JEC COMPOSITES MAGAZINE - Issue #134 - May/June 2020 - 59

Advanced polyurethane resin for wind turbine blades: enhanced performance at lower blade cost

- Responsible consumption and production
- Climate action
The result is a lower levelized cost of energy (LCOE)1. That is, if the production cost for blades is 10% to 15% less, along with lower weight, and
consistent performance is combined with increased length, the LCOE
could be further reduced.
The shape and dimensions of wind turbine blades are determined by the
aerodynamic performance required to efficiently extract energy from the
wind, and by the strength required to resist the forces on the blade. The
latter is where a stronger material could play its role.
Figure 1 shows blade length and weight development trends over the past
years.
Stiffness, compressive strength and fatigue characteristics are the design-driving parameters for modern blade designs. All the developments
that extend the limits of existing glass/epoxy laminates are welcomed by
rotor blade designers and manufacturers.
Over the past years, a number of challenges have been identified for the
structural design of blades:
- Slender blades offer very limited internal space to accommodate the
blade structure.
- Blade weight should be as low as possible (in order to reduce loads and
minimize cost).
- Long, slender blades often face tip-to-tower clearance problems, indicating the need for high-performance glass and carbon fibre as well as
sophisticated analysis methods.

Fig. 2: This wind turbine, with its PU wind blades,
continues to run well while experiencing intensive use

Fig. 3: Initial viscosity at different temperatures

Fig. 4: Viscosity curves at different temperatures

Research and new materials
propel development and progress
In 2009, based on a grant from the US Department of Energy (US DoE),
Covestro began research on an innovative polyurethane (PU) solution
to allow the wind power industry to move beyond a major challenge for
its future development. Given wind power growth trends in China and
Europe, this effort clearly holds promise. Wind blades account for 20 to
25% of wind turbine costs, so reducing the cost of blades can support a
reduced LCOE for the wind industry.
The development process started from PU resin chemistry research, extending to PU infusion process development. The first commercial product was introduced in 2019. A significant component of the research associated with this breakthrough was conducted by Covestro and partners in
the entire wind energy industry value chain. These included wind turbine
and blade manufacturers, a fibreglass supplier, a machine producer and
WINDnovation, a blade designer.
The use of polyurethane (PU) resins is a key development in the advance
of blade technology. Because lightweight design allows for longer rotor
blades, essential design requirements can be met with the new resins.
WINDnovation initially expressed interest in enhancing blade design by
exploiting PU's characteristics and advantages. As that work indicated,
PU resin is suitable for wind turbine blade manufacturing, and it offers
cost advantages.
For easy assembly, it also is possible to substitute PU as a replacement for

another infusion resin, or to consider full design implementation to maximize overall savings. Covestro is currently working with WINDnovation
to calculate the full design advantages and options of using PU resin.
Figure 2 shows a wind turbine installation at the Datang wind farm in
northern China. The 55.2 m wind blade with a PU spar cap and shear
web has been operating since October 2018. The accompanying graph
shows the kWh of electricity generated by the installation through midMay 2019.

Agile designability and its advantages
The new PU resin offers advantages such as fast infusion and fast curing
performance in the production of the very large-fibre composites used in
blades. The initial viscosity of PU resin is very low, offering the advantage
of fast infusion during wind blade production. Figure 3 shows the viscosity of PU resin is only 58 mPa.s at 25°C, which is significantly lower than
that of conventional epoxy infusion resin.
wUnder the same conditions, polyurethane resin has better flow characteristics than epoxy resin due to its low viscosity, which can result in greater infusion speed. Figure 4 shows the viscosity of PU is below 600 mPa.s
at 25°C 140 minutes after mixing, which is suitable for large wind blade
production. Particularly at the beginning, at 90 minutes, the viscosity of
polyurethane remains below 300 mPa.s, which means PU will be infused
much faster than conventional epoxy infusion resin during wind blade
production.
The curing behaviour of polyurethane resin offers additional advantagN°134 May - June 2020

/ jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #134 - May/June 2020

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