Transcritical CO2 Heat Pump System Research Articles

Experimental study on performance and compressor characteristics of transcritical CO2 heat pump system

The accelerated advancement of the economy has led to a heightened focus on the issue of energy conservation and reduced consumption within heat pump systems. An experimental investigation was conducted to analyze the impact of discharge pressure and outlet water temperature on the performance of the transcritical CO2 single-stage heat pump system and vapor injection heat pump system. The experimental results show that the heating capacity first increases and then tends to be constant or even slightly decreases with the increase of discharge pressure in single-stage heat pump system. When subjected to the same operating conditions, the coefficient of performance of vapor injection heat pump system exceeds that of single-stage heat pump system. Compared with single-stage heat pump system, the performance coefficient of vapor injection heat pump system has increased by an average of 6.21% and the heating capacity has increased by an average of 9.54%. Additionally, the volumetric efficiency of the compressor substantially decreases when producing high-temperature hot water. Semi-empirical models for evaluating the compressor performance in both single-stage heat pump system and vapor injection pump system are established through the parameterization of the volumetric efficiency with respect to the pressure ratio and compressor speed. These models effectively forecast compressor performance.

Performance analysis of transcritical CO2 and common medium low-temperature air energy heat pump

To comply with the development of the “dual-carbon” goal, a new refrigerant for heat pump products is necessary. Firstly, mathematical models of enhanced vapor injection air energy heat pump and transcritical CO2 heat pump systems were established, and they were calculated using thermodynamic methods. The results show that when CO2 is used as the refrigerant, the discharge temperature is 30% and 22.7%, which is higher than that of R410A and R134a, and 4.9% lower than that of R32. At the compressor outlet, the volume flow rate of CO2 is 59.0%, 60.8%, and 64.2% less than R410A, R32, and R134a, respectively. In terms of power consumption, the difference between CO2 and R410 A is within 6%. The power consumption of the CO2 system is 0% ∼ 6.4% higher than that of the R32 system and 29.2% ∼ 46.5% lower than that of the R134a system. The COP of the CO2 system is within 6% of that of the R410A and R32 systems, which is 41% ∼ 87% higher than that of the R134a system. When the ratio of CO2 / R32 is 0 ∼ 0.2 and 0.9, the COP of the mixed refrigerant is better than that of the single refrigerant CO2.

Comparison of different strategies for operating a solar-assisted ground-source CO2 heat pump system for space and water heating

This study quantifies and compares operational strategies as applied on a solar-assisted ground-source trans-critical CO2 heat pump (CO2 SAGSHP) system for simultaneous space and water heating, optimized on an hourly basis over the course of a week. Four (4) boundary conditions, representing spring, summer, autumn, and winter, were considered. To enable the use of a dynamic model for optimization simulations, the use of a previously developed surrogate Artificial Neural Network (ANN) model of a Modelica CO2 heat pump model was evaluated for use in this study. The simulation results from the system model that applies the surrogate ANN model demonstrated strong alignment with those generated by the full Modelica model, yielding low root mean square errors and mean absolute percentage errors. Furthermore, it accelerated the speed of results generation for a single forward run by 6–15 times, depending on the boundary conditions. It can be inferred from the results of the optimization runs that when warm climate conditions dominate, implementation of the on/off operation with thermal energy storage (TES) should be prioritized, given that it could potentially yield greater operational cost benefits (12% to 58 % cost reduction) compared to hourly discharge pressure optimization (0.5 to 1.5 % cost reduction). Conversely, when colder seasons are longer, optimizing the CO2 heat pump's discharge pressure becomes more practical (6.5 % to 6.9 % cost reduction) than on/off operation, particularly when only a small TES is installed (1.2 % to 2.6 % cost reduction). The optimization of the mass flow rate of the supply side working fluid of a CO2 SAGSHP system exhibited minimal benefit, except notably in the summer where it generated an even better cost reduction than controlling the discharge pressure. This shows the practicality of this strategy for conditions where relatively high solar irradiation and relatively low heat demand are concurrently expected. In general, weekly reductions in operational costs were more substantial when both the discharge pressure and the on/off operation of the CO2 heat pump were concurrently optimized, especially when using a larger TES tank. Hence, both should be applied if there are no practical or cost limitations that restrict their implementation. While previous studies on CO2 heat pumps primarily emphasized efficiency optimization via discharge pressure control, this study shows that comparable or greater operational cost reductions can also be achieved from other strategies, depending on the boundary conditions.

Optimal discharge pressure and performance characteristics of a transcritical CO2 heat pump system with a tri-partite gas cooler for combined space and water heating

The transcritical CO2 heat pump holds promise for achieving carbon neutrality in buildings, primarily due to its high energy efficiency in hot water heating. However, using it for space heating often leads to efficiency losses. A heat pump equipped with a tri-partite gas cooler can address this issue by providing both space heating and hot water while maintaining lower refrigerant temperatures at the gas cooler exit. Nonetheless, there is limited research on the characteristics of a CO2 heat pump system designed for simultaneous space and hot water production. This work fills this gap by analyzing the impacts of operational and system configuration, considering the variation of space heating to hot water production loads. A theoretical model is presented and verified with the experiments. Results show that under the specified conditions, the heat load ratio has minimal influence on the optimal discharge pressure. Increasing the water outlet temperature from 60 °C to 80 °C leads to a nearly 14% decrease in maximum COP, with a corresponding 12.2% increase in the optimal pressure. A 10 K rise in water inlet temperature results in approximately a 21% decline in maximum COP. Evaporation temperature has little impact on the optimal discharge pressure, while maximum COP increases by roughly 63% with a rise in evaporation temperature from -10 °C to 10 °C. Space heating water temperature notably affects the optimal discharge pressure. Integration of an IHX enhances system COP but has marginal impact on the optimal discharge pressure. These findings offer insights for designing and optimizing transcritical CO2 heat pumps for simultaneous space and water heating applications.

Energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump for simultaneous domestic hot water and space heating

The transcritical CO2 heat pump system experiences significant throttling losses in space heating operation due to the higher temperature of returning water. This paper presents an energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump integrating a tri-partite gas cooler, which is an effective method to match CO2 temperature glide with water for simultaneous domestic hot water and space heating (DHW+SH) production. A pinch point-based numerical model is developed to find the optimal discharge pressure. This validated model is then utilized to investigate the impacts of water inlet temperature and ambient temperature. Two configurations are examined: one with only DHW supply and the other with DHW+SH supply. The results indicate that combining SH with DHW enhances COP by 7.5% with a 7.9% reduction in discharge pressure at 10 °C ambient temperature and 10 °C water inlet temperature. At a water temperature of 10 °C, exergy efficiency improves by 4%. The compressor accounts for 54%–60% of the total exergy loss. The DHW+SH system exhibits an average exergy destruction reduction of 7.6%. With each 5 °C rise in ambient temperature, the DHW+SH system’s total exergy destruction cost rate decreases on average 7.7% compared to the DHW system. Furthermore, the combined exergy destruction cost rate of GC2 and GC3 in DHW+SH is significantly lower (by 48.2%) than only GC3 in DHW. The exergoeconomic factors of the compressor, gas cooler 2, and gas cooler 3 emphasize the need to decrease their costs to enhance cost-effectiveness.

Experimental verification of the novel transcritical CO2 heat pump system and model evaluation method

This work proposes a novel method for verifying the accuracy of the numerical simulation model of the compression/ejector transcritical CO2 heat pump system using convolutional neural networks. The method focuses on converting the original unequal input conditions into equal input conditions and comparing the simulation results with the experimental prediction results of each component. The validation results reveal the following findings: 1) The root mean square errors of the gas cooler and the water source evaporator are 9.16 °C and 11.47 °C respectively, indicating that future work should focus on correcting of the CO2 heat transfer coefficient calculation method; 2) The verification results of the evaporator indicate that suggesting the need to incorporate an air dynamic change module in the air source input; 3) The root mean square error of the CO2 outlet pressure in the ejector is 462.45 kPa. It can be inferred from the pressure variation trend that the limit pressure ratio of the compressor is the main factor affecting the accuracy of the ejector model. Overall, this article presents a novel and effective method for verifying the accuracy of numerical models.

Performance evaluation of transcritical CO2 desiccant heat pumps for electric vehicles

Transcritical CO2 heat pump systems are eco-friendly and have excellent heating performance, which makes them suitable for electric vehicles. The recirculation mode can save energy but requires a dehumidification function to ensure driving safety. Therefore, this paper proposes a transcritical CO2 desiccant heat pump, which differs from the typical transcritical CO2 heat pump system by incorporating an indoor evaporator and a full-pass throttle valve. The flow area of the full-pass throttle valve can be adjusted to obtain the appropriate dehumidification rate. In addition, this study suggested using the desiccant heat pump coefficient of performance (DHCOP) to evaluate the overall energy efficiency of the heat pump system. We have investigated the dehumidification rate and optimal discharge pressure under different operating conditions. The results show that even under the harsh operating conditions at -10 °C, the HDCOP of the transcritical CO2 desiccant heat pump can still exceed 2.1, and reach 3.12 at a moderate condition of Ta=5 °C. The transcritical CO2 heat pump desiccant system demonstrates significant energy-saving potential.

Transcritical-transcritical cascade CO2 heat pump cycles for high-temperature heating: A numerical evaluation

Industrial heat pumps are becoming increasingly popular as a low-emissions technology due to the ability to leverage renewable electricity at high efficiencies. While heat pumps are already well-established for process temperatures below 100 °C, high-temperature heat pumps that can produce greater temperatures are limited by high pressure and temperature requirements that surpass the performance of conventional heat pump compressor technologies. Alongside this, new heat pump cycles which are being developed need to be compatible with low GWP and low ODP refrigerants. This paper presents a numerical evaluation on the thermodynamic performance of high-temperature heat pumps for supplying 200 °C sink temperatures with large temperature glides. Four new transcritical–transcritical cascade CO2 heat pump systems are modelled in this study which aim to limit the pressure requirements required to achieve high sink temperatures. An air heating case study for milk powder spray drying is presented. To achieve a COP of 3, all cycles had a similar maximum air outlet temperature between 110 °C and 113 °C. The transcritical-transcritical cascade cycle with internal heat exchangers on both the top and the bottom cycles achieved the lowest compressor discharge pressure (13.47 MPa maximum) for this case. The same cycle also achieved the highest COP of 2.22 when air outlet temperature was fixed at 200 °C (i.e., complete replacement of external heating). For such a heat pump to be economically feasible for heating air to 200 °C, an electricity-to-reference price ratio equal to or below 2.06 is required.

Analysis of the internal flow features of a CO2 transonic nozzle and optimization of the nozzle shape profile

Two-phase flow Laval nozzles are simple and have promising applications, but the internal flow is intricate and encompasses physical phenomena like phase transition, transonic and Mach waves. Therefore, a 1D mathematical model for validating and designing a CO2 two-phase flow transonic nozzle was developed and validated based on experiments. The findings demonstrated that a grid density of 0.292 mm was appropriate and that the maximum error of the model was less than 6.0 %, with an average error of less than 2.5 %. Additionally, it was discovered that when a phase transition occurred at the nozzle throat, whether through condensation or evaporation, there was a minimum critical pressure ratio in the nozzle; when the phase transition took place in the nozzle diffusion chamber, there was a minimum total pressure ratio. Finally, the Laval nozzle was designed to utilize the isostatic pressure gradient method (IPG), which gave a more significant pressure drop than a conventional nozzle while eliminating the shock wave strings at the nozzle throat. Under the typical operating conditions of a transcritical CO2 heat pump system, the ejector performance was boosted by an average of 5.44 %.

Performance of an automotive reversible CO2 heat pump system during periodic frosting-defrosting cycles

Electric vehicles (EVs) have seen rapid growth in the global market share due to the demand for low-emission commuting. However, EVs face the challenge of range anxiety in cold weather due to the capacity degradation of lithium-ion batteries and the increased demands of cabin and battery heating. Heat pump (HP) technology improves the heating efficiency compared to resistive heating, but in cold and humid weather, frost can grow on the outdoor evaporator and reduce the efficiency of the HP system. In this paper, experiments are carried out to assess the impacts of ambient temperature and relative humidity (RH) on the capacity and efficiency of a transcritical CO2 HP system during the continuous frosting-defrosting cycles under some challenging operating conditions (ambient temperature below 0 °C and RH over 80%). Moreover, the mass of frost accumulated during the frosting period, and that of water retained, drained, and vaporized during the defrosting period are measured and analyzed. The results show that when the ambient temperature decreases from 0 to −10 °C, the peak value of the heating capacity in the first frosting or HP mode period reduces from 4.85 to 3.54 kW, and the coefficient of performance (COPhp) drops from 1.61 to 1.47, but the operating time of the first frosting period significantly increases from 32 to 108 min. Similarly, lowering RH from 90% to 80% at 0 °C also helps to extend the operating time of the frosting period in one frosting-defrosting cycle from 32 to 84 min. In addition, with the current commonly used defrost initiation criterion, which is the air-side pressure drop across the outdoor evaporator increasing by five times, the heating capacity and system efficiency drop by less than 10% at the end of one frosting period. This indicates the potential for a longer operating time of the HP system which could reduce the undesirable impact on the passengers’ comfort during the reversed cycle defrosting.

High temperature transcritical CO2 heat pump with optimized tube-in-tube heat exchanger

CO2 heat pump operating on tanscritical cycle is widely adopted to provide hot water whose temperature is as high as about 90 °C. Recently, it is recognized that high temperature heat pump has promising application in various industries such as papermaking, chemical, automotive, and metallurgical. Then, in this paper a model of a tube-in-tube heat exchanger was established, and its parameters including tube length, fin height, fin number, fin thickness and helix angle are optimized through simulation to enhance its heat transfer performance, Accordingly, a transcritical CO2 heat pump system which can provide about 100 °C pressurized hot water is proposed and investigated. Subsequently, an experimental setup is constructed and operated under ambient temperatures, inlet and outlet temperatures of cooling water to demonstrate the feasibility of obtaining hot water at 100 °C. When the ambient temperature and inlet water temperatures are 40 °C and 9 °C respectively, the highest COP of 3.64 is obtained in experiments and the corresponding simulated value is 3.87. When the ambient temperature is 40 °C, inlet and outlet water temperatures are 9 °C and 85 °C, respectively, the study recorded the highest COP of 4.47. It is worth noting that both the heating capacity and COP increase with rising ambient temperature.

Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field

Refrigerant substitution is an urgent need in the context of reducing carbon emissions and slowing global warming. CO2 is now being proposed as a promising solution based on its excellent properties and system performance, especially in low-temperature environments. This paper presents an overview of recent advances in system configuration and operation characteristics to improve the performance of transcritical CO2 heat pump and refrigeration systems. The paper first introduces the basic research background, system cycle, and thermodynamic characteristics. Secondly, CO2 cycle improvements with single modifications and modification combinations are reviewed. Then, some important operation characteristics and control methods are discussed. Additionally, the paper provides a detailed description of the development of transcritical CO2 heat pump and refrigeration systems in the vehicle field. At the end of this review, conclusions and opportunities for future work in this field are presented.

Experimental and economic analysis of optimization potential for annual operating characteristics of air source transcritical CO2 heat pump system

In this study, an experimental system of air source transcritical CO2 heat pump system was established for investigating annual operating characteristics. A new operating strategy was proposed based on experimental results, and the economic analysis was studied based on different operating strategies. The results showed that the ambient temperatures had the slight effect on the system performance for summer and transition seasons conditions, which had the greater influence on that for winter conditions. When the ambient temperatures are lower, there were the obvious variations of the system heating capacity. The coefficient of performance decreases with the rise of water outlet temperatures. The annual operating expenses significantly reduced from 659.71 Chinese yuan to 514.72 Chinese yuan, which accounted for approximately 78% of the before-optimization cost. The optimized cycle was verified to be preferable, and the presented discussion could provide a promising solution for optimizing transcritical CO2 system.

Decarbonization performances of a transcritical CO2 heat pump for building heating: A case study

An experimental transcritical CO2 heat pump system was implemented in the severely cold region, Harbin (China), and the energy and exergy performances as affected by the expansion valve opening were compared and the environmental and economic performances of CB (Coal-fired Boiler), GB (Gas-fired Boiler), EB (Electric Boiler), and HP (Heat Pump) were analyzed. The results showed that the system COP decreased by around 0.2 when the expansion valve opening was increased from 60% to 80% with the highest system COP of 2.40. The system exergy destruction and exergy efficiency increased from 5.12 and 33% to 5.61 kW and 37% when an increase in the expansion valve opening from 60% to 80%, respectively. The CO2 and pollutants emissions of HP were 52% ∼ 92% lower than CB and EB. The life-cycle cost of HP for a 15-year working period was the lowest (36.57 × 104 CNY) and it was 84%, 77%, and 39% of CB, GB, and EB, respectively. The results revealed that the transcritical CO2 heat pump system has better environmental and economic performances than CB, GB, and EB, its emissions were 52% lower than CB and EB and its life-cycle cost was only 39% of EB.

Hybrid offline and online strategy for optimal high pressure seeking of the transcritical CO2 heat pump system in an electric vehicle

The optimal discharge pressure corresponding to the highest coefficient of performance in a transcritical CO2 heat pump system is influenced by several factors and is difficult to determine. This paper presents and implements a novel hybrid method connecting an improved offline part and an online part in series for optimal high-pressure seeking in a transcritical CO2 heat pump system. Feedforward-feedback control is used to improve the dynamic characteristics of the offline part. The existence of the offline part decreases the gap between the final optimal value and initial value of the online part. The subsequent online part ensures the precision and robustness of the entire method. Different optimal high-pressure seeking methods are compared using a high-accuracy system model under various conditions. The results reveal that the hybrid method decreases the cabin temperature settling time by up to 34 and 807 s compared with the traditional offline and online methods, respectively. The long optimal pressure-seeking process of the traditional online method is shortened by 4934 s, and the corresponding average power consumption is reduced by 235 W between 0 and 5000 s by using the novel hybrid method. The novel hybrid method achieves a fast speed and high precision under different operating conditions, making the electric vehicle more comfortable and energy-efficient. The method can be optimized continuously by replacing offline or online parts with other advanced offline or online methods.

The Collaborative Optimization of the Discharge Pressure and Heat Recovery Rate in a Transcritical CO2 Heat Pump Used in Extremely Low Temperature Environment

Considering the excellent environmental properties and heating capability under wide running conditions of the natural fluid CO2, the transcritical CO2 heat pump system has widely been used in the application of water heaters, commercial heating and cooling, electric vehicle thermal management, etc. Since the performance was highly affected by the discharge pressure and heat recovery rate in a transcritical CO2 system, the collaborative optimization of these two parameters was analyzed in detail in this study. The results showed that the optimal value of the system heating COP, which was the ration of heating capacity to power consumption, was better under a higher heat recovery rate and relatively lower discharge pressure, which is why these kinds of operating conditions are highly recommended from the perspective of collaborative optimization. Additionally, the heat recovery rate had a positive effect on the system performance when the discharge pressure was lower than its optimal value, while the heat recovery rate would present a passive effect on the system performance when the discharge pressure was higher than its optimal value. The relevant conclusions of this study provide a good theoretical basis for the efficient and stable operation of the transcritical CO2 heat pump technology under the conditions of a wide ambient temperature range.

Comprehensive study on EEVs regulation characteristics of vapor-injection CO2 heat pump for electric vehicles

The regulation characteristics of electronic expansion valves (EEVs) are critical to improving the comprehensive energy efficiency of the heat pump system for electric vehicles. In this paper, a vapor-injection transcritical CO2 heat pump system for electric vehicles was first established to experimentally study the impacts of EEV1 and EEV2 openings on the overall system performance. The results show that the cooling capacities of the vapor-injection system are respectively increased by 3.13% / 5.75% with the EEV1’s opening of 15% / 25% and injection ratio of 0.0975 / 0.8305, compared against these of the basic system without vapor injection. As the injection ratio rises, the cooling capacity increment tends to increase while the cooling coefficient of performance (COP) declines from 1.7 to 1.51. Then, the regulation characteristics of EEV1 and EEV2 were quantitively analyzed, indicating that the evaporation pressure is mainly determined by EEV2’s openings. By adjusting EEV1’s opening, the COP of the CO2 vapor-injection heat pump can be maximized at 1.9, with EEV1’s opening at 20% and outdoor/indoor ambient temperature at 0°C / 20°C. When adjusting EEV2’s opening, the maximum injection pressure corresponds to the maximum COP of 1.84, with EEV2’s opening at 65% under 35°C/ 27°C. When the COP reaches the maximum, the refrigerant at the outlet of EEV1 is just saturated liquid under cooling conditions, while it is located in the two-phase region under heating conditions. Besides, the optimal COP regulation strategy of the CO2 vapor-injection heat pump was further formulated based on two modes, as the strategy in the cooling mode differs from that in the heating mode.

Potential assessment of transcritical CO2 secondary loop heat pump for electric vehicles

The excellent heating performance and environmental advantages of the transcritical CO2 heat pump system make it a great fit for the electric vehicles (EVs). However, the high operating pressure leads to the possibility of excessive CO2 concentration in the cabin. To ensure the safety of passengers, a secondary loop (SL) system is proposed. In this study, we make a comparison between the traditional direct (TD) system and the SL system for an EV. The results show that the optimal coolant flow rate, with a range of 0.56 to 1.36 L/min in heating mode and 5.60 to 8.80 L/min in cooling mode, can increase the COP of the SL system in all working situations. In the heating mode, the COP of the SL system is 13.98% to 21.96% lower than that of the TD system, and the cruising range of the EV is reduced by 1.4 km ∼ 10.0 km. In cooling mode, the COP of the SL system is 16.95% to 22.45% lower than that of the TD system, and the cruising range has a reduction of 7.7 km ∼ 20.9 km. Results demonstrate that running the SL system in cooling mode rather than heating mode is more detrimental to EVs.

Experimental performance of a vapor-injection CO2 heat pump system for electric vehicles in −30 °C to 50 °C range

Efficient and wide-range heat pump system is essential to improve the driving range and comfort of electric vehicles. In this paper, a vapor-injection transcritical CO2 heat pump system with a flash tank for electric vehicles was built up, and the heating and cooling performance in a wide range from −30 °C to 50 °C were experimentally studied. The maximum coefficient of performance (COP) at −30 °C/ 20 °C was 1.45, and the maximum COP at 50 °C/ 27 °C was 1.08. By comparing with the basic system without injection, it was found that the heating performance of the vapor-injection system was significantly improved, and the lower the outdoor temperature, the greater the improvement. The COP and heating capacity of the vapor-injection system were increased by 45.5 % and 75.7 % respectively at −30 °C/ 20 °C. Under the cooling conditions, only when the outdoor temperature was lower than 40 °C, the vapor-injection system had an advantage, and the maximum increments of COP and cooling capacity at 35 °C/ 27 °C were 9.5 % and 16.8 %, respectively. Further, a relationship model between vapor injection and compressor structure and operating conditions was established according to the working principle of the twin rotary compressor. The experimental verification showed that the deviation was within 8 %, which could provide a basis for the construction of the CO2 vapor injection heat pump system model.

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.