Investigation of peak wind loads on tandem heliostats in stow position Matthew J. Emes * , Farzin Ghanadi, Maziar Arjomandi, Richard M. Kelso School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia article info Article history: Received 21 July 2017 Received in revised form 12 January 2018 Accepted 20 January 2018 Keywords: Stowed heliostat Wind load Atmospheric boundary layer Gap ratio 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 x u =c of less than 5, whereas tandem loads were 30% lower than an isolated heliostat at L x u =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. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction The concentrating solar thermal (CST) power tower (PT) is one of the most promising renewable technologies for large-scale electricity production. Although the intermittency of solar irradi- ation is a practical limitation of CST systems, PT plants can be deployed with thermal energy storage or as a hybrid system with existing fossil fuel power plants for a base-line power supply [1]. PT systems consist of a field of heliostat mirrors reflecting sunlight to the top of a beam-up or beam-down tower containing a receiver. Heliostats are arranged in rows on one side of an anti-polar facing cavity receiver in a polar field or surrounding a cylindrical receiver in a surround field. The main limitation of PT systems is their significantly larger levelised cost of electricity (LCOE). The LCOE of a conventional molten-salt receiver PT plant was estimated by NREL [2] to be 0.14 USD/kWh in 2015, but this could be further reduced to 0.1 USD/kWh with near-term advanced heliostats at $97/m 2 in a 2017 tower configuration [3]. In comparison, base-load energy systems, such as fossil fuel power plants, have an LCOE in the range of 0.06e0.13 USD/kWh in 2011 [4]. To reduce the LCOE of PT sys- tems there is a need to lower the capital cost of a PT plant, of which the largest cost is the heliostat field, with an estimated contribution of between 40% and 50% [1 ,5e7]. During operation heliostat mirrors are inclined with respect to the horizontal and are exposed to large drag forces and overturning moments that are directly proportional to the wind speed with a large projected frontal area to the wind [8]. Heliostats are aligned parallel to the ground in the stow posi- tion during periods of high wind speeds to minimise the frontal area and thus the drag forces, as shown for a tandem arrangement in Fig. 1 . A cost analysis of quasi-static wind loads by Emes et al. [9] found that the heliostat cost of a PT plant in Alice Springs (central Australia) was reduced by 18% by lowering the design wind speed for stowing the heliostats from 22 m/s to 13 m/s for only a 2% reduction in capacity factor of the heliostat field. While the oper- ating wind load can be reduced by changing the stow wind speed, the survival wind speed that a heliostat is designed to withstand in the stow position cannot be varied based on the maximum site wind speed. Hence, there is a significant potential to minimise the capital cost and LCOE of a PT plant through optimisation of the structural design of heliostats in the stow position. This paper in- vestigates the sensitivity of peak wind loads on a heliostat in stow position (Fig. 1) to: (1) the chord length (c) and elevation axis height * Corresponding author. E-mail address: matthew.emes@adelaide.edu.au (M.J. Emes). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2018.01.080 0960-1481/© 2018 Elsevier Ltd. All rights reserved. Renewable Energy 121 (2018) 548e558