Propulsion contribution of a wing sail on popular shipping routes

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Andrei Traian Tudor

Supervised by Dr. Paul Gilbert and Dr. Michael Traut

MEng Mechanical Engineering

School of Mechanical, Aerospace and Civil Engineering

Abstract

The shipping industry faces major challenges as a result of climate change concerns and

CO2 emissions targets imposed on this sector. Given the observed discrepancy between

CO2 emissions targets and projected forecasts by 2050, this dissertation aims to assess

the propulsive power contribution potential of wing sail technology as a means to

address the expected increase CO2 emissions in this sector. A numerical model of the

wing sail is devised for and applied on five shipping routes, using wind data from Met

Offices Unified Model. Results show that the small wing sail achieves an average

propulsive power between 170 and 704 kW and the large wing sail achieves an average

propulsive power between 453 and 1852 kW across the five routes considered.

Assumptions and major sources of uncertainty are thoroughly discussed throughout

this study.

Table of contents

Introduction......................................................................................................................... 1

1.1 Climate change and Shipping Sector ............................................................................. 1

1.2 Wind-assisted propulsion .............................................................................................. 2

1.3 Aims and objectives ...................................................................................................... 3

1.4 Structure ........................................................................................................................ 4

2.Methodology and Theory ................................................................................................. 6

2.1 Methodological approach to literature review ............................................................. 6

2.2 Numerical Method ........................................................................................................ 7

2.2.1 Forces acting on a rigid wing ...................................................................................... 7

2.2.2 Boundary layer profile of wind across sea ................................................................. 9

2.2.3 Propulsion Contribution ........................................................................................... 12

2.2.4 Wing sail dimension and aerodynamic parameters ................................................ 16

2.3 Control system ............................................................................................................ 22

2.3.1 Preventing excessive oscillation ............................................................................... 22

2.3.2 Switching between lift mode and full drag mode .................................................... 23

2.4 Assessment of propulsive power ................................................................................ 25

3. Literature Review .......................................................................................................... 27

3.1 Climate Change ........................................................................................................... 27

3.1.1 Global warming caused by CO2 emissions................................................................ 27

3.1.2 Natural climate variations ........................................................................................ 28

3.1.3 Unequal distribution of climate change effects ....................................................... 30

3.1.4 Present and future effects of climate change .......................................................... 30

3.1.5 Intergovernmental protocols regarding climate change ......................................... 33

3.2 Shipping Industry ......................................................................................................... 35

3.2.1 Overview of the shipping industry and growth trends ............................................ 35

3.2.2 C02 emissions ........................................................................................................... 37

3.2.3 Methods to reduce CO2 emissions in the shipping industry .................................... 41

3.3 Wind assisted propulsion ............................................................................................ 45

3.3.1 Historical use of wind propulsion ............................................................................. 45

3.3.2 Types of wind assisted propulsion ........................................................................... 46

3.3.3 Previous research on the topic of wind assisted propulsion ................................... 47

3.3.4 Commercial implementation and existing prototypes ............................................ 50

3.3.4.1 Flettner rotor ......................................................................................................... 50

3.3.4.2 Towing kite ............................................................................................................ 51

3.3.4.3 Wing sail ................................................................................................................ 53

4. Results ........................................................................................................................... 54

4.1 Sensitivity analysis- Tornado Chart ............................................................................. 54

4.2 Polar Plots .................................................................................................................... 57

4.3 Results on five shipping routes ................................................................................... 63

5. Discussion ...................................................................................................................... 68

5.1 Major findings ............................................................................................................. 68

5.2 Comparison to Flettner rotor and towing kite ............................................................ 69

5.3 Limitations of this study .............................................................................................. 72

5.4 Recommendations for future work ............................................................................. 74

6. Conclusion ..................................................................................................................... 76

7. Project management ..................................................................................................... 78

Appendix 1 ......................................................................................................................... 79

References ......................................................................................................................... 82

1

1. Introduction

This research is placed in the context of increased climate change concerns arising from

the extensive use of fossil fuels and dynamic growth of global shipping volumes. Section

1.1 will briefly discuss these previously mentioned developments and their likely effect

on the shipping industry, section 1.2 will review previous results in wind assisted

propulsion and justify the choice of wing sails and Section 1.3 introduces the main

research aim and additional objectives. Section 1.4 covers the structure of this study.

1.1 Climate change and Shipping Sector

From 1880 to 2012 earth and ocean surface temperature increased by 0.85 C on a

global average, (IPCC Synthesis Report, 2014). There is strong consensus in the scientific

community that this temperature increase is a result of human activity, specifically the

emission of greenhouse gases (most notably CO2) as a result of burning fossil fuels.

Increasing concerns regarding the effects of climatic changes resulted in a legally

binding intergovernmental protocol known as Kyoto, with the purpose of reducing CO2

emissions of signatory countries with 5% by 2012 compared to 1990 base level

(Ghezloun et al, 2013). These protocols represent significant steps forward in

addressing climate change; however it is worth noting that industrial sectors of

international nature do not fall under the standards of Kyoto protocol, as their

emissions cannot be attributed to a single country (UNFCCC, 2014). Prominent

examples include the aviation sector, which accounts for approximately 2% of global

2

CO2 emissions, and the shipping sector which accounts for 2-3% of global emissions.

While these sectors are in many ways similar, as both are experiencing steady growth,

only the latter succeeded in implementing a legally-binding agreement known as

MARPOL Annex VI, 2011, which aims to reduce CO2 emissions with 50% by 2050.

However, this ambitious target requires substantial investments to improve energy

efficiency. There are numerous methods to improve energy efficiency of a vessel,

ranging from the use of biofuels and natural gas to voyage optimization, speed

reduction and engine upgrade. Various degrees of implementation of these measures

are considered in a forecast of CO2 emissions by IMO, with results showing that no

scenario meets the targets required by MARPOL Annex VI, 2011 (Third IMO GHG Study,

2014).

1.2 Wind-assisted propulsion

One alternative way of reducing CO2 emissions is exploiting wind energy for the

purpose of propulsion. There are three types of wind assisted propulsion technologies:

Flettner rotor, towing kite and wing sails (Rojon and Dieperink, 2013). The propulsive

power contribution of the first two is estimated by some studies with the use of

numerical models integrated with wind velocity data. Schlaak et al (2009) estimate the

propulsive power contribution of a 600 m2 towing kite to be of 256 kW, a result which

is obtained with the use of sea trials measurements incorporated in the numerical

model. Traut et al (2013) estimate the propulsive power contribution of the Flettner

rotor to be between 193 kW and 373 kW, acknowledging the fact that power

3

contribution can be increased by using multiple rotors. Such results suggest that these

two technologies are potential candidates for addressing the discrepancy between

international shipping CO2 targets and existing forecasts, especially given that they

achieved, on a small scale, commercial implementation.

Limited research exists, however, on the topic of wing sail propulsive power, despite

promising small-scale results such as Rynne and Ellenrieder (2009) experimentally test a

4.2m wing sail powered keel-boat and report superior aerodynamic performance over

the traditional sail and capability of harnessing wind regardless of its apparent

direction.

The purpose of this dissertation is not to consider practicalities of design but rather to

provide an estimation of propulsion power that is provided by wing sails. Limitations of

this numerical model arise from simplification of aerodynamic equations and boundary

layer profile, as well as from the assumption of ideal airfoil behavior. These limitations

are further discussed in the Methodology section (chapter 2).

1.3 Aims and objectives

The main research aim of this dissertation is to devise a numerical model that

calculates the power contribution of a wing sail towards the propulsion of a ship using

historical wind velocity data on popular trading routes. Additional research objectives

are:

4

To investigate the climate change landscape and the latest developments with a

focus on identifying the main factors which impact the shipping industry

To identify the effects of this factors in the shipping industry, and additionally

explore industry-specific trends which likely to impact compliance with climate

change regulations in the long run

To conduct an analysis of energy efficiency improving measures in the shipping

industry with a focus on assisted wind propulsion

To review previous research on the use of wing sails for propulsive power

contribution and draw on previous findings for devising of numerical

model/optimization of control system

To devise a control system that is in charge of the angular displacement of the

wing sail to provide reliability of operation and maximization of driving force

To compare the wing sail propulsion contribution with the towing kite and

Flettner rotor on popular shipping routes

1.4 Structure

Chapter 2 covers the methodology and theory of this study, with Section 2.1

referring to the chosen approach in review of literature and Section 2.2 referring to

the method employed in devising the numerical model. Chapter 3 cover literature

review and is divided into three sections: Section 3.1 focuses on the broader topic

of climate change, Section 3.2 reviews the effects of climate change in the shipping

sector and Section 3.3 focuses on wind assisted propulsion as a measure to reduce

5

CO2 emissions. Chapter 4 presents a sensitivity analysis of parameters influencing

propulsive power of wing sails and results on five popular shipping routes. Chapter

5 covers the discussion of results in Chapter 4, while Chapter 5 deals with the

conclusions of this study. Chapter 6 covers project management. Appendix 1

presents the code employed to quantify propulsive power contribution.

6

2. Methodology and Theory

2.1 Methodological approach to literature review

The preliminary approach that is employed to answer the main research questions

involves reviewing relevant literature on the three following topics: Climate change

(Section 3.1), Shipping Industry (Section 3.2) and Wind assisted propulsion (Section

3.3). The main purpose is to identify the major developments in the broader context

(Climate Change).

A review of literature on the area of interest (Shipping Industry) is conducted to identify

the effects of these previously mentioned developments in relation to sector specific

trends.

The main research area (Wind Assisted Propulsion) is reviewed critically with a focus on

previous research on the abatement potential and commercial implementation of

specific technologies which may be able to address the discrepancy between

CO2emissions targets and forecasts. A knowledge gap is identified in the area of wing

sail assisted propulsion, as limited research was conducted on the subject of mitigation

potential. This subject is addressed with the use of a numerical model which is

integrated with wind velocity data to provide an estimate of wing sail technology

propulsive power contribution. The main features and assumptions of the numerical

model are presented in section 2.2.

7

2.2+Numerical Method

This section focuses on the theory and method used for the numerical model

development. Section 2.2.1 presents forces acting on a rigid wing and quantifies

apparent wind angle and magnitude as a function of measurable parameters. Section

2.2.2 covers the boundary layer profile of wind across sea. Section 2.2.3 goes on to

integrate previous findings from Section 2.2.1 and Section 2.2.2 in order to derive a

formula for propulsive power contribution under two different assumptions (apparent

wind boundary layer across sea is/is not identical to true wind boundary layer) and in

two different modes of operation (lift-inducing mode and full drag mode). Section 2.2.4

discusses the choice of wing sail section and presents the optimum values for lift and

drag coefficient (CL and CD) and the corresponding angle of attack . Finally, Section

2.2.5 proposes a control system for effective and reliable operation of the wing sail and

Section 2.2.6 discusses the approach taken for the assessment of propulsive power on

popular shipping routes.

2.2.1 Forces acting on a rigid wing

A pressure gradient is formed across an airfoil-like shape which is placed at an angle in

a fluid flow. This pressure gradient generates two forces that act on the airfoil, a lift

force perpendicular to the relative flow velocity (or apparent wind) and a drag force

parallel to the relative flow velocity. These forces are expressed as (J.D. Anderson,

1984):

8

=

=

Where FL and FD are the lift and drag force respectively, U is the average apparent wind

velocity across planform area, A is the planform area and CL and CD are the lift and drag

coefficients respectively.

Figure 1-Forces acting on a rigid wing (source: Rynne andEllenrieder, 2010)

To calculate the magnitude and direction of lift and drag forces, the apparent wind

magnitude and direction should be known. These two parameters have to be expressed

based on measurable parameters such as true wind speed, ship speed and true wind

angle.

Apparent wind speed can be calculated from true wind speed Vt, ship speed Vs and true

wind angle using the law of cosines as:

9

= + +

Apparent wind angle can be calculated as:

= ( +

) = (

+

)

To express apparent wind angle as a function of Vt, Vs and only, the following

equation is used:

=

(

+

+ + )

2.2.2 Boundary layer profile of wind across sea

One important aspect when considering lift and drag force on an airfoil is the velocity

profile of air. Laminar flow develops a boundary layer profile: flow speed equals 0 at

sea level and 99% of freestream velocity at a distance from sea level defined as

boundary layer thickness (Massey and Smith, 2005). An illustration of this phenomenon

can be seen in figure 2 below.

10

Figure 2-Wind velocity gradient (source: rcsailing.net)

Velocity profile of air above sea level depends greatly on calmness of the sea, as this

influences roughness and has an impact on velocity profile. The equation below is

employed for vertical velocity profile, as it describes relatively accurately the velocity

profile in steady sea from 0.006 meters up to 200 meters. The height segment of

interest for the study is the 10 to 120 meters portion.

= (

)

Where u is wind speed at height z in meters and U10 is the wind speed at 10 meters

(Faltinsen, 2006).

Integration of the equation above results in the following empirical relationship:

= ()

= . (/

/)

11

(where H is wing sail height and Hmax is the z-coordinate of the sail tip, Hmin is the z-

coordinate of the sail base)

Volumetric flow rate can equally be expressed as:

=

(where Uavg is the average across the height segment H of interest ranging from Hmin to

Hmax)

Therefore it can be concluded that:

= . (/

/)/

which can be easily integrated with both the historical wind velocity data and the

driving force equations.

For the purpose of preliminary analysis and sensitivity study, it is assumed that the

boundary layer profile of the apparent wind speed across the sea is identical to the

boundary layer profile of the true wind speed. This assumption is made for simplicity.

However, it is worth noting that in reality the apparent wind boundary layer profile is

smoother than the true wind boundary layer profile and presents less relative variation

with height due to being partly dependent on ship speed Vs which is constant across

the sail wingspan. This assumption is not made when calculating the mitigation

potential using historical wind velocity data, for which purpose the actual propulsive

power contribution equation is presented later in this section.

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2.2.3 Propulsion Contribution

The wing sail operates in two different modes for efficient propulsion: a lift inducing

mode and a drag inducing mode. Propulsion contribution in each of the two modes is

analyzed below:

1. Lifting mode

The equations for lift and drag force can be written in the following form to account for

the variable velocity profile above the sea:

=

()

=

()

Recalling the equation from section 2.2.2 for average apparent wind speed across a

height segment, the equations above can be written as:

= .

[(

/

/)]

= .

[(

/

/)]

Vector summation of the lift and drag force gives:

| | = =

+

13

And the angle of this force (Fnet) relative to the lift force (FL) is given by:

= [

]

Net force can be further expressed as:

= .

[(

/

/)]

+

Referring to figure 2, the net force has two components with respect to ship direction

of motion, driving force (FAR) acting parallel to the ship course and a side force (FAS)

acting perpendicular to it. These forces are dependent on both the apparent wind angle

relative to the ship course and the optimum angle of attack ( is a function of ).

= ( )

= ( )

These equations can be further expressed as:

= .

[(

/

/)]

+ ( )

= .

[(

/

/)]

+ ( )

At this point, side force is neglected as resistance to sideways motion is significantly

higher than resistance to forward motion.

14

Propulsion contribution is quantified as:

=

Where Vship is the ship speed in course direction and Pprop is the propulsion contribution

of the wing sail.

Propulsion contribution can be further expressed as (for the purpose of sensitivity

analysis):

= .

[ (

)]

+

(

[])

For the purpose of quantifying mitigation potential, a different equation for propulsive

power contribution (which provides a more realistic apparent wind boundary layer

profile across the sea) is used.

Starting from the formula for average true wind speed across the planform area:

= = . (/

/)/

And using the formula for apparent wind angle:

=

(

+

+ + )

15

Propulsive power can be written as:

=

Which can be equally expressed as:

=

(

+ + )

+ (

[])

2. Full drag mode

From section 2.2.2 (Forces acting on a rigid wing), the driving force that generates

propulsion was expressed as:

= ( )

Where is the apparent wind angle and is a function of airfoil properties and is

expressed as:

= []

It can be observed from the above equation for driving force that for a range of

angles the driving force can become very small or actually oppose the ship propulsion.

Rynne and Ellenrieder (2010) found, using the same airfoil type that for an average

angle of attack of 10 and a angle greater than 135 it is more advantageous to

reposition the wing sail perpendicular to the wind such that it acts like a parachute to

take advantage of the drag force.

Recalling the equation for drag force from section 2.2.2:

16

=

When the wing sail is in full drag mode, the actual driving force that contributes

towards propulsion is:

=

( )

In the full drag mode, the drag coefficient also changes to its maximum. The maximum

drag coefficient value can be interpolated from the following equation (Timmer, 2010):

= . . /

This equation yields a maximum drag coefficient of roughly 1.5.

In full drag mode, propulsion contribution equals:

=

( )

With U and as determined in section 2.2.3.

2.2.4+Wing sail dimensions and aerodynamic parameters

Two wing sail sizes are inspected for preliminary results, with wing span of H=70m and

H=110m (not including mast height below the wing) and chord length c=20m and

c=12.7m respectively. This selection of dimensions results in an aspect ratio of 5.5,

relatively similar to values encountered in other research on the topic of assisted wind

17

propulsion. Traut et al (2014) employs an aspect ratio of 7 for the Flettner rotor, Rynne

and Ellenrieder (2010) employ an aspect ratio of 5 for a wing sail). Throughout chapters

4 and 5, the wing sail with span of H=70m and c=12.7m is referred to as the small sail

and the wing sail with span of H=110m and c=20m is referred to as large sail.

Therefore, planform area A equals 2200 m2 and 889 m2 for the two wing sail sizes

considered, similar to the values employed by G. Klaus et al, 2007 in a study of cloth

sails propulsive power contribution.

Figure 3-NACA 63(2)-015 type airfoil (source: airfoiltools.com)

The NACA 63(2)-015 type airfoil shape (an illustration of its profile can be seen in figure

3) is selected for subsequent calculations, as extensive data regarding NACA profiles

aerodynamic performance is currently available. Additionally, it is in line with other

similar studies and has been found to provide satisfactory lift to drag ratio compared to

other airfoil shapes.

A rectangular planform area, rather than triangular has been selected for two reasons:

the rectangular planform airfoil produces less induced drag due to less spillage

near the tip

18

ease of calculations

However, it should be mentioned that the triangular planform airfoil has its center of

pressure located at a lower location compared to the rectangular airfoil. Considering

the wind velocity profile discussed in section x, wind speed will be smaller at this lower

location and thus the triangular airfoil will minimize heeling moment (compared to the

rectangular airfoil) (Rynne and Ellenrieder, 2010). However, given that this research

mainly focuses on ships of 400t and more, this heeling moment is neglected in

calculations.

A rotating mast to support and raise the entire wing (up to a height where air flow does

not interfere with ship configuration) is proposed because it yields three simultaneous

advantages:

1. Increase in the available deck space which is otherwise occupied by the rotating

wing

2. Increase in total wetted surface area as otherwise flow interferes with the ship

configuration and affects wing local efficiency in the lower region

3. Increase in propulsive power as wind speeds increase with height as a result of

the atmospheric boundary layer

Two wing sail sizes are inspected for preliminary results, with wing span of H=70m and

H=110m (not including mast height below the wing) and chord length c=20m and

c=12.7m respectively.

19

This selection of dimensions results in an aspect ratio of 5.5, relatively similar to values

encountered in other research on the topic of assisted wind propulsion (Traut et al,

2014 employs an aspect ratio of 7 for the Flettner rotor, Rynne and Ellenrieder (2010)

employ an aspect ratio of 5 for a wing sail).

Therefore, planform area A equals 2200 m2 and 889 m2 for the two wing sail sizes

considered, similar to the values employed by G. Klaus et al, 2007 in a study of cloth

sails propulsive power contribution.

The NACA 63(2)-015 has the following distribution of coefficients of lift and drag in

relation to the angle of attack (see Figures 3, 4, 5). These plots are used as guidelines

for selecting the advantageous range of angle of attacks at which the wing sail is

positioned when operating in the lift-inducing mode.

Recalling equations for driving force (FAR) and side force (FAS) from section 2.2.2:

= ( )

= ( )

Efficient performance of a sailing vehicle requires maximization of the driving force and

minimization of the side force. For a given apparent wind angle :

(

) = (( )

( )) =

(( ))

20

Given that is constant, it is apparent that must reach its minimum for the

maximization of (FAR/FAs). Now, recalling the equation for angle between net force and

lift force:

= [

]

It is apparent that for to reach its minimum, (CD/CL) must reach its minimum (Rynne

and Ellenrieder, 2010). Inspecting the graph in figure 4 below, it is observable that

(CD/CL) has an approximate minimum in the CL region of 0.9 to 1.2 and CD region of 0 to

0.06. The corresponding values of the angle of attack for these regions (found from

inspecting figures 5 and 6) are located in the region of 7.5 to 15-which is the optimum

regime for effective performance of the selected wing sail.

Figure 4-Coefficient of lift vs Coefficient of drag for the NACA 63(2)-015 type airfoil (source:

airfoiltools.com)

21

Figure 5-Coefficient of drag vs Angle of attack for the NACA 63(2)-015 type airfoil (source:

airfoiltools.com)

Figure 6-Coefficient of lift vs Angle of attack for the NACA 63(2)-015 type airfoil (plot obtained

from airfoiltools.com)

22

2.2.5+Control system

The wing sail is placed on a rotating mast which is able of controlling its angular

position. According to wind direction which is determined with the use of multiple

sensors, the wing sail must be optimally positioned to maximize its aerodynamic

performance for the purpose of increasing propulsive power.

As such, a control system is proposed. The control system has two main features as

follows:

1. A feature that prevents excessive oscillation of the wing sail as a result of rapidly

changing apparent wind direction.

2. A feature that positions the wing sail in full drag mode so that it acts as a

parachute (fully separated flow, zero net circulation) when the wind conditions

make the full drag mode more advantageous for propulsion than the lift-

inducing mode. Additionally, the wing sail is collapsed if the wind force opposes

ship propulsion.

2.2.5.1+Preventing excessive oscillation

To achieve the ideal performance of an airfoil, flow needs to be stabilized across it. This

however is not achieved when the wing sail is permanently positioned at the optimum

angle of attack in relation to the apparent wind angle, as this will cause the airfoil to

rotate indefinitely, especially if apparent wind is changing rapidly. The proposed

solution is a control system that positions the wing sail at the optimum angle of attack

23

relative to the average true wind speed of the previous 10 minutes. While this measure

is likely to result in a lower propulsive power than the theoretical maximum, it

addresses the instability issue and prevents the sudden oscillation of the wing sail. The

proposed solution takes into account true wind speed and not apparent wind speed on

purpose, specifically to eliminate inefficiencies when the ship itself is changing course.

For example, if true wind speed is constant and the ship changes its direction, this will

result in a change of apparent wind angle. Therefore, it is preferable in such a case for

the wing sail to rotate instantaneously to the new optimum angular position to capture

maximum propulsion because wind conditions are steady and the movement is only

imposed by the change in ship course.

2.2.5.2+Switching between lift mode and full drag mode

When the wing sail is in full drag mode, the actual driving force that contributes

towards propulsion is:

=

( )

Now, to establish whether trimming the wing sail in full drag mode is advantageous

(and what is the range of apparent wind angles for which it makes sense), it is useful to

compare the driving force in the normal mode with the driving force in full drag mode.

To do so, it is useful to recall the following equation for driving force in normal mode:

24

=

+ ( )

To establish the range of angle for which switching between normal lifting mode and

full drag mode is advantageous, the following inequality needs to be solved:

>

Which is equivalent with:

( ) >

+ ( [

])

This equation can be further simplified to obtain:

( ) > +

( [])

Which can be written empirically as:

. ( ) > 1.1045( . )

This can be solved iteratively to obtain that should be greater than 148 to make

switching between normal lifting mode and full drag mode viable. This result is also

consistent with the findings of Rynne and Ellenrieder (2010).

Additionally, if the wind conditions are not favourable (eg head wind) or if the wind is

too slow to obtain positive propulsive power out of it, the wing sail is collapsed to

prevent additional power being required from the ship engine.

Conditions for collapsing the wing sail imply that the driving force is negative:

25

=

+ ( ) < 0

Which is equivalent with:

( ) < 0

This inequality gives the wind conditions for which it is preferable to collapse the wing

sail.

2.2.6 Assessment of propulsive power

The previously presented numerical model is used in a first stage to perform a

sensitivity analysis to identify critical parameters and develop a general understanding

of the wing sails potential in propulsive power contribution.

In a second stage, this numerical model is combined with Met Offices Unified Model

wind data (with a resolution of less than 1) to calculate propulsion contribution across

5 shipping routes, away and return: Dunkirk (France) to Dover, London to Milford

Haven, Varberg (Sweden) to Gillingham, Tubarao (Brazil) to Grimsby, and Yantian

(China) to Felixstowe. The route selection is identical to that of Traut et al, 2014, a

similar study on propulsion contribution of Flettner rotor and towing kite, and was

chosen with the specific purpose of comparing the wing sail performance to that of the

other two wind assisted propulsion technologies. Routes (including passing through the

Strait of Malacca and Suez Canal from China to Europe) follow the shortest path and

are represented by a set of latitude-longitude coordinates. Additionally, ships typically

26

serving the chosen routes, at both full speed and slow steaming speed are considered

as follows: a RoRo ferry running between Dunkirk and Dover (full speed 9.3 m/s, slow

steaming 7.4 m/s); a products tanker carrying distillates between London and Milford

Haven (full speed 6.2 m/s, slow steaming 4.9 m/s); a general cargo carrier transporting

wood chips between Varberg and Gillingham (full speed 5.7 m/s, slow steaming 4.5

m/s); a bulker bringing iron ore from Tubarao to Grimsby (full speed 7.2 m/s, slow

steaming 5.8 m/s); and a container vessel serving the Far East trade between Yantian

and Felixstowe (full speed 11.8 m/s, slow steaming 9.5 m/s). Propulsive power was

averaged between two data sets: one from January 2013 and one from July 2013. The

code employed to quantify propulsive power contribution using wind data is presented

in Appendix 1.

27

3. Literature Review

The Literature review chapter covers the following sections: section 3.1 covers the

broader topic of climate change, section 3.2 gives a broad overview of the shipping

sector before focusing on the recent developments in the shipping industry as a result

of climate change concerns and section 3.3 goes on to discuss wind assisted propulsion

technology, with an emphasis on previous results from research and industry.

3.1 Climate Change

3.1.1 Global warming caused by CO2 emissions

According to Dunlap (2013), the topic of climate change has been a matter of public

debate in recent years, despite the fact that the scientific community largely agrees on

the fact that global warming is caused by human activities. From 1880 to 2012 earth

and ocean surface temperature has increased by 0.85 C on a global average (IPCC

Synthesis Report, 2014).Several studies have linked this increase in temperature with

the emissions of greenhouse gases (most notably CO2), starting as early as 1896 when

Swedish Nobel chemist Arrhenius proved that doubling the C02 concentration in the

atmosphere would cause a 4-5 Kelvin increase in temperature. More recent research

on the topic of climatic effects caused by an increase in CO2 concentration reported

revised findings. For example, Plass (1956) uses a general circulation model to prove

that doubling the CO2 atmospheric concentration causes a 3.8 Kelvin increase in

temperature, while Augustsson and Ramanathan (1976) used a radiative-convective

28

model to estimate that doubling the CO2 concentration in the atmosphere would cause

a 2 Kelvin increase in temperature. While the exact effect of CO2 emissions on global

temperatures cannot be accurately predicted due to the climate system complexity, all

of these studies agree on the fact that an increase in CO2 concentration causes global

warming to a certain extent.

3.1.2 Natural climate variations

However, these findings do not necessarily reflect the extent to which global warming

is affecting Earth and the pace at which these changes are taking place. As such, many

skeptical scientific or journalistic publications have contradicted the relevance of such

findings by arguing that the global increase in temperature can be attributed to natural

variations in climate (Dunlap, 2013). Indeed, natural climate variations are known to

have significantly impacted global temperatures historically. For example, The Medieval

Warm Period or the Ice Age had temperatures with significant deviation from historical

averages (Hunt, 1998). These two phenomena (human-caused global warming and

natural climate variation) are simultaneously affecting the global climate and it is

difficult to predict the extent to which each of them is influencing the current climate

changes.

29

Figure 7-Surface temperature historical data (IPCC, 2014). Two main warming time intervals can

be observed, one from 1910 to 1945 and another from 1976 onwards. The warming rate during

the second period has been twice that of the first (source: Walther et al, 2002)

This difficulty arises from the fact that scientific understanding of low-frequency

variability in climate change is limited, as changes are occurring gradually over

centuries and recorded meteorological data only exists for the past 125 years, and

some of it is unreliable or incomplete (Reek et al, 1992). However, using mathematical

models of the land-ocean-atmosphere system to predict global temperature, it was

found that increases in temperature from 1880 to 2012 are unprecedented compared

to any variations in the past 1000 years, thus linking the current global warming trend

to human activity (Stouffer et al, 1994). What is more, the IPCC 2014 Report concluded,

with a high level of certainty, that more than half of the global averaged surface

temperature increase from 1951 to 2010 is attributed to human activity, such as the

emission of greenhouse gas concentrations (IPCC, 2014).

30

3.1.3 Unequal distribution of climate change effects

While global average earth and ocean surface temperature increase might not be the

best parameter to indicate the severity of current climate changes and threats of global

warming, it is worth noting that the effect of climatic changes are not averagely

distributed. Indeed, Manabe and Wetherald (1980) use a simplified general circulation

model to prove that doubling the atmospheric CO2 concentration causes poleward

retreat of highly reflective snow cover, especially at high latitudes and discretionary

changes in mean precipitation rates. It is thus more relevant to inspect the potential

effects of climate change in regions which are most vulnerable in order to gain a

meaningful understanding of its severity.

3.1.4 Present and future effects of climate change

Research and investigations from diverse areas show that climate change has a

significant impact on the world as a whole, including ecosystems, human communities,

economy, but with an uneven distribution of effects. A review of studies of these

effects has been put together with the purpose of understanding the complexity of

climate changes and the degree of vulnerability of various systems.

On the topic of wildlife ecosystems, Rahel and Olden (2008) discuss how climate

change affects the aquatic system while humans facilitate the spread of various species

through canal construction, aquariums, stockings and international shipping. The

distribution and abundance of numerous aquatic species are harmed due to altered

31

thermal regimes, reduced ice cover in lakes, altered streamflow regimes, increased

salinity, and increased water-development activities in the form of canal and reservoir

construction (Rahel and Olden, 2008, p. 522). Terrestrial ecosystems are affected as

well: climate changes determine negative loss of species density and diversity.

According to Climate Change 2014: Impacts, Adaptation and Vulnerability report by

IPCC, a loss in the number of plant, bird and mammal groupings will occur in Southern

Europe, while in areas of high altitude the density will rise. 5 to 9% of European

mammals are at risk of extinction in 21st century if migration is not considered (Kovats

and Valentini, 2014).

Climate change has a negative impact on economy and human-made infrastructure, as

reported by recent research. Studies show that the effects of climate change, such as

extreme weather, are growing, harming the environment in different ways while it

affects the world economy (Tol, 2009). Ruth et al 2007 consider agriculture, energy and

transportation to be most affected industries. For instance, Midwest floods in 1993 in

USA caused damages worth $6-8 billion. Also, Hurricane Katrina damaged 2,100 oil

platforms and over 15 000 miles of pipelines as well as hundreds of thousands of

houses. Revenues worth nearly $11 billion were lost.

On the topic of climate change affecting agriculture, it has been found that heat waves

damage soil; therefore irrigation systems are needed in order to prevent destruction of

human settlements, agricultural or technical crops (Kovats and Valentini, 2014). Cereal

yields are likely to increase in Northern Europe and to decrease in South. Yields of

wheat, for example, have suffered because of observed warming since 1980s. Dairy

32

production is also at risk due to the heat stress in lactating cows (Kovats and Valentini,

2014). What is more, weather changes such as hot and cold weather extremes lead to

transportation damages and economic loss as it needs adaptation and reparation or

even replacement. Damaged rail infrastructure from high temperatures has been

experienced and it is expecting to increase in Europe.

Also, because of excessive rainfall, the risk of coastal and river floods has increased in

Europe in the past years. On the topic of climate change effect distribution on human

coastal habitats, Nicholls et al (1999) found that, as a result of a sea-level rise of 40cm,

assuming increased coastal protection, 55 million people would be flooded in south-

east Asia, 21 million people would be flooded in south Asia, Indonesia, Philippines and

New Guinea, 14 millions in Africa and 3 million in the rest of the world.

Figure 8-Historical sea level change (source: IPCC, 2014). A 0.4 m increase in sea-level from

1999 levels would cause a significant increase in flooded population, particularly in south and

south-east Asia (Nicholls et al, 1999).

33

Furthermore, climate change can affect human health, inducing various human

infections. The World Health Organization argues that 150 000 lives have been claimed

annually due to anthropogenic climate change for the past 30 years. Various diseases

occur for the reason that climate fluctuates constantly: for instance, cardiovascular

mortality or respiratory illnesses are the result of heatwaves (Patz et al, 2005).Kalkstein

and Greene (1997) found that increased frequency of heat waves attributed to global

warming would increase heat-stress mortality in cities located at mid-to-high latitudes

in the temperate region.

3.1.5 Intergovernmental protocols regarding climate change

In order to reduce greenhouse gases emissions, several developed countries adopted

the Kyoto Protocol, which is an extended treaty of the 1992 United Nations Framework

Convention on Climate Change (UNFCCC). Entering to force on 16th February 2005, the

Kyoto Protocol was not adopted by United States, nor Southern countries, hence, only

countries that represent 33% of total CO2 emissions in the world committed to respect

the treaty. In 2011, Canada was the first that announced its withdrawal and in

December 2012, the Kyoto Protocol expired (Ghezloun et al, 2013).

The main goal of the Kyoto Protocol was to fight global warming by reducing with 5%

CO2 emissions by 2012 compared to the 1990s level. Also, the members of the treaty

were committed to create and adopt policies to minimize greenhouse gases emissions,

policies that should be reviewed on a regular basis (such as annual reports). Each

member of the Kyoto Protocol was flexible to choose its methods in order to meet their

34

gas reduction obligations. According to Bashkamov et al (2001), there were three main

flexibility mechanisms that could have been used: firstly, clean development

mechanism projects were designed to reduce CO2 emissions through renewable energy

commercialisation, fuel switching, etc. (World Bank, 2010). Secondly, joint

implementation was another mechanism that allowed developed countries to invest in

CO2 emission reduction in any other country part of the treaty, where this action may

be cheaper. Lastly, International Emissions Trading tolerated emissions trading

between countries, giving them the opportunity to diminish emissions in the most

economically efficient way (Bashkamov et al, 2001).

However, according to Klimenko et al (2006), due to the fact that important countries-

which are also the 21st centurys major CO2 emitters- such as USA, China and India

refused to adopt the treaty, the Kyoto Protocol was not, in the end, a success.

Several treaties and accord have been discussed and considered after the Kyoto

Protocol expired. The Copenhagen Accord supports the continuation of the Kyoto

Protocol and is based on the idea that climate change is a significant issue of this

century. It also recognizes that the rise of the global temperature has to be limited to

2degrees Celsius and the role of diminishing emissions coming from deforestation and

forest degradation. What is more, the Copenhagen Accord would agree to raise $100

billion per year by 2020 from various sources, being delivered by respecting a

governance structure, in order to help developing states to reduce CO2 emissions. A

Copenhagen Green Climate Fund would be also taken into account to support policies

and projects and to activate as a fiscal mechanism (Copenhagen Accord, 2013).

35

However, Ghezloun et al (2013) argue that states such as China, India and USA need to

be convinced to join this framework in order to obtain results. Also, citizen participation

would be essential to prove transparency and acceptance of verdict took by the states.

Yet, the accord is not legally binding. Even though progress and recognition of the fact

that climate change might be an irreparable danger to societies and the planet, there is

no new binding structure to follow the Kyoto Protocol (Ghezloun et al, 2013).

3.2 Shipping Industry

3.2.1 Overview of the shipping industry and growth trends

According to Lun et al (2013) shipping activities involve physical movement of cargo

from production to construction sites, by which means they facilitate global trade and

economic development. The shipping industry can be categorized into international

shipping, which represents shipping activities between ports of different countries, and

domestic shipping, which represents shipping activities between ports of the same

country. Both of these categories exclude military and shipping vessels, according to

the IPCC 2006 Guidelines. Total shipping includes both domestic and international

shipping and fishing, but excludes military vessels (Buhaug et al, 2009).

In the context of increased market globalization and variety of production sites,

international trade has been growing dramatically (Robinson, 2002). An idea of the

pace of international trade growth can be drawn from the World Trade Organization

Secretariat, which reports that international trade grew at approximately twice the

36

growth rate of global economy from the 1990s until the 2008 and at approximately the

same pace as global economy from 2010 to 2013 (see figure 9). The international

shipping industry transports approximately 90% of world trade and is therefore a major

instrument sustaining economic growth (UNFCCC, 2014).

Figure 9-Historical trade and GDP growth. Average export growth is approximately twice the

average GDP growth (WTO, 2014).

Figure 10-World trade forecast (source: UNFCCC, 2014)

37

Given the factors presented above it is apparent that international shipping has strong

growth perspectives in the long run and, given that international shipping has a

proportion of more than 80% in total shipping (Third IMO GHG Study, 2014), it is

expected that the entire industry will experience steady growth as a result.

3.2.2 CO2 emissions

The shipping industry accounted for 3.1% of global CO2 emissions during years 2007-

2012, while international shipping accounted for 2.6% of global CO2 emissions in the

same period. Total shipping CO2 emissions have totaled, on average, 1,016 million

tonnes (Buhaug et al, 2009).

Figure 11-International and domestic shipping CO2 emissions for years 2007-2012 (source: Third

IMO GHG Study, 2014)

At this point it is worth noting that the shipping industry is already the most efficient

mode of transport, having significantly less CO2 emissions in terms of grams per tonne-

km. While various types of ships have CO2 emissions in the range of 3-7.9

(grams/tonne-km), depending on ship type, air freight averages at 435 grams/tonne-km

38

and road transport (40 tonnes or larger trucks) average at 80 grams/tonne-km.

(UNFCCC, 2014).

Due to the nature of international shipping industry, CO2 emissions cannot be clearly

attributed to a particular nation, as acknowledged by the Kyoto Protocol. However, as

this industrial sector is being regulated by the International Maritime Organization

(IMO), a legally binding agreement known as MARPOL Annex VI, Chapter 4 was adopted

in July 2011 and entered into force in January 2013 (UNFCCC, 2014). This agreement

covers 94% of world fleet.

The main target of this agreement is to reduce CO2 emissions with 50% by 2050, with a

preliminary target of reducing CO2 emissions with 20% by 2020 (MARPOL Annex VI,

Chapter 4, 2011). In terms of energy efficiency of new ships, preliminary targets of 10%,

20%, 30% improvement in energy efficiency were imposed to be met by 2020, 2025,

2030 respectively (Marpol Annex VI, Chapter 4, 2011).

In order to achieve these targets, it was agreed that all new and existing ships must

implement the Ship Energy Efficiency Management Plan (SEEMP) and Energy Efficiency

Operational Indicator (EEOI). The SEEMP is an operational mechanism aimed at

shipping companies to enable them manage and record fleet efficiency by using the

EEOI. The SEEMP includes guidelines and best practices for efficient fleet management

in terms of fuel consumption, and recommends at each stage of the plan new

technologies and operational practices to improve ship performance. In addition, the

EEOI is employed by ship owners to calculate efficiency based on an initial set of

39

operational parameters and to quantify for incremental changes in efficiency from the

initial value if any of the operational parameters change.

Given the strong growth perspective of the shipping sector presented in Section 3.2.1,

it is of importance to look into the expected trend of resulting CO2 emissions to gain a

broader understanding of the role played by the shipping industry in the context of

increasing climate change concerns. The third IMO GHG Study (2014) provides a range

of scenarios for CO2 emissions from the shipping industry up to 2050 by considering

the most important drivers of maritime transport and efficiency trends in order to

project energy demand in the sector.

Figure 12-CO2 emissions projection (source: Third IMO GHG Study, 2014)

As it is apparent from figure 12 above, there are a range of scenarios regarding CO2

emissions until 2050 with a wide range of possible outcomes. The degree of scatter

between separate scenarios is due to the range and complexity of factors influencing

CO2 emissions. The 16 scenarios are resulted from alternative coupling of 3 major

factors scenarios influencing CO2 emissions as follows:

40

1. Scenarios 1-8 assume the implementation of additional regulations that

encourage the shipping industry to use LNG as fuel; LNG increases steadily

up to a 25% share in total fuel used by the shipping industry by 2050. As it is

a cleaner fuel, lower emissions are expected. Scenarios 9-16 assume no

additional regulations in this regard; thus, LNG has a fuel proportion of 8%

out of the total fuel.

2. Scenarios 1-4 and 9-12 assume an improvement in fleet energy efficiency of

60% due to MARPOL Annex VI Regulations, whereas scenarios 5-8 and 13-16

assume an improvement of only 40%.

3. The third factor relates to 4 representative CO2 concentration pathways

(RCP) that are each coupled with a specific socio-economic scenario.

Scenarios 1, 5, 9, 13 assume high economic growth coupled with high fossil

fuel consumption, scenarios 2, 6, 10, 14 assume high economic growth with

low fossil fuels dependency, scenarios 3, 7, 11, 15 assume low economic

growth and moderate fossil fuel consumption while scenarios 4, 8, 12, 16

assume high fossil fuel consumption and low economic growth due to

unequal distribution of wealth globally (Third IMO GHG Study, 2009).

Average percentage increase in emissions across scenarios 1-16 is 7% by 2020, 29% by

2030 and 95% by 2050 (Third IMO GHG Study, 2009). Contrasting these findings with

the targets imposed by MARPOL Annex VI Agreement, it is apparent that the average of

scenarios 1-16 does not meet the objective of reducing CO2 emissions with 20% by

2020 or with 50% by 2050 (using 2012 CO2 emissions as base value). Increasing

discrepancy can be observed between imposed targets and the forecast in 2020 (-20%

41

target compared to +7% average of forecasts) as opposed to 2050 (-50% target

compared to +95% average of forecasts). What is more, it is observable from figure 10

that the most optimistic scenarios in terms of CO2 emissions growth (decreasing

dependence on fossil fuels, sustainable economic growth, increased energy efficiency

of ships and increasing use of LNG as a fuel) fail to meet the MARPOL Annex VI targets

for both 2020 and 2050. This discrepancy between a legally-binding agreement and the

IMO forecast can be attributed to high growth of the shipping sector, as discussed in

section 3.2.1, coupled with no adoption of alternative propulsion technologies to

reduce CO2 emissions in the shipping sector (as assumed by the IMO forecast).

3.2.3 Methods to reduce CO2 emissions in the shipping industry

Previous research illustrates that a variety methods to reduceCO2 emissions in the

shipping industry have been considered so far. According to Buhaug et al (2009)

technical and operational measures could reduce the emissions rate by 25% to 75%

below the actual levels. The study refers to four key categories of methods for

diminishing gas emissions from shipping:

1. Improvement of energy efficiency by using the same energy consumption in different

ways- this can apply to both design and operation of ships.

In terms of concept and design, flexibility to allow upgrades during ships lifetime is

considered to be an important feature when taking into account maximizing energy-

efficiency. For example, if larger ships might be more efficient per tonne-mile than

42

smaller ones when loaded, smaller and better-adapted ships can reach higher overall

efficiency due to the fact that they can be utilized in different ways.

Optimization of the underwater hull is another method to improve energy efficiency.

For instance, energy can be saved by redesigning the superstructure of the hull so it can

reduce heeling. Also, if the weight of the hull is reduced, the wetted surface area is

concentrated as well, thus it becomes a way to save energy. Light materials such as

aluminum or carbon and glass-fiber are used in ship construction in order to reduce

ships weight. According to Buhaug et al (2009), optimizing the behavior of the hull in

still water can lead to savings of 5-20%, even greater for smaller ships.

Furthermore, energy efficiency can be enhanced by upgrading engines, replacing

turbochargers and using power turbines to capture engine waste heat or to be driven

by an exhaust side-stream. Energy efficiency can be improved through the use of

various enhancements such as vanes, fins, ducts, high-efficiency rudders, vane wheels,

asymmetric rudders, contra-rotating propellers. These devices recover propeller

rotational energy and potential savings are in the range of 5-10% of the ship propulsion

power.

In addition, managing operational aspects is a way of improving energy savings: for

instance, spending less time in ports through more efficient cargo handling and

mooring can reduce emissions. Also, voyage optimization can be achieved by: selecting

optimal routes considering weather and currents, optimizing ballast (avoiding

unnecessary ballast) and optimizing trim (operating at the right trim). Recent studies

estimated savings of 0.6% of total fuel for trim and ballast optimization.

43

What is more, ships and cargos need electric power to sustain the crew as well as

numerous auxiliary systems (cooling-water pumps, ventilation fans, etc.). In order to

save energy, different measures should be taken into account: optimization of steam

plant, of the fuel separator or detection and repair of leaking steam and compressed-

air system.

Further research conducted by Det Norske Veritas (2010) adds that speed reduction is

another operational measure that can reduce CO2 emissions in shipping. This method is

of significant interest for the reason that hull resistance increases exponentially with

speed. Thus, even a modest speed reduction can substantially decrease required

propulsion thrust. Less required thrust means lower fuel consumption and reduced

emissions to air (Det Norske Veritas, 2010, p.10). By reducing the speed of ships, the

transit time between ports is more likely to increase; therefore the overall cargo

delivery time rises. Hence, customer acceptance is required in order to adopt this

method. Other factors such as fuel costs and the market should be taken into

consideration as well.

2. Use of biofuels and natural gases

According to Buhaug et al (2009), biofuels do not have the capacity to significantly

reduce emissions of CO2 from shipping, mostly because of technical issues (such as

acidity or lack of water-shedding which can lead even to engine shutdown), cost issues

(as these are more expensive than petroleum fuels) and also because of lack of

availability.

44

While LNG combustion is less CO2 intensive than oil-based fuels, while also emitting less

SOx, NOx and particulate matter it does produce more CH4 emissions which decreases

the global warming benefit from 25% to 15%. The implementation potential is currently

limited, as LNG fuelling is mainly relevant for new ships and it poses additional

bunkering issues. However, increasing regulatory measures regarding SOx and NOx

emissions will provide an incentive for the implementation of LNG fuelling, especially

given the fact that LNG is significantly cheaper than oil-based fuels (Buhaug et al, 2009)

3. Use of emission-reduction technologies- for example, using chemical conversion

and other means in order to reduce emissions

This method is not considered feasible for reducing CO2 emissions, being mainly

relevant to pollutants within exhaust gases such as NOx, SOx, Particulate Matters, CH4

and NMVOC (Buhaug et al, 2009).

The fourth section focuses on the use of wind and solar power. Solar cells are not a

feasible option for improving energy efficiency, as a result of their low specific capacity

with respect to surface area (Buhaug et al, 2009).

Wind assisted propulsion is discussed in detail in the following section (3.3).

45

3.3aWind assisted propulsion

3.3.1 Historical use of wind propulsion

During the second half of the 19th century steam propulsion started to gain attention to

the detriment of sail propulsion especially for economic reasons thus becoming the

preferred option in maritime trade (Harley, 1971; Pollard and Robertson, 1979; Starkey

and Jamieson, 1998). This was the moment in the history of transition from sail to

steam propulsion when the struggle for new technologies and new economic incentives

gathered around the existence of a sailing ship effect. The main idea behind this

concept involves the continuous innovation of an older technology to supplement its

capabilities and to extend its economic features (Freeman and Soete, 1997). This

concept was implied in many research papers at various time points in the history of

sailing (Ward, 1967; Rosenberg, 1972, 1976; Hall, 2004).

By the end of the 19th century the productivity evolution determined a significant

diminish of the freight rates for many routes and cargo models which caused a

decrease by 20-30% in the cost of sea carriage by weight (North, 1958; Harley, 1988;

Clark and Feenstra, 2003; Mohammed and Williamsons, 2004; Fouquet, 2008). As

opposed to this trendline the ocean transport increased significantly due to economy

expansion. The main drivers of the economic efficiency increase are due to ships

technology especially to improvements in propulsion and hull construction (Davis,

1972, Ville, 2004).

46

During the 19th century the sail and steam technologies faced together the demands for

maritime transportation. It was only by the end of 19th century when the steamships

became seaworthy for a certain category of cargoes. The sailing ship remained at that

moment just a good alternative for cost inefficient and small trades (Rosenberg 1972).

In 1912, after 100 years from the first steam vessel appearance, the figures for sailing

vessels accounted for 8510 units while for steamers reached 12,000 units (Brian

Mitchell, 1988, British Historical Statistics).

Increasing fuel costs, combined with technological advancements resulted in fuel

expenditure as a percentage of the total cost of running a ship to grow from 10% in

1900 to between 25-60% by 2000 (Hamer, 2005). This fact together with increasing

regulatory measures to reduce CO2 emissions from the shipping industry have resulted

in a revival of the interest in wind assisted propulsion. The following sections provide a

breakdown of wind assisted propulsion technologies and then go on to explore relevant

research in this area, existing prototype designs and technologies that achieved

commercial implementation.

3.3.2 Types of wind assisted propulsion

Three types of wind assisted propulsion currently exist: sail, towing kite and Flettner

rotor (Rojon and Dieperink, 2013). The towing kite is installed to the bow of the ship

and provides propulsion power directly from the wing (Det Norske Veritas, 2009). The

Flettner rotor is a vertical, cylindrical rotor that is spinning and takes advantage of the

Magnus effect to convert wind into propulsive power (Crist, 2009). Sails can be further

47

divided into two categories: traditional cloth sails and wing sails (Rojon and Dieperink,

2013). Wing sails have an airfoil-like shape similar to aircraft wings and increased

aerodynamic efficiency compared to traditional sails, having increased capability of

maximizing lift force and minimizing drag force (Buhaug et al, 2009).

3.3.3 Previous research on the topic of wind assisted propulsion

On the topic of appropriate airfoil section for maximizing performance of a sailing craft,

Partida (1996) conducts a comparative computational analysis. The study concludes

that the elliptical leading edge is superior in any configuration to the circular leading

edge and introduces the concept of a semi-rigid wing sail which, according to

Partida(1996) is superior to the fully rigid shapes employed in sailing (Partida, 1996).

This study provides useful considerations for airfoil section selection with the purpose

of optimizing thrust, providing performance parameters for all designs considered.

Rynne and Ellenrieder (2010) cover the topic of design and preliminary validation of a

wind and solar powered autonomous surface vehicle (Rynne and Ellenrieder, 2010).

Although this research is not specifically in line with the topic of wind assisted

propulsion, it is included in this section because of relevant design and optimization

considerations regarding the overall system integration. This study suggests that a 4.2

m monohull keel-boat powered by a 5m high wing sail can operate autonomously given

winds in the range 7-10 knots. Rynne and Ellenrieder (2010) support their selection of

the wing sail by presenting evidence to prove its superior aerodynamic properties over

the cloth sail. The wing sail is optimized with the use of wind sensors that adequately

48

adjust the sail position (Rynne and Ellenrieder, 2010). What is more, the study

introduces two modes of operation for the wing sail, one which is lift-generating at

apparent wind angles below 135 and one which is drag-generating at apparent wind

angles above 135 (Rynne and Ellenrieder, 2010).

While this study provides some useful guidelines regarding the effective operation of a

wing sail, it is unlikely that autonomous operation will be achieved for large cargo ships.

It is thus useful to inspect previous research into how wind propulsion translates from

small scale experiments to large scale vessels. On this topic, experimental testing is

uneconomical but numerical models were devised to estimate the abatement potential

of wind assisted propulsion in order to assess their feasibility. Schlaak et al (2009)

devises a mathematical model and corroborates it with sea trial data and recorded

wind velocities across different routes to estimate propulsive power contribution of the

SkySails 600 m2 towing kite (more information on this technology is provided in the

Commercial implementation & existing prototypes section), reporting average energy

savings between 5 and 21% at a ship speed of 15 knots and between 10 and 36% at a

ship speed of 13 knots on a North Atlantic route, for a vessel with average fuel

consumption of 315 l/h at 12 knots (Schlaak et al, 2009). Because of inclusion of sea

trials measurements in the development of the mathematical model, this results of this

study can be considered of higher accuracy. However, relatively similar results have

been obtained in studies that use different methodology and do not employ

experimental data. For example, Traut et al (2014) devise a numerical model of a

towing kite and use wind data for five shipping routes to conclude that average power

contribution ranges between 127 kW and 461 kW, which compares favorably with

49

results from Schlaak et al (2009) which report the average propulsive power to be of

256 kW (Traut et al, 2014 and Schlaak et al, 2009). This comparison of results for two

different methodologies suggests that the mitigation potential of wind assisted

propulsion can be estimated to a reasonable degree of accuracy without sea trials

measurements.

Traut et al (2014) employ a similar methodology to that used for towing kites for the

Flettner rotor and report the propulsive power to average in the range of 193 kW to

373 kW. This figure is comparatively similar to the power contributed by the towing

kite, however the study reports that the Flettner rotor technology can achieve higher

propulsive powers than the towing kite if two or more rotors are used (Traut et al,

2014).

On the topic of propulsive power contribution of cloth sails, F. Klaus et al, 2007 analyze

the mitigation potential of 5 different sail types, using a simplified numerical model and

weather data from ECMWF (European Centre for Medium-Range Weather Forecasts),

reporting fuel savings of 15% at a speed of 15 knots and 44% at a speed of 10 knots for

a generic product tanker. Considering the findings of Rynne and Ellenrieder (2010)

(which were presented earlier in this section) regarding the superiority of wing sails

over cloth sails, it is expected that higher propulsive power contribution can be

achieved theoretically if one employed the same methodology as Klaus et al(2007).

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3.3.4 Commercial implementation and existing prototypes

3.3.4.1 Flettner rotor

The Flettner rotor was successfully implemented on a commercial cargo ship by the

Finnish marine engineering company Norsepower Ltd and began sea trials. An 18

meters high Flettner rotor was installed on the M/V Estraden cargo ship.

Figure 13-M/V Estraden cargo ship retrofitted with a single Flettner rotor (source:

norsepower.com)

The company installs Flettner rotors of three different heights (18, 24 and 30m) and

claims that the technology can achieve reductions in fuel cost in the range of 5-30%

without lowering the operating speed. Rotors are fitted with wind and GPS sensors

which provide real time data to an automation unit for performance optimization. The

rotors are powered from the vessel electric grid.

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Figure 14-Layout of Flettner rotor equipment (source: norsewind.com)

3.3.4.2 Towing kite

One of the most successful prototypes of the towing kite is developed by SkySails

Gmbh, a German company which already achieved successful commercial

implementation following an investment of 50 million euros, as per the company

website (skysails.info). The company claims their technology is able to replace up to 2

MW of propulsive power from a ships main engine.

Figure 15-SkySail technology (source: Hamer, 2005)

52

The largest prototypes, which are currently in development stage and have an area of

2000 to 5000 m2, are reportedly able to generate between 1 and 1.15 kW per m2, thus

halving fuel consumption of a 200 m vessel at a speed of 15 knots ( Hamer, 2005).

Figure 16-Mitigation potential of SkySails technology (source: Hamer, 2005)

The solution provided by SkySails involves harnessing the wind at height of up to 500

m, which is favourable for fuel savings as winds are stronger and less variable at such

heights (Hamer, 2005). The towing kite, which can be installed on both new and

existing ships, is fully automated and its position relative to the ship course is optimized

in real time to maximize propulsive power. Unlike the Flettner rotor and the wing sail, it

has the added benefit of not exerting aerodynamic on the ships structure in case of

weak or no wind (Hamer, 2005). In addition, apart from benefits in energy efficiency,

sea trials determined that the towing kite improves the ship stability and reduces

heeling (Hamer, 2005).

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3.3.4.3 Wing sail

The Danish maritime architecture company Knud E. Hansen created a prototype design

for a ship using wing sails and claims that the vessel can be solely propelled by wing

sails at a ship speed of 13 knots when the true wind speed is 9 m/s at an angle of 100

from course (Hamer, 2005). On a voyage from Rotterdam to New York, this ship

prototype is reportedly capable of reducing fuel consumption with 27% if travelling at a

speed of 13 knots (Hamer, 2005)

Figure 17-Wing sail technology (source: Hamer, 2005)

The design involves the use of retractable flaps on the wing trailing edge which

generate extra thrust in favourable winds but which can be retracted to reduce

aerodynamic drag when engine power is solely used. Additionally, the slat in front of

the mast has the purpose of smoothing airflow and thus increasing thrust. The wing

sails are rotated hydraulically to take advantage of different apparent wind angles.

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4. Results

4.1 Sensitivity analysis-Tornado Chart

A sensitivity analysis is performed to account for uncertainties in values of parameters

influencing the propulsive power contribution. The base parameters are provided in

table 1 below for the two wing sail dimensions, with minimum and maximum variation.

Table 1-Base values of parameters used in sensitivity analysis with minimum and maximum

variation. In fields where more than one value appear (eg 115/85) the first value corresponds

to the larger sail dimensions and the second value corresponds to the smaller sail dimensions

Parameter Base value Minimum Maximum

Apparent Wind

speed at 10m height

(m/s)

15 3 27

Maximum height

(m)

115/85 60/70 170/100

Wing sail height (m) 110/70 60/40 140/100

Lift coefficient 1.2 0.6 1.8

Apparent wind angle

(radians)

1.5708 0.1 3.04

Wing sail area (m2) 2200/889 1400/600 3000/1178

Ship speed (m/s) 12 10 14

Minimum height (m) 15 10 20

55

Air density (kg/m3) 1.2754 1.2 1.35

Drag coefficient 0.06 0.01 0.11

Propulsive power

(kW)

15,374.4/2841.83

/

/

Tornado plots are produced to identify critical parameters which can significantly

influence. Tornado plots calculate how propulsive power changes when one parameter

varies between two limits, while the other parameters remain constant. Left and right

hand side values show how propulsive power changes if all parameters remain constant

and the parameter in question varies from minimum value (left) to maximum value

(right). Vertical line is indicative of base value for all parameters.

The base scenario was chosen to be the most favourable in terms of apparent wind

angle (wind perpendicular to ship course) and angle of attack (the ship is positioned at

the optimum angle of attack for maximum CL/CD ratio).

The major uncertainty, as expected, is related to the apparent wind speed as a drop in

wind speed from 15 m/s to 3 m/s reduces the propulsive power contribution by 95.96%

from its base value to 1123.142 kW.

Apparent wind angle is another major source of uncertainty because, as seen from

figure 6, an unfavourable angle can reduce propulsive power contribution by 94.95%

from its base value. However, for a range of 148, it is preferable to rotate the wing

sail such that it is perpendicular to the wind and acts as a parachute, causing a fully

separated flow and zero net circulation, as described in Section 2.2.5.2.

56

2841.83 kW {0.11}

3008.06 kW {1.35}

3188.51 kW {10}

3315.47 kW {14}

3765.67 kW {1178}

4346.43 kW {100}

2841.72 kW {1.57}

4262.75 kW {1.8}

8703.12 kW {40}

9207.54 kW {27}

2841.83 kW {0.01}

2673.83 kW {1.2}

2499.39 kW {20}

2368.19 kW {10}

1917.99 kW {600}

1681.48 kW {70}

142.33 kW {0.1}

1420.92 kW {0.6}

1392.49 kW {100}

113.67 kW {3}

0 2000000 4000000 6000000 8000000 10000000

Drag Coefficient {dimensionless}

Air Density {kg/m^3}

Minimum Height {m}

Ship Speed {m/s}

Wing Sail Area {m^2}

Maximum Height {m}

Apparent wind angle {radians}

Lift Coefficient {dimensionless}

Wing sail Height {m}

Apparent Wind Speed at 10m {m/s}

Propulsive Power Sensitivity Analysis-Small wing sail (all values in kW)

7533.46 kW {0.11}

7974.10 kW {1.35}

8149.22 kW {10}

8789.03 kW {14}

10,272.89 kW {3000}

7533.15 kW {1.57}

11,300.18 kW {1.8}

20,926.27 kW {60}

19,868.40 kW {170}

24,408.39 kW {27}

7533.45 kW {0.01}

7088.088 kW {1.2}

6913.13 kW {20}

6277.88 kW {10}

4794.02 kW {1400}

377.30 kW {0.1}

3766.73 kW {0.6}

3,843.60 kW {140}

1321.07 kW {60}

301.34 {3}

0 5000000 10000000 15000000 20000000 25000000 30000000

Drag Coefficient {dimensionless}

Air Density {kg/m^3}

Minimum Height {m}

Ship Speed {m/s}

Wing Sail Area {m^2}

Apparent wind angle {radians}

Lift Coefficient {dimensionless}

Wing sail Height {m}

Maximum Height {m}

Apparent Wind Speed at 10m {m/s}

Propulsive Power Sensitivity Analysis-Large wing sail (all values in kW)

57

4.2 Polar Plots

This section presents polar plots which are useful for quantifying the variation of

propulsive power contribution depending on apparent wind angle and an additional

parameter, which was chosen by inspecting the results of the sensitivity analysis from

section 4.1 and choosing the more sensitive ones. The chosen parameters are apparent

wind speed and coefficient of drag, as they represent a source of uncertainty when

estimating potential power contribution. Two sets of polar plots are produced for the

large and small wing sail respectively.

Figures 18, 19, 20 and 21 show multiple curves corresponding to various wind speeds

for a ship speed of 12 m/s (upper half) or 6 m/s (lower half). Propulsive power is

plotted against apparent wind angle.

As expected, an increase in wind speed as well as an increase in ship speed cause an

increase in propulsive power in accordance to the equation for propulsive power.

However, it is observable that a 100% increase in ship speed has less impact than a

100% in wind speed, which is due to the fact that the relation between wind speed and

power is quadratic (whereas the relation between ship speed and power is linear).

These results are in accordance to findings from the tornado charts (section 4.2), which

show that propulsive power is more sensitive to wind speed compared to ship speed.

Figures 22, 23, 24 and 25 show multiple curves corresponding to different coefficients

of lift (upper half) and drag (lower half). Propulsive power contribution is plotted

against apparent wind angle , at 15 m/s wind speed, 15 m/s ship speed, 0.1 coefficient

of drag (constant for the upper half) and 1.1 coefficient of lift (constant for the lower

58

half). It is observable that propulsive power contribution increases with coefficient of

lift given constant coefficient of drag. This result is in accordance with the aerodynamic

efficiency theory, which states that the efficiency of a lifting surface is dependent on its

capability of maximizing lift and minimizing drag (Larson and Eliasson, 2000).

Variation in drag coefficient has less effect on propulsive power compared to the

variation in lift coefficient. There is little variation in the drag coefficient curves radius

and the only observable change is in the angle of attack , which influences the

apparent wind angle at which maximum propulsive power is achieved. These results

are in accordance with those obtained in the tornado plot analysis (section 4.2) and are

mainly due to the fact that the relative percentage change considered for the drag

coefficient is smaller than the one considered for the lift coefficient. However, a

variation of drag coefficient between 0.05 and 0.045 is in accordance with the airfoil

section characteristics, as it can be observed from figures 4, 5 and 6 (section 2.2.4).

Figure 18-Large wing sail propulsive power depending on apparent wind speed and ship speed

59

Figure 19-Large wing sail propulsive power depending on apparent wind speed and ship speed

(zoom in)

Figure 20-Small wing sail propulsive power depending on apparent wind speed and ship speed

60

Figure 21-Small wing sail propulsive power depending on apparent wind speed and ship speed

(zoom in)

Figure 22-Large wing sail propulsive power depending on coefficient of lift and coefficient of

drag

61

Figure 23-Large wing sail propulsive power depending on coefficient of lift and coefficient of

drag (zoom in)

Figure 24-Small wing sail propulsive power depending on coefficient of lift and coefficient of

drag

62

Figure 25-Small wing sail propulsive power depending on coefficient of lift and coefficient of

drag (zoom in)

Propulsive power contribution of the wing sail achieves promising values from the

sensitivity analysis conducted. Instantaneous propulsive powers of approximately 6000

kW for the large wing sail and 3000 kW for the small wing sail can theoretically be

achieved. However, it is worth noting that these results are achieved in highly

favourable conditions of apparent wind speed (15 m/s) and angle , under the

assumption of ideal aerodynamic behavior of the airfoil (smooth flow which allows the

wing sail to operate at optimum lift and drag coefficients) and identical boundary layer

for both true wind and apparent wind.

High variation is observed in propulsive power when apparent wind speed decreases to

3 m/s and all other parameters remain constant, in which case results show a

63

propulsive power contribution of less than 100 kW for the large wing sail and less than

50 kW for the small wing sail at the optimum apparent wind angle .

4.3 Results on five shipping routes

Average propulsive power contribution was calculated according to the methodology

presented in sections 2.2 and 2.3.

Table 2-Propulsive power contribution of wing sail along 5 s