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.

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.