Search Results (85)

Adaptive feedback can destabilize a loop that would be stable under any fixed gain, so the speed at which the gain adapts is itself a design parameter. We study this effect in a minimal three-dimensional model motivated by the photocatalytic degradation of microplastics: a pollutant concentration is driven toward a setpoint by an ultraviolet (UV) actuator whose gain adapts online. The model has a single bilinear nonlinearity, so the local analysis can be carried out in closed form. Under an explicit feasibility condition, the system has a unique positive equilibrium. The Routh–Hurwitz criterion shows that this equilibrium is locally asymptotically stable below an explicit critical adaptation speed ${\kappa }_c$ and unstable above it. At $\kappa ={\kappa }_c$, a purely imaginary eigenvalue pair crosses the imaginary axis transversally, and a Hopf bifurcation occurs, with an explicit onset frequency. The first Lyapunov coefficient is computed in closed form; it separates a supercritical onset, for well-damped actuators, from a subcritical onset with hysteresis, for weakly damped actuators. Numerical experiments confirm the predicted limit cycle and the classification. All the stability results established here are local. Full article

As the global energy sector and maritime industry transition toward carbon neutrality, Carnot batteries have emerged as a critical technology for flexible, long-duration energy management. Carnot batteries, which operate on a Power-to-Heat-to-Power principle, present a highly promising alternative to conventional electrochemical batteries. The present study provides a review of Carnot battery architectures, systematically evaluating their thermodynamic cycles, working fluids, Thermal Energy Storage media and key turbomachinery components. A comparative assessment of the current literature illustrates that system selection requires balancing round-trip efficiency, Energy Storage Density and Technology Readiness Level. According to the findings of the present study, high-temperature Brayton cycles offer robust baseline efficiencies of 60–80% whereas subcritical Rankine cycles benefit from commercial maturity and can achieve efficiencies exceeding 200% when integrated with cryogenic heat sinks like LNG. It should be clarified that efficiency values exceeding 100% represent “Apparent Round-Trip-Efficiencies (RTE)” resulting from the thermodynamic contribution of external exergy streams, such as LNG cryogenic cold, rather than standalone cycle efficiencies, which are strictly below 100%. In addition, volumetric energy density varies drastically based on the physical phase of the storage medium, scaling from under 1 kWh/m³ for unpressurized water to over 385 kWh/m³ for advanced thermochemical systems. Although most configurations currently remain in the prototyping phase, the technology holds transformative potential for the maritime sector. Carnot batteries can deliver a self-contained, zero-emission electrical power supply to cover the vessel’s electrical load requirements during harbor stays and transit within Emission Control Areas (ECAs) by dynamically capturing and storing shipboard waste heat during open sea transit. Full article

Resistance spot welding (RSW) is the primary joining technology for automotive advanced high-strength steels (AHSSs), where the inherent severe thermal cycles profoundly alter the heat-affected zone (HAZ) microstructure, leading to localized variations in mechanical properties. Characterizing the spatial gradients in microstructure and the resulting mechanical properties remains a major challenge in weld failure analysis due to the small size of the HAZ and its complex thermal history. In this study, the HAZ of two prominent AHSS grades, a first-generation press hardening steel (PHS1500) and a third-generation quenching and partitioning steel (Q&P1180), was physically simulated using a Gleeble^® 3180 thermomechanical simulator to achieve precise control over the localized thermal cycles. The investigation first evaluated the role of thermal cycle duration, governed by the welding time parameter (300 ms vs. 800 ms), on the microstructural evolution of the PHS1500 steel. Increasing the weld time from 300 ms to 800 ms reduced the cooling rate under the nominal 1400 °C condition from approximately 3000 K/s to 2500 K/s; however, no marked change was observed in the overall microstructural and hardness trends within the investigated range. Subsequently, using the 300 ms thermal profile as a reference baseline, a comparative metallurgical study was conducted between PHS1500 and Q&P1180. Under the same 300 ms thermal history, the maximum hardness reduction relative to the corresponding base material was approximately 42% for PHS1500 and 12% for Q&P1180. The hardness minima were located within FE-estimated temperature ranges close to the Ac₁ region for PHS1500 and around 600 °C for Q&P1180, respectively. This comparison highlighted the distinct microstructural responses of the two generations across the upper-critical (UCHAZ), inter-critical (ICHAZ), and sub-critical (SCHAZ) zones. Moreover, microhardness profiles were correlated with the microstructural findings, establishing a correlation among the simulated thermal history, the observed microstructural evolution, and localized mechanical performance. Full article

This study presents a green approach for polyethylene terephthalate (PET) upcycling using a biphasic system of subcritical water and castor oil. This system enables efficient conversion without an external catalyst and facilitates product separation. Hydrolysis of castor oil generates fatty acids in situ, which catalyze PET conversion to selectively produce low-molecular-weight oligoesters (Mn ≈ 1500 g/mol). These oligoesters are inherently immiscible with the bio-medium at room temperature, allowing straightforward separation by centrifugation. Orthogonal experiments show that temperature is the dominant factor affecting PET conversion, with the optimal conditions being 200 °C, a water-to-oil mass ratio of 1:5, and a reaction time of 10 h for complete conversion. Under the practical optimum condition, the castor oil phase remained highly effective over five consecutive depolymerization cycles. The functionalized oligoesters were used in polyurethane synthesis. At 6% loading, they gave adhesives with a T-peel strength of 8.09 N/15 mm and tensile strength of 25.14 MPa, and excellent damp-heat stability (only 0.62% loss in T-peel and 0.47% in 180° peel after aging). Thermogravimetric analysis confirmed enhanced thermal stability, with increases of 19.52 °C in T_5% and 16.50 °C in T_50% compared with the unmodified system. These results demonstrate the practical potential of the obtained oligoesters in high value adhesive applications. Full article

This study presents a comparative thermodynamic assessment of four widely used CO₂ refrigeration configurations, namely, the basic cycle (BC), internal heat exchanger cycle (IHEX), mechanical subcooling cycle (MSC), and ejector cooling cycle (ECS), operating under both subcritical and transcritical conditions. The investigated systems were analyzed using validated numerical models developed in the Engineering Equation Solver (EES) under evaporating temperatures ranging from −30 °C to +5 °C and gas cooler temperatures ranging from 30 °C to 50 °C. For each operating condition, the refrigeration cycles were thermodynamically optimized in order to maximize the coefficient of performance (COP). The results indicate that an increasing gas cooler temperature significantly reduces the COP of all investigated systems, whereas an increasing evaporating temperature improves cycle performance. Among the investigated configurations, the MSC system exhibited the highest thermodynamic performance improvement, particularly under severe transcritical operating conditions characterized by high gas cooler temperatures and low evaporating temperatures. The ECS configuration also provided considerable performance enhancement by reducing throttling-related thermodynamic losses and compressor pressure ratio. In contrast, the IHEX configuration yielded comparatively moderate but relatively stable performance improvement with lower system complexity. In addition to the thermodynamic comparison, a simplified engineering-oriented practical assessment framework based on a relative cost index (RCI) was introduced to comparatively evaluate implementation complexity, control requirements, maintenance considerations, and relative investment burden of the investigated systems. The results indicate that, although the MSC configuration provides the highest thermodynamic performance, it is also associated with the highest implementation complexity and relative investment requirement, whereas the IHEX configuration offers a simpler and lower-cost alternative with moderate performance enhancement. The present study provides engineering-oriented comparative guidance regarding the thermodynamic performance, practical applicability, and operational suitability of advanced CO₂ refrigeration systems under varying climatic and operational conditions. Full article

The introduction of sustainable practices into waste management can have a favorable environmental impact, increase resource value, and yield economic gains. Hydrothermal technologies have strong potential for the production of up-cycled ingredients from biowaste (amino acids, sugars, phenols, pharmacologically active compounds, etc.), enabling high energy recovery (50–80%) from biowaste with net-negative carbon emissions. This review discusses the use of subcritical and supercritical water technologies for sustainable valorization of biowaste and conversion of biomass into high-value chemicals and biofuels. The potential for the extraction/generation of bioactive compounds from plant and animal waste is presented, emphasizing the efficiency, compound stability, and bioactivity of the fractions obtained. The possibilities of simultaneous extraction of added-value compounds and hydrolysis of feedstock biopolymers by these technologies are elaborated. The review further addresses the production of biofuels through hydrothermal carbonization for solid fuels, hydrothermal waste liquefaction for liquid fuels, and supercritical water gasification for gaseous fuels. The paper highlights the environmental and economic advantages of technologies based on sub- and supercritical water over conventional chemical and fermentative routes, emphasizing their contribution to a circular bioeconomy by converting biowaste into value-added products and sustainable energy sources. Full article

To improve the flexibility of coal-fired power units and support renewable energy integration, molten salt thermal storage has been widely considered a promising retrofit option. However, under unified operating conditions, the comparative effects of different heat-release pathways and steam extraction ratios on flexibility, full-cycle thermodynamic performance, and economic performance have not been sufficiently clarified. In this study, a thermodynamic model of a 600 MW subcritical coal-fired power unit coupled with a two-tank molten salt thermal storage system was established in Ebsilon and validated against the design heat-balance data under typical load conditions, with maximum relative deviations of 0.06% for power output, 0.95% for main steam flow rate, and 1.24% for heat consumption rate. Three representative heat-release pathways were comparatively investigated under identical heat-storage conditions, with steam extraction ratios ranging from 2% to 18%. The results show that increasing the extraction ratio raises the thermal storage capacity from 9.762 to 84.636 MWh and enhances the downward peak-shaving capability, but weakens the full-cycle thermodynamic performance. Among the three schemes, Scheme 2 exhibits the strongest upward peak-shaving performance, with upward peak-shaving energy increasing from 2.893 to 24.395 MWh, and also yields the highest annual net profit (0.546–4.342 million CNY). Scheme 3 exhibits the best full-cycle thermal and exergy efficiencies, with full-cycle thermal efficiency of 42.76–41.56% and full-cycle exergy efficiency of 38.34–37.27%. In addition, Schemes 1 and 2 show significantly higher round-trip efficiencies than Scheme 3, with Scheme 2 becoming more advantageous at higher extraction ratios. Scheme 1 exhibits the shortest static payback period (7.12–7.63 years) and the highest internal rate of return (12.77–11.65%). These results indicate that the three schemes have distinct advantages in peak-shaving performance, full-cycle thermodynamic performance, and economic performance, and provide a comparative basis for engineering selection and parameter optimization of molten-salt-based flexibility retrofits in coal-fired power units. Full article

In this paper, we investigate the multi-scale dynamics of a singularly perturbed Rosenzweig–MacArthur model with a generalist predator and identify dynamical phenomena, including equilibrium bifurcations, supercritical or subcritical singular Hopf bifurcations, canard explosion bifurcations and homoclinic bifurcations. Specifically, the system exhibits a globally stable node, a headless canard cycle evolving into a homoclinic cycle, a headed canard cycle encompassing either a headless canard cycle or a homoclinic cycle, and so on. Notably, near the boundary equilibrium, these cycles exhibit a diminutive beard-shaped structure whenever it aligns with the transcritical non-normally hyperbolic point. The numerical simulations confirm the occurrence of a canard explosion, relaxation oscillation, and an inverse canard explosion phenomena not previously reported in singularly perturbed systems with both a transcritical point and a canard point. In brief, our results demonstrate that the generalist predation can cause richer bifurcations and dynamics. Full article

During the operation of nuclear power plants, nuclear fuel undergoes significant compositional changes. After several cycles of use, the fuel must be removed and stored. Currently, spent fuel is stored mainly in pools or casks, and it is necessary to demonstrate the subcriticality of these systems. Spent nuclear fuel has a complex composition, and because computational codes are typically validated using fresh-fuel experiments, subcriticality assessments are usually performed conservatively with fresh-fuel compositions. These approaches demonstrate subcriticality but are very conservative and can lead to storage system designs that are more expensive or have reduced capacity. This paper focuses on the validation of computational codes using nuclear power plant critical start-up tests (referred to as reactor criticals). These tests include spent fuel and are well documented, allowing them to serve as validation experiments. Codes validated using reactor criticals can be applied to systems containing spent fuel calculation if sufficient similarity is demonstrated. Similarity is evaluated using the SCALE TSUNAMI-IP module, which is widely used for this purpose. Based on a database containing dozens of reactor criticals and similarity analyses, we developed a methodology for demonstrating the subcriticality of spent-fuel storage systems. Full article

Biopolymers from renewable sources are increasingly explored to reduce the carbon footprint of materials and mitigate plastic pollution. This review synthesizes the last five years of progress across the biopolymer value chain, comparing plant, microbial/fermentation, fungal, and marine/algal resources and critically assessing greener extraction and fractionation routes (ultrasound and microwave intensification, subcritical water, supercritical CO₂ with co-solvents, ionic liquids, deep eutectic solvents including natural deep eutectic solvents, and enzymatic or bio-mediated processes). We emphasize yield-selectivity trade-offs, scalability, energy demand, and solvent recovery. Downstream, we summarize purification and performance tuning via crosslinking, derivatization, blending/plasticization, and nanocomposites, and we map advanced characterization to targeted functional properties to bridge processing choices with end-use performance. Applications are organized across food and agriculture, biomedical and pharmaceutical technologies, packaging, and cosmetics, with cross-cutting attention to safety and regulatory compliance, quality-by-design, techno-economics, and life-cycle assessment. Key bottlenecks are feedstock variability, viscosity and recyclability limitations of designer solvents, and persistent gaps in barrier and thermal properties versus petrochemical benchmarks, compounded by uneven composting and recycling infrastructure. Promising directions include low-viscosity or switchable solvents, data- and artificial intelligence (AI)-guided process optimization, engineered biopolymers, and circular end-of-life strategies that align material design with realistic recovery routes. Full article

The paper presents a comparative economic and environmental assessment of high-temperature steam turbine technologies (subcritical, supercritical, ultra-supercritical, and advanced ultra-supercritical cycles) within the energy transition. The research employs a model-based analysis to evaluate the cost of electricity production across countries with divergent environmental, social and governance (ESG) strategies, reflected in their carbon pricing mechanisms. The developed model estimates the economic feasibility and optimal timing for the transition to high-efficiency technologies, based on the projected fuel cost dynamics. Within the framework of the model, the optimal energy transition timings for implementing advanced ultra-supercritical steam turbine technologies were established: 2031 for the energy transition model in the Russian Federation (a country with developing ESG principles) and 2018 for the model in the Czech Republic (a country with an emerging ESG strategy). The results indicate that while carbon pricing mechanisms influence economic feasibility, hydrocarbon fuel costs remain the predominant factor. The study concludes that the enhancement of conventional generation technologies aligns with all three pillars of the ESG framework and facilitates the transition to a sustainable development model for the energy sector and the national economy. Full article

In line with the circular economy approach and the pursuit of sustainable solutions for spent coffee grounds, this study investigates the valorization of spent coffee grounds as a source of cellulose-based enzyme immobilization carriers. Considering that global coffee consumption generates approximately 6.9 million tonnes of spent coffee grounds annually, their disposal represents both an environmental challenge and an opportunity for value-added applications. A multistep extraction process, including Soxhlet extraction followed by sequential subcritical extraction with ethanol and water, and alkaline treatment, led to the production of cellulose-enriched carriers. The carriers obtained were characterized by their morphology, porosity and surface properties and subsequently used for the two lipases immobilization, Burkholderia cepacia (BCL) and Pseudomonas fluorescens (PFL), using three techniques: adsorption and covalent binding via direct and indirect methods. The immobilized lipases were analyzed for key biochemical and operational properties and compared with each other and with their free enzymes. Based on their stability, catalytic activity, and reusability, the lipases immobilized by adsorption were identified as the most efficient biocatalysts. These immobilized enzymes were then used in two selected reactions to demonstrate their practical utility: cocoa butter substitute synthesis using PFL and the enzymatic pretreatment of wastewater from the oil processing industry using BCL. Both immobilized lipases showed excellent catalytic performance and maintained their high activity over four consecutive reuse cycles. Full article

Concentrated solar power technology offers an effective pathway for large-scale renewable electricity generation in areas with abundant solar resources. This study develops and evaluates a cascaded multigeneration system that integrates a solar power tower with a subcritical-CO₂ Brayton cycle, a bottoming Rankine cycle, and a multi-effect distillation unit designed for Mediterranean conditions. The system achieves a maximum net power output of 23.48 MW and a freshwater production rate of 14.25 kg/s during peak summer conditions. The analysis reveals that high solar availability enables CO₂ mitigation of up to 27,434.55 kg-CO₂/h, thereby emphasizing the strong environmental benefits of the proposed system. The integrated Sb-CO₂–Rankine–Desalination configuration attains overall thermal efficiencies of 0.39–0.43, while exergy efficiencies reach 0.58 and 0.73 for the Sb-CO2 and Rankine subsystems, respectively. Sensitivity analysis further confirms the strong pressure dependence of the Sb-CO₂ cycle, with an optimum operation condition around 10 bar and 1100 °C, whereas the Rankine cycle exhibits a steady efficiency enhancement with increasing pressure. The proposed system achieves a simple payback period of 15.3 years, demonstrating its economic feasibility under Mediterranean climatic conditions. Full article

To fully recover abundant waste heat and reduce the operation cost in liquid-cooled data centers, a Carnot battery consisting of a heat pump (HP) and organic Rankine cycle (ORC) is proposed. Due to the existence of different cycle states for HPs and ORCs, four different cycle combinations are considered. To evaluate and compare their performances, thermo-economic models are developed. Under the design conditions, the optimal working fluid combinations are first determined for each battery. On this basis, thermodynamic and economic performances of the four batteries are analyzed in detail. The results indicate that the system consisting of a subcritical HP/transcritical ORC achieves the highest round-trip efficiency at 76%. Notably, the round-trip efficiency of the system can exceed 100% at low ORC condensing temperatures. Additionally, the system cost is about 767–796 USD/kW∙h, depending on the cycle combinations. Furthermore, the effects of operating parameters on system performances are also investigated. Finally, with the objective of maximum round-trip efficiency, key parameters of four batteries are optimized. The results reveal that the system with a subcritical HP/subcritical ORC attains a maximum round-trip efficiency of 83% after optimization. These research results contribute to the development of green data centers and the reduction of power costs. Full article

The recycling of carbon-fiber reinforced polymers (CFRPs) presents significant challenges due to their thermosetting matrix, which complicates end-of-life management and often results in energy-intensive disposal or significant waste accumulation. Despite advancements in recycling methods, knowledge gaps remain regarding their sustainability and economic viability. This study undertakes a comprehensive Life Cycle Assessment and Environmental Life Cycle Costing analysis of four key recycling techniques: mechanical recycling, pyrolysis, solvolysis, and high-voltage fragmentation (HVF). By using the SimaPro software, this study identifies mechanical recycling and HVF as the most sustainable options, with the lowest cumulative energy demand (CED) of 5.82 MJ/kg and 4.97 MJ/kg and global warming potential (GWP) of 0.218 kg CO₂eq and 0.0796 kg CO₂eq, respectively. In contrast, pyrolysis imposes the highest environmental burdens, requiring 66.3 MJ/kg and emitting 2.84 kg CO₂eq. Subcritical solvolysis shows more balanced environmental impacts compared to its supercritical counterpart. Cost analysis reveals that for mechanical recycling and pyrolysis, material costs are negligible or zero. In contrast, solvolysis and HVF incur material costs primarily due to the need for deionized water. Regarding energy costs, pyrolysis stands out as the most expensive method due to its high energy demands, followed closely by solvolysis with supercritical water. Full article