Two-stage Cycle Research Articles

Selection of working fluids and heat pump cycles at high temperatures: Creating a concise technology portfolio

As global efforts to address climate change intensify, industrial processes are of the highest interest due to their substantial contributions to greenhouse gas emissions and use of fossil energy. The deployment of high-temperature heat pumps (HTHPs) presents a significant potential for mitigating emissions and decreasing primary energy consumption in industrial applications. However, for widespread adoption, further technological development is crucial to enhance the technology's availability at higher supply temperatures. This study is dedicated to optimising HTHPs by exploring the intricate interplay between working fluids and heat pump cycle design, with the goal of establishing a technology portfolio that guarantees high performance across a wide range of high-temperature conditions. The research evaluates 16 working fluids in 10 HTHP configurations across a broad spectrum of high-temperature conditions, with source temperatures from 0 °C to 100 °C and temperature lifts from 20 K to 150 K. A polynomial estimating the Lorenz efficiency was proposed based on these configurations and temperature conditions. A portfolio of heat pumps using either isobutane, isopentane, or hexane, in a one-stage cycle layout with a suction line heat exchanger, a two-stage cycle layout with an open intercooler using R-718, and cascade configurations using these working fluids, consistently demonstrated the highest Lorenz efficiency for 95% of the evaluated temperature conditions. Including ammonia and carbon dioxide elevated this to 98% of temperature conditions, improving particularly at large sink temperature glides. While underscoring the necessity for an approach considering both COP and production costs, the study highlights the COP as the most important aspect. The marginal improvement observed by including HFOs highlights the potential of a natural working-fluid-based portfolio to achieve high performance, instilling optimism for the future of sustainable high-temperature heat pump applications.

Performance analysis of double-stage heat pump and refrigeration cycles

Multistage refrigeration systems are investigated to/enhance the performance of vapor compression refrigeration cycles based on our previous work on multistage low-grade thermal energy conversion. A double-stage heat pump and refrigeration cycles (D-S) is expected to reduce the irreversible loss in the heat exchange process. However, certain characteristics of independently configured systems, namely, compressor power and coefficients of performance (COPs), have not been sufficiently clarified. This study aims to elucidate highly advanced heat pump and refrigeration systems by comparing D-S, a basic single-stage heat pump and refrigeration cycles, and a two-stage compression cycle. D-S shows higher COPs for both the refrigeration and heat pump cycles compared with the other cycles, especially at high thermal conductances. The reduction of the irreversible loss in the heat exchange process is confirmed via the entropy generation rate; in particular, the COP increases with a decrease in the entropy generation rate. The evaporation and condensation temperatures of the refrigerant approach the temperature of the heat source when the thermal conductance exceeds 150 kW/K for the refrigeration cycle. The heat exchangers exergy destruction of D-S is 169 kW, S-S is 189 kW, the two-stage cycle is 184 kW/K when the thermal conductance is 100 kW/K.

Experimental investigation of two-stage NH3–H2O resorption heat storage system with solution concentration difference

Widespread application of solar energy is severely restricted to its instability in time and space. Heat storage system is considered to be one of the most effective pathways to achieve long-term heat supply. Among all the heat storage technologies, solution concentration difference storage technologies stand out for low heat loss and high energy density. By substituting condenser and evaporator in conventional absorption cycle with a high-pressure absorber and low-pressure generator, a resorption cycle could be established. Compared with absorption heat storage cycle, working pressure of resorption heat cycle is lower, making it possible to utilize low-pressure heat source. Two-stage heat source cycle is further proposed to lower working pressure and upgrading supply water temperature. Resorption energy storage cycle with solution concentration difference is established and studied. A prototype based on two-stage ammonia-water resorption heat storage cycle was set up and tested. The result shows that the constraint relation between ambient temperature and heat source temperature of the proposed system. In single-stage mode, the lowest working ambient temperature is 5 °C when heat source temperature is 90 °C. The lowest working ambient temperature could be declined to 0 °C when heat source temperature rises to 120 °C. In two-stage mode, the lowest working ambient temperature is 0 °C when heat source temperature is only 110 °C, which proves the adaptability of two-stage cycle. The highest ESCOP appears in the 15 °C–120 °C single-stage mode, which is 0.742. The highest COP is 1.324 under the same condition. The highest energy density occurs in two-stage mode, which is 467.07 kJ/kg when ambient and heat source is set as 15 °C, 120 °C. The advantage that two-stage mode could enlarge solution concentration difference is verified.

District heating utilizing waste heat of a data center: High-temperature heat pumps

Data centers are energy-intensive facilities with substantial low-grade waste heat. High-temperature heat pumps can be critical in boosting the data center’s waste heat for district heating, improving the system-level energy efficiency of data centers, and reducing CO2 emissions in district heating. This study built thermodynamic models to assess high-temperature heat pumps with six configurations using low global warming potential refrigerants to supply heat up to 120 °C. The heat pump configurations include single-stage or two-stage cycles with advanced components, such as internal heat exchanger, economizer, flash tank, or parallel compressor. The refrigerants include R1234ze(Z), R1233ed(E), R1224yd(Z), R600, and R600a, and R245fa is used as a reference. A case study was carried out to recover the waste heat from the Frontier high-performance computing data center and provide hot water for district heating at the US Department of Energy’s Oak Ridge National Laboratory campus. The optimized performance of high-temperature heat pumps is characterized with various effectiveness of internal heat exchangers, and the operating parameters of economizer or flash tank, as well as their combination. The results show that the configurations of two-stage cycles with internal heat exchanger + flash tank and internal heat exchanger + economizer/parallel-compressor provide the highest coefficient of performance under scenarios of the maximum allowable value and a fixed value (0.3) of the internal heat exchangers’ effectiveness, respectively. R1234ze(Z) and R600a are the most promising refrigerants, considering trade-offs between the coefficient of performance and the volumetric heating capacity. The single-stage cycle with internal heat exchanger + economizer/parallel-compressor using R1234ze(Z) is recommended for utilizing Fronter’s waste heat in district heating. A one mega-watt high-temperature heat pump will reduce 33,100–33,200 metric tons of CO2 emission annually, corresponding to 85.4 %–85.6 % of equivalent CO2 emissions from natural gas boilers. This study provides good guidelines for designing and deploying high-temperature heat pumps to support sustainable data centers and decarbonize district heating in the US.

Absorption Chiller Efficiency Increasing at Waste Heat Recovery of Cogen-eration Plants using a Vapor Compression Chiller as a Backup Cold Source

Cooling efficiency in a trigeneration cycles is one of the most frequently problems. A single-stage hot water absorption chiller is a universal solution for the gas engine waste heat utilization. How-ever, trigeneration centers exploitation experience shows the need for a more detailed technical and economic analysis to achieve maximum efficiency of the three-generation cycle. The purpose of this article is to substantiate the use of two-stage absorption cycles increasing the utilization rate of waste gas heat. The study presents the results of assessing the efficiency of using energy resources on the example of a three-generation center with more than 14 years of exploitation experience, including two single-stage absorption chillers (Q0= 0.65 MW each) and a chiller with a screw compressor (Q0= 0.8 MW). A comparison of actual energy consumption with designed indicators was carried out. The influ-ence of external and internal factors on the efficiency of heat recovery from a gas engine in single-stage absorption refrigeration machines with a water-heated generator is considered. The dependence of the change in the average annual cooling coefficient during the calendar year was obtained. Based on the study, proposals were formulated to improve the energy efficiency of refrigeration supply, giving an increase in refrigeration capacity of 6.1% with a commensurate reduction in the load on the screw chiller. The option of using a combined two-stage cycle chiller (type 2) under similar operating conditions showed a COP=1.43, which provides savings of 67.45 MWh of electricity or 5.4%, on an annualized basis (assuming only waste heat is used). Additional advantages of such a cycle include increased accu-racy and stability of maintaining the water temperature at the absorption chiller evaporator due to more flexible adjustment of the temperature of the lithium bromide solution in the high-temperature generator.

Performance analysis of water refrigerant heat pump with different configurations for high-temperature application

Water is a promising working fluid for high-temperature heat pumps (HTHPs) with the advantages of being cheap, safe, stable, environmentally friendly, non-toxic, and non-flammable. HTHPs using water as a refrigerant not only can reduce greenhouse gas emissions in the industrial sectors by recovering waste heat but also have no adverse effect on the environment if the refrigerant leaks. This article evaluates the safety, energy, and exergy performance of six designed HTHP configurations with water refrigerant by analyzing the discharge superheat, power consumption, heating capacity, COP, and exergy efficiency—at different condensation and evaporation temperatures. The results indicated that water injection and intercooling can effectively decrease the discharge superheat to ensure safety performance. Among these systems, two-stage cycles can supply higher output temperatures and have better system performance compared to single-stage cycles at high-temperature lifts and large compression ratio conditions. The two-stage system with a flash tank (TS-FT) has the biggest heating capacity, the best COP, and the best exergy efficiency. With an evaporation temperature of 80°C and condensation temperature of 140°C, the COP is 4.14 and the exergy efficiency is 70.9% for the TS-FT. Compared with the single-stage ordinary system (SS-OS), the COP of the TS-FT has an increment of 36.6%. The exergy efficiency of the TS-FT is 19.8% higher than that of the SS-OS. Considering the aforementioned theoretical analysis, the TS-FT with the best safety, energy, and exergy performance is the optimal HTHP system with water refrigerant for high-temperature applications.

Influence of composition of binary mixed working fluids on the performance of cascade heat pump

The optimized design of the cascade heat pump facilitates the recovery and reuse of waste heat in printing and dyeing wastewater. This paper constructs a cascade heat pump system with two-stage cycles sampling the same binary mixed working fluids. The effects of three kinds of combinations of working fluids, including R134a/R152a, R134a/R22, and R152a/R22, on the COP and heating power, are discussed through the thermodynamic cycle process simulation when the component ratio varies. It is found that, for the R134a/R152a mixed working fluids, the system performance can be adjusted approximately linearly by changing the component ratio. For the two mixed working fluids of R134a/R22 and R152a/R22, the performance of the cascade heat pump system is closer to the case of using pure R134a and R152a as corresponding working fluids. The system’s heating capacity is significantly reduced, compared with the case of pure R22 employed. Besides, with the component ratio of working fluid increasing, the COP and heating power change with opposite trends.

A unified approach for the thermodynamic comparison of heat pump cycles

The flexible heat pump cycle introduces a heat storage device into the Evans-Perkins cycle to recover, store, and reuse part of the sensible heat carried by the hot liquid refrigerant from the condenser, achieving a higher coefficient of performance than the latter. In this paper, we develop a unified approach, namely cycle superposition to allow comparison of the flexible heat pump cycle with other performance-enhancing cycle layouts including two-stage cycles with intercooling, subcooling, flash gas removal, or their combinations. We show that under ideal conditions, the flexible heat pump cycle is thermodynamically similar to two-stage heat pump cycles with full subcooling or flash gas removal, but no intercooling. From the energy recovery perspective, the two-stage cycles recover and reuse some sensible heat carried by hot liquid refrigerant simultaneously using their high-stage compressor, whereas the flexible heat pump cycle decouples the recovery and reuse of such heat in time using a heat storage. However, the irreversible heat transfer via real heat exchangers during charging and discharging processes will reduce the benefits of the flexible heat pump cycle. The effectiveness of all these performance-enhancing methods strongly depends on the characteristics of refrigerants.

A Comprehensive Review of Organic Rankine Cycles

It has been demonstrated that energy systems driven by conventional energy sources like fossil fuels are one of the main causes of climate change. Organic Rankine cycles can help to reduce that impact, as they can be operated by using the industrial waste heat of renewable energies. The present study presents a comprehensive bibliographic review of organic Rankine cycles. The study not only actualizes previous reviews that mainly focused on basic cycles operating on subcritical or supercritical conditions, but also includes the analysis of novel cycles such as two-stage and hybrid cycles and the used fluids. Recuperative and regenerative cycles are more efficient than reheated and basic single-stage cycles. The use of two-stage cycles makes it possible to achieve higher thermal efficiencies and net power outputs of up to 20% and 44%, respectively, compared with those obtained with single-stage cycles. Theoretical studies show that hybrid systems, including Brayton and organic Rankine cycles, are the most efficient; however, they require very high temperatures to operate. Most organic Rankine cycle plants produce net power outputs from 1 kW up to several tens of kW, mainly using microturbines and plate heat exchangers.

Thermodynamic performance assessment and application feasibility analysis of small heat pump water heater using CO2/R41 azeotropy mixture as refrigerant for sustainable development

In order to improve the application possibility of CO2/R41 azeotropy mixture in the refrigeration industry, the optimum mass ratio of composition is defined and determined according to the thermodynamic properties and safety. On the basis of the first and second laws of thermodynamics, a model is proposed to evaluate the performance of CO2/R41 single-stage transcritical cycle (CO2/R41 cycle) in which a throttling valve is integrated and CO2/R41 azeotropy refrigerant is adopted. The performance of CO2/R41 cycle at the optimum mass ratio of CO2/R41 azeotropy mixture is then compared with that of a CO2 two-stage transcritical cycle with throttling valve (CO2 two-stage cycle) and that of a CO2 single-stage transcritical cycle with expander (CO2 expander cycle). The results show that the optimum mass ratio of CO2/R41 azeotropy mixture is 0.583/0.417. Compared with CO2 two-stage cycle and CO2 expander cycle, CO2/R41 cycle has the advantages of a simpler structure, lower optimum high pressure, appropriate discharge temperature of the compressor, and higher COP. Therefore, the CO2/R41 azeotropy mixture with the mass ratio of 0.583/0.417 is eco-friendly and can be considered as a good alternative refrigerant for application in the refrigeration industry. The simple CO2/R41 cycle shows great feasibility to replace the complex CO2 two-stage cycle and CO2 expander cycle.

Exergy analyses of two and three stage cryogenic cycles

Abstract Cryogenics has an important influence on industry and science. In this study, optimum working conditions are obtained by applying exergy analysis and local optimization methods to two- and three-stage vapor compression cascade cryogenic cycle. The first and second laws of thermodynamics, exergy analysis, and local optimization methods are applied to the two- and three-stage cascade cryogenic cycle. By considering the needs and demands, it is possible to create new cycles by adding new devices and/or new stages to these cycles. The results of the optimum operating conditions are obtained for the two- and three-stage vapor compression cascade cryogenic cycle. It is seen that to achieve high COP values and high efficiency; it is necessary to reduce the compression ratio of the compressor as much as the fluid allows. For the two-stage cycle, the minimum total work required for cryogenic cooling is around P 7 = 2,400 kPa. The COP value is 0.30 between P 7 = 2,400 and 2,800 kPa, and the maximum exergy efficiency is obtained around 0.235. It is seen operating the first-stage compressor at high pressures increases the total losses of the entire cycle from 7,500 to 18,550 kW. The increase in total exergy losses is around 247%, and operating the first-stage compressor at high pressures increases the exergy efficiency of the entire cycle. The increase in total exergy efficiency is around 160%. When the second-stage compressor is operated at low pressure, the COP value increases by 2%, the exergy efficiency increases by 20%, and the exergy losses decrease by around 40%.

The prediction for optimum combination of high pressure and intermediate pressure on a small refrigerated cabinet with CO2 transcritical two-stage cycle

It is universally acknowledged that the use of two-stage cycle can effectively improve the comprehensive performance of the CO2 refrigeration system. There exists an optimum combination of high pressure and intermediate pressure in a CO2 refrigeration system with transcritical two-stage cycle where the system yields the best coefficient of performance (COP). In this paper, a novel double-iteration method is used to predict the optimum combination of high pressure and intermediate pressure of a CO2 refrigerated cabinet system with transcritical two-stage cycle and the accuracy of prediction is experimentally verified. Then, the validated thermodynamic model is employed to identify the calculation accuracies of related existing correlations for calculating the optimum high pressure and the optimum intermediate pressure under various conditions. The results show that the thermodynamic model has enough accuracy to predict the optimum combination of high and intermediate pressures with an error less than 2.07%. Among the several major existing correlations, ′′Sarkar + Yari′′ (-10 °C≤Te≤10 °C, 30 °C≤Tgc,o≤50 °C) display the most accurate results to calculate the combination of high and intermediate pressures of a CO2 transcritical cycle. Meanwhile, the difference of thermodynamic performance predicted by the model and ′′Sarkar + Yari′′ correlations can be negligible (approximate to 0).

From bodies in hammock bundles to commingled burnt remains: an archaeothanatological case study of a two-stage burial cycle at Toca do Alto da Serra do Capim (Middle-Late Holocene, Northeastern Brazil)

From bodies in hammock bundles to commingled burnt remains: an archaeothanatological case study of a two-stage burial cycle at Toca do Alto da Serra do Capim (Middle-Late Holocene, Northeastern Brazil)

Research on visualization and diagnosis method of civil aircraft WQAR data

Abstract WQAR data has the characteristics of large scale, high dimension and many random interference factors, which makes it extremely difficult to use WQAR operational data for flight quality monitoring, expert unknown anomaly detection and PHM. This paper proposes a civil aircraft WQAR data visualization display scheme and the overall framework of fault diagnosis adopts a two-stage cycle to research and recycle and improve the data preprocessing requirements, visualization and diagnostic process methods, etc., and use the civil aircraft operation data to verify the proposed theoretical model and diagnose the failure status. Results shown that the theoretical framework and method can accurately predict the health status of aircraft engines, and can effectively solve the problems of high dimension and many interference factors of WQAR data.

Quantification and comparison of COP improvement approaches for large-scale ammonia heat pump systems

• Compares five approaches to improve the COP of large-scale ammonia heat pumps. • Evaluates both one-stage and two-stage HP cycles connected in series. • A pinch analysis optimizes the heat exchanger network between HP and heat sink. The number of large-scale heat pumps (HP) implemented in Danish district heating networks has increased in recent years. The installations are often custom-made solutions designed carefully to obtain a high Coefficient of Performance (COP). This study aims at quantifying and compare five approaches for improving the COP of a heat pump, with a one-stage ammonia cycle used as the reference case with sink temperatures of 50 °C/80 °C (in/out) and source temperatures of 10 °C/3 °C. The approaches include increased complexity of cycle layout, optimization of intermediate pressure in two-stage cycles, a connection of HP cycles in series, optimization of load share between heating capacity for HPs in series, and optimal design of heat exchanger networks (HEN). The latter is based on an implementation of pinch analysis to HP HENs. The results revealed an improvement in COP by +15.8% obtained with increased complexity of cycle layout (two-stage cycle with a low-stage desuperheater) compared to a COP of 2.98 for the reference case. Connecting up to three one-stage HPs and two-stage cycles in series improved COP by 4.42% and 24.5%, respectively. The benefit of optimal load share and intermediate pressure was less than 0.5 % points for all configurations compared to reference values based on theoretical approaches. Exploiting the potential for optimal HEN increased the potential to 10.4% for one-stage cycles in series and nearly 29% for two-stage cycles increasing the COP to a maximum of 3.84.

Energy, Exergy, and Environmental (3E) Analysis of Hydrocarbons as Low GWP Alternatives to R134a in Vapor Compression Refrigeration Configurations

The phase-down of hydrofluorocarbons and substitution with low global warming potential values are consequences of the awareness about the environmental impacts of greenhouse gases. This theoretical study evaluated the energy and exergy performances and the environmental impact of three vapor compression system configurations operating with the hydrocarbons R290, R600a, and R1270 as alternatives to R134a. The refrigeration cycle configurations investigated in this study include a single-stage cycle, a cycle equipped with an internal heat exchanger, and a two-stage cycle with vapor injection. According to the results, the alternative hydrocarbon refrigerants could provide comparable system performance to R134a. The analysis results also revealed that using an internal heat exchanger or a flash tank vapor injection could improve the system’s efficiency while decreasing the heating capacity. The most efficient configuration was the two-stage refrigeration cycle with vapor injection, as revealed by the exergy analysis. The environmental impact analysis indicated that the utilization of environmentally-friendly refrigerants and improving the refrigeration system’s efficiency could mitigate equivalent CO2 emissions significantly. The utilization of hydrocarbons reduced the carbon footprint by 50%, while a 1% to 8% reduction could be achieved using the internal heat exchanger and flash tank vapor injection.

Exergy analysis of a vortex tube expansion two-stage transcritical N2O refrigeration cycle

In the present article, two-stage transcritical cycle with expansion valve (TSTCEV) and two-stage transcritical cycle with vortex tube (TSTCVT) are compared mainly on the basis of the second law of thermodynamics. Natural refrigerant nitrous oxide (N2O) is used in both cycles for thermodynamic analysis. The gas cooler and evaporator temperatures are varied from 35°C to 55°C and −55°C to 5°C, respectively. The effects of various operating parameters on the optimum intercooler and gas cooler pressures, the coefficient of performance (COP), exergy destruction and the exergetic efficiency are presented. A component-wise exergy analysis of TSTCEV and TSTCVT shows that the use of the vortex tube instead of the expansion valve reduces the total exergy destruction and increases the exergetic efficiency as well as COP. The exergetic efficiency of TSTCVT is on average 9.17% to 11.68% higher than that of TSTCEV for considered operating conditions. Furthermore, the optimum intercooler and gas cooler pressures in TSTCVT are lower than those of TSTCEV. These results offer significant help for optimum design and the selection of appropriate operating conditions of TSTCVT using refrigerant N2O.

Fproi-GAN with Fused Regional Features for the Synthesis of High-Quality Paired Medical Images.

The use of medical image synthesis with generative adversarial networks (GAN) is effective for expanding medical samples. The structural consistency between the synthesized and actual image is a key indicator of the quality of the synthesized image, and the region of interest (ROI) of the synthesized image is related to its usability, and these parameters are the two key issues in image synthesis. In this paper, the fusion-ROI patch GAN (Fproi-GAN) model was constructed by incorporating a priori regional feature based on the two-stage cycle consistency mechanism of cycleGAN. This model has improved the tissue contrast of ROI and achieved the pairwise synthesis of high-quality medical images and their corresponding ROIs. The quantitative evaluation results in two publicly available datasets, INbreast and BRATS 2017, show that the synthesized ROI images have a DICE coefficient of 0.981 ± 0.11 and a Hausdorff distance of 4.21 ± 2.84 relative to the original images. The classification experimental results show that the synthesized images can effectively assist in the training of machine learning models, improve the generalization performance of prediction models, and improve the classification accuracy by 4% and sensitivity by 5.3% compared with the cycleGAN method. Hence, the paired medical images synthesized using Fproi-GAN have high quality and structural consistency with real medical images.

Exergy analysis of vapour compression-absorption two-stage refrigeration cycle

Exergy analysis of vapour compression-absorption two-stage refrigeration cycle

Upper-limit of performance improvement by using (quasi) two-stage vapor compression

Both quasi two-stage and two-stage compressions are important ways for improving the energy efficiency of the vapor compression heat pump system, especially for high compression ratio conditions. Many experiments and simulations have been conducted to investigate the performance improvement of two-stage or quasi two-stage systems. In most occasions, positive results were obtained. However, the performance improvements reported by different studies vary significantly. Actually, the results obtained by different researchers are barely comparable, because operating conditions, refrigerants, cycles, and economizers heavily affect the performance of (quasi) two-stage system, which hinders the potential evaluation in cycle-selection period and perfectness assessment of developed (quasi) two-stage systems. In this study, a general analytical expression for the (quasi) two-stage vapor compression system is proposed, by which the upper-limit of performance improvement of (quasi) two-stage systems using different refrigerants under different operating conditions can be calculated quickly. Based on that, the potential of adopting (quasi) two-stage cycle and the maturation of the developed prototypes can be easily identified by comparing the test performance with the upper-limit improvement.