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5 Polymer Chemistry and Wind Energy

5 Polymer Chemistry and Wind Energy

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87



Polymers and Sustainable Energy



power to grow rapidly as a sustainable energy source. Additional applications will be

covered as they are presented in later chapters.

When it comes to harnessing wind to create work, we have come a long way and

polymer chemistry has played a huge role in that progress. Wind power has obviously

been around since sailing ships and windmills were common in Europe in the 1700s.

But, in the United States, creation of an energy grid and cheap fossil fuels led to the

disappearance of the functioning farmstead windmill over the course of the nineteenth century. Only recently have technology and the view of wind power as a clean

energy alternative led to the resurgence of wind energy, primarily in the form of modern wind turbines and expansive wind farms, both onshore and offshore. According

to the American Wind Energy Association, in 2012 wind energy made up more than

40% of new electrical generating capacity in the United States (Rich 2013).

Historically, windmill blades were made of wood, wood and cloth, or metal (steel

or aluminum). The weight and strength limitations of these materials limited the

size of the blade and, hence, the maximum power output. Advancements in polymer

chemistry have paved the way for the development of the large horizontal axis wind

turbine (HAWT) for the generation of power over the past two decades (Figure 4.28;

Figure 4.29 shows the nomenclature associated with the modern HAWT). Recent

work has shown that the greater the capacity of the turbine, the greener the electricity: global warming potential per kWh for wind-generated electricity was reduced by



Wind electricity generation by region: 1985–2009

Billion kilowatt hours

300



World



250

200

Europe



150



North America



100



Asia and Oceania

50

0

1985



Other

1990



1995



2000



2005



2009



Other: This category includes South and Central America, Eurasia,

the Middle East, and Africa.



FIGURE 4.28  Growth in wind electricity generation. (From U.S. Energy Information

Administration. 2013. International Energy Statistics. U.S. EIA 2011 [cited 2 July 2013]. Avail­

able from http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=6&pid=29&aid=12.)



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Chemistry of Sustainable Energy



Rotor blade



Swept area, A



Rotor diameter, D

Nacelle



Hub height, h



Tower



Foundation



FIGURE 4.29  Nomenclature of wind turbines. (Reprinted with permission from Caduff,

M. et al. 2012. Wind power electricity: The bigger the turbine, the greener the electricity?

Environ. Sci. Technol. 46:4725–4733. Copyright 2012, American Chemical Society.)



14% with every cumulative production doubling (Caduff et al. 2012). Today’s turbine

blades are as long as 75 m (for comparison, the wingspan of an Airbus A380 is 80 m)

and can weigh around 20 metric tons. The evolution of the modern wind turbine has

been rapid, with small 50 kW machines being dwarfed by the 2–3 MW standard

of today (Figure 4.30). The Enercon E126, rated at 7.58 MW, is one of the world’s

largest and has a hub height of 135 m with a rotor diameter of 127 m. It is predicted



240

220

200

180

160

140

120

100

80

60

40

20



Emerging

Today’s technology



500 kW

d ~ 40 m



Statue of

liberty



1.5–2 MW

Airbus A380 2–3 MW

d ~ 80 m wingspan ~ 80 m d ~ 90 m



5–7 MW Washington

d ~ 126 m monument



Height/diameter (m)



Conceptual



8–10 MW

d ~ 165 m



FIGURE 4.30  The evolution of wind power. (Reprinted with permission from Merugula,

L., V. Khanna, and B.R. Bakshi. 2012. Reinforced wind turbine blades—An environmental life cycle evaluation. Environ. Sci. Technol. 46:9785–9792. Copyright 2012, American

Chemical Society.)



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Polymers and Sustainable Energy

Carbon spar caps



Fiberglass skins

Balsa or foam core



Shear webs

(glass over core)



FIGURE 4.31  Structure of a wind turbine blade. (From Griffin, D.A. and T. Ashwill.

2009. Blade system design study part II: Final project report (GEC). Albuquerque, NM, US

Department of Energy, 152.)



that by 2025, rotor diameters will be 160–165 m with blades of 100–125 m in length

(New Energy Externalities Developments for Sustainability 2008).

This amazing growth in the length and strength of wind turbine components is a

result of advances in materials science, including the development of polymer matrix

composites. Polymer composites are strong, cross-linked thermoset polymer resins

laminated with reinforcing fiber (usually glass or carbon, vide infra). The combination of the compressive and tensile strength of the fibers with the moldable properties of the polymer makes the combined matrix much stronger than the individual

components, allowing for the dramatic growth of wind power as turbines improve

in both power rating and efficiency. Today’s ever-larger wind turbine blades must be

tough and stiff but not brittle, with the blade overall exhibiting reasonable flexibility

(the blades must not bend to the degree that they will hit the tower while rotating!).

A typical turbine blade is an integrated whole made up of an overlaying shell, a foam

core, and the structural spar that runs the length of the blade, with some form of

protective skin that coats the blade (see Figure 4.31).

In addition to increasing the overall dimensions of the rotor diameter by manufacturing ever-larger blades, tower height is another focus for improvement in wind

turbine efficiency (Giannis et al. 2008). Thus, wind turbines keep getting bigger (in

terms of swept width) and taller. Increasing tower height has two advantages: there

is less turbulence from interactions at ground level, and longer and better designed

blades that have a larger sweep area can be used. In any case, these improvements

are a result of the continuing evolution of the basic materials that make up these

structural components: the polymer resins and the reinforcing fibers.



4.5.2 Resins

Most of the polymers used in the manufacture of turbine blades are thermosetting

polyester, vinyl ester, or epoxy resins. Orthophthalic or isophthalic unsaturated

polyester resins (see Figure 4.32) incorporate alkene functionality that provides

additional reactive sites for cross-linking. Addition polymerization is carried out in

styrene to allow cross-linking (curing) to take place between the styrene monomer

and the alkene units of the unsaturated polyester (Figure 4.33).



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Chemistry of Sustainable Energy

O





O



O



O



O



O



O







O

n

Orthophthalic polyester

(isophthalic = meta isomer)



FIGURE 4.32  Phthalic polyester resin structure.



O





Cross-linking across

double bonds

O



O

O



O



O



O







O

n



Cross-linking across

double bonds

O





O



O



O

O



O



O







O

n



FIGURE 4.33  Sites for cross-linking between styrene and an unsaturated polymer.



Polymerization of vinyl esters (where the reactive vinyl substituent is at the ends

of the main polymer chain) leads to a resin that is more resistant to hydrolysis due

to the smaller proportion of ester linkages (Figure 4.34). Both unsaturated polyester

and vinyl ester cross-linked resins suffer from shrinkage during the curing process,

a fate to which epoxy resins are less prone. In addition, the absence of ester functionality makes these types of polymers especially resistant to hydrolysis, an important

consideration in exterior applications such as turbine blades. Figure 4.35 shows an

example of a bisphenol A epoxy prepolymer resin, prepared from the step-growth

polymerization of bisphenol A to epichlorohydrin. Addition of a polyfunctional



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Polymers and Sustainable Energy

H3C



CH3



OH



O



O

O



O

n



O



OH



FIGURE 4.34  Example of a vinyl ester polymer.

H3C



CH3

+



O



Cl



NaOH



OH



HO



Epichlorohydrin



Bisphenol A



H3C







O



CH3



O



O

OH



O

n



FIGURE 4.35  Epoxy resin made from epichlorohydrin and bisphenol A.



amine hardener such as diethylenetriamine (H2NCH2CH2NHCH2CH2NH2) cures

the resin, leading to a throughly cross-linked, strong, and tough polymer network.

In an effort to generate a somewhat renewable version of an epoxy resin, researchers

prepared a polymer composite made from an epoxidized soybean oil and 1,1,1-tris(phydroxyphenyl)ethane triglycidyl ether (THGE-PE) comatrix cross-linked with a variety of amine hardeners and strengthened by incorporation of flax fiber (Figure 4.36)

(Liu et al. 2006). Use of the amine curing agent triethylenetetramine (TETA) gave

the best results, providing a more environmentally benign polymer with sufficient

strength to be used in, for example, the automotive or construction industries.



4.5.3 Reinforcing Fibers

The polymer matrix is reinforced with fibers to make a synergistic whole that is

stronger than each of the individual components. Glass and carbon are the two main

types of fibers that have been used in polymer matrix composites, but natural fibers

such as flax (as noted above) and jute, consisting mostly of cellulose (Figure 4.37),

have been explored. Glass and carbon fiber each has its advantages and disadvantages and there is a wide variety of types within each category. Which fiber is used

greatly impacts the weight, strength, and cost of the turbine blade. In general, carbon

fibers have both a higher strength and modulus than glass, but glass fiber-reinforced



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Chemistry of Sustainable Energy



O



(CH2)7



O



O (CH )

24

O



O



O

(CH2)7



O

O



O



(CH)4CH3



NH2



O

O



O



(CH)4CH3



N



H 2N



NH2



Triethylenetetramine

(TETA)



(CH2)7CH3

Epoxidized soybean oil

CH3

O



O



O



THPE-GE

(cross-linker)

O



O



O



FIGURE 4.36  Components of a “green” polymer composite.



polymer matrices have the significant advantage of lower cost and ease of processing

(the incorporation of glass fibers is more forgiving during the manufacturing process). Glass, however, is considerably heavier than carbon and the glass/epoxy composites are reaching their limit with respect to blade size and increased efficiency.

The use of carbon fibers, while substantially more costly, can lead to a stiffer blade,

allowing placement of the rotor closer to the tower. To minimize the cost of using

carbon fiber and maximize the benefit of lightweight strength, these fibers are often

incorporated at strategic structural areas in the blade (typically as a cap for the spar).

As turbine blades become larger, the use of carbon fibers (or glass–carbon hybrids)

is becoming increasingly common despite their increased costs, and it has been predicted that turbine blades will contain up to 50% carbon fiber by 2025 (New Energy

Externalities Developments for Sustainability 2008).

The use of reinforcing fibers in polymer matrix composites two distinct areas

of chemistry: inorganic (glass) and organic ­(carbon). Glass, of course, is an ancient

ceramic material of many different ­compositions. Common soda lime glass is made

by heating sand (the source of SiO2) to the molten state along with various other

inorganic additives (a flux, to lower the melting temperature, and a stabilizer, to

increase the glass’ resistance to attack by moisture). By allowing the molten mixture

to cool back down to a transparent, rigid state, glass is formed. The cooling process

is controlled so that ordered crystallization fails to take place and an amorphous

solid results.

To make glass fibers, the molten glass is forced through a tiny hole to form fibers

in diameters of 2–15 microns. As the molten glass is pulled, the three-­dimensional

structure becomes oriented, increasing the strength and stiffness of the fiber



O



O



HO



H



H O



H



O



H



H

H



H



H



H



O



O



H



H



H



O



O



H

O



H O



O



H



H O



O



HO



O



O



O



O



H



FIGURE 4.37  Cellulose (dashed lines indicate hydogen bonding).











H HO



H



O



H



O



H



H



H



O



H



O



O



H



H



H



O



H



H



H



O

H



O



H



H



HO



O



O



H



H O



O



H



H



O

O



HO



O



O H



O



OH



O







O







Polymers and Sustainable Energy

93



94



Chemistry of Sustainable Energy



TABLE 4.2

Glass Fiber Properties

Composition (wt.%)



E-Glass



S-Glass



SiO2

Al2O3

CaO

MgO

Na­2O + K2O

B2O3

Fe2O3



52–56

12–16

16–25

0–5

0–2

5–10

0–0.8



64–66

24–25

0–0.2

9.5–10

0–0.2



0–0.1



Tensile strength (MPa, 23°C)

Young’s modulus (GPa, 23°C)



3445

72.3



4890

86.9



Source: Data from Hartman, D., M.E. Greenwood, and D.M. Miller.

1996. High strength glass fibers, AGY Technical Paper.



(Carraher 2000). The compositions of two types of glass commonly used in polymer

matrix composites, E-glass and S-glass, are shown in Table 4.2. S-glass is a stiffer

and stronger glass developed for use in more demanding conditions; as a result,

S-glass is seeing greater use in wind turbine blade manufacture. However, it is more

costly and difficult to process than E-glass.

Carbon fibers, while more recently developed, are (as noted above) increasingly

replacing glass in turbine blade applications. Carbon fibers are most commonly

produced from atactic polyacrylonitrile fibers (PAN, Figure 4.38) that contain at

least 85% acrylonitrile with 6–9% of an additional acid comonomer (Committee

on High-Performance Structural Fibers for Advanced Polymer Matrix Composites

2005; Peebles 1995). After polymerization to make the PAN precursor, the material

is carefully treated in a sequence of steps that convert the polymer to filaments, then

bundled filaments (a “tow”) that are stretched to further orient the molecules along

the axis and enhance the strength and modulus of the final product. Next, the stabilization step takes place by heating the acrylonitrile copolymer in air to 200–400°C.

While the exact mechanism is still unknown, during this step inter- and intramolecular nitrile cross-linking occurs to stabilize the PAN fiber. Extensive spectroscopic

studies have led to proposed models for the stabilized fibers; two examples are shown

in Figure 4.39 (Johnson et al. 1972; Usami et al. 1990). In addition to stabilization,

mechanical changes take place as the fiber is further oriented and strengthened at

temperatures above its glass transition temperature (Peebles 1995). The stabilized

CN



CN



CN









FIGURE 4.38  Polyacrylonitrile.



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Polymers and Sustainable Energy

O





NC



N

H



N



x



N

H



N



N

H



y



N



N

H



N







N

zH



Johnson (1972)







CN



O



O



N



N



N



N



NH2



CN



Usami (1990)



FIGURE 4.39  Proposed structures of stabilized polyacrylonitrile.



fibers are then carbonized in an oxygen-free atmosphere at temperatures of up to

2600°C (Committee on High-Performance Structural Fibers for Advanced Polymer

Matrix Composites 2005). Under these conditions, the fibers cannot combust but

instead undergo a decomposition mechanism in which most of the heteroatoms (noncarbon atoms) are lost as small molecules such as carbon monoxide, carbon dioxide, ammonia, HCN, nitrogen, and water. As a result, primarily elemental carbon

remains, with some nitrogen retained for flexibility (Peebles 1995).

The mechanical properties of carbon fibers are significantly different from those

of glass and depend primarily on the degree of carbonization, the orientation of

the carbon planes, and the degree to which the material is crystallized. The tensile

strength for a carbon fiber has been reported to be as high as 290 GPa, whereas that

for S-glass is 4.9 GPa (Table 4.1 and Committee on High-Performance Structural

Fibers for Advanced Polymer Matrix Composites 2005).



4.5.4  Carbon Nanotubes and Polymer Matrix Composites

Recent studies have focused on polymer nanocomposites where at least one of the

reinforcing materials in the matrix has dimensions in the nanoscale range (e.g., carbon nanofibers or nanotubes). Carbon nanotubes can be single-walled (SWCNT)

(Figure 4.16), double-walled (DWCNT) (Figure 4.40), or multiwalled (MWCNT).

They are attractive additives for polymer matrices because of their shape (they possess an extremely high ratio of height to width) and exceptional mechanical strength:

SWCNTs alone have been shown to demonstrate Young’s modulus on the order of

1 TPa (Schnorr and Swager 2011) and a tensile strength that is stronger than highstrength steel (Chou et  al. 2005). Furthermore, their incorporation into polymer

matrix composites imparts enhanced thermal stability as well.

The applications of CNT-reinforced polymer matrix composites extend throughout sustainable energy research, from turbine blades to fuel cells. However, this area

of research is still in its infancy as there are considerable hurdles to overcome. One



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Chemistry of Sustainable Energy



FIGURE 4.40  Image of a double-walled carbon nanotube. (Reprinted with permission

from Sugai, T. et al. 2003. New synthesis of high-quality double-walled carbon nanotubes by

high-temperature pulsed arc discharge. Nano Lett. 3 (6):769–773. Copyright 2003, American

Chemical Society.)



challenge associated with the use of CNTs is that, as with glass and carbon fibers,

the inherent strength of the CNT-reinforced composite depends on the orientation of

the CNTs, a challenging processing problem. Yet perhaps the greatest challenge is

the tendency for the CNTs to bundle together by virtue of their strong intermolecular

attractions. As a result, it is difficult to disperse the CNTs throughout the polymer

matrix and without excellent dispersion to distribute the load, the strength advantage

of using CNTs is lost. This dispersion can be enhanced, however, by functionalizing

the carbon nanotube. Zhu et al. modified SWNT by carboxylation with H2SO4/70%

HNO3 followed by fluorination, then mixed the functionalized SWNT with an epoxy

resin of bisphenol A cured with an aromatic diamine. A 1 wt.% load of the nanotubes

led to a remarkable 18% increase in tensile strength and a 24% increase in modulus

over the untreated resin (see the following table) (Zhu et al. 2003).

Epoxy Formulation

Neat epoxy resin

Epoxy resin + unfunctionalized SWNT

Epoxy resin + functionalized SWNT



Young’s Modulus (MPa)



Tensile Strength (MPa)



2026

2123

2632



83.2

79.9

95.0



The improved dispersion of the SWNT was attributed to covalent bonding of the

functionalized nanotubes to the epoxy matrix through the carboxyl group. Similar

improvements have been seen with the incorporation of amino-functionalized

DWCNTs to a bisphenol-A epoxy matrix, at even lower load levels (0.1 wt.%) (Gojny

et al. 2004).

The future of wind energy will be one of continued optimization in design and

materials with the expectation that individual turbine capacity will increase to

10–12 MW. New designs continue to be advanced, including those based on jet



Polymers and Sustainable Energy



97



FIGURE 4.41  A vertical axis wind turbine. (With permission from Windspire Energy,

LLC. www.windspireenergy.com)



engines (CleanTechnica 2010) and a bladeless design (Williams 2012). The vertical

axis wind turbine (Figure 4.41) is likely to overtake the more conventional HAWT in

the coming years due to the fact that it can produce up to 10 times more energy on

the same amount of land (Islam et al. 2013). However, simultaneous development of

energy storage technologies (Section 6.10) is imperative due to the sporadic nature

of wind energy. The environmentally benign synthesis of the polymer composites is

another imperative if wind energy is to be truly sustainable; this concept of “green

chemistry” is briefly described in the next section.



4.6  GREEN CHEMISTRY

The astonishing growth in wind energy would not be possible without the development of polymer matrix composites, and future improvements in efficiency will

require continued advancement in this area. As we will see in the later chapters,

polymers are also integral to the development and improving efficacy of fuel cells

and organic photovoltaic devices, and of course biomass (as in cellulose) is natural

polymer chemistry. Design and synthesis of polymers with perfected properties of ion

conductivity, absorption of the solar spectrum, strength, and processing ability will

be crucial as sustainable energy solutions evolve. Perhaps even more important will

be the development of new green chemistry methods of synthesizing polymers. While

our focus is on energy, the importance of green chemistry cannot be overlooked in a

sustainable future. Green chemistry means designing methods of preparing, purifying, and using chemicals that are energy efficient, water-conservative, and generally



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