Booster Refrigeration System Research Articles

Reinforcement Learning Control of Transcritical Carbon Dioxide Supermarket Refrigeration Systems

Commercial refrigeration systems consume a substantial amount of electrical energy, resulting in high indirect global warming impact due to greenhouse gases emissions. The multitude of different system configurations, system complexity, component wear, and changing operating conditions make efficient operation of this kind of refrigeration systems a difficult task. This paper presents an investigation of machine learning for supervisory control of a supermarket refrigeration system. In particular, a reinforcement learning algorithm for a CO2 booster refrigeration system is designed by exploiting the Matlab-based “SRSim” simulation tool. The reinforcement learning controller learns to operate the refrigeration system based on the interaction with the environment. The analysis shows that learning control is a feasible model-free technique to find a suitable control strategy for demand-side management in a smart grid scenario.

Energetic and economic analysis of trans-critical CO2 booster system for refrigeration in warm climatic condition

The performance of five CO2 booster refrigeration systems is analysed and compared in this paper for supermarket application. The investigated configurations include a standard booster system, abbreviated as BC1, a booster system with parallel compressor (BC2), a booster system with flooded low temperature evaporator (BC3), a booster system with work recovery expander (BC4) and a booster with parallel compressor along with flooded low temperature evaporator and work recovery expander (BC5). Annual hourly averaged temperature variations at four prominent cities across the world are taken as case study. Simulation indicates advantage of the proposed system BC5 over BC1 configuration. Work recovery unit is found to have the highest potential in the annual energy savings in BC5, followed by parallel compressor and flooded evaporator. The maximum annual energy savings is found to be 22.16% for BC5 in New Delhi. Economic analysis reveals recovery time of less than four years for the additional investment made in BC5. The slope of recovery time is found steeper at lower tariff compared to that at higher tariff.

Experimental evaluation of transcritical CO2 refrigeration with mechanical subcooling

Transcritical CO2 booster refrigeration systems have seen a growing interest in recent years as they can provide two-stage refrigeration with R744, which has a global warming potential of 1.0, as the working fluid. However, the efficiency of these systems, particularly in transcritical operation, is often not as good as the efficiency of comparable conventional equipment. One possible way to improve performance in hot climates is through the use of a mechanical subcooler. This study describes laboratory testing of a lab-scale transcritical booster system, with and without mechanical subcooling and in transcritical and subcritical operating modes. In addition, a new steady-state model for generalized vapor compression systems was developed and is presented here. The model is used to simulate the system tested in the current article. The testing showed a significant improvement in both capacity and efficiency at all operating conditions with the subcooler. The solver was able to match capacity, power, and efficiency to within 3% for all test conditions. The model was used to perform a parametric study of subcooler capacity on the cycle; the results show that added subcooling substantially improves overall coefficient of performance and reduces the need for bypass flow, but the overall coefficient of performance benefit diminishes as the efficiency of the main cycle increases.

Energy analysis of alternative CO2 refrigeration system configurations for retail food applications in moderate and warm climates

Refrigeration systems are crucial in retail food stores to ensure appropriate merchandising of food products. This paper compares four different CO2 refrigeration system configurations in terms of cooling performance, environmental impact, power consumption and annual running costs. The systems studied were the conventional booster refrigeration system with gas bypass (reference system), the all CO2 cascade system with gas bypass, a booster system with a gas bypass compressor, and integrated cascade all CO2 system with gas bypass compressor. The weather conditions of London, UK, and Athens, Greece, were used for the modelling of energy consumption and environmental impacts to represent moderate and warm climatic conditions respectively. The control strategies for the refrigeration systems were derived from experimental tests in the laboratory on a conventional booster refrigeration system. The results from the analysis showed that the CO2 booster system with gas bypass compressor can provide best performance with 5.0% energy savings for the warm climate and 3.65% for the moderate climate, followed by the integrated cascade all CO2 system with gas bypass compressor, with 3.6% and 2.1% savings over the reference system for the warm and moderate climates respectively.

Advanced exergy analysis of a R744 booster refrigeration system with parallel compression

In this paper, the advanced exergy analysis was applied to a R744 booster refrigeration system with parallel compression taking into account the design external temperatures of 25 °C and 35 °C, as well as the operating conditions of a conventional European supermarket. The global efficiencies of all the chosen compressors were extrapolated from some manufactures' data and appropriated optimization procedures of the performance of the investigated solution were implemented.According to the results associated with the conventional exergy evaluation, the gas cooler/condenser, the HS (high stage) compressor and the MT (medium temperature) display cabinet exhibited the highest enhancement potential. The further splitting of their corresponding exergy destruction rates into their different parts and the following assessment of the interactions among the components allowed figuring out the real achievable improvements. The avoidable irreversibilities of the HS compressor and that of the MT evaporator were mainly and completely endogenous, respectively. On the other hand, the gas cooler/condenser could be predominantly improved by decreasing the inefficiencies of the MT evaporator. As regards the auxiliary compressor, large enhancements were attainable through the drop in the irreversibilities occurring in the remaining components.

Design optimisation of CO2 gas cooler/condenser in a refrigeration system

As a natural working fluid, CO2 has been widely applied in refrigeration systems where heat is conventionally released to ambient through external airflow. Owing to its extraordinary thermophysical properties, especially a low critical temperature, the CO2 heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system.To achieve these targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system. Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localised heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed.

Design Optimisation of CO2 Gas Cooler/Condenser in a Refrigeration System

As a natural working fluid, CO2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO2 heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system.To achieve these targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system. Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localized heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed.

Numerical model for thermoeconomic diagnosis in commercial transcritical/subcritical booster refrigeration systems

Transcritical/subcritical booster refrigeration systems are increasingly installed and used in Danish supermarkets. The systems operate in both transcritical and subcritical conditions dependent on the heat rejection performance and the ambient conditions. The plant consists of one refrigerant cycle supplying refrigerant for evaporators in both chilled and frozen display cases. In the paper, thermoeconomic theory is used to establish the cost of cooling at each individual temperature level based on operating costs.With a high amount of operating systems, faulty operation becomes an economic, and environmental, interest. A general solution for evaluation of these systems is considered, with the objective to reduce cost and power consumption of malfunctioning equipment in operation. An analysis of the use of thermoeconomic diagnosis methods is required, as these methods may prove applicable. To accommodate the analysis, a numerical model of a transcritical booster refrigeration plant is considered in this paper. Additionally the characteristic curves method is applied to the high pressure compressor unit of the refrigeration plant. The approach successfully determine whether an anomaly is intrinsic or induced in the component when no uncertainties are introduced in the steady state model.

Thermodynamic analysis of transcritical CO 2 booster refrigeration systems in supermarket

Due to less environmental impact, the CO 2 booster refrigeration system has been widely applied in the modern supermarket as a substitute for the conventional R404A multiplex system. However, the performance efficiency of the CO 2 system still requires further improvement in order to save energy; thus, one of the most efficient techniques would be to investigate and employ the optimal controls for refrigerant high side pressures at various operating states. In this paper, the possible parameters affecting system efficiency of the CO 2 system in the transcritical cycle at a higher ambient air temperature are identified through thermodynamic analysis, but cannot be quantified mathematically because of the high non-linearity involved. Instead, sensitive analyses of the system by means of the thermodynamic model is used to examine the effects of parameters including high side refrigerant pressure, ambient air temperature, refrigerant intermediate pressure, and medium and low evaporating temperatures, superheating, effectiveness of suction line heat exchanger, and compressor efficiency on system performance. Consequently, the optimal high side pressure in the transcritical cycle is established and derived as a function of three important parameters consisting of ambient air temperature, the effectiveness of suction line heat exchanger and compressor efficiency. In addition, optimal operating parameters such as the intermediate pressure are also proposed to improve the system performance.

Performance evaluation and optimal design of supermarket refrigeration systems with supermarket model “SuperSim”. Part II: Model applications

As described in Part I, the supermarket simulation software “SuperSim” with its integrated refrigeration, building and HVAC system models, can be used to evaluate, compare and optimize alternative supermarket refrigeration systems. In Part II the model was used to evaluate and compare the performance of a CO 2 booster refrigeration system with that of a conventional R404A multiplex system in a supermarket application. Floating head pressure control was implemented for both systems when they were in subcritical cycles. For the CO 2 system, when the system was in transcritical cycle due to higher ambient air temperature, the head pressure was optimized through extensive thermodynamic cycle analysis as a function of ambient air temperature. The performance of the CO 2 booster system in the supermarket was then simulated during a one year period and compared with that of the R404A system. As a result, the system performance will benefit from a lower ambient temperature and a sizeable heat recovery for the CO 2 system.