Optimal Discharge Pressure Research Articles

Performance analysis and thermal comfort evaluation of a CO2 transcritical air-conditioning system for cabin thermal management of electric vehicles

This study investigates the effects of different operating conditions on system performance and optimal discharge pressure by establishing a numerical model of CO2 cabin thermal management system and validating the model with experimental data. Besides, two different optimal discharge pressure formulas are attained based on different parameters. It is found that the multi-parameter formula is more accurate than the other with a maximum deviation of 2.64%. Then, the control strategy is designed based on this formula to investigate the coupled effects of thermal comfort and economical performance under different driving patterns and different ambient temperatures. The elevation of PMV indicator can weaken the declining rate of power and energy consumption, enhance the driving range and reduce annual operation cost in cooling mode (AOCc) of vehicle. The boundary of thermal comfort zone (PMV = 0.5) exhibits excellent economical efficiency due to its higher growth rate of driving range (6.3%) and reduction rate of AOCc (7.9%) from 0 to 0.5 PMV at the ambient temperature of 35 °C. And this improvement is more prominent under higher ambient temperature.

Thermodynamic analysis of a modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler

In the application scope of large-scale supermarkets, the practicality of transcritical CO2 refrigeration device has been confirmed to be quite satisfying. A modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler (MTC) is proposed in this paper. In the modified cycle, the ejector recovers expansion works and reduces irreversible losses in the throttling process. The subcooler provides subcooling degree for the CO2 entering the low-temperature (LT) evaporator, thus increasing the refrigeration capacity and improving the COP of the modified cycle. Thermodynamic analysis has shown that the exergy efficiency (ηex) and COP of MTC under given operating condition has been enhanced by 13.7 % and 14.2 % compared to BTC. The discharge temperature at the outlet of high-pressure compressor in MTC is decreased by 8.3℃. Under typical operating condition, the optimal discharge pressure of MTC is 9.07 MPa, which is lower than that of BTC. The correlation to calculate optimal discharge pressure for single-stage CO2 refrigeration cycle is also suitable for MTC under given operating conditions. The MTC has also shown better performance under variable operating conditions. For MTC, when the temperature at the outlet of gas cooler increases from 35 – 45℃, the COP and ηex are enhanced by 14.1 % - 16.1 % and 12.8 % - 14.2 % compared to BTC, respectively. When gas cooler outlet pressure decreases from 11.0 to 7.5 MPa, the COP and ηex are enhanced by 14.0 % - 22.8 % and 13.4 % - 18.7 %. As the evaporating temperature at the cold side of subcooler increases from -25 to -15℃, the COP and ηex increase from 1.82 to 1.98 and 27.4 % to 29.7 %. With the ratio of refrigeration capacity (Rec) between medium-temperature evaporator and low-temperature evaporator varies from 0.7 to 1.2, the COP and ηex are improved by 10.7 % - 16.3 % and 10.7 % - 15.4 % compared to BTC. There is the maximum exergy loss at gas cooler in the MTC, whereas that of BTC is located in the expansion valve before LT evaporator. The economic analysis shows the cost per unit of exergy of MTC is decreased by 11.5 % under typical operation condition. According to simulation results, the modified cycle has better performance in severe working conditions such as high gas cooler outlet temperature and low gas cooler outlet pressure in the given range of working conditions compared to BTC.

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

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

Experimental study and exergy analysis of electric vehicle single-stage compression secondary throttling CO2 air-source heat pump system in cold climate

A novel CO2 air-source heat pump system (ASHPs) with a secondary throttling based on a single-stage compression was proposed to further improve the heating energy efficiency of electric vehicles (EVs) for heating in cold climates. The heating characteristics of the secondary throttling system (STS) and conventional throttling system (CTS) at different indoor air supply temperatures were compared. The STS performance characteristics were investigated under varying ambient temperature, outdoor air velocity, and exergy analysis was performed on the system and its components. A fitting correlation equation for optimal discharge pressure was obtained based on experimental data. The results demonstrate that implementing STS significantly reduces CO2 ASHPs energy consumption (Wcom), enhances the coefficient of performance (COP), and reduces exergy destruction. When the indoor air supply temperature reaches 45 °C, using STS can result in an 8.1 % reduction in Wcom, a 9.4 % increase in COP, and a 6.1 % improvement in the exergy efficiency (Exη) compared to that of CTS. Furthermore, the advantages of STS become more pronounced with higher indoor target air supply temperatures. As the ambient temperature decreases, the exergy efficiency of the STS reaching the target air supply temperature decreases. The velocity of outdoor air significantly affects the STS performance, and increasing outdoor air velocity can effectively enhance the Exη of STS. The optimal discharge pressure fitting equation for the STS was obtained through multiple linear regression analysis. These findings can provide ideas for architectural optimization and design of CO2 ASHPs for EVs, which are important for further improving system performance.

Optimal discharge pressure and performance characteristics of a transcritical CO2 heat pump system with a tri-partite gas cooler for combined space and water heating

The transcritical CO2 heat pump holds promise for achieving carbon neutrality in buildings, primarily due to its high energy efficiency in hot water heating. However, using it for space heating often leads to efficiency losses. A heat pump equipped with a tri-partite gas cooler can address this issue by providing both space heating and hot water while maintaining lower refrigerant temperatures at the gas cooler exit. Nonetheless, there is limited research on the characteristics of a CO2 heat pump system designed for simultaneous space and hot water production. This work fills this gap by analyzing the impacts of operational and system configuration, considering the variation of space heating to hot water production loads. A theoretical model is presented and verified with the experiments. Results show that under the specified conditions, the heat load ratio has minimal influence on the optimal discharge pressure. Increasing the water outlet temperature from 60 °C to 80 °C leads to a nearly 14% decrease in maximum COP, with a corresponding 12.2% increase in the optimal pressure. A 10 K rise in water inlet temperature results in approximately a 21% decline in maximum COP. Evaporation temperature has little impact on the optimal discharge pressure, while maximum COP increases by roughly 63% with a rise in evaporation temperature from -10 °C to 10 °C. Space heating water temperature notably affects the optimal discharge pressure. Integration of an IHX enhances system COP but has marginal impact on the optimal discharge pressure. These findings offer insights for designing and optimizing transcritical CO2 heat pumps for simultaneous space and water heating applications.

Cooling performance and optimization of a thermal management system based on CO2 heat pump for electric vehicles

Efficient integrated thermal management scheme is crucial for improving the driving range, thermal comfort, and safety of electric vehicles. Compared with the synthetic refrigerant heat pump indirect cooling/heating on the vehicle electrical system, the natural refrigerant CO2 heat pump direct thermal controlling integrated into the thermal management system has advantages in the thermal cycle performance and environmental friendliness. The present study proposed a thermal management system based on CO2 heat hump with multiple working modes for enhancing vehicle energy utilization coefficient. The system thermal performance under the World Light Vehicle Test Cycle was investigated via energy and exergy analyses. The operational control strategies of the thermal management system were optimized for the thermal controls of the cabin, battery and motor. The results show that the minimum exergy destruction of the thermal system occurs at the optimal discharge pressure corresponding to the highest coefficient of performance. Ensuring two-phase latent heat transfer for battery cooling is the most effective measure to maintain battery temperature uniformity via dynamic collaborative regulation of expansion valves’ openings and compressor speed. Compared with constant expansion valve’s opening, the coefficient of performance of the system can be improved by a maximum of 10% with the saturated CO2 vapor at the outlet of battery cooling plate. Due to the low thermal capacitance and minimal thermal inertia of driving motor, its cooling performance is significantly influenced by the heat generation rate and refrigerant flow. Optimal motor cooling efficiency is achieved when controlling the refrigerant quality to 0.85 at the cooling passage outlet. Under the World Light Vehicle Test Cycle, the thermal management system effectively regulated the temperatures across different operating modes while operating at optimal discharge pressures.

Study on the mechanism of the optimal discharge pressure for a transcritical CO2 system

The optimal discharge pressure is the essential factor affecting the operating performance of the transcritical CO2 system. This paper presents a detailed study on the mechanism of the optimal discharge pressure from the perspective of fixed gas cooler outlet temperature and the heat transfer pinch point. The fixed gas cooler outlet temperature mainly analyzes the mechanism of the optimal discharge pressure from the enthalpy difference of isotherm in the supercritical region. The heat transfer pinch point mainly analyses the optimal discharge pressure at low inlet water temperatures, in which the subcritical region close to the critical point is considered. The effect of different parameters on optimal discharge pressure and system performance is discussed. The results indicate that the unique CO2 physical properties and the pinch point in the gas cooler cause the optimal discharge pressure of the transcritical CO2 system. The physical properties mainly affect the optimal discharge pressure at the high inlet water temperature, while the pinch point mainly affects the optimal discharge pressure at the low inlet water temperature. The inlet and outlet water temperatures are the most significant factors affecting the system's performance.

Energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump for simultaneous domestic hot water and space heating

The transcritical CO2 heat pump system experiences significant throttling losses in space heating operation due to the higher temperature of returning water. This paper presents an energy, exergy, and exergoeconomic analyses of an air source transcritical CO2 heat pump integrating a tri-partite gas cooler, which is an effective method to match CO2 temperature glide with water for simultaneous domestic hot water and space heating (DHW+SH) production. A pinch point-based numerical model is developed to find the optimal discharge pressure. This validated model is then utilized to investigate the impacts of water inlet temperature and ambient temperature. Two configurations are examined: one with only DHW supply and the other with DHW+SH supply. The results indicate that combining SH with DHW enhances COP by 7.5% with a 7.9% reduction in discharge pressure at 10 °C ambient temperature and 10 °C water inlet temperature. At a water temperature of 10 °C, exergy efficiency improves by 4%. The compressor accounts for 54%–60% of the total exergy loss. The DHW+SH system exhibits an average exergy destruction reduction of 7.6%. With each 5 °C rise in ambient temperature, the DHW+SH system’s total exergy destruction cost rate decreases on average 7.7% compared to the DHW system. Furthermore, the combined exergy destruction cost rate of GC2 and GC3 in DHW+SH is significantly lower (by 48.2%) than only GC3 in DHW. The exergoeconomic factors of the compressor, gas cooler 2, and gas cooler 3 emphasize the need to decrease their costs to enhance cost-effectiveness.

Improving the long-term performance of electric vehicles CO2 heat pump through correcting discharge pressure

Electric vehicle CO2 heat pumps are an environmentally friendly and energy-efficient solution, especially in cold regions, where they show exceptional heating capabilities and attract widespread attention. The optimal discharge pressure has consistently been the focus of research for enhancing energy efficiency in CO2 heat pump systems. The impact of declining heat transfer efficiency during the long-term operation on the optimal discharge pressure has yet to be thoroughly investigated. Establishing a simulation model for electric vehicles CO2 heat pump. Considering how performance degradation on both sides of the heat exchanger affects the optimal discharge pressure over the lifecycle. Proposing a mathematical correction method to adjust the initial discharge pressure and realize COP loss recovery from a control strategy perspective. The results demonstrate a gradual growth in the optimal discharge pressure, leading to a 15% additional COP loss after 10 years. The extent of heat transfer efficiency deterioration played a crucial role in determining the optimal discharge pressure deviation rate. Correcting the discharge pressure can minimize COP loss from a control strategy standpoint. Theoretically, under the specified test conditions, COP loss can be maximally recovered by approximately 2.48% (within 4 years) and 25.48% (within 10 years). This study contributes to the sustained reduction of electricity consumption in CO2 heat pump systems, thereby prolonging the energy-saving advantages of electric vehicles.

Theoretical investigation of a novel distributed compression cycle for CO2 trans-critical system

Efficient cooling of the trans-critical CO2 vapor compression cycle to improve system performance is a crucial research topic. This paper proposes an innovative approach called distribution compression (DC) to achieve the same effect as subcooling in a trans-critical CO2 thermodynamic cycle. In the DC system, the trans-critical CO2 at the gas cooler outlet is no longer further cooled but subjected to a secondary pressure boost and releases heat in ordinary heat sink conditions as in the gas cooler. Thermodynamics calculated the change in performance of the DC system under different working conditions with different secondary boost ratios. The optimal secondary boost ratio related to evaporation and gas cooler outlet temperatures under optimal discharge pressure based on the baseline system was obtained. The results show that compared with the baseline system, the DC system can effectively improve the system performance, and the maximum refrigeration COP increase is between 8.2 % ∼ 10.76 %, and the maximum heating COP increase is between 5.37 % ∼ 6.34 %. The cooling or heating capacity increase can reach up to about 26 %. The ideal secondary boost ratio in a DC system is not highly demanding, and the increased input power for the second input power for the secondary boost will not exceed 20 % relative to the baseline system. Comparing the COP of the DC system over current systems that only use a single subcooling technology still shows advantages. The DC system provides a new idea to improve the performance of the trans-critical CO2 vapor compression cycle.

Real-time multi-variable optimization of the interstage subcooling vapor-injection based transcritical CO2 HPWH: An integrated model predictive control approach

The interstage subcooling vapor-injection technique has been introduced to transcritical CO2 heat pumps to alleviate performance degradation under conditions of low ambient temperature and high water-outlet temperature. This type of solution holds considerable appeal and finds utility across a wide range of applications. However, the efficacy and efficiency of such systems are contingent upon their management, which frequently requires the implementation of suitable control systems. The primary objective is to enhance the overall system operational performance, resulting in tangible advantages in terms of economic viability, energy conservation, and environmental sustainability. To achieve these objectives, one can utilize both explicit and implicit optimization methods ranging from Model Predictive Control (MPC) to Extremum Seeking Control (ESC), each with its strengths and weaknesses. This paper compares the performance of an ad hoc integrated MPC strategy and a multi-variable ESC one. Based on the results of the system operation employing the latter strategy, a significant linear correlation was discovered between the optimal medium pressure and the optimal discharge and suction pressure. Consequently, an intrinsic self-optimization strategy for the medium pressure was developed. Subsequently, a dynamic system identification was performed on the new system, incorporating this proposed strategy. To optimize the system Coefficient of Performance (COP) while maintaining the water outlet temperature, ensuring the thermal load constraint, the MPC strategy was employed. The main focus of the MPC was to determine the optimal Electronic Expansion Valve (EEV) opening degree and water pump settings. The integrated MPC strategy was evaluated and compared to the multi-variable ESC strategy under various conditions, including fixed design conditions, changing ambient conditions, and step-change water outlet temperature. The results clearly demonstrated the effectiveness and superiority of the integrated MPC.

Driving range evaluation of electric vehicle with transcritical CO2 thermal management system under different battery temperature controls

Improper battery temperature will lead to reduced battery discharge efficiency and electric vehicle driving range. Endeavors to find an efficient and precise battery temperature control method for the transcritical CO2 thermal management system of electric vehicles, two evaporation temperature control methods for battery cooling were proposed. First, the effects of the control methods on battery temperature, optimal discharge pressure and coefficient of performance (COP) were simulated under typical operating conditions. Subsequently, a comparative simulation was conducted on the battery temperatures and COP of the two control methods under full cooling conditions. The results revealed that equal evaporation temperature control induced a reduction in battery temperature to 11 °C (suboptimal levels) under conditions of low ambient temperature and vehicle speed. Conversely, while unequal evaporation temperature control could maintain the battery temperature within the optimal range of 20–45 °C, it would result in a maximum COP drop of more than 20% under certain conditions. Furthermore, the effects of the two control methods on driving range under different ambient temperatures and vehicle speeds were studied. The reasults indicated that the ultimate influence of uneuqal evaporation temperature control on the quarterly average driving range was virtually negligible, peaking at 1.35%. Given the dual-objective constraints pertaining to the driving range and the battery cooling performance, it was concluded that uneuqal evaporation temperature control was the optimal choice for direct battery cooling.

Performance evaluation of transcritical CO2 desiccant heat pumps for electric vehicles

Transcritical CO2 heat pump systems are eco-friendly and have excellent heating performance, which makes them suitable for electric vehicles. The recirculation mode can save energy but requires a dehumidification function to ensure driving safety. Therefore, this paper proposes a transcritical CO2 desiccant heat pump, which differs from the typical transcritical CO2 heat pump system by incorporating an indoor evaporator and a full-pass throttle valve. The flow area of the full-pass throttle valve can be adjusted to obtain the appropriate dehumidification rate. In addition, this study suggested using the desiccant heat pump coefficient of performance (DHCOP) to evaluate the overall energy efficiency of the heat pump system. We have investigated the dehumidification rate and optimal discharge pressure under different operating conditions. The results show that even under the harsh operating conditions at -10 °C, the HDCOP of the transcritical CO2 desiccant heat pump can still exceed 2.1, and reach 3.12 at a moderate condition of Ta=5 °C. The transcritical CO2 heat pump desiccant system demonstrates significant energy-saving potential.

Pseudo-optimal discharge pressure analysis of different CO2 electric vehicle heat pumps based on cabin thermal model

The discharge pressure of electric vehicle heat pumps, aiming for peak COP values, is termed the pseudo-optimal discharge pressure for dynamic regulation, distinguishing it from the optimal discharge pressure at a constant compressor speed. A theoretical model was presented to analyze the impacts of operational parameters considering the variation of cabin loads with operating conditions. Based on simulation results, the effects of ambient temperature, cabin temperature, drive speed, and supply air flow rate on the pseudo-optimal discharge pressure were investigated for single-stage and two-stage compression systems, respectively. Among them, the ambient temperature had the highest degree of influence, more than 45 % in different systems. Heat recovery in electrical systems introduced slight variations, less than 1 bar, to the pseudo-optimal discharge pressure by affecting the evaporation temperature. While the trend remains consistent across the different systems, the values of pseudo-optimal discharge pressure are higher for the two-stage compression. At −10/20 °C, two-stage compression provides a 49.42 % higher COP compared to single-stage while maintaining a safe discharge temperature. In addition, the heating efficiency under the improved prediction can be increased by 18.49 % compared to the existing research. Predictions for the pseudo-optimal discharge pressure provide a basis for effective and cleaner control of electric vehicle heat pumps.

A novel control method for the automotive CO2 heat pumps under inappropriate refrigerant charge conditions

CO2 heat pumps are increasingly widely used in automotive air conditioning, and the demand for refrigerant charge varies greatly under different modes and operation conditions. However, due to the limited space in vehicles, the accumulator cannot fully meet the refrigerant balance requirements for the variable operating conditions, and states of inappropriate refrigerant charge were frequently observed. To enhance system performance and safety under inappropriate charge conditions, an experimental setup and a mathematical model with a maximum error of 6 % were established to investigate the operating characteristics under inappropriate charge conditions. An investigation into the interplay among discharge pressure, the appropriate refrigerant charge range, and refrigerant distribution was conducted. It elucidated how refrigerant charging states (undercharged, adequately charged, or overcharged) manifest in terms of refrigerant distribution and optimal discharge pressure. A novel control method based on refrigerant distribution regulation has been proposed. The proposed control logic could increase the system IPLV by 36.8 % when the refrigerant charge deviates from the appropriate range by 12.12 %, and the greater the dysregulation in the refrigerant charge, the greater the improvement. This control logic would be more enhanced if dysregulation was considered.

Energetic analysis and performance improvement algorithm of transcritical CO2 heat pump water heater system

Transcritical CO2 heat pump water heaters have been widely studied by researchers for its environmentally friendly nature and energy efficiency. To analyze the impact of various operating parameters on the heating performance of transcritical CO2 heat pump water heater systems, simulation models were developed and their accuracy was verified through experiments. Results indicated that the discharge pressure and CO2 temperature at the gas cooler outlet (Tgc,out) had a significant effect on the system performance, the coefficient of performance (COP) increased notably with higher evaporation temperatures, and the optimal discharge pressure varied depending on the evaporation temperature. The system equipped with intermediate heat exchanger demonstrated a lower optimal discharge pressure and a higher COP compared to the basic system. The inclusion of an intermediate heat exchanger in a system, the maximum COP is 2.99 % to 3.83 % higher than the basic system under the same conditions. It has been observed that the introduction of intermediate heat exchanger significantly improved the system performance when Tgc,out were 35 °C, 40 °C and 45 °C, corresponding to discharge pressures lower than 10.75 MPa, 11.167 MPa and 11.625 MPa. According to the research results, the optimal discharge pressure equation was fitted and an improved algorithm was proposed. Compared to the experimental values, the improved algorithm exhibits a maximum error of 8.81 % and 7.55 % for determining the optimal discharge pressure and temperature, and the improved algorithm is effective in engineering applications. Consequently, the simulation data holds great importance in guiding the design of transcritical CO2 heat pump water heater systems.

Modeling and investigation of the performance of a solar-assisted ground-coupled CO2 heat pump for space and water heating

The rise in the popularity of heat pumps should be accompanied by the increase in the utilization of environmentally-safe working fluids. To push for wider uptake of hybrid heat pump systems that use natural working fluids, information about their performance and operating characteristics should be made available. This study models a CO2 solar-assisted ground-coupled heat pump (SAGCHP) system and investigates its performance for space and water heating. Sensitivity analysis, parametric study, and long-term performance simulation were implemented, with the system’s seasonal performance factor (SPF), levelized cost of heating (LCOH), and ground temperature change (GTC) considered as performance indicators. It was seen that ensuring a good combination of design and operating specifications can result in an SPF (∼3.5) and LCOH (0.184 USD/kWh) that are comparable with those of SAGCHP systems that use conventional working fluids. The heat pump’s high-side pressure and output temperature exhibited notable effects on all the performance indicators. Below the optimal operating pressure, a 5% increase in the heat pump’s high-side pressure brought about a ∼7–10% improvement to the SPF, a ∼3–5% reduction to the LCOH, and a ∼6–10% increase to the GTC. Reducing the output temperature by 5% increased the SPF by ∼7–8%, decreased the LCOH by ∼3%, and increased the GTC by ∼6%. Parametric studies identified the presence of optimal heat pump discharge pressure and heat source circulation rate that should be used for operations. Long-term simulation shows that managing the GTC ensures the longevity of the system. Rather than oversizing the borehole heat exchangers (BHEs), it is more practical to reduce ground temperature decline by increasing the BHE spacing or by adding solar collectors.

Data-driven model predictive control of transcritical CO2 systems for cabin thermal management in cooling mode

The transcritical CO2 cabin thermal management system has gained significant attention in the field of electric vehicles due to its outstanding heating performance and environmental advantages. However, ensuring its optimal operation in real-time during vehicle operation poses a challenge. Amongst these challenges, controlling the optimal discharge pressure is particularly difficult. In this paper, we propose a novel model predictive controller that focuses on the cabin cooling mode. The controller utilizes a high-fidelity data-driven dynamic model of the transcritical CO2 system, coupled with a dynamic thermal model of the cabin. By simultaneously controlling the compressor, electronic expansion valve, and indoor fan, the proposed controller enables the cabin thermal management system to operate in real-time at the optimal discharge pressure while ensuring passenger comfort, thereby minimizing the total power consumption of the system. Additionally, two model predictive control strategies, focused on comfort and energy-saving, respectively, are introduced. Through simulations under various conditions over a 6-hour period, comparing the PI controller, the comfort priority model predictive controller reduces energy consumption by 13.33%, and the energy-saving priority model predictive controller achieves a 20.27% reduction. The proposed novel model predictive controller exhibits energy-saving advantages.

Applicability evaluation of internal heat exchanger in CO2 transcritical cycle considering compressor operation boundaries

CO2 transcritical cycle has been widely promoted as a green energy utilization technology. Internal heat exchanger (IHX) is under consideration for cycle efficiency improvement. However, the interaction between IHX and compressor in CO2 cycle has not yet been fully explored. In this regard, compressor operating boundaries are introduced as constraints to IHX applicability evaluation in CO2 cycle. It indicates that considering compressor limits, IHX can either benefit or attenuate the CO2 cycle performance, depending on operating conditions. By reducing optimal discharge pressure, IHX helps CO2 cycle avoid the upper pressure limit, further improving performance by 5%. Over a wide range, however, IHX increases compressor discharge temperature, indirectly resulting in up to 50% performance degradation. The major factors affecting the duality of IHX in CO2 cycle are compressor discharge temperature limit, IHX efficiency and motor heat loss. Finally, performance-oriented IHX design guidelines for mainstream single-stage CO2 applications are provided. For automotive air conditioner, optimum IHX efficiency improves cycle performance by up to 30% in cooling mode. By contrast, IHX contribution is negligible in heating mode. For heat pump water heater and refrigeration system, both can benefit from IHX, particularly when compressor discharge temperature tolerance is high.

OPTIMIZATION OF TECHNICAL AND ECONOMIC PARAMETERS FOR THE NATURAL GAS LIQUEFACTION PROCESS AT A LOW-TONNAGE UNIT WITH A MIXED REFRIGERANT

Liquefaction technologies of natural gas using a mixed refrigerant cycle provide a high liquefaction coefficient at low specific energy consumption, but are characterized by high requirements for the organization of production. There is no methodology for simultaneous optimization of the technical and economic parameters for the production process of liquefied natural gas and the composition of the refrigerant in publications, and therefore a combined optimization criterion is needed to achieve the minimum cost of liquefied natural gas. The aim of the work is to determine optimal pressure for natural gas inlet compression, taking into account the minimization of the complex criterion – the conditionally variable part of the cost of liquefied natural gas. The results were obtained using an applied calculation program, and the method of differential evolution was chosen as an optimization method. The optimization was carried out using the OCMR PEGAZ 1.0 program. The article considers the influence of the discharge pressure of the low-pressure natural gas inlet compressor on the technical characteristics of heat exchange equipment, the energy efficiency of the liquefaction process and the economic characteristics of a low-tonnage liquefied natural gas production plant under conditions of optimized composition and flow rate of mixed refrigerant. A method of optimizing process parameters taking into account economic indicators is proposed, regularities of changes in the optimal composition of mixed refrigerant depending on the discharge pressure of the natural gas inlet compressor are determined. It is determined that with an increase in the discharge pressure of the natural gas inlet compressor, the optimal content of components in the mixed refrigerant changes as follows: methane and n-pentane decrease, and ethane and isobutane increase. It is shown that the optimal discharge pressure range of the natural gas inlet compressor is 6,0±0,5 MPag, which leads to the best total energy efficiency of inlet compression and circulation of mixed refrigerant, equal to 0,25 kWh/kg of LNG, and its cost is minimal. The assessment of capital costs at this pressure will allow choosing the optimal technology of natural gas liquefaction at the FEED stage, which will reduce the cost of production of liquefied natural gas and reduce the payback period to 5 years.