Special Issue on “CFD Applications in Renewable Energy Systems”

The global energy landscape is undergoing a critical transformation driven by the urgent need to mitigate climate change, reduce greenhouse gas (GHG) emissions, and ensure long-term energy security. Fossil fuel-based energy systems remain the dominant source of global electricity and heat, contributing more than 70% of total carbon dioxide emissions worldwide [1]. To counteract their impact, renewable energy—primarily solar, wind, hydro, and marine—must rapidly replace traditional energy sources across all sectors, including electricity generation, transportation, and hard-to-abate sectors. Yet, integrating and optimising renewable energy systems poses significant engineering and physical challenges due to their inherent variability, nonlinear behaviour, and complex fluid-dynamic interactions. In this context, Computational Fluid Dynamics (CFD) has emerged as a powerful modelling approach enabling detailed simulations of flow, turbulence, heat transfer, chemical reactions, cavitation, and multiphase dynamics. These capabilities are essential for improving the design, efficiency, and reliability of renewable energy systems while also reducing experimental costs and development time. CFD enables researchers and engineers to analyse complex flow phenomena in renewable energy systems, including wind turbine wakes, vortex shedding in tidal and hydro turbines, flame dynamics in hydrogen combustors, or convective and radiative heat transfer in solar collectors, among others. Furthermore, advances in high-performance computing, turbulence modelling, and integration of optimization and machine learning algorithms have greatly expanded the predictive capabilities of CFD [2,3].

The aim of this Special Issue is to showcase recent developments and novel applications of CFD tools in the simulation of renewable energy systems. The accepted contributions are organised into five topics: (1) tidal energy, (2) hydraulic energy, (3) wind energy, (4) combustion processes, including hydrogen, and (5) solar energy.

Regarding tidal energy, da Silviera et al. investigated the performance of overtopping wave energy converters (WECs) using CFD simulations under both regular and irregular wave conditions along the southern coast of Brazil [4]. They demonstrated that device geometry—particularly, the ramp height-to-length ratio—strongly influences water capture volume and energy conversion efficiency. Optimising this geometric parameter significantly improved performance under realistic sea states, consistent with earlier CFD-based studies that emphasised the importance of wave focusing and overtopping thresholds in WEC efficiency [5,6]. A second contribution from the same research group introduces and validates the WaveMIMO methodology for generating realistic irregular waves in CFD simulations, using spectral data from Brazilian coastal sites. The study systematically examines key parameters—mesh refinement, time step, velocity boundary discretization—and provides theoretical guidance to improve accuracy. Validation results show that the optimised setup reproduces irregular waves with up to 8% better fidelity compared to previous approaches, enhancing the reliability of coastal and wave energy systems modelling [7]. An additional noteworthy contribution by Thangaraj et al. evaluated seven hybrid Wells turbine designs (based on a NACA0015 airfoil) across a range of inlet fluid velocities representative of coastal regions. CFD was used to assess torque, drag, and hydrodynamic pressure, ultimately selecting the best-performing design for further study. This optimal configuration was then refined through profile modifications (such as stepped-back or zigzag airfoils), which delivered about 15.19% higher hydropower output compared to the baseline design under identical conditions. The study also coupled hydrodynamic modelling with structural, modal, and vibration analyses (including piezoelectric vibration harvesting patches) to ensure that the design maintains structural integrity and offers additional energy harvesting potential [8].

In hydro energy, Lu et al. conducted a comprehensive CFD and experimental study on cavitation in pump–turbines operating under part-load conditions, with a focus on entropy generation and vortex dynamics [9]. Using advanced flow diagnostics, such as Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD), and the Q-criterion, the study reveals the formation and evolution of small-scale vortices that intensify cavitation and energy dissipation. The results show that the entropy production rate correlates directly with hydraulic loss mechanisms, providing valuable insight into system inefficiencies. In a related study, Yang et al. studied the effect of sediment concentration and particle size on cavitation in a Venturi flow channel, combining high-speed photography with a gas–liquid–solid numerical mixture model. Their findings demonstrate that increasing the sediment concentration has a stronger influence than increasing the particle size on reducing the cavitation number, extending the cavitation cloud length, accelerating the cavitation evolution cycle, and intensifying shearing and re-entrant jet effects near walls. Numerical predictions align reasonably well with experimental observations, although simulations tend to overestimate the duration of the cavitation cloud evolution cycle [10].

Three contributions in this Special Issue focus on advancing the understanding of wind energy devices. Malael and Stratila conducted Large Eddy Simulations to analyse the dynamic starting behaviour of a vertical-axis wind turbine (VAWT) enhanced with passive vortex control [11]. Their results show that incorporating a vortex-trap design improves lift generation, accelerates self-starting, and enhances torque output. This study builds on earlier CFD research investigating passive flow control mechanisms for improving both vertical- and horizontal-axis wind turbines’ starting process [12]. Xu et al. evaluated the aerodynamic effects of dielectric barrier discharge (DBD) plasma actuators installed on wind turbine blade rudders. By varying the input frequency, they demonstrate that plasma excitation increases lift at low angles of attack and modifies flow separation at higher angles, especially under crosswind conditions. [13]. Contreras et al. used CFD with FENSAP-ICE 2022 R1in ANSYS to evaluate the impact of surface roughness and turbulence models on the aerodynamic performance of the S809 airfoil under both rime and glaze ice scenarios [14]. Their findings revealed that ice accretion significantly increases drag and reduces lift, primarily due to roughness-induced surface modifications, while the choice of turbulence model resulted in relatively minor differences (under ~10%). The beading roughness model was shown to more accurately capture aerodynamic changes during ice growth, especially in representing the spatial variation of roughness across the iced airfoil.

Hydrogen combustion in micro-scale systems presents unique challenges due to high diffusivity, flame instability, and temperature gradients. In this Special Issue, CFD was applied to simulate hydrogen-fuelled micro-planar combustors with dual cylindrical geometry and recirculating channels [15]. The study reports significant improvements in combustion efficiency, temperature uniformity, and wall cooling, including a 580 K reduction in exhaust temperature and a 14% increase in radiation efficiency. These results highlight the potential of CFD to support the development of safer and more efficient hydrogen combustion systems for portable and stationary applications [16,17]. Two additional contributions focus on diesel-based fuels. Zhou et al. developed a detailed chemical-kinetics mechanism (187 species, 735 reactions) to model the oxidation behaviour of ternary fuel blends of hydrogenated biodiesel, ethanol, and diesel under varied temperatures (900–1400 K); an equivalence ratio of 1.0; and a pressure of 1.0 MPa [18]. Their results show that, at lower temperatures, increasing the biodiesel and ethanol content reduces OH radical production and slows reactivity, while, at higher temperatures, the same increase enhances OH formation and promotes oxidation. This study also identified oleic and stearic methyl esters as better surrogates for hydrogenated biodiesel kinetics than methyl decanoate and highlights the central role of the CH₂O → HCO → CO pathway in both low- and high-temperature oxidation. Valiño et al. implemented a 3D CFD model using an Eulerian–Eulerian two-phase flow formulation combined with a probability density function (PDF) to represent particle size distribution and deposition in a BOSCH automotive diesel filter. The model employs the Brinkman–Darcy approximation for flow through the filter media, showing that under typical operating conditions, this approach simplifies porous medium modelling without significant loss of accuracy. The resulting simulations qualitatively (and, in early stages, quantitatively) reproduce the deposition patterns seen in used filters, particularly the preferential deposition of particles in fold pleats and near inlet slots [19].

Regarding solar energy, a CFD-assisted experimental study investigated the thermal performance of two flat-plate solar air heaters, emphasising the influence of geometry on air temperature distribution and collector efficiency. Numerical results were validated against experimental data, showing good agreement and confirming that geometry-induced flow patterns significantly influence heat transfer [20].

In summary, this Special Issue highlights the essential role of CFD in advancing renewable energy technologies. Across different energy sources—tidal, hydraulic, wind, combustion, and solar—the selected studies demonstrate how CFD delivers critical insights into fluid–thermal interactions, energy conversion processes, and system-level optimization. Looking ahead, the integration of CFD with AI-driven optimization, experimental validation, and real-time data from digital twins offers significant potential. Future research should also address key challenges, such as uncertainties in turbulence models, computational cost reduction, and scale-bridging simulations. We hope this collection inspires further innovation in CFD applications for clean energy.

We thank all the contributions sent to this Special Issue and the support of all the editors at MDPI that helped us in this process.