Abstract Because of the intermittency problem of wind power, co-located energy storage for wind farms is becoming more common, especially for grids with high shares of wind energy. Currently, most co-located wind farm energy storage is achieved with Lithium-Ion Batteries (LIB). LIB provides high round-trip efficiency but has high costs, which limit the average capacity duration to about 2 hours. The two primary energy storage technologies that offer lower cost for capacity are Pumped Hydro Storage and Compressed Air Energy Storage, but both are challenged by topographical and geographic limitations, which limit site opportunities. Herein, a new concept Turbine-integrated Isothermal Compressed Air Energy Storage (TICAES) is proposed, which allows much more site opportunities by using underground reservoir that are small enough (e.g., 40,000 m 3 ) to allow deployment in bedded salt formations, which are relatively common and often located at or near wind farms leverages. TICAES further leverages existing components in the nacelle (gearbox, generator, etc.) and employs a near-isothermal reciprocating compressor/expander for high efficiency. However, this new concept requires further validation of techno-economic feasibility and consideration of complex nacelle integration issues.

An inertial particle separator (IPS) reduces sand and particulate ingestion in gas turbine engines by splitting the inlet airflow: a core leg ideally free of particles and a scavenge leg that carries contaminants. This study evaluates a novel IPS design with reduced flow turning using an axisymmetric-sector wind tunnel. Separation efficiency (eta) was tested across three flow splits (beta) and three standard dust types-ISO 12103-1 A2 Fine, AFRL-03, and ISO 12103-1 A4 Coarse-with a maximum uncertainty of 4%. For the ISO A4 Coarse dust, eta trends matched those from prior large-scale systems. Efficiency increased with beta for all dusts, with ISO A2 Fine showing a steeper eta-beta gradient than the others. Particle-size analysis revealed that lower-Stokes-number particles are harder to separate, especially at low beta. The introduction of axisymmetry significantly affected performance, reducing eta at low-flow splits compared to planar configurations. These results offer benchmark data for modeling and insight into how particle size and flow symmetry affect IPS performance.

Compressed air energy storage is a potential solution to the challenges of intermittency for wind turbines. Herein a novel concept is explored, Turbine-Integrated Isothermal Compressed Air Energy Storage (TICAES) which employs the wind-driven rotor as the motor for energy storage and employs the existing nacelle generator for energy regeneration. TICAES also achieved high efficiency by compressing and expanding the air with near-isothermal processes. In this study, this near-isothermal air compression and expansion is explored for an offshore wind turbine by using reciprocating compressor/expander with sprayed water droplets so the processes are nearly isothermal. The goal for such a spray-based system is to mitigate heat loss in the system by utilizing the high heat capacity and large surface area of water in the form of small droplets. A direct-drive approach is employed whereby the rotor speed (7.5 RPM) is also used for the compressor and expander speeds to avoid the need for a gearbox, since gearboxes can be problematic for offshore wind turbines. The slower speed of the rotor also allows higher heat transfer. This work also expands on previous spray-based studies by considering higher pressure ratios and by considering full-scale conditions consistent with a direct-drive offshore wind turbine for turbine-integrated compressed air energy storage. The spray-based numerical model is validated with experiments and is then used to assess the performance of a Megawatt-scale compression/expansion system for various droplet mass loadings and droplet diameters. The results suggest a range of system parameters that can yield isothermal roundtrip efficiencies greater than 90%. However, further work is needed to investigate the impact of multidimensional flow effects, non-linear piston velocities, cycle frequency, the influence of droplet collisions, and the differences between compression and expansion efficiencies. Investigating increased cycle frequencies is especially recommended to help reduce the very large displacement volumes.

As the world’s electric grids include greater shares of wind energy, the intermittency of wind power is becoming more problematic. As such, low-cost, high-efficiency, long-duration energy storage will be increasingly critical for electric grids with high wind shares. One storage option is utility-scale (e.g., 500 MW) Advanced Adiabatic Compressed Air Energy Storage (AACAES). However, AACAES requires large reservoir volumes (e.g., 690,000 m3), has low round-trip efficiency, and includes a complex thermal storage system to recover compression heat. This study introduces Advanced Isothermal Compressed Air Energy Storage (AICAES), which eliminates the need for dedicated thermal energy storage while improving efficiency through near-isothermal, spray-assisted reciprocating compression and expansion while also allowing elevated temperatures in the underground and surface reservoirs. To evaluate the proposed approach, reference AACAES and AICAES systems are defined, each rated at 500 MW with 8 h of storage duration. Results indicate that AICAES can increase round-trip efficiency by approximately 7

Rivers pose challenges for renewable energy device deployment due to shallow depths and variable, unsteady flow conditions. Vertically oscillating hydrofoil turbines appear well suited to accommodate changing flow conditions, produce high efficiencies, and reduce impact on wildlife. However, no studies exist on the impact of freestream turbulence on oscillating hydrofoil turbine performance. In this study, a semi-passive hydrofoil turbine is experimentally tested in a water channel, operating at chord-based Reynolds numbers ranging from 69000 to 91000. A passive turbulence grid was incorporated upstream to assess performance in a nearly uniform turbulence intensity profile with a turbulence intensity of approximately 5% and turbulent integral length scales on the order of 0.01 m. The velocity field just upstream of the hydrofoil was first characterized using hot-wire anemometry. Baffle boards were placed on top of the flow to mitigate free surface effects. Experiments with the hydrofoil device were then conducted with and without the turbulence grid over various flow speeds to obtain hydrofoil kinematics and force data. The results indicate that increased turbulence levels enhance power production by increasing heave velocities and vertical forces throughout one oscillation period. The most benefit was observed on the downstroke, where heave range/speed increased due to gravity and inertia. As a result, hydrodynamic efficiency increased by up to 12% with an observed maximum value of 52%.

An inertial particle separator (IPS) is a device used to remove particulates from an airflow by exploiting the inertia of particles and geometric constraints to manipulate the incoming inflow of an engine. It is commonly employed in gas turbine engines to protect internal components from damage caused by ingested debris, dust, and sand. In this study, we investigate the unsteady flow behavior and particle separation efficiency of an IPS using Large Eddy Simulation (LES) and Improved Delayed Detached Eddy Simulation (IDDES). The LES approach captured complex flow structures in the scavenge channel, including rippling effects and vortical structures upstream of the splitter that were previously undetected. Particle separation efficiency was examined across a wide range of sizes, showing a sigmoidal trend: near-zero efficiency for particles smaller than 10 mu m and over 95% separation for particles larger than 10 mu m. Efficiency for very large particles (>100 mu m) slightly decreased due to particle-wall interactions. The computational efforts demonstrated stable predictions of separation efficiency across time snapshots, with little sensitivity to mesh density or turbulence models. However, the model overestimated separation efficiency for smaller particles (6.5-13 mu m) compared to experimental results, indicating areas for future refinement. Overall, the study underscores the accuracy of LES for IPS simulations while identifying potential improvements for smaller particle predictions.

A barrier to the adoption of floating offshore wind turbines is their high cost relative to conventional fixed-bottom wind turbines. The largest contributor to this cost disparity is generally the floating substructure, due to its large size and complexity. Typically, a primary driver of the geometry and size of a floating substructure is the extreme environmental load case of Region 4, where platform loads are the greatest due to the impact of extreme wind and waves. To address this cost issue, a new concept for a floating offshore wind turbine’s substructure, its moorings, and anchors was optimized for a reference 10-MW turbine under extreme load conditions using OpenFAST. The levelized cost of energy was minimized by fixing the above-water turbine design and minimizing the equivalent substructure mass, which is based on the mass of all substructure components (stem, legs, buoyancy cans, mooring, and anchoring system) and associated costs of their materials, manufacturing, and installation. A stepped optimization scheme was used to allow an understanding of their influence on both the system cost and system dynamic responses for the extreme parked load case. The design variables investigated include the length and tautness ratio of the mooring lines, length and draft of the cans, and lengths of the legs and the stem. The dynamic responses investigated include the platform pitch, platform roll, nacelle horizontal acceleration, and can submergence. Some constraints were imposed on the dynamic responses of interest, and the metacentric height of the floating system was used to ensure static stability. The results offer insight into the parametric influence on turbine motion and on the potential savings that can be achieved through optimization of individual substructure components. A 36% reduction in substructure costs was achieved while slightly improving the hydrodynamic stability in pitch and yielding a somewhat large surge motion and slight roll increase.

ABSTRACTModern wind turbines have been continuously growing in size due to the increased power generation and reduced costs associated with larger rotors and more abundant wind resources offshore. In order to effectively implement pitch control on blades that are longer, more flexible, and heavier than ever before, modern electric pitch systems must provide enough torque to overcome blade pitch inertia and loads while providing suitable control response frequencies. Despite this need, there is limited published research on the sizing of such pitch systems at extreme scales. This study models peak pitching power and pitch actuator torque requirements in Regions 2 and 3 turbulent wind conditions. The developed model considers blade pitch response, pitching moments, pitch system dynamics, and blade aeroelasticity. The model is applied using an integrated wind turbine code used to simulate the turbine response of a 25‐MW offshore reference turbine with advanced pitch control under standardized turbulent wind conditions. The results show that the fastest pitch response requirements occur in Region 3 wind speeds just above the rated wind speed and that the peak pitch actuator torque requirements are correlated with maximum pitching moments. The model is extended to turbines ranging from 5 to 50 MW to develop a simplified scaling power law based on only the product of blade mass and mean chord length. This scaling law predicts maximum pitch actuator torque and maximum power consumed from pitch actuation based on results from computational simulations of multiple extreme‐scale reference turbines. This study provides useful insights for the design and sizing of pitch systems in large‐scale wind turbines.

To prevent particles from entering aircraft engines, Inertial Particle Separators (IPS) use a flow bend to split the intake into a particle-rich scavenge flow and an ideally clean core flow. While planar IPS configurations have been widely studied, they do not capture the axisymmetric nature of operational systems. To address this, a novel, vertically oriented, axisymmetric-sector IPS wind tunnel was built and characterized. The test section is a 50-degree sector geometry viewed from the incoming plane that enables detailed optical access while preserving realistic flow features of a full-scale and fully axisymmetric design, scaled to 37% of nominal dimensions. Surface oil streak visualizations, a high-fidelity control system for scavenge-to-core flow management, and particle separation efficiency tests with two test dusts (ISO 12,103-1 A4 Coarse Test Dust and C-Spec) were conducted. Flow visualization revealed larger and more prominent corner vortices compared to planar tests, stabilizing in size with increasing scavenge ratios. Particle separation efficiency showed moderate sensitivity to injection location, with efficiency generally lower than planar results. Scavenge leg pressure drop was the primary contributor to total system pressure drop. These findings provide new insights into axisymmetric IPS behavior and will help guide future separator designs.

An Inertial Particle Separator (IPS) is a device used in aircraft engines to remove sand and other particulates from ingested air, protecting engine components from erosion. This study presents a comprehensive experimental investigation of an axisymmetric IPS system in terms of particle separation efficiency. The effects of different particle sizes (A2, AFRL03, and A4 test dusts) and flow split ratios on separation efficiency were examined, incorporating rigorous uncertainty and repeatability analyses. A refined startup procedure was developed to minimize particle reingestion, significantly improving result consistency and accuracy. The study provides detailed experimental data for a reduced-flow-turning IPS geometry designed to minimize pressure losses. Results demonstrate the significant influence of particle size distribution and Stokes number on separation performance, with larger particles (A4) exhibiting higher separation efficiencies compared to smaller particles (A2). A robust dataset for validating advanced computational models is presented and findings contribute to ongoing efforts to enhance engine protection and performance in challenging operational environments, particularly for rotorcraft and other aircraft operating in sandy conditions.

Within their flight envelope, hypersonic vehicles can be exposed to environmental and atmospheric conditions that include meteorological particles and these particles can be damaging to a vehicle’s forebody, engine components, optical sensors, and radomes. Ice particles can be particularly problematic due to their large diameters and high capability for surface erosion and ice particles tend to appear at altitudes consistent with supersonic speeds for hypersonic vehicles. The objective of this study is to understand the trajectories and impacts for typical ice particles on the nose cone of a hypersonic vehicle traveling at supersonic speeds. The particle drag model used to determine the trajectory in a compressible flow considers effects of particle shape and Reynolds number (which were found to be greater than effects due to relative Mach number). Discrete velocity and temperature of the particles are tracked from up to the point of impact for a variety of ice sizes and shapes. The size was based on a volume-equivalent diameter, which is directly proportional to particle mass. The results indicate that particle trajectories are only weakly deflected by the surrounding flow before wall impact. Compared to column-like shaped ice, plate-like shaped ice particles impact the vehicle body at a somewhat lower speed and result in a lower impact fraction. This is a result of the larger drag coefficients for plate-like shaped ice. The inertia vs. drag balance can be incorporated using a Stokes number scaling for both velocity changes and collection efficiency. However, both shapes of particles have very high inertias for typical atmospheric sizes and thus tend to impact at speeds and angles consistent with freestream conditions. In addition, the ice particle temperature stays near freezing up to impact so that melting (phase change) effects are expected to be small. Finally, the temperature changes depend on particle mass and are relatively insensitive to particle shape.

As conventional upwind wind turbines grow larger, the increased mass and flexibility of the longer blades present challenges concerning costs, structural loads, and safety constraints such as tower clearance. At extreme scales, wind turbines in a downwind configuration may provide a feasible alternative to address these challenges by allowing lightweight, flexible blades that can reduce capital costs and blade loads while maintaining safety margins. Downwind turbine blades suffer from increased fatigue loading due to the tower shadow effect. In this study, novel downwind, three-bladed wind turbine designs at 25 MW rating with lightweight, flexible blades are evaluated and compared in terms of power production and structural loading. To obtain a baseline performance, a standard collective blade pitch wind turbine controller is implemented for the two downwind and one upwind turbine designs. Individual pitch control is then added for the downwind turbines to reduce structural fatigue on the turbine blades. In summary, the two downwind turbine designs that differ in rotor pre-coning and shaft tilt angles using collective and individual pitch control are compared against a conventional upwind turbine with collective pitch control at the same scale under turbulent wind conditions.

To minimize the levelized cost of energy (LCOE) of wind turbines, advanced co-design strategies are required that also consider the contribution of active blade pitch control to overall energy production and wind turbine cost. Thereby, the demanded closed-loop performance drives the requirements on the blade pitch actuation system, which needs to be carefully balanced. To enable this, an extended LCOE measure is developed in this paper using stochastic estimates for quantifying pitch actuation cost in terms of pitch power and closed-loop performance in terms of net energy production. Additionally, the impact of blade pitch deflections on structural loads and hence cost is evaluated considering both collective and individual pitch control. The interdependencies between the different design objectives are revealed in a case study carried out on a 25MW wind turbine, demonstrating the guidance for engineers toward cost-effective and efficient wind turbine designs.

To characterize the performance of icephobic coatings for aerospace applications, various shear-based techniques have been used. Generally, these techniques are conducted in conjunction with comparison tests on metals. In this study, a review of the various approaches for measuring adhesion for static and impact ice for metal and icephobic surfaces was done. This review indicated that many details of the test conditions either varied significantly among studies or were omitted. To address this uncertainty, new measurements were taken to examine in-situ ice-shear-adhesion strength for impact and static ice with various surfaces, using a consistent icing-research-tunnel facility with well-characterized and detailed test conditions. The results for the two different metals tested revealed a significantly higher ice adhesion for static ice compared to that for impact ice. However, the tested self-lubricated icephobic coating significantly reduced ice adhesion strength for both impact and static ice and this performance was retained after multiple icing tests. Based on the methodology review and the current experimental study results, it is recommended that future ice adhesion studies fully characterize the following: the apparatuses for shear measurement, which include protocols and procedures used; the surface chemistry and roughness; the thermal conditions of the air, water, and surface; and for impact ice, the droplet conditions such as velocity and size in order to ensure repeatability within a study and comparison across studies.

Continuously increasing offshore wind turbine scales require rotor designs that maximize power and performance. Downwind rotors offer advantages in lower mass due to reduced potential for tower strike, and is especially true at large scales, e.g., for a 25 MW turbine. In this study, three 25 MW downwind rotors, each with different prescribed lift coefficient distributions were designed (chord, geometry, and twist) and compared to maximize power production at unprecedented scales and Reynolds numbers, including a new approach to optimize rotor tilt and coning based on aeroelastic effects. To achieve this objective the design process was focused on achieving high power coefficients, while maximizing swept area and minimizing blade mass. Maximizing swept area was achieved by prescribing pre-cone and shaft tilt angles to ensure the aeroelastic orientation when the blades point upwards was nearly vertical at nearly rated conditions. Maximizing the power coefficient was achieved by prescribing axial induction factor and lift coefficient distributions which were then used as inputs for an inverse rotor design tool. The resulting rotors were then simulated to compare performance and subsequently optimized for minimum rotor mass. To achieve these goals, a high Reynolds number design space was developed using computational predictions as well as new empirical correlations for flatback airfoil drag and maximum lift. Within this design space, three rotors of small, medium and large chords were considered for clean airfoil conditions (effects of premature transition were also considered but did not significantly modify the design space). The results indicated that the medium chord design provided the best performance, producing the highest power in Region 2 from simulations while resulting in the lowest rotor mass, both of which support minimum LCOE. The methodology developed herein can be used for the design of other extreme-scale (upwind and downwind) turbines.

This book is a modern presentation of multiphase flow, from basic principles to state-of-the-art research. It explains dispersed fluid dynamics for bubbles, drops, or solid particles, incorporating detailed theory, experiments, simulations, and models while considering applications and recent cutting-edge advances. The book demonstrates the importance of multiphase flow in engineering and natural systems, considering particle size distributions, shapes, and trajectories as well as deformation of fluid particles and multiphase flow numerical methods. The scope of the book also includes coupling physics between particles and turbulence through dispersion and modulation, and specific phenomena such as gravitational settling and collisions for solid particles, drops, and bubbles. The eight course-based chapters feature over 100 homework problems, including theory-based and engineering application questions. The final three reference-based chapters provide a wide variety of particle point-force theories and models. The comprehensive coverage will give the reader a solid grounding for multiphase flow research and design, applicable to current and future engineering. This is an ideal resource for graduate students, researchers, and professionals.

An Inertial Particle Separator (IPS) is a non-barrier system installed at the inlet of a gas turbine to mitigate effects of particulate ingestion on the engine. This study presents experimental data for a wide range of flow path designs and test dusts. The experiments were conducted in a bifurcating wind tunnel with a rectangular cross-section and aggressive flow turning, injecting particles with a range of sizes and shapes. A new flow path geometry exhibited higher particle separation efficiency for a variety of test dusts over a range of operating conditions, including off-design cases. The new flow path used a smaller scavenge channel than previous designs in order to reduce flow separation, though not so small as to stop large particles from entering. Data postprocessing highlights the importance of particle Stokes number, scavenge mass flow ratio, and scavenge flow expansion ratio as key parameters in IPS design.

An Inertial Particle Separator (IPS) can help reduce performance degradation of gas turbine engines used in aircraft when operating in dusty environments. In particular, an IPS geometry uses flow curvature to separate particles from the core flow path, so they are ideally fully directed into the scavenge flow path. To better understand the multiphase flow physics and performance of an IPS system, an axisymmetric-sector vertical bifurcated wind tunnel was developed and tested. This wind tunnel was designed to emulate the fluid and particle physics associated with the axisymmetric flow bifurcation for typical operational conditions. The wind tunnel employs a 50-degree sector angle to allow optical accessibility combined with a ratio of throat height to throat radius of 0.17. The tunnel uses a geometry representative of typical systems in terms of air flow velocities but with a size of about 40% (based on throat height) relative to full-scale systems. The wind tunnel air flow is achieved with two exhaust blowers and a calibrated control system, which allows for high mass-flow stability and a wide range of flow split ratios between the core and scavenge flows. To characterize the tunnel’s aerodynamic performance, measurements were taken for mass-flow stability and surface flow visualization to investigate corner flows and separation regions. The results indicate that the flow is highly stable and has separation regions qualitatively similar to those seen in 2-D facilities.

A broad range of experimental data and resolved-surface simulations were surveyed in terms of their Richardson-Zaki exponents for solid particles (spherical, ellipsoidal and irregular in shape) along with spherical drops and bubbles (both clean and contaminated). For these data sets, the Richardson-Zaki exponent often reasonably represents the volume fraction effect on group particle drag increase for moderate volumetric concentrations, ranging from 3% to 30%. A separable effects assumption was used to determine the influence of Reynolds number on the Richardson-Zaki exponent for spheres (including solid spherical particles as well as clean and contaminated spherical bubbles) based on a hydro-static pressure gradient for a homogeneous mixture of particles in a liquid, and this method compared well with experiments. A similar influence was found for non-spherical fluid particles with moderate deformations. However, highly deformed bubbles can cause a drag reduction at high concentrations, owing to deformations aligned with the flow, so a Richardson-Zaki exponent is no longer valid. In contrast, non-spherical solid particles introduce an additional complexity due to changes in particle orientation as volume fraction increases, especially at low Reynolds numbers, which instead accentuates the drag increase due to volume fraction.