Optimum Gas Cooler Pressure Research Articles

Energy, exergy, environmental (3E) analyses and multi-objective optimization of vortex tube coupled with transcritical refrigeration cycle

The present study deals with the thermodynamic investigation of vortex tube coupled with trans-critical vapour compression refrigeration cycle (TVTC), followed by environmental analysis and multi-objective optimization. In this research, effect of various operating and design parameters is studied on the performance of TVTC. Furthermore, a comparison is made between the outcomes of TVTC and simple trans-critical vapour compression refrigeration cycle (TVCR). Results show that the optimum gascooler pressure for TVTC is observed to be lower than that of TVCR. Also, the cooling capacity and COP of TVTC are observed to be 10.1 % to 21.1 % and 2.3 % to 11.3 %, respectively, greater than those of TVCR. Moreover, the exergetic efficiency of TVTC is 2.3 % to 11.3 % higher than that of TVCR for the investigated range of evaporator and gascooler exit temperatures. The environmental penalty cost (per unit cooling capacity) of TVTC is 3.5 % to 12.2 % lower than that of TVCR. Furthermore, the coefficient of structural bond is calculated in order to choose the most sensitive parameters for system's performance. Additionally, genetic algorithm-based multi-objective optimization has been performed, with the evaporator temperature serving as the primary determining factor in establishing the optimal solution. This finding can guide the development of TVTC-based systems for a wide range of applications.

Parametric analysis of hybrid elastocaloric – CO2 cooling system

This study examines the potential application of elastocaloric refrigeration (eC) technology for the subcooling of CO2 in transcritical single-stage refrigeration cycles. Elastocaloric refrigeration, a solid-state refrigeration technology, possesses significant untapped potential due to its environmentally friendly characteristics, primarily its lack of harmful operational fluids. However, its direct stand-alone application has been limited due to the relatively small temperature spans produced by current proof-of-concept elastocaloric devices. Efforts of the scientific community have focused on extending the temperature difference that eC systems can provide, while this work offers an alternative to that challenge, offering an application solution for the current state of technology. The study proposes for the first-time a unique integration of eC technology and CO2 cooling systems, aiming to capitalise on the respective weaknesses of these technologies and transform them into strengths. In the proposed solution, an eC device operates as an external agent to subcool CO2, with the objective of enhancing the energy performance of the refrigeration system. This concept is motivated by the recent advancements in CO2 cooling systems and the growing recognition of subcooling as a promising method to boost the performance of such systems. The hybrid system’s performance was evaluated across various ambient temperatures, ranging from 20 °C to 35 °C, and at an evaporating level of −15 °C. It is evaluated by means of a calculation model based on the data obtained experimentally from an elastocaloric regenerator and an experimental CO2 plant. The system‘s energy efficiency was analysed in comparison to a non-subcooled CO2 cycle, and a third cycle comprising both eC subcooling and an expander for energy recovery from the expansion process. The results demonstrated a considerable increase in the coefficient of performance (COP) with the use of eC subcooling: increments of 2.7 % at 20 °C, 4.5 % at 25 °C, 9.6 % at 30 °C and 13.1 % at 35 °C, where more significant increments were observed at higher ambient temperatures. The eC Subcooler with expander reaches increments of 7.5 %, 11.1 %, 18.1 % and 22.2 % respectively. Additionally, the eC subcooler allowed a reduction in the optimum gas-cooler pressure by up to 5 bar at the highest environment temperature. Despite these promising results, the study underscores the necessity for further optimisation and improvement of elastocaloric devices. Additionally, it emphasizes the importance of energy recovery strategies from the expansion process in transcritical CO2 cooling system.

Numerical performance of a water source transcritical CO2 heat pump with mechanical subcooling

In order to improve their efficiency, transcritical CO2 heat pumps need to resort to the use of a subcooling method. Among the different subcooling methods, dedicated mechanical subcooling (DMS) systems and internal heat exchangers (IHX) are currently the more promising technologies. This paper presents a numerical study of a transcritical water source CO2 heat pump during hot water generation using different subcooling methods. Ten different configurations, including both, IHX and DMS, separately and combined in different layouts, some of them not studied previously, are analyzed numerically under the same operating conditions in order to compare their performance. A description of the numerical model is presented: compressors are modeled using the performance curves provided by their manufacturers, expansion valves are modeled as isenthalpic, and heat exchangers are modeled by deriving correlations for the evaporation/condensation pressure and heat transfer rate obtained using a 1D cell-by-cell discretization method previously applied to all heat exchangers. Results are presented for different water heating conditions and show that in most configurations analyzed, the use of a DMS does not improve the performance of the system compared to the base system with IHX. There is only an improvement in the efficiency for two of the configurations analyzed, those in which the main CO2 cycle and the DMS cycle are coupled by the water flowing first through the evaporator of the auxiliary cycle and then through the gas cooler of the main cycle. Specifically, compared to the base cycle with IHX, the configuration that provides the best results (Conf. F* according to the nomenclature used in this work) gives average improvements of around 26% in efficiency and almost 160% in the heating capacity, while the optimum gas cooler pressure is reduced by an average of 12%. Even more, compared to the best performance system previously studied by other authors (indirect DMS without IHX, Conf. F in this work) this configuration improves the efficiency by almost 8.5%, with a decrease in the total capacity lower than 1% and similar gas cooler pressure. The results also show that the auxiliary compressor capacity and the way in which the water is distributed among the main and the auxiliary cycle have an important influence on the efficiency of the system, although that influence depends on the configuration studied. For the configuration that provides the best efficiency (Conf. F*), the optimum efficiency is obtained when the auxiliary compressor capacity is similar to the capacity of the main compressor (55% of the total heating capacity comes from the auxiliary cycle), and the water is mostly heated in the auxiliary cycle (85% of the water flow heated in the condenser of the auxiliary cycle, 15% heated in the gas cooler of the main cycle).

Experimental determination of the optimum working conditions of a commercial transcritical CO2 refrigeration plant with a R-152a dedicated mechanical subcooling.

Abstract Transcritical CO2 plants combined with subcooling systems are the focus of several researches in the last years with the objective of improving their performance. Among the subcooling systems, the Dedicated Mechanical Subcooling system (DMS) is one of the most interesting because it greatly improves the overall COP and the cooling capacity of the system. This work presents the experimental study of a transcritical CO2 plant working with an R-152a DMS. The plant was tested at different pressure and subcooling conditions in order to determine the working conditions where the COP of the plant is maximum. The optimal operation conditions are determined for three ambient temperatures 25.0 °C, 30.4 °C and 35.1 °C and three cold sink temperatures (-1.3 °C, 3.8 °C and 10.0 °C). The measured values go from 6.5 kW to 7.3 kW for glycol inlet temperature -1.3 °C, from 7.6 kW to 8.4 kW for 3.8 °C and from 8.8 kW to 9.8 kW for 10.0 °C. Optimum COP goes from 1.51 to 1.95 for -1.3 °C, from 1.69 to 2.21 for 3.8 °C and from 1.86 to 2.52 for 10.0 °C. Optimum gas-cooler pressure has a higher dependence on the heat rejection level, being higher when higher the heat rejection level is, but it slightly depends on the evaporation level. Optimum subcooling degree is both dependent on the water inlet temperature and on the glycol inlet temperature. Two correlations are proposed to determine the optimal pressure and subcooling degree for the CO2 plants working with DMS as a function of the gas-cooler outlet temperature and the evaporation level.

Thermoeconomic and feasibility analysis of novel transcritical vapor compression-absorption integrated refrigeration system

In this paper, a novel structure of transcritical vapor compression-absorption integrated refrigeration system (TVCAIRS) is proposed based on its thermoeconomic viability. The integration of single effect vapor absorption refrigeration system (VARS) (with H2O-LiBr fluid couple) in transcritical vapor compression refrigeration system (TVCRS) (with R744 refrigerant) provides a subcooling of 5 °C in the proposed configuration. Hence, the overall COP and exergetic efficiency of TVCAIRS is 28.6% and 26.9%, respectively higher than the equivalent standalone TVCRS with optimum gas cooler pressure and generator temperature. Besides, the requirement of external cooling water for the gas cooler, absorber and condenser cooling of TVCAIRS is 12.3% less than the equivalent TVCRS. The direction for the further process improvement of TVCAIRS is also presented. Further, the thermodynamic performance of proposed system is also investigated for low temperature refrigeration application and hot climate conditions. The comparative results showed that the present TVCAIRS is thermodynamically more efficient as compared to the equivalent TVCRS and conserves more water resources with effective recovery of waste heat for the sustainable development.The techno-economic analysis of proposed TVCAIRS shows a shorter breakeven point and payback period of 1.3 yr. and 1.8 yr., respectively with total annualized cost of INR 5,641,614 (17.8% less than the equivalent TVCRS). Further, selection diagram for TVCAIRS and equivalent TVCRS shows that TVCAIRS is more feasible as compared to the TVCRS under low temperature and hot climate conditions and its viability strongly increases, if financial incentives of reduced interest rate are offered with longer life designs and operation duty.

Energetic and exergetic studies of modified CO2 transcritical refrigeration cycles

Abstract Thermodynamic analysis including energetic and exergetic analysis is carried out employing Engineering Equation Solver for the five modified cycles: dual expansion cycle, internal heat exchanger cycle, work recovery cycle, work recovery with internal heat exchanger cycle and vortex tube expansion cycle. Contours are developed to study the effect of gas cooler temperatures and evaporator temperatures on the system performance and optimum gas cooler pressure. The modified cycle with work recovery turbine offers relatively higher COP and higher exergetic efficiency with lower compressor discharge pressure. The exergy loss in compressor, gas cooler, throttle valve and vortex tube (VT) are considerably higher than that in internal heat exchanger (IHX), evaporator and turbine. It is observed that COP of modified cycle with VT is slightly less than that with IHX, whereas the cycle with work recovery turbine brings the highest COP with the improvement of 25% at the gas cooler exit temperature of 305 K and evaporator temperature of 248 K.

Thermodynamic Analysis of a CO2 Refrigeration Cycle with Integrated Mechanical Subcooling

Different alternatives are being studied nowadays in order to enhance the behavior of transcritical CO2 refrigeration plants. Among the most studied options, subcooling is one of the most analyzed methods in the last years, increasing cooling capacity and Coefficient Of Performance (COP), especially at high hot sink temperatures. A new cycle, called integrated mechanical subcooling cycle, has been developed, as a total-CO2 solution, to provide the subcooling in CO2 transcritical refrigeration cycles. It corresponds to a promising solution from the point of view of energy efficiency. The purpose of this work is to present, for the first time, thermodynamic analysis of a CO2 refrigeration cycle with integrated mechanical subcooling cycle from first and second law approaches. Using simplified models of the components, the optimum operating conditions, optimum gas-cooler pressure, and subcooling degree are determined in order to obtain the maximum COP. The main energy parameters of the system were analyzed for different evaporation levels and heat rejection temperatures. The exergy destruction was analyzed for each component, identifying the elements of the system that introduce more irreversibilities. It has been concluded that the new cycle could offer COP improvements from 11.7% to 15.9% in relation to single-stage cycles with internal heat exchanger (IHX) at 35 °C ambient temperature.

Performance characteristics of a two-stage transcritical N2O refrigeration cycle with vortex tube

ABSTRACTThe thermodynamic analysis has been presented in this article using nitrous oxide as the refrigerant in a two-stage transcritical cycle with the vortex tube (TSTCVT) instead of the expansion valve and its results are compared with the two-stage transcritical cycle with the expansion valve (TSTCEV). The evaporator and the gas cooler temperature ranges in both the cycles have been considered between −55°C to 5°C and 35°C to 60°C for the analysis. Gas cooler and intercooler pressures are simultaneously optimised to obtain the maximum cooling coefficient of performance (COP). The COP of the TSTCVT improves by 1.97–27.19% in comparison to TSTCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature reduce the COP of TSTCVT. The comparison of refrigerants N2O and CO2 in TSTCVT shows that N2O exhibits higher cooling COP, higher second law of efficiency and lower optimum gas cooler pressure under the considered operating conditions.

Thermodynamic analysis of a novel ejector-cascade refrigeration cycles for freezing process applications and air-conditioning

Waste heat from the gas cooler is a form of free energy, which can be utilized to drive an ejector cooling cycle. This paper presents a new CO2 ejector-cascade refrigeration cycle. The effects of important parameters on the thermodynamic performance of the new cycle are theoretically investigated based on energetic and exergetic analyses. Furthermore, the performance comparison of the proposed cycle and conventional cycle is carried out. The theoretical study shows that the new cycle exhibits a reasonable value of COP (coefficient of performance) and system second law efficiency. For the same cooling capacity, the improvements of the maximum COP and second law efficiency could reach 37% and 12%, respectively, over those of the conventional cascade cycle under the given operating conditions and at the optimum gas cooler pressure.

Thermodynamic analysis of double-compression flash intercooling transcritical CO2 refrigeration cycle

The performance of double-compression flash intercooling (DCFI) transcritical CO2 refrigeration cycle is evaluated by thermodynamic analysis in comparison with the double-compression external intercooling (DCEI) cycles. The effect of expander isentropic efficiency and position on the performance of DCFI cycle is investigated. Six double-compression refrigeration cycles are studied in the paper. The results show that the DCFI cycle performs better than DCEI cycle when the throttle valve is used as the expansion device. The replacement of the throttle valve with the expander yields less coefficient of performance (COP) improvement for DCFI cycle compared with DCEI cycle, especially at low evaporating temperatures or high gas cooler outlet temperatures. The COP of the two cycles is nearly equivalent when the isentropic efficiency of the expansion processes reaches 1.0. The DCFI cycle with only one throttle valve replaced with an expander shows lower COP compared with the corresponding DCEI expander cycle. The replacement of the main throttle valve with an expander results in higher COP compared with the auxiliary throttle valve replacement in the DCFI cycle. The substitution throttle valve for expander in the DCFI cycle is not economical compared with DCEI cycle. Furthermore, the optimum gas cooler pressure, the optimum intermediate pressure and the mass flow rate ratio are also discussed in the paper.

Experimental Investigation of a Combined Power Refrigeration Transcritical CO2 Cycle

Optimization studies of the CO2 transcritical combined power refrigeration cycle are presented here. Steady state thermodynamic analysis including energy is carried out at various operating conditions employing a precision in-house property code CO2 PROP. CO2 transcritical combined power refrigeration cycle exhibits better performance than that of CO2 transcritical refrigeration cycle alone. However, optimum gas cooler pressure does not change with the type of the system; it remains same for both the system; refrigeration and combined cycle. However, system performance is significantly affected by the evaporator pressure and compressor isentropic efficiency unlike the CO2 transcritical refrigeration system Combined cycle exhibits significant improvement in COP at relatively higher evaporative temperature. However, at lower evaporator temperature cooling cycle shows better performance.

Optimal control of the gas-cooler pressure of a CO2 heat pump using EEV opening and outdoor fan speed in the cooling mode

The objective of this study is to investigate the control methods of the gas-cooler pressure in a CO2 heat pump. The cooling performance of the CO2 heat pump was measured by varying the refrigerant charge amount, EEV opening, compressor frequency, and outdoor fan speed at various outdoor temperatures. The effects of the EEV opening and the outdoor fan speed on the gas-cooler pressure and the COP were analyzed by using the experimental data. In the standard cooling condition at the compressor frequency of 45 Hz, the optimum gas-cooler pressure and the maximum COP were 9200 kPa and 3.04, respectively, at the optimum EEV opening of 41% and outdoor fan speed of 500 rpm. As the compressor frequency increased from 45 Hz to 55 Hz at the standard cooling condition, the optimum outdoor fan speed increased from 500 rpm to 700 rpm.