Cross-flow heat exchangers are widely used in thermal and energy systems for their compactness and structural simplicity; however, their thermal–hydraulic performance remains strongly constrained by geometric configuration, flow regime, and pressure-drop penalties. This review systematically examines more than four decades of research on cross-flow and compact heat exchangers, covering theoretical, numerical, and experimental investigations. The effects of geometric modifications—such as fin and tube shape, pitch, orientation, and surface interruption—are critically analyzed, revealing that non-uniform, flow-disturbing geometries can enhance heat transfer by 15–50%, albeit often at the cost of increased hydraulic resistance. Studies of mechanical vibration and flow oscillation demonstrate notable enhancements in heat transfer in low-Reynolds-number and buoyancy-dominated regimes when vibration parameters are optimally tuned. The integration of porous media, including metal foams and packed spheres, has shown substantial performance gains, often exceeding 40–90%, though significant pressure-drop challenges accompany this approach. More recently, artificial intelligence and data-driven optimization techniques have emerged as powerful tools for balancing thermal enhancement and hydraulic penalties. Despite these advances, key gaps persist in condensation-dominated applications, low-Reynolds-number regimes, long-term reliability, and experimentally validated coupled thermal–hydraulic optimization. This review consolidates existing knowledge, identifies unresolved challenges, and outlines future research directions towards high-efficiency, application-specific cross-flow heat exchanger design.

The transition from fossil fuels to renewable energy is vital for addressing climate change and ensuring energy security. Hybrid renewable energy systems (HRES), particularly those integrating solar photovoltaic (PV) and wind power, have emerged as a promising solution to overcome the intermittency and variability of individual sources. This study develops a comprehensive simulation and optimization framework for hybrid PV–wind systems, incorporating advanced energy storage options such as lithium-ion batteries and ultracapacitors. Using high-resolution meteorological and load data, both grid-connected and off-grid configurations are analyzed to evaluate system reliability, cost-effectiveness, and adaptability across different climates. A special focus is given to Kuwait, where high solar irradiance and moderate wind resources align with national energy diversification goals under Kuwait Vision 2035. The results highlight the technical and economic feasibility of hybrid systems, showing significant improvements in energy yield, load matching, and levelized cost of energy (LCOE) compared to standalone technologies. Furthermore, the study underscores the importance of intelligent control strategies, advanced component technologies, region-specific optimization, and explicit planning and performance evaluation insights in ensuring sustainable and resilient deployment of hybrid renewable systems.

The accelerated deployment of photovoltaic (PV) systems in Malaysia has raised critical concerns regarding end-of-life (EOL) panel waste, particularly in states like Kedah where large-scale solar installations are concentrated. Despite growing attention to solar energy, limited infrastructure and governance mechanisms exist for managing decommissioned PV panels. This study presents an integrated approach to optimizing EOL PV waste management in Kedah, Malaysia, by incorporating lifecycle-based environmental and economic analysis. With a projected increase in PV waste by 2034 and beyond, the research applies a combination of Life Cycle Assessment (LCA), Life Cycle Costing (LCC), Multi-Criteria Decision Analysis (MCDA), and Geographic Information Systems (GIS)-based modeling to assess and optimize each phase of the waste management process from uninstallation, transportation (T$_1$ and T$_2$), and collection center operations to recovery facilities (RF). Results show that optimized routing, strategic load consolidation, and selective frame dismantling at collection centers (CC) can reduce transport related emissions by up to 35% and operational costs by over 20%. The integration of Circular Economy (CE) principles and Extended Producer Responsibility (EPR) frameworks ensures material recovery (aluminum and silicon), improves traceability, and aligns the model with national regulatory standards. This research proposes a scalable, policy-aligned optimization framework that enhances environmental performance and cost efficiency in Malaysia's emerging PV waste sector.

Photovoltaic thermal (PVT) systems have emerged as a promising solution for enhancing overall energy efficiency by simultaneously generating electricity and heat, addressing the performance limitations of conventional photovoltaic modules operating under elevated temperatures. This study reviews thermal management strategies, techno-economic feasibility, and environmental sustainability of PVT systems through a comprehensive bibliometric and technical analysis. A systematic approach was employed, integrating bibliometric mapping of global research trends with a detailed classification of thermal management technologies, including air cooling, water cooling, nanofluids, phase change materials (PCMs), heat pipes, and refrigerant-based systems. The review further examines cost performance trade-offs and life-cycle environmental impacts to evaluate the viability of large-scale implementation. Results indicate that advanced cooling approaches particularly nanofluid-enhanced systems and PCM composites effectively improve thermal conductivity and stabilize module temperatures, enabling combined electrical and thermal efficiencies exceeding 85% and, in some hybrid configurations, approaching 94%. Techno-economic assessments reveal that optimized system designs and integration with heat pumps can lower the levelized cost of energy and shorten payback periods, while environmental evaluations demonstrate reductions in carbon footprint and energy payback time. Despite these advantages, challenges persist regarding material stability, cost, and end-of-life recyclability. This study highlights the need for integrated optimization frameworks that account for energy, exergy, and life-cycle impacts. Future research should prioritize cost-effective nanomaterials, AI-driven control strategies, and advanced hybrid configurations to accelerate commercialization and support global decarbonization efforts.

Phase change materials (PCMs) are highly effective in storing and releasing thermal energy during phase transitions, making them critical for thermal energy storage (TES) systems, particularly for renewable energy sources such as solar and wind. They have been reported in energy storage, especially in renewable energy systems such as solar and wind. However, despite their potential, their practical use is limited by low thermal conductivity and slow heat transfer rates. These limitations reduce the efficiency of PCMs in applications requiring rapid thermal responses, such as solar thermal storage and electric vehicle (EV) battery cooling. This review synthesizes and compares recent numerical and experimental studies on PCM enhancement techniques. A significant challenge across these studies is the lack of uniform operating conditions, which complicates the identification of the most effective methods for specific TES applications. The review highlights several strategies to improve PCM performance, including the use of metal foams (MFs), nanoparticles (NPs), and fins. MF has been shown to significantly improve thermal conductivity, increasing it by up to 200% for calcium chloride hexahydrate and 100% for paraffin, while also reducing melting times by 84.9% compared to pure paraffin. NPs, like copper oxide (CuO) and aluminum oxide (Al$_2$O$_3$), can enhance thermal conductivity by up to 122% relative to pure PCM. However, higher concentrations of NPs may increase viscosity, which slightly hinders heat transfer. Fins provide a cost-effective method to enhance heat transfer. The addition of fins has been shown to reduce melting times by 65.5% at 3600 seconds, making them an ideal choice for applications where cost is a key consideration. Hybrid systems combining MFs and NPs achieve the greatest performance improvements. For instance, using 3% NPs and a 60% porosity in copper MF increases thermal conductivity by 37.7% and reduces the melting time by 87.03%. Further improvements are observed when using MF with 85–90% porosity and 10–15% NPs, achieving a 90% reduction in melting time. This demonstrates the synergistic effect of combining these two techniques. In conclusion, hybrid methods combining MFs and NPs offer an efficient and cost-effective approach for enhancing PCM performance in TES applications. By integrating the strengths of these techniques, multiple performance limitations can be addressed simultaneously, providing a viable solution for large-scale TES systems.

Bio-oil from lignocellulosic biomass pyrolysis cannot be applied as biofuel because it is generally corrosive due to its high organic acid content. The organic acid content of heavy fractions can be reduced by fractionation and by esterification with alcohol to form ester compounds. This study aims to produce high-quality fuel by optimizing the bio-oil: methanol ratio using a magnesium metal oxide modified H-zeolite catalyst (HAAZ/MgO) to convert sago dregs bio-oil. This study was carried out in several stages: pyrocatalytic sago dregs were heated to 350–500 °C, then the bio-oil was filtered and fractionally distilled at 91–110 °C. HAAZ/MgO catalyst was successfully synthesized according to Fourier transform infrared spectroscopy (FTIR) characterization, showing absorption at 3300–3700 cm^-1, the emergence of hydroxyl group (-OH) stretching vibrations originating from silanol groups (Si-OH) and Brønsted acid sites (Si-OH-Al), 1641–1649 cm-1 as H-O-H bending vibrations, 1053–1223 cm⁻¹ asymmetric stretching vibrations of Si-O-Si and Si-O-Al bonds, and 1350–1450 cm⁻¹ indicating the presence of MgO-zeolite. The X-ray diffraction (XRD) spectrum of HAAZ/MgO shows diffraction peaks at 2θ = 20.86°, 25.67°, 26.65°, and 27.74°. The presence of MgO does not damage the HAAZ structure and is evenly dispersed. The fractionated distillate was esterified by reflux at 65 °C. at the ratio of bio-oil distillate to methanol (1:6, 1:8, and 1:10) using HAAZ/MgO catalyst. The esterified biofuel showed the best yield at a 1:10 ratio, with 72.22 ± 1.11% (v/v). The esterification process demonstrated the HAAZ/MgO catalyst's good performance, yielding dimethyl and methyl esters. In addition, the physicochemical properties of bio-oil, including pH, viscosity, and API gravity, increased significantly after esterification, while water content, density, specific gravity, and viscosity decreased. Meanwhile, the higher heating value (HHV) of the esterified biofuel increased from 43.55 to 45.15 MJ/kg. Improvements in these parameters indicate that the esterification process plays an important role in enhancing biofuel quality, making it a feasible and efficient renewable energy source.