Thermo, Volume 6, Issue 1 (March 2026) – 22 articles

Cogeneration system optimization in refineries confronts the challenge of simultaneously integrating design parameter selection and topological configuration. The literature typically addresses these aspects separately: parametric optimization with fixed topology or configuration optimization for specific nominal conditions. This work develops a comprehensive methodology integrating exhaustive parametric exploration with superstructure-based optimization through mixed-integer nonlinear programming (MINLP), applied to the Miguel Hidalgo refinery in Tula, Mexico. The systematic procedure generates superstructures considering all viable expansion and tempering routes under steam quality restrictions ($x\ge 0.88$), evaluating 84–105 combinations of generation pressure ($P_{\mathrm{HRSG}}=70$–140 bar) and superheater outlet temperature ($T_{s_4}=500$–560 °C). The analysis reveals three topologically distinct configurations identified as generating maximum power under different operating conditions and characterizes how transitions between high-performing configurations occur at discrete thermodynamic thresholds that correlate with constraint activation contradicting the conventional assumption of continuous parameter-configuration relationships. Multi-criteria evaluation positions Configuration 1 as the recommended design, generating 25% increase in electric generation, 11% improvement in utilization factor ($UF$: $0.640\to 0.710$) and 20% reduction in specific fuel consumption ($SFC$: $0.259\to 0.207$ kg/kWh). The methodology is directly generalizable to other refineries through universal thermodynamic principles, with a systematic five-step procedure applicable to any multi-pressure steam demand profile. The characterization of discrete transition phenomena and the associated methodology for their thermodynamic explanation challenges the conventional assumption of continuous parameter–configuration relationships in optimization approaches, with immediate implications for the design of flexible cogeneration systems in refineries worldwide. Full article

Carnot batteries based on the coupling of high-temperature heat pumps (HTHPs) and Organic Rankine Cycles (ORCs) emerge as promising solutions for large-scale and long-duration energy storage, enabling sector coupling and the valorization of industrial waste heat. In such systems, the charging subsystem plays a dominant role, as variations in heat pump performance influence the round-trip efficiency more strongly than comparable variations in the ORC. This work presents a thermodynamic assessment of Rankine-based HP–ORC Carnot batteries focusing on the influence of heat pump configuration and working fluid selection. System performance is evaluated using the heat pump coefficient of performance, volumetric heat capacity, ORC efficiency, and Carnot battery round-trip efficiency through a grid-search optimization over a wide range of storage outlet and waste heat source temperatures. The results show that single-stage configurations are optimal at low to moderate temperature lifts, while two-stage and cascade systems become advantageous at higher lifts. Among the investigated fluids, R-601 provides the highest round-trip efficiencies at elevated storage temperatures, whereas R-600 enables more compact systems due to its higher volumetric heat capacity. These findings provide design guidance for selecting heat pump configurations and working fluids in industrial waste-heat-assisted Carnot battery applications. Full article

Ensuring the required microclimate parameters is the most critical task in hot climates. In pig farms, air cooling is provided by means of steam-compression chillers or evaporative cooling, which is the simplest way to cool the air. The implementation of evaporative cooling depends largely on the interaction of the media involved in this process. This paper considers the process of interaction of cooling water with the surface of a cellular polycarbonate heat exchanger. A mathematical model describing the process of wetting the sprayed surface of the heat exchanger is obtained. The authors determined the theoretical water flow rate required to provide air cooling for a given operation mode. Experimental trials of a recuperative heat recovery unit with a heat exchanger made of cellular polycarbonate equipped with a water evaporative cooling system were carried out. The authors conducted a comparative assessment to evaluate the effectiveness of evaporative cooling in a heat recovery unit equipped with a polycarbonate heat exchanger versus panel evaporative systems using wetted paper pads at pig farms in the Vladimir and Tambov regions of Russia. The panel evaporative coolers provided a temperature reduction of 11.3 °C without any splashing effect. Under the same operating conditions, the heat recovery unit achieved an inlet air temperature reduction of 10.5 °C, accompanied by splashing. When the water flow rate supplied for evaporation was reduced until the splashing ceased, the cooling temperature drop decreased to 10.1 °C, which is 11% lower, compared with the paper pads. The study revealed characteristic operating modes for the unit that ensure effective air cooling, depending on the cooling water flow rate. Since the prevailing temperature during the system’s main operating time is significantly lower than the design temperature (the absolute temperature maximum), to achieve effective cooling of the supply air without splashing or excessive water waste, the cooling circuit water should circulate at a flow rate within 40 to 63% of the maximum design value. Alternatively, an automated control system should be employed to regulate the water supply based on outdoor air temperature and humidity. Full article

Based on Jaffer’s (2023) heat engine analysis of natural convection, this investigation mathematically derives a novel, comprehensive formula predicting the natural convective heat transfer from an inclined cylinder given its length, diameter, angle, and Rayleigh number and the fluid’s Prandtl number and thermal conductivity. The present formula was tested with 93 inclined cylinder measurements having length-to-diameter ratios between 1.48 and 104 in nine data-sets from three peer-reviewed studies, yielding (data-set) root-mean-squared relative error values between 1.9% and 4.7%. Full article

While twisted tape inserts are widely used for heat transfer enhancement, the specific impact of tape thickness remains under-explored. This study provides a systematic numerical investigation into the thermo-hydraulic performance of a double-pipe heat exchanger equipped with twisted perforated tape (TPT) inserts of varying thicknesses (1, 1.5, and 2 mm). Using a validated 3D SST k−ω model across Re = 1000–12,000, the research establishes a mechanistic distinction between flow regimes. The results indicate that the 2 mm TPT yields the highest enhancement, achieving a 78.6% increase in the average Nusselt number (Nu_avg) and a 67.8% improvement in the overall heat transfer coefficient at Re = 12,000. Quantitative analysis of secondary flow intensity and turbulence kinetic energy confirms a transition from geometry-induced swirl at low Re to turbulence-driven shear at high Re. Despite a pressure drop penalty of up to 3.26 times the plain tube, the thermal performance factor remained above unity for all cases, peaking at 1.17 at Re ≈ 4000. These findings establish tape thickness as a first-order design variable for optimizing high-performance thermal systems. Full article

Thermodynamic cycles used with external combustion are typically based on compression, heating, expansion and cooling, admitting variants to enhance efficiency or power. This paper carries out a thorough theoretical study of isochoric heating and non-adiabatic expansion processes and proposes a new thermodynamic cycle based on three instead of four stages. The compressor is removed because the working fluid (a gas) is pressurized by heating it isochorically. A novel concept of an engine is proposed (patent ES2992009A, WO 2025/257447), and it shows potential for power generation that has to be explored. Full article

Thermal stratification strongly affects the efficiency and operational reliability of sensible thermal energy storage (TES) tanks in energy systems. This study numerically investigates the combined influence of inlet configuration and mass flow rate on the charging performance of a vertical cylindrical TES tank (H = 3 m, D = 1 m) using transient CFD simulations. Five inlet designs—open, orifice, groove, shower, and shower-groove are analyzed at three flow rates: $Q_1$ = 0.0003 m³/s, ${Q_2=Q}_1/2$, and $Q_3=Q_1/3$. System performance is evaluated using key thermal and stratification metrics. Increasing the flow rate from $Q_3$ to $Q_1$ enhances convective heat transfer and energy and exergy efficiencies, but significantly intensifies mixing and degrades thermal stratification. At $Q_1$, the groove inlet achieves the highest capacity ratio and exergy efficiency (0.87), while exhibiting increased mixing. Reducing the flow rate to $Q_2$ and $Q_3$ limits inlet-induced momentum, leading to improved stratification for all configurations. The shower-groove inlet reaches a maximum stratification level (tail factor) of 1.13 at $Q_3$, indicating superior thermal layering, albeit with lower energetic efficiency (≈0.40–0.45). The groove inlet provides the best overall compromise at $Q_2$, combining high efficiency with stable stratification. These results demonstrate a clear efficiency-stratification trade-off and highlight the importance of selecting inlet-flow combinations according to application-specific objectives. Full article

The rapid evaporation of liquid droplets across a normal shock wave is a phenomenon of critical importance in advanced propulsion and clean energy systems, such as NH₃ supersonic separation. The conventional Spalding d²-law is commonly used to model such phenomena, but it is not suitable for predicting the complete vaporization of sub-micron droplets, particularly as evaporation approaches the free-molecular regime. To address this issue, this paper introduces a novel time-dependent one-dimensional CFD model, which is used to analyze the shock structure, the non-equilibrium heat and mass transfer between the liquid and gas phases, and the evolution of the droplets’ size through the shock. The model describes the evaporation of NH₃ sub-micron droplet sprays across a stationary normal shock for various fractions of the liquid phase. The Gyarmathy evaporation model is utilized to accurately account for the transition from diffusion-governed to free-molecular regimes, alongside a new two-phase Rankine–Hugoniot shock jump formulation. The study reveals the influence of a steady normal shock on the physical structure of a droplet-laden flow, including the existence of an initial droplet size swelling through the shock, and quantifies the subsequent complete evaporation of the suspended droplets. The maximum swelling throughout the shock is up to 17%, which corresponds to the case with 8% liquid phase mass fraction in the flow. The model provides acceptable accuracy in calculating the two-phase parameters in high-speed flows and can be extended for modeling more complex, multidimensional detonation and propulsion systems. Full article

Thermochemical conversion of plastic waste to hydrogen and synthesis gas represents a potential pathway for energy recovery from heterogeneous waste streams. The feasibility and performance of such systems are fundamentally governed by thermodynamic constraints and heat-management requirements. This review critically examines the thermodynamic and heat-integration aspects of plastic waste conversion to hydrogen and syngas, with emphasis on pyrolysis, steam reforming, gasification, and system-level behaviour. Key thermodynamic features of plastic pyrolysis, reforming, and gasification are discussed, including reaction endothermicity, equilibrium limitations, temperature effects, and product distribution trends. The role of steam reforming and water–gas shift reactions in enhancing hydrogen yield is assessed from equilibrium and energy-demand perspectives. Heat integration emerges as a critical determinant of overall efficiency, with recoverable waste heat present at multiple process stages offering opportunities for internal heat recovery. Energy and exergy analyses identify dominant sources of irreversibility and enable comparison of plastic-derived hydrogen systems with conventional thermochemical hydrogen production routes. Quantitatively, conventional steam methane reforming achieves energy efficiencies of 65–75% and exergy efficiencies of 60–70%, whilst plastic-derived systems without extensive heat integration report 45–60% and 40–55%, respectively. Key challenges include limited thermodynamic property data for real plastic-derived mixtures, insufficient reconciliation of equilibrium and kinetic behaviour, incomplete system-level heat-integration analysis, and scarcity of comprehensive exergy-based evaluations. This review provides a thermodynamic framework for assessing the opportunities and limitations of hydrogen production from plastic waste. Full article

The integration of Foamed Concrete (FC) into 3D Concrete Printing (3DCP) processes facilitates the design of energy-efficient building envelopes. However, strategies for optimizing material porosity and printing topology to balance winter and summer performance remain underexplored. This study presents a 2D numerical thermal analysis of an innovative 3D-printed building envelope block characterized by sinusoidal internal partitions. Through a parametric variation in porosity (ranging from 10% to 50%) and internal geometry (amplitude and period of the partitions), 45 distinct configurations were simulated. Performance was evaluated by calculating the steady-state thermal transmittance (U) and the periodic thermal transmittance (Y_ie) under dynamic climatic conditions. The results demonstrate that porosity is the governing parameter; increasing porosity from 10% to 50% reduces U by 31% and, contrary to traditional assumptions for massive structures, also improves Y_ie by 12.3%. These outcomes are physically driven by the drastic reduction in thermal conductivity, which overcompensates for the loss of thermal mass, leading to a net reduction in overall thermal diffusivity. While internal topology plays a secondary role, its optimization allows for fine-tuning dynamic damping without compromising insulation. The study confirms that 3D printing with foamed concrete enables the overcoming of the traditional trade-off between insulation and thermal inertia. High-porosity configurations (50%) with optimized internal topology emerge as the most effective solution, simultaneously guaranteeing beneficial steady-state and dynamic thermal performance for sustainable buildings. Full article

Photovoltaic–thermal (PVT) collectors often experience limited heat extraction under laminar cooling conditions, and the influence of controlled flow pulsation on full-scale PVT performance has not been clearly established. This study experimentally investigates a water-cooled PVT system operated under pulsating flow using an indoor solar simulator to quantify its thermal and electrical response. Flow pulsations were generated using a solenoid valve at frequencies of 0.25, 0.5, 1, and 2 Hz across inlet flow rates of 1–4 L/min, with average irradiance maintained between 700 and 800 W/m². System performance was benchmarked against uncooled and continuous-flow reference cases. Pulsating operation reduced the PVT surface temperature and produced a clear enhancement in thermal performance relative to continuous flow, while electrical efficiency exhibited a smaller but consistent improvement that followed the same thermal trend. A pulsation frequency of 0.5 Hz yielded the most favorable results, achieving thermal efficiencies exceeding 50% at higher flow rates without any measurable increase in average pressure drop. Electrical efficiency stabilized at approximately 9.82%, slightly higher than that obtained under continuous-flow operation. The results indicate that low-frequency pulsating flow can significantly improve thermal energy extraction in PVT systems under controlled conditions, with modest associated electrical gains, and provide a basis for further investigation of flow-modulation strategies for thermally driven PVT applications. Full article

Recently, different techniques have been proposed for the scheduling and loading problems in cooling plants with chillers in a parallel configuration. Two broad groups can be considered: the online control-based group and the offline optimization-based group. The first group is exemplified by Model Predictive Control, where the selection of control moves provides a solution to both scheduling and loading. The second group includes Optimal Chiller Loading and Optimal Chiller Sequencing algorithms. They usually derive operating plans with some lead time in a batch-like fashion for long horizons. Both groups use forecasts of important factors such as the cooling demand and ambient conditions; hence, they have to deal with inaccuracies in the forecasts. In this paper, a comparison among these two groups is made considering demand uncertainties. The severity of the uncertainty is shown to play a role in the results as well as the controller tuning in the case of the predictive approach. The results are favorable to OCS with respect to overall consumption (up to 15%) but uses more on/off changes in the chiller’s operation (double in some cases). Full article

The development of deep eutectic solvents (DESs) is a key issue for the realization of green and efficient metal extraction processes. The present study aims to experimentally construct the phase diagram of the binary system consisting of tri-n-octylphosphine oxide (TOPO) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (HBTA) and, thus, determine its eutectic composition for the solvent extraction of Li⁺. Differential scanning calorimetry was used to characterize the phase transitions (melting temperatures and enthalpies) over the entire composition range of the binary mixture. Its eutectic composition was established at HBTA:TOPO mass ratio of 60:40. For further validation of the eutectic composition from the experimentally measured thermal effects for melting of different HBTA:TOPO mass ratios, a Tammann diagram was also constructed. Only mixtures with HBTA:TOPO mass ratios of 70:30, 60:40 (eutectic composition), and 50:50 were liquids at 30 °C, while at room temperature of 25 °C, the 70:30 mixture formed crystals. All three mixtures, which were liquids at 30 °C, were found to extract Li⁺ effectively. However, at a room temperature of 25 °C, only the eutectic mixture (60:40 mass ratio) extracted Li⁺ effectively, while the mixture with HBTA:TOPO mass ratio of 50:50 formed crystals when mechanically agitated and, therefore, was deemed as unsuitable for Li⁺ extraction. Full article

A firewood stove’s combustion chamber can withstand temperatures of 1500 °C. To prevent the deterioration of a firewood stove due to excessive heat, it is necessary to use thermal insulation materials that stop heat transfer to the walls. These materials must be economical and durable. This work examines the materials used in the construction of combustion chambers of firewood stoves in southern Mexico and Central America. This field study collects information and samples of materials used in the manufacture of firewood stoves. Heat transfer experiments are conducted, and the thermal properties of each material are analyzed. As a result, methodology and information is provided for the manufacture of future plancha-type firewood stoves used in the study area, such as pine wood (pinus chiapensis) which is mainly used as casing for firewood stoves in coniferous forest areas; in addition, the use of wood ash as thermal insulation material is proposed since it does not present direct costs and has a thermal conductivity between 0.10 and 0.20 W/m°C and a melting point greater than 1500 °C. The next layer proposed is hollow brick, a high-temperature-resistant material that can be used as support due to its mechanical strength and low thermal conductivity of 0.6 W/m°C. Finally, the use of calcium hydroxide as a coating material is proposed, applied in the form of a paste or paint to detail the imperfections of the combustion chamber construction as it resists temperatures above 1000 °C. Full article

Double complex salts (DCSs) of the composition [Co(NH₃)₆][Fe(CN)₆] are a promising precursor for the preparation of catalysts for the hydrogenation of carbon oxides (CO and CO₂) by Fischer–Tropsch synthesis. The specific surface area is an important parameter for catalysts. Our article investigates the influence of mechanochemical activation (MCA) on this DCS in order to determine the conditions for obtaining the largest specific surface area of the intermetallic compound, a product of the DCS thermolysis. In this work, the effect of MCA on the physicochemical properties of the DCS [Co(NH₃)₆][Fe(CN)₆] and the products of its thermal decomposition in an argon atmosphere were investigated. It was shown that MCA leads to partial reduction of Fe⁺³ to Fe⁺², changes in the coordination of ammonia, amorphization of the structure and a decrease in the thermal stability of DCS. Thermolysis at 650 °C of samples subjected to MCA for 10 min results in the formation of nanocrystalline intermetallic compound Co_0.5Fe_0.5. The results demonstrate the potential of using MCA to control the properties of functional materials based on DCS. Full article

This work presents a straightforward procedure for constructing the solution to the steady-state energy-transfer process in a system of two convex, opaque, gray bodies, with the aim of determining the temperature distribution within these bodies when separated by a vacuum. The methodology proposed in this work combines a sequence of elements that are functions obtained from the solution of uncomplicated, well-known linear, uncoupled heat transfer problems, thereby enabling solutions to be obtained using tools found in basic engineering textbooks. Specifically, these well-known problems resemble classical conduction-convection heat transfer problems, in which the boundary condition is described by the noteworthy Newton’s law of cooling. The limit of sequences of elements that are solutions to straightforward linear problems corresponds to the original, complex, coupled nonlinear problem. The convergence of these sequences is mathematically proven. The phenomenon (considered in this work) encompasses those involving black bodies. Since each element of the sequence arises from a well-known linear problem, numerical approximations can be used to obtain it, yielding a simple and powerful tool for simulations. Some presented results highlight the importance of considering thermal interaction between the two bodies, even in the absence of physical contact. In particular, the alterations in the temperature distributions of two separate gray bodies are explicitly shown to result from their thermal interaction. Full article

To balance the quantity of heat generated and consumed, thermal energy storage systems are crucial for power plants and district heating systems. Particularly when phase transitions and pressure variations are not adequately covered in the existing literature, their work frequently takes place under complicated, changing temperature and fluid dynamic settings. The goal of this research is to create a thermodynamic model that incorporates the effects of steam condensation, steam injection, and heating failures to describe the transient behaviour of temperature and pressure in pressure vessels containing single-phase and two-phase fluids. To account for nonlinear, temperature-dependent steam properties, as well as initial and boundary constraints, the study proposes energy balance models for hot water and saturated steam cases. Numerical simulations evaluating sensitivity to parameter changes are presented alongside analytical solutions for isochoric and isobaric systems. The model also includes direct steam injection heating and the use of a heat exchanger. It explains the changes in temperature and pressure that occur in thermal energy storage systems over time, including significant events such as steam cushion collapse and condensate drainage. According to the sensitivity analysis, the main factors influencing the system’s safety limitations and transient dynamic phenomena are thermal power, heat exchanger capacity, and thermal insulation efficiency. The proposed thermodynamic model closes a major gap in the literature by providing reliable predictions of the transient behavior needed for the safe design and reliable operation of pressure vessels utilized for heat storage in district heating networks. This model can be used by engineers and researchers to optimize system design and steer clear of risky operational situations. Full article

Improving the architecture and control strategies of thermal management systems (TMSs) is crucial for minimizing energy consumption in heating and cooling components, thereby enhancing the driving range of Battery Electric Vehicles (BEVs). This study presents a holistic approach for developing an Integrated Thermal Management System (ITMS) based on an Octo-valve-type architecture, designed to efficiently manage the thermal demands of both the cabin and powertrain components. Empirical data were collected under various heating and cooling scenarios across a wide operating temperature range (−20 °C to 40 °C), and these data were used to parametrize and validate key ITMS components. Experimental results demonstrated that the parametrized simulation model closely replicated the cabin and battery thermal behavior observed in vehicle tests, particularly under cooling conditions. Minor deviations, such as cabin temperature overshoot during heating scenarios, were attributed to duct thermal effects and control tuning limitations. Overall, the optimized Octo-valve-based ITMS architecture exhibited thermal trends consistent with literature references and effectively validated the proposed control strategy, demonstrating improved thermal efficiency and potential range enhancement for BEVs across diverse environmental conditions. Furthermore, ITMS energy consumption over the indicated temperature range is quantified in this research paper. Full article

This work proposes the design, construction, and field test of a vacuum seawater desalination system (VSDS) driven by an evacuated tube solar collector (with a total absorption area of 1.86 m²) under tropical climatic condition (Thailand ambient at latitude 13°43′06.0″ N, longitude 100°32′25.4″ E). The VSDS prototype was designed and constructed to be driven by hot water, which is produced by two heat source conditions: (1) an electric heater for laboratory tests and (2) an evacuated tube solar collector for field tests under real climatic conditions. A comparative experimental study to assess the ability to produce fresh water between a conventional dripping/pipe feed column and spray falling film column is proposed in the first part of the discussion. This is to demonstrate the advantage of the spray falling film distillation column. The experimental method is implemented based on the batch system, in which the cycle time (distillation time) considered is 10–20 min so that heat loss via the concentrated seawater blow down is minimized. Later, the field test with solar irradiance under real climatic conditions is demonstrated to assess the freshwater yield and the system performance. The aim is to provide evidence of the proposed vacuum desalination system in real operation. It is found experimentally that the VSDS working with spray falling film provides better performance than the dripping/pipe feed column under the specified working conditions. The spray falling film column can increase the distillated freshwater volume from 1.33 to 2.16 L under identical cycle time and working conditions. The improvement potential is up to 62.4%. The overall thermal efficiency can be increased from 33.7 to 70.8% (improvement of 110.1%). Therefore, the VSDS working with spray falling film is selected for implementing field tests based on real solar irradiance powered by an evacuated tube solar collector. The ability to produce fresh water is assessed, and the overall performance via the average distillation rate and the thermal efficiency (or Gain Output Ratio) is discussed with the real solar irradiance. It is found from the field test with solar time (8.00–16.00) that the VSDS can produce a daily freshwater yield of up to 4.5 L with a thermal efficiency of up to 19%. The freshwater production meets the requirement for international standard drinking water criteria, indicating suitability for household/community use in tropical regions. This work demonstrates the feasibility of VSDS working under real solar irradiance as an alternative technology for sustainable fresh water. Full article

In this study, the thermomechanical behavior of a variable cross-section rod with fixed ends is studied using an analytical method. A rod with a radius that varies quadratically with length is considered, and it is thermally insulated on its side surface. Heat flow is applied to the left end of the rod, and heat exchange with the environment occurs at the right end. Based on the obtained temperature distribution, the thermal strains, stresses, displacements, and total elongation are determined. The results highlight the influence of boundary thermal conditions on the thermomechanical response of the rod, which is of interest for the design and analysis of structural elements operating under conditions of uneven thermal stress. Full article

The low thermal conductivity of phase-change materials (PCMs) remains a key limitation in latent heat thermal energy storage systems, leading to slow melting and incomplete energy recovery. To address this challenge, this study explores extended Y-Fin geometries as a novel heat transfer enhancement strategy within a concentric-tube latent heat thermal energy storage configuration. Six fin designs, derived from a baseline Y-shaped structure, were numerically compared to assess their influence on the melting and solidification behavior of stearic acid. A two-dimensional transient enthalpy–porosity model was developed and rigorously verified through grid, temporal, and residual convergence analyses. The results indicate that fin geometry plays a critical role in enhancing heat transfer within the PCM domain. The extended Y-Fin configuration achieved the fastest melting time, 28% shorter than the baseline Y-Fin case, due to improved thermal penetration and bottom-region accessibility. Additionally, the thermal performance was evaluated using nano-enhanced PCMs (10% Al₂O₃ and CuO in stearic acid) and paraffin wax. The addition of Al₂O₃ nanoparticles significantly improved thermal conductivity, while paraffin wax exhibited the shortest melting duration due to its lower melting point and latent heat. This study introduces an innovative fin architecture combining extended conduction paths and improved convective reach for efficient latent heat storage systems. Full article