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Renewable and Sustainable Energy Reviews 13 (2009) 428438

www.elsevier.com/locate/rser

A review of surface engineering issues critical to

wind turbine performance
N. Dalili a, A. Edrisy a, R. Carriveau b,*
a

Department of Mechanical, Automotive and Materials Engineering, University of Windsor,

401 Sunset Avenue, Windsor, Ontario N9B 3P4, Canada
b
Department of Civil and Environmental Engineering, University of Windsor, 401 Sunset Avenue,
Received 17 August 2007; accepted 7 November 2007

Abstract
Wind turbine performance can be significantly reduced when the surface integrity of the turbine blades is compromised. Many frontier highenergy regions that are sought for wind farm development including Nordic, warm-humid, and desert-like environments often provide conditions
detrimental to the surface of the turbine blade. In Nordic climates ice can form on the blades and the turbine structure itself through a variety of
mechanisms. Initial ice adhesion may slightly modify the original aerodynamic profile of the blade; continued ice accretion can drastically affect
the structural loading of the entire rotor leading to potentially dangerous situations. In warmer climates, a humid wind is desirable for its increased
density; however, it can come at a price when the region supports large populations of insects. Insect collisions with the blades can foul blade
surfaces leading to a marked increase in skin drag, reducing power production by as much as 50%. Finally, in more arid regions where there is no
threat from ice or insects, high winds can carry soil particles eroded from the ground (abrasive particles). Particulate-laden winds effectively sandblast the blade surfaces, and disrupt the original skin profile of the blade, again reducing its aerodynamic efficiency. While these problems are
challenging, some mitigative measures presently exist and are discussed in the paper. Though, many of the current solutions to ice or insect fouling
actually siphon power from the turbine itself to operate, or require that the turbine be stopped, in either case, profitability is diminished. Our survey
of this topic in the course of our research suggests that a desirable solution may be a single surface engineered coating that reduces the incidence of
ice adhesion, insect fouling, and protects the blade surface from erosive deterioration. Research directions that may lead to such a development are
discussed herein.
Keywords: Wind energy; Surface engineering; Turbine icing; Insect fouling; Blade surface erosion; Materials science

Contents
1.
2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .
Effects of icing . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Problems due to icing . . . . . . . . . . . . . . .
2.1.1. Full stop of the turbine . . . . . . . .
2.1.2. Disruption of aerodynamics . . . . .
2.1.4. Decreased fatigue life . . . . . . . . .
2.1.5. Human safety risks . . . . . . . . . . .
2.2. Wind turbine performance in cold weather
2.2.1. Icing types (ice formation) . . . . .
2.2.2. In-cloud icing . . . . . . . . . . . . . .
2.2.3. Precipitation icing . . . . . . . . . . .

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* Corresponding author. Tel.: +1 519 253 3000.

doi:10.1016/j.rser.2007.11.009

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429
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2.3.

3.
4.
5.

6.

Ice accretion modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.1. Ice accretion profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. Current turbine blade icing solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1. Active methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2. Passive methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effects of insect contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Current solutions for insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effects of erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Current solutions for erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential surface engineering research directions for each of icing, insect fouling, and blade surface erosion
5.1. The physics of ice adhesion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1. Effect of temperature on ice adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2. Effect of surface roughness on ice adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3. Effect of surface contamination on ice adhesion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4. Effect of type of ice on ice adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2. Physics of an anti-stick surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3. Physics of an erosion resistant surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1. Normal, elastic/plastic impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2. Oblique, elastic/plastic impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4. Potential coating candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nomenclature
d
D
H
WA

projected distance of ice throw

diameter of turbine rotor
height of nacelle

Greek symbols
g1
surface energy per unit area of material 1
g2
surface energy per unit area of material 2
g12
surface energy per unit area of interface between
materials 1 and 2
glv
surface energy per unit area of interface between
liquid and vapor
u
contact angle

1. Introduction
Wind energy is a renewable power source that produces no
known significant atmospheric pollution. Currently the worldwide capacity of installed wind energy stands at more than
40,000 MW with a 30% growth rate predicted over the next
decade. Maintaining such growth necessitates research into the
management of economic and environmental risks associated
with the operation of large-scale commercial wind ventures.
Wind turbine technology has a unique technical identity and
subsequently has unique demands in terms of methods used for
design [1]. A focal variable critical to the economic optimization
of wind power production is the evolution of the wind turbine
blade. Fundamental to the efficient extraction of power from the
wind are the structural and aerodynamic properties of the blade.
Until relatively recently it was enough to design blades with a

429

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desirable aerodynamic profile and a durable yet responsive

structure. However, with the expansion of wind infrastructure, an
increasing number of turbine installations have pushed their way
into three distinct climatic zones, icy Nordic environments,
humid regions that support large insect populations, and desert
environments with sand-laden winds. Each of these environments can create significant operational issues for a wind farm;
critical to the comprehension and potential resolution of all three
issues are the composition, specifically the surface properties of
the turbine blade. Currently, most manufacturers employ epoxy
or polyester matrix composites reinforced with glass and/or
carbon fibres; though polyester and glass fibres remain the
material of choice due to their lower capital cost, despite the
superior mechanical properties of epoxy and carbon fibres. A
compatible gelcoat is also typically applied on the finished blade
to improve surface erosion resistance. The wind energy industry
presently faces three major challenges concerning the surface
and the erosion of blades by sand and water droplets. We have yet
to uncover much literature that addresses the surface engineering
issues facing wind turbines in a holistic sense. This is likely due
to the complexity associated with a multidisciplinary challenge
that calls on expertise from materials science, aero-, thermo-, and
structural-dynamics. We endeavour to provide details of the
current challenges confronting the industry, then discuss present
solutions, and propose potential research directions.
2. Effects of icing
Wind turbines installed in cold climates face icing issues
over their service life. Ice accretion on wind turbines,
particularly turbine blades, can be detrimental to turbine

430

performance, durability, and the safety of those in the vicinity

of operating iced turbines. In the following sections the issues
with icing, ice adhesion, and accretion in cold weather are
discussed.

long-term detrimental effect on generator and blade mechanical

health. It was also conceded that the increase in power may
have been strictly due to the increased air density associated
with the cold icing temperatures.

2.1.4. Decreased fatigue life

The operation of a wind turbine with an imbalance caused by
icing experiences an increase in the loads imposed on all
turbine components. Although the extreme loads are treated at
the components [4,5]. Details of these effects were illustrated in
the WECO (Wind Energy Production in Cold Climates) project
[6]. They found the following:

Prior to addressing causes and measures of mitigation, the

significant problems icing causes commercial wind generation
are discussed.
2.1.1. Full stop of the turbine
In some cases, severe icing has led to the complete stop of
the turbineresulting in significant energy loss. This is the
simplest of all loss mechanisms to quantify, and certainly an
event all operators wish to avoid at nearly any cost. The
ppelbo wind turbine in Sweden, for example, reported 7
A
weeks of turbine stoppage in the winter of 20022003. The
Swedish statistical incident database contains a total of 1337
such stop records reported to have occurred between 1998 and
2003resulting in a total downtime of 161,523 h. Ninety-two
of the incidents (7%) were related to the cold climate and
resulted in 8022 h (5%) of production loss. The reported lowtemperature downtime for all cold climates was 669 h (8%)
while the equivalent for icing events was 7353 h (92%).
Downtimes reported due to icing in Finland between 1996 and
2001 resulted in 1208, 495, 196, 581, 739 and 4230 h for 19, 21,
29, 38, 61 and 61 turbines, respectively [2].
2.1.2. Disruption of aerodynamics
Antikainen and Peuranen [3] performed a comprehensive
study of the effects of icing time and shapes on the aerodynamic
balance of turbine blades, and the results suggested that both
mass and aerodynamic imbalance can occur even in the early
stages of icing. Jasinski et al. [2] established that even the onset
of ice accretion could cause a slight increase in surface
roughness that, in turn, increases the drag coefficient
reducing power production. Field data, wind tunnel simulations
and numerical analyses have been used to describe the ice
formation on wind turbine blades, as well to calculate the
aerodynamic coefficients and power curves for iced wind
turbines and study the corresponding loads. Ice accretion on
blades nearly always decreases power production, and at harsh
sites the annual power loss is typically in the range of 2050%
[4].
Jasinski et al. [2] demonstrated that under certain conditions,
stall regulated wind turbines that develop ice accretion
experienced a temporary increase in power production as a
consequence of delayed stalling. Though ice accretion typically
reduces airfoil efficiency, the rime ice that had formed on the
subsequently was postulated to increase the maximum lift
coefficient and delayed aerodynamic stall to a higher angle of
attack. As a result, maximum power development increased
beyond typical limits. Though seemingly desirable, this has a

 Asymmetric masses will cause unbalance.
 Ice accretion will increase the excitation of edgewise
vibrations.
 Resonance may occur due to the changed natural frequencies
2.1.5. Human safety risks
Ice thrown from rotating blades poses a serious safety issue,
particularly when the wind power plant site borders public
roads, housing, power lines, and shipping routes. Ice throw has
been studied using both theoretical models and collected
experimental data, such as questionnaires sent to operators in
various parts of Europe and observations made at the WECO
test sites [6]. These studies have prompted a recommendation
that for sites with a high probability of icing, the distance
between the turbine and the nearest object should adhere to the
following equationwith the effect of slopes taken into
account for mountainous sites:
d 1:5D H

(1)

where d is the projected distance ice can be thrown, D is the

diameter of the rotor and H is the height of the nacelle. This
equation should be considered a rough estimate, but it provides
a general idea of the area at risk [6,7].
2.2. Wind turbine performance in cold weather
In spite of all the difficulties mentioned above, there exists
increasing interest in cold-climate wind turbines. Air density
variations affect the power output of wind turbines, and based
on the equation of state for an ideal gas, air at 30 8C is 26.7%
denser than at 35 8C. Power is proportional to air density;
accordingly the power output increases at low temperatures [8],
and the higher energy output has sparked an increasing interest
in sites at both higher altitude and latitudedespite the
limitations caused by harsh weather conditions. There are three
major concerns when operating in cold weather:
 Impact of low temperatures on the physical properties of
materials.
 Ice accretion on structures and surfaces.

 The presence of snow in the vicinity of a wind turbine [6].

Ice accretion represents the most significant threat to the
integrity of wind turbines in cold weather. The amount and
type of accreted ice depends on a variety of factors such as
climate condition, the shape and kind (stationary or moving) of
the turbine component subjected to icing and the surface
characteristics of the component. Fig. 1 illustrates an example
of ice accretion on a wind turbine blade in Germany.
2.2.1. Icing types (ice formation)
Different kinds of icing occur at different sites, the two main
types of ice accumulation being in-cloud icing and precipitation
icing [9,10]. Frost, a third method of ice accumulation, is not
believed to cause any problems for wind turbines. The two main
categories can be further subdivided from in-cloud icing into
rime or glaze ice; and precipitation ice can be further
subdivided as wet snow or freezing rain variants.
2.2.2. In-cloud icing
In-cloud icing occurs with the impact of super-cooled droplets
on a surface. Super-cooled droplets appear in clouds with
ambient temperatures of 20 to 35 8C [11]. These droplets

431

freeze on surface contact. The temperature and size of the

droplets determines whether the ice formed will be considered
hard rime, soft rime, or glaze [2,6]. Rime is formed when supercooled droplets impact the surface and the thermal energy
released is removed quickly enough by wind and radiation such
that no liquid water is present on the surface. Rime ice is white,
and breaks off more easily than glaze. Soft rime forms when the
droplet size is small and the water content in the air is low. It has a
lower density than hard rime, due to larger air gaps between the
frozen particles. Likewise, hard rime forms as a result of
medium-sized droplets with higher water content in the air;
subsequently, it is harder largely due to better bonding and
smaller air gaps between the frozen particles [7,9,10].
Glaze is formed when the thermal energy released during
droplet surface impact is not removed as quickly as it is for
rime, and some portion of the droplets remain as liquid water.
As the amount of trapped air is very low, glaze is a solid cover of
clear ice with a higher density and is significantly more difficult
to remove than rime [7,9,10].
2.2.3. Precipitation icing
Precipitation icing occurs when precipitation either rain or
snow freezes after striking the surface. Wet snow can stick to a
surface if the temperature is between 0 and 3 8Ca condition
that ensures that the snow will have some liquid water present,
allowing the snow crystals to bind together when they come
into contact on the surface. While the binding strength of wet
snow is often low when it first forms, it can become very hard if
the temperature subsequently falls below 0 8C [2,5].
When rain falls at temperatures below 0 8C it leads to glaze,
freezing rain, or drizzle. This often occurs in connection with a
temperature inversion where cold air is trapped near the ground
beneath a layer of warmer air [7,9,10]. This can also occur in
the case of a rapid air temperature rise where an objects
temperature is below freezing even though the air temperature
is above freezing.
2.3. Ice accretion modeling
To aid in predicting the formation and effects of ice accretion
some empirical and statistical models for the accretion of ice on
turbine blades have been developed by the aerospace industry.
These programs were developed to model the leading edge
icing of aircraft wings, i.e. TURBICE and LEWICE [2]. Such
packages utilize wind speed, temperature, accretion time, liquid
water content and droplet size as input data then simulate the
shape and amount of resulting ice on the aerofoil [2,9].
Wind tunnel tests can also be carried out using artificial
iced profiles with various types and amounts of ice accretion to
model accretion effects. This information can be used as input
for models, which then predict load and power output for iced
rotor blades [6]. Wind tunnels have also been used for the
simulation of ice accretion on turbine blades.

2.3.1. Ice accretion profile

the rest of the blade profile because of the stagnation point in

432

additional mechanical complexity associated with such a

system would add significantly to the maintenance burden for
the 20-year life span of the turbine [4].
2.4.1.2. Active anti-icing systems. Most active methods that
have been developed at least to a prototype stage have been
based on thermal systems that remove the ice by applying heat
through several methods:

Fig. 2. Serrated ice accretion on turbine blade edge formed by an alternating

series of ice breaking and reforming events; Tammelin [12].

take on a saw-tooth appearance; this is due to the aerodynamic
forces that are sufficient to break ice fragments off, then
subsequent ice accretions build until the leading edge takes on
the serrated appearance shown in Fig. 2.
2.4. Current turbine blade icing solutions
In recent years, extensive research has been undertaken to
identify and model ice prevention methods. Most of these
methods are taken from the aviation industry and can be
classified in two categories: active and passive. Passive
methods rely on the physical properties of the blade to prevent
ice accumulation while active methods rely on an external
system applied to the blade. Two types of systems can be
employed to prevent icingspecifically de-icing and antiicing. The former removes the ice from the surface after its
formation, while the latter prevents the initiation of icing [13].
These systems can be either passive or active and are described
in the following sections.
2.4.1. Active methods
Active protection methods require supplied energy to
perform, and include thermal, chemical, and pneumatic
techniques and act as de-icing or anti-icing systems.
2.4.1.1. Active de-icing systems. Small aeroplanes often
use mechanical de-icing systemsinflatable rubber boots on
the leading edge of the wing that expand and contract on iceprone areas of the aircraft. These systems have not proven
pragmatic for wind turbine application for several reasons;
obvious ones include the increased aerodynamic interference
and noise that would be caused by an inflated boot. Further, the

 Electrical resistance heating. The electrical ice protection

system consists of a heating membrane or element that is
applied to the blade surface. The membrane or element is
laminated into the blade structure. These thermal ice
prevention systems are simple and have been used successfully in the aerospace industry for many years. Similar
heating systems were developed for wind energy application
in the mid-1990s [4]. At present, there are many commercially available electrical blade heating options. The Finnish
blade heating system carbon fibre elements mounted to the
blades near the surface has the widest operating experience
with installation in 18 turbines at various sites with a total of
nearly 100 operating winters [2]. A JE system of direct
resistance heating has been shown to work effectively as well,
but has not yet seen mass production [7].
 Indirect heating of the surface. This method heats the inside
of the blade with warm air or a radiator, and the heat is
conducted to the outer surface. Such a system has been
installed on an Enercon turbine in Switzerland [14].
 Microwave heating. These systems are based on microwave
energy technology, but have not yet to our knowledge, been
successfully implemented [2].
 A system where the blade surface is protected by a layer of
clean air has been conceived. This system utilizes air flow
from inside the blade pushed through rows of small holes near
the leading and trailing edges to generate a layer of clean and,
if necessary, heated air directly around the blade surface. This
layer of air would deflect the majority of water droplets in the
air and would melt the few droplets that managed to strike the
surface [14].
The following points highlight the inherent disadvantages
connected with these heating methods:
 Leading edge heating elements will not help de-icing when
active stall-controlled turbines are at a standstill (e.g. during
icing conditions combined with low-wind speeds).
 Positioning the heating elements at the leading edge can also
cause potential structural issues with the blades. Rotor
structure. Further to this, the aerodynamic driving forces and
superposed edgewise vibrations, caused by a low damping of
the natural frequency in this direction, are added to the
stress in the wires or fibres of the heating elements. This is
particularly true when the heating elements are made from




carbon fibrewhose Youngs modulus is much higher when

compared to the glass fibres of todays rotor blade structures.
In other words, the heating fibres end up bearing a large
portion of the loads. Special technical solutions are required
in order to avoid these effects and prevent cracks in the
heating elements [4].
The electrical heating elements metal or carbon fibre can
attract lightning strikes at an exposed site.
The airfoil contour must be kept free from waviness to avoid
unnecessary disturbances of the laminar flow around the
leading edge during ice-free conditions. Electrical fibres and
laminate embedding may increase surface roughness [4].
For blade cavity forced warm air thermal systems, the
efficiency of the system wanes as turbine blades increase in
size; shell structures become thicker and thermal resistance
rises. In practice, this means that very high temperatures are
needed inside the blades to keep the outer surfaces free of ice,
even in mild conditions. Considering the maximum operating
temperatures of thermoset composites, using such temperatures inside the blade structure to keep the blades free of ice
would be a significant challenge [14,15].
Should one blade heater fail, a significant mass imbalance
may be imposed on the rotor as each blade may have different
icing loads; such a situation occurred for Yukon Energy in
1996 [15].
In some cases, the run-back water on the blade during icing
and blade heating can freeze after it passes the heated area.
Ice formed from run-back water has a high density and may
be dangerous when shed from the blades in big pieces. It can
also accumulate at positions where it may be aerodynamically harmful [15].
Current anti-icing technology consumes electricity. The
break-even cost of such a heating system depends on how
much energy production is lost due to icing and the price of
electricity. Therefore, when the financial benefits of a blade
heating system are evaluatedicing time, severity of icing,
and potential wind resources need to be known. Based on
early work in Europe, thermal anti-icing requires power equal
to at least 25% of the turbines maximum power. Recent work
conducted in Europe, however, indicated that the early
estimates in anti-icing power requirement can be revised
down. Current claims suggest that the power requirement
ranges between 6 and 12% of the output for smaller
commercial scale turbines [6].

2.4.2. Passive methods

Passive methods take advantage of the physical properties of
the blade surface to eliminate or prevent ice, and are similar to
active methods in their ability to act as de-icing or anti-icing
systems.
2.4.2.1. Passive de-icing systems
. A system designed with blades flexible enough to crack the
known to help shed the ice, to our knowledge there is little
published information on this subject. The disadvantage of
trying to crack the ice loose is that thin layers of ice can

433

enough to crack loose from just the vibration of the blade; it
would likely also compromise the aerodynamic properties of
 An electro-expulsive system that depends on very rapid,
electromagnetically induced vibrations has recently been
certified for use on Raytheons Premier I business jet. This
system holds promise for smaller, new, general aviation
airplanes but there remains a lack of practical information for
wind turbines [16].
 Other passive systems such as active pitching of the blades,
start stop cycles and facing the blades into the sun are used to
remove ice from turbine blades. Though these methods may
work in light icing environments, few studies have been
published to validate their efficacy, and such methods may
damage the turbine and/or reduce power production [17].
2.4.2.2. Passive anti-icing systems
. In 1996, Yukon Energy painted their blades with a black
coloured coating called StaClean1 [15]. StaClean1 is
purported by the manufacturer to be non-wetting, slicker
than Teflon1, highly impact and abrasion resistant. Such
qualities would be very advantageous in minimizing ice
accretion. Subsequently this was reported to be effective at
reducing icing issues [18]. Though, the black colour did not
significantly increase blade surface temperatures during
winter months [19]. In fact in many instances the sections of
the blade coated only with the manufacturers original gelcoat actually rose to a marginally higher temperature than
sections coated with the black StaClean1.
Special coatings that reduce the shear forces between the ice
and the blades surface are not new; as icing is a substantial
problem for aircrafts as well. While development of an ideal
surface coating for icing mitigation still eludes the aerospace
and wind turbine industry, it remains the only method among
those discussed herein that may prevent the adhesion of insects
and the erosion of the leading edge.
3. Effects of insect contamination
Stall control is not very accurate in practical application, and
many stall-controlled turbines do not meet their specifications
[20]. The power of wind turbines operating in high winds has
been known to drop for no known reason, causing production
losses from 25 to 50%a phenomenon referred to as a Double
Stall or a Multiple Stall [20]. Corten et al. [2022] attributed
this multiple power level occurrence to the insect theory, which
states that these levels correspond to different degrees of insect
contamination. Fig. 3 illustrates an example of a wind turbine
blade contaminated by insects. A low contamination level
decreases the power by 8% of the design value; while at high
levels it can be decreased by up to 55%. In this study a device
called a stall flag was employeda hinged flap that opens up
in a separated airflow to uncover an individual reflector.
Employing a projector as a light source, they measured the
separated flow from the intensity of reflected light.

434

N. Dalili et al. / Renewable and Sustainable Energy Reviews 13 (2009) 428438

Fig. 3. Insect collisions foul the leading edge of wind turbine blade (M.D.
Burns/Oak Creek Energy Sys.).

The hypothesis has been validated by three crucial experimentsthe observation of the decrease of the stall angle due to
contamination, monitoring the progress of the power level over
time that showed a decrease after each period of low wind, and
the application of artificial roughness to turbine blades, which
prompted the power curve to drop in a way that mirrored the
multiple power level observations. Insects prefer to fly in
conditions of high air humidity, low wind and temperatures
above about 10 8C. Under these circumstances, they will
Efficient aerodynamic performance is best achieved when
the airflow is attached to the aerofoil section. Flow separation
can take place under a high angle of attack or due to the
presence of roughness near the leading edge. In the latter case,
the primary flow detaches from the airfoil at a point called
the separation point, and beyond this point the airflow near
the aerofoil actually proceeds away from the primary flow
directiona state called reversed flow or separated flow.
Generally, such flow separation is detrimental to the
performance of the aerofoil and should be avoided [23].
Insects rarely fly in high winds, so turbines operating in
steady high-wind conditions do not become contaminated,
and their power levels remain constant. Although, with high
winds, the angle between the flow and the blades increases and
the aerodynamic suction peak (the area of minimum pressure and
maximum air velocity) shifts to the leading edge; and if the
leading edge is already coated with dead insects (from a lowwind period), the resulting power output will fall. The greater the
contamination at the suction peak, the sooner the blades will stall
and the more lift will be lost. Thus after each period of low wind,
the amount of insect contamination may change and cause a
different power level to be produced in high wind [2022].
3.1. Current solutions for insects
The most common solution for reducing the effect of insects
and air pollutants on the blades is to wait for rainfall to wash the

blades. There are some services that offer cleaning machines

that can be hoisted up and down the bladescleaning and
this system is that the wind turbine must be stopped, so the
resulting power loss will simply add to the losses created by the
insects [24]. Another blade washing technique involves
pumping water up through the tower and spraying it into the
wind and through the blade tipa solution that can be
implemented while the turbine is in operation [25].
The compromise of the aerodynamic efficacy through insect
fouling is a result of the increasing surface roughness. In the
past, airfoils had been more susceptible to the effects of
roughness. Recently, improved families of airfoils designed
specifically for horizontal-axis wind turbines have been
patented, wherein the airfoils maximum lift coefficient is
designed to be largely insensitive to leading edge roughness
[26].
Of the options discussed, the most appropriate means of
insect adhesion prevention may be through the application of
non-stick coatings that alleviate the adhesion by providing a
low-friction, low surface free energy and smooth surface on
which insects have great difficulty settling. Even if settling does
eventually occur on the surface, the bonding between the
foulants and the surface would be so weak that the shear forces
on the airfoil may easily remove them.
4. Effects of erosion
Wind carrying large amounts of sand and water droplets can
roughness. These effects deteriorate aerodynamic performance
and reduce machine power output in the same manner as insect
contamination. The potential for erosion depends on the force at
which the particulate matter impacts the airfoil. Geometric
shapes and the relative velocities of both the airfoil and
the impacting particle determine the impact force of the
particulates. Wind speed and rotational speed of the blade
determine impact velocity. The rotational speed of the blade is
at its maximum at the outboard tip of the airfoil, which is the
furthest from the rotational axis [27].
According to van Rooij and Timmer [28] the effect of
roughness on a blades aerodynamic performance depends on
the geometric design of the blade, such that a blade may be
adapted to induce minimal energy loss. However, surface
roughness changes during operation due to contamination
(insects) or erosion, and will typically always lead to
unpredicted energy losses.
4.1. Current solutions for erosion
The common technique for improving an airfoils erosion
resistance is to apply an elastometric material to the leading
edge of the airfoil in the form of a tape. These tapes must be
replaced frequently however; since they fail to adequately
absorb the impact energy of the particulate matter. On the other
hand, these tapes are suitable for small turbines, though they
will increase the aerodynamic drag on the blade surface [29].

435

For example, the 7.5 kW Bergey EXCEL-R wind turbine

blades are protected from erosion with a special polyurethane
leading edge tape and the blades are painted with an aircraftquality polyurethane paint [30].
There are other innovations that are worth noting, despite the
fact that they are not yet commonly available. One is the
vacuum infusion of large thermoplastic composite blades,
which show higher resistance against erosion for areas like the
leading edge and blade tip [31]. Another is the application of a
nanocomposite layer to the leading edge of an airfoil [27].
Adding the nano-sized reinforcing particles to an elastomer
allows the nanocomposite to absorb the impact energy over a
larger volume when compared to a pure elastomeric material.

Since the binding energies of H2O molecules and different

solids are expected to be similar in ice and water, it may be
supposed that the values of WA on different ice-solid interfaces
will be a function of wetting angle of water on these solids.
Petrenko and Whitworth [33] collected measurements made by
other investigators and concluded that there is a correlation
between angle of contact and adhesion strength, although its
deviation is high [33]. The results of NASA LEWICE and BF
Goodrich studies also indicate that hydrophobicity does not
necessarily produce poor ice adhesion, but shows that hydrophilic materials have in fact substantial ice-adhesive properties
[34,35]. The following items contribute to the poor correlation
between ice adhesion and contact angle:

5. Potential surface engineering research directions for

each of icing, insect fouling, and blade surface erosion

 The plastic deformation that is associated with fracture on the

interface will absorb energy, so the assumption that the force
required for adhesive failure depends only on WA takes no
account of this energy.
 The electrostatic component of WA is different for water and
ice, unlike the previous assumption [33].

Novel surface engineering applied to blade surfaces has been

identified here as a versatile solution that potentially addresses
the majority of the aforementioned problems. In order to further
investigate the effect of surface engineering, the physical or
mechanical properties of surfaces that have low ice adhesion,
anti-stick and erosion resistance properties must first be
understood. These properties are described in the following
sections.
5.1. The physics of ice adhesion
Using special coatings to reduce ice adhesion requires a
comprehensive understanding of the fundamental physics
involved. The physical process of adhesion can be attributed
to three different kinds of bonds. First is covalent or chemical
binding, which holds atoms in molecules together and acts only
over distances of the order of 0.10.2 nm. For most of the
materials particularly the blade material there is no affinity
between water molecules and the surface. The second type of
bonds is due to Lifshitzvan der Waals forces, which have no
considerable effect on adhesion. It is electrostatic forces
between two solids which contain non-compensated spatial
distributions of charge that can contribute most to the adhesion
[32].
Exerting an external force on the ice-solid interface can lead
to cohesive breaks (breaks that happen within the ice) or
adhesive breaks (breaks that happen in the ice-solid interface
and occur when the strength of adhesion is lower than the ice
strength). The strength of adhesion between two materials is
defined as the free energy required to separate a boundary of
unit area between the materials, i.e. the work of adhesion:
W A g 1 g 2  g 12

(2)

where g1, g2 and g12 are the surface energies per unit area of
materials 1, 2 and the interface, respectively. Work of adhesion
for liquidsolid interfaces can be described as a function of the
angle of contact, u and the surface free energy between liquid
and vapour, glv:
W A g lv 1 cos u

(3)

There are many parameters that affect the adhesion strength

of ice to surfaces; unfortunately most of them are not well
understood. Some of these parameters are mentioned as
follows.
5.1.1. Effect of temperature on ice adhesion
Scavuzzo et al. mentioned that decreasing temperature
between 0 and 4 8C will increase ice adhesion strength, but at
this temperature ice adhesion reaches its maximum and further
temperature reduction will not have an effect on adhesion [34].
The results of shear tests between stainless steel and ice in the
form of frozen snow show that the shear strength increases with
falling temperatures as low as 13 8C, at which point the
adhesive strength of ice becomes higher than its cohesive
strengththerefore at lower temperatures failure occurs within
the ice and decreasing temperature will have no effect on shear
strength [36]. Alternatively, adhesion of ice to some plastics
(PTFE, polystyrene, perplex and a thick layer of stearic acid)
were found to be independent of temperature [37].
5.1.2. Effect of surface roughness on ice adhesion
Increasing surface roughness can boost the shear strength by
a factor of 10 from a mirror polished surface to a machined one
[32].
5.1.3. Effect of surface contamination on ice adhesion
The presence of a contaminant film or particle will decrease
will be reduced, though these contaminants can act as ice
nucleating points and increase ice accumulation [37].
5.1.4. Effect of type of ice on ice adhesion
It is reported that the adhesive shear strength of rime ice is
much weaker than that of glaze ice [34]. In order to measure the
adhesion strength of ice to different coatings, and to introduce a
material, the tests must be performed with atomically clean and

436

flat surfaces under a constant climate condition. The results of

some of the tests performed to evaluate the ice adhesion
properties of materials are mentioned in Section 5.4.
5.2. Physics of an anti-stick surface
In order to increase the anti-stick properties of a substrate, its
surface energy, friction coefficient and roughness must be
decreased. If there is a relation between anti-icing and icephobic properties, applying an ice-phobic coating will satisfy
the anti-stick requirements as well.
5.3. Physics of an erosion resistant surface
Material removal during erosion depends on the properties
of the target material, the physical and chemical characteristics
of erodent particles and the exposure condition. With respect to
the impact angle, solid particles erosion falls into two
categories:
 Erosion at normal impact angles (a  908)
 Erosion at oblique impact angles (08 < a < 908)
Generally two modes of ductile and brittle erosion are
observed, neither of which directly match the traditional
grouping of materials according to their failure [37]. In ductile
mode, weight loss due to erosion has a maximum at low
impingement angles while in brittle mode, the maximum is at
high impact angles (a  908).
Elastomers and ductile polymers generally exhibit ductile
erosion behavior, but elastomers present a much lower weight
loss compared to that observed for ductile thermoplastics. This
difference in behavior can be traced to the mechanism by which
material removal happenstearing and fatigue for rubbers,
cutting and chip formation for ductile metals and polymers and
brittle fracture for brittle polymers. The phenomena that occur
during turbine blade erosion fall into two categories:
 Normal, elastic/plastic impact
 Oblique, elastic/plastic impact
5.3.1. Normal, elastic/plastic impact
This type of impact is most frequent, here the impact energy
converts to plastic energy and heat (by internal friction) with the
latter displaying a larger proportion. Increasing the ductility of
impact partners will increase the number of stress cycles to
failurethereby decreasing erosive wear. In the case of
elastomers, a much lower amount of kinetic energy will be
absorbed on impact. Elastomer failure is attributed to their
Possions ratio (0.5). The surface tensile stresses arising from
frictional forces cause fine cracks to grow progressively in the
surfaceconsequently decreasing the friction coefficient and
alleviating the erosion rate [38,39].
5.3.2. Oblique, elastic/plastic impact
At lower impact angles, the parallel component of velocity
to the surface becomes higher and micro-cutting and micro-

ploughing mechanisms (which are related to the relative

hardness of impact partners) start playing a role. This is the
reason for the higher erosion rate of the ductile matrix at lower
impact angles, while brittle materials are not so easily cut by
particles and therefore have a lower erosion rate at lower impact
angles. Elastomers eroded at glancing incidence show
formation of tears and cracks perpendicular to the erosion
direction. Impact particles slide on the surfaces, producing
deforming ridges at the first stages of erosion and causing the
growth of fatigue cracks from the base of each ridge [38].
While the erosive wear mechanisms that can take place in
polymer materials are different, there is no definition for the
anti-erosive properties of these materials. A large number of
factors affect the erosion resistance of a polymer including the
crystallinity, physical and chemical network characteristics,
thermal conductivity, glass transition temperature, mechanical
properties (such as hardness, tensile strength, modulus of
elasticity, fracture toughness, yield stress, rebound resilience,
friction coefficient, ultimate strength and elongation) and the
degree of work hardening and softening by heat generation.
None of these factors have a common effect on polymers,
which can be attributed to high strain rates in the impact of
erosive particles and the change in some of these properties
during erosion, such as the friction coefficient and hardness.
A number of models have been proposed to relate the
abrasive wear resistance of polymers to other mechanical
properties. One of the earliest of these is commonly known as
the RatnerLancaster correlation [39], and it predicts that the
wear coefficient is inversely proportional to the product of
stress and strain at tensile break, this is an estimate of the energy
to fracture. Another model by Friedrich [40] proposes that the
brittle index in the form of H/GIc, where GIc is the fracture
energy and H is the hardness, this is an indicator of erosion
resistance. Nevertheless, Budinski [41] studied these models
for 21 plastics/elastomers and found poor correlation between
experimental data and these models.
It follows that materials with anti-erosive properties ought to
be extremely hard and tough such that the impacting particle is
unable to make any impression on the surface. Conversely
however, the material should also be highly elastic so that the
kinetic energy of the particle is harmlessly dissipated. Thus, no
general statement for erosion properties of polymers can be
the appropriate combination of properties necessary for a
material to be deemed truly anti-erosive.
5.4. Potential coating candidates
A wide range of coatings claiming anti-icing properties have
been tested by NASA LEWICE and BF Goodrich research
centers, including a hydrophobic treatment for aircraft windshields (PPG Surface Seal), a Dow Chemical Anti-Stick
developed water-based fluorocarbon, a second Dow coating,
Freedom Ceram-Kote MTM and a commercial ceramic-epoxy
coating used in petroleum, marine and sewage applications to
protect against corrosion and erosion. Only the Dow Anti-Stick
appeared to reduce the adhesion of ice when compared with the

uncoated surface. Perhaps the most interesting result of these

tests was the discovery that Teflon provided no reduction in
adhesion. This could be assumed to be due to its high pore
density, which probably encouraged strong mechanical bonding with ice [34]. Meanwhile, in research performed on a group
of thermoplastics and coatings to reduce ice accretion on Corps
hydropolymer, including Avetal, Derlin, Teflon, UHMWPE,
PSX-700, Inerta and Envelon; the highest reduction in ice
accretion was observed with Teflon [42].
One of the most common materials with an ice-phobic claim
are siliconepoxy-based resins in which the silicon part
provides the low ice adhesion properties and the epoxy part
provides the erosion and impact properties [43]. Claims have
also been made regarding their resistance to erosion and
fouling, but these claims have yet to be conclusively
substantiated.
Current research is heading towards nanocomposite
coatings, polymers reinforced by minute, nanometre-scale
particles. This is because nanocomposites create very high
contact angles with water and the reinforced polymer acts to
absorb and dissipate the higher impact energy caused by the
repeated impact of particulate matter, which produces higher
erosion resistance. One particular type of nanocomposite has
been proposed that benefits from electrically conductive
particles that can be used to raise the temperature of surface it
coats [44]. Another proposed coating is a molecular
combination of organic and inorganic constituents. According
to TPL Inc., the inorganic component will impart hardness
and durability while the organic part will impart flexibility
and icephobicity [45]. Another proposed material is a liquid
and/or solid anti-icing filler and/or oil that are combined with
erosion resistant silicone and/or fluorocarbon elastomeric
materials to create erosion resistant anti-icing coatings [46].
A mixture of Rain-X and MP55 PTFE (powder Teflon) has
also been claimed to be an outstanding coating to reduce ice
adhesion to surface of the space shuttle [47]. An anti-icing
coating has also been developed in Cape Cod Research, Inc.
for navy applications called PCM-based icephobic coatings
[48]. Presently, detailed information for most of these
coatings remain proprietary, consequently most of the data
sheets and chemical compositions are classified and
unattainable.
6. Conclusions
Ice, insects, and erosion represent significant economic
issues for commercial wind turbine operation, as they can
decrease the aerodynamic efficiency of wind turbine blades,
provoke sudden shutdowns, and contribute to unscheduled
maintenance requirements. The significance of these concerns
will likely increase as more turbines are being located in
frontier climates to take advantage of higher wind velocities
and/or densities.
The development of a single holistic solution to the
problematic surface engineering issues facing commercial
wind turbines is a formidable undertaking. The multidisciplinary nature of the research may reveal the reason for relative

437

paucity of literature that relates the three major issues together.

We have attempted to summarize the detailed difficulties that
each issue produces for the industry. Further we have compiled
many of the currently available solutions that have been
reported, and have attempted to bring to light some relevant
considerations for future research on this topic, and in this
process have arrived at some general conclusions.
1. Neither passive nor active measures against icing have
proven completely effective in preventing initial icing and
subsequent ice accretion.
2. As active measures against icing (or insects) represent a
significant initial capital investment, and still require
subsequent power to operate, resources might be better
devoted to the development of a passive solution capable of
providing some resolution to each of ice, insects, and
erosion.
3. A single application coating material could provide a
multipurpose solution that may at least reduce the frequency
of unscheduled shutdowns and maintenance issues.
4. Promising directions for future coatings research include
further testing of siliconepoxy-based resins for their iceadhesive properties, and the continued development of new
nanocomposites and combined organic/inorganic constituent
materials.
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