Search Results (14)

Rail transit as a high-energy consumption field urgently requires the adoption of clean energy innovations to reduce energy consumption and accelerate the transition to new energy applications. As an energy-saving fluid machinery, the ejector exhibits significant application potential and academic value within this field. This paper reviewed the recent advances, technical challenges, research hotspots, and future development directions of ejector applications in rail transit, aiming to address gaps in existing reviews. (1) In waste heat recovery, exhaust heat is utilized for propulsion in vehicle ejector refrigeration air conditioning systems, resulting in energy consumption being reduced by 12~17%. (2) In vehicle pneumatic pressure reduction systems, the throttle valve is replaced with an ejector, leading to an output power increase of more than 13% and providing support for zero-emission new energy vehicle applications. (3) In hydrogen supply systems, hydrogen recirculation efficiency exceeding 68.5% is achieved in fuel cells using multi-nozzle ejector technology. (4) Ejector-based active flow control enables precise ± 20 N dynamic pantograph lift adjustment at 300 km/h. However, current research still faces challenges including the tendency toward subcritical mode in fixed geometry ejectors under variable operating conditions, scarcity of application data for global warming potential refrigerants, insufficient stability of hydrogen recycling under wide power output ranges, and thermodynamic irreversibility causing turbulence loss. To address these issues, future efforts should focus on developing dynamic intelligent control technology based on machine learning, designing adjustable nozzles and other structural innovations, optimizing multi-system efficiency through hybrid architectures, and investigating global warming potential refrigerants. These strategies will facilitate the evolution of ejector technology toward greater intelligence and efficiency, thereby supporting the green transformation and energy conservation objectives of rail transit. Full article

Two-phase expansion processes have emerged as a promising technology for enhancing energy efficiency in power generation, refrigeration, waste heat recovery systems (for example, partially evaporated organic Rankine cycle, organic flash cycle, and trilateral flash cycle), oil and gas, and other applications. However, despite their potential, widespread adoption is hindered by inherent challenges, particularly energy losses that reduce operational efficiency. This review systematically evaluates the current state of two-phase expansion technologies, focusing on the root causes, impacts, and mitigation strategies for expansion losses. This work used Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). Using the PRISMA framework, 52 relevant publications were identified from Scopus and Web of Science to conduct the systematic review. A preliminary co-occurrence analysis of keywords was also conducted using VOSviewer version 1.6.20. Three clusters were observed in this co-occurrence analysis. However, the results may not be significant. Therefore, the extended work was done through a comprehensive analysis of experimental and simulation studies from the literature. This study identifies critical loss mechanisms in key components of two-phase expanders, such as the nozzle, diffuser, rotor, working chamber, and vaneless space. Also, losses arising from wetness, such as droplet formation, interfacial friction, and non-equilibrium phase transitions, are examined. These phenomena degrade performance by disrupting flow stability, increasing entropy generation, and causing mechanical erosion. Several losses in the turbine and volumetric expanders operating in two-phase conditions are reported. Ejectors, throttling valves, and flashing flow systems that exhibit similar challenges of losses are also discussed. This review discusses the mitigation and the strategy to minimize the two-phase expansion losses. The geometry of the inlet of the two-phase expanders plays an important role, which also needs improvement to minimize losses. The review highlights recent advancements in addressing these challenges and shows optimization opportunities for further research. Full article

This investigation attempts to develop a tower solar collector-based system designed for the cogeneration of cooling and desalination. The traditional organic Rankine cycle (ORC) integrated with the ejector refrigeration cycle generates limited power and cooling at a single temperature. Acknowledging their limitations, our present study uses an organic flash cycle (OFC) supported by solar heat combined with the two-phase ejector cycle and the reverse osmosis (RO) desalination unit. Since the OFC turbine is fed with two extra streams of fluid, therefore, it provides greater power to run the compressor of the ejector and pumps of the RO unit, resulting in the production of cooling at two different temperatures (refrigeration and air conditioning) and a higher mass flow rate of fresh water. A mathematical model is employed to assess the impact of coil curvature ratio, Rib height, and direct normal irradiation (DNI) on the temperature of the collector’s oil outlet. ANSYS-FLUENT conducts numerical simulations through computational fluid dynamics (CFD) analysis. The results indicate an ultimate increase in oil outlet temperature of 45% as the DNI increased from 450 to 1000 W/m² at a curvature ratio of 0.095 when employing the 1st Rib. Further, a steady-state energy and exergy analysis is conducted to evaluate the performance of the proposed cogeneration, with different design parameters like DNI, coil curvature ratio, rib height, and OFC turbine inlet pressure. The energetic and exergetic efficiencies of the cogeneration system at DNI of 800 W/m² are obtained as 16.67% and 6.08%, respectively. Exergetic assessment of the overall system shows that 29.57% is the exergy produced as cooling exergy, and the exergy accompanied by freshwater, 68.13%, is the exergy destroyed, and 2.3% is the exergy loss. The solar collector exhibits the maximum exergy destruction, followed by the ejector and RO pumps. Integrating multiple technologies into a system with solar input enhances efficiency, energy sustainability, and environmental benefits. Full article

Recent research in the liquefied natural gas (LNG) industry has concentrated on reducing specific power consumption (SPC) during production, which helps to lower operating costs and decrease the carbon footprint. Although reducing the SPC offers benefits, it can complicate the system and increase investment costs. This review investigates the thermodynamic parameters of various natural gas (NG) liquefaction technologies. It examines the cryogenic NG processes, including integrating NG liquid recovery plants, nitrogen rejection cycles, helium recovery units, and LNG facilities. It explores various approaches to improve hybrid NG liquefaction performance, including the application of optimization algorithms, mixed refrigerant units, absorption refrigeration cycles, diffusion–absorption refrigeration systems, auto-cascade absorption refrigeration processes, thermoelectric generator plants, liquid air cold recovery units, ejector refrigeration cycles, and the integration of renewable energy sources and waste heat. The review evaluates the economic aspects of hybrid LNG systems, focusing on specific capital costs, LNG pricing, and capacity. LNG capital cost estimates from academic sources (173.2–1184 USD/TPA) are lower than those in technical reports (486.7–3839 USD/TPA). LNG prices in research studies (0.2–0.45 USD/kg, 2024) are lower than in technical reports (0.3–0.7 USD/kg), based on 2024 data. Also, this review investigates LNG accidents in detail and provides valuable insights into safety protocols, risk management strategies, and the overall resilience of LNG operations in the face of potential hazards. A detailed evaluation of LNG plants built in recent years is provided, focusing on technological advancements, operational efficiency, and safety measures. Moreover, this study investigates LNG ports in the United States, examining their infrastructures, regulatory compliance, and strategic role in the global LNG supply chain. In addition, it outlines LNG’s current status and future outlook, focusing on key industry trends. Finally, it presents a market share analysis that examines LNG distribution by export, import, re-loading, and receiving markets. Full article

This article proposes a heating method based on heat pump technology to address the large amount of low-grade waste heat generated by a certain type of ultra-high voltage direct current (UHVDC) converter valve. Thermal performance calculations for two systems, a basic vapor compression heat pump system (BVCHPS) based on thermal expansion valve throttling and an ejector-enhanced heat pump system (EEHPS) are analyzed. The research results show that the EEHPS exhibits superior COP and exergy efficiency when generating hot water above 80 °C using a heat source below 50 °C. Additionally, mathematical modeling analysis identifies optimal structural parameters such as nozzle throat diameter, throat area ratio, and nozzle outlet diameter for the ejector in its design state. The low-temperature waste heat recovered from the UHVDC converter valves can be further used in engineering applications such as heating, refrigeration, seawater desalination, and sewage treatment. Full article

An ejector refrigeration system is a promising heat-driven refrigeration technology for energy consumption. The ideal cycle of an ejector refrigeration cycle (ERC) is a compound cycle with an inverse Carnot cycle driven by a Carnot cycle. The coefficient of performance (COP) of this ideal cycle represents the theoretical upper bound of ERC, and it does not contain any information about the properties of working fluids, which is a key cause of the large energy efficiency gap between the actual cycle and the ideal cycle. In this paper, the limiting COP and thermodynamics perfection of subcritical ERC is derived to evaluate the ERC efficiency limit under the constraint of pure working fluids. 15 pure fluids are employed to demonstrate the effects of working fluids on limiting COP and limiting thermodynamics perfection. The limiting COP is expressed as the function of the working fluid thermophysical parameters and the operating temperatures. The thermophysical parameters are the specific entropy increase in the generating process and the slope of the saturated liquid, and the limiting COP increases with these two parameters. The result shows R152a, R141b, and R123 have the best performance, and the limiting thermodynamic perfections at the referenced state are 86.8%, 84.90%, and 83.67%, respectively. Full article

CO₂ is regarded as an effective and environmentally friendly refrigerant. Using a CO₂ ejector is a proven method for enhancing the effectiveness of a transcritical CO₂ refrigerant system. However, the complex internal flow of a CO₂ ejector, involving supersonic effects, phase change effects, metastable effects, and so on, makes it difficult to understand. In order to summarize the current state of the technology and knowledge gaps, this work provides a comprehensive literature review on CO₂ ejectors. In the first part, mathematical modelling and simulation calculations of CO₂ ejectors are presented, and an overview and classification of ejector models are given. In the second part, the structural optimization part of the ejector is described in detail, and the nozzle structure, the mixing chamber length, improvements to multi-jet systems, and the impact of these factors on the system performance are analyzed. In the third part, flow visualization is used to study the complex flow phenomenon, and the effect of the shock wave on the entrained rate of the ejector is discussed. Finally, the paper outlines the relationship between all ejector technologies, working fluids, and ejector performance and makes valid recommendations for further research and development of CO₂ ejectors. Full article

The trigeneration plants for combined cooling, heating, and electricity supply, or integrated energy systems (IES), are mostly based on gas reciprocating engines. The fuel efficiency of gas reciprocating engines depends essentially on air intake temperatures. The transformation of the heat removed from the combustion engines into refrigeration is generally conducted by absorption lithium-bromide chillers (ACh). The peculiarity of refrigeration generation in food technologies is the use of chilled water of about 12 °C instead of 7 °C as the most typical for ACh. This leads to a considerable cooling potential not realized by ACh that could be used for cooling the engine intake air. A refrigerant ejector chiller (ECh) is the simplest in design, cheap, and can be applied as the low-temperature stage of a two-stage absorption-ejector chiller (AECh) to provide engine intake air cooling and increase engine fuel efficiency as result. The monitoring data on gas engine fuel consumption and power were analyzed in order to evaluate the effect of gas engine cyclic air cooling. Full article

Absorption Refrigeration Systems (ARS) are potential alternatives to direct expansion (DX) refrigeration systems. This review focused on the incorporation of an ejector into absorption refrigeration cycles to constitute Hybrid Ejector-Absorption Refrigeration Systems (HEARS). The ejector adds several advantages to the absorption refrigeration systems depending on its location in the cycle. The two prevalent configurations of HEARS are Triple pressure level (TPL-HEARS), and Low Pressure Condenser (LPC-HEARS). Previous studies revealed the preference of the latter configuration as it allows lower circulation ratios, enhances the refrigeration effect, and could achieve a COP up to 1. Moreover, LPC configuration is suitable with single, double, and variable-effect absorption systems with a COP of above unity. In turn, the TPL-HEARS notably enhances the absorption process, particularly when a variable geometry ejector is utilized. This configuration could obtain a COP around 1.1, but only with high-density refrigerant vapor. Lately, to attain the advantages of both configurations, some studies investigated the viability of adding two ejectors to the cycle. This paper meticulously reviews investigations conducted on the emerging dual ejectors-absorption refrigeration technology. This paper reveals the general performance trend and the maximum attainable COP by each type of hybrid ejector-absorption refrigeration system. DEARS and Ejector-driven absorption refrigeration systems (ED-ARS) could achieve COP that ranges between 1.2 and 1.46. The use of a flash tank and a RHE is essential in NH₃/H₂O HEARS. At high generator temperatures (of 120–170 °C), DEARS was found to be the system with less complexity and best performance. Nevertheless, the performance of the DEARS might drop significantly if the heat source temperature is fluctuating. Thence, the variable-effect HEARS is considered the best alternative. The capability of HEARS to be integrated with different power generation cycles is also highlighted. Finally, the review presents possible future research opportunities to improve the absorption refrigeration technology. Full article

The energy-saving potential of heat pump technology is widely recognized in the building sector. In retrofit applications, especially in old and historic buildings, it may be difficult to replace the existing distribution and high-temperature emission systems. Often, historical buildings, especially the listed ones, cannot be thermally insulated; this leads to high temperatures of the heat carrier fluid for heating. In these cases, the main limits are related, on the one hand, to the reaching of the required temperatures, and on the other hand, to the obtaining of good performance even at high temperatures. To address these problems, a suitable solution can be a two-stage heat pump. In this work, a novel concept of a two-stage heat pump is proposed, based on a transcritical cycle that uses the natural fluid R744 (carbon dioxide) with an ejector system. The second refrigerant present in the heat pump and used for the high-temperature stage is the R1234ze, which is an HFO (hydrofluoro-olefin) fluid. This work aims to present the effective energy performance based on real data obtained in operating conditions in a monitoring campaign. The heat pump prototype used in this application is part of the H2020 Cheap-GSHP project, which was concluded in 2019. Full article

The rising of the global energy demand requires the use of alternative energy conversion systems employing renewable sources. In the refrigeration and air conditioning fields, heat driven ejector systems represent a promising way to produce the cooling effect by using available low-grade temperature sources. In this paper, a thermo-economic analysis of a waste heat recovery hybrid ejector cycle (WHRHEC) was carried out. A thermodynamic model was firstly developed to simulate a WHRHEC able to obtain chilled water with a cooling load of 20 kW, by varying the working fluids and the pinch point values in the heat exchangers. Specific single- and two-phase heat transfer correlations were used to estimate the heat transfer surface and therefore the investment costs. The operative ranges that provide a reasonable compromise between the set-up costs and the cycle performances were then defined and compared to the current waste heat-driven technologies, such as absorption chillers and organic Rankine cycles (ORCs) coupled with vapor compression cycles (VCCs). The last part of the paper presents an economic analysis providing the map of the design (plant size) and contingent (specific cost of energy, waste heat availability) variables that lead to the economic convenience of a WHRHEC system when integrated to a conventional VCC plant. Full article

The paper presented a mathematical assessment of selected refrigerants for the ejector cooling purpose. R1234ze(e) and R1234yf are the well-known refrigerants of hydrofluoroolefins (HFOs), the fourth-generation halocarbon refrigerants. Nature working fluids, R600a and R290, and third-generation refrigerant of halocarbon (hydrofluorocarbon, HFC), R32 and R152a, were selected in the assessment. A detail mathematical model of the ejector, as well as other components of the cycle, was built. The results showed that the coefficient of performance (COP) of R1234ze(e) was significantly higher than R600a at the same operating conditions. R1234yf’s performance was compatible with R290, and both were about 5% less than the previous two. The results also indicated that R152a offered the best performance among the selected refrigerants, but due to the high value of global warming potential, it did not fulfill the requirements of the current European refrigerant regulations. On the other hand, R1234ze(e) was the most suitable working fluid for the ejector cooling technology, thanks to its overall performance. Full article

The adoption of the EU F-Gas Regulation 517/2014 and the resulting development of the multi-ejector concept have led carbon dioxide to take center stage as the sole refrigerant (R744) in several applications. Therefore, a knock-on effect on the number of supermarkets relying on “CO₂ only” refrigeration systems has been experienced. Additionally, a global consensus of commercial multi-ejector based R744 units is also intensifying as a consequence of both the promising results obtained and the other measures in force for environment preservation. Furthermore, the multi-ejector concept is expected to offer significant energy savings in other high energy-demanding buildings (e.g., hotels, gyms, spas) as well, even in warm climates. In this investigation, the evolution of R744 ejector supported parallel vapor compression system layouts for food retail applications was summed up. Furthermore, their technological aspects, the results related to the main theoretical assessments and some relevant field/laboratory measurements were summarized. Also, the experience gained in the adoption of the multi-ejector concept in transcritical R744 vapor-compression units aimed at other energy intensive applications was presented. Finally, the persistent barriers needing to be overcome as well as the required future work were brought to light. Full article

Naval surface ships should use thermally driven heating and cooling technologies to continue the Navy’s leadership role in protecting the marine environment. Steam ejector refrigeration (SER) or steam ejector heat pump (SEHP) systems are thermally driven heating and cooling technologies and seem to be a promising technology to reduce emissions for heating and cooling on board naval surface ships. In this study, design and thermodynamic analysis of a seawater cooled SER and SEHP as an HVAC system for a naval surface ship application are presented and compared with those of a current typical naval ship system case, an H₂O-LiBr absorption heat pump and a vapour-compression heat pump. The off-design study estimated the coefficient of performances (COPs) were 0.29–0.11 for the cooling mode and 1.29–1.11 for the heating mode, depending on the pressure of the exhaust gas boiler at off-design conditions. In the system operating at the exhaust gas boiler pressure of 0.2 MPa, the optimum area ratio obtained was 23.30. Full article