Savonius rotor, a class of drag-driven vertical axis wind turbine, has been extensively investigated mainly to calculate the torque and power coefficients (C T and C P ) by various investigators. Hitherto, studies related to lift and drag characteristics are very few and have mainly been restricted to a semicircular-bladed rotor. A deeper investigation into the drag and lift coefficients (C D and C L ) can result in the better design of rotor blades leading to an increment in C T and C P . In view of this, in the present investigation, C D and C L of an elliptical-bladed rotor with vent augmenters have been studied numerically. Initially, two-dimensional (2D) unsteady simulations using an ansys fluent solver is carried out to estimate the instantaneous C D and C L . The shear stress transport (SST) k–ω turbulence model is selected to solve the Reynolds averaged Navier Stokes (RANS) equations. Finally, three-dimensional (3D) unsteady simulations are carried out for the vented elliptical-bladed rotor. The unsteady simulations are performed for the nonvented elliptical- and semicircular-bladed rotors at the identical condition in order to have a direct comparison. From the unsteady simulations, the average C D for the vented elliptical profile is found to be 1.45; whereas, the average C D for the nonvented elliptical and semicircular profiles is found to be 1.43 and 1.35, respectively.

This study uses the Reynolds-averaged Navier–Stokes (RANS) equations to validate a canopy model by computing a fully developed wind flow within and above a horizontally homogeneous dense forest as in the work of Dalpé and Masson. The model is paired with a modified k– ε turbulence closure. A set of boundary conditions (BCs) that rely on the law of the wall for a sustainable atmospheric boundary layer (ABL) is used. All simulations are conducted in the open source software OpenFOAM v.2.4.0 (OpenCFD Ltd (ESI Group)). Two practical aspects are considered in the validation process. First, an accurate leaf area index (LAI) integration to exactly fit the wind shear is evaluated. Since the physical foliage parameters may not be accessible for all type of forests, a generic leaf area density α distribution is tested. The results of this test show that a generic distribution is sufficient for preliminary analyses to improve accuracy of wind flow predictions over forested terrain. Second, the approach of Dalpé and Masson is limited to cyclic BCs which are not practical for real sites. For cases without cyclic BCs, imposing a proper slope on the inlet velocity profile is of high importance. This condition can be achieved through adjustment of the roughness length at the inlet.

This study presents an innovative idea to augment heat transfer to an air heater using helicoidal finned arrangement. A parametric analysis of the helicoidal shaped fin geometry is considered with helicoidal pitch ratio of 0.1666–0.3, fin diameter ratio of 1.75–2. For the placement of the fin beneath the absorber plate, longitudinal pitch ratio ranging from 0.0416 to 0.1666 are used. The flow Reynolds number used for the study ranges from 4800 to 25,000. The effects of helicoidal pitch ratio, wire diameter ratio and longitudinal pitch ratio on Nusselt number and friction factor have been discussed. It is seen from the analysis that there is a significant improvement in Nusselt number for the case of helicoidal fin of wire diameter ratio of 1 when compared to base model as well as straight fin model for the operating range of Reynolds number. It is also observed from the analysis that for the helicoidal fin configuration of helicoidal pitch ratio of 0.2333, friction factor appears to be moderate. Flow and roughness parameters for roughened solar air heater have been optimized using thermal-hydraulic enhancement factor (THEF). The study reveals that by the use of helicoidal fins, maximum enhancement in the Nusselt number is found to be 2.21 times when compared to the base model for longitudinal pitch ratio of 0.0416, helicoidal pitch ratio of 0.166 for a fixed wire diameter. The improvement obtained in performance corresponding to increased Nusselt number establishes the efficacy the helicoidal fin design for the absorber plate.

A solar radiation model is applied to a low temperature water-in-glass evacuated tubes solar collector to predict its performance via computational fluid dynamics (CFD) numerical simulations. This approach allows obtaining the transmitted, reflected, and absorbed solar radiation flux and the solar heat flux on the surface of the evacuated tubes according to the geographical location, the date, and the hour of a day. Different environmental and operational conditions were used to obtain the outlet temperature of the solar collector; these results were validated against four experimental tests based on an Official Mexican Standard resulting in relative errors between 0.8% and 2.6%. Once the model is validated, two cases for the solar collector were studied: (i) different mass flow rates under a constant solar radiation and (ii) different solar radiation (due to the hour of the day) under a constant mass flow rate to predict its performance and efficiency. For the first case, it was found that the outlet temperature decreases as the mass flow rate increases reaching a steady value for a mass flow rate of 0.1 kg/s (6 l/min), while for the second case, the results showed a corresponding outlet temperature behavior to the solar radiation intensity reaching to a maximum temperature of 36.5 °C at 14:00 h. The CFD numerical study using a solar radiation model is more realistic than the previous reported works leading to overcome a gap in the knowledge of the low temperature evacuated tube solar collectors.

An accurate assessment of the amount solar radiation incident at specific locations is highly complex due to the dependence of available solar radiation on many meteorological and topographic parameters. Reunion Island, a small tropical French territory, intends to deploy solar energy technologies rapidly. In this context, the variability and intermittency of solar irradiance in different regions of the island is of immediate interest if the generated energy will be integrated in the existing energy network. This paper identifies different features of spatial and temporal variability of daily global horizontal irradiance (GHI) observed on Reunion Island. For this purpose, trends in the mean daily as well as seasonal variability of GHI were investigated. Furthermore, the intermittency and multifractal behaviors of the spatial daily GHI change were examined. Analyzing this daily variability is crucial to day-ahead forecasting of solar resource for better managing solar integration in the power grid, particularly in small island states with isolated power systems. Results revealed that the difference in cumulative GHI for two successive days ranges between −10 and 10 kW/m 2 /day while the highest and lowest variability of daily change occurs during summer and winter, respectively. The decorrelation distance, which gives a measure of the distance over which the variability at distinct geographic locations become independent of one another at a given timescale, was also calculated. It was found that the average decorrelation distance for day-to-day GHI change is about 22 km, a smaller value than that calculated by the previous studies using much sparser radiometric networks. The Hurst exponent, fractal co-dimension, and Lévy parameter, which describe solar radiation intermittency, were also evaluated for Reunion Island.

The ribbed three-dimensional solar air heater (SAH) model is numerically investigated to estimate flow and heat transfer through it. The numerical analysis is based on finite volume approach, and the set of flow governing equations has been solved to determine the heat transfer and flow field through the SAH. For detailed analysis, rib chamfer height ratio (e′/e) and rib aspect ratio (e/w), two innovative parameters, have been created and considered along with the commonly used roughness parameter, i.e., relative roughness height, e/D. The parameters e′/e, e/w, and e/D are varied from 0.0 to 1, 0.1 to 1.5, and 0.18 to 0.043, respectively, but the value of P/e is kept constant for the entire investigation at 12. A good match is seen in Nusselt number (Nu) and friction factor (f) by comparing the predicted results with the experimental ones. With the variation of roughness parameters, distinguishable change in Nu and f is obtained. The highest value of thermohydraulic performance parameter (TPP) observed is 2.08 for P/e, e′/e, e/w, and e/D values of 12, 0.75, 1.5, and 0.043, respectively, at Re of 17,100. The developed generalized equation for Nu and f has shown acceptable percentage deviation under the studied range of parameters.

This study presents an experimental investigation on the effects of winglets on the near wake flow around the tip region and on the tip vortex characteristics downstream of a 0.94 m diameter three-bladed horizontal axis wind turbine (HAWT) rotor. Phase-locked 2D particle image velocimetry (PIV) measurements are performed with and without winglets covering 120 deg of azimuthal progression of the rotor. The impact of using winglets on the flow field near the wake boundary as well as on the tip vortex characteristics such as the vortex convection, vortex core size, and core expansion as well as the resultant induced drag on the rotor are investigated. Results show that winglets initially generate an asymmetric co-rotating vortex pair, which eventually merge together after about ten tip chords downstream to create a single but nonuniform vortex structure. Mutual induction of the initial double vortex structure causes a faster downstream convection and a radially outward motion of tip vortices compared to the baseline case. The wake boundary is shifted radially outward, velocity gradients are diffused, and vorticity and turbulent kinetic energy levels are significantly reduced across the wake boundary. The tip vortex core sizes are three times as big compared to those of the baseline case, and within the vortex core, vorticity and turbulent kinetic energy levels are reduced more than 50%. Results show consistency with various vortex core and expansion models albeit with adjusted model coefficients for the winglet case. The estimated induced drag reduction is about 15% when winglets are implemented.

Advanced and model-based control techniques have become prevalent in modern wind turbine controls in the past decade. These methods are more attractive compared to the commonly used proportional-integral-derivative (PID) controller, as the turbine structural flexibility is increased with multiple and coupled modes. The disturbance accommodating control (DAC) is an effective turbine control approach for the above-rated wind speed region. DAC augments the turbine state-space model with a predefined disturbance waveform model, based on which the controller reduces the impact of wind disturbances on the system output (e.g., rotor speed). However, DAC cannot completely reject the wind disturbance in certain situations, and this results in steady-state regulation errors in the turbine rotor speed and electric power. In this paper, we propose a novel wind turbine pitch control using optimal control theory. The obtained feedback and feedforward control terms function to stabilize the turbine system and reject wind disturbances, respectively, derived systematically based on the Hamilton–Jacobi–Bellman (HJB) equation. Simulation results show that the proposed method achieves desired rotor speed regulation with significantly reduced steady-state errors under turbulent winds, which is simulated on the model of the three-bladed controls advanced research turbine (CART3) using the FAST code.

Wind turbines are subjected to fatigue loading during their entire lifetime due to the fluctuating excitation from the wind. To predict the fatigue damage, the design standard IEC 61400-1 describes how to parametrize an on-site specific wind climate using the wind speed, turbulence, wind shear, air density, and flow inclination. In this framework, shear is currently modeled by its mean value, accounting for neither its natural variance nor its wind speed dependence. This very simple model may lead to inaccurate fatigue assessment of wind turbine components, whose structural response is nonlinear with shear. Here we show how this is the case for flapwise bending of blades, where the current shear model leads to inaccurate and in worst case nonconservative fatigue assessments. Based on an optimization study, we suggest modeling shear as a wind speed dependent 60% quantile. Using measurements from almost one hundred sites, we document that the suggested model leads to accurate and consistent fatigue assessments of wind turbine blades, without compromising other main components such as the tower and the shaft. The proposed shear model is intended as a replacement to the mean shear, and should be used alongside the current IEC models for the remaining climate parameters. Given the large number of investigated sites, a basis for evaluating the uncertainty related to using a simplified statistical wind climate is provided. This can be used in further research when assessing the structural reliability of wind turbines by a probabilistic or semiprobabilistic approach.

Computational fluid dynamics (CFD) simulations of wind turbine wakes are strongly influenced by the choice of the turbulence model used to close the Reynolds-averaged Navier-Stokes (RANS) equations. A wrong choice can lead to incorrect predictions of the velocity field characterizing the wind turbine wake and, consequently, to an incorrect power estimation for wind turbines operating downstream. This study aims to investigate the influence of different turbulence models, namely the k – ε , k – ω , SST k – ω , and Reynolds stress models (RSM), on the results of CFD wind turbine simulations. Their influence was evaluated by comparing the CFD results with the publicly available experimental measurements of the velocity field and turbulence quantities from the Sexbierum and Nibe wind farms. Consistent turbulence model constants were proposed for atmospheric boundary layer (ABL) and wake flows according to previous literature and appropriate experimental observations, and modifications of the derived turbulence model constants were also investigated in order to improve agreement with experimental data. The results showed that the simulations using the k– ε and k– ω turbulence models consistently overestimated the velocity and turbulence quantities in the wind turbine wakes, whereas the simulations using the shear-stress transport (SST) k– ω and RSMs could accurately match the experimental data. Results also showed that the predictions from the k– ε and k– ω turbulence models could be improved by using the modified set of turbulence coefficients.

Leading edge erosion is a considerable threat to wind turbine performance and blade maintenance, and it is very imperative to accurately predict the influence of various degrees of erosion on wind turbine performance. In the present study, an attempt to investigate the effects of leading edge erosion on the aerodynamics of wind turbine airfoil is undertaken by using computational fluid dynamics (CFD) method. A new pitting erosion model is proposed and semicircle cavities were used to represent the erosion pits in the simulation. Two-dimensional incompressible Reynolds-averaged Navier–Stokes equation and shear stress transport (SST) k–ω turbulence model are adopted to compute the aerodynamics of a S809 airfoil with leading edge pitting erosions, where the influences of pits depth, densities, distribution area, and locations are considered. The results indicate that pitting erosion has remarkably undesirable influences on the aerodynamic performance of the airfoil, and the critical pits depth, density, and distribution area degrade the airfoil aerodynamic performance mostly were obtained. In addition, the dominant parameters are determined by the correlation coefficient path analysis method, results showed that all parameters have non-negligible effects on the aerodynamics of S809 airfoil, and the Reynolds number is of the most important, followed by pits density, pits depth, and pits distribution area. Meanwhile, the direct and indirect effects of these factors are analyzed, and it is found that the indirect effects are very small and the parameters can be considered to be independent with each other.

The fluid flow characteristics and heat transfer in triangular duct solar air heater (SAH) have been studied experimentally and numerically for Reynolds number range from 4000 to 18,000. In the present paper, three different models of triangular duct solar air heater were considered, namely, model 1 with simple triangular duct, model 2 with rounded corner on one side of the triangle with fixed radius of curvature of 0.39 times the duct height as flow passage, and model 3 with rounded corner on one side of the triangular duct with roughness on the absorber plate of SAH. The absorber plate and apex angle values are assumed as constant in all the three models of SAH, i.e., 160 mm and 60 deg, respectively. The three-dimensional numerical simulations were performed by discretization of computational domain using finite volume method (FVM) and are analyzed with the help of computational fluid dynamics (CFD) code. Experiments were performed to validate numerical results by comparing absorber plate temperature along the length of the SAH. A detailed analysis of different models of solar air heater was carried out by solving flow governing equations numerically on ansys fluent 12.1. A close match has been observed between the simulated and experimental results of SAH with maximum percentage deviation of approximately ±5% in absorber plate temperature. The rounded apex improves velocity distribution near the corner region and helps in improving heat transfer. In the three studied models of solar air heater, the best performance is observed in the case of model 3.

The working principle of particle-based solar receivers is to utilize the absorptivity of a dispersed particle phase in an otherwise optically transparent carrier fluid. In comparison to their traditional counterparts, which use a solid surface for radiation absorption, particle-based receivers offer a number of opportunities for improved efficiency and heat transfer uniformity. The physical phenomena at the core of such receivers involve coupling between particle transport, fluid turbulence, and radiative heat transfer. Previous analyses of particle-based solar receivers ignored delicate aspects associated with this three-way coupling. Namely, these investigations considered the flow fields only in the mean sense and ignored turbulent fluctuations and the consequent particle preferential concentration. In the present work, we have performed three-dimensional direct numerical simulations of turbulent flows coupled with radiative heating and particle transport over a range of particle Stokes numbers. Our study demonstrates that the particle preferential concentration has strong implications on the heat transfer statistics. We demonstrate that “for a typical setting” the preferential concentration of particles reduces the effective heat transfer between particles and the gas by as much as 25%. Therefore, we conclude that a regime with Stokes number of order unity is the least preferred for heat transfer to the carrier fluid. We also provide a 1D model to capture the effect of particle spatial distribution in heat transfer.

Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.

The problem of particle deposition on the glass window of a solar receiver has restricted its continuous operation by reducing solar radiation transmission. A rigorous attempt has been made in this analysis by exploring the understanding of particle deposition mechanisms and their mitigating strategies. A simplified form of a fluidized bed solar receiver (FBSR) having the same flow phenomena of FBSR is chosen for the numerical analysis. In the numerical analysis, the turbulent flow in the receiver is investigated by renormalized group (RNG) theory based k–ε models. The validation of the numerical model is carried out by measuring the turbulent flow properties using a turbulent flow instrumentation (TFI) Cobra probe. The results of this analysis revealed that mass flow into the secondary concentrator of the receiver was reduced significantly when the ratio between the outlets and inlet areas was 0.5, and the ratio between the aperture and receiver diameter was 0.41. When using window shielding jets, only 5% of the inlet mass as a window jet was sufficient to prevent any particle deposition on the glass window, however, the number of jets was found to be an important factor. At a constant mass flow rate, increasing the number of window shielding jets reduced the suction pressure from the core to the aperture, which helped to restrict the inlet flow in the secondary concentrator.

Site-specific assessment of wind turbine design requires verification that the individual wind turbine components can survive the site-specific wind climate. The wind turbine design standard, IEC 61400-1 (third edition), describes how this should be done using a simplified, equivalent wind climate established from the on-site distribution functions of the horizontal mean wind speeds, the 90% quantile of turbulence along with average values of vertical wind shear and air density and the maximum flow inclination. This paper investigates the accuracy of fatigue loads estimated using this equivalent wind climate required by the current design standard by comparing damage equivalent fatigue loads estimated based on wind climate parameters for each 10 min time-series with fatigue loads estimated based on the equivalent wind climate parameters. Wind measurements from Boulder, CO, in the United States and Høvsøre in Denmark have been used to estimate the natural variation in the wind conditions between 10 min time periods. The structural wind turbine loads have been simulated using the aero-elastic model FAST. The results show that using a 90% quantile for the turbulence leads to an accurate assessment of the blade root flapwise bending moment and a conservative assessment of the tower bottom for-aft bending moment and low speed shaft torque. Currently, IEC 61400-1 (third edition) neglects the variation in wind shear by using the average value. This may lead to a nonconservative assessment of blade root flapwise fatigue loads, which are sensitive to wind shear. The results in this paper indicate that using a 75% quantile for the wind shear at each wind speed bin leads to an appropriate, but conservative, assessment of the fatigue loads. However, care should be taken when using this approach for components where low or negative wind shears can lead to large fatigue loads. This is the case for some drivetrain components where a lower quantile may be required.

The objective of this study is to investigate how different volumetric projection techniques used in actuator-line modeling affect the unsteady blade loads and wake turbulence statistics. The two techniques for the body-force projection radius are based on either (i) the grid spacing or (ii) the combination of grid spacing and an equivalent elliptic blade planform. An array of two National Renewable Energy Laboratory 5-MW turbines separated by seven rotor diameters is simulated for 2000 s (about rotor 300 revolutions) within a large-eddy simulation (LES) solver of the neutral and moderately convective atmospheric boundary layer (ABL). The statistics of sectional angle of attack (AOA), blade loads, and turbine power histories are quantified. Moreover, the degree of unsteadiness of sectional blade loads in response to atmospheric and wake turbulence is computed via a reduced frequency based on the rate-of-change in sectional AOA. The goal of this work is to make the wind energy community aware of the uncertainties associated with actuator-line modeling approaches.

High-speed vehicle motion on the highways produces localized winds whose energy can be harnessed. These local winds have less variability especially if the highway traffic is constant. The idea of extracting energy from highway winds has been conceptualized in many studies before. However, the feasibility of this idea has never been tested using analytical, computation, or experimental methods. In this study, we numerically compute the amount of power that can be extracted from local highway winds due to vehicular motion. A unsteady Reynolds-averaged Navier–Stokes (URANS) method is used for modeling the atmospheric boundary layer (ABL). Realistic computer-aided design (CAD) models of cars and trucks separated by spacing information obtained from the existing standards are used to model the vehicle motion. A vertical axis wind turbine (VAWT) is used for extracting energy from the wind. The entire framework of ABL, vehicles, and turbine is simulated using overset grids and multiple translating and rotating frames of reference. Many vehicle motion scenarios were compared to the case of an isolated wind turbine. The initial results show a significant increase in the power that can be extracted by these turbines. The average extracted power increases about 317% when compared to the case without any vehicular motion. Field measurements or wind tunnel studies are required to provide validation for the computations and to determine if more advanced turbulence modeling methodologies have to be employed for these studies.

The solar chimney is a natural ventilation technique that has the potential to save energy use in buildings as well as maintain comfortable indoor quality. The objective of the current study was to examine the effects of the wall-solar chimney on airflow distribution and thermal conditions in a room. In the current work, computational fluid dynamics (CFD) was used to model a solar chimney. The solar chimney was modeled three-dimensionally for a more realistic simulation of fluid and thermal conditions. Experimental and numerical data from literature were used to validate the current model, and the results agreed very well. The current study showed that the flow in the solar chimney system can be either laminar or turbulent depending on the parameters of the system, and that the effect of the chimney inlet was more significant than that of the chimney width (air gap between the glass and absorber) on the flow regime. This study also developed a new characteristic Rayleigh number (R a * ) relating the chimney inlet and width, which showed good consistency with the prediction of the flow regime. The investigations of R a * and the flow regime indicated that the flow becomes turbulent for R a * ∼ 0.8 × 10 8 . Finally, the potential improvements of the designs were discussed by observing the flow and thermal conditions of the room.

In this study, Reynolds-averaged Navier–Stokes (RANS) simulations are performed using the k-ε and k-ω shear stress transport (SST) turbulence closure schemes to investigate the interactions of horizontal-axis wind turbine (HAWT) models in the neutrally stratified atmospheric boundary layer (ABL). A comparative study of actuator disk, actuator line, and full rotor models of the National Renewable Energy Laboratory (NREL) 5 MW reference turbine is presented. The open-source computational fluid dynamics (CFD) code openfoam 2.1.0 and the commercial software ansysfluent 13.0 are used for simulations. Single turbine models are analyzed for turbulent structures and wake resolution in the downstream region. To investigate the influence of the incident wind field on very large turbine blades, a high-resolution full rotor simulation is carried out for a single turbine to determine blade pressure distributions. Finally, simulations are performed for two inline turbines spaced 5 diameters (5D) apart. The research presented in this study provides an intercomparison of three dominant HAWT models operating at rated conditions in a neutral ABL using an RANS framework. Furthermore, the pressure distributions of the highly resolved full rotor model (FRM) will be useful for future aeroelastic structural analysis of anisotropic composite blade materials.