Reducing bio-fouling on tidal turbines using microstructures

1 eBU: Online Journal, 1(1) December 2015, PP. 1-X
Reducing bio-fouling on horizontal axis tidal turbines using
engineered surface microstructures
Adam Branna
a
Faculty of Science and Technology, Department of Design and Engineering, Bournemouth
University, BH12 5BB, UK
ABSTRACT
This journal paper explores existing methods of anti-fouling in various industries, and discusses the viability
of using novel surface solutions to combat bio-fouling on horizontal axis tidal stream generators, increasing
the reliability and financial viability of such systems. It is found that microtopographies show excellent
antifouling properties in test conditions. The real life implications of fouling on microstructures are discussed,
and it is suggested that the application of engineered surface microstructures will not be financially viable
until and automated solution for applying them to large scale structures is found.
1. INTRODUCTION
In a time where the UK’s demand for electricity is
expanding, with its steadily growing population, and
heating and transport have become increasingly
electrified (UK Gov 2012), there is a growing interest
in renewable electricity generation.
Tidal energy is an emerging process of renewable
electricity generation, currently in the pre-
commercialisation phase, with only a few plants
nationwide (UK Gov 2012). Tidal energy is divided
into two sub-categories, tidal range and tidal stream.
Tidal range solutions allow a reservoir to be filled by
the incoming tide, and then allow the reservoir to
drain once the tide has receded, turning turbines to
create electricity. Tidal stream solutions capture the
kinetic energy of the flowing water (TEL 2015).
There are a number of different technologies
available for tidal stream energy conversion,
including oscillating hydrofoils, horizontal axis tidal
turbines (HATTs) and vertical axis turbines (EMEC
2008). The most widespread technology is
horizontal axis, with 11 full scale systems in
operation (Green Rhino Energy 2015). HATTs may
be bottom anchored, or mounted on floating tethered
platforms.
Figure 1.Example of a horizontal axis tidal stream energy
converter in Ireland, the SeaGen S (Sea Generation ltd.
2015).
The UK is the world leader in the development of the
emerging tidal stream sectors, and the government
have set aside “a ring fenced revenue support for
100MW of wave and tidal stream up to 2019” (UK
Gov 2014). Although at present, government reports
show the contribution to be negligible, (UK Gov
2015), the government predicts that by 2020, tidal
energy will have begun to play an increasing role in
electricity generation. (UK Gov 2011). In light of this,
it is important that we understand the potential
problems and barriers to the widespread adoption of
tidal stream technology. One such problem, and the
problem that will be focused on in this paper, is bio-
fouling.
Despite having significant merits for the marine eco-
system, biofilms are better known for their
detrimental effects on manmade structures. (Davey
et al 2000; Killea 2014). Biofouling has plagued the
shipping industry for centuries (Field 1981; Hamm
2009; AkzoNobel 2013), colonising submerged
sections of the hull and generating drag. A study
performed by Schultz (2011, pg91) on a boats hull
reported increased resistance of 9% with light
fouling, increasing to 69% with heavy calcareous
fouling. Such drastic increase in resistance would
significantly affect the performance of the blade, by
reducing the lift drag-ratio (Almukhtar 2012). Orme
et al. (2001, p.99) conducted an experiment
simulating the effects of barnacle growth on a marine
turbine blade. Results showed that low levels of
barnacle fouling caused losses of 20%, and higher
levels could cause a 70% decrease in efficiency. As
well affecting the hydrodynamics of the blades,
fouling can induce microbial influenced corrosion
(MIC). MIC is defined as the acceleration of
corrosion due to the presence of biofilms (Beech
2000; Beech 2004; Mijle-Meijer 2009). MIC can
result costly unexpected maintenance, potentially
increasing the cost of electricity produced. Turnock
(2009) states, ‘The key to the success of tidal
turbines is their ability to operate in the ocean for
extended periods with minimal intervention (15-20
years)’. It is clear that for such lifespans, an effective
antifouling solution must be chosen.

2 Reducing the biofouling of tidal turbines using surface microstructures
2. Biofouling
In the marine environment, ‘Biofouling’ is the
colonisation of submerged surfaces by unwanted
organisms such as bacteria, barnacles and algae.
(AMBIO 2010). In our context, biofouling is the
attachment and growth of undesirable molecules
and organisms to submerged surfaces. (Rittschof
2009). Biofouling has 4 main stages (see figure.1),
which occur in succession to one another. The first
layer, the conditioning film (often referred to as
‘biofilm’), is comprised of media on a microbial scale;
proteins, polysaccharides, organic and inorganic
substances. (Bruijs 2006). Organisms on this scale
either ‘choose’ where to settle (motile), or change
shape to ‘fit’ the micro topography of the substrate.
In the marine environment, practically all surfaces
are eventually colonized with a bio-film (Turnock
2009). Once the conditioning film has colonised the
substrate layer, it attracts bacteria, micro-algae and
fungi, which form the 2nd and 3rd layers, and along
with the conditioning film, form the micro-fouling
community. Finally, invertebrates, seaweed and
other animals are attracted, and form the
Macrofouling community. It is important to note, that
all layers of fouling need to attach to the substrate
itself in order to feed, grow and reproduce, rather
than settling on top of the conditioning film (Field
1981) (Bruijs 2006). Furthermore, Walker et al.
(2014, p.266) state that, “The tenacity of the biofilm
is affected by a range of variables including, but not
limited to, composition, substrate, and a host of
environmental factors. It follows that a solution must
pertain to the substrate itself.
2.1 State of the art antifouling measures
Presently the majority of antifouling solutions falls
under two categories; chemically active antifouling
(biocide containing) coatings, and non-toxic
antifouling (biocide free). (AkzoNobel 2014a). Both
systems are paints, usually applied in two layers.
The primer applied directly to substrate normally
contains anti-corrosive functions. The topcoat, the
layer with which marine life will interface with,
changes depending on the system. (Chambers
2006).
2.11 Chemically active antifouling
Chemically active coatings work by secreting
biocides. Biocides are chemical substances or
mixtures used for the elimination of living
organisms through chemical or biological action
(European environmental bureau 2014). The main
driver dictating development in biocidal antifouling
technology is legislation (AkzoNobel 2010). There
are two key techniques for controlling the release of
antifouling compounds from a coating by using
either a soluble or insoluble matrix (Chambers
2006) (see figure. 3). Chemically active antifouling
technologies can be subdivided into 2 more
categories; Contact leaching, and soluble matrix
(Bressy 2009). Antifouling systems are distinctive
from each other in both their microstructure and
rosin content. In coatings where the rosin content is
low, the coatings are “contact leaching” coatings
(AkzoNobel 2014b; Dennington 2015) Contact
leaching coatings are mechanically tough, with a
maximum service life of 18-24 months, (AkzoNobel
2014b) (Pei and Ye 2015). The service life is
proportional to the amount of biocide incorporated,
the coating thickness remains the same while the
biocide depletes (Bressy 2009). Contact leaching
coatings consist of an insoluble matrix structure,
‘filled’ with a soluble medium, rosin, and biocides.
The rosin and biocide diffuse upon contact with
seawater, leaving an empty skeleton of insoluble
material, which is what the coating owes its
mechanical strength to. The leaching of biocides is
non-liner: the biocide will leach rapidly when freshly
applied, and become less effective with time
(Berendsen 1989).
Figure 2. Schematic view of the four main stages of marine
biofouling. (Bruijs 2006)
Figure 3. Schematic of (a) soluble matrix biocide releasing
coating and (b) insoluble biocide releasing coating.●
Antifoulant loaded,○ depleted antifoulant. (Chambers 2006)

3 Adam Brann
Coatings in which the rosin level is high are
“soluble matrix” coatings. There are two main types
of soluble matrix coatings; Controlled depletion
polymer, and self-polishing copolymers. Controlled
depletion coatings allow seawater to penetrate the
paint, releasing the biocides by diffusion
(Goodes.L.R, 2014). They are designed to release
biocides at a constant rate (Kiis.S, 2009), thus
providing stable and relatively predictable
antifouling.
Self-polishing copolymers (SPC’s) also
have a soluble microstructure. The soluble matrix
allows seawater to penetrate the matrix, and
through hydrolysis, strip the biocide from the
polymer, which is chemically linked to the biocide
(see figure.3) (Chambers 2006) (Dennington 2015).
As the surface depletes, the resin ‘smooth’s over’
microscopic fissures caused by the erosion of the
coating, leaving a smoother underlying surface
(Omae 2003; Kiis 2009), hence the name ‘self-
polishing’. The reduced skin friction is obviously a
benefit to the hydrodynamic properties of the
surface. SPC’s have the best long term
performance of chemically active anti-fouling’s
(Bressy 2009), and have a service life of up to 60
months (AkzoNobel 2010).
In 2008, the most popular and effective SPC,
Tributyltin-Tin (TBT) was banned from the shipping
industry for its adverse effects on the marine
ecosystem. (Doyle 2014). SPC now contains less
virulent biocides. The prohibition of TBT spawned
new interest in the development of foul release
coatings (Altar 2003).
2.12 Non-Toxic Antifouling
Foul release coatings are biocide free,
hydrophobic, ‘non-stick surfaces. A combination of
low surface tension and low wettability makes it
difficult for organisms to adhere to the surface of
the coating (Atlar 2003; Baier 2006; Carman 2006;
Magin et al. 2010). Biofouling that manages to
adhere does so very weakly, and is removed by the
shear forces incurred by the movement of water
past the surface (Candries 2001). Current coatings
require 10-15 knots of flow perpendicular to the
surface to remove fouling (Solomon 2014;
AkzoNobel 2015a). Experiments by Baier (2006)
found that
biofouling organisms exhibited the lowest relative
adhesion on materials with surface tensions
approximately 22 mN/m (see figure.4). Current foul
release polymers are normally
Polydimethylsiloxane (PDMS) based (Magin et al.
2010; Lindholt 2015; Thorlaksen et al. 2015) due to
its surface tension - 20-23 mN/m (Potter 1996;
Grunde et al. 2008). Thickness also plays an
important role in foul release coatings – it has been
shown that barnacles can cut through coatings
below ~100µm, and adhere securely to the
underlying primer (Candries 2001; Altar 2003).
Above this thickness there is no increase in fouling
release properties (Altar 2003). Experiments by
Candries.M (2001) showed that foul release
coatings incurred less drag than SPC coatings,
when first applied. As there is no chemical change,
polishing function or any film degradation, the
service life of the coating is governed by the
amount of wear the coating is subjected to
(AkzoNobel 2015b).
3. Surface Microstructure
The microstructure of a material is simply described
as the appearance of a material on a nano-micro
meter scale. (ASM 1985; Cavendish laboratory
2013). Examples of antifouling microstructures can
be found in nature; sharkskin consists of a matrix of
‘denticles’ (figure. 5) that reduce its drag whilst
swimming, by
disrupting the flow of water (Wen.L et al. 2014). The
lotus leaf has a very rough microstructure, which is
Figure 4. Self-Polishing copolymers (Dennington 2015)
Figure. 6. Examples of surface microstructures in nature. Left:
CGI of the lotus leafs' microstructure. Right: Shark skin
denticles (Thieckle 2015; Sewell 2015)
Figure 5. The Baier Curve (Baier 2006)

responsible for its super-hydrophobic, self-cleaning
surface (Figure. 5). The periostraca on blue mussels
and crabs are effective antifouling surfaces (Magin
et al. 2010). Epoxy resin replicates of the periostraca
reduced fouling for 3-4 weeks (Bers 2004). Magin et
al. (2010) suggests that the short term performance
implies that natural antifouling is a combination of
chemistry and micro topography. Using modern
techniques, it is possible to construct ‘man-made’
engineered surface microstructures. Engineered
microstructures take on two forms; Micro-
topographical, and self-assembled monolayers.
3.11 Topographical Microstructures
It is possible to impose microscopic patterning on a
surface using a variety of techniques such as laser
etching, photolithography, and nano-imprinting
(Lee.W et al. 2004; Groenendijk 2008; Magin et al.
2010). The topography on a micro-scale affects
mechanical friction, optical properties, and
bioadhesion (Lee.W et al. 2004) as well as the
wettability of a surface (Groenendijk 2008; Lin.F et
al. 2011; Lamberti 2012).
The pattern with which the substrate is imprinted has
an effect on the properties of the material. In an
experiment by Carman et al. (2006), it was found that
different micro patterns (see figure 7.) on treated
PDMS (PDMSe) exhibited different contact angles
with water droplets. Pits and ridges increased the
contact angle by approximately 7°. Channels, and a
bio-inspired antifouling surface, Sharklet AFTM ,
increased the contact angle by 25° and 27°
respectively, compared to smooth PDMSe. Carman
(2006) suggests that wettability and settlement of
biofilm agents (spores, bacteria) can be modelled
using the same principles; surfaces that encourage
the Cassie-Baxter wetting state (Cassie and Baxter
1944) will encourage cells to ‘bridge’ the peaks of the
microstructures. Similarly, a surface that encourages
the Wenzel wetting state, will encourage cells to
pack into the ‘valleys’ of microstructure (Wenzel
1936; Carman 2006). The state of bridging offers the
alien cells less surface area to adhere to, and
reduces the strength of the adhesion. (Carman et al.
2006; Athavale 2010).
3.12 Current applications
The SharkletTM micropattern has applications in the
health sector, for reducing bacterial infection on
invasive equipment, such as catheters and
endotracheal tubes (Sharklet 2015). The pattern can
be applied to ‘high touch surfaces’ in hospitals such
as doors, in the form of an adhesively backed film.
An anti-bacterial surface that works through
topography alone pre-empts the immunisation of
superbugs such as MRSA to existing chemical
solutions. Carman et al. (2006) found that a
SharkletTM microstructure was able to reduce the
settlement of zoospores by up to 85% compared to
smooth control surfaces. In another study, Mann
(2014) found that the SharkletTM pattern reduced
MRSA attachment by 98%. In the aerospace sector,
Lufthansa Technik is currently experimenting with
imprinting riblet microstructures into paints on
aircraft to reduce turbulence perpendicular to the
direction of airflow (Gubisch 2013; Lufthansa
Technik, 2015a). Using, ‘Simultaneous stamp
hardening’. Simultaneous stamp hardening works in
a similar way to nano imprinting (citation), but
instead of a stainless steel mould, a transparent
silicon negative of the microstructure is applied
directly to the freshly painted lacquer (Lufthansa
Technik 2015b). The coating is then immediately
cured with UV light (Gubisch 2013; Lufthansa
Figure 7. Examples of different topographical patterns A:
Pillars, B:Pits, C:Channels, D:Ridges, E:Sharklet AFTM
(Carman 2006)
Figure 8. a) Wenzel state b) Cassie-Baxter state (Bridging)
(Rodriguez 2012)

5 Adam Brann
Technik 2015b).A robotic arm is being developed to
automatically apply the coating, as applying the
coating manually to large surface areas is not
deemed cost effective. The coating is expected to
increase fuel efficiency by 1%, and will finish in 2017
(Lufthansa Technik 2015a).
3.2 Self-assembling monolayers
Self-assembling monolayers (SAMs) are ‘grown’ on
substrates (Mahajan 2010). They are organic
assemblies formed by the absorption of molecules
from a liquid or gas (Schreiber 2000; Love et al.
2005; Mahajan 2010). The absorbed molecules ‘self-
assemble’ to form a regular crystalline layer around
1-3µm thick on a metallic substrate (Love et al.
2005). SAMs can be configured to be hydrophobic
or hydrophilic. Unlike topographical microstructures,
the production of SAMs does not require specialist
equipment, and are easy to prepare (Schreiber
2000; Love et al. 2005). A SAM consists of 2 main
parts; a metallic layer, usually gold for its chemical
inertness (Love et al. 2005; Nugraha et
al. 2015), but other metals such as silver, copper and
palladium have been extensively studied (Laibinis et
al. 1991; Love et al. 2003). The metallic layer is often
applied as a film on a glass (Taglietti 2014) or silicon
substrate (Love et al. 2005; Zhang 2012). The
second layer is comprised of a chain of alkanes. The
composition of the alkane chain depends on the
solution that the substrate is exposed to. The chains
are fragile due to their minute scale.
SAMs have been shown to repel the settlement of
proteins (Chapman et al. 2000; Ederth et al. 2011),
as well as displaying excellent antifouling against
cyprids (Barnacle larvae), polysaccharides and algal
spores (Ederth 2011). Hydrophilic SAMs are
expected to be effective against foulers that secrete
hydrophobic adhesive, and hydrophilic SAMs are
effective against foulers with hydrophobic adhesive
(Li et al. 2015).
DISCUSSION
Topographical microstructures: A possible
solution?
Microtopographies have been shown to reduce the
settlement of bacteria on their surfaces. The
SharkletTM micro-pattern has demonstrated high
resistance in test conditions, reducing settlement up
to 98% over a set amount of time. In the situation of
a tidal turbine, reducing the settlement alone is not
good enough. Over a longer period of time, fouling
will eventually form, albeit slower on a surface with
resistance. The surface needs to have a way of
removing fouling from its surface. Coatings must
have a method of removing settled growth. Biocidal
paints remove growth by killing it, foul release
coatings rely on the flow of water to self-clean.
With respect to self-cleaning, topographical
microstructures would be expected to function in a
similar way to a foul release coating. If organisms do
manage to adhere, the coatings rely on the shear
force of the water to remove the fouling. It follows
that the hydrodynamic environment in which these
coatings will be applied is of great importance to their
efficacy.
The speed of the water past the anchor surface of
the turbine is dependent on the tidal stream speed.
The relative speed over the rotating blades surface
is the resultant of the angular speed of rotation and
the tidal stream speed, which changes along the
length of the blade (see figure 11). The current in
which most existing turbines operate is 2-3ms-1
(PMSS 2006; Power Technology 2015). On a static
structure, such as the anchoring pillar of a bottom
mounted turbine, or the platform of a surface
mounted tethered turbine, the speed of flow would
be too low for existing foul release paints, which
require at least 5ms-1. In the absence of
experimental proof, it’s impossible to categorically
state that a topographical microstructure could
achieve foul release at speeds of 2-3ms-1. It may be
speculated that a microstructure that encourages a
‘bridging’ state provides less surface area to adhere
but retains the critical surface tension of
approximately 22Mn/M; would encourage adhesive
bonds of weaker strength than on foul release
coatings. In reality, while this may be true in a
controlled laboratory environment, it is unlikely to
work for a marine environment. Current tests haven’t
taken into account the presence of other organisms,
sediment, or inorganic particles. It is possible that
particles of sediment or microscopic organisms
smaller than the channel width of the micro pattern
could settle in the channels, producing a surface that
Figure 10. Schematic diagram of an ideal crystalline SAM (Love
et al. 2005)
Figure 9. Riblet microstructure impressed in lacquer using
'simultaneous stamp hardening' (Lufthansa Technik 2014)

is likely to encourage a Wenzel-style state of fouling,
rather than the preferred Cassie-Baxter state. Media
that settles in the channels will become sheltered
from the flow to a degree, if the pattern isn’t aligned
perfectly with the current and as a consequence,
exacerbate the fouling situation.
Microstructures have been shown to enhance flow
properties by reducing cross axial drag, as shown in
nature, by the microstructure of sharkskin, and in
aerospace by Lufthansa Techniks’ riblet
microstructure. These microstructures are suited to
high speed turbulent flow conditions, in laminar flow
hydraulically smooth surfaces produce less drag.
Tidal stream currents are normally turbulent (Lynn
2014), but relatively low speed. Should the
microstructure reduce drag in this condition, the
turbine would be able to achieve higher efficiency.
Regardless of whether microtopographies can
achieve this, their viability still hinges on their ability
to effectively deter fouling and self-clean; fouling
would eventually incur more drag than the
microtopography reduces.
Application of microtopographies
Work by Bers (2004) demonstrated the fact that
fouling is not deterred by the micro pattern alone. It
follows that the substrate into which they are printed
must also have antifouling properties. Previous
studies suggest PDMS is the most effective
substrate to use.
The application of surface microstructures to
horizontal axis tidal turbines (HATTs) is a complex
problem. I suggest that topographical
microstructures could be applied in to horizontal axis
tidal turbines in two ways, either as a cladding,
where the topography is prepared on PDMS ‘tiles’,
which are then adhered to the surfaces of
components, or are imprinted into a foul release
style paint, using a process similar to simultaneous
stamp hardening. The cladding method would
comply with traditional methods of creating micro
topographies, imprinting or photolithography, and
damaged sections could be easily stripped from
HATTs surface and replaced with a new tile.
However, this method has a number of flaws. The
PDMS substrate that they are normally imprinted
onto is inherently ‘non-stick’, thus making it difficult
to stick to surfaces with conventional adhesives. The
selection of adhesive is further complicated by the
submarine environment it has to endure. There
would inevitably be discontinuities in the pattern at
the interface between tiles, affecting the AF
properties of the pattern. Finally, adhering the tiles to
a surface with a relatively tight radius, such as the
leading edge of a turbine blade, would warp the
pattern, elongating the ‘valleys’ of the
microstructure, allowing spores and microbes to
settle within the valleys rather than bridging them.
The simultaneous stamp hardening method offers
more promise, it could be assumed less discontinuity
in the pattern would occur. A bespoke paint would
need to be formulated with similar properties to foul
release coatings, but also able to cure rapidly in UV
light. At present the process is carried out by hand.
Application of engineered microstructures to HATTs,
or any large scale assemblies will be extremely
labour intensive in the absence of automation. The
cost-effectiveness of both solutions will be vastly
increased in the presence of an automatic
application process. A robotic solution such as the
arm being developed by Lufthansa Technik would
dramatically increase the financial viability of
applying microtopographies to large scale
structures.
Applying microtopographies to existing turbines
would be extremely difficult, not least because of the
awkward location HATTs are in. Retrofitting
permanently submerged areas of the turbine would
be impossible. Turbines which allow the drivetrain to
be raised above water, such as the Seagen S (see
figure 1) would allow retrofit in-situ. Alternatively, the
turbines could be dismantled, taken ashore and
retrofitted. The cost in labour and machine downtime
for this option would be exorbitant.
SAMs
The ability of SAMs to repel proteins and
polysaccharides is desirable, as the formation of
biofilms relies on such constituents. As biofouling is
a successive process, this would affect macro
foulers and invertebrates, and the ability to repel
cyprids has already been proven. The effectiveness
against all forms of biofouling is yet to proven – in a
marine environment SAMs would be exposed to a
diverse array of foulers. An antifouling solution must
cater for this; a SAM that exhibits excellent
antifouling properties to settlers with hydrophobic
adhesive is useless if it is colonized by foulers with
hydrophilic adhesive. For this reason, the use of
SAMs can be dismissed as a general solution for the
whole HATT assembly. I would suggest that SAMs
have the potential to be excellent anti-slime
coatings; slime is a thick biofilm, comprised of layers
of microbial scale foulers, which SAMs have been
shown to resist.
CONCLUSIONS
 Microtopographies are effective at
providing antifouling for specific motile
spores.
 Microtopographies may not be suited to
cope with the diverse array of fouling the
marine environment provides.
 For the coating of large scale structures
with microtopographies to become
financially viable, an automated process
must be adopted.
 Self-assembled monolayers lack the
versatility to perform well in the marine
environment.

7 Adam Brann
FURTHER WORK
Investigation into the shear forces required to
remove fouling from topographical microstructures.
 The effect of mechanical damage on the
antifouling efficacy of microtopographies.
 Methods for automatically applying
microstructures to large scale
components
 Investigation into the adhesion strength
of fouling organisms on
microtopographies, and ascertain a
critical foul release speed
 Reducing cross axial drag at low flow
speeds using topographical
microstructures
ACKNOWLEDGEMENTS
I’d like to thank Ben Thomas for providing me with
an initial idea for this paper, and support throughout.
I’d also like to thank Adam Roberts for helping to
provide me with an initial direction.
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