Row Of Turbines Research Articles

Effect of layout on asymptotic boundary layer regime in deep wind farms

This paper presents wind tunnel data for 56 different layouts of a scaled wind farm with 100 porous disk models. The data indicates that a nonuniform streamwise spacing between turbine rows can increase power output in the fully developed and entrance regions of large wind farms.

ЗНИЖЕННЯ РІВНЯ ДИНАМІЧНИХ НАПРУГ У РОБОЧИХ ЛОПАТКАХ ТУРБІНИ ЗА РАХУНОК ВИБОРУ ГЕОМЕТРІЇ СОПЛОВОГО АПАРАТУ

Interacting between the next rows of the turbine creates a circumferential flow non-uniformity, which leads to origination of resonance dynamic stresses on rotor bladings with frequencies z·fn, where fn - a rotor rotation frequency, z - number of stator vanes. At projection and development of the engine is not always possible detuning from a resonance as the spectrum of eigenfrequencies of rotor blades can be wide enough in relation to a band of working rotor speed. Reduction of exterior exciting forces can be one of ways of a reduction of dynamic stresses in rotor blades. For attenuation of these forces intensity was possibly use of a stator vanes with a different spacing, and also with the blades inclined in a circumferential direction. In the given article numerical research for choice a distribution law of stator vanes spacing and a declivity angle of its blades, allowed to diminish amplitude of the unsteady air forces acting on rotor blades with frequency z·fn are presented. As object for examinations the stage of the air starter turbine, the containing 26 nozzle vanes disposed with different spacings, and 40 rotor blades without a binding has served. Rotor blades and turbine disk of the air starter are made for a single whole of an aluminium alloy. This work was executed stage by stage: in the beginning the angular disposition of vanes blades, giving maximum decrease of exciting forces on rotor blades, by results of unsteady flow calculation in the turbine was chosen; then for the found geometry of a vanes the slope angle of its blades in the circumferential direction, giving the maximum decrease of exciting forces on rotor blades was chosen. The viscous gas unsteady flow was modelled in the computational domain including all blade passages of turbine rows - 26 channels in a nozzle and 40 channels in the rotor wheel. By results of calculation dependence of decrease unsteady force acting on blades and changes of turbine efficiency from a slope angle of vanes is presented. Reduction of dynamic stresses level in rotor blades of the turbine at the expense of decrease of aerodynamic exciting forces amplitude is attained. The numerical result is confirmed experimentally in rig test by decrease of resonance stresses on explored frequencies.

Exploring the benefits of vertically staggered wind farms: Understanding the power generation mechanisms of turbines operating at different scales

AbstractWind farms are known to modulate large scale structures in and around the wake regions of the turbines. The potential benefits of placing small hub height, small rotor turbines in between the large turbines in a wind farm to take advantage of such modulated large‐scale eddies are explored using large eddy simulation (LES). The study has been carried out in an infinite wind farm framework invoking an asymptotic limit, and the wind turbines are modeled using an actuator line model. The vertically staggered wind turbine arrangements that are studied in the present work consist of rows of large wind turbines, with rows of smaller wind turbines (ie, smaller rotor size and shorter hub height) placed in between the rows of large turbines. The influence of the hub height of the small turbines, in particular, how it affects the interactions between the large and small turbines and consequently their power, along with the multiscale dynamics involved, has been assessed in the current study. It was found that, in the multiscale layouts, the small turbines at lower hub heights operate more efficiently than their homogeneous single‐scale counterparts. In contrast, the small turbines with higher hub heights incur a loss of power compared with the corresponding single‐scale arrangements.

Large eddy simulations of the effect of vertical staggering in large wind farms

AbstractIn order to study the effect of vertical staggering in large wind farms, large eddy simulations (LES) of large wind farms with a regular turbine layout aligned with the given wind direction were conducted. In the simulations, we varied the hub heights of consecutive downstream rows to create vertically staggered wind farms. We analysed the effect of streamwise and spanwise turbine spacing, the wind farm layout, the turbine rotor diameter, and hub height difference between consecutive downstream turbine rows on the average power output. We find that vertical staggering significantly increases the power production in the entrance region of large wind farms and is more effective when the streamwise turbine spacing and turbine diameter are smaller. Surprisingly, vertical staggering does not significantly improve the power production in the fully developed regime of the wind farm. The reason is that the downward vertical kinetic energy flux, which brings high velocity fluid from above the wind farm towards the hub height plane, does not increase due to vertical staggering. Thus, the shorter wind turbines are effectively sheltered from the atmospheric flow above the wind farm that supplies the energy, which limits the benefit of vertical staggering. In some cases, a vertically staggered wind farm even produced less power than the corresponding non vertically staggered reference wind farm. In such cases, the production of shorter turbines is significantly negatively impacted while the production of the taller turbine is only increased marginally.

Actuator-line CFD modelling of tidal-stream turbines in arrays

CFD modelling of tidal turbines in arrays is described and assessed against experimental studies of turbines operating either at constant speed or constant torque. Rotor blades are represented by rotating actuator lines, whilst supports are represented by partially-blocked-out cells. For a single turbine the model successfully reproduces towing-tank measurements of thrust and power coefficients across a range of tip-speed ratios. For two turbines staggered streamwise, it is demonstrated that loads may be reduced or augmented, according as the downstream turbine is in the wake or bypass flow of the upstream turbine. When the downstream turbine is partially in the wake, individual blades are subject to large cyclic load fluctuations. Array performance is evaluated by comparison with experimental data, modelling up to 12 turbines in up to three staggered rows. The speed of each turbine is continuously adjusted in response to flow-induced torque. Distribution of thrust coefficients within the array is well reproduced, but there is greater discrepancy in angular speed. With actuator representation of blades, the choice of turbulence model has little effect on load coefficients for an isolated turbine or row of turbines, but a significant effect on the wake, and hence on downstream turbines in an array.

Mean kinetic energy replenishment mechanisms in vertical-axis wind turbine farms

We analyze kinetic energy (KE) transport allowing wind speed recovery behind a row of wind turbines, showing that KE replenishment is significant after about the 6th row. Before that there is net negative transport out of the turbine layer, and initial advection of KE is the main energy source.

Applications of satellite winds for the offshore wind farm site Anholt

Abstract. Rapid growth in the offshore wind energy sector means more offshore wind farms are placed closer to each other and in the lee of large land masses. Synthetic aperture radar (SAR) offers maps of the wind speed offshore with high resolution over large areas. These can be used to detect horizontal wind speed gradients close to shore and wind farm wake effects. SAR observations have become much more available with the free and open-access data from European satellite missions through Copernicus. Examples of applications and tools for using large archives of SAR wind maps to aid offshore site assessment are few. The Anholt wind farm operated by the utility company Ørsted is located in coastal waters and experiences strong spatial variations in the mean wind speed. Wind speeds derived from the Supervisory Control And Data Acquisition (SCADA) system are available at the turbine locations for comparison with winds retrieved from SAR. The correlation is good, both for free-stream and waked conditions. Spatial wind speed variations along the rows of wind turbines derived from SAR wind maps prior to the wind farm construction agree well with information gathered by the SCADA system and a numerical weather prediction model. Wind farm wakes are detected by comparisons between images before and after the wind farm construction. SAR wind maps clearly show wakes for long and constant fetches but the wake effect is less pronounced for short and varying fetches. Our results suggest that SAR wind maps can support offshore wind energy site assessment by introducing observations in the early phases of wind farm projects.

Power output efficiency in large wind farms with different hub heights and configurations

Abstract Large-eddy simulation (LES) is used to investigate the effect of the spatial arrangement of a utility-scale wind turbine array on the power outputs. Eight turbine-array layouts are considered, where each has 120 turbines installed in 30 rows with aligned or staggered configurations along the wakewise direction. We perform the LESs of neutrally-stratified atmospheric boundary layer over the eight large wind farms with the turbines arranged with a perfectly-aligned configuration, four laterally-staggered configurations, and three vertically-staggered configurations. Unlike the alignment of the turbine micro-siting in the aligned wind farm, both the laterally-staggered and vertically-staggered configurations lead to the misalignment of the turbines with staggered arrangement in the lateral and vertical directions. Simulation results show that the power outputs in the wind farms have obvious decreases to 45–65% within the first 12 turbine rows and retain within that range in the rest. In general, the staggered wind farms produce more power than the aligned wind farm. The laterally-staggered wind farm configurations due to the better adaptability in spatial configuration provide higher power production in the first 6th turbine rows. Moreover, the vertically-staggered configuration causes clear reduction in the maxima of the velocity deficit and the turbulence intensity inside the farm wake flow.

Preliminary Design Method for Dense-Gas Supersonic Axial Turbine Stages

A fast preliminary design methodology for supersonic organic Rankine cycle (ORC) stator and rotor axial turbine blades with low degree of reaction is presented. First, the stator and rotor blade mean-line profiles are designed by using the two-dimensional (2D) method of characteristics (MOC), extended to gases governed by general equations of state (EOS). We focus more specifically on working fluids with medium to high molecular complexity, operating at thermodynamic conditions such that the fundamental derivative of gas dynamics Γ is lower than one in a significant portion of the flow field. For rotor blades, MOC is combined with a free-vortex method to achieve a smooth deflection of the supersonic incoming flow. A numerical approach is developed for solving the unique incidence problem in the case of gases governed by general EOS. Both stator and rotor blade geometries designed according to the inviscid MOC model are subsequently corrected to account for the development of viscous boundary layers by solving the compressible integral boundary layer equations extended to dense gases. The resulting blade designs are assessed by means of computational fluid dynamics (CFD) simulations based on a high-order finite volume solver equipped with advanced thermodynamic and transport-property models. Properly accounting for dense gas and viscous effects at an early design stage is found to improve the expected performance of ORC turbine rows significantly and delivers valuable baseline profiles for any further optimization.

Behavioral accommodation of nesting hawks to wind turbines

ABSTRACTThe most direct effects of wind energy development on birds are rotor blade strikes and collisions with wind turbines. Probability of a blade strike is affected by the rates at which a bird visits the rotor swept zone (RSZ), and mitigated by the behaviors it uses to avoid being struck. Between 2011 and 2013, we studied the occurrence and nature of avoidance responses of 3 nesting ferruginous hawks (Buteo regalis), 5 Swainson's hawks (B. swainsoni), and 2 red‐tailed hawks (B. jamaicensis) to turbines built within their territories. Two of 10 breeding males we tracked with satellite telemetry were turbine‐collision fatalities, one during the study and the other in the following year. Collectively, nesting hawks displayed no macro‐avoidance of turbine projects and flew adjacent to or within RSZs at high rates ( = 0.8 visits/hr, 1.3 min/visit; n = 387 visits during 434 hr of observation) synchronized with morning and afternoon peaks in foraging activity. Passes through rotors accounted for 9.8% of responses. The turbine that hawks approached most closely from collective encounters on each territory accounted for an average of 29% of all visits to RSZs and visitation was greater than expected at turbines <0.8 km from nests (P < 0.001). As rotor speed increased, the frequency that hawks in RSZs flew between turbine rows increased relative to other avoidance behaviors and hawks that were potentially distracted (i.e., hunting or aggressive chasing) increased avoidance flights over and between turbines. The ability of hawks to employ evasive behaviors mitigated blade collision but allowed for close, hourly interaction with turbines that increased risk of collision. Where nesting populations of hawks are at risk, particularly ferruginous hawks, pre‐construction siting of wind turbines needs to account for nest locations and expected high rates of hawk visitation to turbines <800 m from nests. On existing projects collision risk can be reduced through operational shutdown of turbines in this zone during 2 daily peaks in hawk foraging in the vicinity where hawks nest, but a greater effectiveness can be achieved by shutdown of specific turbines associated with highest regular use. © 2018 The Wildlife Society.

Wind Farm Blockage and the Consequences of Neglecting Its Impact on Energy Production

Measurements taken before and after the commissioning of three wind farms reveal that the wind speeds just upstream of each wind farm decrease relative to locations farther away after the turbines are turned on. At a distance of two rotor diameters upstream, the average derived relative slowdown is 3.4%; at seven to ten rotor diameters upstream, the average slowdown is 1.9%. Reynolds-Averaged Navier-Stokes (RANS) simulations point to wind-farm-scale blockage as the primary cause of these slowdowns. Blockage effects also cause front row turbines to produce less energy than they each would operating in isolation. Wind energy prediction procedures in use today ignore this effect, resulting in an overprediction bias that pervades the entire wind farm.

Modeling of horizontal axis wind turbine wakes in Horns Rev offshore wind farm using an improved actuator disc model coupled with computational fluid dynamic

In order to increase the accuracy and ability to predict the total energy gained in wind farms, it is necessary to use accurate wake models. In the present study, an improved methodology is applied on actuator disc in order to take all the operational and geometrical characteristics into account such as airfoil type, angular velocity, twist, and chord distribution. In wind farms, turbines are affected by upstream ones resulting in a non-uniform upstream velocity for each turbine. However, in literature, for all the wind turbines in a wind farm, thrust coefficient curve at undisturbed wind speed is used in order to estimate the upstream speed resulting in some errors. This weakness of actuator disc is resolved by a hybrid methodology based on blade element momentum theory and mass conservation coupled with computational fluid dynamics to be independent of thrust coefficient curve and calculate more accurate incoming velocity according to operational condition. It has been observed that by using the developed model and considering the details of wind turbines and more accurate incoming wind speed, in spite of steady state simulation and low computational cost, the interaction between different turbines is well-described. For turbulence modeling, standard k-∊ turbulence model is used and it is shown that the error of power estimation in second row of turbines especially in 270° wind angle is decreased significantly. In large wind sectors, such as 270±10°, ±15°,222±10°, ±15° and 312±5°, ±10° the proposed model performs as well as LES simulation. The developed methodology is practical for designing and optimization of new wind farms even if the technical specifications such as thrust curve would not be available from manufacturer.

Exploring a better turbine layout in vertically staggered wind farms

Vertical staggering of wind turbines can lead to an increased power production in the entrance region of a wind farm because downstream turbines are consequently outside the wakes of preceding turbines. We perform large eddy simulations of different vertically staggered wind farm configurations for which we keep the average turbine hub height the same. We find that the turbine power output in the entrance region of the wind farm is significantly higher when the first turbine row is elevated than when the first turbine row is lowered. The reason is that this allows the first high turbine row to fully benefit from the strong winds at a high elevation. In the fully developed region of the wind farm the power production of the vertically staggered wind farms is similar to the power production of the corresponding reference aligned wind farm, while the normalized power fluctuations can be significantly higher than in the reference wind farm.

Wake behind an offshore wind farm observed with dual-Doppler radars

We present dual-Doppler measurements of the wake behind an offshore wind farm. The measured wind speed in the wake is compared with two wake models, one resolving the single wakes from individual turbines, the other describing the wind farm as a distributed area of increased roughness. We analyse a case with steady wind direction approximately aligned with the turbine rows. The radar measurements allow us to track the evolution of the wakes through the wind farm and further downstream. In the case studied here the wake region extends at least 17 km downstream of the wind farm. Both models are in good agreement with observations provided a coastal gradient is accounted for. The generality of this conclusion is tested by comparing modelled wind speeds with observations aggregated over multiple cases with a similar inflow wind direction.

On the interaction of very-large-scale motions in a neutral atmospheric boundary layer with a row of wind turbines

Recent experiments have revealed the existence of very long streamwise features, denoted as very-large-scale motions (VLSMs), in the thermally neutral atmospheric boundary layer (ABL) (Hutchins et al., Boundary-Layer Meteorol., vol. 145(2), 2012, pp. 273–306). The aim of our study is to elaborate the role of these large-scale anisotropic patterns in wind-energy harvesting with special emphasis on the organization of turbulent fields around wind turbines. To this end, we perform large-eddy simulation (LES) of a turbine row operating under neutral conditions. The ABL data are produced separately in a very long domain of $240\unicode[STIX]{x1D6FF}$, where $\unicode[STIX]{x1D6FF}$ is the ABL thickness, to ensure a realistic representation for very large scales of $O(10\unicode[STIX]{x1D6FF})$. VLSMs are extracted from the LES database using a cutoff at streamwise wavelength $\unicode[STIX]{x1D706}_{x}=5\unicode[STIX]{x1D6FF}$, or $\unicode[STIX]{x1D706}_{x}=50D$ in terms of turbine diameter. Reynolds averaging of low-pass filtered fields shows that the interaction of VLSMs and turbines produce very-long-wavelength motions in the wake region, which contain approximately $20\,\%$ of the resolved Reynolds shear stress, and $30\,\%$ of the resolved streamwise kinetic energy in the shear layers. To further elucidate these statistics, we conduct a geometrical analysis using conditional averaging based on large-scale low- and high-velocity events. The conditional eddies provide evidence for very long (${\sim}10\unicode[STIX]{x1D6FF}$) and wide (${\sim}\unicode[STIX]{x1D6FF}$) streak–roller structures around the turbine row. Although all of these eddies share the same streak–roller topology, there are remarkable modifications in the morphology of the conditional eddies whose cores are located sideways to the turbines. In these cases, the turbine row pushes the whole low- or high-momentum streak aside, and prevails as a sharp boundary to the low–high-momentum streak pair. In this process, accompanying rollers remain relatively unaffected. This creates a two-way flux towards the turbine row. These observations provide some insights about the high lateral spreading observed in the large-scale Reynolds stress fields.

Assessment of the Malaysian tidal stream energy resource using an upper bound approach

In this paper, an upper bound approach is used to determine the maximum power available to tidal stream turbines placed at five sites along the west coast of Peninsular Malaysia. A depth-averaged hydrodynamic model of the Malacca Strait is built and validated against field measurements. Actuator disc theory is then used to introduce rows of tidal stream turbines as line sinks of momentum and to determine the maximum time-averaged power available to rows of both moderately sized and very large turbines, placed strategically at the locations of highest naturally occurring kinetic energy flux. Results suggest that although the Malaysian tidal stream energy resource is not large enough to make a significant contribution to the country’s energy mix, there may yet be opportunities to use low-speed tidal turbines in small-scale and off-grid electricity generation schemes. Methods are described in detail and links to source codes and results are provided to encourage the application of this simple, yet effective resource assessment methodology to other promising tidal energy sites.

On the Evolution of the Integral Time Scale within Wind Farms

A wind-tunnel investigation was carried out to characterize the spatial distribution of the integral time scale ( T u ) within, and in the vicinity of, two model wind farms. The turbine arrays were placed over a rough wall and operated under high turbulence. The two layouts consisted of aligned units distinguished only by the streamwise spacing ( Δ x T ) between the devices, set at five and ten rotor diameters d T (or S x = Δ x T / d T = 5 and 10). They shared the same spanwise spacing between turbines of 2.5 d T ; this resulted in arrays of 8 × 3 and 5 × 3 horizontal-axis turbines. Hotwire anemometry was used to characterize the instantaneous velocity at various vertical and transverse locations along the central column of the wind farms. Results show that T u was modulated by the wind farm layout. It was significantly reduced within the wind farms and right above them, where the internal boundary layer develops. The undisturbed levels above the wind farms were recovered only at ≈ d T / 2 above the top tip. This quantity appeared to reach adjusted values starting the fifth row of turbines in the S x = 5 wind farm, and earlier in the S x = 10 counterpart. Within the adjusted zone, the distribution of T u at hub height exhibited a negligible growth in the S x = 5 case; whereas it underwent a mild growth in the S x = 10 wind farm. In addition, the flow impinging the inner turbines exhibited T u / T i n c u < 1 , where T i n c u is the integral time scale of the overall incoming flow. Specifically, T u → β T i n c u at z = z h u b , where β < 1 within standard layouts of wind farms, in particular β ≈ 0.5 and 0.7 for S x = 5 and 10.

Flow Adjustment Inside and Around Large Finite-Size Wind Farms

In this study, large-eddy simulations are performed to investigate the flow inside and around large finite-size wind farms in conventionally-neutral atmospheric boundary layers. Special emphasis is placed on characterizing the different farm-induced flow regions, including the induction, entrance and development, fully-developed, exit and farm wake regions. The wind farms extend 20 km in the streamwise direction and comprise 36 wind turbine rows arranged in aligned and staggered configurations. Results show that, under weak free-atmosphere stratification ( Γ = 1 K/km), the flow inside and above both wind farms, and thus the turbine power, do not reach the fully-developed regime even though the farm length is two orders of magnitude larger than the boundary layer height. In that case, the wind farm induction region, affected by flow blockage, extends upwind about 0.8 km and leads to a power reduction of 1.3% and 3% at the first row of turbines for the aligned and staggered layouts, respectively. The wind farm wake leads to velocity deficits at hub height of around 3.5% at a downwind distance of 10 km for both farm layouts. Under stronger stratification ( Γ = 5 K/km), the vertical deflection of the subcritical flow induced by the wind farm at its entrance and exit regions triggers standing gravity waves whose effects propagate upwind. They, in turn, induce a large decelerating induction region upwind of the farm leading edge, and an accelerating exit region upwind of the trailing edge, both extending about 7 km. As a result, the turbine power output in the entrance region decreases more than 35% with respect to the weakly stratified case. It increases downwind as the flow adjusts, reaching the fully-developed regime only for the staggered layout at a distance of about 8.5 km from the farm edge. The flow acceleration in the exit region leads to an increase of the turbine power with downwind distance in that region, and a relatively fast (compared with the weakly stratified case) recovery of the farm wake, which attains its inflow hub height speed at a downwind distance of 5 km.

Experimental study on power curtailment of three in-line turbines

A dataset of wind tunnel power and wake flow measurements on a setup of three aligned model wind turbines is presented. The power outputs of the three turbines are in good agreement with measurements from a full-scale wind farm of similar inter-turbine spacing. A comparison of the wake flow behind the first row and the second row shows a significantly higher mean velocity loss behind the second row justifying a further power drop from the second to the third row turbine. Curtailing the front row turbine to smaller than rated tip speed ratios resulted in insignificant total power gains below 1%. Curtailments of both the first and second row turbine indicate that the best combined array power results are achieved for slightly lower than rated tip speed ratios. Although power curtailment is observed to have a rather small potential for power optimization of a wind farm, it could be an effective method for load distribution at constant farm power.

Comparison of wind farm large eddy simulations using actuator disk and actuator line models with wind tunnel experiments

We compare wind farm large eddy simulations with the EPFL wind tunnel measurement by Chamorro and Porté-Agel (Bound-Lay. Meteorol. 136, 515 (2010) and Energies 4, 1916 (2011)). We find that the near turbine wake, up to 3 turbine diameters downstream, of a single turbine is captured better with the actuator line method than using the actuator disk method. Further downstream the results obtained with both models agrees very well with the experimental data, confirming findings from previous studies. For large aligned wind farms we find that the actuator disk model predicts the wake profiles behind turbines on the second and subsequent rows more accurately than the wake profile behind the first turbine row. The reason is that the wake layer profile that is created at hub height in very large wind farms is closer to the assumptions made in the actuator disk model than the logarithmic profile found in the inflow conditions. In addition, we show that, even in relatively coarse resolution simulations, adding the effect of the turbine nacelle and tower leads to a significant improvement in the prediction of the near wake features at 1 and 2 diameters downstream.