1

Comparative analysis of winch-based

wave energy converters

Aleksandar Nachev

Bachelor Thesis MMK 2017:04 MKNB 090

KTH Industrial engineering and management

Machine Design SE-100 44 STOCKHOLM

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Acknowledgments

I would like to thank my supervisors, Dr Richard Neilson and Dr. Ulf Sellgren,

for their advice and support.

I would like to thank the Dr. Panagiotis Kechagiopoulos and the Erasmus teams

in the University of Aberdeen, UK, and the Royal Institute of Technology, Sweden, for

their technical support during my time abroad and the preparations beforehand.

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BachelorThesis MMK 2017:504 MKNB 090

Comparative analysis of winch-based wave energy converters

Alexandar Nachev

Approved

2016-04-20

Examiner

Ulf Sellgren

Supervisor

Ulf Sellgren

Commissioner

KTH Machine Design

Contact person

Ulf Sellgren

Abstract

Renewable energy sources are probably the future of the mankind. The main

points advocating wave energy in particular include its huge potential, low

environmental impact and availability around the globe. In order to harvest that energy,

however, engineers have to overcome, among others, the corrosive sea environment and

the unpredictable storms as well as secure funding for research and development. A lot

of effort has been put into building and testing WECs after the oil crisis in the 1970s

and is now being used as a starting point to create the modern alternatives to fossil fuels.

The purpose of this thesis is to analyze the available winch-based wave energy

converters (WECs) in comparison with other devices of similar scale, such as linear and

hydraulic point absorber WECs. First, an introduction to the physics and geography of

waves is presented, covering wave formation and particle motion. Then a number of

designs, their working principles and history are described – oscillating water columns,

floating or bottom-fixed devices using translational or rotational motion. Environmental

and economic effects in the long term have to be taken into account. Narrowing down to

point absorbers, different power take-off systems are available – linear, hydraulic or

winch-based, each of them with specific advantages and disadvantages. Focusing on

winch-based systems, three different concepts are described – the counter weight, the

counter buoy and the Lifesaver devices. A set of comparison criteria, covering

technical, economic and environmental aspects of the device performance, is prepared.

Using this set, the three concepts are compared and results are analyzed.

Keywords: winch, PTO, wave, energy

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Examensarbete MMK 2017:04 MKNB 090

Jämförande analys av vinschbaserade vågenergikonverterare

Aleksandar Nachev

Godkänt

2017-04-20

Examinator

Ulf Sellgren

Handledare

Ulf Sellgren

Uppdragsgivare

KTH Maskinkonstruktion

Kontaktperson

Ulf Sellgren

Sammanfattning (in Swedish)

Förnybara energikällor är förmodligen människans framtid. De viktigaste

argumenten för just vågenergi är att vågenergi har en särskilt stor potential, en liten

miljöpåverkan och att den finns tillgänglig i hela världen. För att kunna skörda denna

eneri måste ingenjörerna emellertid övervinna den korrosiva havsmiljön och de

oförutsägbara stormarna, samt säkerställa finansiering för forskning och utveckling.

Efter oljekrisen på 1970-talet, har stora ansträngningar har gjorts för att bygga och

prova WEC och det pionjärarbetet används nu som utgångspunkt för att skapa moderna

alternativ till fossila bränslen.

Syftet med denna avhandling är att analysera de tillgängliga vinschbaserade

vågenergiomvandlarna (WEC) och att jämföra dessa med andra liknande enheter, såsom

WEC-linjer med linjära och hydrauliska system (WEC). Först ges en introduktion till

vågornas fysik och geografi, en beskrivning som innefattar vågbildning och

partikelrörelse. Därefter beskrivs ett antal konstruktionslösningar, deras arbetsprinciper

och historia - oscillerande vattenkolumner, flytande eller bottenfasta enheter, som

baseras på translaterande- eller roterande rörelser. Miljömässiga och ekonomiska

långtidseffekter måste beaktas för denna typ av system. Om vi begränsar oss till

punktabsorberare (PTO), så finns olika typer av kraftuttagssystem tillgängliga - linjära,

hydrauliska eller vinschbaserade, var och en med specifika fördelar och nackdelar. Med

fokus på vinschbaserade system beskrivs tre olika koncept - motvikt, motverkande boj

och ”Lifesaver”-enheter. En uppsättning jämförelsekriterier, som täcker tekniska,

ekonomiska och miljömässiga aspekter av enhetens prestanda definieras. Med hjälp av

dessa kriterier jämförs de tre koncepten och resultaten analyseras.

Keywords: energi, vinsch, PTO, vågenergi

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List of figures

Figure 1: Wave formation diagram.

Figure 2: Simplified wave model

Figure 3: Deep and shallow water waves; water particles motion.

Figure 4: Six degrees of freedom.

Figure 5: Classification of the spectrum of ocean waves according to wave period.

Figure 6: World wave energy resource.

Figure 7: Seasonal variation in wave energy intensity.

Figure 8: Wells turbine schematic.

Figure 9: Shoreline oscillating water column (OWC) plant.

Figure 10: Backward Bent Duct Buoy.

Figure 11: WEC classification according to orientation to incoming waves.

Figure 12: Singe body WEC.

Figure 13: Two body WEC.

Figure 14: Wave Star WEC – an example of a multi body system.

Figure 15: Salter’s duck.

Figure 16: Bottom hinged WEC.

Figure 17: Bristol cylinder.

Figure 18: Tapered channel, Norway.

Figure 19: Wave dragon over-topping device.

Figure 20: Hose-pump WEC.

Figure 21: Pelamis WEC.

Figure 22: Bottom-fixed heaving WEC – the Archimedes wave swing (AWS).

Figure 23: Pendulor WEC.

Figure 24: Counter weight WEC.

Figure 25: Pulley-gear system.

Figure 26: Counter buoy device configuration.

Figure 27: Capstan equation.

Figure 28: BOLT Lifesaver.

Figure 29: Winch-based PTO.

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List of tables

Table 1: WEC classification according to location.

Table 2: Sea state and device parameters.

Table 3: Environmental impact of WECs.

Table 4: Cost comparison between Lifesaver and the next-generation device by FO.

Table 5: Winch-based WECs comparison.

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Abbreviations

CB – counter buoy

CCL – creative commons license

CW – counter weight

ECMWF – European Centre for Medium Range Weather Forecasts

EPRI TAG – Electric Power Research Institute Technical Assessment Guide

FO – Fred.Olsen

LIMPET – Land Installed Marine Power Energy Transmitter

OWC – oscillating water column

PGS – pulley gear system

PTO – power take-off

SWL – still water level

WEC – wave energy conversion (converter)

List of symbols

A – wave amplitude

d – distance to seabed

D – buoy diameter

H – wave height

Hs – wave height

L – wavelength

T – wave period

Te – energy period

P – designed power

Pw – wave power

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Contents

Acknowledgments 3

Abstract 4

Sammanfattning (in Swedish) 5

List of figures 6

List tables 7

Abbreviations 8

List of symbols 8

1. Introduction 11

1.1 Background 11

1.2 Motivation 12

1.3 Challenges 13

1.4 Thesis outline 15

1.5 Methodology and delimitations 15

2. Frame of reference 16

2.1 Wave physics 16

2.1.1 Wave formation 16

2.1.2. Wave structure 17

2.1.3. Water particle motion and relation to WEC motion 19

2.1.4. Types of waves 20

2.2. World wave energy resource 21

2.3. WECs classification 25

2.3.1. Classification according to location-based 25

2.3.2. Classification according to energy extraction method 26

2.3.2.1. Oscillating water column – OWC 26

2.3.2.1.1. Fixed OWC 27

2.3.2.1.2. Floating OWC 28

2.3.2.2. Oscillating bodies 29

2.3.2.2.1 Translational 30

2.3.2.2.2. Rotational 33

2.3.2.2.3. Mixed 35

2.3.2.3. Other 36

2.4. Existing WECs around the world 37

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2.4.1. Europe 38

2.4.2. North America 42

2.4.3. Asia 43

2.4.4. Australia 44

2.4.5. Alternative uses of WECs 44

2.5. PTO systems in point absorbers 44

2.5.1. PTO systems 44

2.5.2. Alternative to electric generators 46

2.6. Control mechanisms 47

2.7. Existing winch-based PTO systems 48

2.7.1. Counter weight 48

2.7.2. Counter buoy 52

2.7.3. Lifesaver 54

2.7.4. Aquaharmonics 57

2.8. Environmental impact 58

2.9. Economy of WEC 61

3. Comparative analysis of winch-based PTO systems 63

3.1. Comparison criteria 63

3.2. Comparison and analysis of existing WECs 64

4. Conclusion and future work 67

5. Bibliography 68

Appendix 1 – Coefficients of importance 71

Appendix 2 – Fred. Olsen design guidelines 73

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1. Introduction

1.1. Background

The main renewable source of energy on Earth is the Sun. All forms of

electrical, mechanical or chemical energy are converted from solar radiation. In recent

years, mankind has become more aware of the harmful effects of fossil fuels on the

planet (air, water and soil pollution); hence, a global tendency has emerged to replace

fossil fuels with cleaner renewable energy sources. Even though wave energy converters

(WECs) are not as common as wind turbines yet, there is huge potential for energy

extraction. The global theoretical energy wave power resource corresponds to is

approximately 8*106 Twh/year (Rodrigues, 2008; Brooke, 2003). Producing the same

amount of energy using fossil fuels would result in releasing 2 million tones of CO2 into

the air (Rodrigues, 2008). Even though only 10 – 15 % of this energy is practically

available due to various reasons such as location or extraction efficiency (Rodrigues,

2008) this could still be a major contributer to global energy supply.

A French man named Girard and his son were the first people to attempt

capturing wave energy. They obtained a patent in 1799 and since then more than 1000

different devices were patented (McCormick, 1981; Ross, 1995).

The first of the modern technologies were developed by Yoshio Masuda (1925-

2009). A former Japanese navy officer, Masuda started his research around 1940s and

developed a navigation powerbuoy that used air turbine and could be classified as

floating oscillating water column (OWC). It was commercialized in Japan, 1965 (and

later in the USA) and a similar but much larger device, Kaimei, was built and tested in

the 1970s (Falcão, 2014).

After the oil crisis in 1973, a lot of effort was made in research and

development of large scale renewable energy systems. A landmark paper was published

by Stephen Salter, University of Edinburgh, in 1974 that described the ‘duck’ - a wave

energy system using rotating gyroscopes (Salter, 1974) . A year later, the British

Government started a R&D programme in wave energy. The Norwegian prototype plant

at Toftestallen started feeding electricity into the grid in 1985 and a formal programme

was initiated in Denmark in 1990s (Falcão, 2014). By the year 2000, major projects

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such as Islay LIMPET (UK) and Pico OWC (European wave energy project) were

already connected to the grid. (Brooke, 2003).

In order be able to better describe the wave energy resource, studies started in

the countries developing wave energy technologies, e.g. UK. In 1991, a study by the

European commission was started to review the background on wave theory required

for the exploitation of the resource. The result of this study, called WERATLAS

(European Wave Energy Atlas), remains the basic tool for wave energy planning in

Europe.

1.2. Motivation

Wave energy converters have been around for more than 200 years and yet there

are few commercially competitive designs. Despite the fluctuating intensity of the

funding in the field, however, constant research has been done since 1940s. DuPlesis

(2012) lists some of the advantages of wave energy (based mainly on Clement, 2002,

and Ocean atlas) in the list below:

Wave energy has the highest energy density among renewable energies;

There is a limited negative impact on the environment;

The natural seasonal variability follows electricity demand;

There is a negligible demand on land use;

The energy supply is secure;

Wave energy power plants may serve as artificial reefs;

WECs will stimulate declining industries such as shipbuilding, job creation in

construction, maintenance and operations, etc., leading to local economic

development;

Waves can be generated at a location and travel long distance with negligible

energy losses hence allowing for the usage of WECs around the globe

(advantage over wind and solar energy);

There are no carbon dioxide emissions during operation;

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Behind the WEC may exist a calm-sea area that could be used for the

development of aquaculture or recreational activities;

WECs may be used on remote islands, as a power source for navigation buoys,

or water pumping for fish farms;

WECs can be built into harbour walls, therefore having the dual uses of the

protection of harbours and the generation of electricity;

WECs have long operation life time;

A WEC may reduce grid stability problems as wave energy converters may

make use of synchronous generators that allow reactive power control.

1.3. Challenges

Despite the abundance of energy in the oceans, it is not an easy task to harvest it.

Here are some of the reasons:

In terms of the wave resource:

Waves’ phase, period and direction vary with location, season, time of the day

or because of other unpredictable factors (storms, earthquakes, etc) (Falcão,

2014) which makes waves an intermittent source of energy;

Due to the complexity of the phenomenon, theoretical models are not able to

perfectly predict wave behavior which in turn leads to imperfect designs;

Periodicity of waves can reach 50-years for a particular site (Clement, 2002);

meanwhile, wave energy-specific data has been actively collected for only a few

decades; more time and data is needed;

The irregular, slow motion of the waves is incompatible with the conventional

electrical generators that require a few hundred times greater frequency.

(Clement, 2002).

In terms of the energy extracting technology:

Real ocean tests or wave tanks are required for the development and testing of

WECs. Either of those is costly and hence makes design iterations expensive.

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Ocean tests are also constrained by strict weather windows for maintenance,

repair and rebuild. Wave tanks are not easily available and might slow down the

testing process (Sjolte, 2014);

Unlike ships, WEC are usually fixed at a specific site and have to be designed

for the worst case scenario. This could mean loading as high as 100 times the

average. If structural strength is underestimated, the device can be easily

damaged. An overestimation, on the other side, leads to unnecessary high costs.

(Clement, 2002);

In order to maximize efficiency, each device is designed based on the most

frequent waves for a specific location. This is an obstacle for mass production

and relocation of devices hence keeps the prices higher (Drew, 2009);

Except for the high irregular loads, corrosive sea water, high pressures or UV

radiation might be additional constrains that have to be accounted for (Drew,

2009);

The incoming waves have a specific range of wavelengths and periods, which

limits the maximum power of most of the WEC technologies (Sjolte, 2014);

As the energy generation is likely to vary in intensity usually some kind of

energy storage and power smoothing systems are needed along with other

components of the WEC.

Since many factors have to be taken into account (efficiency, survivability, cost)

and the technology is yet immature, the overall design is a difficult task.

Construction and maintenance may be challenging offshore.

In terms of compatibility to the grid:

Wave power offshore is a few times higher than near shore areas. However,

offshore devices need transmission cables to connect the WECs to the grid. That

increases the capital and maintenance costs (Du Plessis, 2012);

Due to high design and production costs most of the current WECs are not

competitive on the current energy market (Clement, 2002);

Fluctuating power from WECs have to be adapted to the grid (Du Plessis, 2012);

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Other:

Legal issues might arise if more than one party would like to use the same

resource (once a wave passes through a WEC, its energy is transferred to the

device hence a hypothetical device down the road will receive less energy);

Lack of funding / investors due to the high risks and low investment return rate;

Some of the WECs are not powerful enough and a group of several devices has

be deployed (creating a wave energy farm) in order to reach the desired power

rating; this would require additional work and funding;

WECs might affect marine life, shipping, tourism and fishing.

1.4. Thesis outline

The purpose of the thesis is to review the current state wave energy conversion

with a focus on winch- (or drum-) based devices; to compare and analyze those devices

to other similar scale devices; and answer the following research questions:

What are the advantages and disadvantages of the winch-based systems

compared to linear systems?

What are the design requirements of a winch-based PTO.

1.5. Methodology and delimitations

The methods used in the creation of this thesis include collection, analysis, and

restructuring of existing knowledge in the forms of paper-based and electronic souses;

as well as critical analysis and comparison of different concepts.

The thesis will not include detailed design, construction or testing of any

devices.

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2. Frame-of-reference

2.1. Wave physics

2.1.1 Wave formation

Wave formation starts with solar radiation. Due to the uneven heating of the

Earth’s surface, volumes of air expand and pressure differential is created. The

differential is the reason for the air flow also known as wind and the friction between

the water and air transforms the motion into small waves, called capillaries. Provided

favorable conditions are present, the waves grow bigger accumulating more and more

energy. Hence, wave energy can be seen as concentrated solar energy: from about 1000

W / m2 originally to between 10 - 100 kW per meter crest length (Du Plessis, 2012).

The three main factors governing the wave size are the wind speed, the length of

time during which the wind acts on the water surface and the ‘fetch’ - the distance over

which the wind blows (Figure 1). The winds creating a wave front can be either random,

such as a storm, or regular, such as trade winds. The presence of these constant winds

leads to locations with more energetic wave climates such as South-West coasts of

South Africa, South America, Australia, New Zealand, Europe and the North-West

coasts of North America (Du Plessis, 2012).

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Figure 1: Wave formation diagram

2.1.2. Wave structure

Real ocean waves are a complex phenomenon. In this thesis will be considered a

simplified model of the waves where they are two-dimensional in x-z plane (Figure 2),

also called ‘plane waves’ as their front moves in parallel planes. Since only

axisymmetrical buoys will be analyzed, this model will be sufficient.

A wave is described by two main parameters: period (T) and wave height (H).

Other parameters (wavelength, speed) could be derived from those. The shape of the

water surface could be represented by a cosine function depending on the time and

distance from a reference point, η(x,t). Still water level (SWL; z = 0) represents a

perfectly calm sea. The highest point of the wave is called crest and the lowest – trough.

The distance between the SWL and the crest is called amplitude (A) and the difference

between the crest and trough is the wave height (H) (note that H = 2 A). Wavelength (L)

is the distance between two equivalent points on the curve, for example, two

neighboring crests. The distance from the seabed to SWL is d.

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Figure 2: Simplified wave model

Real seas contain waves with random height, period or direction and remain

constant for only a short period of time. Using statistical approach, a sea state can be

described by analyzing the whole wave spectrum. The resultants are significant wave

height (Hs) and energy period (Te). Significant wave height is the average height of the

highest one-third waves; energy period is the mean wave period compared to the

spectral distribution of wave energy transport (Brooke, 2003).

To determine the power that waves contain the following relationship for deep

water waves given by Brooke will be used:

Pw = 0.5 Hs2

Te [ kW/m ] (Eq. 1)

where Hs is the significant wave height, Te is the energy period and Pw is the wave

power. Alternative sources give different coefficient in front of H, most commonly 0.42

(Rodrigues, Baris).

It can be seen that the wave height has significant part in the amount of energy

in the wave. However, if the wave steepness, H/L, is greater than 1/7 the wave

becomes unstable and breaks which limits the energy that can be carried.

Waves are differentiated according to the ratio between wavelength and water

depth (Figure 3):

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if d > L / 2 waves are called deep water waves. It is assumed that seabed has no

effect on the wave behavior in this case as wave motion decreases exponentially

with depth; at d = L / 2 motion is negligible;

if d < L / 20 waves are called shallow. With increasing depth waves vertical

motion decays and only horizontal motion is present at seabed;

if L / 20 < d < L / 2 waves are called intermediate.

In the near shore regions wave height and period are significantly lower

compared to offshore waves. From an energy perspective it is preferable to locate a

WEC offshore. Hence only deep water waves will be considered in this thesis.

Figure 3: Deep and shallow water waves; water particles motion.

2.1.3. Water particle motion and relation to WEC motion

As can be seen on Figure 3, water particles move along circular paths. There are

both vertical and horizontal forces (functions of x and t), the combination of which

would act on any body submerged in the water. The intensity of these forces and

therefore the displacement of the water particles decreases with depth hence at the SWL

forces are greatest. In general, a floating body has 6 degrees of freedom – 3 translational

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and 3 rotational (Figure 4). For the purposes of this thesis only heave, surge and pitch

will be considered.

Figure 4: Six degrees of freedom.

2.1.4. Types of waves

As wind blows over the water surface and gravity acts as a restoring force, the

wave height and period increase. Starting with capillary waves (negligible wave height,

period ~ 0.1 sec), then ‘chops’, ‘swells’ (waves traveled away from the wind generation

zone; they have the highest wave height and hence carry the greatest amount of energy;

these are the primary target of the WECs) and finally ‘seiches’ (relatively long period,

1-2 min). Tsunami and tide are not wind-generated waves; even though they have

significant wave height, they appear rarely and will only be considered as the ‘worst

case scenario’ in the design process.

An important point to mention is the ability of the waves to travel long distances

with losing very little of their energy. This means even if there are no winds on a

particular location waves can still be present and hence WECs used.

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Figure 5: Classification of the spectrum of ocean waves according to wave period. Image: Walter H.

Munk. CCL

2.2. World wave energy resource

The power of ocean waves is usually described in kW/m wave crest. Using

Eq. 1, only significant wave height and energy period are required to determine the

power. The value refers to the average power that passes through cross section one

meter in width and infinite depth, perpendicular to the wave length. Typically, a good

location will generate between 20 – 70 kW/m (Baris). Depending on the type of WEC,

between 20% and 50% of that energy can be captured. However, point absorbers have

the ability to resonate with the waves and can capture up to 6·π times their width with

respect to the incoming energy (Sjolte, 2014).

There are several ocean atlases that can be used as data sources. In the case of

offshore projects, the most commonly used is WERATLAS, the European Wave

Energy Atlas – a European Commission funded project, focused on North-Eastern

Atlantic Ocean and the North, Norwegian, Barents and Mediterranean Seas. It uses

wind-wave numerical simulations validated by in situ measurements in questionable

ares. Another atlas is Furgo OCEANOR. They use data from the European Centre for

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Medium Range Weather Forecasts (ECMWF) which is then validated and calibrated

against satellite data. OCEANOR offers offshore wave parameters and full directional

spectra time series data as well as up to 50-years associated statistics.

As waves approach the shore their characteristics are modified by seabed effects,

refraction, diffraction or sheltering by islands. Therefore, detailed maps for specific sites

are usually produced only when needed. An exception is ONDATLAS which covers the

Portugal coast that has a relatively straight bottom profile. Due to its location and

climate, Ireland can meet 75% of its electric needs from wave energy. The Ireland's

marine renewable energy atlas covers small area around the island, focusing on the

Atlantic coast, and is updated hourly. Information about waves’ height, period, mean

power, etc is available.

The wave energy distribution is uneven across the globe. The highest values for

wave energy are between 40 – 60 degrees in both hemispheres (Figure 6). The southern

hemisphere, however, is subject to much lower seasonal variation and hence could be

described as more resourceful. For a more detailed picture we should mention the

British Isles, Iceland, Greenland, and somewhat lower energy levers at the western coast

of the US and Canada in the north; in equatorial waters power levels vary between 15 –

20 kW/m annually, highest off Peru and Ecuador; south: Southern Chile, South Africa

and the south coasts of Australia and New Zealand.

According to Cruz (2008) the points with highest levels of wave energy are

about 48°S 90°E with annual average above 140 kW/m; and 57°N 21°W with 90 kW/m.

Both of these locations, however, are too far away from the shore and the water depth

exceeds what is currently technologically practical – 50 – 60 m depth.

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Figure 6: World wave energy resourse. Image: Cruz, 2008.

Wave intensity varies along the year, differently for north and south hemisphere.

A point to note is that higher energy levels are present during the winter period which

matches with the higher electricity demand.

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Figure 7: Seasonal variation in wave energy intensity. Image: Cruz, 2008.

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2.3. WECs classification

2.3.1 Classification according to location-based

In this thesis the WECs will be classified according to location and working

principle. This classification does not pretend to be the only correct one or to cover all

aspects of the devices but it gives an overall idea about the similarities and differences

between the devices.

According to location there are three types of WECs: shoreline, near shore and

offshore. Each of them is characterized in Table 1 below.

Table 1: WEC classification according to location.

Shoreline (fixed to or embedded in the shoreline):

Advantages Disadvantages

close to utility network;

easy to build and maintain;

lower chance of damage due to severe waves;

coastal structures can support bigger and heavier

devices hence cheaper materials can be used;

no need for long transmission cables or moorings;

presence of ‘hot spots’ - local high energy regions

created by diffraction and refraction near shore (if

location is carefully chosen).

near shore waves carry less energy;

tidal range can be an issue;

devices are site specific and require

knowledge of the shoreline

geometry and geology and cannot

be designed for mass

manufacturing;

design must comply with

preservation the coastal scenery.

Near-shore (devices in water shallower than ¼ wavelength (Duckers, 2004)):

Advantages Disadvantages

usually attached to seabed which gives a good

stationary base.

shallower waters allow for less

energy harvest.

Offshore (devices in water deeper than 1/3 wavelength (Falnes, 2007); 40 m and deeper):

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Advantages Disadvantages

deep waters offer higher energy content for

extraction;

deep water floating devices are said to have

greater structural economy (Korde, 2000).

harder construction and

maintenance;

devices have to be designed for

extreme weather conditions which

adds to their cost.

2.3.2. Classification according to energy extraction method

In order to be able to extract energy from the oscillating waves, a WEC must

have a stable reaction point. Depending on the design this could be the seabed or a

specific part of the device.

2.3.2.1. Oscillating water column – OWC

Oscillating water columns use the motion of the waves to run water or air

turbines. Commonly used is the Wells turbine, developed in 1976 by Alan Wells in

Queen’s University, Belfast, UK. The main advantage of the turbine is that it works

regardless the flow direction. This, however, leads to lower efficiency because of the

large angle of attack to the blades.

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Figure 8: Wells turbine schematic. Image: Dominik Lenné, CCL.

Alternatives to the Wells turbine are the I.A. Babinsten’s self-rectifying impulse

turbine and a device by G.D. Filipenco, (US patent number: US3912938 A), both

patented in 1975 (McCormick, 1981).

2.3.2.1.1. Fixed OWC

OWC can be either fixed or floating. In the first case, a partly submerged

concrete or steel structure creates a chamber at the end of which is the air turbine. As

incident waves reach the device, water level in the chamber rises, pushing the trapped

air and creating a flow through the turbine. The rotation of the turbine is turn drives an

electrical generator.

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Figure 9: Shoreline oscillating water column (OWC) plant.

Due to the relatively easier construction compared to open sea devices, fixed

OWC were one of the first types of WEC to reach the national grid as early as 1980s

(Falcão,2014). Full scaled projects were developed in Norway (in Toftestallen, 1985),

Japan (in Sakata, 1990), India (in Vizhinjam, 1990), Portugal (Pico, On the Azores,

1999) and the UK (LIMPET in Islay, Scotland, 2000). The biggest of all such projects,

called OSPREY, was destroyed by the see shortly after being deployed near the Scottish

coast in 1995. The installed power capacity of such devices usually varies between 60

and 500 kW (reaching 2 MW for OSPREY (Falcão,2014)). OWC can be fixed to

breakwaters. This benefits both sides during the construction.

An improvement to the OWC design was the introduction of a collector that

would increase the area of operation of the OWC. A large parabolic-shaped collector

was developed by the Australian company Energetech and a prototype (Oceanlinx Mk

1) was tested in 2005.

2.3.2.1.2. Floating

An alternative to the fixed OWC are the floating ones, located in near-shore

waters where wave energy is greater. First of those devices was developed by Yoshio

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Masuda in Japan in the 1970s. The barge, called Kaimei, had thirteen open-bottom

chambers built into the hull, with a water plane area of about 45 m2 each. During the 3

year testing period a variety of air turbines were tested, both self-rectifying and one-

directional with additional rectifying valves. Along with Japan as the lead partner,

contributors from the UK, Canada, Ireland and the USA were involved.

A later project of Masuda was the Backward Bent Duct Buoy (BBDB). It

consisted of a tube with the air turbine at the end and a buoyant body to keep the device

afloat.

Figure 10: Backward Bent duct Buoy.

A 1:4th-scale model of the BBDB has been tested in Galway Bay, western Ireland since

2006. It was equipped with a horizontal-axis Wells turbine and later upgraded to an

impulse turbine.

2.3.2.2. Oscillating bodies

Depending on their orientation to the incoming waves, devices can be described

as attenuators, terminators or point absorbers. Attenuators lie parallel to the

predominant wave direction and ‘ride’ the waves. An example is a ‘sea snake’-like

device called Pelamis. Terminators are devices with their principal axis perpendicular to

the predominant wave direction, intercepting the waves. Wave roller is an example. The

third type, point absorbers, are small compared to the wave length and hence the wave

direction is not important. An example – Powerbuoy.

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Figure 11: WEC classification according to orientation to incoming waves.

2.3.2.2.1 Translational

Using the heave or surge motion, the translational devices can be single-body or

multi-body devices.

Single-body WEC use the seabed or a fixed structure as a reference point. They

can be submerged (Archimedes wave swing) or floating, using linear or rotary

generators, purely mechanical or hydraulic PTOs (see chapter 2.5.1.). Such devices have

been developed around the world (Denmark, Sweden, USA) as early as 1990. Because

they are relatively smaller compared to fixed OWC, lower rated power can be achieved.

Often multiple devices are combined in a ‘farm’ to reach a certain power output.

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Figure 12: Singe body WEC.

Multi-body systems use the motion of one body relative to another to extract

energy. For example, Power buoy has a floater moving against a heave plate and hence

driving a generator (Figure 13). There is no need for bottom-based components such as

support structures or heavy duty moorings, which is an advantage. However, control

issues may rise as the dynamics is more complex. Other devices using similar principle

are IPS Buoy and Wavebob.

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Figure 13: Two body WEC.

A different approach to the multi-body concept has the WaveStar. The device is

consists of two rectilinear arrays of floaters located on both sides of a bottom-standing

platform. The waves make the buoys heave up and down pumping oil in a hydraulic

system. Scale models were deployed in Denmark in 2006 and 2009. Sinnpower,

founded in 2014, has a similar concept but the structure is floating. Multiple modules,

each comprising a buoy, a rod and reaction structure, are connected in a steel matrix. As

the front rows heave up, the rear heave down, creating relative motion against each

other. The company has projects in Greece, India and the Caribbean sea.

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Figure 14: Wave Star WEC – an example of a multi body system. Image: WaveStarEnergy

2.3.2.2.2. Rotational

The main types of devices using rotational motion are either pitching floaters or

bottom-hinged systems.

In the case of pitching devices most of the energy comes from relative rotation

of the devices. A well known example and one of the first constructed pitching devices

was the ‘nodding duck’ developed by Stephen Salter in Edinburgh in 1970s. Many of

these rather small devices (less than a meter in diameter and 6 m in length) are

connected in a row and the relative rotation between the inner circle and the outer

floater creates electricity.

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Figure 15: Salter’s duck.

The bottom hinged devices are mostly terminators (Figure 16). They use the

horizontal force of the incoming waves to drive a generator or a hydraulic pump. An

example could be the Wave roller, developed by AW-Energy Ltd.

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Figure 16: Bottom hinged WEC.

2.3.2.2.3. Mixed

Some designs try to maximize the energy extraction and include both rotational

and translational motion. The Bristol cylinder, developed in 1970s by D. V. Evans

comprises of a submerged cylinder connected to hydraulic rams that absorbs motion in

both vertical and horizontal direction.

36

Figure 17: Bristol cylinder.

2.3.2.3. Other

Over-topping devices are deigned to collect ocean water and use it to run

conventional low head water turbines (such as the Kaplan turbine). In the case of

Norwegian Tapered channel (or Tapchan) incoming waves are directed into a reservoir

through a narrowing channel – the collector. The reservoir is held above mean sea level

so that a pressure head is created for the turbine to run. The design was created in 1980s

in Norway and a prototype was tested between 1985 and 1991.

There are alternative designs that use the similar over-topping principle but on a

floating platform (e.g. Wave Dragon).

37

Figure 18: Tapered channel, Norway.

Bombora wave power, an Australian ocean energy company, founded in 2012,

has developed a membrane pump, called mWave. The device consists of a elongated

bottom fixed structure with inflatable rubber membranes, positioned at an angle to the

wave front. As waves pass over the device, the rubber membranes force air to circulate

through a generator and then return for the next cycle.

2.4. Existing WECs around the world

Since the oil crisis in 1970s the research in wave energy has never fully stopped,

even though there are waves of enthusiasm and disappointment. The first nations to

engage in the area were Japan in Asia and Ireland, UK, Norway, Sweden, Denmark and

Portugal in Europe. However, along the way many more countries joined and many

devices were designed and tested. The current chapter is an overview of th world’s

effort to tame the waves.

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2.4.1. Europe

DENMARK

Lying on the North sea coast, Denmark has annual resource between 7 and 24

kW/m (Clement, 2002). The Danish wave energy programme started in 1996 after a

successful wind energy experience. The Point Absorber Wave Energy Converter,

developed by Rambøll, has already gone through a few scale models. The devices

consists of a floater that activates a piston pump. The floater is attached to a suction cup

via a polyester rope. Another notable device is the floating over-topping Wave Dragon.

It comprises of a collector and a Kaplan turbine similarly to Tapchan and Oceanlinx.

Figure 19: Wave dragon over-topping device. Image: CC Erik Friis-Madsen.

IRELAND

Ireland is one of the richest countries in terms of wave energy. According to

Clement (2002), ‘total incident wave energy is around 187.5 TWh’ (compared to 30

TWh in Denmark). Due to the available resource, a number of governmental and private

organizations as well as universities are focused on wave energy research. Focus on

wave and climate prediction methodology.

NORWAY

Norway has a long west coast exposed to the Atlantic ocean which gives the

country plenty of sites and wave energy to explore. Its investigation started back in

1970 with Tapchan (see chapter 2.3.2.3.). In the 1970, Kvaerner Brug AS and the

Norwegian Institute of Technology developed a phase controlled buoy with hydraulic

PTO, funded by Royal Norwegian Council for Scientific and Industrial Research.

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During the development a the hydraulics were replaced by pneumatic system and scale

models were build. Most recently was developed ConWEC (Controlled WEC). The

device explores the combines usage of OWC and a float and targets near-shore to

offshore areas.

PORTUGAL

With annual wave power rates of 30 – 40 kW/m, Portugal is a relatively good

candidate for WECs deployment. Portuguese government supports the R&D of wave

energy through various channels (Ministries of Science, Ministry of Economics) and

universities have been working on wave energy devices since 1978. The country also

hosted the first EU OWC project called PICO and located in the Azores. Rated 400

kW, Pico was designed for both testing and energy supply. The project was built

between 1992 and 1999. Since then the a number of improvements have been made.

However, technical difficulties and unsteady funding made the project stop and start

often. There are plans for future development.

SWEDEN

Although Sweden has less good areas for utilizing wave energy, back in 1976

was founded Wave Energy Research Group including Chalmers University and the

private consultant Technocean. In 1980 was developed the first full scale point absorber

buoy ‘Elskling’ funded by IPS. Another interested design was the Hosepump, tested

between 1983 – 1986. The Hosepump is a two body oscillating system with an elastic

hose between the floater and the damper. Check valves are attached at both ends. As the

system oscillates, the hose’s volume decreases, pumping water through a hydraulic

system which in turn drives a generator. A floating over-topping device, Floating Wave

Power Vessel, was deployed near Stockholm in the 1980s. Although there is not active

research programme today, such programmes were carried out between 1980 – 1986

and 1989 – 1996. Nowadays, active Swedish companies on the market are IPS (that

developed IPS buoy), Sea Power International and CorPower (founded 2009 by Stig

Lundbäck).

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Figure 20: Hose-pump WEC.

UNITED KINGDOM

Similarly to Ireland, UK has a great wave energy potential on its west coast. The

R&D of wave energy started shortly after 1973 oil crisis with S. Salter and his nodding

duck. Meanwhile more that ten other wave energy projects were initiated (Clement,

2002). The government support was strongest between 1973 – 1983 and 1999 – 2003

(R&D received about £3 million for the period).

One of the well known WECs – Pelamis – is also located in the UK. The 130 m

long sea snake is a terminator that uses hydraulic rams inside hinged joints. A 375 kW

device is being developed by Ocean Power Delivery Ltd.

Figure 21: Pelamis WEC. Image: Wikipedia.

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Located on the Isle os Islay, LIMPET (Land Installed Marine Powered Energy

Transformer) is a modular OWC. Using the experience from previous projects,

LIMPET consists of three water columns and is equipped with two counter rotating

Wells turbines, each rated 250 kW.

Two main structures support the work on wave energy R&D – Marine Energy

Technology Network (METN), serving as a network for universities, companies,

consultants etc; and the ‘Sea Power Association’ working closely with METN and the

British Wind Energy Association (BWEA).

OTHER

Although most of the other European countries have less contribution to the

wave energy research in the recent years, there are some projects worth mentioning.

Archimedes Waves Swing (AWS) is a Dutch submerged heaving system that utilizes

the variation in hydrostatic pressure as waves past above. Developed by Teamwork

Technology BV, a 2 MW prototype has been completed.

Figure 22: Bottom-fixed heaving WEC – the Archimedes wave swing (AWS).

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Despite the number of projects that were undertaken in France during the

1980s, the country has reoriented to dynamic absorption problem solution (the

development of wave absorbing devices – paddles) and optimal control systems for

OWC devices.

Greece has a 16 000 km long coast and despite the rather low energy waves

there are a number of hot spots thanks to the many islands. There is also WEC market

since many of these islands rely on diesel engines (Clement, 2002). However, wind

energy has already taken over the governmental support and all research in wave energy

systems is mainly done in universities or the Centre for Renewable Energy Sources.

Similarly to Greece, Italy has relatively low energetic coastlines and R&D focus

is at universities. Two devices to be mentioned are the ISWEC (Inertial Sea Wave

Energy Converter), developed in University of Rome, which is suitable for closed sea

applications where the wave power is low to moderate. The second, ENERMAR, is

worked upon by Ponte di Archimede nello Stretto di Messina and utilizes marine

currents. Currently a 130 kW prototype is being constructed.

2.4.2. North America

Despite good energy climate on the west coast, the USA has rather low interest

in WECs. In the 1990s the country invested in building and testing the McCormick air

turbine as part of the International Energy Agency’s contribution to Kaimei. McCabe

Wave pump and the Ocean Power Technologies are currently the two bigger projects in

the country. McCabe Wave pump is a pitching device that consists of forward, central

and aft pontoons, hinged together. Hanging below is a damper plate (multi-body

device). The incoming waves and the damper plate make the hinged pontoons rotate

against the central one. A prototype was deployed in Ireland in 1996.

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2.4.3. Asia

India, Sri Lanka and China focus mainly on OWC. China currently tests

BBDB converters. S.D.E. Ltd. in Israel has developed a device that utilizes both the

kinetic and potential energy of a wave to generate hydraulic pressure and hence

electricity.

Japan was one of the first countries to research WEC. A project from the 1980s

called Pendulor (Figure 23) is one of the first hinged terminator type WEC. Currently,

except for the relatively small unmanned buoys that are being charged by wave power,

there is only one big project – the Mighty Whale. The Mighty Whale (50x30x12 m,

displacement 4400 t), a device developed by Japan Marine Science and Technology

Center, consists of a three air chambers located side by side at the front and buoyancy

tanks. Each air chamber is connected to a Wells air turbine that drives an electric

generator with total rated power of 110 kW. The device was deployed in 1998 and

tested for several years.

Figure 23: Pendulor WEC. Image: MDPI

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2.4.4. Australia

Being completely surrounded by water, Australia has the conditions for wave

energy devices. However, because of the relatively low energy demand there is very

little done on wave energy. A project to be mentioned is the OWC by Oceanlinx. A few

pototypes were tested at Port Kembla since 2005 (Figure 10).

2.4.5 Alternative uses of WECs

Wave energy devices are not competitive on the general energy market yet due

to the expensive development and capital costs. However, WECs might be a cheaper

alternative for some isolated islands where the main source of electrical power comes

from diesel engines. Their substitution with WECs will have positive effects on the

economy, environment and overall well-being of such areas. More, instead of electrical

energy, WECs can be designed to produce fresh water for the household, livestock and

crops. This can be achieved by modifying a hydraulic system to create the pressure,

required for reverse osmosis, and desalinate sea water. The McCabe wave pump is an

example of a device designed to produce potable water (Brooke, 2003). Alternative uses

of the WEC might be aeration of stagnant bottom water, a reserve power source in sea, a

recharge station offshore for buoys or other autonomous devices (Brooke, 2003).

2.5. PTO systems in point absorbers

2.5.1. PTO systems

Point absorbers are usually devices relatively small compared to a wave length.

They can be floating or bottom fixed, usually located near- to offshore. Particularly

important component is the power take-off (PTO) system which is responsible for

converting the wave energy into electrical energy. Usually the PTO consists of a

45

primary and a secondary conversion system because of the differences in frequency

between waves and conventional electrical generators.

As of today, the most common way to generate electricity is by using rotary

generators with the linear generators being an alternative. One of those two options is

usually chosen for the secondary conversion. There is greater choice regarding the

primary conversion – hydraulic driven generators, rods or cables attached to linear

generators, gearboxes as a link between linear motion and rotary generators, etc. Each

of those, however, has specific advantages and weaknesses, some of which are

described below.

Linear generator systems advantages:

allow for direct conversion of the waves’ heaving motion into electricity;

no rotating parts or high pressure valves.

Disadvantages:

end stops problem: as the system oscillates, the moving component of the linear

generator exerts forces on the fixed component at every cycle, which leads to

fatigue; in the case of an extremely strong waves, the system can be damaged

(Figure 12);

rods buckling/bending (especially if side loads are present).

Cable and drum systems advantages:

no end stops;

direct conversion of rotary motion to electricity.

Disadvantages:

drum diameter is directly related to cable bending and hence fatigue; however,

larger drums are heavier and require bigger buoys;

the drum has to be rewound at the end of every cycle in order to start the next.

Hydraulic systems advantages:

The hydraulic systems are well adjusted to slow, high load forces;

there are less moving parts compared to a purely mechanical system which leads

to longer lifespan for the device;

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hydrostatic systems traditionally have high peak efficiency (< 80 % (Du Plessis,

2012);

most hydraulic components are readily available.

Disadvantages:

because of the high pressures, hydraulic fluid containment is possible;

sea water could be used as a hydraulic fluid, however challenges may rise due to

sealing and leakage, working temperature and pressure, deposition of solids,

biological growth, lubrication or corrosion (Du Plessis, 2012);

despite the high peak efficiency, the frictional loses, leakages and and

compressibility lead to much lower real efficiencies; moreover, it is unlikely that

a device will spend much time working at its ideal conditions due to the

variability of the wave characteristics;

end stops problem.

2.5.2. Alternative to electric generators

The main devices used to convert the mechanical power of the waves to

electrical are the linear and rotary electric generators and some hydraulic motors. A

more recent idea is the usage of piezoelectric materials and dielectric polymers. These

materials, called also ‘smart materials’ (Tanaka, 2015), are able to produce electricity in

result of some mechanical deformation, such as bending, stretching, etc. This

technology usually consists of a base material, such as silicon rubber, that would

provide the mechanical support and durability; and a piezoelectric film, which will

create the electric charge. The lack of complex mechanical devices, such as a generator,

may reduce costs and help the wave energy industry become competitive. Some

research has been carried out, e.g. Tanaka (2015), Zurkinden (2007), Matsuda (2013).

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2.6. Control mechanisms

During the design process specific design parameters (or a narrow range of

those) is selected in order to maximize energy extraction. In the case of point absorbers,

resonance concept can be used to further increase the efficiency of the device. At

resonance, the oscillating motion is matched with the incoming waves resulting in a

substantial energy transfer due to the increase amplitude of oscillation and forces.

(Drew 2009). However, in real seas the conditions vary greatly hence the device’s

oscillating frequency is matched with the waves only in a fraction of the time. To

overcome the problem, engineers have developed different control mechanisms. An

example of an active control mechanism is the latching. The idea, proposed by Budal

and Falnes in 1980, is to stall (i.e. latch) the device until the wave phase is appropriate

for resonance. There could be different levels of tuning, from sea-state to independent

wave control (fast tuning). The mechanism can be only applied to a device whose

natural frequency is higher than the wave excitation frequency. Although this could be

restricting in general, latching control is widely applicable to point absorbers, which

smaller mass usually meets the requirements. Research on latching has been carried out

by Babarit, Clement, Falcão, Valerio, Korde, etc. (Drew 2009).

An alternative is the so-called declutching (unclutching, dubbed unlatching)

where the primary element of the system is allowed to float freely during some parts of

the cycle. The PTO component and hence the energy extraction is only used at the

desired conditions. For example, in the case of a hydraulic system, valves could be open

to allow for free flow without energy extraction. In an experiment including the

SEAREV device, the efficiency was seen to improve by a factor of two for some wave

conditions (Drew 2009).

Reactive loading control is another control mechanism aiming to broaden the

efficiency range both below and above the resonance frequency. The main tool it uses to

maximize energy absorption is the adjustment of the dynamic parameters of primary

converter (spring constant, inertia, damping).

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2.7. Existing winch-based PTO systems

In this section a few variations of the winch-based PTO will be compared. All

devices use electro-mechanical systems. The main difference is in the mechanisms used

to rewind the drum.

2.7.1. Counter weight

The device was developed by Sung-Hoon Han et al. as a cooperation between

Division of Naval Architecture and Ocean Systems Engineering, Korea Maritime and

Ocean University and the Department of Convergence Study on the Ocean Science and

Technology, Republic of Korea. The experiment and its outcomes are presented in the

Journal of Ocean Engineering and Technology in February 2016 (Han, 2016).

Although the paper describes more of a concept than a final commercial design,

the device can be classified as a fixed point absorber with electromechanical PTO (also

based on the prototype tests carried out). It consists of five main components – buoy,

gearbox, counter weight, generator and flywheel – and a support structure.

Figure 24: Counter weight WEC.

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Working principle

The study evaluates only the primary efficiency of the system. Additional

resistance is added to account for the generator. Also, it will be assumed that the support

is a fixed structure hence motion of the pulley/gear system (PGS), flywheel and

generator can be neglected. The buoy has mass greater than the mass of the counter

weight (CW). The two are connected through the PGS so that as the waves lift the buoy

up, the CW sets the generator in motion; as the buoy heaves down, the latter rotates the

generator and lifts the CW, preparing the system for the next cycle. The wave energy is

first converted to torque, rotating the flywheel, and then to electricity, rotating the

generator. There is a possibility to regulate the CW mass which in turn can be used to

optimize the response rate of the buoy.

The novel concept in the design is the bidirectional PGS that allows for energy

extraction no matter which way the buoy moves. This is achieved thanks to clutches in

both pulleys, allowing rotation in one direction and stop rotation in the opposite

direction. The configuration of the pulleys can be seen on Figure 25. The rotation of the

pulleys is translated to the flywheel through a gearbox. The gear ratio converts the slow

heaving motion to a higher speed rotation, required by the generator; the flywheel is

used as an intermediate energy storage which helps to steady the energy fluctuations

from the waves. The CW also is the rewind mechanism for the system.

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Figure 25: Pulley-gear system.

Experimental conditions

The Korean team designed and build a 1:20 scale model which was tested in a

two-dimensional basin (sized at 25 m (L) × 1 m (B) × 0.7 m (D)). The wave parameters

used were obtained from the scaled annual wave parameters for the coast of Jeju Island,

South Korea, namely wave period of 0.9 ~ 2.0 s and wave height of 0.10 m. The

respective buoy diameter was 0.5 m. The buoy hemispherical shape and designed size

were adopted from the Danish Wave Star WEC. Five different combinations of buoy

mass, counter mass and flywheel inertia were tested in order to find the best

combination and mass ratios. (Buoy mass: 9.3 or 18.6 kg; CW mass: 2, 6 or 12 kg;

Flywheel moment of inertia: 0, 0.0038 or 0.0060 kg.m2

). Generally, the best mass ratio

of buoy/CW is 3/2.

Results

For wave periods about 1 s, best power generation was achieved without

flywheel (inertia = 0). In the more extreme conditions of 1,7 – 2 s, the presence of a

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flywheel improved the power generation, however, with lower maximum compared to

the 1st case.

The increase of the buoy and CW masses, maintaining the ratio, leads to

increase in power generation for wave periods between 1 – 2 s, although the effect is

exactly opposite for the lower wave periods.

In the study was also used the concept of capture width ratio (CWR) – the

fraction of the wave energy that is actually converted to the primary conversion system.

This is an estimate of the efficiency of the PTO system. According to the study, the

CWR calculated was 45% (at up-scaled wave period = 5.36 s). The up-scaled power

generated was 95 kW (with the largest buoy and CW masses, wave period 5.36 s). The

equation used to calculate the CWR is given below:

CWR = P / (D x Pw) (Eq. 2)

where P is designed power (dependent on torque and speed), D is the buoy diameter and

Pw is the wave power (dependent on water density, wave height and period).

The study concludes that both power generation and efficiency are strongly

dependent on specific wave conditions. Hence, selecting the optimal masses is crucial

for good results. The findings of the study could be used as a starting point for future

projects including the counter weight concept.

Counter-weight WEC summary:

Advantages:

bidirectional energy absorption;

no energy is wasted rewinding the winch (done by gravity);

equalized energy input to generator thanks to the flywheel.

Disadvantages:

bidirectional gearbox is rather complicated piece of equipment with many

moving parts hence lower durability and higher maintenance costs;

requires a support structure;

pendulum effect: the hanging freely mass can exert unwanted and hardly

predictable motion that can damage the device;

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survivability: the limited length of the cables, supporting the CW, can lead to

damage in the case of a higher-than-expected wave heights; on the other side,

increase in the cable length leads to greater pendulum effect.

2.7.2. Counter buoy

The second device to be presented is very similar to the counter-weight,

however, using a buoy instead of a mass and buoyancy force instead of gravity. The

project was presented at the World Renewable Energy Congress in Linköping, Sweden,

in 2011. It is a joint project between scientists from University of Hawaii, USA, INSA,

France and Chalmers University of Technology, Sweden.

The device is designed as a low cost point absorber with electromechanical PTO

which aims to charge electrical vehicles. It utilizes the heaving motion of the waves and

converts it into electricity through a permanent magnet AC generator. The total cost of a

2 kW WEC is said be less than $ 2000, including the generator and cables (Foster,

2011).

Working principle

The device can be separated into three main components: the floater, the PTO

unit and the counter-buoy. The PTO unit (including the motor/generator, a drum and an

inertial wheel) could be located at the seabed, however, this leads to difficulties related

to leaks, deployment and maintenance. To overcome these obstacles, the designers

suggest to put the PTO unit on a column, fixed to the seabed with a concrete anchor-

base. In this case, the PTO is above the mean sea level which makes it accessible and

does not require watertight housing. Additional structure, however, has to be

present/built.

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Figure 26: Counter buoy device configuration.

The floater is at the water surface and the counter-buoy is submerged

underwater. To allow for undisturbed motion of the two buoys, additional pulleys are

located at the seabed, guiding the buoys away from each other. The buoys are connected

with a single cable that has been turned around the drum enough times to provide

friction and prevent unwanted sliding. Given the tension on both sides of the drum and

the friction coefficient, the capstan equation can be used to determine the number of

turns required. Once this number is reached, the drum will only turn when the floater

oscillates. The floater is the main power generator and the smaller buoy provides the

tension to keep the cable on the drum from sliding and also rewinds the floater’s cable.

As in the counter-weight device, the floater is larger than the counter buoy (and exerts

larger forces). Therefore the counter buoy only moves upward when the cable on the

floater side is slack (which is on the downward floater motion).

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Capstan equation (also called the Eytelwein's formula):

T2 = T1 * exp( μ * θ ) (Eq. 3)

where:

T2 = tension on the counter buoy side (Thold)

T1 = tension on the floater side (Tload)

μ = friction

φ = angle around the drum [rad]

Figure 27: Capstan equation. Image: Wikipedia.

Counter-buoy WEC summary (assuming PTO unit is above mean water level)

Advantages:

easy access to the PTO unit;

good survivability, provided counter buoy has sufficient cable length;

lightweight and low cost buoys.

Disadvantages

requires additional support structure;

(inverse) pendulum effect on the counter buoy;

multiple pulleys will lead to cable fatigue and shorter lifespan.

2.7.3. Lifesaver

BOLT Lifesaver is the latest point absorber device by the Norwegian company

Fred. Olsen. Developed and first tested between 2010 – 2012, the design is based on

more than ten years of experience in wave energy industry. The device was

manufactured in the UK and tested between 2012 – 2014 at Falmouth, UK, and from

2016 onwards near the Hawaii, USA.

The Lifesaver is named after the shape of the hull – a flat, hollow cylinder,

consisting of five separate modules that are assembled together. Each of the modules

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can carry a winch-based PTO system, including a drum, belt drive gearbox and a

motor/generator. In the latest prototype tested only three of the five PTO units are used,

with total power of 30 kW.

Figure 28: BOLT Lifesaver. Image: Fred.Olsen

Working principle

Similar to the previous devices, the Lifesaver is a heaving point absorber with

electromechanical PTO system. A relative motion between the floating body (the hull,

including all PTOs) and the seabed fixed moorings drives the rotation of the drums and

hence the generators. During the downward motion of the hull, generators act as motors

in order to rewind the drums and complete the cycle. Although this requires energy

input and hence decreases the overall efficiency of the WEC, using springs, counter

weight/buoy or hydraulic systems was found to make the device unnecessary complex,

less durable and more expensive. A guiding system is also present in order to negate the

effects of rolling or pitching and prevent cable entanglement or high lateral forces. Once

the energy is collected by the generators, it is transferred to a common DC-bus and then

to the grid. Additional battery bank serves as a long term storage to keep the system

running in the case of a calm sea. The device is equipped with a number of on-board

monitoring systems that deliver real time data and help maintenance and control.

However, the control and optimization of the drum motion in reaction to the ever-

changing sea state is still an issue.

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Figure 29: Winch-based PTO. Image: Fred.Olsen.

Since BOLT Lifesaver is one of the first winch-based WECs so far in its

development, some of the components and mechanism had to be developed during the

design process. Examples of such are:

a several million bend cycles winch line developed in collaboration with Gates

Corporation between 2010 – 2016;

a high ratio, belt based gear box developed in collaboration with Gates

Corporation with the focus on low maintenance requirements, accessibility and

high corrosion resistance;

proprietary software based on the LabView software platform from National

Instruments.

Table 2: Sea state and device parameters.

Parameter Value Unit

Wave height 2.75 m

Wave period 6.5 s

Hull outer diameter 16 m

Hull inner diameter 10 m

Hull height 1 m

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Hull dry weight 35 tones

Single PTO dry weight 3 tones

Buoyancy 1200 (total); 650 (available) kN

Lifesaver WEC summary

Advantages:

independent PTO units account for reliability and flexibility of maintenance;

simple rewinding system;

practically unlimited cable length allows for good survivability.

Disadvantages:

energy usage to rewind the drum hence lower efficiency;

control issues.

2.7.4. Aquaharmonics

“Aquaharmonics” is the name of the team that won the 2015 – 2016 US Wave

Energy Prize – an 18-month public design-build-test competition sponsored by the U.S.

Department of Energy. The main goal of the competition is to encourage the design of

efficient WEC devices which would be competitive on the energy market. The grand

price of $1.5 million attracted than 90 teams and provided ideas for improving

performance and reducing costs. “Aquharmonics” includes two engineers – Alex

Hagmuller and Max Ginsburg.

The working principle of the device is very similar to the Lifesaver – a heaving

buoy reacting against some mooring at the seabed through a pretensioned cable. The

vertical motion is converted to rotation with a horizontal drum, located in the buoy, and

then to electrical energy through a generator. Main focus of the design is the control

system. Starting from latching and declutching, the design evolved to “proportional

integral control, using coefficients based on the peak frequency of the wave spectrum –

the force demanded by the generator was the sum of a constant times velocity and a

constant times displacement” (Wave power conundrums, 2017). Although the WEC

operates with lower loads and hence produces less energy, lower CapEx and OpEx are a

58

significant advantage as well as greater survivability. Different modes of operation were

also available such as power capture or lower loads (in the case of harsh sea).

2.8. Environmental impact

For the purposes of this thesis, the environmental analysis will focus on WECs

classified as ‘point absorber’ - relatively small compared to wave length and located

offshore.

Wave energy is usually considered a clean renewable energy source. The limited

research done on environmental impact, however, combined with the short time that

WEC devices have been in use, only allows for partial description of possible

environmental effects.

In terms of environmental impact research, a few notable projects can be

mentioned: the WaveNet and the Coordinated action on ocean energy project (CA-OE).

WaveNet was carried out between 2000 and 2003 as a “European Commission

Thematic network to share understanding and information on the development of ocean

energy systems” (Wavenet, 2003). Including many European universities (in the UK,

Ireland, France, Sweden, Denmark, etc.), the study was one of the first to address

legislative, environmental and sustainability issues. The Coordinated action on ocean

energy project was carried out between October 2004 and December 2007, once again

supported and funded by the European Commission. Its main objectives include

developing R&D policies regarding WEC design, coordination of efforts in key projects

and collection and analysis of data from ocean energy systems in real seas.

Despite some effort that has been made to incorporate the WECs in coutry-level

legislations concerning coastal structures, shipping or electricity production, there are

few at the European union or international level. The lack of common guidelines at the

early stage of WEC development increases the possibility of conflict of interests as the

industry grows and allows for environmentally harmful use of the wave energy resource

and environment due to lack of knowledge or negligence. Hence extensive research and

monitoring should be undertaken.

There are a number of effects that a WEC can have on its immediate

surroundings. Those can vary in nature and severity during the installation, operation,

maintenance or decommissioning phase of the device life. The impact also depends on

59

the scale of the device – a prototype test, a single device, a small array or a wave energy

farm. Finally, along with the converter itself, all cables, moorings, energy hubs and

deployment/decommissioning ships affect the sea environment.

Environmental effect can be separated into three main areas:

Effect on abiotic system

Some larger WEC and especially wave energy farms may alter the water

currents and hence change the sediment deposition patterns. Large seabed fixed devices

or the moorings used for floating ones can also alter the seabed. The waves behind a

WEC have 10-15% lower wave height (Cruz, 2008), which can reduce coastal erosion

and interfere surfing. During the deployment and decommissioning phase increased

presence of ships may temporally increase the turbidity. Bad practices can pollute the

water with hydraulic and other oils.

Effect on biotic system

Seas are full of life – benthos, fish and marine mammals. These could be

attracted or displaced by the noise a WEC emits. Electromagnetism from high voltage

lines on seabed can affect the migration or feeding habits of some animals (Cruz, 2008).

On the other hand, floating WEC devices can serve as artificial nesting locations for sea

birds; bottom-fixed devices can act as artificial reefs; restricted fishing in the area can

help the local fish population serving as sanctuary for vulnerable species.

Some measures to mitigate negative impact include burying electricity cables in

order to decrease the electromagnetic radiation; observation of

installation/decommissioning phases by a marine life specialist who can delay the

disturbing works (usually high noise levels are present) in case there are sensitive

animals nearby; design of low-noise WECs, etc.

Effect on human activities

Fishing can be negatively impacted if the WEC devices occupy larger area.

WECs can visually impact tourism as the floating ones could be seen from the shore.

However, as the industry grows, wave energy can generate employment in many sectors

such as production and services. WECs can serve as guidance at night or they can be a

navigational hazard for shipping.

60

A major issue will be the conflict of uses – existing structures such as pipes can

be an obstacle for WECs; fishing or sand mining can be negatively affected if a wave

energy away or a farm are located in the area; surfing is directly affected due to the

reduction in wave height; the presence of biologically or historically important sites will

limit the available locations for WECs’ deployment.

Cruz (2008) summarizes the environmental impact of WECs in the following table:

Table 3: Environmental impact of WECs.

Actions Installation Operation Decommiss -

ioning

Environmental factors S C M D S C M D S C M D

Coastal processes x x x x x x x

Abiotic Water quality x x x x x x x x x x

Air quality x x x x

Benthos x x x x x

Fish x x x x x x x x x

Biotic Marine mammals x x x x x x x x x

Other aquatic fauna x x x x x

Marine birds x x

Flora x x x x x

Conflict of uses x x x x x x x x x x x x

Socioeconomic Cultural resources x x

Socio-eco x x x x x x

Visual x

Legend: S – ships; C – cables; M – moorings; D – device

61

2.9. Economy of WEC

According to Brooke (2003), the most accurate method of calculating energy

production cost, and hence being able to compare different devices, is using the US

Electric Power Research Institute Technical Assessment Guide (EPRI TAG ™). “The

cost of energy is computed by leveling a power plant’s annual revenue requirement over

the service life of the plant and dividing it by the plants’ annual output. If the energy

would be sold at this price, the total collected revenue would have the same present

value as the sum of all fixed charges and expenses paid out during the plants life,

allowing for discounting of future costs and revenues” (Brooke, 2003). Hence a single

term can be used to compare various designs, namely the “levelized” cost of energy.

The revenue requirement consists of 1. fixed charges and 2. variable costs and expenses.

The fixed charges do not depend on the amount of energy produced and include

construction, installation and regular maintenance of the device, power transmission

cables, staff payment, etc. The variable costs are directly related to the amount of

energy produced and can include consumable items such as filters, lubricants or fuel,

depending on the specific design. If sales exceed costs, the difference is considered a

profit and is subject to taxes. Important to note is that the majority of the costs are site

specific.

The wave energy industry is not yet mature and hence it could not be used as

costs data source (Brooke, 2003). This requires some mechanism to estimate the capital

costs of a WEC. Three commonly used are ‘costing by analogy’ approach, ‘bottom-up’

approach and the ‘parametric costing’ approach. The first uses average figures from

similar projects and adjusts them based on size, complexity, etc. In th case of a

relatively new ideas where analogs are hard to find, the use of this approach may be

limited. An alternative is the ‘bottom-up’ method: using the detailed drawings and

construction plans, a complete work breakdown is established. However, this method

can only be used when more detailed design has been created. Somewhere in between

these two is the ‘parametric costing’ method that uses functional relationship between

item characteristics (weight, size, etc.) and ts cost. Relationships are derived based on

past experience and an outline drawing is required as well as specifications and unit

rates for labour, transport, materials, etc.

62

In addition to WECs, some designs incorporate both wave and wind energy in

order to create more reliable devices and lower the energy prices.

The majority of WEC companies believe more research is required before wave

energy is ready to compete on the market (Sjolte, 2014). Similar to the wind energy

industry, however, wave energy economics has shown improvement over time.

Developing better understanding of the phenomenon as well as good practices and

manufacturing methods helps lower the capital and operational costs. Taking the

Lifesaver for example, the device was built as a prototype with the purpose of gaining

operational experience. Since little effort was made to optimize the device for low cost

energy production, there is huge potential for cost reduction. In table X are presented

the costs, based on a detailed design, of the current Lifesaver WEC and a next

generation device (Sjolte, 2014):

Table 4: Cost comparison between Lifesaver and the next generation device by FO.

Item Lifesaver Next

generation Unit

Floater 7.7 1.6 k€/kW

PTO 8 2.2 k€/kW

Mooring and auxiliary 7.6 2.1 k€/kW

Installation - 0.95 k€/kW

Operational cost 5.3 0.38 k€/(kW.yr)

Sum CapEx 23.3 5.9 k€/kW

Sum OpEx (NVP, 20 yr, 4%) - 5.2 k€/kW

Sum CapEx + OpEx - 11.1 k€/kW

63

3. Comparative analysis of winch-based PTO systems

3.1. Comparison criteria

WECs have been designed and improved for many years now and therefore a

great number of ideas are present. However, for financial or other reasons, not many of

those have reached the model testing stage. For example, this thesis considers three

distinct designs and only one of them (Lifesaver) has been built and tested

(Aquaharmonics WEC will not be analyzed further because of the early stage it is in;

and the similarity to Lifesaver). Since it is inefficient and financially challenging to

built and test every WEC design, a theoretical method for comparison and analysis must

be developed.

For the comparison criteria to be determined, it is important to define the goals a

WEC device aims to achieve. Once these are defined, a more detailed list of parameter

related to these goals can be created. These criteria could also be considered a starting

point for more specific design requirements.

The aim of the WECs is to produce reliable, low cost, clean energy from the

waves. Parameter that would affect the ‘cleanness’ could be the materials and methods

used for manufacturing, the presence of fuel, oils or exhaust gases. In terms of

reliability, the ability of the device to adapt to the changing conditions, its robustness

and survivability are important to account for. Factors affecting the cost are many,

including efficiency, mass, maintenance requirements, materials and manufacturing

methods, etc. The following list of criteria for comparison summarizes and extends the

points mentioned above:

WECs comparison criteria:

1. cost (per unit energy produced; for development and construction; for

installation, maintenance and decommissioning) ;

2. mass and dimensions (of the buoys, PTOs, any additional structures);

3. materials, manufacturing methods and availability of existing components;

4. robustness (device reaction to corrosion, fatigue, temperature change, etc.);

5. survivability (in extreme conditions);

64

6. design complexity and maintenance frequency requirement (are there multiple

components, moving parts, etc.);

7. efficiency (energy losses might occur in wave – mechanical/hydraulic

conversion as well as in mechanical – electrical conversion and transport);

8. adaptability to wave conditions (efficiency over a broad specter of wave

parameters);

9. environmental impact (see chapter 2.8).

These criteria are not equally important. For example, variation in efficiency,

design complexity and adaptability to wave conditions can be tolerated provided the

overall performance meets the desired goals. However, there might be legal, economic

or moral reasons that would restrict the freedom in terms of cost or environmental

impact.

3.2. Comparison and analysis of existing WECs

In order to select the best design, the considered devices are ordered best to

worst receiving 3, 2 or 1 points respectively for each of the comparison criteria. If there

is no undeniable winner for some of the criteria, 2 points will be allocated to either two

of the devices (where the third receivs 1 or 3 pts depending on its relative performance)

or to all three devices. ‘Coefficients of importance’ are based on the subjective

importance of each parameter (justification provided in Appendix 1); these coefficients

would be later multiplied by the allocated points to give the final score. Justification for

the decision is also provided.

Abbreviations

Counter weight device – CW;

Counter buoy device – CB;

BOLT Lifesaver – Ls;

Coefficient of Importance – C.o.I;

Weighted score – w.s. (score * C.o.I)

65

Table 5: Winch-based WECs comparison.

Criteria C.o.I CW CB Ls w.s.

CW

w.s.

CB

w.s.

Ls Justification

1 0.9 1 2 3 0.9 1.8 2.7

All devices require solid mooring; both CW

and CB require additional structure to be build;

CB uses relatively low cost buoys;

2 0.3 1 2 3 0.3 0.6 0.9

Ls only carries the PTO and the floater; in

addition, CB uses light buoys and CW uses

relatively large mass;

3 0.5 1 2 2 0.5 1 1

All devices require high load durable cables;

CW requires a specific bidirectional gearing

system;

4 0.9 2 2 1 1.8 1.8 0.9

All devices experience cable fatigue; CW and

CB are fixed to a structure above SWL and

could be better insulated from the sea water;

5 1 2 2 3 2 2 3

CW and CB experience the (inverse) pendulum

effect which limits their survivability;

meanwhile, Ls could have practically infinite

cable length

6 0.5 2 2 3 1 1 1.5

CW (PSG) and CB (multiple pulleys and

capstan) have rather complicated machinery;

Ls uses the generator which simplifies the

design;

7 0.8 3 2 1 2.4 1.6 0.8 CW utilizes energy production bidirectionally;

Ls uses energy to wind up the drum;

8 0.9 2 2 3 1.8 1.8 2.7

Regulation of the mass or buoyancy in CW and

CB is an option; however, the direct link

between the motor and the cable gives Ls an

advantage;

9 0.6 2 2 3 1.2 1.2 1.8

The installation and presence of the large

structures, required by CW and CB, will affect

the environment more compared to the Ls.

Final scores: 11.9 12.8 15.3

66

The result of the comparison shows that the Lifesaver is the best of the

compared designs. A design guide is available on the Bolt wave Power website and a

summarized version is presented in Appendix 2.

Results

Following the comparison, some key points have been identified:

All devices are prone to cable fatigue issues;

All devices use unconventional gear systems which is likely increase the overall

cost;

Both counter weight and counter buoy have a similar problem with balancing

between the pendulum effect and survivability in harsh sea;

Due to either support structure construction or underwater installation and

maintenance, the counter buoy design would most likely be commercially

inefficient;

Lifesaver trades energy production for simpler design and avoids drum rewind

issues such as pendulum effect, hydraulic leaks or spring wear;

Provided enough cable is present on the drum, Lifesaver has the best chances to

survive the hundred year wave (a hypothetical wave with greater wave height

and energy, appearing once every 100 years)

Lifesaver has been tested and developed for more than 5 years which ensures

well-thought design and less unexpected complications;

Hence, Lifesaver is currently the best winch-based PTO design – a buoy, moored to the

seabed, a drum-based PTO and a motor/generator with electrical hub to store the

produced energy.

67

4. Conclusion and future work

The thesis shows that the light and simple design of the Lifesaver is the right

way for future winch-based systems. The lack of heavy structures allows for cheaper

construction and installation; the compact and versatile PTO is the perfect alternative to

mass-, spring- or hydraulic-based energy storage. Compared to linear systems, the end-

stop problem is eliminated and survivability in severe storms is enhanced; the linear

generator is replaced by the more efficient rotary generator; the structure is lighter. No

matter the type of the device, however, wave energy has been an inspiring field of

research for many years and surely has its place in the future energy market.

In terms of winch-based devices, further research in computer-based wave

modeling will allow for better understanding of the devices’ behavior and hence for

optimal design. New rotary generators, adaptable to the low frequency wave

oscillations, or alternatives, such as the piezoelectric systems, should be developed. A

common system for classification and comparison of WECs will make it easier for

investors to choose the appropriate project. Further investigation on the specifics of the

the design requirements, such as cost evaluation, environmental impact, material and

manufacturing choice and design complexity and maintenance solutions will greatly

affect the performance, lifespan and cost of the future WECs.

68

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(University of Stellenbosch)

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[Online]. Wikipedia. Available at:

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energy production”, Renewable Energies and Power Quality Journal, Vol. 1,

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[27] Salter S.H., 1974, “Wave power”, Nature, 249:720-4.

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modeling, design and operation”, PhD Thesis (Norwegian University of Science

and Technology)

[29] Tanaka, Y., Oko, T., Mutsuda, H., Patel, R., McWilliam, S. and Popov,

A.A., 2015, “An Experimental Study of Wave Power Generation Using a

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71

Appendix 1

Coefficients of importance

To the criteria selected to compare the WECs were allocated coefficients that

would be a sign of how important those criteria are in the design process. These

coefficients were subjectively selected based on the author’s thoughts and experience.

The criterion with the highest priority (coefficient of 1.0) is the survivability –

how well the device handles harsh conditions. It would not matter how efficient or

durable a device is, if it only operates until the next storm. Right after (0.9) are cost,

robustness and adaptability to wave conditions. Provided that the WEC are trying to

deliver energy for a land-based consumer, the devices will compete with all other

energy sources such as fossil fuels, wind and nuclear power, which are all ahead in their

development and hence can offer lower prices. Robustness is important due to the

constant effect that the elements have on the device – salty and acidic sea water, solar

radiation (some plastics might alter their properties under UV light) and frequent loads.

A need for constant maintenance would increase the overall cost of a device because of

both materials , repair works and travel. Adaptability to wave conditions refers to the

bandwidth of operation of the device. This is essential due to the time- and location-

based variation of the wave parameters. In the case of a very narrow operational

bandwidth, the WEC is practically working in a fraction of the time hence producing

less energy. Next (0.8) is the efficiency of a device. The efficiency determines what

percentage of the available energy will reach the end consumer. Losses may occur

anywhere in the system, from the PTO to the transport power cables and will affect the

overall cost. Environmental impact (0.6) should be an important design criteria because

one of the main points for wave energy advocates its cleanness. Replacing fossil fuels

with devices that poison the oceans is an expensive enterprise with no actual benefit.

Additionally, a harmful WEC may provoke legal or social reaction. Design complexity

and building issues (materials, manufacturing and components) were placed towards the

end of the list (0.5) because if the device performs generally good enough, it is of little

importance to the consumer how complicated it is or how was it made. In terms of

design, however, the lack of certain mechanisms or components (such as a specific

72

gearbox or a cable) may require additional time and effort. On the other side, if a WEC

is build using existing components the cost and maintenance quality will be at better.

Finally, the size (0.3) of a WEC will affect construction and maintenance costs as well

as the environmental impact. However, as point absorbers are relatively small devices

(compared to the shore-based OWC) and are located offshore, provided good overall

performance, mass and dimensions are not an issue.

Criticism

The determination of both C.o.I values and the scores are highly subjective; the

rating system allows for unclarity about the distance in quality between 1st, 2

nd and 3

rd

place. The ‘undeniable winner’ rule attempts to account for that, however, as C.o.I are

multiplied there is another issue – the difference between 1/2/2 and 2/2/3 will grow

proportionally with the C.o.I.

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Appendix 2

Fred. Olsen design guidelines

On their website (BOLT Lifesaver system), Fred. Olsen have posted a list of

‘rules’ to be followed during the WEC design process. The years of experience with

their previous devices are summarized as follows:

1. As a first cost reduction step all structures that are not required for energy

production should be removed (such as support);

2. Devices should be light-weight in order to reduce ballast and submerged mass

(moorings) costs;

3. Following the rules above, the company advices against multi-body floating

systems (such as heave plates) where passive bodies will add inactive mass;

4. Similarly to additional mass, all moorings should be included in the energy

extraction process (provided the system behavior is fully understood);

5. The company advices against the use of hydraulic PTOs due to unsatisfactory

cost, efficiency and controllability as well as limited ability for filtering

unwanted behavior of the system;

6. Device stroke length should be unlimited to allow for greater wave heights and

eliminate the end stop problem;

7. The company advices against subsurface devices since the submersion

complicates design and maintenance;

8. As WEC are rather complex devices, reliability and durability should come

before maximizing energy production.

Even though the list above is limited to one company’s experience only, Fred. Olsen

have been testing various designs, including bottom fixed multi-body system, a floating

point absorber, hydraulic and electric PTOs, etc.