Integral length scales in a low-roughness atmospheric boundary layer

This paper discusses the integral length scales in a low-roughness atmospheric boundary layer (ABL), based on the high-fidelity measurements of wind velocity. Results from the analysis shows that longitudinal integral length scales follow a linear relationship with height in a low-roughness ABL that deviates significantly from semi-empirical Engineering Sciences Data Unit (ESDU) 85020 model derived for open country and urban terrains with larger surface roughness heights. Although the model accurately predicts the integral length scales non-dimensionalised relative to the boundary layer thickness for the majority of the profile, they are over-predicted by more than double in the lowest 10% of the ABL, corresponding to the atmospheric surface layer (ASL). The analysis shows that the largest eddies at lower heights in the ASL over a very low roughness desert terrain have length scales similar to the characteristic lengths of physical structures positioned on the ground, which corresponds to the maximum wind loads for buildings. Hence, it is recommended that the integral length scales in the ASL are characterised over an estimated range at each of the four terrain categories in AS/NZS 1170.2 to ensure that buildings and other large physical structures can be optimised in terms of their size and location.M.J. Emes, M. Arjomandi, R.M. Kelso and F. Ghanad

Optimisation of the size and cost of heliostats in a concentrating solar thermal power tower plant

Concentrating solar thermal (CST) power tower (PT) is one of the most promising renewable technologies for large-scale electricity production, however the main limitation of PT systems is their significantly larger levelised cost of electricity (LCOE) relative to base load energy systems. One opportunity to lower the LCOE is to reduce the capital cost of heliostats through optimisation of the size and position of heliostat mirrors to withstand maximum wind loads during high-wind conditions when aligned parallel to the ground in the stow position. Wind tunnel experiments were carried out to measure the forces on thin flat plates of various sizes at a range of heights in a simulated part-depth atmospheric boundary layer (ABL). Calculated peak wind load coefficients on the stowed heliostat showed an inverse proportionality with the chord length of the heliostat mirror, which suggests that the coefficients could be optimised by increasing the size of the heliostat mirror relative to the sizes of the relevant eddies approaching the heliostat. The peak lift coefficient and peak hinge moment coefficient on the stowed heliostat could be reduced by as much as 23% by lowering the elevation axis height of the heliostat mirror by 30% in the simulated ABL. A significant linear increase of the peak wind load coefficients occurred at longitudinal turbulence intensities greater than 10% in the simulated ABL. Hence, the critical scaling parameters of the heliostat should be carefully considered depending on the turbulence characteristics of the site.Matthew Emes, Farzin Ghanadi, Maziar Arjomandi, Richard Kels

Hinge and overturning moments due to unsteady heliostat pressure distributions in a turbulent atmospheric boundary layer

Available online 09 October 2019Non-uniform pressure distributions on the heliostat surface due to turbulence in the atmospheric boundary layer (ABL) have a significant impact on the maximum bending moments about the hinge of and pedestal base of a conventional pedestal-mounted heliostat. This paper correlates the movement of the centre of pressure due to the mean and peak pressure distributions with the hinge and overturning moment coefficients using high-frequency pressure and force measurements on a scale-model heliostat within two simulated ABLs generated in a wind tunnel. The positions of the centre of pressure were calculated for a range of heliostat elevation-azimuth configurations using a similar analogy to those in ASCE 7-02 for monoslope-roof buildings, ASCE 7-16 for rooftop solar panels, and in the literature on flat plates. It was found that the maximum hinge moment is strongly correlated to the centre of pressure movement from the heliostat central elevation axis. Application of stow and operating load coefficients to a full-scale 36m² heliostat showed that the maximum hinge moment remains below the stow hinge moment at maximum operating design gust wind speeds of 29 m/s in a suburban terrain and 33 m/s in a desert terrain. The operating hinge moments at elevation angles above 45° are less than 60% of the stow loads with a constant 40 m/s design wind speed. The results in the current study can be used to determine heliostat configurations and appropriate design wind speeds in different terrains leading to the maximum design wind loads on the elevation drive and foundation.Matthew J. Emes, Azadeh Jafari, Farzin Ghanadi, Maziar Arjomand

Turbulence length scales in a low-roughness near-neutral atmospheric surface layer

Published online: 14 Oct 2019.This paper investigated the integral length scales of turbulence in a low-roughness atmospheric surface layer (ASL), characterised by very smooth terrain in the Utah desert during near-neutral conditions, and evaluated the Engineering Sciences Data Unit (ESDU) 85020 and 86010 predictions for the turbulence length scales in a lowroughness ASL. The correlation integral method was used to estimate the integral length scales of the velocity components with longitudinal, lateral and vertical separations from sonic measurements on a vertical tower and spanwise array in the Surface Layer Turbulence and Environmental Science Test (SLTEST) field experiment. It was found that the longitudinal integral length scales calculated using near-neutral SLTEST data followed a logarithmic relationship with height proportional to the mean velocity profile with approximately constant integral time scale, however the sizes of the longitudinal components of the energy-containing eddies in the low-roughness flat terrain were 2–3 times smaller than those previously measured during field experiments in open country terrains. The calculated length scales with longitudinal separations over the very smooth terrain characteristics of the salt flats at Dugway were not consistent with those predicted by ESDU 85020. In contrast, the scaling of the lateral and vertical components of the three-dimensional turbulence structure with respect to the longitudinal component in the low-roughness ASL were consistent with similarity theory predictions in ESDU 86010 that the scaling ratios are independent of terrain roughness. Furthermore, this confirms the large dependence of the longitudinal turbulence length scales on the upstream terrain roughness and highlights the large variation of turbulence length scales observed at different low-roughness sites in the literature.Matthew J. Emes, Maziar Arjomandi, Richard M. Kelso and Farzin Ghanad

Effect of turbulence characteristics in the atmospheric surface layer on the peak wind loads on heliostats in stow position

This study investigates the dependence of peak wind load coefficients on a heliostat in stow position on turbulence characteristics in the atmospheric surface layer, such that the design wind loads, and thus the size and cost of heliostats, can be further optimised. Wind tunnel experiments were carried out to measure wind loads and pressure distributions on a heliostat in stow position exposed to gusty wind conditions in a simulated part-depth atmospheric boundary layer (ABL). Force measurements on different-sized heliostat mirrors at a range of heights found that both peak lift and hinge moment coefficients, which are at least 10 times their mean coefficients, could be optimised by stowing the heliostat at a height equal to or less than half that of the mirror facet chord length. Peak lift and hinge moment coefficients increased linearly and approximately doubled in magnitude as the turbulence intensity increased from 10% to 13% and as the ratio of integral length scale to mirror chord length Lux/c increased from 5 to 10, compared to a 25% increase with a 40% increase in freestream Reynolds number. Pressure distributions on the stowed heliostat showed the presence of a high-pressure region near the leading edge of the heliostat mirror that corresponds to the peak power spectra of the fluctuating pressures at low frequencies of around 2.4 Hz. These high pressures caused by the break-up of large vortices at the leading edge are most likely responsible for the peak hinge moment coefficients and the resonance-induced deflections and stresses that can lead to structural failure during high-wind events.Matthew J. Emes, Maziar Arjomandi, Farzin Ghanadi, Richard M. Kels

Correlating turbulence intensity and length scale with the unsteady lift force on flat plates in an atmospheric boundary layer flow

Available online 11 April 2019The correlation between turbulence intensity and length scale and the lift force on a horizontal flat plate in an atmospheric boundary layer flow is investigated in this study. Experiments were conducted in a large-scale wind tunnel to measure the peak loads on flat plate models of various chord length dimensions at different heights within simulated atmospheric boundary layers. The peak lift force coefficient on the flat plates was correlated with both turbulence intensity and length scale. The results show that the peak lift force coefficient on the flat plate is a function of vertical integral length scale (Lxw) and vertical turbulence intensity (Iw) in terms of a parameter defined as Iw (Lxw/c)²·⁴, where is the chord length of the plate. An increase in this turbulence parameter from 0.005 to 0.054, increases the peak lift force coefficient from 0.146 to 0.787. The established relationship is then used to predict the peak wind loads on full-scale heliostats within the atmospheric surface layer as a case study. It is found that decreasing the ratio of heliostat height to the chord length dimension of the mirror panel from 0.5 to 0.2 leads to a reduction of 80% in the peak stow lift force coefficient, independent of the terrain roughness.Azadeh Jafari, Farzin Ghanadi, Maziar Arjomandi, Matthew J. Emes, Benjamin S. Cazzolat

Turbulence characteristics in the wake of a heliostat in an atmospheric boundary layer flow

The mean and spectral characteristics of turbulence in the wake flow of a flat plate model resembling a heliostat in the atmospheric boundary layer flow are investigated in a wind tunnel experiment. Mean velocity and turbulence kinetic energy were characterized in the wake of a heliostat model at three elevation angles up to a distance of eight times the characteristic dimension of the heliostat panel. An increase in turbulence intensity and kinetic energy was found in the wake flow, reaching a peak at a distance equal to approximately twice the characteristic dimension of the heliostat panel. Furthermore, spectral and wavelet analysis of velocity fluctuations in the wake showed that the dominant mechanism in the immediate downstream of the plate was the breakdown of large inflow turbulence structures to smaller scales. In the end, the wake-induced turbulence patterns and wind loads in a heliostat field were discussed. It was found that compared to a heliostat at the front row, the heliostats positioned in high-density regions of a field were subjected to a higher turbulence intensity and, consequently, larger dynamic wind loading. The results show that it is necessary to consider the increased unsteady wind loads for the design of a heliostat in high-density regions of a field, where the gap between the rows is less than three-times the characteristic length of the heliostat panel.Azadeh Jafari, Matthew Emes, Benjamin Cazzolato, Farzin Ghanadi, and Maziar Arjomand

An experimental investigation of unsteady pressure distribution on tandem heliostats

The unsteady surface pressure distribution on heliostats in a tandem arrangement is investigated in this experimental study. The differential pressure on the panel of a heliostat model is measured for a range of gaps between the two tandem heliostats, varying from 1 to 7 times the chord length dimension of the panel. The heliostat models are placed in a simulated turbulent atmospheric boundary layer in the University of Adelaide wind tunnel. The measured surface pressures are analysed and compared with those of a single heliostat, at three elevation angles of 30°, 60° and 90°. The results showed that the peak pressure distribution on the tandem heliostat differs significantly from the single heliostat. Regions of large-magnitude pressure occur near the edges of the panel at smaller gap ratios. Large unsteady variations of the position of the centre of pressure are found for the tandem heliostat at gap ratios equal to and less than 5, which lead to an increase of the hinge moment relative to the single heliostat. The peak hinge moment coefficient on a tandem heliostat is found to be 40% and 70% larger than the coefficient on the single heliostat at elevation angles of 30° and 60°, respectively. The results therefore indicate the importance of the unsteady wind loads in different rows of a field for the design of heliostats as they vary significantly from the loads on a single heliostat dependent on the field arrangement.Azadeh Jafari, Matthew Emes, Benjamin Cazzolato, Farzin Ghanadi, Maziar Arjomand

Towards testing of a second-generation bladed receiver

A bladed receiver design concept is presented which offers a >2% increase in overall receiver efficiency after considering spillage, reflection, emission and convection losses, based on an integrated optical-thermal model, for a design where the working fluid is conventional molten salt operating in the standard 290–565°C temperature range. A novel testing methodology is described, using air and water to test the receiver when molten salt facilities are not available. Technoeconomic analysis shows that the receiver could achieve a 4 AUD/MWhe saving in levelised cost of energy, but only if the bladed receiver design can be implemented at no additional cost

A summary of experimental studies on heliostat wind loads in a turbulent atmospheric boundary layer

The aerodynamic loads on heliostats have been investigated through an extensive range of experimental studies at the University of Adelaide in association with the Australian Solar Thermal Research Institute (ASTRI). Applied modelling techniques using spires and roughness elements were adopted for generation and characterisation of the temporal and spatial turbulence fluctuations, matching those in the lower region of the atmospheric boundary layer (ABL) where full-scale heliostats are positioned. Heliostat wind loads were found to be highly dependent on the critical scaling parameters of the heliostat and the turbulence intensities and scales in the ABL flow. The peak drag and lift coefficients on heliostats followed a similar variation with elevation and azimuth angles to those previously reported in the literature at a similar turbulence intensity. However, the current study revealed a linear increase of the peak drag and lift coefficients on heliostats in operating and stow positions with a parameter defined by the product of the turbulence intensity and the ratio of the turbulence length scales to the heliostat chord length.Maziar Arjomandi, Matthew Emes, Azadeh Jafari, Jeremy Yu, Farzin Ghanadi, Richard Kelso, Benjamin Cazzolato, Joe Coventry, Mike Collin