Transcritical Operation Research Articles

High-fidelity model development of CO2 booster refrigeration systems in supermarkets using field measurements

Supermarket refrigeration systems adopting traditional refrigerants with high global warming potential (GWP) have impacts on global warming for indirect and direct greenhouse gases (GHG) emissions. CO2 is a popular low-GWP alternative. The transcritical operation of CO2 systems worsens their energy performance, but provides recoverable heat as a heat source to reduce gas consumption. To evaluate operation performance, data-driven models, trained by historical data, are weak in implementation with datasets outside the scope of training data; in contrast, theoretical models have better extrapolation ability to calculate all operation conditions of CO2 systems at supermarket. Existing theoretical modeling approaches often lack validation against the limited public-access data, which reduces model reliability for further applications, and adopt oversimplified inference methods for unmeasured variables, which increases the risks of breaking thermodynamic laws and lowering model accuracy. This study therefore develops a steady-state theoretical model for CO2 booster refrigeration systems validated against field measurements from three UK supermarkets. The available measurements are utilized to the best level to ensure model accuracy and physical interpretability. Proposed methods to infer missing variables in CO2 systems include condenser outlet temperature, evaporating temperature, compressor isentropic efficiency and compressor mass flow rate. Results show that proposed inference methods enhance the abilities of the proposed modelling approach to ensure data integrity, avoid breaking thermodynamic laws, and improve model accuracy by reflecting real-time actual values of unmeasured variables rather than rough assumptions. The proposed modeling approach provides satisfactory accuracy validated using high-resolution measurements across the whole year from three real supermarkets.

Thermodynamic analysis of an enhanced ejector vapor injection refrigeration cycle for CO2 transcritical operation at low evaporating temperatures

The main drawback associated with CO2 refrigeration systems is related to their performance reduction during transcritical operation at warm climate conditions, which may be compensated by better cycle architectures such as the split-cycle with subcooling or the flash-tank configuration, among others. Specifically, the use of standard gas-ejectors together with parallel compressors provides even better efficiency improvements, not being able to use them with low-temperature evaporators to prevent the triple point inside the ejector. This paper proposes an enhanced cycle with a gas ejector for two-stage compressor architectures with vapor injection from the flash-tank, which is able to operate at low evaporating temperatures and that provides a greater performance improvement the more severe the climate conditions are. The methodology conducted is based on a thermodynamic analysis that includes parametric evaluation and cycle optimization, comparing the results to a conventional CO2 transcritical cycle with flash-tank and dynamic vapor injection architecture. The main results show that a maximum Coefficient of Performance improvement of 17.5% is achievable for transcritical operation at -40 °C evaporating temperature. The compressor displacement capacity required with the enhanced cycle is up to 9% lower for the same refrigeration demand, reducing the electrical consumption as well as the compressor expenditure. Moreover, greater vapor injection mass flow rates are obtained by the gas-ejector injection with discharge temperature reductions up to 18%, enhancing the system reliability.

Thermodynamic evaluation of CO2 for ultra-low temperature refrigeration

Carbon dioxide (CO2, R744) is the only refrigerant in the safest class by the ASHRAE 34 Standard in the group of natural refrigerants, with zero ozone depletion potential and a global warming potential of 1. It has been recently proposed for commercial refrigeration and heat pumps. Ultra-low temperature (ULT) refrigeration considers two-stage cascades with hydrofluorocarbon synthetic refrigerants (R404A/R23 high and low-temperature stages, HTS and LTS, respectively) and, lately, hydrocarbons (R290/R170). This paper examines the potential of R744 in ULT refrigeration cascade configurations in combination with other promising refrigerants. R744 is proposed in the medium temperature stage (MTS) of a three-stage cascade and the HTS of a two-stage transcritical operation (subcritical and transcritical with and without ejector, respectively). The operational and energy performance are compared with standard two- and three-stage ULT refrigeration cascades. Also, the cycles have been optimized, changing the main parameters as cascade heat changer temperatures or gas cooler pressure to maximize COP. This optimization and all the models have been made with Python, extracting the thermodynamic properties of REFPROP. The results show that in the HTS, the coefficient of performance (COP) is 39 % lower than the same two-stage cascade cycle with R290. In the MTS of a three-stage cascade, COP is 10 % lower than the same three-stage cascade cycle with R290. The ejector increases the COP by 38 % in the transcritical HTS, but remains below the hydrocarbon two-stage cascade. The choice of alternative refrigerants in the other stages does not significantly vary the COP results. Technological advancements in single subcritical and transcritical R744 configurations should be transferred to ULT refrigeration cascades to increase competitiveness and take advantage of its environmental and safety characteristics.

Comprehensive experimental performance study on a small-capacity transcritical R744 vapour-compression refrigeration unit equipped with an innovative ejector

Ejector-equipped transcritical R744 condensing units are believed to lead to a low-to-zero commercial refrigeration sector. In order to overcome the persisting barrier to their wider adoption represented by the lack of an affordable ejector control technique, the novel pulse-width modulation (PWM) ejector, being low cost, simple and invulnerable to clogging was recently implemented. However, additional experimental evaluations are needed. Therefore, in this experimental work the performance of two PWM ejector-equipped transcritical R744 condensing units, i.e. with and without overfed evaporator, was carried out. The experimental assessment was implemented at the medium temperature (MT) of about -5 °C, heat sink temperatures from 30 °C to 40 °C and compressor speeds from 40 Hz to 60 Hz.The outcomes obtained revealed that the PWM ejector can effectively control the high pressure in transcritical operating conditions, regardless of the selected heat sink temperature and compressor speed. In addition, at the same cooling capacity, the PWM ejector-equipped R744 system was found to permit energy savings between 7.0% and 11.1% without overfed evaporator and between 11.5% and 16.3% with overfed evaporator compared to the standard R744 unit (i.e. with vapour by-pass valve and without ejector), respectively. Finally, higher values of coefficient of performance (COP) were found to be offered by the PWM ejector compared with its today's available competitors.

Next generation of ejector-supported R744 booster systems for commercial refrigeration at all climates

The pernicious effects of synthetic refrigerants on different environmental aspects leave natural refrigerants as the only alternative for vapour compressions systems. Among natural refrigerants, CO2 (R744) has become the preferred choice for commercial refrigeration at almost any location and climate. However, efficient R744 refrigeration systems for warm climates have a great level of complexity, implementing technologies such as mechanical subcooling or ejector that increase the investment costs. The goal of the novel hybrid configuration presented in this work is to simplify ejector-supported R744 commercial refrigeration systems while maintaining all the benefits of the ejector implementation in transcritical operation mode. Moreover, utilization of a low-pressure accumulator layout in the subcritical mode removes all the practical challenges related to the booster layout operating at low ambient temperatures. This solution is based on: (i) MT and LT compressor suction groups, (ii) non-superheated MT evaporation with increased evaporation temperature, and (iii) ejector utilization throughout the year. The ejector is actively operated as a high-pressure-control device at elevated ambient temperatures ('summer mode'), while it is passive and acts as a check-valve at lower ambient temperatures ('winter mode'). The experimental campaign performed proved that this novel system configuration is more energy efficient than booster systems with parallel compressors at any condition, improving the COP by around 40 % at the most extreme gas cooler outlet temperatures tested, i.e., 40 °C (summer mode) and 10 °C (winter mode).

CONCENTRATING SOLAR POWERED TRANSCRITICAL CO2 POWER GENERATION CYCLE FOR THE UNION TERRITORY OF LADAKH, INDIA

High solar irradiation, cloud-free dry climate, abundant barren land, and low ambient temperature make the Union Territory of Ladakh, India, suitable for concentrating solar thermal power (CSP) plants. The present study comprehensively analyzes a 5 MW transcritical CO<sub>2</sub> Rankine cycle power tower CSP plant. Low ambient temperature of the region allows transcritical operation, which provides high cycle efficiency. The study focuses on five aspects: solar field and thermal energy storage (TES), thermodynamic cycle simulation, turbomachines, and off-design performance analysis. Modeling and optimization of the solar field are undertaken to capture the diurnal and annual variations of direct normal irradiation levels using System Advisor Model open source software. Molten salt TES is integrated to overcome the dynamic variations of solar energy by providing stable operations and additional hours. The effect of storage sizes, starting from no storage to 12 hours, on the solar field size and specifications is also assessed. An in-house algorithm is developed for thermodynamic cycle optimization, exergy analysis, and off-design operations. The turbomachines of the cycle are designed using in-house meanline codes, and 3D CFD simulations are conducted for efficiency estimations. The effects of ambient temperature variations on the low side saturation pressure, cycle efficiency, and power output are evaluated. The proposed plant offers annual optical efficiency of 54.1%, thermal efficiency of 36.5%, and overall efficiency of 19.7%.

Detailed numerical investigation of the CO[formula omitted] two-phase ejector 3-D CFD model based on the flow visualisation experiments

3-D CFD-based simulations of CO2 flow inside two-phase ejectors are commonly used with various turbulence models. However, none of the previously published studies examined the influence of a local turbulence phenomenon on the simulation results. Therefore, in this study, a detailed numerical analysis of CO2 local flow behaviour was compared to experimental flow visualisation data as well as to turbulence approach validation. Furthermore, the kinetic behaviour of mixed streams in the boundary layer was examined under transcritical operating conditions for motive nozzle pressures above 80.0 bar. The mixing process mapped by different turbulence models showed differences in recapturing the angle of suction flow in the mixing chamber. Furthermore, the different flow shapes of the simulated motive expanded flow were observed. The best prediction of the mixed two-phase flows was obtained using the Reynolds stress model with linear pressure-strain approach with the suction mass flow rate discrepancy of 2%. The higher pressure lift caused the suction flow to be more perpendicular towards the motive flow, elongating the suction flow course in the mixing chamber and intensifying the mixing process. A further increase in the pressure lift eventually leads to the possible occurrence of suction reverse flow.

Evaluation of Various Ejector Profiles on CO2 Transcritical Refrigeration System Performance.

This study examines the potential impact of the different ejector profiles on the CO2 transcritical cooling system to highlight the contribution of the multi-ejector in the system performance improvement. The research compares the implementation of an ejector-boosted CO2 refrigeration system over the second-generation layout at a motive flow temperature of 35 °C and discharge pressure of 90 bar to account for the transcritical operation mode. The result revealed a significant energy saving by reducing the input power to the maximum of 8.77% when the ejector was activated. Furthermore, the multi-ejector block could recover up to 25.4% of the expansion work losses acquired by both ejector combinations VEJ1 + 2. In addition, the behavior of the multi-ejector geometries and operation conditions greatly influence the system exergy destruction. The analysis shows a remarkable lack of exergy destruction during the expansion process by deploying the ejector in parallel with the HPV.

Large eddy simulations of reacting and non-reacting transcritical fuel sprays using multiphase thermodynamics

We present a novel framework for high-fidelity simulations of inert and reacting sprays at transcritical conditions with highly accurate and computationally efficient models for complex real-gas effects in high-pressure environments, especially for the hybrid subcritical/supercritical mode of evaporation during the mixing of fuel and oxidizer. The high-pressure jet disintegration is modeled using a diffuse interface method with multiphase thermodynamics, which combines multi-component real-fluid volumetric and caloric state equations with vapor–liquid equilibrium calculations for the computation of thermodynamic properties of mixtures at transcritical pressures. Combustion source terms are evaluated using a finite-rate chemistry model, including real-gas effects based on the fugacity of the species in the mixture. The adaptive local deconvolution method is used as a physically consistent turbulence model for large eddy simulation (LES). The proposed method represents multiphase turbulent fluid flows at transcritical pressures without relying on any semi-empirical breakup and evaporation models. All multiphase thermodynamic model equations are presented for general cubic state equations coupled with a rapid phase-equilibrium calculation method that is formulated in a reduced space based on the molar specific volume function. LES results show a very good agreement with available experimental data for the reacting and non-reacting engine combustion network benchmark spray A at transcritical operating conditions.

Influence of operating parameters on the COP of an R600 high-temperature heat pump

Process heat supply by high-temperature heat pumps exploiting waste heat is an effective method for enabling the decarbonisation of industrial processes. Key aspects for the application of this technology are the supply temperature levels and efficiencies that can be achieved. This work investigates an R600 (n-butane) high-temperature vapour compression heat pump (HTHP) capable of utilizing waste heat at temperature levels between 40 and 60°C and lifting it to supply temperatures between 110°C in sub-critical and 160°C in trans-critical operation. The HTHP prototype is designed as a single-stage cycle, comprising an inverter-driven suction-gas-cooled reciprocating compressor, low-pressure accumulator and internal heat exchanger (IHX) for suction gas superheating. The influence of the operating parameters high pressure and suction gas superheat (“internal parameters”) plus the heat source temperature, the heat sink inlet and outlet temperatures and the compressor speed (“external parameters”) on the COP and heating capacity are studied using data from both the prototype and the simulation model. The optimum high pressure depends on the operating conditions, an increase of suction gas superheat increases the COP. In sub-critical operation, a heating capacity of 30.7 kW at a COP of 4.4 is reached at a source inlet temperature of 60°C heating the sink from 80 to 110°C. Trans-critical operation enables a heat sink outlet temperature of 160°C, offering a heating capacity of 24.2 kW at a COP of 3.1.

Hybrid two–stage СО2 transcritical mechanical compression–ejector cooling cycle: Thermodynamic analysis and optimization

In the paper, the main results of thermodynamic analysis and optimization of the hybrid two–stage carbon dioxide (СО2) transcritical mechanical compression–ejector cooling cycle are presented. The proposed hybrid cooling cycle contains a two–stage CO2 transcritical mechanical compression refrigeration machine and an ejector cooling machine using refrigerants R245ca, R601b, and R1233zd(E). The operation of the ejector cooling cycle is ensured by using some part of the heat rejected from the compressed superheated CO2 vapor of the mechanical compression refrigeration cycle. The cold obtained in the ejector cooling machine is used for subcooling the high–pressure CO2 vapor after the gas cooler. In addition, the intermediate cooling of the CO2 vapor after the low–pressure stage of the mechanical compression refrigeration cycle is realized by preheating and boiling the refrigerant of the ejector cooling cycle in the additional generator–precooler before entering the main generator–precooler. This paper presents a method for determination of the optimal design parameters and performance of the proposed hybrid cooling cycle. The results showed an increase in efficiency of the two–stage CO2 transcritical mechanical compression refrigeration machine in the proposed cooling system of up to 32.7% compared to the conventional system at transcritical operating conditions with high an environmental temperature.

Lumped parameter modelling of two-phase ejectors: numerical implications of the equilibrium assumptions

The use of carbon dioxide as refrigerant is attracting a growing attention and is a cutting-edge research topic. In spite of its many advantages, carbon dioxide has a major shortcoming, viz., low critical temperature. Owing to the low critical temperature, carbon dioxide cycles encompass both the sub-critical and the trans-critical operation conditions; the trans-critical operating conditions are characterized by high thermodynamic losses, requiring particular attention in the integrated component/system design criteria. In this perspective, in recent years, ejector technology has been widely recognized as a promising technical solution to support the deployment of carbon dioxide cycles, by reducing throttling losses. Unfortunately, the large variation in system operations as well as the changes in sub-critical and trans-critical operating conditions makes the numerical simulation of carbon dioxide ejector-based system a cutting-edge challenge. This paper contributes to the present day discussion on the validation of lumped parameter models for carbon dioxide ejectors. A model taken from the literature has been tested against literature data and the equilibrium assumptions, underlying the modelling approach have been tested.

Modeling and experiments for a CO2 ground-source heat pump with subcritical and transcritical operation

CO2-based ground-source heat pumps (GSHPs) have the potential to be very environmentally friendly, since GSHPs operate with high energy efficiency, and CO2 has no ozone depletion potential (ODP) and a low global warming potential (GWP). We developed a prototype CO2 liquid-to-air GSHP to investigate its performance potential in residential applications. Further, we developed a detailed model of the system that simulates both cooling and heating operation; the model is the primary focus of this report. The model simulates both subcritical and transcritical operation since the system regularly operates near and above the critical temperature of CO2 (30.98 °C) during heating and cooling operation. The model considered both the refrigerant-side thermodynamic and transport processes in the cycle, as well as the air-side heat transfer and moisture removal.We performed cooling tests for the prototype CO2 GSHP that included those from the International Standards Organization (ISO) 13256-1 standard for liquid-to-air heat pumps, as well as extended tests at additional entering liquid temperatures (ELTs). The model predicted the measurements within 0.5% to 6.7% for COP, 1.0% to 3.6% for total capacity, and 3.3% to 4.9% for sensible capacity. We compared the measured cooling performance to published performance data for a commercially-available R410A GSHP and found that for ELTs below 20 °C, the CO2 GSHP has a higher cooling COP and total capacity than the R410A GSHP. At the ‘standard’ cooling rating condition (ELT 25 °C), the CO2 GSHP COP was 4.14 and the R410A GSHP COP was 4.43. At ‘part-load’ conditions (ELT 20 °C) the CO2 GSHP COP was 4.92 and the R410A GSHP COP was 4.99. In the future, the model can be used to investigate methods to improve the CO2 GSHP performance to meet or exceed that of the R410A system over a wider range of ELTs; possible studies include replacing the electronic expansion valve (EEV) with an ejector, optimizing the charge, and optimizing the heat exchanger geometry and circuiting.

Thermodynamic analysis of the optimal operating conditions for a two-stage CO2 refrigeration unit in warm climates with and without ejector

The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.

Effect of dedicated mechanical subcooler size and gas cooler pressure control on transcritical CO2 booster systems

Dedicated Mechanical Subcooling (DMS) is one of the most investigated and effective strategies applied to increase the performance of CO2 commercial refrigeration systems in transcritical operation. Further performance benefits can be obtained by a reduction of the gas cooler pressure of the main cycle at transcritical conditions. In this work the most important parameters for the design and operation of such a system, i.e. the DMS cooling capacity, the subcooling degree and the gas cooler pressure, are considered and their effect on the annual energy use of the plant is estimated in warm and hot climate conditions by means of a validated model. DMS is also compared to the parallel compression scheme and subcooling performed through a water chiller dedicated to HVAC. DMS results to be the most effective solution among those investigated, and the choice of the best design and operating parameters allows further energy saving and cost reduction.

Thermo-Economic Optimization of Organic Rankine Cycle (ORC) Systems for Geothermal Power Generation: A Comparative Study of System Configurations

The suitability of organic Rankine cycle (ORC) technology for the conversion of low- and medium-grade heat sources to useful power has established this as a promising option in geothermal power-generation applications. Despite extensive research in this field, most of which has focused on parametric analyses and thermodynamic performance evaluations, there is still a lack of understanding concerning the comparative performance of different plant configurations from both thermodynamic and economic perspectives. This study seeks to investigate the thermo-economic performance of subcritical and transcritical geothermal ORC power-plants, while considering a range of working fluids and the use of superheating and/or recuperation. A specific case study based on the exploitation of a medium-temperature geothermal heat source (180 °C, 40 kg/s) is conducted. Multi-objective optimization is performed to maximize the power/exergy efficiency (i.e., resource use) and minimize the payback period. The optimization results of different configurations are compared and the influence on system performance of superheating, recuperation, and subcritical vs. transcritical operation are evaluated. The results reveal that superheating is preferable for working fluids with low critical temperatures, but would hinder the performance of fluids whose critical temperature is higher. Recuperation is not attractive under most operating conditions, since the thermal performance improvement and cooling water saving cannot compensate the cost associated with the installation of the additional heat exchanger. Finally, transcritical ORC system are favoured thanks to the better thermal match between the heat source and the working fluid in these configurations. A more generalized geothermal heat source is then considered to explore the optimal configuration over a range of heat sources, which indicates that non-recuperated transcritical-cycle systems with working fluids whose critical temperature is close to the heat-source temperature are generally favourable.

CO2 Refrigeration and Heat Pump Systems—A Comprehensive Review

An increased awareness of the impacts of synthetic refrigerants on the environment has prompted the refrigeration industry and researchers worldwide to seek better alternatives in terms of technical, economic and environmental performance. CO2 refrigerant, also known as R744, has re-emerged as a potential alternative to existing refrigerants with its zero ozone depletion potential (ODP) and impressively low global warming potential (GWP). A refrigeration system utilising this refrigerant, however, suffers performance degradation when it operates in warm or hot climatic regions due to its inevitable operation in the supercritical region. In addition, the CO2 refrigerant properties necessitate the need for components designed to withstand very high operating pressures. These challenges have not been let unnoticed; related industries and researchers are actively involved in research and development of various components and systems which in turn encourages increased applications of these systems. In this paper, a comprehensive review of CO2 refrigeration systems and the state of the art of the technology and its applications in various industries is presented. In particular, the paper reviews recent research and developments on various aspects of CO2 systems including cycle modifications, exergy analysis of the systems, system modelling, transcritical operation consideration and various existing and potential applications.

Zero Gradient Control for R-744 refrigeration cycles

This paper proposes a method called Zero Gradient Control, which is used to control the heat rejection pressure and the rotational speed of the fans of the gascooler of R-744 refrigeration cycles nearly energy optimal. The basic concept is to develop algebraic terms for the partial derivatives of the COP with respect to the heat rejection pressure and the speed of the fans. These equations are evaluated online with measurement signals and directly fed to decentralized controllers with setpoint zero. The control structure is the same for both sub- and transcritical operation, requires little parameterization and can be applied to a wide range of systems. Using Zero Gradient Control, there is no need to calculate setpoints, which simplifies the mathematical effort in the implementation. Measurements for various scenarios validate the method even in the case of atypical system behavior or gascooler fouling.

Thermal storage subcooling for CO2 booster refrigeration systems

Transcritical CO2 supermarket refrigeration equipment is becoming more common as a near-zero global warming potential solution. Much attention is being given to improvements in efficiency for these systems, especially in transcritical operation to counteract reduced efficiency during hot outdoor temperatures. One approach to improving cycle efficiency is through the use of dedicated mechanical subcooling. This article examines further enhancement to the subcooler for the booster cycle by adding the use of thermal storage. By integrating a thermal energy storage system into the subcooling loop, the efficiency enhancements of dedicated subcooling may be maintained, while shifting power consumption to off-peak hours, significantly improving efficiency, and reducing power during hours with hot outdoor temperatures. Transient modeling was used to simulate a supermarket system with a thermal storage subcooler with different phase change material (PCM) and heat exchanger configurations. The results show a reduction of power corresponding to the power of the subcooler, with the possibility of further power reduction if the thermal storage system delivers lower leaving refrigerant temperatures. The power reduction is in the range of 8% to 16% for the duration of the storage discharge.

An Object-Oriented R744 Two-Phase Ejector Reduced-Order Model for Dynamic Simulations

The object-oriented two-phase ejector hybrid reduced-order model (ROM) was developed for dynamic simulation of the R744 refrigeration system. OpenModelica software was used to evaluate the system’s performance. Moreover, the hybrid ROM results were compared to the results given by the non-dimensional and one-dimensional mathematical approaches of the R744 two-phase ejector. Accuracy of all three ejector models was defined through a validation procedure for the experimental results. Finally, the dynamic simulation of the hybrid ROM ejector model integrated with the R744 refrigeration system was presented based on the summer campaign at three different climate zones: Mediterranean, South American and South Asian. The hybrid ROM obtained the best prediction of ejector mass flow rates as compared with other ejector models under subcritical and transcritical operating conditions. The dynamic simulations of the R744 ejector-based system indicated the ejector efficiency variations and the best efficiency at the investigated climate zones. The coefficient of performance (COP) varied from 2.5 to 4.0 according to different ambient conditions. The pressure ratio of 1.15 allowed a more stabilised system during the test campaign with an ejector efficiency from 20% to over 30%.