Ndaona Chokani

Full-Scale Wind Turbine Near-Wake Measurements Using an Instrumented Uninhabited Aerial Vehicle

In this paper, the first-ever measurements of the wake of a full-scale wind turbine using an instrumented uninhabited aerial vehicle (UAV) are reported. The key enabler for this novel measurement approach is the integration of fast response aerodynamic probe technology with miniaturized hardware and software for UAVs that enable autonomous UAV operation. The measurements, made to support the development of advanced wind simulation tools, are made in the near-wake (0.5D–3D, where D is rotor diameter) region of a 2 MW wind turbine that is located in a topography of complex terrain and varied vegetation. Downwind of the wind turbine, profiles of the wind speed show that there is strong three-dimensional shear in the near-wake flow. Along the centerline of the wake, the deficit in wind speed is a consequence of wakes from the rotor, nacelle, and tower. By comparison with the profiles away from the centerline, the shadowing effects of nacelle and tower diminish downstream of 2.5D. Away from the centerline, the deficit in wind speed is approximately constant ≈ 25%. However, along the centerline, the deficit is ≈ 65% near to the rotor, 0.5D–1.75D, and only decreases to ≈ 25% downstream of 2.5D.

High resolution simulations of increased renewable penetration on Central European transmission grid

High spatial and temporal resolution optimal power flow simulations of the 2013 and 2020 interconnected grid in Central Western Europe and Central Eastern Europe regions are undertaken to assess the impact of an increased penetration of renewables. In contrast to prior studies, the present work models the individual transmission lines and power plants within the two regions. It is shown that the planned 2020 developments of the AC grid reduce the mean loading of the grid from 34% to 24%. Most impacted are north-south lines in Germany. The planned developments of HVDC lines further reduce the loading on the AC grid to 22%, as the HVDC lines provide a low-loss transmission solution for the abundant wind energy in northern Germany. Physical loop flows in central Europe increase by 15% for planned 2020 penetration levels of renewables; however these loop flows are reduced by 15% with addition of German HVDC lines.

Seven-Sensor Fast-Response Probe for Full-Scale Wind Turbine Flowfield Measurements

The unsteady wind profile in the atmospheric boundary layer upstream of a modern wind turbine is measured. The measurements are accomplished using a novel measurement approach that is comprised of an autonomous uninhabited aerial vehicle (UAV) that is equipped with a seven-sensor fast-response aerodynamic probe (F7S-UAV). The autonomous UAV enables high spatial resolution (∼6.3% of rotor diameter) measurements, which hitherto have not been accomplished around full-scale wind turbines. The F7S-UAV probe developed at ETH Zurich is the key-enabling technology for the measurements. The time-averaged wind profile from the F7S-UAV probe is found to be in very good agreement to an independently measured profile using the UAV. This time-averaged profile, which is measured in moderately complex terrain, differs by as much as 30% from the wind profile that is extrapolated from a logarithmic height formula; therefore, the limited utility of extrapolated profiles, which are commonly used in site assessments, is made evident. The time-varying wind profiles show that at a given height, the velocity fluctuations can be as much as 44% of the time-averaged velocity, therefore indicating that there are substantial loads that may impact the fatigue life of the wind turbine’s components. Furthermore, the shear in the velocity profile also subjects the fixed pitch blade to varying incidences and loading. Analysis of the associated velocity triangles indicates that the sectional lift coefficient at midspan of this modern turbine would vary by 12% in the measured time-averaged wind profile. These variations must be accounted in the structural design of the blades. Thus, the measurements of the unsteady wind profile accomplished with this novel measurement system demonstrate that it is a cost effective complement to the suite of available site assessment measurement tools.

Time-resolved entropy measurements using a fast response entropy probe

This paper describes a recently developed miniature fast response entropy probe and its application in the turbomachinery facilities at ETH Zurich. The development of the probe is motivated by the need to more clearly document the loss generation mechanisms in the harsh environment of turbomachines. The probe is comprised of a piezoresistive sensor and a pair of thin-film gauges that measure the unsteady pressure and temperature, respectively. The unsteady relative entropy can thus be determined. The design, manufacture and calibration of the probe are first presented in detail. Its application to detail the unsteady entropy field, and associated losses, in a centrifugal compressor, axial turbine and film cooling flows are then described.

Comparison of Performance and Unsteady Loads of Multimegawatt Downwind and Upwind Turbines

The benefits and drawbacks of a multimegawatt downwind compared to upwind wind turbine are assessed using unsteady, three-dimensional (3D) computational fluid dynamics. For the same operating conditions, the downwind turbine has a 3% higher output power and a similar mean flapwise root bending moment. However, in comparison to the upwind turbine, the downwind turbine has a 3% higher thrust and a factor 3 larger peak-to-peak unsteady loading. These features arise due to higher flow incidences on the blade, higher axial velocities ahead of the rotor, and higher loading on the inboard span of the blade when the downwind turbine is compared to the upwind turbine. Overall, it is concluded that the downwind turbine configuration may be better suited for the design of multimegawatt offshore wind turbines.

Effect of Flow Inclination on Wind Turbine Performance

This work examines the effect of flow inclination on the performance of a stand-alone wind turbine and of wind turbines operating in the wakes of upstream turbines. The experimental portion of this work, which includes performance and flowfield measurements, is conducted in the ETH dynamically-scaled wind turbine test facility, with a wind turbine model that can be inclined relative to the incoming flow. The performance of the wind turbine is measured with an in-line torquemeter, and a 5-hole steady-state probe is used to detail the inflow and wake flow of the turbine. Measurements show that over a range of tip-speed ratios of 4―7.5, the power coefficient of a wind turbine with an incoming flow of 15 deg inclination decreases on average by 7% relative to the power coefficient of a wind turbine with a noninclined incoming flow. Flowfield measurements show that the wake of a turbine with an inclined incoming flow is deflected; the deflection angle is approximately 6 deg for an incoming flow with 15 deg inclination. The measured wake profiles are used as inflow profiles for a blade element momentum code in order to quantify the impact of flow inclination on the performance of downstream wind turbines. In comparison to the case without inclination in the incoming flow, the combined power output of two aligned turbines with incoming inclined flow decreases by 1%, showing that flow inclination in complex terrain does not significantly reduce the energy production.

Wind Resource Assessment Using a Mesoscale Model: The Effect of Horizontal Resolution

Prospecting for wind farm sites and pre-development studies of wind energy projects require knowledge of the wind energy resource over large areas (that is, areas of the order of 10’000 km2 and greater). One approach to detail this wind resource is the use of mesoscale numerical weather prediction models. In this paper, the mesoscale Weather Research and Forecasting (WRF) model is used to examine the effect of horizontal grid resolution on the fidelity of the predictions of the wind resource. The simulations are made for three test cases, Switzerland (land area 39’770 km2), Iowa (land area 145,743 km2) and Oregon (land area 248’647 km2), representing a range of terrain types, from complex terrain to flat terrain, over the period from 2006–2010. On the basis of comparisons to the data from meteorological masts and tall communication towers, guidelines are given for the horizontal grid required in the use of mesoscale models of large area wind resource assessment, especially over complex terrain.Copyright

Tin ion and neutral dynamics within an LPP EUV source

The life-time of normal incidence collectors used in LPP EUV sources has been computationally investigated. A two-dimensional/ axisymmetric hydrodynamic-particle code is used to model the plasma expansion from the laser-droplet interaction up to the collector optic. The plasma is formed from the interaction of a Nd:YAG laser, operating at the fundamental frequency, with 50μm tin droplets. The simulation results show non-uniform mass-density distributions at the end of the laser pulse. As the expansion continues up to the collector, the non-uniformities continue to develop. Sn5+ is the most energetic ion impinging on the collector, with kinetic energies up to 7keV. The sputtering yields for Sn ions onto Mo and Si show a strong dependence on both the ion energy and their impact angle. The deposition of neutral tin atoms on the collector has also been assessed with a large scale hydrodynamic simulation. These results are used to investigate the build-up of tin vapor at the irradiation site.

Improved modelling of demand and generation in high resolution simulations of interconnected power systems

In order to accomplish the reliable planning and operation of large interconnected power systems with high penetration of renewables, knowledge of the impacts on individual transmission lines and power plants within the system is required. This may be accomplished using high-resolution optimal power flow simulations with which different scenarios can be assessed. This paper presents detailed numerical weather prediction models that can be used for accurate predictions of solar and wind generated electricity. Furthermore a high-resolution power demand model that accounts for the spatial and temporal variations of different sectors is presented. The models are demonstrated in the case of one-year hourly resolution simulation of the central European power system.

Assessment of Wind Turbine Performance in Alpine Environments

The performance of the wind turbine at the Guetsch Alpine Test Site, Switzerland, is analysed using 10-minute averaged data over the period of a year, following IEC 64100-12-1 recommendations. The predicted Annual Energy Production using the manufacturers power curve is found to be 20% larger than the measured production, which represents a large investment risk. This could be minimised by applying a correction to the power curve during the project development stage. Through photographic analysis of icing events on the blades at the Guetsch Alpine Test Site, icing is found to cause only a 1.6% reduction in Annual Energy Production. Analysis shows that the energy production is 62% −180% higher than the annual average when temperatures are between −1 and −9°C, and a 50% reduction is estimated if the wind conditions remain at the 10°C levels for the entire year. This indicates the benefit of lower temperatures at this site and provides a strong argument for the further development of wind farm projects in alpine environments. A power curve correction technique for the accurate prediction of performance in alpine environments is thus being developed through controlled experiments.