CO2 Heat Pump Research Articles

Parametric studies and structural optimization of a PCM tank integrated with CO2 heat pumps

Parametric studies and structural optimization of a PCM tank integrated with CO2 heat pumps

Characteristic analysis and performance optimization of a transcritical CO2 heat pump for high-temperature heating: An experimental study

Based on the improved experiment, a study on the characterization and optimization of transcritical CO2 heat pumps for high-temperature heating is conducted through the combined analysis perspective of thermal quality and heat capacity, as well as the proposed indicator. The result shows that: In the heat transfer process of the gas cooler, the thermal quality of CO2 is closely related to the variation of discharge pressure and compression ratio, and the temperature rise of water shows a corresponding three stages; An increase in the heating temperature (th) requires a higher level of thermal quality on the CO2 side, which exacerbates the heat capacity mismatch in the gas cooler; The elevated compressor speed (ncom) can effectively alleviate this mismatch, but an excessive ncom can instead cause a reduction in system COP; In the study, the COP of the system is improved by up to 26.12 % by optimizing the throttle opening benchmark, and it is further enhanced by up to 19.91 % through the synergistic control strategy optimization; Consequently, the optimized COP can be obtained 2.204 when the required th is 100 °C; Furthermore, the introduction of IHX significantly benefits the high-temperature heating performance of the system and the COP lifting studied ranges 3.70%–14.12 %.

Numerical investigation of high temperature heat pump integrated hybrid cooling system

Over 67 % of the energy demand within the pharmaceutical and semiconductor industries is for their cleanrooms, driven by stringent temperature and humidity control requirements. Despite high energy usage, heating, ventilation, and air-conditioning (HVAC) units consisting of conventional vapor compression systems (CVCS) are favoured for their simplicity. Hybrid cooling systems (HCS) offer potential efficiency gains by separating latent and sensible loads. Still, reliance on fossil-fuel-fired or electric heaters to regenerate the desiccant makes it energy intensive and reduces CO2 mitigation benefits. This study explores high-temperature heat pumps to simultaneously cool process air and heat regeneration air, thereby using multiple benefits. Heat pumps with condensing temperatures of 80 °C, 90 °C, and 120 °C and an ejector-enhanced trans-critical CO2 heat pump were compared with CVCS and HCS. The 120-bu and CO2HP configurations demonstrated a system COP of 2.94 and 3.22, respectively, surpassing CVCS, which has a system COP of 1.44. These setups achieved CO2 emission reduction of 51 % and 55.2 %, compared to CVCS, respectively.

A quasi-two-stage trans-critical CO2 heat pump with in-cycle thermal storage for performance enhancement

To decarbonise the heating sector, there is a strong demand for environmentally friendly and cost-effective heating technologies. Trans-critical CO2 heat pumps offer a sustainable alternative to traditional vapor compression cycles that rely on environmentally harmful synthetic refrigerants. However, a significant challenge within trans-critical CO2 heat pumps is their high throttling loss during expansion, leading to substantial flash gas production and subsequently higher recompression power consumption. In this paper, we apply our Flexible Heat Pump cycle concept to trans-critical CO2 heat pumps to address this issue. A thermal storage unit is introduced into a traditional trans-critical CO2 heat pump cycle to recover heat from the hot super-critical CO2 leaving the gas cooler during charging mode. Once it is fully charged, the stored heat is then used as a temporary heat source for the heat pump’s operation during the discharging mode. Since the recovered heat has a higher temperature than ambient. The resultant flexible trans-critical CO2 heat pump (FlexiCO2-HP) reduces throttling losses and the compressor’s power consumption, leading to a higher average COP (Coefficient of Performance). The obtained results of the FlexiCO2-HP are compared against a single-stage trans-critical CO2 heat pump. For gas cooler temperatures ranging from 35 °C to 60 °C and across various discharge pressures, its COP is potentially 10 % to 40 % higher than the standard cycle under ideal conditions. In addition, the proposed system demonstrates a lower optimal gas cooler pressure across different gas cooler temperatures. Furthermore, this study compares the FlexiCO2-HP with other trans-critical CO2 systems, including those with expansion work recovery and two-stage configurations. The results demonstrate that the FlexiCO2-HP offers improvements in COP for most conditions, though not all. It has the potential to achieve a comparable COP when compared with ejector-based and expander-based heat pumps or two-stage systems, but it is much simpler in design without introducing an extra expander or compressor.

Optimal high-pressure correlation for transcritical CO2 cycle in direct expansion solar assisted heat pumps

Heat demand may be met sustainably by a solar-assisted heat pump that CO2 as a refrigerant. This is made possible by the employment of an ecologically benign refrigerant and a renewable energy source that enhances system performance. At the ideal high pressure, the CO2 heat pump running in a transcritical cycle will function at its highest coefficient of performance (COP). In that way, this study aims to present correlations for calculating the optimum high pressure in a CO2 direct expansion solar assisted heat pump (DX-SAHP). In this study a mathematical model for compressor, solar evaporator and gas cooler was developed and validated experimentally. The mean difference between the mathematical model and experimental results are −3.9%. Two new equations are proposed to calculate the optimum high pressure in a CO2 DX-SAHP. The first one considered environment temperature, water outlet temperature and evaporating temperature, which are easy to measure. These facilities the use of correlation to control the heat pump. The second one considered environment temperature, water outlet temperature and solar radiation, which are more suitable for designing CO2 DX-SAHP. A data base with 100 optimum points was used to curve fitting. Another data base with 50 optimum points was used to compare the results obtained from curve fitting and correlations for the optimum high pressure available in the literature. The proposed correlations shown a maximum error lower than 10.2% despite of the correlations available in the literature from which the errors are about 30%.

Refrigerant charge estimation method based on data-physic hybrid-driven model for the fault diagnosis of transcritical CO2 heat pump system

The transcritical CO2 heat pump system is a promising and excellent heating scheme. However, the operating pressure of the transcritical CO2 heat pump system is high, and refrigerant leakage is a frequent problem. It is worth noting that the operating principle of CO2 heat pump systems is unique. Thus, conventional refrigerant charge estimation methods for subcritical systems cannot be directly applied to transcritical systems. In this study, a novel refrigerant charge estimation method for the transcritical CO2 heat pump system is proposed. Based on the heat and mass transfer characteristics of the transcritical system, the grey box model constrained by physical laws is constructed. Based on the machine learning algorithm, the correction model of the grey box model is built. The hybrid model combines the advantages of the high generalization ability of the physic-driven model and the nonlinear fitting ability of the data-driven model. The results show that the novel estimation method can accurately predict the refrigerant charge, enabling fault diagnosis of refrigerant leakage. The data-driven correction model can improve the prediction accuracy of the grey box model. The Mean Relative Error of the hybrid-driven model can be controlled within 5 %.

Investigation on ejector design for CO2 heat pump applications us-ing Dymola

In this paper, the Dymola modelling tool is used to study the influence of ejector design onto the whole heat pump cycle working with carbon dioxide. The cycle is built using the components provided by the TIL Modelica library. It is found that the ejector models in TIL are quite limited, namely by their inability to properly capture the on-design plateau and rapid decrease in performance in off-design operation. Therefore, an in-house state-of-the-art ejector model, originally developed in Python, is implemented as a Dymola object. This model is then calibrated onto CO2 experimental data. The operation of a simple CO2 heat pump system is investigated, with focus on the ejector sizing at fixed geometry. It is found that there exists an ejector size that maximises the COP of the cycle. Furthermore, critical ejector pressure is not reached at this optimum COP point; the ejector is operating well under the on-design regime.

Numerical investigation of the heating efficiency of CO2 heat pump water heater system in cold environments

Numerical investigation of the heating efficiency of CO2 heat pump water heater system in cold environments

Structural analysis and optimization of flattened tube gas cooler for transcritical CO2 heat pump systems

The performance of gas cooler is acknowledged as a vital factor in CO2 heat pump systems. Consequently, its optimal design can significantly impact the overall efficiency, attributed to the pronounced changes in thermophysical properties of CO2 during pseudo-critical operating conditions. This research examines, for the first time, various configurations of flattened tubes (FTs) utilized as the conduit for CO2 flow within the gas cooler. It is established that changing the cross-sectional configuration represents an effective approach for enhancing the thermal performance of the gas cooler, resulting in a significant increase of up to 60 % in heat transfer coefficient. All FTs exhibit superior overall performance across most CO2 cooling conditions compared to circular tubes. Regarding the performance index, the overall improvement can be reached up to 31.8 % and 18.6 % under near-critical and trans-critical conditions, respectively. Furthermore, an optimization is conducted using BBD-RSM for a specific alternating FT due to its potential to enhance the thermal performance not only on the CO2 side but also on the cold side of the gas cooler. The analysis highlights the significant positive influence of the minimum axis on the thermal and hydraulic specifications of gas cooler, followed by the transition length.

Research on the performance and thermal comfort of CO2 heat pump heating system in high altitude area

Research on the performance and thermal comfort of CO2 heat pump heating system in high altitude area

Multi-factor coupling optimization of heat recovery effectiveness in a transcritical CO2 heat pump

In the background of energy shortage and climate change, transcritical CO2 heat pump(HTP) technology has attracted lots of attention because of its energy-saving and environmentally friendly advantages. In this research, an experimentally verified simulation model of transcritical CO2 HTP is established to investigate multi-factor coupling optimization of heat recovery effectiveness(ηIHX). First, the coupling optimization mechanism of ηIHX and discharge pressure(pdis) is analyzed. Moreover, this research explores the influence of ηIHX on heating capacity, power consumption, discharge temperature(tdis), and internal heat exchanger(IHX) cost, and further proposes a comprehensive heat recovery index to optimize the above factors. Based on this index the optimal heat recovery effectiveness(ηIHX,opt) for each operating condition is obtained. Also, a failure boundary for the coupling optimization of the ηIHX is also indicated. In addition, the optimal discharge pressure(pdis,opt) prediction correlation for different ηIHXs is proposed, which can be used for heat recovery effectiveness collaborative optimization control. Finally, a general method for evaluating IHX is provided. Taking Xi'an as an example, the optimal heat recovery area(AIHX,opt) of this HTP system is 0.42m2, with which the optimized HTP system operates safely at extreme operating conditions, resulting in an annual lucre of 1,211 CNY.

Study on electric vehicle thermal management system using phase change materials and CO2 heat pump waste heat recovery under cold conditions

The thermal management system plays a pivotal role in electric vehicle. The CO2 heat pump technology, which performs well at low temperatures and environmentally friendly, has emerged as one of the most promising solutions for optimizing the thermal management system of electric vehicles. However, at −30 ℃ to −15 ℃, the CO2 heat pump faces issues with excessively high discharge temperatures and limited Gas Cooler heat exchange area, thereby there is potential of waste heat recovery and utilization. This paper proposes a thermal management system that utilizes PCM heat storage to recover waste heat from the CO2 heat pump. The system aims to insulate the battery during winter parking, reducing the battery’s preheating energy consumption and enhancing the vehicle’s overall energy utilization efficiency. This paper explored the waste heat characteristics of CO2 heat pump under various conditions. Subsequently, Numerical structural optimization analysis of the PCM heat storage was conducted. Finally, an economic analysis of the PCM heat storage is performed, considering the boundary condition of the urban climate. The result showed that Under the heat production control strategy, the PCM heat storage has a high heat storage rate at −30 ∼ -20 ℃ ambient temperature, reaching 1470 W ∼ 2709 W, and the heat storage time is less than 1 h. At −15 ℃, where the energy saving is maximized, the system battery coupled with the PCM heat heat storage consumes only 0.88 kWh for preheating, saving 44 % compared to the 1.58 kWh consumed by the baseline. The PCM heat storage demonstrates a better energy-saving effect in colder areas.

Cooling heat transfer attributions of supercritical CO2 in a spiral groove tube casing heat exchanger: A numerical investigation

Abstract To improve the performance of the CO2 heat pump water heater, the spiral groove tube casing heat exchanger is used as a gas cooler. At present, the flow mode of supercritical CO2 (SCO2) flows between inner and outer tube channels is mainly adopted. However, the efficiency of the gas cooler was studied rarely when the SCO2 flows in the inner tube channel (ITC). So, the heat transfer of SCO2 in the two flow channels are studied and compared in this paper. A physical model of the cooling heat transfer of SCO2 is established for the spiral groove tube casing. The impact of SCO2 pressure, the mass flow ratios of SCO2 and water on the heat transfer attributions of SCO2 in the tube are analyzed using numerical simulation. The outcomes designate that the flow channel in the exchanger can affect the heat transfer attributions of SCO2. When the mass flow ratio of SCO2 becomes lower, the average heat transfer coefficient (h) of SCO2 flowing between inner and outer tube channels is higher, with about 2.09%. As the mass flow ratio of SCO2 rises, the average h of SCO2 flowing in an ITC is higher, with about 3.90%. Moreover, both the safety of the system operation and the heat transfer attributions of the functioning medium should be considered, the flow mode of SCO2 flows in the ITC is recommended.

Effects of the different water tank location on the performance of CO2 heat pump with thermal energy storage for space heating

Effects of the different water tank location on the performance of CO2 heat pump with thermal energy storage for space heating

The advantage of running a direct expansion CO2 heat pump with solar-and-air simultaneous heat sources: experimental and numerical investigation

Dual-source solar-air heat pumps represent a promising solution for overcoming the limitations associated with single-source utilization, thereby enhancing heat pump performance. However, running the heat pump by alternatively employing the more advantageous source requires the integration of a controller capable of continuously monitoring and predicting the heat pump's performance in response to dynamic environmental and operational variables. Even so, a selective alternate operation does not allow to get the maximum possible performance from the use of the two heat sources. A different approach to address this challenge is the simultaneous utilization of the two sources, by properly combining two evaporators in the CO2 circuit. This paper presents an experimental investigation of a dual-source heat pump using CO2 as refrigerant, which can operate in three different evaporation modes: air-mode (using a finned-coil evaporator), solar-mode (using a photovoltaic-thermal PV-T evaporator), and simultaneous-mode (using both the evaporators simultaneously). The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators. Experimental data indicate that the heat pump operating in simultaneous-mode allows to increase the evaporation pressure and the coefficient of performance compared to operation in air-mode or solar-mode. The measurements have been employed for validating a model of the system, capable of predicting steady-state and dynamic performance under various environmental and operational conditions. Simulation results show that the simultaneous-mode operation can be outperformed by the solar-mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature. Finally, the impact of the number of PV-T collectors and solar irradiance on the heat pump performance has been simulated and discussed. On this regard, the simultaneous use of the two heat sources adds more flexibility to the system and its design, because even the availability of a small solar area can contribute enhancing the performance over the mere air source heat pump.

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.

Enhancing control disorder and implementing V2X-Based suppression methods for electric vehicle CO2 thermal management systems

In recent years, the development of electric vehicles (EVs) thermal management systems has underscored the crucial role in ensuring driving safety and optimizing driving range has become increasingly prominent. However, the inherent dynamic complexity of EV operation coupled with automatic control systems, can sometimes lead to unstable behavior, resulting in performance degradation and safety risks for compressors and batteries. To effectively address this issue, an evaluation was conducted on the dynamic control characteristics of an EV thermal management system utilizing CO2 as the refrigerant in this study. Through mathematical modeling and experimental analysis, the erratic nature of the dynamic thermal process was first identified. The underlying reasons were elucidated, focusing on system control characteristics and intrinsic mechanisms. It was found that control disorder could induce abnormal actions in thermal management system components like compressors and expansion valve, leading to significant performance decline and issues such as liquid carryover in compressor suction. Furthermore, specific control disorder regions of CO2 heat pumps for EVs were delineated, providing a framework for assessing the likelihood of system control disorder. Notably, control disorder was more likely to occur under conditions of low indoor air flow rate, high ambient temperature, and low supply air temperature. Given the widespread nature of this issue and the lack of suitable solutions, two control disorder suppression schemes were developed using V2X technology and validated through simulation. Results showed that adoption of V2X communication technology prevented an average of 70.1 % COP degradation, ensuring stability and safety of compressors and batteries under various operating conditions. The research provides useful information for exploring the dynamic characteristics of CO2 thermal management systems, offering a novel approach to enhance the system stability and efficiency.

Experimental characterization and numerical modelling of an air to water CO2 heat pump under different outdoor and operating conditions for domestic hot water generation

This work assesses the influence of different outdoor and operating conditions on the performance of an air-to-water CO2 heat pump for domestic hot water (DHW) generation. For that purpose, an air to water CO2 heat pump with internal heat exchanger (IHX) was developed and characterized in an experimental campaign under well-controlled lab conditions. The system includes an in-house control system implemented to operate at the optimal gas cooler pressure. The heat pump and the control system are tested for DHW generation. The heat pump uses a finned-tubes heat exchanger as evaporator and plate heat exchangers as gas cooler and internal heat exchanger. The numerical model used a 1dimensional, cell by cell discretization method to characterize the performance of both, the gas-cooler and the evaporator under a wide range of values of their key variables. The IHX is also modelled with the same strategy. ARHI equations are considered to describe the compressor performance. The experimental data obtained are used to validate a numerical model of the heat pump under different air velocities, air temperatures and air humidity at the evaporator. The effectiveness of the control system proposed is experimentally confirmed during the experimental tests. Results show a good agreement between the numerical simulations and the experimental data what permits to validate the model and to assess the performance of the transcritical heat pump under different climate conditions.

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.

The comprehensive research on the transcritical CO2 thermal management system used in thermostatic transport vehicles

Natural fluid CO2 has become the preferred target of refrigerant alternative in the electrified transportation field in recent years to replace positive temperature coefficient (PTC) heaters, reduce energy consumption, and respect the international ban on Hydrofluorocarbons (HFCs) refrigerants. Thermostatic transport vehicles (TTVs) are well suited to exploit the benefits of CO2 due to their working conditions with a broad range of ambient temperatures, extensive transit distances, and substantial heating capacity requirements in cold climates. In this paper, a transcritical CO2 heat pump air-conditioning system used in TTV was constructed initially with proper control logic. Then following the assessment of the system’s steady-state performance, the dynamic operation process under high thermal inertia was thoroughly examined and analyzed in detail. Ultimately, the optimization of the dynamic response process was realized under varying heat flow boundary conditions. The primary results demonstrated that the rationally designed CO2 thermal management system performed admirably in various operating conditions, particularly in the heating mode where the coefficient of performance (COP) exceeded 3.2. Addressing the high-frequency fluctuation caused by the cargo thermal inertia, the temperature could be maintained with only two starts and stops within 3×105 seconds when the system control logic was refined. The optimized thermal management system of TTV efficiently allowed cargo to achieve the proper temperature as quickly as possible and sustain stability. This study provides an effective theoretical basis for the promotion of the transcritical CO2 thermal management system in the field of TTVs.