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Search Results (496)

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Keywords = waste heat recovery system

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21 pages, 3334 KiB  
Article
Market Research on Waste Biomass Material for Combined Energy Production in Bulgaria: A Path Toward Enhanced Energy Efficiency
by Penka Zlateva, Angel Terziev, Mariana Murzova, Nevena Mileva and Momchil Vassilev
Energies 2025, 18(15), 4153; https://doi.org/10.3390/en18154153 - 5 Aug 2025
Abstract
Using waste biomass as a raw material for the combined production of electricity and heat offers corresponding energy, economic, environmental and resource efficiency benefits. The study examines both the performance of a system for combined energy production based on the Organic Rankine Cycle [...] Read more.
Using waste biomass as a raw material for the combined production of electricity and heat offers corresponding energy, economic, environmental and resource efficiency benefits. The study examines both the performance of a system for combined energy production based on the Organic Rankine Cycle (ORC) utilizing wood biomass and the market interest in its deployment within Bulgaria. Its objective is to propose a technically and economically viable solution for the recovery of waste biomass through the combined production of electricity and heat while simultaneously assessing the readiness of industrial and municipal sectors to adopt such systems. The cogeneration plant incorporates an ORC module enhanced with three additional economizers that capture residual heat from flue gases. Operating on 2 t/h of biomass, the system delivers 1156 kW of electric power and 3660 kW of thermal energy, recovering an additional 2664 kW of heat. The overall energy efficiency reaches 85%, with projected annual revenues exceeding EUR 600,000 and a reduction in carbon dioxide emissions of over 5800 t/yr. These indicators can be achieved through optimal installation and operation. When operating at a reduced load, however, the specific fuel consumption increases and the overall efficiency of the installation decreases. The marketing survey results indicate that 75% of respondents express interest in adopting such technologies, contingent upon the availability of financial incentives. The strongest demand is observed for systems with capacities up to 1000 kW. However, significant barriers remain, including high initial investment costs and uneven access to raw materials. The findings confirm that the developed system offers a technologically robust, environmentally efficient and market-relevant solution, aligned with the goals of energy independence, sustainability and the transition to a low-carbon economy. Full article
(This article belongs to the Section B: Energy and Environment)
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24 pages, 3325 KiB  
Article
Multi-Energy Flow Optimal Dispatch of a Building Integrated Energy System Based on Thermal Comfort and Network Flexibility
by Jian Sun, Bingrui Sun, Xiaolong Cai, Dingqun Liu and Yongping Yang
Energies 2025, 18(15), 4051; https://doi.org/10.3390/en18154051 - 30 Jul 2025
Viewed by 235
Abstract
An efficient integrated energy system (IES) can enhance the potential of building energy conservation and carbon mitigation. However, imbalances between user-side demand and supply side output present formidable challenges to the operational dispatch of building energy systems. To mitigate heat rejection and improve [...] Read more.
An efficient integrated energy system (IES) can enhance the potential of building energy conservation and carbon mitigation. However, imbalances between user-side demand and supply side output present formidable challenges to the operational dispatch of building energy systems. To mitigate heat rejection and improve dispatch optimization, an integrated building energy system incorporating waste heat recovery via an absorption heat pump based on the flow temperature model is adopted. A comprehensive analysis was conducted to investigate the correlation among heat pump operational strategies, thermal comfort, and the dynamic thermal storage capacity of piping network systems. The optimization calculations and comparative analyses were conducted across five cases on typical season days via the CPLEX solver with MATLAB R2018a. The simulation results indicate that the operational modes of absorption heat pump reduced the costs by 4.4–8.5%, while the absorption rate of waste heat increased from 37.02% to 51.46%. Additionally, the utilization ratio of battery and thermal storage units decreased by up to 69.82% at most after considering the pipeline thermal inertia and thermal comfort, thus increasing the system’s energy-saving ability and reducing the pressure of energy storage equipment, ultimately increasing the scheduling flexibility of the integrated building energy system. Full article
(This article belongs to the Special Issue Energy Efficiency and Thermal Performance in Buildings)
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37 pages, 1099 KiB  
Review
Application Advances and Prospects of Ejector Technologies in the Field of Rail Transit Driven by Energy Conservation and Energy Transition
by Yiqiao Li, Hao Huang, Shengqiang Shen, Yali Guo, Yong Yang and Siyuan Liu
Energies 2025, 18(15), 3951; https://doi.org/10.3390/en18153951 - 24 Jul 2025
Viewed by 316
Abstract
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 [...] Read more.
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
(This article belongs to the Special Issue Advanced Research on Heat Exchangers Networks and Heat Recovery)
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49 pages, 4131 KiB  
Review
Municipal Solid Waste Gasification: Technologies, Process Parameters, and Sustainable Valorization of By-Products in a Circular Economy
by Nicoleta Ungureanu, Nicolae-Valentin Vlăduț, Sorin-Ștefan Biriș, Mariana Ionescu and Neluș-Evelin Gheorghiță
Sustainability 2025, 17(15), 6704; https://doi.org/10.3390/su17156704 - 23 Jul 2025
Viewed by 399
Abstract
Gasification of municipal solid waste and other biogenic residues (e.g., biomass and biowaste) is increasingly recognized as a promising thermochemical pathway for converting non-recyclable fractions into valuable energy carriers, with applications in electricity generation, district heating, hydrogen production, and synthetic fuels. This paper [...] Read more.
Gasification of municipal solid waste and other biogenic residues (e.g., biomass and biowaste) is increasingly recognized as a promising thermochemical pathway for converting non-recyclable fractions into valuable energy carriers, with applications in electricity generation, district heating, hydrogen production, and synthetic fuels. This paper provides a comprehensive analysis of major gasification technologies, including fixed bed, fluidized bed, entrained flow, plasma, supercritical water, microwave-assisted, high-temperature steam, and rotary kiln systems. Key aspects such as feedstock compatibility, operating parameters, technology readiness level, and integration within circular economy frameworks are critically evaluated. A comparative assessment of incineration and pyrolysis highlights the environmental and energetic advantages of gasification. The valorization pathways for main product (syngas) and by-products (syngas, ash, tar, and biochar) are also explored, emphasizing their reuse in environmental, agricultural, and industrial applications. Despite progress, large-scale adoption in Europe is constrained by economic, legislative, and technical barriers. Future research should prioritize scaling emerging systems, optimizing by-product recovery, and improving integration with carbon capture and circular energy infrastructures. Supported by recent European policy frameworks, gasification is positioned to play a key role in sustainable waste-to-energy strategies, biomass valorization, and the transition to a low-emission economy. Full article
(This article belongs to the Special Issue Sustainable Waste Process Engineering and Biomass Valorization)
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28 pages, 5208 KiB  
Article
ORC System Temperature and Evaporation Pressure Control Based on DDPG-MGPC
by Jing Li, Zexu Gao, Xi Zhou and Junyuan Zhang
Processes 2025, 13(7), 2314; https://doi.org/10.3390/pr13072314 - 21 Jul 2025
Viewed by 298
Abstract
The organic Rankine cycle (ORC) is a key technology for the recovery of low-grade waste heat, but its efficient and stable operation is challenged by complex kinetic coupling. This paper proposes a model partitioning strategy based on gap measurement to construct a high-fidelity [...] Read more.
The organic Rankine cycle (ORC) is a key technology for the recovery of low-grade waste heat, but its efficient and stable operation is challenged by complex kinetic coupling. This paper proposes a model partitioning strategy based on gap measurement to construct a high-fidelity ORC system model and combines the setting of observer decoupling and multi-model switching strategies to reduce the coupling impact and enhance adaptability. For control optimization, the reinforcement learning method of deep deterministic Policy Gradient (DDPG) is adopted to break through the limitations of the traditional discrete action space and achieve precise optimization in the continuous space. The proposed DDPG-MGPC (Hybrid Model Predictive Control) framework significantly enhances robustness and adaptability through the synergy of reinforcement learning and model prediction. Simulation shows that, compared with the existing hybrid reinforcement learning and MPC methods, DDPG-MGPC has better tracking performance and anti-interference ability under dynamic working conditions, providing a more efficient solution for the practical application of ORC. Full article
(This article belongs to the Section Energy Systems)
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13 pages, 9148 KiB  
Article
Investigation of Thermoelectric Properties in Altermagnet RuO2
by Jun Liu, Chunmin Ning, Xiao Liu, Sicong Zhu and Shuling Wang
Nanomaterials 2025, 15(14), 1129; https://doi.org/10.3390/nano15141129 - 21 Jul 2025
Viewed by 299
Abstract
An altermagnet, characterized by its distinctive magnetic properties, may hold potential applications in diverse fields such as magnetic materials, spintronics, data storage, and quantum computing. As a prototypical altermagnet, RuO2 exhibits spin polarization and demonstrates the advantageous characteristics of high electrical conductivity [...] Read more.
An altermagnet, characterized by its distinctive magnetic properties, may hold potential applications in diverse fields such as magnetic materials, spintronics, data storage, and quantum computing. As a prototypical altermagnet, RuO2 exhibits spin polarization and demonstrates the advantageous characteristics of high electrical conductivity and low thermal conductivity. These exceptional properties endow it with considerable promise in the emerging field of thermal spintronics. We studied the electronic structure and thermoelectric properties of RuO2; the constructed RuO2/TiO2/RuO2 all-antiferromagnetic tunnel junction (AFMTJ) exhibited thermally induced magnetoresistance (TIMR), reaching a maximum TIMR of 1756% at a temperature gradient of 5 K. Compared with prior studies on RuO2-based antiferromagnetic tunnel junctions, the novelty of this work lies in the thermally induced magnetoresistance based on its superior thermoelectric properties. In parallel structures, the spin-down current dominates the transmission spectrum, whereas in antiparallel structures, the spin-up current governs the transmission spectrum, underscoring the spin-polarized thermal transport. In addition, thermoelectric efficiency emphasizes the potential of RuO2 to link antiferromagnetic robustness with ferromagnetic spin functionality. These findings promote the development of efficient spintronic devices and spin-based storage technology for waste heat recovery and emphasize the role of spin splitting in zero-magnetization systems. Full article
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21 pages, 1897 KiB  
Article
Simulation of Conventional WWTPs Acting as Mediators in H2/CO2 Conversion into Methane
by Rubén González and Xiomar Gómez
Environments 2025, 12(7), 245; https://doi.org/10.3390/environments12070245 - 16 Jul 2025
Viewed by 507
Abstract
CO2-biomethanation was studied in the present manuscript by considering the direct injection of hydrogen into a conventional anaerobic digester treating sewage sludge within a simulated wastewater treatment plant (WWTP). The plant was simulated using the Python 3.12.4 software, and a Monte [...] Read more.
CO2-biomethanation was studied in the present manuscript by considering the direct injection of hydrogen into a conventional anaerobic digester treating sewage sludge within a simulated wastewater treatment plant (WWTP). The plant was simulated using the Python 3.12.4 software, and a Monte Carlo simulation was conducted to account for the high variability in the organic content of the wastewater and the methane potential of the sludge. Two modes of operation were studied. The first mode involves the use of an anaerobic digester to upgrade biogas, and the second mode considers using the digester as a CO2 utilization unit, transforming captured CO2. Upgrading biogas and utilizing the extra methane to generate electricity within the same plant leads to a negative economic balance (first scenario). A hydrogen injection of 1 L of H2/Lr d (volumetric H2 injection per liter of reactor per day) was required to transform the CO2 present in the biogas into methane. The benefits associated with this approach resulted in lower savings regarding heat recovery from the electrolyzer, increased electricity production, and an additional oxygen supply for the waste-activated sludge treatment system. Increasing the injection rate to values of 5 and 30 L of H2/Lr d was also studied by considering the operation of the digester under thermophilic conditions. The latter assumptions benefited from the better economy of scale associated with larger installations. They allowed for enough savings to be obtained in terms of the fuel demand for sludge drying, in addition to the previous categories analyzed in the biogas upgrading case. However, the current electricity price makes the proposal unfeasible unless a lower price is set for hydrogen generation. A standard electricity price of 7.6 c€/kWh was assumed for the analysis, but the specific operation of producing hydrogen required a price below 3.0 c€/kWh to achieve profitability. Full article
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16 pages, 1648 KiB  
Article
Biomimetic Stator Vane Design for Radial Turbines in Waste Heat Recovery Applications
by Fuhaid Alshammari, Ibrahim Alatawi and Muapper Alhadri
Biomimetics 2025, 10(7), 463; https://doi.org/10.3390/biomimetics10070463 - 15 Jul 2025
Viewed by 297
Abstract
Organic Rankine Cycle (ORC) systems are widely used for converting low-temperature waste heat into useful power, but their overall efficiency depends heavily on the turbine’s performance, particularly the stator vane design in radial turbines. This study introduces a biomimetic approach to turbine design [...] Read more.
Organic Rankine Cycle (ORC) systems are widely used for converting low-temperature waste heat into useful power, but their overall efficiency depends heavily on the turbine’s performance, particularly the stator vane design in radial turbines. This study introduces a biomimetic approach to turbine design by implementing cambered stator vanes inspired by bird feather geometry. These specially shaped vanes are added to a radial inflow turbine and compared to a traditional design that uses straight (symmetric) vanes. The new cambered design helps guide the airflow more effectively, leading to higher tangential speeds and better energy transfer. Simulations show that this design increases the turbine’s power output from 388.6 kW to 394.87 kW and improves the system’s overall efficiency from 8.78% to 10.12%. A detailed study of different camber levels found that moderate curvatures (around 8–12%) gave the best results. Overall, this study demonstrates that implementing biomimetic cambered stator vanes in radial turbines can significantly enhance turbine performance and improve cycle-level efficiency in ORC systems for waste heat recovery. Full article
(This article belongs to the Special Issue Nature-Inspired Science and Engineering for Sustainable Future)
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22 pages, 4469 KiB  
Article
Thermal and Exergetic Performance Analyses of a Heat Pipe Heat Exchanger Using CMC/Co3O4-Based Non-Newtonian Nanofluids
by Duygu Yilmaz Aydin
Appl. Sci. 2025, 15(14), 7831; https://doi.org/10.3390/app15147831 - 12 Jul 2025
Viewed by 321
Abstract
This study presents an experimental evaluation of the thermal and exergetic performance of an air-to-air heat pipe heat exchanger using a cobalt oxide (Co3O4)-based non-Newtonian nanofluid, with the additional incorporation of carbon black (CB). Nanofluids were synthesized via a [...] Read more.
This study presents an experimental evaluation of the thermal and exergetic performance of an air-to-air heat pipe heat exchanger using a cobalt oxide (Co3O4)-based non-Newtonian nanofluid, with the additional incorporation of carbon black (CB). Nanofluids were synthesized via a two-step method and tested under turbulent flow conditions across varying Reynolds numbers. The results demonstrated that increasing the Co3O4 nanoparticle concentration and adding CB substantially improved both the thermal and exergetic performance compared to deionized water. Specifically, maximum thermal efficiency improvements of 62.7% and 75.4% were recorded for nanofluids containing 1% and 2% Co3O4, respectively. The addition of CB further enhanced the thermal efficiency, achieving a maximum improvement of 79.2%. Furthermore, the maximum reduction in thermal resistance reached 61.4% with CB incorporation, while the 2% Co3O4 nanofluid achieved a maximum decrease of 50.2%. The use of nanofluids led to a significant reduction in exergy loss, with exergy-saving efficiencies reaching up to 33.6%. These findings highlight the considerable potential of Co3O4- and CB-based hybrid nanofluids in advancing waste heat recovery technologies and enhancing the thermodynamic performance of air-to-air heat pipe heat exchanger systems. Full article
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27 pages, 7623 KiB  
Article
A Ladder-Type Carbon Trading-Based Low-Carbon Economic Dispatch Model for Integrated Energy Systems with Flexible Load and Hybrid Energy Storage Optimization
by Liping Huang, Fanxin Zhong, Chun Sing Lai, Bang Zhong, Qijun Xiao and Weitai Hsu
Energies 2025, 18(14), 3679; https://doi.org/10.3390/en18143679 - 11 Jul 2025
Viewed by 279
Abstract
This paper proposes a ladder carbon trading-based low-carbon economic dispatch model for integrated energy systems (IESs), incorporating flexible load optimization and hybrid energy storage systems consisting of battery and thermal energy storage. First, a ladder-type carbon trading mechanism is introduced, in which the [...] Read more.
This paper proposes a ladder carbon trading-based low-carbon economic dispatch model for integrated energy systems (IESs), incorporating flexible load optimization and hybrid energy storage systems consisting of battery and thermal energy storage. First, a ladder-type carbon trading mechanism is introduced, in which the carbon trading cost increases progressively with emission levels, thereby providing stronger incentives for emission reduction. Second, flexible loads are categorized and modeled as shiftable, transferable, and reducible types, each with distinct operational constraints and compensation mechanisms. Third, both battery and thermal energy storage systems are considered to improve system flexibility by storing excess energy and supplying it when needed. Finally, a unified optimization framework is developed to coordinate the dispatch of renewable generation, gas turbines, waste heat recovery units, and multi-energy storage devices while integrating flexible load flexibility. The objective is to minimize the total system cost, which includes energy procurement, carbon trading expenditures, and demand response compensation. Three comparative case studies are conducted to evaluate system performance under different operational configurations: the proposed comprehensive model, a carbon trading-only approach, and a conventional baseline scenario. Results demonstrate that the proposed framework effectively balances economic and environmental objectives through coordinated demand-side management, hybrid storage utilization, and the ladder-type carbon trading market mechanism. It reshapes the system load profile via peak shaving and valley filling, improves renewable energy integration, and enhances overall system efficiency. Full article
(This article belongs to the Special Issue Hybrid Battery Energy Storage System)
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18 pages, 2433 KiB  
Article
Thermodynamic Assessment of the Pyrometallurgical Recovery of a Pb-Ag Alloy from a Mixture of Ammonium Jarosite–Lead Paste Wastes
by Jose Enrique Sanchez Vite, Alejandro Cruz Ramírez, Manuel Eduardo Flores Favela, Ricardo Gerardo Sánchez Alvarado, José Antonio Romero Serrano, Margarita García Hernández, Teresita del Refugio Jiménez Romero and Juan Cancio Jiménez Lugos
Recycling 2025, 10(4), 136; https://doi.org/10.3390/recycling10040136 - 8 Jul 2025
Viewed by 589
Abstract
A previously pyrometallurgical process, developed to obtain a Pb-Ag alloy and a slag rich in sulfur from the recycling of a mixture of industrial wastes of jarosite and lead paste, was thermodynamically assessed at 1200 °C. The industrial jarosite sourced from a Mexican [...] Read more.
A previously pyrometallurgical process, developed to obtain a Pb-Ag alloy and a slag rich in sulfur from the recycling of a mixture of industrial wastes of jarosite and lead paste, was thermodynamically assessed at 1200 °C. The industrial jarosite sourced from a Mexican zinc hydrometallurgical plant corresponded to an ammonium jarosite with a measurable silver content. The specific heat capacity (Cp) of the ammonium jarosite was obtained from TGA and DSC measurements, as well as the thermodynamic functions of enthalpy, entropy, and Gibbs free energy. The Cp was successfully modeled using polynomial regression, with a second-degree polynomial employed to describe the low-temperature behavior. The thermodynamic data generated were input into the thermodynamic software FactSage 8.2 for modeling of the lead paste–ammonium jarosite-Na2CO3-SiC system and represented by stability phase diagrams. The thermodynamic assessment of the pyrometallurgical process predicted compounds formed at high temperatures, showing that a Pb-Ag alloy and a slag rich in Na, S, and Fe (NaFeS2 and NaFeO2) were obtained. The compounds formed evidence of the effective sulfur retention in the slag, which is crucial for mitigating SO2 emissions during high-temperature treatments. The experimental compounds, after solidification, were determined by X-ray diffraction measurements to be Na2Fe(SO4)2 and Na2(SO4), which reasonably match the thermodynamic assessment. The heat capacity of the ammonium jarosite provides essential thermodynamic insights into the compositional complexities of industrial waste, which are particularly relevant for thermodynamic modeling and process optimization in pyrometallurgical systems aimed at metal recovery and residue valorization. Full article
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20 pages, 1810 KiB  
Article
Optimization of Arrangements of Heat-Storage Bricks in a Regenerative Combustion System by Tree Search
by Tsai-Jung Chen, Ying-Ji Hong, Sheng-Chuan Chung and Chern-Shuh Wang
Appl. Sci. 2025, 15(13), 7490; https://doi.org/10.3390/app15137490 - 3 Jul 2025
Viewed by 215
Abstract
When there are several different types of heat-storage ceramic bricks (checkers) that can be arranged in a regenerative combustion system, one must find an optimal arrangement (with the highest long-term Waste Heat Recovery Ratio) of these checkers, possibly of different types, in this [...] Read more.
When there are several different types of heat-storage ceramic bricks (checkers) that can be arranged in a regenerative combustion system, one must find an optimal arrangement (with the highest long-term Waste Heat Recovery Ratio) of these checkers, possibly of different types, in this regenerative combustion system. However, the number of possible arrangements of checkers in a heat regenerator could be huge. For example, when 5 different types of checkers are available for each of 14 positions in a heat regenerator, the total number of possible arrangements of checkers is 6,103,515,625. It is impractical to completely evaluate the efficiency of each of the 6,103,515,625 arrangements of checkers by 3D CFD simulations on Ansys Fluent. Here, we propose an optimization algorithm by tree search to tackle this optimization problem. This tree search method is motivated by the recent applications of Artificial Intelligence, based on combination of Deep Learning with Monte-Carlo Tree Search, to the incredibly complicated board game Go. Empirical evidence shows that this simple tree search algorithm leads to fast convergence of an optimization search and successfully suggests the optimal arrangement of checkers. This simple tree search method/algorithm may effectively enhance the thermal efficiency of a regenerative combustion system. Full article
(This article belongs to the Section Applied Thermal Engineering)
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35 pages, 5144 KiB  
Systematic Review
A Systematic Review of Two-Phase Expansion Losses: Challenges, Optimization Opportunities, and Future Research Directions
by Muhammad Syaukani, Szymon Lech, Sindu Daniarta and Piotr Kolasiński
Energies 2025, 18(13), 3504; https://doi.org/10.3390/en18133504 - 2 Jul 2025
Cited by 1 | Viewed by 354
Abstract
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 [...] Read more.
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 article belongs to the Special Issue Design and Experimental Study of Organic Rankine Cycle System)
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18 pages, 8224 KiB  
Article
Cascaded Absorption Heat Pump Integration in Biomass CHP Systems: Multi-Source Waste Heat Recovery for Low-Carbon District Heating
by Pengying Wang and Hangyu Zhou
Sustainability 2025, 17(13), 5870; https://doi.org/10.3390/su17135870 - 26 Jun 2025
Viewed by 271
Abstract
District heating systems in northern China predominantly rely on coal-fired heat sources, necessitating sustainable alternatives to reduce carbon emissions. This study investigates a biomass combined heat and power (CHP) system integrated with cascaded absorption heat pump (AHP) technology to recover waste heat from [...] Read more.
District heating systems in northern China predominantly rely on coal-fired heat sources, necessitating sustainable alternatives to reduce carbon emissions. This study investigates a biomass combined heat and power (CHP) system integrated with cascaded absorption heat pump (AHP) technology to recover waste heat from semi-dry flue gas desulfurization exhaust and turbine condenser cooling water. A multi-source operational framework is developed, coordinating biomass CHP units with coal-fired boilers for peak-load regulation. The proposed system employs a two-stage heat recovery methodology: preliminary sensible heat extraction from non-saturated flue gas (elevating primary heating loop (PHL) return water from 50 °C to 55 °C), followed by serial AHPs utilizing turbine extraction steam to upgrade waste heat from circulating cooling water (further heating PHL water to 85 °C). Parametric analyses demonstrate that the cascaded AHP system reduces turbine steam extraction by 4.4 to 8.8 t/h compared to conventional steam-driven heating, enabling 3235 MWh of annual additional power generation. Environmental benefits include an annual CO2 reduction of 1821 tonnes, calculated using regional grid emission factors. The integration of waste heat recovery and multi-source coordination achieves synergistic improvements in energy efficiency and operational flexibility, advancing low-carbon transitions in district heating systems. Full article
(This article belongs to the Section Energy Sustainability)
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20 pages, 845 KiB  
Article
Designing a Waste Heat Recovery Heat Exchanger for Polymer Electrolyte Membrane Fuel Cell Operation in Medium-Altitude Unmanned Aerial Vehicles
by Juwon Jang, Jaehyung Choi, Seung-Jun Choi and Seung-Gon Kim
Energies 2025, 18(13), 3262; https://doi.org/10.3390/en18133262 - 22 Jun 2025
Viewed by 349
Abstract
Polymer electrolyte membrane fuel cells (PEMFCs) are emerging as the next-generation powertrain for unmanned aerial vehicles (UAVs) due to their high energy density and long operating duration. PEMFCs are subject to icing and performance degradation problems at sub-zero temperatures, especially at high altitudes. [...] Read more.
Polymer electrolyte membrane fuel cells (PEMFCs) are emerging as the next-generation powertrain for unmanned aerial vehicles (UAVs) due to their high energy density and long operating duration. PEMFCs are subject to icing and performance degradation problems at sub-zero temperatures, especially at high altitudes. Therefore, an effective preheating system is required to ensure stable PEMFC operation in high-altitude environments. This study aimed to mathematically model a shell-and-tube heat exchanger that utilizes waste heat recovery to prevent internal and external PEMFC damage in cold, high-altitude conditions. The waste heat from the PEMFC is estimated based on the thrust of the MQ-9 Reaper, and the proposed heat exchanger must be capable of heating air to −5 °C. As the heat exchanger utilizes only waste heat, the primary energy consumption arises from the coolant pumping process. Calculation results indicated that the proposed heat exchanger design improved the overall system efficiency by up to 15.7%, demonstrating its effectiveness in utilizing waste heat under aviation conditions. Full article
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