JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 35

blade is 3.5 m in length. The external
profile and material lay-up of the blade are
depicted in Fig. 3. The blade consists of
a core, with structural blister caps and an
outer shell. A combination of multi-axial
fabrics is employed to achieve the desired
structural performance.

Fig. 3: Profile and lay-up of the composite blades

Blade manufacture
Two identical blades were manufactured
using flax and E-glass as reinforcements
(Fig. 4) employing the same stacking
sequence. Low-twist flax rovings were sourced from Safilin (France). The blades were
manufactured using an unsaturated polyester resin in a two-stage Light Resin Transfer
Moulding (LRTM) process. Both blades
took about 1.5 hours to infuse showing that
using plant fibre reinforcements does not
significantly alter cycle times. Post-cure was
conducted at 40°C for 2 hours. A manufacturing advantage of using flax over E-glass
is that the former doesn’t cause itching
during handling and is non-hazardous if
inhaled.

Fig. 4: Images of the a) flax and b) E-glass blades

Comparison of materials cost
Fig. 5b presents the difference in materials cost of making the flax and E-glass
blades. The materials cost for the flax blade
amounts to some £240, making it about
3 times more expensive than the E-glass
blade (~£85). The materials cost of the
core and the polyester resin are almost
identical for the two blades, amounting to
~£65. Fig. 5 shows that despite requiring
less fibre in a flax blade, the fibre cost of
the flax blade is 8 times that of the E-glass
blade. Hence, it is the cost of the fibre reinforcement which is causing the cost disparity between the flax and E-glass blades.
Unlike E-glass, costs of flax reinforcements
increase tremendously with processing
steps. This is because i) E-glass reinforcements are an established mature market,
and ii) the processing (and incurred
costs) of flax and E-glass reinforcements is
different. Under current market conditions,
flax reinforcements are more expensive
than E-glass reinforcements at every stage:
raw fibre, yarn/roving, and multi-axial
fabric (Table 1). Note that the prices of
the flax reinforcements quoted here are
based on small quantities and prices reduce
significantly with higher quantities (> 5
tonnes). Nonetheless, it begs mention
that for structural applications flax is a
sustainable replacement to E-glass, but not
a low-cost alternative. Jute, on the other

Fig. 5: Comparison of the a) mass and
b) materials cost of the flax and E-glass blades
Tab. 1: Flax is costlier than E-glass at every stage

Cost of
reinforcement

Flax

Eglass

Ratio

Raw fibre [£/kg]

1.50

1.30

1.2

Yarn/roving [£/kg] 10–13 1.30

7.7–10.0

Aligned fabric [£/m2] 13–22 1.80

7.2–12.2

hand, is a sustainable and low-cost (yarn
and fabric cost of ~1.1 £/kg) alternative to
E-glass, but jute composites have inferior
mechanical properties compared to flax
composites.

Mechanical testing of blades
Upon the design and manufacture of the
two blades, their structural integrity was
assessed through design load analysis and
full-scale mechanical tests, as per [6, 7].
Description and derivation of test loads
The SWT for which the blades are to be
used is an 11-kW Class-II horizontal-axis
3-bladed model. Design loads for the
blades were determined for the several load

No78 January - February 2013 /

Feature Wind energy

Comparison of mass properties
Fig. 5a shows the difference in mass of the
flax and E-glass blades. Weighing 23.3 kg,
the flax blade is 10% lighter than the Eglass blade (25.8 kg). Directly comparing
the fibre masses allows to truly appreciate
the weight savings that flax provides. While

the E-glass blade has 7.7 kg of fibre, the flax
blade has only 4.2 kg of fibre; that is, a fibre
mass savings of 45%. Note that the volume
of fibre used in both blades is similar. The
centre of gravity Cg from the blade root is
1.26 m for the flax blade and 1.29 m for the
E-glass blade. The Cg is fairly similar for
both blades, as the blade construction and
fabric stacking sequence are identical.

jec composites magazine 35

JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013