CASE STUDY

21REINFORCEDplastics 0034-3617/13 ©2013 Elsevier Ltd. All rights reserved JANUARY/FEBRUARY 2013

NOVA is a two-year renewable

energy feasibility project, supported

by the UK Energy Technologies

Institute (ETI) and the Engineering & Science

Research Council (EPSRC), with fi nancial

support from the European Regional

Development Fund (ERDF). The project aims

to investigate the aff ordability and feasibility

of manufacturing a VAWTS of this design

and scale.

The 50 kW prototype was manufactured at

Cranfi eld University’s Composites Centre,

with support in its design and manufac-

ture provided by key materials suppliers.

These included Scott Bader, whose Crystic®

Crestomer® 1152PA urethane acrylate

structural adhesive has been specifi ed for

bonding the various carbon fi bre and glass

fi bre epoxy composite parts which make up

the two 10 m by 1.9 m rotor sails.

To be a viable, the planned massive

scale up in off shore wind turbine

manufacture will require the rotor blades

to sustain at least 20 years’ service without

maintenance. As such, the structural design

and selection of suitable high quality

materials with these long term performance

capabilities is a critical factor for this project.

By fi rst developing a 50 kW prototype

scaled down demonstrator with embedded

structural strain and air pressure monitoring,

the NOVA project team will be able to

Bonding the bladesThe UK NOVA project team has achieved a key milestone with the

successful construction of the sails for a 50 kW prototype demonstrator

of its concept 10 MW ‘double arm’ vertical axis wind turbine system

(VAWTS). This commercial 10 MW offshore turbine will have two 160 m

long arms supporting two 80 m long V shaped sails. When built, it

would be the heaviest composite construction in the world, weighing

around 160 tonnes.

An artist’s impression of the final design of 10 MW Aerogenerator X offshore wind turbine. The arm and sail structures are made from epoxy resin with carbon and glass fibre reinforcement. (Picture courtesy of Wind Power Ltd.)

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CASE STUDY

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gain an understanding of the engineering

performance and aerodynamic behaviour of

the design in use and extensively test the

composite materials selected in off shore

operational conditions.

The fabrication stage has already helped

the team to address on a much smaller

Figure 1: The carbon fibre epoxy ribs, spars and skins of the box design were bonded together with Crystic Crestomer 1152PA structural adhesive.

Figure 2: All the bond joints in the sail sections use a simple joggle design and shim plates to allow for rapid assembly with minimum fixtures and aerodynamically smooth joints.

Prior to bonding the Cranfield team ‘dry’ assembled the prototype sail spars and skins on the skin mould tool.

The rib sections of the central box were bonding into place at precise locations along the 10 m length of the sail.

For the prototype, the application of adhesive to the spar caps and upper skin was carried out by hand using brushes and spatulas. For large scale commercial production, pneumatic hand guns and bulk dispensing equipment would be used for cost effective manufacturing.

scale, any processing challenges to cost

eff ectively manufacture and assemble the

composite components. These insights can

then be applied to the full sized version to

help ensure its aff ordability and commercial

viability.

The overall turbine assembly has been

designed and constructed to have the best

structural reliability long term in order to

meet the key criteria of being ‘maintenance

free’ during its expected 20 years service

life. The two prototype rotor sails each have

dimensions of 10 m long by 1.9 m wide,

with a maximum depth of 180 mm at the

mid section.

The structural approach used for the

rotor sails is similar to a large commercial

aircraft wing. The sail has a central box

section, designed with tapering thickness

skins, two C spars with ribs and ‘omega’

shaped hat stringers to provide resistance

to buckling (Figure 1). The sail central box

components were manufactured from

multiaxial carbon fi bre fabrics and epoxy

resin using a vacuum infusion moulding

process. To the central box is added glass

fi bre reinforced leading and trailing edge

components.

Bearing in mind that the scaled up 10 MW

wind turbine sails would be 80 m long,

fi nding ways to reduce weight in the

overall sail design was critical. For the

prototype, the rotor sail weight was

signifi cantly reduced by using a structural

adhesive, which had the added benefi t

of providing lower overall manufacturing

costs compared to a jointed sectional and

mechanical assembly design. The carbon

fi bre/epoxy ribs, spars and skins of the box

design were bonded together with Scott

Bader Crystic Crestomer 1152PA adhesive.

The leading and trailing edge components,

separately fabricated from glass fi bre/epoxy

composite, were then bonded onto the

central box of the sail. All the bond joints

used a simple joggle design and shim

plates to allow for rapid assembly with

minimum fi xtures and aerodynamically

smooth joints (Figure 2).

As the adhesive was such a critical factor

in both the manufacturing and long term

off shore performance of the two lightweight

rotor sails, Cranfi eld University Composites

Centre carried out its own detailed study to

evaluate a range of possible adhesives for

this very demanding application.

“The design and scale of the sail rotor

structure means that there are large bond

surface areas and wide bond lines,” explains

Andrew Mills, Nova project leader at

Cranfi eld University Composites Centre.

“The fully cured structural adhesive must,

therefore, have long term performance

properties which meet a number of key

requirements. These include gap fi lling and

providing outstanding peel resistance, while

at the same time being strong and very

tough, with exceptional fl exural strength

properties.”

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CASE STUDY

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The Cranfi eld team carried out adhesive

performance comparative testing on a

range of bond-line thicknesses from 30 mm

down to 2 mm. The test results obtained

showed that the best performing adhesive

for bonding both carbon fi bre and glass

fi bre epoxy laminate substrates was

Crystic Crestomer 1152PA. These fi ndings

are supported by to the quality and long

term performance of Crestomer adhesives,

which have already been proven for over

30 years in many demanding composite

marine applications, such as deck and hull

stringer and bulkhead bonding, endorsed

by both Lloyds and RINA for composite

bonding in a variety of marine applications.

For the 10 m sail prototype, each

component and mould tool was designed

with joggled part fi t, scored fi tting positions

and 1 mm adhesive shims. This provided

accurate fi nal sail dimensions without the

use of complex assembly jigs. As part of

the assembly process to ensure joint

accuracy, after positioning without adhesive,

each part was drilled and the holes used to

either pin or rivet each part together after

adhesive application. All of the adhesive

joints were post cured in a portable taped

foam board oven for 24 hours at 50°C and

joint gaps were subsequently fi lled with

a body fi ller and abraded smooth using a

circular sander to eliminate evidence of the

component joints. ■

Further information

Cranfield University; www.cranfield.ac.uk

Scott Bader; www.scottbader.com/adhesives

Wind Power Ltd; www.windpower.ltd.uk

The leading edge section of the sail was bonded onto the central box. All adhesive joints were post cured in an oven for 24 hours at 50°C.

For an aerodynamically smooth sail surface, all adhesive joint gaps were filled with a body filler and made smooth using a circular sander prior to painting.

Finished, painted 10 m sails minus end plates, which are bolted on at each end to seal the sail units.

The finished sails, including end plates. The sails have designed to sustain at least 20 years’ service without maintenance.

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