Wake Losses Research Articles

Effect of tip spacing, thrust coefficient and turbine spacing in multi-rotor wind turbines and farms

Abstract. Large eddy simulations (LESs) are performed to study the wakes of a multi-rotor wind turbine configuration comprising four identical rotors mounted on a single tower. The multi-rotor turbine wakes are compared to the wake of a conventional turbine comprising a single rotor per tower with the same frontal area, hub height and thrust coefficient. The multi-rotor turbine wakes are found to recover faster, while the turbulence intensity in the wake is smaller, compared to the wake of the conventional turbine. The differences with the wake of a conventional turbine increase as the spacing between the tips of the rotors in the multi-rotor configuration increases. The differences are also sensitive to the thrust coefficients used for all rotors, with more pronounced differences for larger thrust coefficients. The interaction between multiple multi-rotor turbines is contrasted with that between multiple single-rotor turbines by considering wind farms with five turbine units aligned perfectly with each other and with the wind direction. Similar to the isolated turbine results, multi-rotor wind farms show smaller wake losses and smaller turbulence intensity compared to wind farms comprised of conventional single-rotor turbines. The benefits of multi-rotor wind farms over single-rotor wind farms increase with increasing tip spacing, irrespective of the axial spacing and thrust coefficient. The mean velocity profiles and relative powers of turbines obtained from the LES results are predicted reasonably accurately by an analytical model assuming Gaussian radial profiles of the velocity deficits and a hybrid linear-quadratic model for the merging of wakes. These results show that a larger power density can be achieved without significantly increased fatigue loads by using multi-rotor turbines instead of conventional, single-rotor turbines.

Wind Turbine Wake Effects on Wind Resource Assessments – a Case Study

This paper describes a case study of wind turbine wake loss effects on wind resource assessment in cold region. One year wind park SCADA data is used. Computational Fluid Dynamics (CFD) based numerical simulations are carried out for wind resource assessment and estimation of resultant Annual Energy Production (AEP). Numerical results are compared with the field SCADA data, where a good agreement is found. To better understand the wind flow physics and effects of wind turbine turbulence wake loss effects, three different wake loss models are used for the numerical simulations, where results with wake model is found in best agreement with the AEP estimation from field SCADA data. A detailed comparison of all wind turbines is also presented with the gross AEP. A preliminary case study about wind park layout optimization has also been carried out which shows that AEP can be improved by optimizing the wind park layout and CFD simulations can be used as a tool in this regards.

Platform position control of floating wind turbines using aerodynamic force

This paper presents platform position control of utility-scale floating offshore wind turbines in surge and sway directions using aerodynamic thrust force. The position control will be useful in mitigating the wake effect, and thus maximizing the total power capture, of an offshore wind farm. A linear-quadratic-integrator (LQI) controller is designed to achieve the control objectives of platform position transfer to a specified position for the wake loss mitigation, as well as of platform position regulation against disturbances in wind and wave. Power regulation during and after the position transfer is accomplished by the standard constant-power strategy. It is demonstrated, through simulations of a 5-MW wind turbine in the FAST software, that the LQI controller can attain satisfactory platform position transfer and regulation.

Wind farm layout optimization using genetic algorithm and its application to Daegwallyeong wind farm

This paper proposes a new wind farm layout optimization methodology based on a genetic algorithm by implementing a simulation model considering wake effect. This method consists of (1) batch optimization to efficiently obtain a rough wind farm layout for the maximum energy production in a large scale, and (2) post-optimization to obtain a refined layout to further improve the energy production in a small scale. The proposed two-step optimization enables to efficiently optimize wind farm layout and thus can be applicable to layout optimization of large-scale wind farms. A case study with the actual Daegwallyeong wind farm shows that wake loss is improved by 2.3% point after the proposed layout optimization which means about 2.5% more energy production compared with the existing layout.

Optimal Micro-Siting of Wind Turbines in an Offshore Wind Farm Using Frandsen–Gaussian Wake Model

This paper proposes a method to obtain the optimal placement of wind turbines (WTs) in an offshore wind farm (WF). The optimization objective is to minimize the levelized average cost per net electric power generated by a WF with a fixed number of WTs while the distance between WTs is not less than the allowed minimal distance in the far wake region. The WT wake losses have been taken into account, with the Frandsen-Gaussian (F-G) wake model and the optimization problem is subsequently solved by the hybrid grey wolf optimization (HGWO) algorithm. Synthesis methods that contain a special WT ranking strategy for multiple WTs are described in detail. Both the F-G model and Jensen's model are applied in the offshore WF optimization simulation platform for comparison. Simulation results demonstrate that the F-G model is more consistent with real wakes and thus the optimization result is more accurate than the commonly used Jensen's model.

Modeling of the atmospheric boundary layer under stability stratification for wind turbine wake production

The assessment of climatological site conditions, airflow characteristics, and the turbulence affecting wind turbines is an important phase in developing wake engineering models. A method of modeling atmospheric boundary layer structure under atmospheric stability effects is crucial for accurate evaluation of the spatial scale of modern wind turbines, but by themselves, they are incapable to account for the varying large-scale weather conditions. As a result, combining lower atmospheric models with mesoscale models is required. In order to realize a reasonable approximation of initial atmospheric inflow condition used for wake identification behind an NREL 5-MW wind turbine, different vertical wind profile models on equilibrium conditions are tested and evaluated in this article. Wind farm simulator solvers require massive computing resources and forcing mechanisms tendencies inputs from weather forecast models. A three-dimensional Flow Redirection and Induction in Steady-state engineering model was developed for simulating and optimizing the wake losses of different rows of wind turbines under different stability stratifications. The obtained results were compared to high-fidelity simulation data generated by the famous Simulator for Wind Farm Applications. This work showed that a significant improvement related to atmospheric boundary layer structure can be made to develop accurate engineering wake models in order to reduce wake losses.

Simulation study on the flow field of guide vane and impeller of turbo expander

AbstractIn the face of the current energy shortage, organic Rankine cycle (ORC) technology is a way to make efficient use of energy, especially geothermal energy. As the core component of an ORC system, a turbo expander can achieve a high conversion efficiency for geothermal energy and has good development prospects. For low temperature geothermal energy such as 95°C water, R245fa could be chosen as the working fluid. Through preliminary thermal calculations and the MATLAB programming method, the main thermal performance parameters and the aerodynamic parameters, structural parameters, and three‐dimensional modeling of the guide vane and impeller were obtained. The design of the turbo expander was modeled and meshed by the Workbench module using the ANSYS software, and the internal flow path of the expander was numerically simulated by the CFX module. The results showed that there was a small range of wake loss at the trailing edge of the vane flow; impact loss and wake loss occurred at the leading and trailing edges of the impeller blades. At the 30% relative chord strength of the impeller, there was excessive expansion, resulting in a local low‐pressure area and a secondary flow from the hub to the rim.

Study of combined flow control strategies based on a quantitative analysis in a high-load compressor cascade

Micro-vortex generator and a jet flow control technique are combined to improve the flow performance of a high-load compressor cascade, and the injection position is further investigated. The calculated results showed that the reverse flow region of baseline grows with the increment of incidence, and an unstable flow phenomenon rapidly occurs near the leading edge when the incidence rises to +7.9°. Meanwhile, two critical points occur on the end-wall, that is considered to be the main reason for occurring stall. When an injection tube is applied to remove the low energy fluid, the total pressure loss appears drastic reduction, and the occurrence of stall shows a slight delay from +7.9° to +8.7° incidence compared with the baseline. With the micro-vortex generator is introduced in the baseline, the occurrence of stall is delayed from +7.9° to +10.0° incidence due to the mixture of low energy fluid and main flow. The combined application shows an excellent effect in delaying the occurrence of stall and reducing the total pressure loss, and the sudden deterioration of the cascade performance occurs at +9.8° incidence. A quantitative analysis of total pressure loss shows that the secondary flow loss and wake loss contribute to the reduction of total pressure loss, the former is reduced by 31.0% and the latter is decreased by 24.1% at the near stall condition. Moreover, the spanwise injection position can affect both the total pressure loss and stall incidence, and the wider operating range is accompanied by the increment of total pressure loss.

Technoeconomic study for the optimization of turbine size and wind farm layouts

A technoeconomic analysis and optimization of wind turbine size and layout are performed using WAsP software. A case study of a 100-MW wind farm located in Egypt is considered. Wind atlas for Egypt was used as the input data of the WAsP software. Two turbine models of powers 52 and 80 MW are considered for this project. The wind turbine size and distributions are selected based on the technoeconomic optimization, namely minimum wake effect, maximum annual energy production (AEP) rate, optimum cash flow, and payback period. The future worth method is adopted in economic comparison between the two alternatives, and the cash flow diagram provided the payback period and future worth after the lifetime of the plant. The results showed that (1) the AEP dramatically decreases for a wind farm area less than 15 km2; (2) the turbine spacing, spacing-to-diameter ratio, and the setback distances decrease and the wind turbine density and wake losses increase with decreasing the wind turbines size; (3) the total net AEP using G52 is lower than that of using G80 by about 16%; (4) the technoeconomic analysis recommended using G80 as it has higher profit than those of G52 by about $20 million.

Numerical investigation of flow mechanisms of tandem impeller inside a centrifugal compressor

This study numerically investigated a single stage centrifugal compressor ‘‘Radiver’’ with a wedge diffuser and several tandem-designed impellers to explore the flow phenomena within the tandem impeller and the potential to enhance compressor performance. The results demonstrate that tandem design and clocking fraction (λs) significantly affects the compressor performance. The compressor stage with tandem impellers of Series A of boundary layer growth interruption alone are observed to have a widely operating range but efficiency and total pressure ratio penalty compared with that of conventional impeller. The tandem impeller with at least the same impeller efficiency as the conventional design is considered as a critical design criteria so that further modification process based on the flow characteristic of tandem impeller is necessary. In order to restrain the inducer wake and exducer shock losses, parameters modification of blade angle and thickness distributions are necessary and the modified tandem impeller of Series B is obtained. The modified tandem impeller with 25% clocking arrangement shows an 8.45% stall margin increase and maintains the total pressure ratio and efficiency as the conventional design, which proves the potential of tandem impeller to improve compressor stage performance. It is noteworthy that the tandem impellers of Radiver have not shown obviously balanced exit flow field and the fundamental mechanism of stall margin extending of tandem impeller lies on the improved impeller/diffuser matching performance resulting from the incidence angle variation at diffuser inlet.

Short-Term Forecasting of Wake-Induced Fluctuations in Offshore Wind Farms

The increasing share of offshore wind energy traded at the spot market requires short term wind direction forecasts to determine wake losses and increased power fluctuations due to multiple wakes in certain wind directions. The information on potential power fluctuations can be used to issue early warnings to grid operators. The current work focuses on analyzing wind speed and power fluctuation time series for a German offshore wind farm. By associating these fluctuations with wind directions, it is observed that the turbines in double or multiple wake situations yield higher fluctuations in wind speed and power compared to the turbines in free flow. The wind direction forecasts of the European Center for Medium-Range Weather Forecast model are compared with Supervisory Control and Data Acquisition (SCADA) data observations of the turbine yaw. The cumulative probability distribution of the difference in forecasted and observed wind directions shows that for a tolerance of +/−10 ∘ , 71% of the observations are correctly forecasted for a lead time of 1 day, which drops to 54% for a lead time of 3 days. The circular continuous rank probability score of the observed wind directions doubles over the lead time of 72 h.

Numerical study on wet compression in a supercritical air centrifugal compressor

Wet compression is widely used to reduce compression work and improve efficiency in gas turbines. However, wet compression has not been industrially applied in the compressed air energy storage system (only few studies on isothermal compressed air energy storage system exist), which has an urgent demand to reduce the compression work. The high-pressure section of the compressed air energy storage system usually contains supercritical air, and the influence of supercritical wet compression is not yet clear. Thus, in this study, supercritical wet compression was numerically investigated in a centrifugal compressor used for the compressed air energy storage system, and its effects on performance and internal flow were also studied. The results show that the ideal maximum evaporation of water droplets in supercritical air is very low, which limits the maximum available water injection ratio; however, wet compression still has some benefits, such as increasing the total pressure ratio and isentropic efficiency and reducing the compression specific work. Moreover, wet compression reduces the diffuser loss but raises the wake loss in the impeller at the near-stall point. For this compressor, the optimum water injection ratio is 0.3%, which reduces the specific work by 0.65% at the designed pressure ratio.

Wind farm layout optimization for wake effect uniformity

The basic objective of wind farm layout optimization is to maximize the energy produced by wind farms. However, when wind turbines are arranged in a limited space like an onshore wind farm, specific wind turbines may have greater wake exposure than other wind turbines. This phenomenon can be conspicuous in a mixed layout that consists of turbines with different capacities and hub heights. In this study, we developed and tested a new objective function to increase wind farm energy output while making the wake loss of each wind turbine uniform. The purpose of this function is to adjust the wake effects of all of the wind turbines on a wind farm to similar levels, thereby promoting the operational stability of all of the wind turbines. Layout optimization was performed using a simulated annealing algorithm, which is a heuristic method, with actual wind conditions for an existing wind farm in operation. Then, the results obtained using the proposed method were compared with those yielded by layout optimization for energy maximization. The layout generated using the proposed objective function had lower energy output than that obtained by energy maximization. However, this difference was small and the proposed method prevented wake effect concentration on specific turbines by making the wake effect levels uniform.

A design methodology for wind farm layout considering cable routing and economic benefit based on genetic algorithm and GeoSteiner

Wind farm designing is a crucial stage to realize the application of wind energy. This work studies the problem of wind farm layout optimization (WFLO). A new method based on power production, wind distribution, wake loss is proposed to optimize the layout of wind farm. Genetic algorithm (GA) is utilized to optimize the locations of wind turbine in the wind farm. GeoSteiner algorithm is used to optimize the layouts of cable which has important influence on power transmission. The objective function is annual economic benefit (AEB) including annual production benefit (APB) and the costs of energy, cable and land. In the case study, the wind farm size is 3850 m × 3850 m. The number of wind turbines (WTs) of the cases changes from 2 to 58. The capacity achieves 87 MW when the number of WTs is 58. The result shows that the case considering all factors mentioned above has the highest AEB with 1.87 × 109 ¥ per year. There is a 27.01% increase compared with the original case with APB as objective function. Specifically, the investment of cable is 3.68 × 106 ¥ comparing with 4.06 × 106 ¥ of the case only considering APB.

Why curved wind turbine rows are better than straight ones

Wind turbine wake effects in wind farms not only reduce the wind farm power production but also influence the wind farm power dependency on wind direction. In this paper, the wake effects in wind farm layouts consisting of curved and straight wind turbine rows are studied using engineering wake models and a Reynolds-averaged Navier-Stokes model. These models predict a similar annual energy production for both wind farm layouts, but show stronger wake losses in the aligned wind directions for a rectangular wind farm layout, while the wake losses for a curved wind farm layout are more spread out over a larger wind direction sector. An energy system level simulation predicts that the enhanced spreading of wake losses over wind directions results in less hourly variability in energy generation on a Danish energy system level. Thus, our results show that a curved wind farm layout is more favorable compared to a wind farm layout with straight rows.

Wind farm power optimization through wake steering

Global power production increasingly relies on wind farms to supply low-carbon energy. The recent Intergovernmental Panel on Climate Change (IPCC) Special Report predicted that renewable energy production must leap from [Formula: see text] of the global energy mix in 2018 to [Formula: see text] by 2050 to keep global temperatures from rising 1.5°C above preindustrial levels. This increase requires reliable, low-cost energy production. However, wind turbines are often placed in close proximity within wind farms due to land and transmission line constraints, which results in wind farm efficiency degradation of up to [Formula: see text] for wind directions aligned with columns of turbines. To increase wind farm power production, we developed a wake steering control scheme. This approach maximizes the power of a wind farm through yaw misalignment that deflects wakes away from downstream turbines. Optimization was performed with site-specific analytic gradient ascent relying on historical operational data. The protocol was tested in an operational wind farm in Alberta, Canada, resulting in statistically significant ([Formula: see text]) power increases of 7-[Formula: see text] for wind speeds near the site average and wind directions which occur during less than [Formula: see text] of nocturnal operation and 28-[Formula: see text] for low wind speeds in the same wind directions. Wake steering also decreased the variability in the power production of the wind farm by up to [Formula: see text] Although the resulting gains in annual energy production were insignificant at this farm, these statistically significant wake steering results demonstrate the potential to increase the efficiency and predictability of power production through the reduction of wake losses.

The role of sleep and wakefulness in myelin plasticity.

Myelin plasticity is gaining increasing recognition as an essential partner to synaptic plasticity, which mediates experience‐dependent brain structure and function. However, how neural activity induces adaptive myelination and which mechanisms are involved remain open questions. More than two decades of transcriptomic studies in rodents have revealed that hundreds of brain transcripts change their expression in relation to the sleep–wake cycle. These studies consistently report upregulation of myelin‐related genes during sleep, suggesting that sleep represents a window of opportunity during which myelination occurs. In this review, we summarize recent molecular and morphological studies detailing the dependence of myelin dynamics after sleep, wake, and chronic sleep loss, a condition that can affect myelin substantially. We present novel data about the effects of sleep loss on the node of Ranvier length and provide a hypothetical mechanism through which myelin changes in response to sleep loss. Finally, we discuss the current findings in humans, which appear to confirm the important role of sleep in promoting white matter integrity.

Cryogenic surface resistance of copper: Investigation of the impact of surface treatments for secondary electron yield reduction

The surface resistance of copper samples with an amorphous carbon (a-C) coating or with laser surface structuring, the surface treatments of choice for electron cloud suppression in critical cryogenic sectors of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC), has been measured for the first time at a cryogenic temperature using the quadrupole resonator at CERN. Three different frequencies of relevance for evaluating beam impedance effects, namely, 400, 800, and 1200 MHz, have been investigated. No significant increase in surface resistance is observed for the a-C coating, compared to plain copper. In the case of laser structuring, the surface resistance depends on the direction of the surface currents relative to the laser-engraved groove pattern. The increase is minimal for parallel patterns, but in the perpendicular case the surface resistance increases considerably. Radio frequency (rf) heating from wake losses would then also increase in the HL-LHC case; however, the reduction in the power deposited onto the cold surfaces thanks to electron cloud suppression would still outweigh this effect.

Experimental and numerical investigations of trailing edge injection in a transonic turbine cascade

Trailing edge mixing flows associated with coolant injection are complex, in particular at transonic flows, and result in significant aerodynamics losses. A series of tests and calculations on a high-pressure turbine vane cascade with trailing edge injection were conducted to investigate the influence of trailing edge injection on the loss characteristics of the transonic profile. Wake traverses with a five-hole probe and measurements of the pressure distribution on the profile were taken for injection mass flow ratios of 0 and 4.1% under a test Mach number of 0.7. In the meantime, two-dimensional numerical predictions were carried out for exit isentropic Mach numbers of 0.7, 0.85 and 1.1 and injection mass flow ratios of 0, 4.1%, 4.7% and 5.2%. Numerical predictions show a reasonable agreement with the experimental data, and wake total pressure losses and flow angles as well as pressure distributions on the profile were compared to calculations without trailing edge injection, showing a significant effect of trailing edge injection on the wake development and its blockage effect on the flow in the vane cascade passage. The outlet total pressure loss and flow angle are increased with increasing exit Mach number. The trailing edge injection of 4.1% increases the static pressure in the rear part of vane suction side, and then reduces the vane loading, thus causing decreased wake losses at different exit Mach numbers. At transonic flows, the injection also reduces the strength of the shock wave at the trailing edge pressure side. In addition, the injection decreases the outlet pressure circumferential-unevenness at an exit isentropic Mach number of 0.7, while it increases the pressure circumferential-unevenness at higher exit Mach numbers.

The performance of a centrifugal compressor with a tandem impeller in off-design conditions

Centrifugal compressors often suffer relatively low efficiency and a terrible operating range particularly due to the complex flow structure and intense impeller/diffuser interaction. Numerous studies have focused on improving the centrifugal compressor performance using many innovative ideas, such as the tandem impeller, which has become increasingly attractive due to its ability to achieve the flow control with no additional air supply configurations and control costs in compressor. However, few studies that attempted to the investigation of tandem impeller have been published until now and the results are always contradictory. To explore the potential of the tandem impeller to enhance the compressor performance and the underlying mechanism of the flow phenomena in the tandem impellers, this paper numerically investigated a high-pressure-ratio centrifugal compressor with several tandem impellers at off-design operating speeds. The results encouragingly demonstrate that the tandem impeller can achieve a performance enhancement over a wide range of operating conditions. Approximately 1.8% maximum enhancement in isentropic efficiency and 5.0% maximum enhancement in operating range are achieved with the inducer/exducer circumferential displacement of [Formula: see text] = 25% and 50%, respectively. The observed stage performance gain of the tandem impellers decreases when the operating speed increases due to the increased inducer shock, increased wake losses, and deteriorated tandem impeller discharge flow uniformity. In addition, the tandem impeller can extend the impeller operating range particularly at low rotation speeds, which is found to be a result from the suppression of the low-momentum fluid radial movement. The results also indicate that the maximum flux capacity of the tandem impeller decreases due to the restriction of the inducer airfoil Kutta–Joukowsky condition.