Transcritical CO2 Refrigeration Cycle Research Articles

Multi-aspect assessment and optimization of a poly-generation layout composed of a geothermal power plant and a wind turbine

The current research is an effort to propose a novel poly-generation layout by hybridization of wind power and geothermal energy as two renewable heat sources. The geothermal unit comprises a steam flash cycle as the topping system and a Kalina flash cycle as the bottoming system. Hot water is produced through the utilization of the energy of the geothermal water before reinjection to the ground. An E126/7580 wind turbine is accountable for generating the favorable output electricity. Moreover, the waste heat from the generator of the wind turbine is recovered in a trilateral flash cycle. The sum of power produced by the steam flash cycle and the trilateral flash cycle is employed to generate hydrogen in an electrolysis unit. Moreover, the output electricity of the Kalina flash cycle is employed to produce cooling and freshwater in a transcritical CO2 refrigeration cycle and a reverse osmosis desalination system. The final evaluation of the system utilizing energy and exergy-economic analyses and three-objective optimization of the layout revealed an exergetic efficiency of 37.11 % in the optimum case. The configuration produces 771.7 kW, 454.3 kW, 3585 kW, 1.29 kgh-1, and 3 kgs-1 of electricity, cooling, heating, hydrogen, and freshwater, correspondingly. The economic analysis of the system indicates a total cost rate of 140.2 $h-1, a specific cost of products equal to 19.5 $GJ-1, and a payback period of 1.03 years, suggesting an excellent economic performance for such a complex system.

3E analysis and multi-objective optimization of a trans-critical ejector-assisted Co2 refrigeration cycle combined with thermo-electric generator

3E analysis and multi-objective optimization of a trans-critical ejector-assisted Co2 refrigeration cycle combined with thermo-electric generator

Design Analysis of CO2 Refrigeration System for Artificial Ice Rinks

With the increasingly serious environmental problems, the use of natural refrigerants has become an inevitable trend. In this paper, an artificial ice rink with a CO2 refrigeration system is studied, and the ice rink site and refrigeration system process are analyzed. The structure of the ice rink site includes a surface layer, a base layer, and an antifreeze layer. The refrigeration system flow has indirect ice rink refrigeration systems and direct ice rink refrigeration systems. With the application of the CO2 transcritical refrigeration cycle, the efficiency of the ice rink has been greatly improved.

Comprehensive performance assessment and multi-objective optimization of a new combined cooling, heating and power system with CO2 working fluid

In order to utilize the medium-and-low grade energy sources more efficiently, a new combined cooling, heating and power system using CO2 working fluid is proposed in this paper. The system is coupled by supercritical CO2 Brayton cycle and transcritical CO2 refrigeration cycle, and is characterized by the clever arrangement of some heat exchangers which could recover overall waste heat to generate useful heating capacity. Firstly detailed mathematical models are established to investigate the thermodynamic and exergoeconomic performance of the system. Under the preliminary design conditions with 170 °C heat source, the system could achieve values of 115.49%, 41.37% and 16.84$‧GJ−1 in thermal efficiency, exergy efficiency, and cost of unit product exergy respectively. Then comprehensive parameter analysis are conducted and reveal that within some certain ranges, the decrease of turbine inlet pressure and the increase of turbine inlet temperature, turbine outlet pressure, evaporation pressure and ratio of CO2 mass flow rates would lead to higher exergy efficiency and lower unit product exergy cost. Afterwards multi-objective optimizations performed under two operation modes demonstrate the everlasting high-efficiency of the system at different modes. Finally a performance comparison with a similar system in the literature further proves the superiority of the proposed system.

Performance improvement of transcritical CO2 refrigeration cycle by integrating vortex tube expansion: Simulation and optimization

Since the critical temperature of CO2 is low and the working pressure is high, cycles using CO2 as a refrigerant are generally transcritical. Considering the large throttling loss and low system performance of the existing transcritical CO2 cycles, this paper proposes a general parameter matching method for transcritical CO2 cycles with vortex tube by using the self-defined functional blocks that can easily and quickly realize the parameter matching of components in refrigeration cycles with diverse architectures. Based on this new method, simulation and optimization of existing transcritical CO2 systems with or without vortex tube are implemented. A two-stage compression transcritical CO2 system with serial vortex tubes is proposed and proven to have a high cooling performance and less exergy loss in comparison with existing transcritical CO2 systems. Furthermore, the number of vortex tube(s) used and the coupling mode of vortex tube(s) with the transcritical CO2 system are also discussed. The results show that for the transcritical system with evaporators greater than two, the performance of the system with parallel vortex tubes is better than that of serial vortex tubes. When there are two evaporators and vortex tubes with a cold mass rate greater than 0.3, the performance of the system with serial vortex tubes is better than that of parallel vortex tubes. Finally, exergy analysis of the improved and existing transcritical CO2 cycles is conducted which shows that the exergy loss caused by the throttle valve is considerably reduced by using the vortex tube.

Performance Analysis and Comprehensive Evaluation of Solar Organic Rankine Cycle Combined with Transcritical CO2 Refrigeration Cycle

In order to achieve efficient utilization of solar energy resources, this study combines the trans-critical organic Rankine cycle (ORC) power cycle (TORC) with the trans-critical CO2 refrigeration cycle (TCO2). Additionally, a comprehensive three-level index decision evaluation system is developed based on system safety and environmental protection, thermodynamics, and techno-economic performance. The evaluation focuses on typical medium- and high-temperature solar energy applications and considers six organic working gases. The evaluation results demonstrate that the R600 + CO2 solution outperformed the others. This solution achieved a maximum net output power (Pnet) of 1531.31 kW and 2306.43 kW, a maximum coefficient of performance (COP) of 3.16, a predicted payback period of 2.651 years and 2.033 years, and a benefit–investment ratio of 4.533 and 5.773.

Investigation on novel natural gas fueled homogeneous charge compression ignition engine based combined power and cooling system

A significant part of energy of fuel supplied is lost in internal combustion engines in the form of atmospheric discharge of engine exhaust gases which are considered as a big source of engine inefficiency and formation of pollutant emissions. To address this issue, a bottoming cycle combining the transcritical CO2 (T-CO2) refrigeration cycle and the supercritical CO2 (sCO2) power cycle is employed, aiming to produce cooling for food preservation by utilizing the exhaust heat of homogeneous charge compression ignition (HCCI) engine powering the refrigerated truck. The operative variables and their effect on thermal and exergetic efficiency of HCCI engine and the combined system are investigated. At the base case operative conditions, the thermal and exergy efficiencies of natural gas fueled HCCI engine are improved significantly from 48.69% to 61.28% and from 41.14% to 42.79%, respectively, after employing the sCO2 powered T-CO2 refrigeration cycle. Promotion of equivalence ratio from 0.3 to 0.9 enhances the thermal and exergy efficiencies of HCCI engine from 47.44% to 49.54% and from 40.14% to 42.12%, respectively. Increasing of engine speed from 1400 r.p.m to 2200 r.p.m provides marginal improvement in HCCI engine efficiencies but the efficiencies of combined cycle are significantly improved from 57.67% to 65.18% and from 40.64% to 45.06%, respectively. Finally, exergy analysis applied to determine the sources of non-idealities within the system revealed that out 361 kW (100%) fuel exergy supplied to the system, HCCI engine destroys 93.31 kW (25.85%), catalytic convertor destroys15.49 kW (4.29%), and the in-cylinder heat transfer losses and system exhaust losses are found as 35.72 kW (9.91%) and 16.61 kW (4.61%), respectively.

Performance analysis of two-stage CO2 transcritical refrigeration cycles with absorption subcooling and mechanical recooling

Although the two-stage CO2 transcritical refrigeration cycle with dedicated absorption subcooling is favorable for energy saving through employing the heat to reduce the loss of throttling processes, some important issues are essential to address exactly for the performance improvement, e.g., extending the working range of heat sources and enhancing the efficiency for the mid-temperature heat utilization. Thus, the two-stage CO2 transcritical refrigeration cycle with dedicated absorption subcooling and mechanical recooling is presented and studied thermodynamically. Two kinds of the proposed system are considered, e.g., sole-subcooler for the low-temperature heat source (the scheme A) and dual-subcooler for the mid temperature one (the scheme B). The performance of the proposed system and traditional one is compared, and the effect of critical parameters on the novel cycle is analyzed in detail. For the scheme A, the lower limit of heat source temperature is extended from 80 °C to 77 °C, and the compressor power goes down by 3.3%. For the scheme B, the lower limit of heat source temperature is extended from 139 °C to 128 °C, and the power consumption is reduced by 6%. Moreover, the payback period of the scheme A and scheme B is 7.9 years and 6.4 years, respectively. The paper is helpful to increase the performance of two-stage CO2 refrigeration cycle through the heat recovery and utilization, and promote the energy saving of cold storages.

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.

Measurement of critical properties for the binary mixture of R744 (carbon dioxide) + R1234ze(E) (trans-1,3,3,3-tetrafluoropropene)

In the context of global warming and demanding alternative refrigerants, the transcritical CO2 refrigeration cycle was considered a potentially energy-saving and carbon-reducing technology. The use of CO2 mixtures with higher critical temperatures and lower critical pressures has been demonstrated as an effective way to improve the performance of transcritical CO2 refrigeration cycles. To achieve this approach, accurate critical locus parameters are needed for establishing cross-over state equations and calculating thermodynamic properties in the near-critical region. Currently, experimental methods are the most effective way to obtain high-precision critical locus parameters of mixtures. In this work, the critical properties including critical temperature, critical pressure, critical density, and mole fraction of the R744 (carbon dioxide) + R1234ze(E) (trans-1,3,3,3-tetrafluoropropene) binary mixture were measured according to a variable-volume method with a metal-bellows volumeter. The critical point was determined by visual observation of the critical opalescence and reappearance of the vapor-liquid meniscus. The combined expanded measurement (relative) uncertainties of the critical temperature, critical pressure, critical density, and mole fraction were within 50 mK, 0.021 MPa, 0.6%, and 0.012 (k = 2, 0.95 level of confidence), respectively. After comparing the fitting results of the Modified Chueh-Prausnitz (MCP) method, the Modified Wilson (MW) method, and the Redlich-Kister (RK) method. We recommended the Modified Wilson method to correlate the critical properties of the binary system of R1234ze(E)+R744. Our research showed that R1234ze(E)+R744 mixture can achieve a higher critical temperature and critical density simultaneously and a lower critical pressure compared with pure CO2 when the composition of R744 is less than 0.23.

Blade design and analysis of centrifugal compressors for the transcritical carbon dioxide refrigeration cycle

The compressor is one important component in the transcritical CO2 (carbon dioxide) refrigeration cycle. The CO2 compressor has a small volume at the same refrigeration capacity because its refrigeration capacity per unit volume is large. The radial compressor often used in the CO2 refrigeration cycle has complex internal flow and difficult blade modeling. The paper presented a new centrifugal compressor with radial flow, whose blades are straight and easy to process. Taking a single stage compressor blade for example, the effects of the velocity coefficient, energy head coefficient, rotational speed of the moving blade, blade height, inlet radius and radial distance of the blade on the efficiency and pressure ratio were studied. According to the conclusions, the paper designed and simulated the blades of two compressors of the transcritical CO2 two-stage refrigeration cycle. Simulation result showed that the efficiencies of the two compressors were respectively 84.97% and 87.23%.

A novel bi-evaporator cooling system via integration of absorption refrigeration cycle for waste energy recovery from an ejector-expansion trans-critical CO2 (EETRCC) cycle: Proposal and optimization with environmental considerations

In this paper an integrated refrigeration system is designed, in which the waste heat from an ejector-expansion trans-critical CO2 (EETRCC) cooling scheme is used for the generation of cooling not only above but also below the temperature of zero degrees Celsius. Waste heat usage was accomplished via employment of an absorption refrigeration system in conjunction with a CO2trans-critical refrigeration cycle. The absorption refrigeration cycle generates excess cooling above zero temperature utilizing the waste heat from ejector expansion system. Appropriate thermodynamic models are employed for feasibility assessment of integrated refrigeneration system, along with economic considerations to estimate produced cooling price. The results revealed significant better performance of novel integrated framework compared to standalone EETRCC cycle. In the best-case scenario, the levelized cost of cooling for the former is obtained as 105.3 $/GJ and, while the latter produces cooling with a levelized cost of 120.9 $/GJ. Also, it has been concluded that, when the temperature of the second evaporator is raised, the exergy efficiency of the system drops by 3 %, which results in increment of unit cost of the cooling by 6.34 %. In addition, highest exergy destructions are found to be 27.53 kW in the gas cooler and 15.57 kW in the generator.

Effects of Internal Heat Exchanger on Two-Stage Compression Trans-Critical CO2 Refrigeration Cycle Combined with Expander and Intercooling

Because of the limitations of traditional refrigerants, the application of trans-critical CO2 technology in domestic gas conditioners and other fields is becoming increasingly popular. This paper proposes a new CO2 trans-critical refrigeration system. Combining the internal heat exchanger and expander components, as well as the two-stage compression cycle, we analyzed the effectiveness of the expander, internal heat exchanger, and intercooling on system performance under various operating conditions in terms of energy, exergy analysis, and optimal discharge pressure. The system performance can be changed by changing the cycle conditions and internal heat exchanger effectiveness, which reduces system power consumption and the percentage of exergy losses of gas cooler components. Compared to the single-stage compression with expander cycle, the systems cycle power consumption is reduced by 2–15.7% and the maximum system COP is increased by 2.93–6.93%. From the view of energy effectiveness, the system’s maximum COP increases by 3.9% and the percentage of exergy losses of gas cooler decreases by 22.5% with the effectiveness of internal heat exchanger varying. The addition of an internal heat exchanger has resulted in improved system performance, which is important for providing a relevant cycle model for the application.

Thermodynamic performance evaluation of an ejector-enhanced transcritical CO2 parallel compression refrigeration cycle

A parallel compression cycle is one efficient approach to improving the transcritical CO2 refrigeration cycle performance. This paper proposes a modified parallel compression transcritical CO2 refrigeration cycle with an ejector and a sub-cooler to further enhance the cycle performance. The thermodynamic analyses based on the energetic and exergetic analyses are conducted, and the influences of the parameters, including discharge pressure, flash pressure, gas cooler outlet temperature, and evaporation temperature, are discussed. The particle swarm optimization method is used to get the optimal discharge pressure and flash pressure and the temperature reduction in the sub-cooler with respect to the maximum COP. The performance comparisons between the conventional cycle and the modified cycle are presented. The results illustrate that the maximum improvements in COP and volumetric refrigeration capacity reach 15.8% and 17.1%. The utilization of the ejector and sub-cooler could reduce the optimal flash pressure by 16.4–33.0% and the compressor displacement by 11.4% compared with the conventional cycle. The ejector plays a dominant role in performance improvement at a high gas cooler outlet temperature. The exergy destruction analysis shows that the first-largest system exergy destruction ratio of 31.5% in the ejector-enhanced cycle is caused by the gas cooler, and its second-largest exergy destruction is generated in the two parallel compressors instead of the expansion valves in the conventional cycle.

Thermodynamic analysis and multi-objective optimization of a waste heat recovery system with a combined supercritical/transcritical CO2 cycle

This study firstly develops a novel combined cycle system consisting of a supercritical CO2 recompression Brayton cycle and a transcritical CO2 refrigeration cycle to recover waste heat from a marine turbine for both power generation and refrigeration. The pressure drop of the total heat exchanger in the whole cycle is considered in the thermodynamic and economic modeling process. Then, the influence of crucial system parameters on the system performance and economics is investigated through parametric sensitivity analysis. Finally, the system is optimized parametrically by multi-objective optimization. The results show that the higher inlet pressure of the high-pressure compressor helps to improve the thermal efficiency but reduces the exergy efficiency; the low-temperature heat exchanger plays a decisive role in the overall system exergy destruction; the HRHE accounts for a more significant proportion of the system cost, and the pressure drop has the most significant impact on the network of the system. The optimal solution is COP = 3.059, LCOE = 18.348$/(kW/h), and ηWhr = 0.651 when waste heat recovery efficiency, COP, and LCOE are considered optimization objectives.

An investigation into the thermodynamic improvement potential of a transcritical automotive CO2 refrigeration cycle

To identify the underlying reasons for the poor efficiency of a transcritical automotive CO2 refrigeration cycle in hot climates clearly, a thermodynamic analysis of ideal and real cycles was performed. The analysis considered energy and exergy (conventional and advanced) viewpoints, determined the ideal cycle efficiency, and identified the improvement potential of each component. Energy analysis results revealed that the ideal efficiency reaches 25.3 at 35 °C outdoor condition, which is far better than currently available real efficiencies. This indicated excellent potential for efficiency improvement. The transition from a pseudo-optimal high pressure under ideal conditions to an optimal high pressure under real conditions occurred upon the emergence of non-ideal factors. Conventional exergy analysis results indicated that exergy destruction within the throttling process accounts for the largest share (36%) of overall exergy destruction. However, advanced exergy analysis results suggested that irreversibilities in the evaporator and gas cooler are the main sources of exogenous exergy destructions in the compressor and throttling process, and their optimization potentials are typically 2–4 times greater than those of compressor and throttling process. Thus, the two heat exchange processes provide significant opportunities for transcritical automotive CO2 refrigeration cycle performance improvement.

Evaluation of the ejector two-stage compression refrigeration cycle with work performance from energy, conventional exergy and advanced exergy perspectives

Low temperature cold chain is the top priority of logistics development at this stage. CO2 (carbon dioxide) refrigeration system is widely used. However, the low efficiency of CO2 refrigeration system is the biggest obstacle to its development. In this research work, EES (Engineering equation solver) is used to simulate the system, which is widely used in thermodynamic modeling and calculation, a novel trans-critical CO2 refrigeration cycle with ejector for low-temperature storage is proposed. The energy and conventional exergy model of the system are established, and the advanced exergy model is established based on the conventional exergy model, and the actual performance of the system is analyzed, and the optimization potential of the system components is analyzed through the commonness of the two exergy analysis methods. The results show that the exergy destruction of each component of the ejector is the largest in the system, and the optimization potential is the highest in both conventional and advanced exergy models. After studying the influence of main parameters in the system cycle on the system performance and performance, it is found that there is an optimum intermediate pressure and gas cooler pressure in the system to maximize the system performance and efficiency. According to the influence of the change of system parameters on the system, it is found that the outlet temperature of gas cooler has the greatest influence on the optimization of ejector.

Energy analysis and performance comparison of transcritical CO2 supermarket refrigeration cycles

Using carbon dioxide as a refrigerant gains importance as a result of restrictions applied to high-GWP (Global Warming Potential) refrigerants. Although CO2 is an environment-friendly refrigerant, its low critical temperature and high operating pressure lead to high energy consumption, especially in commercial refrigeration. As a result, research and improvements are being made to reduce the energy consumption of transcritical CO2 refrigeration cycles. This paper presents thermodynamic analysis and pressure optimizations to obtain maximum performance for transcritical booster, parallel compression, and ejector expansion supermarket refrigeration cycles. Effects of different operation conditions were also investigated and cycles were compared to each other in terms of performance. It was obtained that up to 47% Coefficient of Performance (COP) improvement can be achieved using ejector expansion cycle with the help of expansion work recovery in the ejector. It was also shown that ejector expansion cycle outperforms other CO2-only cycles without ejector in all conditions.

Thermodynamic study of carbon dioxide transcritical refrigeration cycle with dedicated subcooling and cascade recooling

Stimulated by the fast development of cold chain commodity market and a large amount of demand in cold storages, the application of CO2 refrigeration system is to enhance remarkably. Owing to the low critical temperature of CO2, the dedicated subcooling technology is widely-used in the CO2 transcritical refrigeration cycle to increase the system performance. In this regard, reducing the compressor power of subcooling subcycle and extending the working range of heat source temperature is highly important for the mechanical subcooling technology and absorption one, respectively. Motivated by such purpose, the CO2 transcritical cycle with dedicated subcooling and cascade recooling is presented and the thorough investigation is performed thermodynamically. The proposed system is compared with layouts with dedicated mechanical subcooling and dedicated absorption one at first. Subsequently, the effect of recooling capacity ratio, the critical parameter for the system performance, is analyzed in-depth. Finally, the optimum recooling capacity ratio for different conditions is fitted by the neural network approach. Compared to layouts with dedicated mechanical subcooling and dedicated absorption one, the secondary compressor work goes down by 28.5% and the lower limit of heat source temperature is extended by 4 °C, respectively in the proposed system. The paper is favorable for the development and performance improvement of CO2 transcritical refrigeration systems.

Performance analysis of multi-refrigerant multi-variable environment refrigeration system based on marine cold chamber

This paper is based on the engineering equation solver to calculate the performance of refrigeration system under multi-refrigerant multivariable environment, which can simulate the operating performance of the system under different working conditions and the view of each state point, and provide help for the design and optimization of trans-critical CO2 refrigeration cycle refrigeration units. Using thermodynamic analysis, the system performance is observed by using the equations of conservation of energy, conservation of momentum and conservation of mass as the basis for the change of evaporator pressure, gas cooler pressure and intermediate pressure in the system; the state of each position in the ejector part of the system is calculated; after the calculation is completed, the pressure-enthalpy diagram of the system can be plotted; according to the different data parameters selected, the COP of the system can be plotted with the change of certain data parameters for different parameters. The results showed that the calculation method used is suitable for different refrigerants, compared with the traditional calculation and selection, the advantage of the EES software is that it can use the laws of thermodynamics for calculation and iteration.