International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

In the paper, geometry parameters of a longitudinally-finned vertical shell-and-tube latent thermal energy storage (LTES), which uses paraffin as the phase change material (PCM) and water as the heat transfer fluid (HTF) have been optimized with the objective of maximizing its storage capacity, i.e. the amount of stored and released thermal energy. Three objectives were set: maximization of stored thermal energy in 8 h, maximization of released thermal energy in 12 h and a combination of the two, in which each objective was given equal significance. There geometry parameters were optimized; fin number, fin width and tube diameter. Optimization has been performed using response surface methodology and Box-Behnken approach. Responses have been obtained numerically, through an experimentally validated modeling procedure and solver scheme. The responses for each objective were fitted with a regression polynomial function and the fitness quality was evaluated through a coefficient R2. Optimization procedure offers different optimum values of analyzed parameters for each objective and provides guidance for choosing the favorable values of LTES geometry parameters in order to enhance LTES thermal performance.

The European Union’s REPowerEU strategy places green hydrogen at the center of its plan to eliminate fossil fuels and accelerate the green transition. The strategy targets 20 million tonnes (MTPA) of green hydrogen per year by 2030: 10 MTPA to be produced domestically and 10 MTPA imported. Achieving this requires scaling electrolysis capacity from the current 0.3 GW to 120 GW, a remarkably ambitious, if not unrealistic, target. Current green hydrogen production costs range from 100 to 200 €/MWh, several times higher than natural gas prices, which fluctuate between 20 and 40 €/MWh. In contrast, blue hydrogen, which is produced through natural gas reforming combined with carbon capture and storage (CCS), generally costs between 50 and 100 €/MWh. To bridge the cost gap between hydrogen and fossil fuels, the EU established the Hydrogen Bank with €3 billion to kick-start the market through competitive funding mechanisms. The REPowerEU hydrogen targets have drawn criticism due to limited availability of renewable electricity, underdeveloped infrastructure, and the slow pace of electrolysis deployment. Concerns also focus on the inefficiency of hydrogen use in sectors such as passenger transport, short sea shipping, residential and commercial heating, where direct electrification is significantly more effective. Nonetheless, the EU is advancing regulatory frameworks, developing over 40 Hydrogen Valley Projects, and establishing international import corridors to support market growth. This paper examines REPowerEU’s hydrogen ambitions, balancing its potential as a key decarbonization tool against economic, technical, and logistical challenges that may hinder its realization.

The study reports on a series of numerical simulations conducted to assess how tube diameter affects charging (melting) and discharging (solidification) performance in a shell-and-tube latent thermal energy storage (LTES) with longitudinal fins. In the investigated LTES, water flows through the tubes and serves as the heat transfer fluid (HTF), while paraffin is used as the phase change material (PCM) and fills the shell side. Employing an experimentally validated mathematical model and numerical procedure, LTES charging and discharging performances were investigated for three tube diameters: 28/24, 38/34 and 48/44 mm. LTES performance for different tube diameters was assessed by comparing melting and solidification times, as well as stored and released thermal energies in 8, 9 and 10 h of charging and 12, 13 and 14 h of discharging for each configuration. Results show that larger tube diameters accelerate melting and solidification processes due to increased conductive surface area, but also decrease LTES energy storing capacity as the amount of the PCM reduces as a result of increased tube diameter. The results indicate that tube diameter greatly influences LTES thermal performance and must be chosen carefully for the LTES to be effective.

The objective of this study is to numerically investigate the fluid flow and heat transfer performance of a finned-tube heat exchanger (FTHEX). The analysis focuses on the implementation of three vortex generator (VG) configurations: rectangular winglet (RW), delta-winglet upstream (DWU), and delta-winglet downstream (DWD) — mounted on the fin surface in a “common-flow-up” orientation. Attack angles of 15°, 30°, and 45° are considered for each VG type to evaluate their impact on the heat exchanger’s heat transfer potential and friction losses. The air-side Reynolds number, based on the outside tube diameter, was varied within the range 684 ≤ Re ≤ 1532. The results indicate that among the tested configurations, the RWP setup with an attack angle of 45° achieves the highest enhancement in the airside Nusselt number, with improvements ranging from 20% to 45% compared to the reference configuration, but at the expense of a higher pressure drop. For attack angles αvg = 15° and αvg = 30°, the highest overall performance (TPF factor) is achieved with the rectangular winglet configuration across the entire Reynolds number range. At an attack angle of αvg = 45°, the heat exchanger with downstream delta winglets shows higher TPF values compared to the other configurations, except at Re = 1278.

Compact air-cooled fin-and-tube heat exchangers are widely used in various fields, including the automotive and computer industries, as well as in heating, air conditioning, refrigeration, and process applications. Due to the thermal characteristics of air, the majority of heat transfer resistance occurs on the air side of the heat exchanger. As a result, research in this area primarily concentrates on enhancing the air-side performance. Numerous studies in the literature explore different fin and tube configurations aimed at optimizing the design of these heat exchangers however, pressure drop is sometimes neglected. In this study, a numerical analysis was conducted to investigate air-side pressure drop and heat transfer in various configurations of slotted fin-and-tube heat exchangers whereby heat exchangers with different ellipticity ratios were considered. The numerical model of the three-dimensional, laminar, steady-state problem of air-side flow and heat exchange was done using the finite volume method. The convection-diffusion equations were discretized using the Power Law scheme, and the SIMPLE algorithm was employed to couple pressure and velocity. Simulations were carried out in ANSYS Fluent 18.2. The validation of the proposed model was tested by comparing the numerical results with experimental measurements available in the literature whereby no discrepancies greater than 5% were observed. Four different inlet air velocities ranging from 1 to 4 m/s, corresponding to Reynolds numbers between 558 and 2233 were considered. Both the inlet air temperature and the tube surface temperatures were kept constant at 293 K and 373 K, respectively. The results emphasize potential benefits of using elliptic instead of round tubes in slotted fin and tube heat exchangers to achieve lower air-side pressure drop without penalty of lower heat transfer.

Carbon capture, Utilization and Storage (CCUS) technologies are rapidly evolving as a critical component of global decarbonization strategies, particularly in hard-to-abate sectors such as natural gas processing, power generation, fertilizer, cement and steel production industries. Amine-based absorption systems are currently the most established capture method, widely applied in large point sources for natural gas processing and chemical industries. Alternatives such as membrane separation, adsorption, and direct air capture are also emerging, offering benefits for specific applications. CO2 transport is increasingly diversified, with supercritical CO₂ pipelines and liquefied CO₂ shipping offering scalable and flexible solutions. CO₂ storage is focused on deep saline aquifers and depleted oil and gas fields. Carbon capture costs are project-specific and depend on CO2 concentrations, facility size, and technology complexity, with costs ranging from 30 to 120 US$/tCO2. The CCUS chain will undergo substantial development in the next decades, both in technological maturity and economic viability. As of early 2025, the total global CCUS capacity was 50 million tonnes per annum (MTPA) and is expected to reach 1300 MTPA by 2050. Yet, this will cover only 6% of total global CO₂ emissions, far from any net-zero carbon emissions scenario. By 2050, modularization, improved materials, and process integration are expected to reduce investment costs by up to 30%.