CO2 Heat Pump Research Articles

High-efficiency solution for an open-loop desiccant assisted solar cooling system by integrating trans-critical CO2 heat pumps: A comprehensive techno-economic assessment

Solar cooling systems are classified into open and closed loop cycles. The latter is superior in terms of reliability and commercialization. However, the former, due to unique features to provide sensible and latent cooling separately is still under development.This work proposes a potential improvement in terms of energy performance in solar-assisted open loop cycles. The cooling system is composed of desiccant wheel, indirect evaporative coolers that require regeneration power. Such power is provided by a cutting-edge technology named trans-critical CO2 heat pump, running mostly by photovoltaic modules. Four different configurations are dynamically simulated inside MATLAB SIMULINK environment for the year interval.Based on the main simulation results, the renewable energy share of at least 50% for both electric and thermal is achieved. It could be increased up to 77% electric and 100% for thermal share. Furthermore, the parameters to optimize the cooling system performance are the heat pump COP, the recovery heat exchanger effectiveness, and the regeneration air mass flow rate. Generally, the innovative cooling system behaviour is affected by climate variations. Hence, the COP of the CO2 heat pump is starting from 2.3, which could be improved up to 6.5. It is clearly higher than the available benchmark.

Pinch point variation in the gas cooler of the CO2 heat pump water heater: An experimental and simulation comparative study

The carbon dioxide heat pump operation is strongly impacted by the presence of the pinch point within the gas cooler. The existing findings regarding the pinch point need to be verified due to the deficiencies of the simulation method. Given this, an experimental and simulation comparative study is performed. The pinch point variation characteristics and its connection with the heat pump operation are elucidated. The limitations and accuracy of the simulation model are revealed. The location and number of the pinch point are at most four scenarios. One locates at inside. Two locate at inside and cold end, respectively. One locates at cold end. One locates at hot end. The hot end pinch point is difficult to occur for actual heat pump operating conditions. As the carbon dioxide pressure rises, the pinch point temperature difference first grows slightly and then drops. The experiments demonstrated that the actual best operation of the carbon dioxide heat pump corresponds to one inside pinch point rather than two. The simulation model cannot reproduce this phenomenon well. The accuracy of the simulation model can be greatly improved in case the pinch point location is known, and it is acceptable from the perspective of meeting the engineering permissible error. This study may contribute to a better understanding of the pinch point variation. Moreover, it may play a reference role in the building and optimization of the gas cooler model.

4E analysis and multi-objective optimization of compression/ejection transcritical CO2 heat pump with latent thermal heat storage

To promote the goal of peak carbon dioxide emissions and carbon neutrality, low-energy consumption buildings require innovative technologies and efficient energy management. In this paper, the multi-objective optimization and the energy, exergy, economic and environmental (4E) analyses of the compression/ejection transcritical CO2 heat pump with latent thermal storage (TPE-LTES) system are conducted, and the influences of crucial parameters on the system performance are evaluated. The results show that the optimal comprehensive COP and annual hot water production (mDHW) of the TPE-LTES system are 3.59 and 3110 tons, respectively. Then, the comprehensive exergy efficiency of the TPE-LTES system owns 48.93 % higher than that of the heat pump unit. Furthermore, the optimal life-cycle cost (LCC) and life-cycle CO2 emissions (LCCE) of the TPE-LTES system are 708,073 CNY and 575,512.79 kg, respectively. Moreover, with constant discharge pressure, the evaporative superheat can be preferentially increased to reduce the system LCC and LCCE. Finally, the comprehensive COP and exergy efficiency of the system can be improved by increasing the inlet water temperature, while the increasing inlet water flow rate is the least desirable. This work is helpful to promote the research and application of latent heat storage unit integrated into air source heat pump.

Thermodynamic and economic analysis of a new methanol steam reforming system integrated with CO2 heat pump and cryogenic separation system

A novel design has been developed to optimize the performance of the methanol steam reforming (MSR) system by integration with a CO2 heat pump (HP) and a cryogenic separation system. In the integrated system, the cryogenic separation system separates most of the CO2 from the MSR byproducts in liquid phase and the rest chemical energy of CO is recycled to MSR after combustion. Besides, the CO2 HP system organically connects the MSR process and the cryogenic separation system, which is capable to absorb the waste energy from MSR and also provides cryogenic energy to cryogenic separation system and mid-temperature heat to MSR preheating process. By such incorporation, the waste heat and the energy of CO can be fully utilized and the CO2 from MSR can be exported in liquid phase as a secondary product. Based on a 20,000 ton/year MSR plant, the energy and exergy analyses reveal that the energy and exergy efficiency are promoted by 5.57% and 5.15%, respectively. Besides, the profit of the integrated design is 139.6 M$ higher than that of the conventional system over the lifespan. Furthermore, sensitivity evaluation shows that the optimal reaction temperature is approximately 250 °C and the higher reaction pressure is favorable.

Experimental and numerical analysis of a CO2 dual-source heat pump with PVT evaporators for residential heating applications

Multi-source energy systems are a promising solution to lower the environmental impact of the heating and cooling sector and enhance the exploitation of renewable energy sources. In this context, dual-source solar-assisted heat pumps exploit solar energy and air as the low-temperature heat sources. However, the efficiency of solar-based systems is strictly related to weather conditions, location, and time. Therefore, there is a need for accurate models to be used in dynamic simulations of these systems and perform detailed performance analyses and study the involved energy flows.This paper presents an experimental and numerical investigation of a direct-expansion solar-assisted heat pump (DX-SAHP) operating with CO2 as the refrigerant. The heat pump prototype can work with an air-finned coil heat exchanger or photovoltaic-thermal (PVT) solar collectors as the evaporator. The solar-mode configuration allows the exploitation of the heat from solar radiation to evaporate the refrigerant and to improve the photovoltaic electricity production due to the cooling of the cells up to 8%.A numerical heat pump model, integrated with novel gas-cooler and PVT collectors models, has been developed and implemented as a TRNSYS type for dynamic simulations of the system. The model has been validated with continuous measurements during the heat pump operation in solar and air modes. The proposed model can be used for performing seasonal simulations of a heat pump operating with a transcritical CO2 cycle. Moreover, the outcomes of the analysis show how the configuration of a CO2 heat pump with a direct-expansion air-finned coil heat exchanger or PVT can be used to enhance the performance of the heat pump and increase the electrical efficiency of the photovoltaic cells.

Experimental investigation of refrigerant charge determination for heating performance in a CO2 heat pump system for electric vehicles

Given the scarcity of investigation of refrigerant charge for the heating performance of the CO2 automotive heat pump system, a test of charge determination with a 350–650 g range over a wide operating condition is comprehensively conducted. It is concluded that 450–550 g charge can be an appropriate level. The inefficient charge level is more sensitive for performance deterioration than the excessive level. Each refrigerant charge level can generate ∼30 kg/h mass flow rate lift, and more than 50 K discharge temperature increase in respect of the valve openings. Lifting the compressor speed can generally decrease system coefficient of performance (COP) by ∼18% under −20 °C ambient and by ∼40% under 0 °C ambient, respectively. As to the discharge pressure concerning the control strategies, for a fixed compressor speed, the cabin side air supply temperature in general has a linear relationship (with a fixed value of slope) in respect of the discharge pressure for low discharge pressure domains (less than 70 bar), no matter what the valve settings are or which charge amount level we choose or what the ambient condition is. In addition, with a fixed compressor speed, under the same discharge pressure, with ∼20% refrigerant charge reduction from the appropriate level, the COP generally can be dropped by more than 12%, and the drop is more pronounced under high ambient conditions. What’s more, for a fixed optimal valve setting with different compressor speeds, under the same discharge pressure, ∼20% refrigerant charge loss can provide more than 20% COP drop. This may be helpful for fault diagnosis of system performance deterioration when charge leakage is involved. The conclusions in this study can be served as the guidelines for automotive heat pump systems.

Numerical investigation of the effects of inlet conditions and wall heat transfer on subcritical/transcritical CO2 nozzle performance

In ejector-based refrigeration cycles and CO2 heat pump cycles, inlet flow state and wall heat transfer play an important role in improving nozzle performance. A CO2 two-phase nozzle model was firstly established. Then, the numerical model is validated by experimental data and void fraction correlation, and the effect of condensation coefficient and gas equation of state was analyzed. In addition, the CO2 flow behavior and the thermodynamic performance of the nozzle under different inlet gas phase volume fraction (GVF) and wall conditions (temperature and materials) were studied. The results show that although the GVF can reduce the critical mass flow rate of the nozzle, its adverse impact on the thermodynamic performance of the nozzle is more significant (The GVF varied from 0 to 0.6, with the former decreasing by 10.2% and the latter increasing by 25.1%). Low temperature on the wall will exacerbate the non-equilibrium phase transition, leading to an increase in the critical mass flow rate of the nozzle. At this time, high thermal conductivity walls will also exacerbate exergy losses, by using low thermal conductivity walls can improve the consequences. In addition, the study also found that the inlet supercooling degree (ISCD) should be set between 0 and 7 K, and excessive ISCD would lead to an increase in outlet humidity and a rise in critical mass flow.

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.

Thermal energy storage as a way to improve transcritical CO2 heat pump performance by means of heat recovery cycles

Electrification of the heating sector is a major target of energy transition towards a more sustainable, efficient, and less polluted future. Heat pumps are considered more suitable than electrical heaters or fossil-fueled boilers; however, common refrigerants cause ozone layer depletion, which exacerbates the greenhouse effect. Natural refrigerants, such as CO2, perform comparably and even better than hydrofluorocarbons while minimizing the negative aspects. This study presents a model of a water-heater CO2 transcritical heat pump in a configuration that increases the overall coefficient of performance (COP) by introducing thermal energy storage (TES). The thermodynamic cycle was divided into two separate phases. After heating the TES (charging), warm water was used as the heating fluid in the evaporator to increase the evaporation temperature and pressure of CO2, which reduced the work of the compressor. As the water temperature decreased progressively, the discharge cycles improved the total COP. The case study focuses on dairy processes and suggests a more straightforward and cheaper method to improve cycle efficiency than the current processes, such as ejector-expansion systems or double compression.

High-temperature green hydrogen production: A innovative– application of SOEC coupled with AEC through sCO2 HP

This study evaluates a potential coupling between AEC and SOEC through a CO2 heat pump unit that works in super-critical conditions. In such a way, thermal power requested by SOEC can be provided by recovering heat loss from AEC via the CO2heat pump. The coupled configuration is characterized by renewable electric input. Hence, the power is provided for electrolysis without more external thermal resources. The energy, economic and environmental impacts of such intervention have been determined and compared with SOEC in solo mode. The layout is created and simulated in a MATLAB SIMULINK environment.The simulation results show a potential improvement in the total energy efficiency of the whole system without an external heat resource, up to 68%, which is higher than 65% of SOEC itself, thanks to the heat recovery section that is correlated with sCO2 HP contribution. LCOH varies widely depending on SOEC size from around 2.59 €/kgH2 up to 9.27€/kgH2.

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 study on performance of the trans-critical CO2 heat pump with flash tank vapor injection at variable revolution ratio conditions

The trans-critical CO2 heat pump has a wider application prospect since its advantages of more friendly environment, higher heating temperature and more compact equipment volume. In this study, performance of a trans-critical CO2 heat pump with flash tank vapor injection technique was experimentally studied. The effect laws of revolution ratio and expansion valve opening on the operating parameters were acquired to guide the design of an effective control strategy. Then influences of the discharge pressure, intermediate pressure and injection ratio on heating capacity, COP and heating temperature were analyzed at varied revolution ratio conditions. The results show that compared with revolution ratio = 1, varied revolution ratio can improve the COP by 7%, and decrease discharge pressure by 0.85 MPa. When revolution ratio <1, near revolution ratio ≈0.89, the COP reaches a peak value of 1.98. Compared with the simple two-stage compression cycle, improvements of the heating temperature, heating capacity and COP of the flash tank vapor injection cycle are 16.5%, 26.9% and 8.4%, respectively.

Heat transfer and performance enhancement investigation of biomimetic honeycomb gas coolers in transcritical CO2 heat pumps

CO2 air source heat pump heating technology has characteristics of strong adaptability to outdoor temperature in cold areas and high outlet temperature, which is widely used in heating and domestic hot water supply. A gas cooler is one of the crucial components in CO2 heat pump systems, while its performance and different structural forms will directly affect the performance of the whole CO2 heat pump system. In order to improve its heat transfer efficiency, a biomimetic honeycomb gas cooler is proposed in this paper, the steady-state simulation model of the biomimetic honeycomb gas cooler is established, and the reliability of the model is verified by experimental data. In addition, the effect of structural parameters and operating parameters on the heat transfer performance is studied, also the heat transfer performance of the biomimetic honeycomb gas cooler is compared with that of the internal spiral tube gas cooler and triple tube-in-tube gas cooler. The results show that the performance of the biomimetic honeycomb gas cooler can be effectively improved by reducing the water flow rate. Under the same working condition, the performance evaluation criteria1 of the 1st, 2nd and 3rd layer biomimetic honeycomb gas cooler s are about 193.4%, 239.9% and 357.0% higher than that of the internal spiral tube gas cooler, and about 197.2%, 205.9% and 237.7% higher than that of triple tube-in-tube gas cooler. This research is helpful to provide the reference for the design and optimization of this type of gas cooler and is conducive to the popularization and application of CO2 air source heat pumps.

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.

Energy storage device based on a hybrid system of a CO2 heat pump cycle and a CO2 hydrate heat cycle

A new large-capacity energy storage device (with a storage capacity of several megawatt-hours or more) based on a hybrid cycle of a CO2 heat pump cycle and a CO2 hydrate heat cycle is investigated using an experiment-based numerical analysis. In the charging mode of the CO2 heat pump cycle, the work of the compression process is input with surplus electricity from renewable energy sources. Liquid CO2 is stored in a tank after the condensation process. In the discharge (power generation) mode, the liquid CO2 stored in the tank is heated by outside air, and the expanded gas is supplied to the expander, which then operates the generator to obtain electricity from the expansion work. However, if the outside temperature is low, the expansion of liquid CO2 will be insufficient. The abovementioned CO2 hydrate heat cycle is added to the CO2 heat pump cycle. The resulting energy volume density of the proposed system is 80–140 kWh/m3, with a charge–discharge efficiency of 60%–106%, and the equipment costs should be similar to those of pumped storage. At ambient temperatures of <10 °C, the hybrid system with energy storage by gas hydrates is more effective, with a higher energy volume density and charge–discharge efficiency than previous systems.

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.

Energy, exergy, exergoeconomic and environmental (4E) analysis of cascade heat pump, recuperative heat pump and carbon dioxide heat pump with different temperature lifts

Air source heat pump is recognized as an effective environmental protection and energy-saving technology. The heating temperature lift is one of the key factors affecting the performance of heat pump systems. In this paper, an in-depth quantitative thermodynamic comparison of cascade heat pump, recuperative heat pump and CO2 heat pump for direct-heating and space heating was conducted. Aspen HYSYS was used to invoke advanced genetic algorithm in MATLAB to optimize the system parameters. The temperature matching in heat exchangers was employed to reveal the essence of the performances of different heat pump systems. The recuperative heat pump is more suitable for direct-heating with the temperature lift 60 °C, and its COP is up to 4.81 at the ambient temperature of 20 °C. The cascade heat pump performs best in space heating with the maximum COP of 2.47. The exergoeconomic factor of the compressor is less than 7%, which illustrated that the investment in compressor should be increased to further improve the system efficiency. The annual CO2 and SO2 lowest emissions is 4431 kg and 130 kg of recuperative heat pump for direct-heating, and the cascade heat pump has the lowest annual 4234 kg CO2 and 127 kg SO2 emissions for space heating.

Evaluation of energy-saving potential and cabin thermal comfort for automobile CO2 heat pump

Currently, the heat pump air conditioning system is extensively used in Electric Vehicle (EV). However, most previous simulation studies focused on one-dimensional (1D) system performance rather than three-dimensional (3D) cabin thermal comfort. This study proposed a novel control strategy for the operation of a one- and three-dimensional coupling transcritical CO2 heat pump air conditioning system based on the weighted Predicted Mean Vote (PMV) model. The mathematical characterizations of the multiple thermodynamic systems were achieved by one-dimensional calculation and the cabin thermal environment was described by three-dimensional simulation. The weighted predicted mean vote value was calculated by real-time interactive three-dimensional cabin thermal environment parameters while providing control signals for the one-dimensional refrigeration system. The simulation comparison revealed the influence of the weighted Predicted Mean Vote control strategy on the system’s thermodynamic parameters and energy-saving potential. Results showed that the compressor power consumption of the Predicted Mean Vote control strategy was reduced by 9.1%-33.7%, compared with the method of presetting specific temperatures. The proposed one- and three-dimensional coupling method of cabin thermal management establishes the structured correlation between heat pump air conditioning system and cabin thermal environment. The weighted Predicted Mean Vote control strategy reduces the system power consumption within the thermal comfort range of the passengers, and it provides a novel idea for cabin thermal management during practical engineering.

Energy Conservation and Carbon Reduction of Air Source Heat Pump for District Heating

Using ASHP for space heating is an environmental solution. Based on the concept of Lorenz cycle, an ASHP is proposed. Taking a residential community as an example, this study proposes a thermodynamic model. Then, the energy consumption and carbon emission performance of the heating system are discussed. The results indicate that the primary energy consumption of ASHP is 13.45% less than the coal-fired boiler and the carbon emission reduces by 1203.6 tCO2. This study is meaningful for the development of air source CO2 heat pump for space heating integrated with DMS.