Structural deformation of sandwich composite heliostats

Sulaiman Fadlallah1, Timothy Anderson1 and Roy Nates1

1Department of Mechanical Engineering, Auckland University of Technology, New Zealand

Introduction

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• The sun emits energy at an extremely highand relatively constant rate.

oIf all of this energy could be converted intousable forms on earth, it would be morethan enough to supply the world’s energydemand.

• This demand significantly encouraged thedevelopment of solar power generationtechnologies.

Introduction

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Central tower concentrating solar power (CSP) systems

Central Tower CSP System

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• This emerging technology holds muchpromise for countries with plenty ofsunshine and clear skies.

• Its electrical output matches well the shifting daily demand for electricity.

A huge obstacle prevents the expansion of these systems

COST

Central Tower CSP System

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Heliostats contribute around 50% to the plant’s cost (Kolb et al., 2007)

• Heliostats are the most crucial cost element of central tower CSP systems

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Impact of heliostat primary elements on the total cost (Kolb et al., 2011)

Heliostat Cost Reduction Opportunities

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• Focusing on large-scale heliostats:

• Heavyweight mirror support structure(Steel)

(high-torque drive)

Less structural weight

How can we accomplish this weight reduction ???

Heliostat Cost Reduction Opportunities

Drive cost as a function of torque

capacity (Kolb et al., 2007)

Less drive torque

Less heliostat cost

Sandwich Composites

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• Sandwich composites are becoming anessential part of today’s materials.

• They offer various advantages including:

Aircraft structures

Automotive industry

Satellites

Lightweight.

High fatigue strength.

Corrosion resistance.

Faster assembly.

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Honeycomb Sandwich Composites

Facing

Adhesive

Honeycomb

core

Facing

• High stiffness to weight ratios.

• formed by adhering two thin-facesheets to a low-density honeycombcore.

• The honeycomb core is capable ofwithstanding transverse normal andshear loads, while the faces handleboth compressive and tensile loadsdue to bending. Honeycomb sandwich structure (Abbadi et al., 2009)

How can honeycomb sandwich composites be utilized to

develop a robust, lightweight heliostat mirror support

structure that is capable of withstanding wind loads at

various tilt angles ?????

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Research Question

Method

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Fluid-Structure Interaction (FSI)

CFD FEA

Stage 1: Heliostat Structure Modelling

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Stage 2: Modelling the Flow of Air around the Heliostat Structure

Stage 3: Fluid-Structure Interaction

Heliostat Structure Modelling

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• Considering a typical heliostatconfiguration, the full structure ofthe heliostat have been visualized.

Existing ATS 150 Heliostat

(Mancini, 2000; Kolb et al., 2007)

(a) Full-scale (b) Sandwich composite structure

Figure 1. Sandwich composite-based heliostat

Mirror Glass

Aluminum

Honeycomb Core

Aluminum Sheet

Sandwich composite-based heliostat

Stage 1: Heliostat Structure Modelling

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Stage 2: Modelling the Flow of Air around the Heliostat Structure

Stage 3: Fluid-Structure Interaction

65 m

100 m

45 m

70 m

Fluid Domain

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Air

( 20 m/s )

> Steady state

> Pressure-based solver

> SST K-ω model

Solver Settings

Stage 1: Heliostat Structure Modelling

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Stage 2: Modelling the Flow of Air around the Heliostat Structure

Stage 3: Fluid-Structure Interaction

Importing Pressure Loads from Fluid Solver to Mechanical Solver

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One-way FSI

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Mechanical properties’ calculation(Nast 1997; Gibson and Ashby 1997)

𝐸1 =𝑡3 1 + sin𝜑

12 𝑎3 cos3𝜑cos𝜑3 −

1 + cos𝜑8 1 − 𝑣2

𝐸

𝐸2 =𝑡3 cos𝜑

1 + sin𝜑 𝑎3 sin2𝜑 1 − 𝑣2𝐸

𝐸3 =2 𝑡

𝑎 cos𝜑 1 + sin𝜑𝐸

𝐺12 =𝑡3 1 + sin𝜑

𝑎3 1 − 𝑣2 cos𝜑 6.25 − 6 sin𝜑𝐸

𝐺23 =10 𝑡

9 𝑎 cos3𝜑 1 + sin𝜑𝐺

𝐺13 =2 𝑡

𝑎 cos𝜑 1 + sin𝜑𝐺

𝑣12 =sin2𝜑 1 + sin𝜑 2

12 𝑎3 cos2𝜑cos𝜑3 −

1 + cos𝜑8

𝑣23 =𝑡2 cos2𝜑

2 𝑎2 sin2𝜑 1 − 𝑣2𝑣

𝑣13 =𝑡2 1 + sin𝜑 2

24 𝑎2 cos𝜑cos𝜑3

−1 + cos𝜑

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∗𝑣

1 − 𝑣2

𝜌ℎ𝑜𝑛𝑒𝑦𝑐𝑜𝑚𝑏 =3

2 𝑎 cos𝜑 1 + sin𝜑𝜌

Honeycomb sandwich composite material properties

Aluminum Mechanical Properties

Mechanical Property Value Unit

Modulus of elasticity (E) 6.9E10 Pa

Poisson’s ratio (v) 0.33 -

Shear modulus (G) 2.7E10 Pa

Density (ρ) 2700 kg/m3

Aluminum honeycomb core calculated mechanical properties

Ho

ne

yc

om

b

ce

ll g

eo

me

try

Core angle (φ) 30 deg

Cell wall length (a) 6 mm

Sheet thickness (t) 0.03 mm

Calc

ula

ted

mec

ha

nic

al p

rop

ert

ies

Modulus of elasticity in direction 1 (E1)

2.91E04 Pa

Modulus of elasticity in direction 2 (E2)

2.23E04 Pa

Modulus of elasticity in direction 3 (E3)

5.31E08 Pa

Poisson’s ratio in plane 1–2 (v12)

1.14 -

Poisson’s ratio in plane 2–3 (v23)

1.39E-5 -

Poisson’s ratio in plane 1–3 (v13)

1.81E-5 -

Shear modulus in plane 1–2 (G12)

5.16E03 Pa

Shear modulus in plane 2–3 (G23)

1.54E08 Pa

Shear modulus in plane 1–3 (G13)

2.08E08 Pa

Density of honeycomb core (ρhoneycomb)

15.59 kg/m3

Validation

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Validation of CFD model

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Drag coefficient

Lift coefficient

Validation of FEA model

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Modal frequency results

Results and Discussion

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Reflective surface Back surface

θ =

90°

θ =

60

θ =

30°

θ =

0°

θ =

-30°

θ =

-60°

θ =

-90°

Figure 7. Pressure distribution on the heliostat at wind speed of 20 m/s

Pressure distribution on the heliostat Heliostat surface structural behaviour

(a) θ = 90° (b) θ = 60°

(c) θ = 30° (d) θ = 0°

(e) θ = -30° (f) θ = -60°

(g) θ = -90°

Figure 8. Displacement distribution of the heliostat surface at wind speed of 20 m/s for different tilt angles

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The maximum allowable deflection = ±21.3 mm.

tan ±3.6 𝑚𝑅𝑎𝑑 =𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡

12 (𝐻𝑒𝑙𝑖𝑜𝑠𝑡𝑎𝑡 𝑐ℎ𝑜𝑟𝑑 𝑙𝑒𝑛𝑔𝑡ℎ

Simplified interpretation of the wind load displacement requirement

(Björkman, 2014).

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Highly stressed

regions

Back surface

(θ=30°)

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20%Weight Reduction

Steel-based heliostat Composite material-based heliostat

Total weight of

the mirror support

structure (tonnes)

1.5 1.2

Honeycomb sandwich composite-based

heliostatSteel-based heliostat

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