritical CO2 HPWHs. In the supercritical region, temperature and pressure are independent variables, and an iterative process regarding the compressor isentropic efficiency has to be considered. However, when isentropic efficiency data are not available, an additional procedure is required, using a validated gas cooler model to verify the physical reliability of the numerical solutions. This work aims at presenting base thermodynamic analysis of a novel methodology for non-intrusive refrigerant side characterization of transcritical CO2 HPWHs, exploring the influence of the compressor isentropic efficiency condition.

Real-time energy-efficient operation of a dedicated mechanical subcooling based transcritical CO2 heat pump water heater via multi-input single-output extreme seeking control

In the field of transcritical CO2 systems applied research, dedicated mechanical subcooling (DMS) is one of the most recently investigated and promising solutions, especially for space heating. However, this kind of system brings with it an intrinsic complexity, and to pursue effective and efficient operation, it requires suitable control and optimization strategies. The paper considers a combined transcritical CO2 and R134a -based DMS system and depicts the benefits of using a Multi-Input-Single-Output (MIMO) Extremum Seeking Control (ESC) approach. The real-time optimization strategy was obtained through a Computer-Aided Control System Design (CACSD) tool, exploiting an ad hoc Dymola model of the DMS system that has been validated and tested by using experimental data. The performance of the proposed approach has been compared with standard solutions relying on physical correlations. The MISO-ESC strategy always performed better in terms of system Coefficient of Performance (COP), under both fixed and variable ambient temperature conditions. Finally, the influence of the different refrigerants in the combined DMS system was analyzed from both heating capacity and performance points of view.

Experimental investigation of the extreme seeking control on a transcritical CO2 heat pump water heater

• Model-free real-time maximization of transcritical system COP. • ESC design for optimizing the CO 2 discharge pressure. • Experimental investigation of ESC control under various conditions. • The system COP was maximumly improved by 8.81% under the ESC control. In recent years, CO 2 has been widely considered as one of the most promising substitutes for the HFCs due to the consideration of both the environmental protection and high efficiency. However, the system performance of a transcritical CO 2 cycle is greatly influenced by the discharge pressure, the determination of which has been the most important and hottest issue. Apart from the traditional correlation development through theoretical analysis and experimental data, model-free extreme seeking control method has also been proved to be effective through the simulation investigation in literatures. However, the related experimental investigation is still rare. In this article, we provide the experimental investigation of the ESC method on a transcritical CO 2 heat pump water heater. The ESC controller was designed based on the system dynamics and then implemented on the test rig. Experimental investigation was then carried out under the fixed design condition (inlet water temperature at 40°C and air temperature at 0°C) and the fixed off-design condition (inlet water temperature at 45°C and air temperature at 0°C), the system COP operated by the ESC controller was improved by 7.62% after 4167s, and by 8.81% after 4287s respectively. The experimental results under the continuous step change of the inlet water temperature (40-45-40°C) demonstrated the repeatability and the effectiveness of the ESC control under changing working conditions. The static performance of the heat pump water heater was finally explored under the inlet water temperatures of 40°C and 45°C to furtherly demonstrate that the found discharge pressures of the ESC controller were indeed the optimal discharge pressures of the transcritical CO 2 heat pump.

Investigation on the performance of fluted tube-in-tube gas cooler in transcritical CO2 heat pump water heater

The utilization of fluted inner tube in tube-in-tube heat exchangers is an effective way to improve the heat transfer performance. Based on the segmented model, the performance of a fluted tube-in-tube gas cooler (FTGC) in the transcritical CO2 heat pump water heater was studied in this paper. The simulation model was validated and adopted to numerically investigate the effects of different parameters on the heat transfer performance of the gas cooler, which was compared to the smooth tube-in-tube gas cooler (STGC) with the same geometric parameters. The results showed that the FTGC had higher heat transfer capacity in most conditions. The heat transfer rate first increased and then decreased as the CO2 inlet pressure increased. The temperature difference at the pinch point in the FTGC was lower than that in the STGC. In the FTGC, the pinch point position was mainly influenced by the CO2 inlet pressure and water inlet and outlet temperatures. Regarding the system COP, the improvement of maximum COP could be up to 29.41% by using the FTGC compared to STGC when the water inlet temperature was 10 °C and the water outlet temperature was 80 °C. However, when the water inlet temperature was 40 °C, the improvement of COP could be impaired to 12.98%. Through changing the designs of the FTGC with the fixed total heat transfer area, when the water inlet and outlet temperatures were 10 °C and 60 °C, the highest maximum COP could be obtained with the design used in the experiments.

Experimental and techno-economic analysis of transcritical CO2 heat pump water heater with fin-and-tube and microchannel heat exchanger

• The CO 2 heat pump with fin-and-tube and microchannel heat exchangers is compared. • Coefficient of performance is raised by 16.32–22.62% with microchannel heat exchanger. • The frosting performance of heat pump with two different heat exchangers is discussed. • The primary energy consumption and enviroeconomic cost are reduced with microchannel. • The life cycle cost and sensitivity analysis with unit electricity price is presented. Previously, the conventional fin-and-tube heat exchanger is usually chosen as the evaporator in the air source heat pump. Recently, with the higher heat transfer efficiency, the microchannel heat exchanger is increasingly recognized. Nevertheless, information available in the contrastive analysis of the two heat exchanger types is scarce. Therefore, the effect of the mentioned evaporator type on the air source transcritical CO 2 heat pump was experimentally studied in this paper. Then, with the detailed techno-economic analysis methods, the heat pump with different heat exchangers is evaluated from multiple perspectives. Results showed the heating capacity and coefficient of performance of the microchannel system were increased by 16.5%-37.3% and 16.3%–22.6%, compared with the baseline fin-and-tube system. However, the attenuation degree of heating capacity was more obvious in the fin-and-tube system under the frosting condition. The primary energy consumption, CO 2 emission and corresponding cost of microchannel system are declined by 6.28 tce, 14.4 tons and 77.6 dollars for the comparison with the fin-and-tube system. The maximum values are all presented by direct electric heater whereas the minimum life cycle cost is achieved by coal-fired boiler. The life cycle cost and sensitivity analysis with unit electricity price are also conducted in the economic analysis.

Numerical study on the influence of internal heat exchanger in transcritical CO2 heat pumps under optimal pressure conditions

This paper presents a numerical study on the influence of internal heat exchanger (IHX) exchanging surface in the performance of a transcritical CO2 heat pump water heater at different operating conditions. Five different IHX geometries and four different evaporation temperatures have been studied with water temperature ranging from 10 °C to 60 °C at the gas cooler inlet. The results show a strong influence of IHX characteristics on system’s performance.

Comparison between reverse cycle and hot gas bypass defrosting methods in a transcritical CO2 heat pump water heater

• Reverse cycle defrosting method showed lower power consumption and shorter time. • System returned to steady heating faster with hot gas bypass defrosting method. • Averaged heating capacity with reverse cycle defrosting method was 8.24% lower. • Averaged COP with the reverse cycle defrosting method was 4.61% higher. Defrosting is an important issue that should be considered when an air source heat pump is operating in a cold climate. For the transcritical CO 2 heat pump (TCHP) water heater, the high operating pressure affects the application of defrosting methods. In this paper, the reverse cycle defrosting (RCD) and hot gas bypass defrosting (HGBD) methods were experimentally investigated in an air source TCHP water heater. The dynamic defrosting characteristics, influences on system operation and overall energy performances of the two methods were analyzed. The results suggested that the RCD method presented more complex parameter variations, lower power consumption, shorter defrosting time and larger influence on the system operation; while the HGBD method displayed less impact on the system heating capacity. The electric energy used for the RCD method was only 17.5% of that for the HGBD method. Moreover, the averaged COP with the RCD method was higher than that with the HGBD method by 4.61%. In summary, the RCD method showed evidently higher energy performance and could be a promising solution for the TCHP water heater, and the HGBD method demonstrated its robustness but had a limited energy performance.

Energy and Exergy Analysis of the Air Source Transcritical CO2 Heat Pump Water Heater Using CO2-Based Mixture as Working Fluid

Given the large demand nowadays for domestic hot water, and its impact on modern building energy consumption, air source transcritical CO2 heat pumps have been extensively adopted for hot water production. Since their system efficiency is limited by significant irreversibility, a CO2-based mixture could offer a promising drop-in technology to overcome this deficiency without increasing system complexity. Although many CO2 blends have been studied in previously published literature, little has been presented about the CO2/R32 mixture. Therefore, a proposed mixture for use in transcritical CO2 heat pumps was analyzed using energy and exergy analysis. Results showed that the coefficient of performance and exergy efficiency variation displayed an “M” shape trend, and the optimal CO2/R32 mixture concentration was determined as 0.9/0.1 with regard to flammability and efficiency. The irreversibility of the throttling valve was reduced from 0.031 to 0.009 kW⋅kW−1 and the total irreversibility reduction was more notable with ambient temperature variation. A case study was also conducted to examine domestic hot water demand during the year. Pure CO2 and the proposed CO2 blend were compared with regard to annual performance factor and annual exergy efficiency, and the findings could provide guidance for practical applications in the future.

Approaching optimum COP by refrigerant charge management in transcritical CO2 heat pump water heater

• Transcritical CO 2 HPWH can approach optimum COP without high pressure control. • Charge management method is developed to replace high pressure control. • Average COP loss of the redesigned system without high pressure control is −0.2%. Optimal high pressure is crucial to the performance of transcritical CO 2 heat pump water heater (HPWH). From the perspective of refrigerant charge management, this work aims to approach optimum COP in CO 2 HPWH without high pressure control. By modeling a running CO 2 HPWH, we in-depth investigated the relation between optimal high pressure and refrigerant charge. It was found that the essence of optimal high pressure control is to manage optimum refrigerant charge of the system. If the optimum charge of the system could remain constant under any working conditions, the system would operate automatically at optimum COP with fixed refrigerant charge. Then influences of water inlet temperature, water outlet temperature and ambient temperature on optimum refrigerant charge were numerically analyzed. Finally, system design methods to approach optimum COP by minimizing the variation of optimum charge under variable operating conditions were developed. As a result, the average COP loss of fixed-charge system after resizing heat exchangers is only −0.8%. By comparison, with a high-pressure reservoir at the outlet of gas cooler, the average COP loss can be as low as −0.2%.

Optimal discharge pressure in transcritical CO2 heat pump water heater with internal heat exchanger based on pinch point analysis

Abstract In transcritical CO2 heat pump water heater, the pinch point in gas cooler and the utilization of internal heat exchanger (IHX) both have a significant impact on the optimal discharge pressure and system performance. A theoretical model was developed to investigate the effects, and the results demonstrated that when the water inlet temperature was low, the pinch point prevented the gas cooler outlet temperature from approaching the water inlet temperature at low discharge pressure, which impaired the coefficient of performance (COP). The application of IHX could reduce the gas cooler outlet temperature by 4.9 °C when the water inlet temperature was 10 °C and the discharge pressure was 8.5 MPa. In this condition, combining with the subcooling effect of IHX, the COP was accordingly improved by 26.3%. Regardless of the working condition, using IHX lowered the optimal discharge pressure and enhanced corresponding optimal COP. In addition, parametric analyses were conducted, and the results indicated that the augment of water outlet temperature and ambient temperature resulted in the rise of optimal discharge pressure. The increase of water inlet temperature mostly enlarged the optimal discharge pressure as well. Overall, the optimal discharge pressure varied within a range of 8.53 MPa to 13.38 MPa. The reduction of optimal discharge pressure caused by using IHX intensified with the increase of water inlet and outlet temperature. Nevertheless, the effect of ambient temperature was various depending on the working condition. The reduction of optimal discharge could be up to 1.18 MPa when the effectiveness of IHX was 0.6.

Experimental investigation on the hot gas bypass defrosting in air source transcritical CO2 heat pump water heater

The transcritical CO2 heat pump is widely used due to its higher efficiency. Frost is inevitable at low ambient temperature, resulting in a deteriorated coefficient of performance and heating capacity. Considering the disadvantages and limitations of other defrosting methods, the hot gas bypass defrosting method was experimentally investigated in this paper. The dynamic characteristics of pressure and temperature during the defrosting process are studied in detail and the pressure-enthalpy diagrams are depicted. In addition, energy analysis of the heat supply and energy consumption is presented to evaluate the defrosting efficiency. Furthermore, the influence of ambient temperature is also discussed. The results show that the measured parameters change gradually during the defrosting process except for the initial stage and defrosting time is within the defrosting regulations. The defrosting efficiency is evaluated to be 41.16%–50.84% with variation of ambient temperatures, which indicates that the hot gas bypass defrosting method is suitable for the air source transcritical CO2 heat pump.

The Influence of Internal Heat Exchanger on the Performance of Transcritical CO2 Water Source Heat Pump Water Heater

The characteristics of the transcritical CO2 heat pump water heater (HPWH) system are; a lower inlet hot water temperature (Ti-hw) (sometimes this is lower than the water source temperature), and an outlet gas cooler temperature (To-gc) which is affected by the Ti-hw and often lower than the critical temperature. In order to study the effects of the internal heat exchanger (IHX) on the operational performance of the transcritical CO2 HPWH when To-gc is low, a transcritical CO2 water source HPWH experiment platform is established to conduct experimental research and comparative analysis on the operational performance of the transcritical CO2 water source HPWH, with or without IHX. It is found that, if only the coefficient of performance (COP) and heating at the optimal exhaust pressure of the transcritical CO2 water source HPWH were considered, COP and the heating of the non-IHX system would be slightly higher than those of the IHX system at the lower hot water flow and water source temperature, and this increase was not obvious. At the higher hot water flow rate and water source temperature, COP and the heating of the non-IHX system were also higher than those of the IHX system, and the increase was obvious. The experiment results showed that, near the optimal exhaust pressure, the variation range of COP and heating of the IHX system is relatively small, and the system has a relatively high stability.

Experimental investigation on the influence of internal heat exchanger in a transcritical CO2 heat pump water heater

In this paper, a transcritical CO2 heat pump water heater prototype was experimentally investigated to research the influence of internal heat exchanger (IHX). In addition, the exergy analysis was carried out based on the experimental data. The results showed that the optimal discharge pressure reduced with the application of IHX, and this reduction could be amplified by the decrease of the ambient temperature, the increase of the water inlet temperature and the increase of the water outlet temperature. From the energy point of view, under the optimal discharge pressure, the coefficient of performance (COP) increased by up to 6.65% and the total power consumption decreased by up to 6.22% via using IHX. Then the influence of IHX on the heat pump cycle was discussed. The results of the exergy analysis showed that the exergy destruction of compressor, gas cooler and evaporator increased, and the exergy destruction of expansion valve decreased with the application of IHX. Furthermore, the product exergy and the total exergy destruction increased, and the exergy efficiency of system decreased in all working conditions. In conclusion, the application of IHX enhanced the energy performance and impaired the exergy efficiency in this research.

Experimental study and optimal control strategy of a transcritical CO2 heat pump water heater

In this study, a transcritical CO2 heat pump water heater prototype was experimentally investigated in order to clarify the effects of the opening of electronic expansion valve (EEV), research the optimal discharge pressure and obtain an optimal control strategy for performance optimization. The tests were carried out by regulating the opening of EEV in multiple working conditions. The results showed that the decrease of opening led to reduction in evaporator inlet pressure and gas cooler outlet temperature, augment in discharge temperature, and decrease first then increase in discharge pressure. The optimal opening to reach maximum coefficient of performance (COP) always existed in all working conditions, however, there were two different situations where the corresponding optimal discharge pressure was near or away from the minimal value of the pressure curve. The reason for this phenomenon was the difference in water inlet temperature. Moreover, a correlation for the optimal discharge pressure was established with ambient temperature, water inlet and outlet temperature as independent variables. Furthermore, an optimal control strategy, in which the control of discharge pressure and superheat degree was considered, was proposed based on the correlation. Testing results proved that the optimization of performance could be achieved by using the strategy and the deviations between the COP under the control strategy and the optimal COP measured by experiments were within 5%.

Study on annual energy performance of transcritical CO2 heat pump water heating systems in Shanghai

CO2 heat pump water heating is an energy-efficient technology and being widely applied in Shanghai. However, its energy efficiency after long-term operation has not been well evaluated so far. In this paper, the energy efficiency and potential issues of CO2 heat pump water heater after multi-year service are in-depth investigated. In-situ study of CO2 heat pump water heating systems in Shanghai was carried out. Operating data were collected and field test was conducted. A typical case was chosen for comprehensive analysis. The results show that the annual performance factor (APF) of the actual system is 2.57, relative to 4.12 in its perfect shape. To find out what happened to this system, a simulation model was developed, calibrated and validated with field test data and operating data. In terms of simulations, suggestions were made on the system design and control for long-term performance improvement.

Analysis and comparison of influence factors of hot water temperature in transcritical CO2 heat pump water heater: An experimental study

In order to investigate the factors that affect the hot water outlet temperature on air-source transcritical CO2 heat pump water heater, an experimental research is designed where the expansion valve opening is controlled to adjust the discharge pressure. The experiments are conducted at different compressor speeds, water-inlet flowrates and water-inlet temperatures. Then, a new indicator named unit temperature-lift cost (UTC) is proposed to evaluate the lifting effects of hot water temperature. The results show that the rise of hot water temperature leads to the decrease of the maximum coefficient of performance (COP) but the increase in both the gas-cooler outlet temperature at the optimal status and the optimal discharge pressure, which are mainly caused by the variations of gas-cooler outlet status. At ranges of compressor speeds, water-inlet flowrates and water-inlet temperatures, the UTCs always show a drop with a higher required hot water temperature, and the corresponding average UTCs are 0.054 °C−1, 0.042 °C−1 and 0.099 °C−1, respectively. According to the results, the method of controlling the water inlet flowrate should be taken as a priority to lift the hot water temperature of the system. The compressor speed is suitable to meet the needs of increasing hot water temperature and heating capacity simultaneously. It is not advisable to increase the hot water temperature by adjusting the water inlet temperature.