• Energy Research

Abstract During the operation of a concentrating solar thermal (CST) power tower plant, heliostat mirrors inclined at different angles act as bluff bodies that are exposed to large drag loads from the wind. This experimental study investigates the aerodynamic loads on a heliostat in a tandem configuration, to determine the significance of the shielding effect from an upstream heliostat. To understand the effect of turbulence on the peak wind loads, scale-model heliostats with square facets were positioned within a part-depth atmospheric boundary layer (ABL) with a Power Law velocity profile. Peak drag coefficients on the instrumented downstream heliostat in the tandem configuration were normalized with respect to those on a single (isolated) heliostat. A range of tandem configurations were tested to determine the effects of elevation angle, azimuth angle, and gap spacing between the tandem heliostats. Findings show that peak drag loads are reduced by up to 60% on the downstream heliostat relative to an isolated heliostat at an elevation angle of 90 Â ° and a gap spacing of two chord lengths, but at higher gap spacing the shielding effect is either marginal or non-existent. Peak hinge moment coefficients on a downstream heliostat in tandem are up to seven times the load on an isolated heliostat, with the maximum occurring at 90 Â ° elevation and 180 Â ° azimuth. Base-overturning moment coefficients are less affected, as the changes in the centre of pressure location are relatively small compared to the length of the support pylon. Strouhal number analysis of the fluctuating surface pressures indicated that the dominant frequency of the pressure spectra on the downstream heliostat is over three times the value on an isolated heliostat at 45 Â ° elevation and azimuth angles. Hence, both static and dynamic effects must be considered separately in the wind load design for heliostats at typical operating angles.

Abstract Non-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 36 m2 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.

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.

Abstract 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 L u x / 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.

Abstract The total heat transfer from a finned or ‘bladed’ structure may be expected to rise from that of a flat geometry due to a larger area exposed to ambient air. However, this is not always the case because a trade-off can be found between convection and radiation as the bladed geometry and surface temperature are varied. Furthermore, a mixed convection regime adds complexity to the heat exchange between the surrounding air and the heated bladed structure, which is relevant to various applications such as cooling and solar thermal energy. In this study, mixed convection and radiation heat transfer were determined for a cuboid with isothermal blades protruding from one of its sides. A steady-state simulation based on a three-dimensional SST k– ω turbulence model was performed in OpenFOAM to estimate the average convection heat transfer coefficient as a function of structure orientation, convection regime and bladed geometry. A Monte Carlo ray tracing method was employed to calculate view factors for determining the radiation heat transfer coefficient. Wind tunnel experiments validated the combined numerical approach. An increasing pitch angle (starting from the vertical) gradually increased convection heat transfer until a maximum value at ~ 45 ° and then significantly decreased it, where both effects were due to a flow behaviour in which vortices between the blades formed ( 45 ° ) but then disappeared ( > 45 ° ). The velocity and wall temperature of the mixed convection flow in between the dominantly natural and forced convection regimes were identified. In comparison to a flat geometry, the bladed structure increased the temperature value at which the heat transfer by convection and radiation equalised, and produced a lower total heat transfer coefficient than the flat configuration at high surface temperatures. Furthermore, flow bifurcations were observed between the blades as their length was increased. In contrast, a large number of blades led to a flow transition where most of the incoming air exited from the side apertures to a lid-driven-like convection with flow recirculation occurring within the blades, which created a thermal barrier that decreased convection heat transfer. These results support the design of bladed or finned structures that enhance or reduce heat transfer by mixed convection and/or thermal radiation.

Abstract The 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 ( L w x ) and vertical turbulence intensity ( I w ) in terms of a parameter defined as I w ( L w x c ) 2.4 , where c 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.

Abstract This paper investigates the effects of turbulence in the atmospheric boundary layer (ABL) on the peak wind loads on heliostats in stow position in isolation and in tandem configurations with respect to the critical scaling parameters of the heliostats. The heliostats were exposed to a part-depth ABL in a wind tunnel using two configurations of spires and roughness elements to generate a range of turbulence intensities and integral length scales. Force measurements on different-sized heliostat mirrors at a range of heights found that both peak lift and hinge moments were reduced by up to 30% on the second tandem heliostat when the spacing between the heliostat mirrors was close to the mirror chord length and converged to the isolated heliostat values when the spacing was greater than 5 times the chord length. Peak wind loads on the tandem heliostat were above those on an isolated heliostat for an integral-length-scale-to-chord-length ratio L u x / c of less than 5, whereas tandem loads were 30% lower than an isolated heliostat at L u x / c of 10. The reduced loads on the tandem heliostat corresponded to a shift to higher frequencies of the fluctuating pressure spectra, due to the break-up of large eddies by the upstream heliostat.

Abstract The effect of aspect ratio and head-on wind speed on the force and natural (combined) convective heat loss and area-averaged convective heat flux from a cylindrical solar cavity receiver has been assessed using three dimensional computational fluid dynamics (CFD) simulations. The cavity assessment was performed with one end of the cavity open and the other end closed, assuming an uniform internal wall temperature (i.e. the cavity walls were heated). The numerical analysis shows that there are ranges of wind speeds for which the combined convective heat losses are lower than the natural convective heat loss from the cavity and that this range depends on the aspect ratio of the cavity. In addition, the effect of wind speed on the area-averaged flux of convective heat loss from a heated cavity is smaller for long aspect ratios than for short ones, which indicates that the overall efficiency of the solar cavity receiver increases with the aspect ratio for all conditions tested in this study.

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.