Industrial high-temperature waste heat remains a largely underused high-grade energy source due to the limited techno-economic performance of conventional thermal energy storage (TES) systems. This paper proposes a novel multi-layered hybrid TES concept that combines phase change materials (PCMs) and molten salts to achieve high energy storage density while maintaining adequate heat transfer performance and stable charging-discharging behavior. The design incorporates two concentric layers filled with different melting temperature PCMs and a central core filled with molten salt, which serves as the heat transfer fluid and a high-temperature thermal buffer. The paper analyzes the thermal response, cycling behavior, and techno-economic performance of the hybrid TES for a given PCM pair. It benchmarks it against the two-tank TES across three scenarios: electricity generation with a power block, heat supply to an industrial process, and standalone energy storage. The optimized hybrid design reduces the levelized cost of electricity (LCOE), heat (LCOH), and storage (LCOS) by up to 2.6%, 12.4%, and 33.2%, respectively. The optimization also provides guidelines for the internal tube configuration, tube pitch, and energy storage allocation between the PCM layers. This article demonstrates the strong potential of our hybrid TES approach for cost-effective high-temperature energy storage.

In this work, we numerically investigate a rectangular PCM tank with isothermal heating applied to one vertical wall, while the remaining walls are insulated. The effects of enclosure length (L) and height (H) on melting performance are systematically examined. It is shown that for each length, there exists an optimal height that minimizes the melting time. Moreover, this optimal height H_opt decreases with increasing enclosure length and wall temperature. A theoretical model is developed and validated against simulation results, revealing that the optimal enclosure aspect ratio follows the scaling law AR_opt∼Ra_L^−1/7. This framework provides practical guidance for the design of insert-free PCM enclosures aimed at rapid thermal response and efficient energy storage.

The aim of this study is to quantify and utilize waste heat from wind-farm-powered water electrolysers and ammonia cracking. The port of Rotterdam, as a case study, has been analyzed where the transport of 4.6 Mt hydrogen and water electrolysis, powered by wind farms, is planned. A dynamic model was developed to calculate waste heat from an electrolyser powered by fluctuating electricity inputs from offshore wind power. Moreover, thermal analysis of ammonia cracking process streams was conducted. It was observed that integrating water electrolysis waste heat into the ammonia cracking process is not only a promising novel application for the reuse of the electrolysis waste heat, but also it can potentially enhance cracking efficiency by 2 % while creating synergies within the hydrogen industry. Additionally, waste heat can be used for district heating saving more than 70 % of energy and reducing CO₂ by just as much. In parallel, cold utilisation from ammonia cracking was explored for CO₂ and H₂ compression, as well as industrial cold storage to observe that technical implementation is possible.

It is widely recognized among specialists that PCMs (Phase Change Materials) typically have low thermal conductivity, which significantly restricts their commercial use. This study presents alternative, low-cost, yet effective approaches to enhance the average thermal conductivity of a PCM system (a commercially available paraffin wax with a phase change temperature around 40 °C) intended for thermal energy storage. The system contains 600 g of PCM within an annular space around an inner tube, through which heat is either added to or removed from the PCM. Experiments were conducted to assess the effects of water flow rate and temperature, used as the heat transfer fluid, on the system's performance. The flow rate was varied from 2 to 8 L/min, and the temperature was set between 45 and 55 °C. We tested three types of aluminum-based thermal enhancers: a commercial metal foam, a wire mesh, and irregular aluminum flakes (chips) produced as waste from machining processes. The PCM-only sample required the longest time for both charging and discharging, while the PCM with metal foam had the shortest times. The intermediate solutions, using chips and wire mesh, showed moderate phase change times. To evaluate the economic feasibility, we introduced a performance metric based on cost per phase change rate, showing that these two affordable thermal conductivity enhancers could play a vital role in promoting the broader application of latent thermal energy storage technology across various fields.

Optimization of heat-exchanger manifolds can significantly improve the flow distribution inside their cores, improving the heat exchange and reducing flow obstruction. It also reduces the overall mass of the system and with it, the cost of additive manufacturing. However, during optimization, domains are typically modeled as 2D to minimize computing effort. Likewise, laminar flow is prescribed even when turbulence is expected in operation. The accuracy of such assumptions and their effect on optimized geometry is unclear. In this work, 2D topology optimization was first performed on an inlet manifold for both laminar and turbulent inlet boundary conditions. The resulting geometries were found to be starkly different, illustrating a difference in design concepts for different flow regimes. The laminar flow cases were then topology optimized with a 3D domain that modeled out-of-plane walls. This produced yet more different geometry, showing that these walls cannot be ignored. Experimental validation by testing stereolithography 3D prints proved that 3D optimization involves far more accurate flow modeling and the results are therefore likely to have better flow distribution.

This study examines the integration of Natural Draft Dry Cooling Towers (NDDCT) and pre-cooled NDDCT as innovative heat rejection systems for a 50 MW Concentrated Solar Power (CSP) plant, comparing their performance against conventional alternatives: Mechanical Draft Wet Cooling Towers (MDWCT), Air-Cooled Condensers (ACC), and hybrid pre-cooled ACC systems (media pads and spray nozzles). Analytical models for the cooling systems and their interaction with the CSP plant were developed and validated against results reported in the literature. Using hourly temporal resolution and real climatic data from Granada, Spain, the analysis evaluates the annual impact of these systems on energy generation and water consumption. The results demonstrate that pre-cooled NDDCT increases annual power generation by approximately 1400 MWh compared to NDDCT, with substantial performance improvements during peak summer conditions. Although MDWCT achieves the highest efficiency, it comes at the cost of significant water consumption. Pre-cooled NDDCT stands out as a promising hybrid solution, balancing improved condensation performance (only 2.9% less efficiency compared to MDWCT) with minimal water usage (76.7% less than MDWCT). This study provides valuable insights for optimising CSP plant performance in arid regions, advancing beyond previous efforts in the literature.

Automatic icemakers are integrated into refrigerators to ensure a consistent ice supply and improve energy efficiency. Despite these advantages, a thorough investigation of the automatic icemaking process in domestic refrigerator-freezers is lacking in the literature. This study aims at assessing the performance of automatic icemaking process in a domestic freezer through detailed theoretical, numerical and experimental analyses. A simplistic zero-dimensional transient energy balance model is developed to investigate the heat transfer during different stages of the water solidification process. The convective heat transfer coefficient calculated from the theoretical analysis is used to inform the numerical model. A three-dimensional transient model is proposed to predict the temperature and density variation inside the ice cube modelled as a pyramid. The free surface flow is modelled using volume of fluid method, while enthalpy-porosity method is employed for the water freezing process. The results show a non-uniform temperature distribution throughout the solidification process and that the temperature of the outer frozen layers keeps decreasing with the solidification time. Experiments are conducted to measure the temperature variation of the ice cube. It is shown that the icemaking process is accelerated by around 18 % when the ice-removal temperature is set at −8°C instead of −12 °C, which is a conventional set temperature for ice remover in current domestic freezers.

This paper proposes and simulates a desiccant air cooling system integrated with a vapor compression cooling unit and a heat recovery unit for an office building in Çanakkale, Turkey, during the summer season. The required electrical energy for equipment of the proposed system is supplied by an Solid Oxide Fuel Cells (SOFC) unit using human waste as fuel. Moreover, some of the waste heat generated by the SOFC is used to regenerate the desiccant wheel. The simulation also includes the effects of three different refrigerants for the vapor compression cooling unit. Among the refrigerants, the highest electrical COP was obtained for the system using R1234ze(Z), which is 3.14% and 2.40% higher than the systems using R717 and R1233zd(E), respectively. Additionally, the system using R1234ze(Z) achieved electrical savings of 9.97% and 9.23% compared to the other systems. These electrical savings resulted in fuel savings of 1.19% and 0.90% for R1234ze(Z) compared to R717 and R1233zd(E), respectively. During the summer season, the electricity production from the existing SOFC unit met 82.00% of the total electricity consumption of the desiccant hybrid cooling system. Furthermore, a difference of 3984.56 kWh in primary energy consumption was identified between the desiccant hybrid cooling systems operating with the SOFC and without the SOFC during the summer season.

Among experts, it is well-known that the thermal conductivity of PCMs (phase change materials) is low hence a major limitation for their commercial application. This work proposes alternative, inexpensive, but nevertheless effective solutions to increase the average thermal conductivity of a PCM system (a commercial paraffin wax, having a phase change temperature of about 40 °C) used for thermal energy storage. 600 g of PCM fills an annulus wrapping an inner tube used to either charge or discharge heat to the PCM. The effect of the flow rate and temperature of the water used as heat transfer fluid was experimentally analysed. The flow rate was set to vary between 2 and 8 l min^-1 and the temperature between 45 and 55 °C. We tested three different aluminum-based thermal enhancers: a commercially available metal foam sample, a wire mesh, and irregular flakes (chips) obtained as waste product of machining operations. The PCM-only sample exhibited the longest charging and discharging times, while the PCM + foam sample shortened them the most. The two cost-effective solutions (chip and wire mesh) resulted in intermediate phase change times. A performance indicator, in terms of cost per phase change rate, is proposed to compare different enhancers. It demonstrated that these two cost-effective thermal conductivity enhancing solutions can become a key enabling method to widely deploy latent thermal energy technology widely in many different applications.

In this study, a desiccant-based hybrid cooling system supported by a vapor compression system and a heat recovery unit (rotary heat wheel) was analyzed from energetic and exergetic perspectives during the daily working hours of an office building in Istanbul to meet the desired comfort conditions. We focus on the impact of different refrigerants; namely R32, R1234yf, R290, R134a, R600a, R245fa, and R717, on hybrid rotary desiccant-vapor compression systems. While the highest electricity consumption was obtained in the system using R1234yf, the lowest electricity consumption was achieved with R717. However, with the effectiveness of the desiccant wheel, the best results were obtained for R1234yf with those pertinent to R717 at the other extreme. Considering the total electricity consumption of the system, the highest energetic and exergetic performance parameters were achieved with the use of R717 as the refrigerant. Compared to R1234yf, the daily average energetic performance parameters obtained with R717 increased by 22.3 % for COP _r, 21.8 % for COP _el, and 4.7 % for COP _th. Similarly, compared to R1234yf, the daily average exergetic performance parameters in R717 presented increases of 13.6 % for COP _x,el, 7.5 % for COP _x,th, and 8.1 % for η _x.

The current study presents a novel piezoelectric design that converts vortex-induced vibrations caused by flow across a circular cylinder. This design can be implemented as a passive control method for energy harvesting. It encompasses the novelty of employing circular arc plates around a circular cylinder integrated with flexible splitters downstream. To assess the merits of the proposed design, a comprehensive parametric study for flexible splitters is conducted to specify the optimum layout of piezoelectric harvesters. The analysis is carried out at a low Reynolds number, Re=100, at which accurate non-intrusive experimental assessment of flow structure is difficult. The results indicate that the drag can be halved by adding circular arc plates to the cylinder. Moreover, it is demonstrated that the plate displacement increases by 18.29 folds for dual splitters compared with a single plate. By adding the arc plates, a further increase of 3.97 folds is obtained. Following this, the maximum strain with the inclusion of arc plates is 170.17% higher than the cylinder without arc plates (bare cylinder) for dual flexible splitters downstream, enhancing the energy harvesting performance remarkably. Finally, using the proposed design, the generated electrical power figures out at 3.13μW at 77.7% conversion efficiency.

The low thermal conductivity of phase change materials (PCMs) has limited their widespread use in practical applications. In the present study, different fin structures, namely, rectangular, perforated, and pin were examined to analyze the thermal performance of the melting process in rectangular latent heat storage tanks. Experiments were performed at both horizontal and vertical orientations to evaluate the effectiveness of different fin configurations. Visual observation of the phase change evolution at different time intervals was enabled through a transparent plexiglass shell. Instantaneous heat transfer rate and energy storage were measured using thermocouple readings and melting photographs. The results show that the maximum heat transfer coefficient between the heated wall and PCM is obtained by the pin-finned tank followed by perforated and rectangular-finned tanks. This thermal behavior is justified by the intensification of the upward convection flows through the voids provided by pin fins or perforated fins. Although the rectangular fin structure has the lowest convective heat transfer coefficient, its heat transfer rate is slightly higher than the other structures due to its larger heat transfer area. At a wall temperature of 70℃, the convective heat transfer coefficient and heat transfer rate obtained by the pin fin configuration are respectively 25% higher and 4% less than those of the rectangular fin. It reveals that the pin fin structure provides the most effective heat transfer area compared to its counterparts which have a significantly larger fin volume. In addition, it was found that regardless of the fin configuration, the melting rate in the horizontal tank was significantly higher than in the vertical tank due to the formation of more vortical flow structures within the molten PCM. The melting time in the unfinned horizontal tank was less than those of the vertical finned tanks implying that the tank orientation should be well-chosen to minimize the melting time along with adding fins of various configurations.

This paper uses a combination of numerical and theoretical techniques to evaluate dynamical threshold for cavitation in water triggered by weak tension. Within the test section of a once-through, nonrecirculating cavitation tunnel, a cylinder is installed, behind which cavitation occurs, and once cavitated, water cannot return to the monitoring section. The single-phase water flow has been simulated using an LES-based CFD solver to observe velocity distribution at the throat, pressure distributions before and after the throat, flow separation, and vortex formation downstream of the separation point. Bubble dynamics analysis using the Rayleigh-Plesset equation is conducted for conditions under which bubble nuclei supposed to exist in water grow according to CFD-based pressure histories and along streamlines, revealing that nuclei with O(1 μm)-radius suddenly start growing beyond the throat, largely depending on the streamlines. Finally, dynamical cavitation threshold theory is successfully applied to predict nuclei growth, resulting in rather good agreement with the bubble dynamics analysis.

In this work, melting of a high-temperature inorganic phase change material (PCM) eutectic (with a melting point of 569 °C) within a vertical cylindrical tank has been experimentally investigated. To promote the heat transfer rate, a periodic structure that is constructed by a commercial SS-304 mesh screen has been considered and immersed into the PCM tank. Thermal characteristics of the PCM-periodic structure tank under different initial temperatures (450, 490 and 546 °C) and wall temperatures (620, 640, 660, 680 and 700 °C), are then investigated and reported. The presented experimental data can facilitate practical engineers to find the best operating condition of similar PCM tanks; meanwhile, it can be also employed for the investigation of thermal response of transient heat conduction before melting starts.

In this study, the microstructure of open-cell metal foam was generated and reconstructed, to produce a new generation of open-cell foam, which is called 3D printed open-cell foam. At the current stage of research, nylon powder and plastic acid are utilized as the materials for two different 3D printing technologies: Selective Laser Sintering (SLS) and Fused Deposition Modelling (FDM), respectively. The microstructural properties and surface roughness of the 3D printed open-cell foam are investigated using CAD files and microscope images. The surface smoothness and structure strength are found to be dependent on the printing technologies, material employed, and foam size. However, the SLS technology produced smoother ligament surfaces with fewer residues than using the FDM. The ligaments of the small-size 3D printed open-cell foam at the exact size of the metallic foam, on the other hand, are weak and easily shattered. This study also found that the trends of pressure drop from additive manufacturing methods agreed to the original metallic open-cell foam, which are decreasing with the increase of pore sizes.

The integration of phase change materials (PCMs) and metal foam has been widely concerned recently. To decrease non-uniformity of uniform metal foam-PCMs, adaptive metal foam arrangement strategy with increasing porosity from inside to outside has attracted widespread attention. This work conducted a symmetric simulation model of vertical thermal energy storage (TES) tube validated by experiments, for optimization of adaptive metal foam arrangement in basic design (0.94–0.94–0.94). It was followed by assessing the performance of gradient metal foam structures that included 27 cases with radial foam gradients of larger porosity on the outside and smaller porosity on the inside. Results demonstrated that a smaller difference between the inside and outside subregions resulted in better thermal performance when the same porosity of the intermediate subregion was used. More intense natural convection with stronger liquid paraffin vortex could be obtained by an adaptive arrangement. With the same average porosity, the faster phase change evolution, which was influenced by the maximum promotion of stronger natural convection, was achieved by using a larger intermediate porosity and a larger porosity difference between the inside and outside regions. The optimal strategy (0.87–0.94–0.97) could significantly shorten the melting duration as maximal as 17.15% compared with the original uniform (0.94–0.94–0.94), which contributed to efficient vertical metal foam TES systems, also as light and cost-effective as possible while also avoiding sacrificing thermal capacity.

Air cooling systems are widely used in current data centers owing to their low capital costs and high reliability. To satisfy the increasing rack power density, the optimal air-cooling technology and an economic analysis should be carefully discussed. Therefore, this study discusses four airflow management technologies: Case 1: raised floor and cold aisle containment supply/computer room air conditioning (CRAC) direct return; Case 2: CRAC direct supply/hot aisle containment (HAC) return; Case 3: overhead duct supply/CRAC direct return; and Case 4: overhead duct supply/HAC return. Using a validated model, the thermal and economic performances of each case were compared. Results showed that Case 4 exhibited the best thermal performance, followed by Cases 3, 2, and 1. Case 1 cannot satisfy the heat dissipation requirement when the rack power density is larger than 12.5 kW; whereas only Case 4 can be used when the power density is larger than 15 kW. Regarding location within China, owing to the high ambient temperature, Shenzhen showed the highest annual cost value and power usage effectiveness, followed by Shanghai, Xi’an, Beijing, and Harbin. Finally, Cases 3 and 4 are recommended for application when the rack power density is greater than 10 kW.

Thermal energy storage is increasingly needed in a sustainable world because of its potential of capturing waste heat and being incorporated in solar power plants. For power generation, in particular, as turbine technology advances, a demand for higher temperature thermal energy storage materials also grows. For this purpose, latent thermal energy storage fits in well since it uses phase change materials (PCMs) which generally have a higher energy density compared to their sensible heat counterparts. In the present study, a eutectic Na₂CO₃(41.69%)-(33.1%)KCl-(25.21%)NaCl phase change material (PCM) with a melting temperature of 569 ° C was chosen as the storage material to experimentally assess the performance benefit of using a readily available stainless steel (ss304) wire mesh (as the periodic structure) to enhance heat transfer within the domain. In addition, for discharging, a numerical model was developed and compared with the experimental results. Furthermore, for discharging, a numerical investigation of the influence of the heat transfer fluid (HTF) flow-rate to the rate of heat transfer was performed. Overall, it was experimentally observed that the charging time for the composite case was shortened by about 35%, compared to the pure PCM case. For discharging, in the axial direction, the composite solidification time when compared to the pure PCM case was on average 10% shorter. Regarding the radial discharging performance of the composite, there was only about 5% improvement compared to the pure PCM case, which was expected due to the thermal contact resistance in the radial direction. Discharging experimental results were used to validate a discharging numerical model. Discharging results from the model showed that increasing the flow rate of the heat transfer fluid (HTF) reduced the time for solidification. It was observed that for the HTF flow rate of 5 L/min, 10 L/min, 20 L/min and 30 L/min, the discharge time was shortened by 23%, 30%, 33% and 35%, respectively.

NO_x emission reduction from gas boilers has become a key issue in improving air quality. Combustion air humidification technology is gradually being used to reduce NO_x emissions. However, the NO_x emission reduction effect of gas boilers at a higher combustion air humidity has been studied less. A flue gas with low NO_x emissions and a waste heat recovery system using combustion air humidification technology are proposed in this study. In the ultra-low NO_x mode, the effect of high combustion air humidity on NO_x emission reduction and efficiency of the gas boiler were studied experimentally. In the waste heat recovery mode, the effects of the heat network backwater temperature on the NO_x emission reduction and system efficiency were studied experimentally. Results showed that an increase in air humidity can significantly reduce the NO_x concentration formed by combustion. The ultra-low NO_x mode reduces NO_x emissions from 130 mg/m³ to 23.3 mg/m³ and affects the boiler efficiency slightly. In the waste heat recovery mode, NO_x emissions can be reduced to 39.9 mg/m³ when the backwater temperature of the heat network is 55 °C. This condition improves the efficiency to 93.8%. The analysis results provide suggestions for the selection of the operation modes.

Recently, the supercritical carbon dioxide (SCO₂) power cycle has become a hotspot in the field of energy-efficient utilization. The utilization of additives in the power cycle has been proven to be an effective way to improve the SCO₂ power cycle efficiency. As one of the core components of the system, the influence of CO₂-based mixtures on turbine performance needs to be further explored. In this study, the preliminary design and three-dimensional numerical simulation of a 500 kW radial-inflow turbine (RIT) for small-scale SCO₂ power systems were carried out. Furthermore, the design and off-design performance of high Reynolds number and small size turbine under the change of the CO₂-based binary mixture compositions and mixing ratios were studied. Increasing the amount of nitrogen, oxygen, or helium into CO₂ has a negative effect on the RIT performance, and the appropriate amount of xenon or krypton can improve the turbine efficiency. Moreover, mixtures with higher krypton additions adapt to higher heat source conditions. The loss of the turbine stage passage shows that a large amount of helium greatly reduces the working fluid density, and the high amount of xenon has a great influence on the dynamic viscosity, which all makes the RIT operation deviate from the steady state. Therefore, the CFD model simulation fails indicating that RIT designed based on pure CO₂ may not run smoothly and continuously. The losses in the stage with pure CO₂ and CO₂–Kr mixture were investigated. The results indicate that the losses originated from the stator cannot be ignored and that the improvement of efficiency is mainly owed to the reduction in clearance losses. There is no doubt that the viewpoints proposed in this paper have significant reference value for the practical application of the SCO₂ power cycle using mixtures.