Low-temperature Compressor Research Articles

Advanced Exergy and Exergoeconomic Analysis of Cascade High-Temperature Heat Pump System for Recovery of Low-Temperature Waste Heat

Cascade high-temperature heat pumps (CHTHPs) are often applied to recover low-temperature industrial waste heat owing to their large temperature lift. Through a comprehensive consideration of thermodynamic and economic performance, conventional and advanced exergy and exergoeconomic analyses are employed in this study to evaluate the potential for the improvement in CHTHP systems. The results show that the avoidable endogenous exergy destruction in a CHTHP system accounts for 62.26% of its total exergy destruction, indicating that most of the exergy destruction comes from the components. This suggests that CHTHP systems still have significant potential for improvement. The very low exergoeconomic factor of the total system (only 0.75%) implies that the exergy destruction cost has a great influence on the economic performance of a CHTHP system. The high- and low-temperature compressors are the two components with the highest exergy destruction, accounting for 34.14% and 26.79% of the total exergy destruction in the system, respectively. Moreover, their exergy destruction cost is much larger than that of the other components. Thus, the priorities for improvement should be the high- and low-temperature compressors. The decrease in exergy destruction in compressors produces a reduction in carbon emissions. This comprehensive analysis of thermodynamic and economic performance supplies guidance for the engineering application of CHTHPs in low-temperature waste heat recovery.

Energy and Conventional Exergy Analysis of an Integrated Transcritical CO2 (R-744) Refrigeration System

This study analyses the performance of an integrated transcritical CO2 (R-744) refrigeration system operating in winter conditions within a supermarket in Trento, north Italy. This system fulfils multiple functions, providing the heating (for domestic hot water and space heating), cooling, and freezing capabilities for the supermarket. Energy analysis reveals that the average value of the total coefficient of performance, total COP, over the entire study period is calculated at 2.47. Notably, the medium-temperature (MT) compressor rack exhibits the highest power consumption, especially in sub −5 °C conditions. The auxiliary (AUX) compressor rack and the gas cooler (GC) fan contribute significantly to the electrical power usage. The air conditioning (AC) heating load is consistently high, averaging 41.6 kW, while the domestic hot water (DHW) heating load remains stable at approximately 5 kW. The refrigeration demands include an average MT cooling load of 25.86 kW and a low-temperature (LT) freezing load of 10–15 kW, with an average of 13.76 kW. The current study also delves into exergy analysis, disclosing an overall system exergy efficiency of 22.4%. The AUX compressor rack is identified as the primary exergy destructor, followed by the GC, AC coils, MT compressor rack, and the ejector. The LT compressor rack has the highest exergy efficiency, followed by the MT and LT expansion valves.

An ultralow-temperature cascade refrigeration unit with natural refrigerant pair R290-R170: Performance evaluation under different ambient and freezing temperatures

The global demand for ultralow-temperature (ULT) refrigeration units gets greatly promoted, for storage, transportation, and distribution of COVID-19 vaccines. In this work, a ULT freezer is developed with a cascade refrigeration system (CRS) utilizing environmentally friendly refrigerants R290 and R170. The performance of the ULT freezer is experimentally evaluated under different ambient temperatures Tamb and freezing temperatures Tfreezing. The result shows that once the refrigeration system starts, the freezer enters a pull-down period and then reaches stable or periodic on-off operation. The monitored temperatures present drastic variations in the low-temperature cycle (LTC) and show a relatively stable start-up process in the high-temperature cycle (HTC). The monitored temperature rises when Tamb increases from 16 °C to 32 °C. The increasing Tamb brings about a larger temperature drop crossing the pre-cooled condenser, cascade heat exchanger, and high-temperature condenser, and a smaller temperature reduction through the anti-condensation. As Tfreezing decreases from −60 °C to −86 °C, the suction/discharge gas temperatures increase in the low-temperature compressor, while the other monitored temperatures reduce. The largest temperature non-uniformity in the freezer is 9.19 °C, and the lowest wall temperature can reach −90.52 °C. With Tamb ranging from 16 °C to 32 °C, the power consumption of the freezer increases from 896 W to 912 W. When Tfreezing varies from −60 °C to −86 °C, the CRS’s consumed power reduces from 804 W to 904 W. The present freezer can easily obtain low temperatures e.g., −81 °C, and reach a lower temperature, such as −86 °C with proper improvements to reduce cold loss.

Theoretical analysis of three CO2/C3H8 (R744-R290) cascade refrigeration systems with precooling processes in low-temperature circuits

In a cascade refrigeration system (CRS), the superheat of the vapour discharged from a low-temperature compressor is large for refrigerants with significant adiabatic exponents, and part of the cooling heat in the fluid can be removed using precooling technology. In this study, three CO2/C3H8 cascade refrigeration systems with precooling processes, namely a CRS with an air precooler (CRSA), CRS with a mechanical precooler (CRSM) and CRS with both an air precooler and a mechanical precooler (CRSAM), are proposed. The performances of the proposed systems are compared studied that of a conventional cascade refrigeration system (CCRS) in terms of energy and exergy. The results indicate that a design that arrange precoolers and precooling refrigerant circuits in low-temperature circuits is beneficial, directly reducing the work input of C3H8 compressors and improving refrigeration performance. When the condensing temperature rises from 12 °C to 48 °C, the coefficient of performance (COP) of the CRSA, CRSM and CRSAM enhances by 2.73% to 5.19%, 1.82% to 4.90% and 3.71% to 7.86% compared with the CCRS, and the exergy efficiency averagely improves by 4.01%, 3.41%, and 8.48%, respectively. In addition, the applicable conditions of the different systems are discussed, and the component cost is evaluated in terms of component capacity. The research results may provide guidelines for the practical application of CO2/C3H8 CRS with precooling processes.

Comparison study of conventional and advanced exergy analysis on cascade high temperature heat pump system based on experiment

Cascade high temperature heat pump (CHTHP) has shown remarkable application potential for industrial waste heat recovery owing to its superiority of large temperature lift. Great efforts have been made to optimize the performance of CHTHP by means of conventional exergy analysis (CEA), which was not qualified to uncover the thermodynamic interactions among components. Thus, advanced exergy analysis (AEA) has been adopted to comparatively evaluate the relationships of reversibility among the CHTHP components. Herein, both CEA and AEA were based on the experiment results from a prototype which could realize maximum temperature lift of 100 °C.The CEA results show that total exergy efficiency is 43.23%, while AEA results indicate that endogenous part accounts for 98.27% of the total exergy destruction and 67.69% of it is avoidable. The AEA reveals that the highest improvement priority belongs to low temperature compressor (LTCOM), followed by cascade heat exchanger (CHE) and high temperature compressor (HTCOM) and that low temperature expansion valve (LTEV) and high temperature expansion valve (HTEV) has no improvement potential which is different from the results of CEA theory. Conclusively, AEA can give more detailed guidance about the potential for improvement.

Advanced exergy analysis of NH3/CO2 cascade refrigeration system with ejector

Aiming at the NH3/CO2 Cascade Refrigeration System with ejector (CRSE), this paper establishes the mathematical model of the refrigeration cycle and compares it with the literature based on thermodynamic analysis. Advanced exergy analysis on the CRSE is carried out and the exergy destruction of components in the system are divided into two ways: avoidable/unavoidable exergy destruction and endogenous/exogenous exergy destruction, and combine the two ways to clarify the types and values of exergy destruction in components, which provides a direction for the improvement of the system. The analysis shows that the avoidable exergy destruction of the system accounts for 38.674% of the total exergy destruction, and the proportion of endogenous exergy destruction and exogenous exergy destruction in the total exergy destruction of the system is 77.772% and 22.228% respectively. The improvement potential of the system is very high, and the low-temperature compressor, condenser, and intermediate heat exchanger should be paid special attention to in the improvement.

Efficient control of cascade heat pumps using variable speed compressors

Building load varies continuously throughout the year, causing heat pumps to operate at part load. At such situations, the capacity of the heat pump needs to be controlled to meet the building load. The use of variable speed compressors to control the capacity of heat pumps is found to be the most energy-efficient. However, the efficient control of the capacity of the cascade heat pump using variable speed compressors is lacking in the open literature. This study investigates an energy-efficient capacity control method for a cascade heat pump by adopting variable speed compressors to match the rated capacity of a cascade heat pump at varying indoor and outdoor entering water temperature (EWT) conditions. Three capacity control strategies were employed in this study namely: low temperature (LT) compressor speed control, high temperature (HT) compressor speed control, and simultaneous LT and HT compressor speed control. COP of the cascade heat pump was almost similar in simultaneous control of the LT and HT compressor speeds and HT compressor speed control but significantly higher in simultaneous LT and HT compressor speed control than LT compressor speed control at all outdoor EWT conditions. However, the cascade heat pump had no significant difference in its COP at varying indoor EWT using all three capacity control strategies. It is recommended that water-to-water cascade heat pumps adopt variable speed compressors at the HT side and constant speed compressors at the LT side for efficient and cost effective capacity control at varying building loads.

Stress and Pressure Pulsation Analysis of Low Temperature Compressor Piping System in LNG Vaporizing Station

LNG (Liquefied Natural Gas) vaporizing stations are usually built in the cities and towns, and the BOG (Boiled Off Gas) pressurizing system is a very important element. In the pressurizing system, the severe vibration of the low-temperature reciprocating compressor may lead to a failure of the pipeline system and the equipment. Therefore, this paper analyzes the stress and pressure pulsation of the BOG compressor piping system in the LNG vaporizing station. The beam model was used to establish the pipe model. The static, harmonic and modal analysis were carried out based on the plane-wave theory and the pressure-fluctuation theory, and the influence factors of support spacing, the settlement of the fulcrum foundation, pipe pressure and elbow angle were analyzed. The main conclusions are as follows: (1) the unbalanced excited force caused by pressure pulsation greatly affects the stress of the exhaust pipe and compressor outlet pipe, and has less influence on the stress of the suction pipe and compressor inlet pipe; (2) although unbalanced excited force is generated in the elbow, it also has an impact on the straight pipe stress; (3) adding an expansion joint to the pipe of the BOG compressor can effectively reduce the stress of the pipe its the displacement, and can increase the flexibility of the pipe.