Transcritical Carbon Dioxide Refrigeration Cycle Research Articles

A new multi‐objective optimization of refrigeration cycles (Case study: “Optimization of transcritical carbon dioxide cycle”)

AbstractFor store and maintain products, ingredients, and other perishables at a safe temperature, there is a growing need for low‐temperature refrigeration systems. In this study, a new multi‐objective exergetic criterion is proposed for the optimization of refrigeration cycles, the conventional coefficient of performance (COP) maximization method results in high evaporator temperature, which is in contrast with the philosophy of the invention and design of refrigeration cycles. Thus, it is not a suitable platform for comparing different refrigeration cycles. This research aims to define a practical performance analysis parameter for designing and comparing refrigeration cycles. The novelty of this research is incorporating the impact of evaporator temperature into the COP concept. A performance comparison of the multi‐objective optimization is drawn with that of the maximum COP state on a transcritical carbon dioxide refrigeration cycle. According to the comparison, the evaporator temperature at the optimal state of multi‐objective exergetic optimization was 44 K lower than that of the maximum COP state, which demonstrates the excellent performance of the method. The results of the new multi‐objective exergetic optimization can be used to design low‐temperature evaporation refrigeration cycles. In the end, the flowchart of the suggested performance optimization is presented. Exergy of power at the optimal point of exergetic optimization is about 77% higher than the maximum COP. A comparison of the results obtained from maximum performance coefficient with optimal point of exergetic optimization suggests that the performance coefficient at optimum state of exergetic optimization is about 58% lower than the maximum COP.

Synergetic characteristic of a novel thermally-driven CCHP system based on supercritical and transcritical CO2 cycles

A novel poly-generation system is proposed based on supercritical carbon dioxide (SCO2) Brayton cycle and transcritical carbon dioxide (TCO2) refrigeration cycle to meet the seasonal energy demand in buildings. The thermodynamic model of the combined cooling, heating and power (CCHP) system was constructed based on the laws of thermodynamics, and seasonal operation modes were also analyzed. The Techno-economic performance of CCHP system was analyzed and compared with the single output systems and organic working fluids. The results show that the temperature of the turbine inlet in SCO2 cycle should be close to the heat source temperature and the temperature of the condenser outlet should be close to the critical temperature. The optimal pressure of the condenser outlet for coefficient of performance is about 9.3 MPa, while it is 8.6 MPa for power generation. The optimum thermodynamic and economic performance cannot be achieved simultaneously. The power generation of CO2 is 13.13% worse than that of organic working fluids and coefficient of performance of former is worse than that of latter. As the manufacturing cost goes down over time, the CCHP system based on CO2 will further improve its economic performance in the future.

Thermodynamic investigation of a centrifugal reverse Brayton cycle for refrigeration in air conditioning filed

The reverse Brayton cycle (RBC) is known to exhibit a lower coefficient of performance (COP) when compared to conventional vapor-compression refrigeration systems. To tackle this issue, this study proposes a centrifugal reverse Brayton cycle (CRBC) that features a rotating heat exchanger, which utilizes the conversion mechanism between equal potential energy and pressure energy to enhance system performance. The findings indicate that the COP of the CRBC is roughly inversely proportional to the working fluid density, making CO2 a more suitable option for CRBC systems than conventional working fluids. We propose three different cycles for the CRBC system: the diamond cycle, trapezoid cycle, and rectangle cycle. From both technical and operational perspectives, the trapezoid cycle appears to be the preferred choice for CRBC. When the isentropic efficiency of the CRBC system exceeds 95%, its coefficient of performance (COP) ranges from 5.31 to 5.7, whereas the COP of transcritical carbon dioxide refrigeration cycle (tCO2), air RBC, and CO2 RBC are around 4.3, 1.9, and 0.79, respectively. This suggests that the performance of the CRBC system surpasses that of RBC by a significant margin, while achieving comparable performance to conventional vapor compression refrigeration systems. Moreover, the dimensionless power reflecting the installation capacity requirements for CRBC, tCO2, air RBC and CO2 RBC are around 0.2, 0.35, 5.3 and 1.9, respectively. This indicates that the compressor/expander capacity demand for system CRBC approaches the thermodynamic minimum.

Improvement design and performance assessment of combined cooling and power system using CO2 for waste heat recovery

Waste heat recovery (WHR) is an efficient approach to reducing fossil fuel consumptions and carbon dioxide emissions. In shipboards, gas turbine exhaust can be recovered to simultaneously produce power and cooling for meeting the actual need of marine. The supercritical carbon dioxide (sCO2) power cycles and transcritical carbon dioxide (tCO2) refrigeration cycles have many advantages of compact layout, high efficiency, and environmentally friendly property. Accordingly, combined cooling and power systems (CCPs) integrating sCO2 power cycle with tCO2 refrigeration cycle are considered as competitive and promising WHR systems. However, a large space is still left for CCP to improve its overall performance. In this study, a novel CCP is proposed based on two traditional CCPs. Mathematical models are developed based on the software MATLAB to conduct quantitative parametric analyses, three-objective optimizations and exergy analyses on these systems. The results indicate that compared with two traditional systems, the proposed system can enhance net power output by 40.185% and 35.743% while obtaining a comparable refrigeration capacity and total product unit cost. Exergy analysis indicates that the maximum exergy destructions occur in recuperators and cooler, and the proposed system can significantly reduce exergy destructions in these components compared with two traditional systems.

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%.

Impact of a thermoelectric subcooler heat exchanger on a carbon dioxide transcritical refrigeration facility

To improve the performance of vapour compression refrigeration cycles, the inclusion of a thermoelectric subcooler for low-medium power units has been the focus of recent studies due to its robustness, compactness and simplicity of operation. In thermoelectric systems, it has been demonstrated that the heat exchangers used in the hot and cold side of the thermoelectric modules have a critical impact in the performance of the system. This influence has not yet been studied for thermoelectric subcooling systems in vapour compression cycles. This work, for the first time, evaluates the impact that the heat exchangers of a thermoelectric subcooler, included in a transcritical carbon dioxide refrigeration cycle, have, in the performance of the refrigeration cycle. The influence is quantified in terms of: optimum working conditions, coefficient of performance and cooling capacity. The results show that, through an optimization of the heat exchangers of the thermoelectric subcooler, the performance improvements on the coefficient of performance using this technology are boosted from 11.96 to 14.75 % and the upgrade in the cooling capacity of the system rises from 21.4 to 26.3 %. Moreover, the optimum gas-cooler working pressure of the system is reduced and the optimum voltage supplied to the thermoelectric modules increases.

Performance assessment of an experimental CO[formula omitted] transcritical refrigeration plant working with a thermoelectric subcooler in combination with an internal heat exchanger

Regulations in the refrigeration sector are forcing the transition to low global warming potential fluids such as carbon dioxide in order to decrease direct greenhouse gases emissions. Several technologies have arisen over the past years to compensate the low performance of the transcritical carbon dioxide vapour compression cycle at high ambient temperatures. For low–medium power units, the inclusion of a thermoelectric subcooler or an internal heat exchanger have been proven as effective solutions for enhancing the coefficient of performance. However, the combination of a thermoelectric subcooler and an internal heat exchanger working simultaneously is yet to be explored theoretically or experimentally. This work presents, for the first time, an experimental transcritical carbon dioxide refrigeration facility that works simultaneously with a thermoelectric subcooler and with an internal heat exchanger in order to boost the cooling capacity and coefficient of performance of the refrigeration system. The experimental tests report improvements at optimum working conditions of 22.4% in the coefficient of performance and an enhancement in the cooling capacity of 22.5%. The 22.4% increase in coefficient of performance would result in a decrease of energy consumption along a reduction of the greenhouse gases emissions. The proposed combination of a thermoelectric subcooler and an internal heat exchanger outperforms each of the technologies on their own and presents itself as a great controllable solution to boost the performance and reduce the greenhouse gasses emissions of transcritical carbon dioxide refrigeration cycles.

Energy and Exergy Analysis of Transcritical Carbon Dioxide Refrigeration Cycle

The effects of conventional refrigerants on ozone layer and its effect on global warming are known. Refrigerants based on chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFCs) have been used in refrigeration systems for a long time and are still being used. Instead of these refrigerants, the use of natural refrigerants which are much less harmful to the environment, has been given importance. Carbon dioxide is the first that comes to mind among these fluids. In this study, energy and exergy analysis of transcritical carbon dioxide refrigeration cycle examined as theoretically. Energy and exergy analysis were made for different evaporator temperatures and outlet temperatures from gas cooler. Exergy losses of each component of the transcritical carbon dioxide refrigeration cycle were determined. In addition, the first and second law efficiencies of the refrigeration cycle were determined. When the evaporator temperature is -5 oC and the outlet temperature from the gas cooler is 20 oC, the highest exergy efficiency value was determined as 41.19 in this study.

Proposal and thermoeconomic analysis of a novel combined cooling and power system using carbon dioxide as the working fluid

A novel combined cooling and power (CCP) system integrating a supercritical carbon dioxide recompression Brayton cycle with an ejector transcritical carbon dioxide refrigeration cycle (E-TCRC) is proposed to realize the effective utilization of nuclear power. In the proposed system, a portion of CO2 exiting the pre-cooler is used to drive the E-TCRC for generating cooling and recovering partial waste heat of sCO2 turbine exhaust. The mathematical model and economic model of the proposed system are established under steady-state conditions. Besides, the exergy efficiency and total product unit cost of the system are selected as the main criteria to evaluate system performance. Parametric analysis is applied to study the effects of four key parameters on the system performance. The CCP system, conventional separated cooling and power (C-SCP) system and ejector separated cooling and power (E-SCP) system are optimized by single-objective and multi-objective optimization. Single-objective optimization reveals that the exergy efficiencies of the CCP system are up to 1.08%pt (percentage point), 0.80%pt and 0.47%pt higher than those of the C-SCP system at the corresponding evaporation temperatures (−20 °C, −10 °C and 0 °C). Besides, the CCP system performs better than the E-SCP system at lower turbine inlet pressures. The multi-objective optimization shows that when the evaporation temperature increases from −20 °C to 0 °C, the total product unit cost of the CCP system decreases from 10.087 $/GJ to 9.668 $/GJ, and exergy efficiency increases from 59.25% to 60.97%.

Thermodynamic analysis on the combination of supercritical carbon dioxide power cycle and transcritical carbon dioxide refrigeration cycle for the waste heat recovery of shipboard

With excellent physical properties, carbon dioxide has been widely employed as a working fluid in efficient energy conversion technologies, which are represented by supercritical carbon dioxide Brayton cycle and transcritical carbon dioxide refrigeration cycle. In this contribution, with the aim to recover the waste heat of shipboard, a combined system coupling carbon dioxide Brayton cycle and refrigeration cycle is proposed to simultaneously produce power and cooling. In order to alleviate the temperature mismatch in recuperator and effectively utilize the discharged heat of refrigeration cycle, low temperature recuperator and gas cooler are shared by power and cooling cycles. For this novel waste heat recovery system, thermodynamic and economic models are developed to conduct energy, exergy and economic analysis. Thereafter, key cycle parameters including gas cooler pressure, evaporation temperature and turbine inlet temperature are investigated to reveal the effects on the system performances. The obtained results indicate that the energy and exergy efficiencies of the proposed system are respectively 42.42% and 39.05% under design conditions. The corresponding average energy cost is 9.28 $/GJ. At lower evaporation temperature and higher gas cooler pressure, the advantages of low temperature recuperator can be fully utilized and more work is produced. However, lower cooling capacity is obtained. Furthermore, the turbine inlet temperature has no effects on refrigeration cycle, and the net work decreases with the increase of inlet temperature. These results will be beneficial to improve the design and performance of combined power and cooling system for future shipboard applications.

Thermo-economic analysis and optimization of a novel carbon dioxide based combined cooling and power system

Supercritical carbon dioxide is widely used in power plant and refrigeration system as working fluid. In this paper, a novel combined cooling and power system consists of supercritical carbon dioxide recompression Brayton cycle and transcritical carbon dioxide refrigeration cycle is proposed. In the novel system, the working fluid through the recompressor in supercritical carbon dioxide recompression Brayton cycle is partially replaced by the refrigeration compressor outlet working fluid in transcritical carbon dioxide refrigeration cycle. Mathematical models are established to simulate the combined system under steady-state conditions and the simulated results show that the reactor dominates the exergy destruction followed by the cooler and high temperature recuperator. Effects of five parameters, including turbine discharge pressure, cooler outlet temperature, evaporation temperature, cooling capacity and mass flow fraction are evaluated. The performances of the proposed combined system and the separation system are optimized and then compared from the perspective of thermodynamics. The calculated results show that the combined system’s comparative advantage over separation system will increase with increasing cooling capacity and decreasing evaporation temperature. The exergy efficiency of combined system is up to 2.45% and 5.87% higher than separation system when evaporation temperature is 273.15 K and 253.15 K, respectively. In order to balance the contradiction between the investment and system performance, the multi-objective optimization is conducted with exergy efficiency (to be maximized) and annual cost per heat consumption (to be minimized) as objective functions at different evaporation temperature. Non-dominated sorting generic algorithm-II is employed and Pareto frontier solution for the proposed system is given. Suggestions for engineering practice are provided based on the distributions of some key parameters on Pareto frontier solution.

A novel hybrid dual-temperature absorption refrigeration system: Thermodynamic, economic, and environmental analysis

The applications of multi-evaporator and low temperature refrigeration systems are growing rapidly, highlighting the necessity of developing new configurations that can be economically and environmentally attractive. In this study, a hybrid dual-evaporator absorption refrigeration system for refrigerating and freezing applications is proposed and evaluated from thermodynamic, economic, and environmental viewpoints. In the proposed system, two compressors were employed between the evaporators and absorbers to increase the absorbers pressure to a higher level. The detailed parametric study results demonstrated that only employing a compressor between the higher temperature evaporator and absorber is beneficial improving the coefficient of performance (COP) significantly. However, the economic results showed that the compressor should be implemented carefully because in most design and operating conditions, the required capital and operating costs outweigh the COP improvement. Under specified design conditions, integrating the second compressor with a pressure ratio of 2.5 significantly increased the COP from 0.404 to 0.624 while it caused an increase in the unit production cost of cooling (UPCC) from $0.211/ton-hr to $0.228/ton-hr. The use of flue gas as a heat input was found to have excellent potential to improve the economic and environmental performance of the system. A detailed comparison with a dual-temperature transcritical carbon dioxide refrigeration cycle from the literature demonstrated that, under similar conditions, the system of this study has a substantially lower UPCC than the system in the literature ($0.211/ton-hr and $0.110/ton-hr for the steam and waste heat driven systems against $0.457/ton-hr). Overall, the proposed system (with and without the compressor) of this study showed an excellent potential for dual-temperature refrigeration applications.

Thermoeconomic performance and optimization of a novel cogeneration system using carbon dioxide as working fluid

A novel cogeneration power and refrigeration system using carbon dioxide as working fluid, is proposed, analyzed and optimized. The system is an internally interacting combination of a supercritical carbon dioxide recompression Brayton cycle and a transcritical carbon dioxide refrigeration cycle with an expander. The system proves to perform either as a cogeneration system producing power and refrigeration or only refrigeration system. The effects on system’s thermodynamic and economic performances are quantified of the main decision parameters. The systems performance is optimized for having either maximum first law efficiency or maximum second law efficiency or minimum total product unit cost, for both the cogeneration and refrigeration only situations. It is shown that, for the cogeneration situation, the minimum total product unit cost in cost optimal design case is 3.5% lower than that for the exergy efficiency optimal design cases. For the refrigeration only situation, it is observe that the minimum total product unit cost obtained for the cost optimal design case is 18.9% and 4.2% lower than those for the thermal and exergy efficiencies optimal design cases, respectively. The highest value of refrigeration capacity is obtained for the thermal optimal design case for the system when it produces refrigeration only. At the refrigeration only situation the lowest evaporator temperature is achieved when the system is optimized for maximum exergy efficiency.

Thermodynamic performance evaluation of transcritical carbon dioxide refrigeration cycle integrated with thermoelectric subcooler and expander

New configurations of transcritical CO2 refrigeration cycle combined with a thermoelectric (TE) subcooler and an expander (TES+EXPHM and TES+EXPML) are proposed. The expander can operate between the high-pressure to the vessel pressure, or from vessel pressure to evaporation pressure. A power system is utilized to balance and supply power to thermoelectric subcooler and compressor. Thermodynamic performance optimizations and analyses are presented. Comparisons are carried out with the BASE, EXPHM, EXPML, and TES cycles. The results show that the coefficient of performance (COP) improvement is more notable when the expander is installed between the liquid receiver and the evaporator. Maximum COP is obtained for the new cycles with a simultaneous optimization of discharge pressure and subcooling temperature. The new proposed TES+EXPML cycle shows an excellent and steady performance than other cycles. It operates not only with the highest COP, but also the lowest discharge pressure. Under the working conditions of high gas cooler outlet temperature or low evaporation temperature, the merits of COP improvement and discharge pressure reduction are more prominent. The new cycle is more suitable for the hot regions where the CO2 can not be sufficiently subcooled or the refrigerated space operates at low evaporation temperature.

Comparative Analysis of Typical Improvement Methods in Transcritical Carbon Dioxide Refrigeration Cycle

In this paper, five kinds of typical improvement methods for the performance of SC cycle are compared and analyzed using the developed thermodynamic model based on the first and second laws of thermodynamics. The results shows that SCE cycle has a most positive effect. The COP of SCSLHX cycle only has an increase of 15% in high temperature conditions, conversely TCFGB cycle can improve by 15% and 28% in common and low temperature conditions. TCIGc cycle has a middle but stable increase of 14%~21%, and margin improvement of TCIC cycle is less than 10% in research conditions. The exergy analysis reveals that SCE, SCSLHX, TCFGB and TCIGc cycles improve the COP mainly by reducing throttling losses, while the improvement of COP for TCIC cycle is mainly achieved by reducing the loss of gas coolers. Furthermore, the exergy loss of each component in each cycle is also quantified.

Performance improvement of double-tube gas cooler in CO2 refrigeration system using nanofluids

The theoretical analyses of the double-tube gas cooler in transcritical carbon dioxide refrigeration cycle have been performed to study the performance improvement of gas cooler as well as CO2 cycle using Al2O3, TiO2, CuO and Cu nanofluids as coolants. Effects of various operating parameters (nanofluid inlet temperature and mass flow rate, CO2 pressure and particle volume fraction) are studied as well. Use of nanofluid as coolant in double-tube gas cooler of CO2 cycle improves the gas cooler effectiveness, cooling capacity and COP without penalty of pumping power. The CO2 cycle yields best performance using Al2O3-H2O as a coolant in double-tube gas cooler followed by TiO2-H2O, CuO-H2O and Cu-H2O. The maximum cooling COP improvement of transcritical CO2 cycle for Al2O3-H2O is 25.4%, whereas that for TiO2-H2O is 23.8%, for CuO-H2O is 20.2% and for Cu-H2O is 16.2% for the given ranges of study. Study shows that the nanofluid may effectively use as coolant in double-tube gas cooler to improve the performance of transcritical CO2 refrigeration cycle.

Effect of an Internal Heat Exchanger on Performance of the Transcritical Carbon Dioxide Refrigeration Cycle with an Expander

The effect of the internal heat exchanger (IHE) on the performance of the transcritical carbon dioxide refrigeration cycle with an expander is analyzed theoretically on the basis of the first and second laws of thermodynamics. The possible parameters affecting system efficiency such as heat rejection pressure, gas cooler outlet temperature, evaporating temperature, expander isentropic efficiency and IHE effectiveness are investigated. It is found that the IHE addition in the carbon dioxide refrigeration cycle with an expander increases the specific cooling capacity and compression work, and decreases the optimum heat rejection pressure and the expander output power. An IHE addition does not always improve the system performance in the refrigeration cycle with an expander. The throttle valve cycle with IHE provides a 5.6% to 17% increase in maximum COP compared to that of the basic cycle. For the ideal expander cycle with IHE, the maximum COP is approximately 12.3% to 16.1% lower than the maximum COP of the cycle without IHE. Whether the energy efficiency of the cycle by IHE can be improved depends on the isentropic efficiency level of the expander. The use of IHE is only applicable in the cases of lower expander isentropic efficiencies or higher gas cooler exit temperatures for the refrigeration cycle with an expander from the view of energy efficiency.

Analysis of Combined Power and Refrigeration Generation Using the Carbon Dioxide Thermodynamic Cycle to Recover the Waste Heat of an Internal Combustion Engine

A novel thermodynamic system is proposed to recover the waste heat of an internal combustion engine (ICE) by integrating the transcritical carbon dioxide (CO2) refrigeration cycle with the supercritical CO2power cycle, and eight kinds of integration schemes are developed. The key parameters of the system are optimized through a genetic algorithm to achieve optimum matching with different variables and schemes, as well as the maximum net power output (Wnet). The results indicate that replacing a single-turbine scheme with a double-turbine scheme can significantly enhance the net power output (Wnet) and lower the inlet pressure of the power turbine (P4). With the same exhaust parameters of ICE, the maximumWnetof the double-turbines scheme is 40%–50% higher than that of the single-turbine scheme. Replacing a single-stage compression scheme with a double-stage compression scheme can also lower the value ofP4, while it could not always significantly enhance the value ofWnet. Except for the power consumption of air conditioning, the net power output of this thermodynamic system can reach up to 13%–35% of the engine power when it is used to recover the exhaust heat of internal combustion engines.

Identification of optimum inter-stage pressure for two-stage transcritical carbon dioxide refrigeration cycles

The disadvantage of high discharge temperatures associated with transcritical CO 2 refrigeration cycles can be overcome by adopting two-stage compression with inter-cooling between the stages. This also helps in improving the overall volumetric efficiency and some power saving. This paper considers the criteria of minimum work of compression, equal discharge temperatures at the exit of each stage and minimum exergy loss during inter-stage and gas cooling for identifying inter-stage pressure. It is shown that the equality of stage discharge temperatures is a more useful criterion than the saving in power or improvement in COP because multi-staging has at best a marginal (<10%) effect on the latter. The criterion of minimal exergy loss in heat rejection also yields the same inter-stage pressures as the minimum work criterion. Optimum inter-stage pressure for each condition is identified for several evaporator and gas cooler outlet temperatures and expressed in terms of the inter-stage pressure index of the product of cycle operating pressure limits.

Research and development of a semi-hermetic reciprocating compressor for transcritical CO2 refrigeration cycle

A semi-hermetic reciprocating compressor was designed and developed for the transcritical carbon dioxide (CO2) refrigeration cycle. In addition, a mathematical model was established to simulate the thermodynamic process in this transcritical CO2 compressor. The internal leakage, heat transfer, and flows through the suction/discharge valves together with the valve motion were taken into account in the model. In order to evaluate the overall performance of the developed compressor, a test rig was built to measure the volumetric efficiency, isentropic efficiency, and coefficient of performance (COP), and also recorded the pressure—volume diagram. The calculated indicated power deviated by 4.16 per cent from the measured one, and the maximum deviations of the isentropic and volumetric efficiencies were −2.86 per cent and 4.87 per cent, respectively. Both the calculated and measured data showed that the compressor had a cooling COP of 2.0—3.2. As the pressure ratio increased from 2.3 to 3.5, the volumetric and isentropic efficiencies ranged from 0.70 to 0.86 and from 0.62 to 0.66, respectively. The recorded p— V diagram indicated that the power losses associated with the suction and discharge processes accounted for 4.6 per cent and 6.8 per cent of the total indicated power, respectively. The validated model can be used to investigate the thermodynamic process of the reciprocating CO2 compressor with the aim of enhancing the compressor efficiency.