IN DEGREE PROJECT ENVIRONMENTAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2018

Tidal park within offshore wind parksAn analysis for the potential use of tidal kites within the Aberdeen offshore wind farm.

CHRISTINA MERKAI

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT

Tidal park within offshore wind parks

An analysis for the potential use of tidal

kites within the Aberdeen offshore wind

farm

Christina Merkai

Supervisor

Katrin Grünfeld

Examiner

Ulla Mörtberg

Supervisor at Vattenfall

Lara Pérez Andrés

Degree Project in Environmental Engineering and Sustainable Infrastructure

KTH Royal Institute of Technology

School of Architecture and Built Environment

Department of Sustainable Development, Environmental Science and Engineering

SE-100 44 Stockholm, Sweden

ii

TRITA-ABE-MBT-18498

iii

Summary in Swedish

Havsbaserad vind har visat sig vara en av de mest tillförlitliga och rena energikällorna under senare

år. Inom denna industri har en betydande tillväxt skett, med en ökning med 101% år 2017 jämfört med

2016. Detta relaterar till behovet av säkra elförsörjningssystem, som kan användas för att styra

havsbaserade vindraftverksparker under urkoppling från huvudnätet. Numera används

dieselgeneratorer som reservkälla till havsvindkraftverk i nödsituationer. Men när den marina

förnybara energiindustrin utvecklas, har tidvattenkraftverk potential att ersätta dieselgeneratorer och

ge ett mer hållbar och miljövänlig långtidslösning. Dessutom har de potential att producera extra

el, som antingen kan lagras för framtida användning eller kopplas direkt till distributionsnätet. Denna

rapport erbjuder en teknisk, finansiell och miljömässig bedömning av en potentiell

tidvattenkraftverkspark kopplad till en havsvindpark. Jämförelse med alternativa källor för

strömförsörjning genomförs också. Tre alternativa platser med hög vindstyrka och stora

tidvattenresurser längs Storbritanniens kust och fyra olika grupper av tidvattenanordningar

utvärderas och jämförs med hjälp av kartor och andra tillgängliga marina data. Aberdeen

vindkraftpark och tidvattendrakar väljs för ytterligare undersökning och kostnadsanalys. Sju

tidvattendrakar med genomsnittlig effekt på 700 kW och nominell effekt 3,5 MW kan ge tillräckligt

med el till havsvindkraftverk i tre månader utan nätförbindelse, medan de också kan ge överflöd av

energi dagligen när strömavbrott inte förekommer. Den totala kostnaden för projektet skulle vara

cirka 301,6 MSEK. På grund av läget idag på elmarknaden för förnybar energi, är projektet inte

genomförbart utan höga investeringsrisker. Men den här studien bör utvärderas igen inom en snar

framtid när kostnaden för tidvattenanordningen har minskat.

iv

v

Abstract

Offshore wind has proved to be one of the most reliable and clean energy sources over the last few

years. The industry has experienced a significant growth, with an increase of 101% only in 2017

compared to 2016. This raises the importance of the need for more secure power supply systems,

which can be used for controlling the offshore farms during disconnections from the main grid.

Nowadays, diesel generators are being used to feed auxiliary services of the offshore wind turbines in

situations of emergency. However, as the marine renewable energy industry evolves, tidal energy

parks have the potential to replace diesel generators and provide a more sustainable and eco-friendly

solution for a long-term auxiliary power system. Moreover, they have the potential to produce extra

power, which can be either stored for future use or linked directly to distribution. This report

demonstrates a technical, financial and environmental assessment of a potential tidal park within an

offshore wind park. Comparison with alternative sources for emergency power supply is also

performed. Three alternative locations with high wind speeds and large tidal resource around the UK

coast and four different groups of tidal devices are evaluated and compared for the implementation of

this solution with the use of ArcGIS maps and other accessible marine data. The Aberdeen wind farm

and the tidal kites are selected for further investigation and cost analysis. Seven tidal kites with

average power 700 kW and rated power 3.5 MW can provide adequate power to the offshore wind

farm for three months without grid connection, whereas they can also provide excess of energy on

daily basis when grid disconnection does not occur. The total cost for the project would be

approximately 301.6 MSEK. Due to the current renewable energy market, the project is not feasible

without high investment risks. However, this study should be evaluated again in the near future when

the cost of the tidal device will be further decreased.

vi

vii

Acknowledgments

A large number people has played an important role in the realisation and completion of this master

thesis study and I would like to acknowledge them.

First of all, I would like to thank my two main supervisors, Katrin Grünfeld at KTH Royal Institute of

Technology and Lara Pérez Andrés Lara at Vattenfall, for giving me the chance to write this master

thesis and for their support and help all these months.

I would like to thank Per Holmberg, my second supervisor at Vattenfall, who provided me with

valuable information about tidal technology and energy systems, and with whom I had meaningful

conversations about the study.

Special thanks to Jonas Persson, my section manager at Vattenfall R&D, who, although did not have

the official role of the supervisor in my project, he has been extremely helpful and supportive,

providing me with constant feedback and helping me organise my report in the best possible way.

I would like to thank all the following employees (in alphabetical order) at Vattenfall, whose

contribution has been outstanding and essential in many ways from the first days of this study until

the last day: Aggarwal Rajat, Axelsson Urban, Jagalos Panos, Mawdsley Robert, Kairelis Karolis,

Kitimbo Andrew, Roberts Zoe, Slot Iver.

I would also like to thank Marc Jeroense at MJ MarCable Consulting AB for subsea cable consulting in

regards to cost estimations.

viii

ix

Table of contents

1. Introduction 1

1.1 Background 1

1.2 Climate change, regulations and policies. 1

1.3 Basic theory of tides 2

1.4 Tidal energy 3

1.4.1 Tidal range 3

1.4.2 Tidal stream 4

1.5 Aim and objectives of this study 4

1.6 Benefits of study 5

2. Methodology 5

3. Resource assessment 6

3.1 Site selection 6

3.1.1 Alternative 1- Aberdeen wind farm 8

3.1.2 Alternative 2 - Ormonde wind farm 10

3.1.3 Alternative 3 - Thanet wind farm 11

3.2 Device selection 12

3.2.1 Tidal energy converters (TEC) 12

3.2.2 Device alternatives 14

3.2.2.1 Commercial axial flow or horizontal axis turbines 14

3.2.2.2 Cross-flow turbines 15

3.2.2.3 Enclosed tips (Venturi) 15

3.2.2.4 Tidal kites 16

3.3 Conclusions on the suitability of the different devices for the different sites 18

4. Technical assessment 19

4.2 Case studies 20

4.3 Calculation of load flow in emergency mode 21

5. Cost analysis 22

5.1 Tidal kites 22

5.2 Diesel generators 23

6. Construction/Implementation/Comparison with other alternatives 25

6.1 Physical prerequisites and setup 25

6.2 Project timeline 26

6.3 Stakeholder consultation and participation/ analysis 27

6.4 Regulations and permitting issues 27

x

6.5 Comparison with other alternatives 27

6.5.1 Photovoltaic devices 27

6.5.2 Wave power farms 28

7. Energy storage solutions 28

7.1 Rechargeable battery types 28

8. Environmental impacts 29

9. Monitoring 30

9.1 Software and tools for monitoring 30

10. Uncertainties 31

11. Results and Discussion 32

12. Conclusion 34

13. Recommendations for future studies and further work 34

References 36

Other references 38

Appendix I 42

xi

Abbreviations AGM Absorbed glass mat

APS Auxiliary power supply

BMS Battery management system

CAPEX Capital expenditure

CAN Controller area network

EMF Electromagnetic field

EU European Union

GHG Greenhouse gas

km kilometre

kW kilo Watt

LCOE Levelized cost of energy

m metre

MSEK million SEK

MRE Marine Renewable Energy

MW Mega Watt

OPEX Operating expenditure

OSS Offshore substation

OWF Offshore wind farm

OWT Offshore wind turbine

PPE Personal protective equipment

TEC Tidal energy converter

TRL Technology readiness level

TST Tidal stream turbine

UK United Kingdom

UPS Uninterruptible power supply

VRLA Valve regulated lead acid

xii

1

1. Introduction

1.1 Background

Offshore wind is a young industry, which began in 1991 when the first wind farm was installed off the

Danish coast and now it is one of the most important sources of clean energy worldwide. Only in 2017,

offshore wind power installations grew 101% compared to 2016 (WindEurope, 2018). 560 new

offshore wind turbines were installed across 17 wind farms by increasing the installed capacity to 12.6

GW during the last five years (deCastro et al, 2018;WindEurope, 2018). Europe has a total installed

4149 offshore wind turbines across 11 countries.

Due to the special conditions in the sea, it is important that the turbines operate smoothly and their

components avoid any damage, so they do not pose risks to the surrounding environment. Therefore,

the turbines are in constant need of power for their auxiliary services, such as navigation lights,

sensor, communication, ventilation and heating appliances etc., which during long grid

disconnections is currently provided by the use of diesel generators (Berggren, 2013). However, this

solution is neither environmentally friendly due to high Greenhouse Gas (GHG) emissions nor

financially sustainable due to the unstable fuel prices.

Looking for a new solution, which can provide energy during long time of grid-disconnection and

under no wind conditions, small tidal devices are here investigated as a new solution to support the

offshore wind farms, while avoiding diesel generators. Within the tidal park, the auxiliary services

could be fed on daily basis due to the consistency of the tides. Furthermore, this solution could

increase the total power production of the wind farms. The combination of the offshore wind energy

and tidal energy could result to a 100% green energy system with high power production and high

predictability.

Tidal energy is an emerging renewable energy source, capable of supplying large amounts of energy

worldwide. Although there are many areas with high tidal resource, the UK is currently leading the

development of tidal devices. Half of the tidal resource within Europe is estimated to be found in the

UK. Tidal energy is capable of supplying up to 30% (20 - 100 TWh) of the UK’s annual electricity

demand, however until today, the tidal resource remains unexploited in its highest degree (GOV.UK,

2013). Therefore, it is important to find ways to accelerate the implementation of tidal energy and

improve the sustainable energy mix. With the wise utilization of the tidal energy sources and

technologies, the reduction of the carbon footprint of the current electricity generation will be

maximized, while the impact on human and wildlife activity should be minimized.

1.2 Climate change, regulations and policies.

The global warming and the exhaustion of the fossil fuel have a negative impact on climate change,

which has been noticeable the last few years. The scientific groups worldwide have raised awareness

on the matter and recommend the use of natural resources for power generation. As a result, the

interest in energy generation with the use of wind, solar, wave and tidal sources has increased and

energy technology development is in the centre of attention.

In Europe, there are agreements and policies, which regulate all the newly proposed and implemented

projects in regards to environmental impacts and climate change.

2

Some of these climate action plans are:

● The Environmental Code. Each country can have a slightly different Environmental Code

than the others, but the aim is similar. The Environmental Code aims to promote

environmental responsibility and encourage the development of environmentally friendly

technologies.

● The Paris agreement signed by the 175 countries, which recognises the need to significantly

reduce the global greenhouse gas emissions and aims to limit the temperature increase to

1.5°C (UNFCCC, 2015). The agreement will come into force in 2020.

● Climate Change Act 2008: The UK Government has committed to legally binding targets

in order to reduce carbon emissions by 34% by 2020 and 80% by 2050 (HM Government,

2008; Murray et al, 2017).

● North Sea Countries’ offshore grid initiative (NSCOGI) was established in 2010 and

was signed by the following countries: Belgium, Denmark, France, Germany, Ireland,

Luxembourg, the Netherlands, Sweden and the UK. Nowadays, there is also close cooperation

with Norway. The aim of this initiative is to promote and develop a joined offshore electricity

grid in the wide North Sea area (Eamon, 2015).

The European Union (COM(2015)6317) has introduced the Strategic Energy Technology Plan (SET-

Plan), which aims to improve the performance of the ocean energy sector’s technologies and drive the

sector forward towards commercialisation (Magagna et al, 2016). Member States and stakeholders

have set cost-reduction targets and strategies for tidal and wave energy with the “Declaration of

Intent” (European Commission, 2016). Specifically, for tidal energy, the Levelized Cost OF Energy

(LCOE) target, as presented in the SET-Plan Declaration of Intent, is set to 1.5 SEK/kWh by 2025 and

1 SEK/kWh by 2030, whereas the cost of tidal energy technology has to be reduced by 75% (Magagna

et al, 2016). Further actions towards the commercialisation of the sector have been proposed by the

Ocean Energy Forum (Ocean Energy Forum, 2016) and the Technology and Innovation Platform for

Ocean Energy (TPOcean, 2016; Salvador et al, 2018), which have developed the Ocean Energy

Strategic Roadmap and the Strategic Research Agenda for Ocean Energy, respectively.

1.3 Basic theory of tides

Tides are the periodic changes of the sea level, as a result mainly of the strong gravitational forces of

the Moon and to a lesser extent, the Sun, with respect to the Earth. Gravity pulls the ocean water

towards the moon and creates periodic vertical shifts in these moving water bodies around the earth.

These shifts of water are called “tides”. Tides are the strongest when the moon is in either in New

Moon or the Full Moon position and the Sun, Earth and Moon are all in line (Pugh and Woodworth,

2014). Weaker tides occur when the moon is in the first or third quarter positions.

There are three types of daily tides: diurnal, semi-diurnal and mixed tides. The diurnal tides have a

cycle of high to low water, which takes about 24 hours and 48 minutes, whereas a period of 12 hours

and 24 minutes has been observed for semidiurnal tides. The mixed tides are a combination of the

diurnal and the semi diurnal tides (Samo et al, 2016; Kay, 2017). This difference in the tidal height

between the two daily tides is repeated twice per month as the moon rotates around the earth.

Therefore, in order to address the impact of this variation on power output from the turbines more

efficiently, a 14 day analysis is required.

Apart from the gravitational forces, there is a group of tides, which can be caused due to extreme

atmospheric variations. These tides are known as meteorological tides. One example is storm surge,

which is the outcome of the combination of wind and inverted barometric pressure (Kay, 2017).

Tides can be also divided into spring tides and neap tides (fig.1). Spring tides occur twice a month,

every 14-15 days, when the moon is either new or full (Pugh and Woodworth, 2014). They are defined

3

as the highest and lowest tide (Pugh and Woodworth, 2014). Neap tides are the opposite of the spring

tides. The tidal range between high and low water is smaller and occurs nearest the time of the first

and last lunar quarters (Pugh and Woodworth, 2014).

Regardless if it is a spring tide or a neap tide there are always two high tides: one on the side facing the

moon while the other high tide is on the opposite side of earth. As Earth spins, different areas of the

planet face the moon and this rotation causes the tides to cycle around the planet. The area between

the tide marks, above the low tide and under water at high tide, is called intertidal zone.

Figure 1: Illustration of the neap and spring tides (Ghefiri et al, 2017).

1.4 Tidal energy

Tidal energy or tidal power is the power produced as a result of the phenomenon of the tides.

Specifically, the energy of the tides is converted into power, mainly electricity. Tidal energy is a

predictable form of renewable energy, which can hence provide constant and reliable electricity

supply. It can be predicted 100 years ahead from now. However, it only provides power for around 10

hours each day when the tide is actually moving in and out. Although the tidal energy density is higher

compared to other renewable energy sources, its high capital investment, mainly due to the

demanding technology, has hindered its development. Therefore, tidal energy exploitation remains in

its infancy.

In practice, there are two forms of tidal energy generation, tidal range and tidal stream (Hardisty,

2007). Tidal range is the vertical difference between high and low tide, whereas tidal stream is the

horizontal flow of strong tidal currents due to variations of water levels (Hardisty, 2007; Samo et al,

2016).

The motion of the tidal streams, which is water driven by the gravitational forces in the form of tidal

currents, contains large amounts of kinetic energy.

1.4.1 Tidal range

Tidal range devices often require the creation of a tidal basin, in order to control and use the variation

in water level during the flood and ebb tide for the generation of power.The energy from the tidal

range is captured mostly with dam-like tidal barrages or tidal lagoons. They are usually integrated into

onshore structures, such as harbour walls, breakwaters and other artificial structures near the coast.

This way, they allow simple power transmission and can be easily accessed, resulting in maintenance

cost reduction. However, these structures are built in shallow waters and their construction and

operation costs are quite high. Furthermore, they have significant environmental impacts to the

surrounding area as they can impact the water and sediment movement in large scale, resulting in loss

of habitat for the fish populations (Maulud et al, 2009).

4

There is a need of generating power from tidal flow without the use of expensive methods and

structures such as tidal barrages or the storage of water in a basin. Therefore, the generation of power

from tidal streams in deeper offshore waters is promising.

1.4.2 Tidal stream

Compared to tidal range structures, the tidal stream technology requires very low capital investment

and has less environmental impacts due to their limited size (Johnstone et al, 2013). Tidal stream

energy is captured by tidal stream turbines (TSTs), which are mounted underwater in the ocean or

floated at the sites where strong tidal currents occur (Hardisty, 2007). The turbines use the kinetic

energy directly and let the water pass through them.

The location of tidal stream devices can vary from shallow water (less than 40m deep) to deep water

(more than 40m deep). Once the tidal technology has been proven to be commercially and financially

more viable, the deep water sites will be more easily exploited, whereas now that only applies for the

shallow water sites (O'Doherty et al, 2018). Some of the key issues that can affect and obstruct the

successful development of the TSTs are the local tidal velocity, water column depth and bathymetry,

possible turbulences, local shipping and concerns associated with the marine wildlife (O'Doherty et al,

2018).

1.5 Aim and objectives of this study

This study focuses on answering the following research question: Can tidal devices replace the diesel

generators in order to supply sufficient power supply for the auxiliary services in offshore wind farms?

If the answer is yes, how many tidal devices would be needed, how much power would they generate

and how much would their implementation cost be?

The aim of this study is to conduct a resource, technical and environmental impact assessment of a

potential energy system between offshore wind parks and tidal parks. The main focus lies on the

implementation possibilities in current or future development projects of Vattenfall in the UK. Three

offshore wind farms owned by Vattenfall are being evaluated and compared, whereas the case study of

an offshore wind farm in the area of Aberdeen is being further investigated. Storage options and other

alternatives for emergency power supply are also discussed.

The objectives of the study are the following:

- Characterization and evaluation of the commercial tidal units.

- Availability of the tidal energy source and investigation of potential locations for the project

implementation.

- Investigation of number and size of the tidal units required for supporting an offshore wind

farm.

- Analysis and comparison of total costs for the diesel generator and the tidal units for this

purpose.

- Assessment of possible environmental impacts as a result of the tidal farm construction and

operation.

- State of art of the connection between the tidal devices and the onshore substation with the

presentation of a tidal farm illustration and an estimated project timeline.

- A short comparison between tidal and other energy sources, such as solar and wave energy.

- Presentation and comparison of rechargeable battery types as possible energy storage

options.

- Benefits from tidal units installed with offshore wind farms.

5

1.6 Benefits of study

The study can be used by Vattenfall for improvement of their current offshore wind farms power

production with the addition of tidal devices. The tidal energy production can be utilized for

emergency power for the wind farms as well as additional power to the existing grid. The solution will

also provide 100% green energy, which is important for the current state of climate change. The study

can be also used as guideline for similar future projects, when the tidal devices will be commercially

more mature, technologically more advanced and financially more cost-effective.

2. Methodology

For the purposes of this report, three main methods of data collection were used. The methods

included extensive literature review of scientific papers, the use, evaluation and comparison of

information obtained from ArcGIS maps and personal contact with Vattenfall BA WIND employees in

both Sweden and the UK. In addition, there were short phone contacts with the industry actors and

manufacturers of tidal devices. During these phone calls, questions regarding the characteristics of the

devices were asked, especially the characteristics that were not accessible by literature review or not

publicly available yet. More specifically, the above actors were asked questions about the technological

stage of the tidal devices at the moment of the study, their performance in any tidal conditions as well

as the desirable conditions for each device to perform better. Some offshore wind industry actors were

asked to provide information regarding the power needs of the offshore wind farms and the solutions

that are currently applied in cases of grid disconnection. This information was used for the

comparison of the solutions technically, financially and environmentally. The most relative to this

study information is presented in the report. Some confidential to their companies information was

not provided by the interviewees.

In the report, the following steps are followed:

1. First, three alternative locations are evaluated for the potential implementation of a hybrid

system between offshore wind and tidal park. The three locations are located on the UK coast

and were selected because of the existing offshore wind farms, which are operated by

Vattenfall. This made it easier to collect data about their current operation status and

infrastructure. ArcGIS mapas were reviewed for the tidal resource potential of these locations

in regards with the bathymetry in the area.

2. Later in the report, four different possible tidal device categories are presented and compared.

The most suitable device(s) is then selected and analysed further in the technical assessment

for the energy production calculations and cost estimations.

3. The selection of the most suitable combination of wind farm location and type of tidal device

is based on tidal resource, bathymetry and device technological maturity data.

4. A cost analysis is performed for the tidal devices in case of implementation of the project and

comparison with the currently used diesel generators.

5. Possible environmental impacts of the project are recorded.

6. An estimated project timeline is presented by taking into account weather conditions and

other possible delays in contruction.

6

3. Resource assessment

3.1 Site selection

Sites that are characterised with a current velocity of 2.5 m/s or higher are considered to have

extremely high energy resource potential (Charlier, 2003). From previously conducted assessments,

such sites can be found in some of the following geographical areas around the world (Johnstone et al,

2013): the Amazon, the Arctic Ocean, Bosphorus, the English Channel, Gibraltar, the Gulf of Mexico,

the Gulf of St Lawrence, the Irish Sea, Messina, Rio de la Plata, Sicily and the Straits of Magellan.

Nowadays, thanks to technological advancement and new promising tidal devices, sites in deeper

waters and/or with tidal stream velocity 1- 2.5 m/s could be exploited equally well.

In the scope of this project, the UK coast tidal resource potential is of interest and specifically, three

wind farms located around the UK coast, Aberdeen, Ormonde and Thanet, will be further investigated.

The UK has some of the most suitable sites for tidal energy development in the world. Only around the

Channel islands, five sites have been identified as suitable based on tidal current velocities (mean

neap peak and mean spring peak velocities), bathymetry and available area, according to reports

commissioned by the Carbon Trust (Coles, 2017). Moreover, Scotland has more 25% of Europe’s

potential tidal resources with some of the best resources located across the north-west coast (AREG,

2018). In tidal stream energy resource assessments around the UK, commissioned by the Carbon

Trust, the exploitable resource was estimated approximately 18 TWh per year within 1450 km2 of UK

waters. This amount could meet 5% of the electricity demands in the UK (Black and Veatch, 2004;

Black and Veatch, 2005).

According to Black and Veatch 2005, 60% of the UK tidal resource can be found in depths more than

40 metres, 30% of the resource can be found in depths between 30 - 40m below the sea level, whereas

only 10% of the resource can be found in shallower water with depth less than 30m (Black and Veatch,

2005). However, the current commercial tidal substructures of offshore wind farms (OWF) are

financially limited for maximum water depths of 40-50m. Floating turbine designs with fixed-bottom

design in water over 50m deep are currently in plan to operate, whereas deep water (>50m) farms

could be operating in 4 years time.

As seen in figure 2, the annual wind speed within the UK Continental Shelf is quite high making it

ideal for offshore wind farm installations. The majority of the sites have wind speeds higher than 7.1

m/s in average annually, and only the areas within small distance off the shore present lower wind

speeds in average.

Figure 2: Annual wind speed around the UK coast (ABPmer, 2018).

7

In order to examine whether the possibility of a hybrid system between offshore wind and tidal parks

can be feasible, the tidal resource ArcGIS maps for the UK continental Shelf are presented and

analysed.

The tidal resource ArcGIS maps for the UK Continental Shelf were retrieved online at the ABPmer

website and can be used for a more thorough examination of the tidal potential data around the UK

coast. In figure 3a, the average annual tidal power is presented. The most common annual tidal power

values are within the range of 0.001-1.00 kW/sq.m and less common, but still observed in quite many

areas, are values 1.01-2.00 kW/sq.m. In figure 3b, the annual % exceedance 1m/s of tidal stream

around the UK coast is presented. The annual percentage of exceedance of the 1m/s speed for tidal

streams can be less than 11% (deep purple color) and reach 80% (yellow color) in some sites. These

means that areas with relatively high annual % exceedance of 1 m/s in tidal streams are potential sites

for exploitation of tidal energy.

Figure 3: a. Annual tidal power around the UK coast. b. Annual % exceedance 1 m/s of the tidal stream around the UK

coast (ABPmer, 2018).

8

Figure 4: a. Spring peak flow and b.Neap peak flow (ABPmer, 2018).

In the figure 4 above, the spring peak flow on the left and the neap peak flow on the right are

presented. The spring peak flow values are observed to be within the range of 0.11 m/s and maximum

3.5 m/s, whereas the neap peak flow values start from 0.11 m/s in most areas and can reach up to 2.00

m/s. Regardless the location, it is observed that the the amount of the available energy is during neap

tides is significantly less than during spring tides (Roberts et al, 2016). Therefore, it is of no use

analysing the neap peak velocities in order to estimate the resource potential in this report, only the

spring peak velocities.

Some sites, where currently offshore wind farms are operating, were selected as potential locations for

the installation of tidal energy devices, which in combination with the already installed wind turbines

could provide electricity during disconnections from the grid or to the grid as a hybrid system. These

sites, as mentioned above are analysed further as alternatives 1, 2 and 3.

3.1.1 Alternative 1- Aberdeen wind farm

The Aberdeen wind farm is located in northeast Scotland, United Kingdom (fig.5) in distance 3-5km

from the shore with maximum depth approximately 31m LAT. The area is characterised by high wind

power potential as it is shown in fig. 5. The annual wind speed map indicates wind speed values

between 7.1 to 9 m/s for distance 3-5 km from the shore. For longer distances, the wind speed values

inevitably increase.

9

Figure 5: The annual wind speed map around the coast of Aberdeen (ABPmer, 2018). The Aberdeen wind farm is

marked with a red star.

The tidal range in the area is approximately 4.6 m and surface current speeds can be on average

0.37m/s with a maximum of 1.24 m/s. From the spring peak flow map (fig. 6a), it is noticeable that for

a distance up to 30 km from the shore the tidal stream spring flow can reach 1 m/s and for longer

distances up to 45 km north of Aberdeen it can reach 1.5 m/s. From the annual % exceedance 1m/s

map (fig. 6b), it is obvious that for distance up to 30km from the wind farm the percentage can reach

40% annually. The average depth in the area can reach up to 60m and the mean significant wave

height can be 1.50m. These last two characteristics will be useful in the device decision process.

Figure 6: a. Aberdeen spring peak flow and b. annual % exceedance 1m/s (ABPmer, 2018).

10

3.1.2 Alternative 2 - Ormonde wind farm

The Ormonde offshore wind farm is located in the west coast of England, United Kingdom, in the Irish

Sea, 15km away from the Isle of Walney and has a maximum depth of 21m LAT. According to the

annual wind map in fig.7, the wind speed values can be from 7.1 m/s to 9.5 m/s and in further

distances even higher.

Figure 7: Annual wind speed around the Ormonde wind farm (ABPmer, 2018).

The area around the Ormonde wind farm presents large tidal range (9 m) and current speeds with

average speed 0.35 m/s and maximum surface current speed approximately 1m/s. According to the

spring peak flow map in figure 8a, the tidal stream can reach spring flows of 1m/s within 20 km

southeast or 45 km west off the wind farm and 1,25 m/s within 20km southeast and 54 km west off

the wind farm, whereas the annual % exceedance of 1m/s in the area can reach 20% in distance less

than 10km southeast of the wind farm (fig.8b). These values are lower compared to Aberdeen

indicating less tidal stream resource in relation to the distance from the wind farm. The average depth

in the area can reach up to 40m, whereas the mean wave height can be 1.25 m.

Figure 8: Ormonde wind farm and the surrounding coast. The figure on the left (a) is presenting the spring peak flow,

while the figure on the right (b) is presenting the annual % exceedance 1m/s (ABPmer, 2018).

11

3.1.3 Alternative 3 - Thanet wind farm

The Thanet wind farm is located 12 km from the coast of Thanet district in Kent, east of England,

United Kingdom. The wind speed in the area can reach 9m/s, as it is observed on the annual wind

speed map in figure 9a. The annual tidal power can start from 0.11kW/sq.m. and reach 1.5kW/sq.m

within a distance of maximum 20km from the wind farm (fig. 9b).

Figure 9: a. Annual wind speed map and b. Annual tidal power map (ABPmer, 2018).

Figure 10: a. Spring peak flow map for Thanet. b. Annual % exceedance 1 m/s map for Thanet (ABPmer, 2018).

More specifically, the spring peak flow map (fig. 10a) shows velocities up to 1.25 m/s in the area of the

wind farm and 1.5 m/s within a distance of 4km from the the wind farm. These velocities can be

converted into energy by some devices. Based on the annual % exceedance 1m/s map (fig.10b), the

12

annual percentage of tide stream velocities exceeding 1m/s can reach 40% in the surrounding area.

The average depth in the area is maximum 20-30 m, which is explained by the fact that the Southern

North Sea is a shallow sea with depths lower than 50 m (Vanhellemont and Ruddick, 2014). The mean

significant wave height in the area can be 1.11 m.

Apart from the resource potential, the selection of the right site will be determined by several factors.

These factors are the equipment’s survival, the access for maintenance, the degree of integration with

the power distribution network, as well as, environmental and ecological impacts (Thake, 2005).

3.2 Device selection

The motion of the tidal water, driven by the pull of gravity, contains large amounts of kinetic energy,

which can be extracted by tidal devices and converted into electrical energy.

According to their evolution stages, the tidal devices can be divided into first-generation, second-

generation and third-generation devices (JRC, 2014). The the first-generation tidal devices are

designed for bottom-mounted installations the second-generation devices are designed to exploit the

tidal resources in the mid-high water column, whereas the third-generation devices intend to exploit

the tidal resources that cannot be exploited by the other two devices (JRC, 2014). Many companies are

investing in prototype development and use the Technology Readiness Levels (TRLs) as guide to

minimise any financial and technological obstacles (Murray et al, 2017). The TRLs can determine the

maturity of different types of technology with a scale range between 1 and 9. The most mature

technology has a TRL of 9.

3.2.1 Tidal energy converters (TEC)

Tidal Energy Converters (or Tidal Stream Generators) are simple devices that harness the flow of

currents, the tidal stream, to produce energy.Their design is quite similar to the windmills, they

generate power when the rotor blades turn in either direction. If the turbines can run in both forward

and reverse directions, then the system will produce energy as the tide comes in and as it goes out.

However, the cost of reversible generators is higher than single direction generators. Water is

approximately 830 times denser than air and thus provides more contact with the blades of the rotor,

which translates into more energy being transferred to the turbine with the use of smaller blades

(Bahaj and Myers, 2003). Although there are no extreme flow speeds underwater that could cause the

failure of the tidal devices (Blunden and Bahaj, 2006), there are still large load forces generated by the

water, which the devices must be able to endure without any physical damage.

The tidal turbines can be attached to the seabed of floating near the surface with moorings attached to

the seafloor.

The turbine design can significantly affect the amount of energy that can be derived from a tidal

stream. The most common turbine design type is the Horizontal Axis Turbine (HAT) or Axial Flow

Turbine (fig.11), which, so far, has reached the highest TRL of 8, compared to other designs. The name

is given due to the fact that these turbines sweep through a circular area in the water, while rotating in

parallel with the flow direction axis. These turbines are characterised by rapid spinning, but slow

enough to not be a threat to wildlife, and low torques on the drive train (EMEC, 2018a).

13

Figure 11: Illustration of horizontal axis tidal turbines (Global CCS Institute, 2018).

Another type of design is the Vertical Axis Turbine (VAT) or Cross Flow turbine, which sweeps

through a rectangular area by rotating vertically and about a perpendicular to the flow axis, with water

flowing across each blade twice. These turbines have the advantage that they do not need to be

directed perfectly into the tidal stream and thus can generate electricity in settings where there is tidal

stream variation, which is not quite common. The turbines are the least likely to cause damage to

passing organisms (EMEC, 2018a). An illustration of VATs is presented in figure 12.

Figure 12: Illustration of the vertical axis tidal turbines.

Other generalised models of tidal devices are the oscillating hydrofoil,the archimedes screw, the

enclosed tips (venturi) (fig.13) and tidal kite (EMEC, 2018a) (fig.14) . The oscillating hydrofoil uses the

oscillating movement of the hydrofoil, whereas the archimedes screw uses the spinning movement to

generate electricity. These two models have not presented significant progress and there is not enough

technical information available, therefore they are not further studied in this report. The enclosed tips

utilize the venturi geometry to cause pressure differential, which drives an air turbine and eventually

produce electricity. Although these devices have made considerable progress similar to tidal kites,

they require tidal current velocities higher than 3.5 m/s, which due to the site alternatives and the lack

of commercial-scale devices in the market, which could provide with more technical information, is

not presented in detail in this study. Ongoing projects could prove the commercial viability of the

enclosed tips in the near future. Last, tidal kites are small-size turbines, which have successfully

reached TRLs of 5-7 and have demonstrated a fast-growing development the last few years. They are

also considered to be optimal for low current velocities and high water depths. Their concept is to

produce energy with the help of a wing and turbine arrangement tethered to the seabed, which moves

with the water flow. These devices are presented in more detail further in this report.

Tidal stream generation has the advantage of being affordable to implement, because, in contrary to

tidal range generation, it does not require the construction of a massive dam. The tidal stream

generators have the ability to stand on their own or incorporate into existing structures. This not only

further reduces the cost, but also allows the surrounding environment, including its habitats to remain

the same and not have any significant impacts, due to the limited area that is used for their

construction and operation. Moreover, the tidal blades prevent the marine animals from straying into

their path thanks to their acoustic properties in the water, therefore the tidal devices exhibit less

negative impact compared to the surface turbines. However, tidal stream generators generally cannot

produce as much power as barrage systems and they are sensitive to corrosion from saltwater,

although the advancement of the materials science has made corrosion only a minor problem.

14

The amount of power a turbine can extract from an unbounded fluid flow can be described

mathematically using momentum theory or actuator disc theory (Roberts et al,2016):

(1)

where ρ is the seawater density (1025 kg/m3), A is the total device area (m2), Cp is the power

coefficient (%) or Betz limit and u is the velocity.

The power produced per m2 of total device area, or turbine power density can be then calculated

easily with the following equation:

(2)

When the flow is bounded, usually in tidal channels, the power coefficient of a turbine increases as a

result of the blockage effect (Garrett and Cummins, 2007). However, since the case study areas in this

report are located along the open coastline, the blockage effect on power density will not be

considered.

3.2.2 Device alternatives

Before selecting the most suitable device(s) for our sites, four different are presented and evaluated.

Most of the following information has been obtained from the manufacturers’ webpages and might

not reflect reality.

3.2.2.1 Commercial axial flow or horizontal axis turbines

The performance characteristics of four commercial axial flow turbines that are currently in the

market are presented in table 1. The most advanced technology is SeaGen, which is developed by

Marine Current Turbine. From R&D projects, it has been observed that the average capacity factor for

axial flow turbines would range between 8% to 20%, whereas the SeaGen device has achieved the

highest capacity factors with a maximum of 60% (Magagna et al., 2016).

Table 1: Performance characteristics of selected commercial axial-flow turbines (Roberts et al, 2016; EMEC, 2018b;

Global CCS Institute, 2018; Marine Current Turbines).

Device Atlantis AR1000

SeaGen-S Verdant Gen5

Voith Hydro Ocean

Rated power (kW) 1000

2000 1.68 1000

Rated flow velocity (m/s) 2.65 2.50 2.59 3.00

No. of rotors 1 2 3 1

Rotor diameter (m) 18.0 20.0 5.00 13.0

Rotor swept area (m2/per rotor) 254 314 20 201

Rated Cp 0.41 0.45 0.35 0.40

Weight (ton) 140 60 (only drive trains, tower:site-specific)

N/a 200

15

It is evident that the power output of the devices is dependent on their size and the flow speed.

Although these devices can generate great amounts of energy in flow velocities higher than 2m/s, they

are not favored to generate tidal energy in lower flow velocities.

In shallow waters, the circular swept area of the rotor in the axial flow tidal turbines would be

restricted by the water depth, something that would limit maximum power output and eventually

hamper their economic potential (Roberts et al., 2016).

3.2.2.2 Cross-flow turbines

Performance data on commercially developed cross-flow turbines are less available than for axial-flow

turbines, which is an indication of the lower technological readiness level (TRL) they are at the

moment (Roberts et al., 2016). Therefore, only two academically developed cross-flow turbines are

presented and compared. The devices are not available in the market.

Table 2: Performance characteristics of academically developed cross-flow turbines (Roberts et al., 2016).

Study Göttingen 623 Hydrofoil (Yang and Shu, 2012)

Truss (McAdam et al., 2010)

Orientation Horizontal

Maximum power (kW) 0.095 0.003

Required flow speed (m/s) 1.50 0.30

No. of rotors 1 1

Rotor diameter (m) 0.50 0.50

Rotor length (m) 0.45 0.88

Rotor swept area (m2) 0.14 0.44

Rated Cp 0.41 0.53

The Göttingen 623 Hydrofoil design features a helical rotor and inclined blades, which improve the

performance and starting torque of the device, but are difficult and expensive to construct (Yang and

Shu, 2012; Baker, 1983). The Truss device (McAdam et al., 2010) also features inclined blades.

The cross-flow devices can produce more power for their size than axial turbines, which will be of

benefit in small scale deployments, particularly in shallow waters. However, their stage of

development until today is not enough for these devices to be commercially viable. Further testing of

these devices might be required.

3.2.2.3 Enclosed tips (Venturi)

As mentioned previously, the enclosed tips have demonstrated considerable progress in the last few

years. However, they require tidal flow velocities higher than 3.5 m/s, which in the current study is out

of the scope, due to the three selected sites. The characteristics of one device, BELUGA 9 by Alstom,

are presented on table 3. This specific device is not currently in the market.

16

Table 3: Characteristics of BELUGA 9, an example of enclosed tips tidal device (Global CCS Institute, 2018).

Device name BELUGA 9

Status Full scale prototype

Maximum power (kW) 1000

Required flow speed (m/s) 3.50-4.00

No. of rotors 1

Device diameter (m) 13

Device height (m) 20

Water depth (m) 30

A similar to BELUGA 9 device is Open Centre Turbine, a commercial scale turbine by OpenHydro. It

weighs 300 tonnes, has a 16 metres diameter and rated power at 2MW (OpenHydro, 2018). The

device is available in the market, but not all performance characteristics could be obtained for

comparison with other devices (fig.13).

Figure 13: Illustration of the Open Centre Device (OpenHydro, 2018).

3.2.2.4 Tidal kites

Tidal kites are a unique type of tidal technology. As illustrated in figure 14a. a tidal kite consists of a

small turbine, which is attached to a hydrofoil wing. The system is then bound to the seabed with the

use of a tether, which allows it to “fly” as a kite (Roberts et al, 2016).

The movement of the kite through the water results from the lift force, which is created over the wing

by the tidal stream flow (Roberts et al, 2016). The strain in the tether, in combination with the a

rudder can give a certain “flight” direction to the tidal kite through the the water column. This

movement increases the flowing speed and allows the device to generate greater amounts of power

from lower current velocities by using a smaller turbine (Roberts et al, 2016). The generator, which is

attached to the kite, converts the mechanical energy of the turbine to electrical energy, which is then

transmitted through a cable in the tether to the seabed and then to the shore via a subsea cable.

Minesto claims that the device moves 10 times faster than the water current. In water depths higher

than 50m, tidal kites certainly appear capable of producing considerable amounts of power in regards

to the size of their rotors (Roberts et al, 2016).

The majority of the global tidal energy resource is characterised by low velocity tidal currents, which

flow slower than 2.5 m/s. Deep Green is known to be able to produce electricity from those slower

currents (Minesto, 2017).The advantage of these devices is their ability to produce energy in tidal

currents velocities lower than 2.5 m/s, while horizontal axis turbines cannot (fig.14b). Table 4

summarises the most important parameters of several Deep Green devices.

17

Figure 14: a.Illustration of the Minesto Deep Green system. b. Graph showing the ability of the Deep Green technology

to produce energy in lower velocities than horizontal axis turbines.

Table 4: Technical specifications of the Minesto Deep Green device (O’Driscoll, 2012; Koca et al., 2013; Roberts et al.,

2016).

Device DG8 DG10 DG12 DG14

Wing span (m) 8 10 12 14

Rated power (kW) 120 220 500 850

Rated speed (m/s) 1.3 1.4 1.6 1.7

Tether length (m) 60-80 75-100 80-120 110-140

Installation depth (m) 50-65 60-80 60-120 110-140

Desired speed (m/s) 1.2-1.8 1.4-2.0 1.4-2.2 1.4-2.2

Lower/Upper cut off current (m/s)

0.5/2.5 0.5/2.5 0.5/2.5 0.5/2.5

Rotor diameter (m) 0.67 0.83 1.00 1.15

Weight (tonnes) 2 4 7 11

Devices/km2 50 30 25 16

Clearance (tip to surface) (m)

7.5-10 9.5-12.5 12-16 16-18

Power(W)/m2 5.50 6.60 12.5 13.6

Swept area (m2) 888 1400 2000 2700

Estimated Pd (W/m2) 19 24 44 45

Estimated rotor Cp 0.28 0.29 0.30 0.31

Nacelle length(m) 3 3.75 4.5 5.25

Nacelle diameter (m) 0.5 0.63 0.75 0.88

Nacelle weight (tonnes) 1.2 2.3 4 6.35

18

Minesto also intends to develop an even smaller Deep Green DG100 unit with 4-5m wing span and

rated power of 100 kW. Small scale testing has been performed in the past with devices of rated power

0.3 MW and this year the company’s first utility-scale project has been deployed in Holyhead Deep,

North Wales. It is expected that in the upcoming years, the devices will be used in larger power tidal

arrays as long as the market is ready to commercially accept the technology. Nevertheless,

theTidalKite Power Plant technology has presented a range of positive characteristics, which can be

critical in the scope of this study. The technology is sustainable as it produces entirely renewable

electricity without GHG emissions in the environment. The testing has shown minimal impact on

marine life and no impact on human activity. The power production is reliable due to the fact that it is

100% predictable. Last but not least, the technology is quite advanced, the design is quite simple with

a low kg/kW-installed ratio and standard components, therefore the Capital expenditure (CAPEX) is

low. In phone communication with the manufacturer, the cost of the installations could not be shared,

but it was stated that it will be estimated in a similar way with the wind turbine installations and

performance per MW.

3.3 Conclusions on the suitability of the different devices for the different sites

From the evaluation and comparison of all the proposed devices, there are a few conclusions that can

be reached by taking into consideration the different investigated sites.

The axial flow devices (see section 3.4.2.1) are, until now, the most commercially viable amongst the

tidal devices and have proved to be very energy efficient. However, they require high tidal current

speeds (higher than 2.5 m/s) in order to generate sufficient power. The three investigated sites are

characterised by rather low tidal current velocities, therefore the axial flow devices are not considered

suitable for these locations as they would not be able to perform well and reach their design potential.

The cross flow tidal devices are designed to perform better in low current velocities and their technical

characteristics appear to be promising for the exploitation of low current velocities. However, their

current stage of development is not encouraging for considering them viable in the scope of this study.

The tidal kites are also designed to utilize the low current velocities and their ocean testing has

exhibited encouraging results in power production. The devices are one step away from becoming

commercially available therefore they are considered to be the most suitable devices for the

exploitation of tidal current power in the studied sites. However, these devices require deep sea water

for the installation and taking into consideration this requirement, Thanet and Ormonde wind farms

would not be able to support the installation of tidal kites in reasonable distance from the farm.

According to bathymetry maps as the one in figure 15, the seas water around the Thanet wind farm

does not reach depths higher than 50m, whereas around the Ormonde wind farm, depths higher than

50m can be achieved, but in distance of more than 40 km away from the farm. These distances will

not be cost-effective for the project as only the transmission cable from the tidal devices to the

Offshore substation (OSS) would cost a lot, 60 MSEK on a rough estimation (Jeroense, 2018).

The Aberdeen offshore wind farm, on the other hand, is characterised by high depth values close to

the shore and in distances lower than 20km from the wind farm (fig.15). Specifically, average depth

values of more than 55m and tidal current velocities equal to at least 1.25 m/s can be observed within

10-15 km east off the wind farm or 13-18 km off the shore. The current velocities can be higher than

the values indicated on the ArcGIS maps, since the maps are not completely accurate due to variations

in several factors and tend to present lower values than the real values. Therefore, the installation of

tidal kites in the area could be supported since both depth and current velocity requirements are met.

19

Figure 15: UKSeaMap 2010 bathymetry layer in metres ( McBreen et al., 2011).

4. Technical assessment

In cases of emergency disconnections, the offshore wind turbines are equipped with an

uninterruptible power supply (UPS) system, which consists of lead-acid batteries, and can supply the

turbines with power for maximum 12 hours. However, there are extreme emergency cases when an

OWF might be disconnected from the power grid for up to three months. Therefore, OWFs are legally

bound to have a complete auxiliary service supply, an APS-system that can provide power for the wind

turbine loads during these situations (Berggren, 2013; REpower Systems SE, 2018) .

Offshore wind turbines have both constant and alternating loads that are always in need of

power. The total wind turbine load demand consist of a number of different smaller loads, such as

navigation lights, heating, ventilation and transformer magnetisation loads etc. (Bergreen, 2013).

Figure 16: A schematic picture of an offshore wind farm (Bergreen, 2013).

Nowadays, the auxiliary service power supply is carried out by diesel generators, which can be costly

and environmentally unfavorable due to their high carbon oxide emissions. Therefore, it is important

to provide new, sustainable and environmentally friendly alternatives, which can replace the current

use of diesel generators. Tidal energy devices can be the solution to the problem.

The study is focused on investigating the potential use of tidal devices instead of diesel generators in

OWFs as APS system, which is needed during disconnections from the grid.

20

4.1 Theory of the investigated offshore wind farm system

In order to find a suitable APS-system, alternative scenarios for three offshore wind farms,

owned/operated by Vattenfall AB, are being investigated with the assumption that the average load

power needed can be calculated in similar way as for the REpower 6M wind turbines. Due to

confidentiality reasons, the precise information regarding the load power needs for the turbines in the

three wind farms, MHI Vestas 8 MW, REpower 5M and Vestas V90-3.0 MW, respectively, was not

accessible, whereas for 6M WTs the information was accessible by another master thesis project

(Berggren, 2013). Nevertheless, on the grounds that the turbines are quite similar, the outcome of the

calculations-estimations can represent and be implemented for all three wind farms (Jonkman et al.,

2009). A typical REpower 6M wind turbine has the following characteristics as shown in table 5.

Table 5 : Basic data of REpower 6M wind turbine (Berggren, 2013; REpower Systems SE, 2018).

Wind turbine data cosφ 0.925 Rated Complex power S 6.65 MVA Rated Active power P 6.15 MW

The manufacturer REpower has in every WT placed a UPS-system, which main component is a lead-

acid battery. The UPS-system can provide the wind turbine with the average load demand up to 12

hours (Bergreen, 2013). The total load demand of a REpower 6M WT is shown in table 6. The existing

UPS-system, in combination with a new APS system could be able to provide the power that the OWF

will need for its emergency systems during long-term disconnections from the grid.

Table 6: The load demand in a REpower 6M WT.

Basic WT load data

Average load demand 42 kW

Peak load demand 193 kVA~ 193kW

4.2 Case studies

As mentioned previously, three wind farms owned/operated by Vattenfall AB are used as case study

scenarios for the potential implementation of tidal energy devices as auxiliary power systems, the

Aberdeen Bay wind farm, the Ormonde wind farm and the Thanet wind farm. All three of the wind

farms are located around the UK coast and they were chosen due to their good access in tidal energy.

The Aberdeen Bay wind farm is located in the North Sea, 3 km east off the Aberdeenshire, the

Ormonde wind farm is located in the Irish Sea, 9.5 km west of Barrow in Furness/Isle of Walney and

the Thanet wind farm is located 12 km off the coast of Thanet district in Kent. Some of the most

important characteristics of the three offshore wind farms are summarized in the following table 7.

21

Table 7: Offshore wind farms’ characteristics (4C Offshore, 2018).

Characteristics Offshore wind farm

Aberdeen Ormonde Thanet

Status Partial generation/Under construction

Operational Operational

Site area 19 km2 8.7 km2 35 km2

Maximum water depth

32 m 21 m 25 m

Distance of shore 3 km 9.5 km 12

Turbine capacity 8.4 MW 5.1 MW 3 MW

Number of turbines 11 30 100

Turbine Model MHI Vestas 8 MW REpower 5M Vestas V90-3.0 MW

4.3 Calculation of load flow in emergency mode

For each REpower 6M wind turbine the average load power during emergencies is estimated to be

PWTGEmergency = 42 kW including the WT transformer magnetisation losses. Therefore, the

average load power is calculated for all three different types of wind turbines in the three different

locations. For Aberdeen and MHI Vestas 8MW WTs, the average load power is 56kW, for Ormonde

and REpower 5M WTs is 35kW and for Thanet and Vestas V90-3.0MW WTs is 21kW.

As a result, the total load demand will be equal to POWFEmergency = 56 kW * 11WTs = 616 kW for

Aberdeen, POWFEmergency = 35 kW * 30 WTs = 1.05 MW for Ormonde and POWFEmergency = 21

kW * 100 WTs = 2.1 MW. The results are summarized in table 8. It is obvious that the total load power

needs are much higher for the Thanet wind farm as a result of the high number of the wind turbines

installed in the area.

Table 8 : Calculated average and total load power for the three investigated wind farms.

Wind farm

Average load power/ per turbine (kW)

Total number of the installed wind turbines

Total load power (kW)

Aberdeen 56 11 616

Ormonde 35 30 1050

Thanet 21 100 2100

The tidal devices should be able to provide the total average load power in each farm, in order to be

considered as viable option of substituting the diesel generators. The tidal kites are the devices with

the highest suitability for the three sites considered. Since the Aberdeen wind farm meets all the

requirements regarding the tidal resource and the desired depth for the devices, the farm will be used

as reference for the further calculations.

Deep Green technology has a rated power between 120 and 850 kW per device for tidal velocities

between 1.2 to 2.5 m/s. For the study, the Deep Green 12 (DG-12) is selected, which has desired

velocity 1.4 to 2.2 m/s and rated power 500 kW at 1.6 m/s. The DG-8 and DG-10 could be also more

suitable for the Aberdeen wind farm, but since the ArcGIS maps are not accurate and tend to show

lower current velocities, the DG-12 could operate as fine in the area as the other two devices and it is

22

more energy-efficient. However, more strict measurements should be performed before any

installation, in order to determine the exact tidal current velocity values and sea water depths in the

area. With a capacity factor of 20% (Bernas et al., 2015), the average power per device will be 100 kW.

The capacity factor value has resulted from continuous testing of the tidal kites the previous years in

tidal water velocities of minimum 0.5 m/s. The capacity factor is increasing and will increase more in

the upcoming years, however, in this study the value of 20% is used, as it is a secure value.Taking into

consideration the average load power needs of the Aberdeen wind farm, the number of tidal kites

needed would be approximately 7 with an average power generated 700 kW and rated power 3.5 MW.

These devices can be installed relatively close to each other within 1 km2 area, as 25 devices can be

installed per km2 area (Bernas et al., 2015).

5. Cost analysis In order to estimate the costs required for a tidal current energy array, the following three cost-groups

must be taken into consideration (Rourke et al., 2010):

● Capital costs are all the fixed expenses, which consist of the turbine costs, civil and structural

costs, electrical machinery costs, foundation or mooring costs, cabling costs, grid connection

costs, installation costs, commissioning costs etc. (Rourke et al, 2010).

● Operation and Maintenance (O&M) costs, which represent the total costs needed for

servicing, insurance, telecommunications, taxes and administration (Rourke et al, 2010).

● Financing costs are all the costs caused due to the financial needs of the project and include

repayment of bank loans and investors (Rourke et al, 2010).

5.1 Tidal kites

The cost for devices in an early stage of development are uncertain. According to Bernas et al. (2015),

the price for each tidal kite is approximately 37.6 MSEK. For the project, 7 Deep Green devices are

estimated that should be installed. The cost for these devices will be 7 devices x 37.6 MSEK = 263.2

MSEK. As the Tidal Energy Converter (TEC) market grows, it is expected that the capital cost will

decrease considerably. However, this cost trend is not noticeable yet.

The Aberdeen wind farm is located within a short distance from the shore and therefore there is not

an offshore substation, only onshore. This means that the tidal kite devices would connect to the

onshore substation directly. For this connection a transmission cable of 13-18 km, or to round it up 15-

20 km, would be needed for the connection to the substation and approximately 2 km for the

connection of the tidal devices. In total, the length of the cable needed would be between 17-22 km.

The cable will be three phase 33 kV waterproof cable, since this is the voltage output of the tidal kites,

with cross sectional area 95 mm2. From Appendix I, the cable cost would be 1092 SEK/m and the

installation cost would be 2077 SEK/m. In total, for 17-22 km, the cable cost would be 18.6 - 24.02

KSEK and the installation cost would be 35.31 - 45.69 KSEK. The voltage output of the wind farm in

Aberdeen is 66 kV and due to the distance of the tidal devices from the shore, a transformer will be

needed to increase the voltage from the tidal park to the grid, in order to lower the transfer losses. For

the voltage level of 66 kV, a transformer would cost 6,29 MSEK (Bitowt and Johansson, 2013).

Furthermore, tidal kites have lower operation and maintenance cost than other tidal technologies,

because they weigh less and are easily accessed. The proximity to the shore is an important cost

reduction factor. Routine operation and maintenance costs are estimated to be approximately 31.58

MSEK over a lifetime of 20 years (DTI, 2007), which the expected lifetime for the Aberdeen Bay wind

farm. Unexpected costs are not estimated, however there should be some consideration if the project

will be implemented. To sum up, in table 9, there are the total estimated costs for the tidal devices and

the cabling needed.

23

Table 9: Estimated minimum and maximum costs for the tidal devices and the cabling needed.

Cost item Minimum cost (MSEK) for distance 17km

Maximum cost (MSEK) for distance 22km

7 Deep Green devices 263.20 263.20

33kV cable and cable installation

0.05 0.07

Transformer 6.29 6.29

Routine O&M (20 years) 31.58 31.58

Total 301.62 301.64

From table 9, it is clear that the cost for the tidal power plant does not differ significantly for a

distance difference of 5 km and it is approximately 301.6 MSEK.

The cost of the devices shown on table 9 are retrieved from a project conducted in 2015. These prices

are estimated to have decreased since then, but there are not accessible reports that can provide

concrete information about the exact decrease range. A project with Deep Green devices is currently

implemented in Holyhead Deep, off the coast of North Wales. General information about the project

state that approximately 300 MSEK are secured for the installation of 20 tidal kites in the area, which

means that the cost of 7 tidal kites is probably 105 MSEK, half amount than in 2015 (Minesto, 2017).

However, the reports stating the exact costs for the project are confidential and not publicly

accessible, therefore the prices in this study are cautiously maintained similar to the ones in 2015.

The cost of a marine current energy device can be also expressed per unit of electricity generated

(cost/kWh). Levelized cost of energy estimates for tidal kites have shown electricity cost of 1570

SEK/MWh, whereas new estimates predict 1047 SEK/MWh after 100MW installed tidal stream

capacity, which indicates high cost-effectivess of the Minesto’s technology (Kay, 2017).

In order to reduce the cost of electricity generated thanks to tidal energy, CAPEX cost reduction

should be pursued while improving the device performance (Magagna et al., 2016). A reduction of the

capital costs by 50% could lead to a decrease of total LCOE by 20%-30%, whereas a 50% improvement

of the capacity factor would lead to a 33% cost reduction for tidal energy (Magagna et al., 2016).

As previously mentioned, the electricity generated from the tidal kites can be used for providing not

only emergency power to the wind farm, but also additional power to the grid during normal

operation. This can create profit for the owner of the wind farm. With an average power of 700 kW,

the tidal kites can produce 16800 kWh per day, which equals to 6,132,000 kWh per year and

eventually 122,640,000 kWh in 20 years. With a conservative price for renewable energy

approximately 0.30 SEK/kWh, the tidal kites could generate a profit of total 36.8 MSEK in 20 years.

Here also feed-in tariff can be added.

5.2 Diesel generators

There could be two types of generators installed on the offshore substation (onshore for Aberdeen),

emergency and standby generator. The emergency generators provide support for life safety systems,

such as emergency lighting, fire protection and ventilation, whereas the standby generators provide

power for all the other uses, such as backup power for data centers, auxiliary systems, refrigeration,

and HVAC systems. The size of the emergency generators is usually 200 kW or smaller and for the

standby generators is between 200 kW and 2 MW (Munich RE, 2014).

24

The approximate cost for a commercial generator is 2700 SEK to 4500 SEK per kW (American

Generators, 2015). This means that for an emergency generator 200 kW, the cost would be between

0.54 MSEK and 0.90 MSEK, whereas for a standby generator 1 MW, the cost would be within the

range of 2.7- 4.5 MSEK. This price ranges do not include the costs for the foundation, the installation,

the exhaust funnel, a 20-40 ton tank system and the connection to the remote control system

(Supervisory control and data aqcuisition system SCADA). In order to include these costs as well, the

initial price range is multiplied with 1.3 - 1.45. The total cost for the emergency generator and the

standby generator is estimated to be between 0.7 -1.3 MSEK and 3.5-6.5 MSEK, respectively.

Furthermore, depending on the load at which the generator is operating at, there is different fuel

consumption for the two generator types. With the assumption that the generators will operate at ½

load, the fuel consumption is 29.15 l/h and 137.79 l/h, respectively . Various factors, outside the scope

of this study can increase or decrease the fuel consumption. According to current british fuel prices,

the average price for diesel is 15.3 SEK/litre. However, offshore projects usually pay reduced fuel taxes

and prices depending on their location and consumption. Therefore an average price per litre of fuel of

10 SEK/lt is used for the calculations.

In this study the hypothesis of a yearly use of the generators for 30 days is assumed for the

calculations. Although the disconnection from the grid might not last for 30 days long every year, this

assumption takes into consideration the time that the generators will need for maintenance and

refuelling on yearly basis. Therefore, in 20 years, the generators will use fuel for at least 600 days,

which means 14400 hours in total. The total fuel consumption cost will then be 4.2 MSEK for one

generator 200 kW and 19.8 MSEK for a generator 1 MW. Although there might be no grid

disconnections for long terms of time, there is the need of regular fuel maintenance and keeping the

engine run in order to prevent the generator from failure. For example, for the fuel maintenance the

fuel must change at least once per year. Other fuel needs will depend on daily consumption of the

generator while it is functioning as back-up system or not.The cost of fuel is quite high, nonetheless,

and it can be unpredictable as it is constantly changing.

Furthermore, the diesel generators will need to be maintained in continuous intervals, weekly,

monthly and annually, something that will add up to the estimated cost. Typical maintenance check

can include oil, water and coolant levels or leaks, electrical functionality, engine noise, colour of

exhaust fumes, nuts and bolts tightness and more [30]. Maintenance will be also required during the

grid disconnections when the generators will be used for the auxiliary power supply. Specifically, the

generators must be checked and maintained after every 8 hours, 100 hours, 250-300 and 1200-2000

hours of diesel engine continuous running. Although the substation in Aberdeen is not offshore, an

approximate maintenance estimation cost of 60 MSEK and 70 MSEK, respectively, is used as the

study is aimed for offshore wind farms. This amount includes ship or helicopter transportation cost

from the shore to the offshore platform and it is smaller for the emergency generator due to their

difference in tank size. A farm can be entered from crew transfer vessel (CTV) boat in 1,5 – 2 m wave

height and from helicopter in 25 – 30 m/sec wind (Slot, 2018). A helicopter cost about 30.000 kr per

hour and a CTV about the half, but it is normally hired for longer periods (Slot, 2018). Most of the

companies that rent diesel generators include maintenance services in the rental price. In table 10, a

summary of the costs for both generator types is presented. The maintenance of the generators is

responsible for the biggest amount of the cost.

25

Table 10: Estimated costs for the two generator types.

Generator type Emergency 200kW Standby 1MW

Capital cost (MSEK) 0.7 - 1.3 3.5 - 6.5

Fuel consumption cost for 20 years (MSEK) 4.2 19.8

Maintenance cost (MSEK) 60 70

Total cost per device (MSEK) 64.9 - 65.5 98.3 - 101.3

The total number of 200 kW generators needed to meet the average load requirements of the wind

farm would be four, therefore the total cost for the emergency type of generator would be between

89.6 and 92 MSEK. The maintenance cost is set 70 MSEK for all generators, due to the fact that they

might need maintenance in different times.

Although the standby generator is a much cheaper solution, as it seems from the cost estimations, this

type of generator cannot work for long periods of time without interruption. The main problem they

exhibit is overheating, which will inevitably force the generator to stop and additional maintenance

will be required. That means that more than one standby generator would be needed if the

disconnection from the grid lasts longer than a couple of days. The addition of one more generator

would result to a cost range between 116.6 MSEK and 122.6 MSEK and maybe more if the two

generators require maintenance in different times.

6. Construction/Implementation/Comparison with other alternatives

6.1 Physical prerequisites and setup

The aim of the project is to investigate whether a tidal park will be able to provide enough electricity to

the offshore wind farm for three months without grid connection, how many tidal devices would be

sufficient and how much power would be needed to be generated in that case.

From the technical assessment and the cost analysis, it is concluded that a tidal park with 7 Deep

Green tidal kites of 700 kW average generated power and 3.5 MW rated power is able to provide

enough electricity for three months of disconnection. The tidal kites can be directly purchased by

Minesto, who is currently the only company producing commercial-scale tidal kites. Alll the tidal

generation equipment will be directly purchased from the manufactures. The devices can be easily

transported to the installation spot by small ship, since the distance from the shore is smaller than

other offshore installation projects. The 7 devices will be assembled into an array, which will be

moored on the seabed. The construction and installation of the project might take up to 6 months, if

not taking into consideration bad weather conditions. An illustration of the potential tidal farm close

to the Aberdeen offshore wind farm is presented in figure 17. The distance between the tidal farm and

the onshore substation is estimated to be 15-20km.

26

Figure 17: Illustration of the potential tidal farm close to the offshore wind farm of Aberdeen.

6.2 Project timeline

A GANTT chart with a simplified timeline for the project in Aberdeen is presented in figure 18. A

feasibility study for the tidal power plant in the area is the first step for the optimal implementation of

the project. The collected data will provide more concrete information about the tidal resource, the

bathymetry and the seabed structure. The study might take up to one year. During the first year,

consultation of commercial parties and other stakeholders must be also included in the decision

making process. The second year will be devoted to permits, financing and investment decisions. The

selection and optimization of the tidal technology might take up to two years (2nd and 3rd year),

whereas all the necessary the equipment for installation will be secured during the third year. After

the successful purchase of the equipment and the optimization of the technology, the project will be

ready for installation, in the beginning of the fourth year. This part of the project can take 6 months or

more than one year, if bad weather conditions, accidents or any other emergencies that might force

the work to stop occur. The tidal power plant can be operational from the first part of the fifth year,

according to the estimated project timeline.

Figure 18: Estimated timeline for the tidal power plant in Aberdeen project.

27

6.3 Stakeholder consultation and participation/ analysis

The early engagement and participation of the stakeholders in the project is essential for the the

effectiveness of the project development process. Working with stakeholders throughout

development, deployment, and operation of marine renewable energy processes can promote better

understanding and reduce stakeholder concerns.

The stakeholders can be divided in 3 main target groups according to their interest and involvement

proximity:

1.The first group consists of European/National/Local authorities and public bodies, who are the main

regulators and who can provide permission for the project. Their role is crucial in every stage of the

project development.

2.The second group could consist of the customers and investors, such as utilities,banks and private

financiers.

3.The last group could consist of the technology developers, supply chain and original equipment

manufacturers.

6.4 Regulations and permitting issues

Regulators are faced with significant uncertainty about the potential environmental effects of marine

renewable energy (MRE) devices on the surrounding environments. Because of this uncertainty,

regulators usually take a precautionary approach, requesting extensive pre- and post-installation data

to determine the significance and potential adverse effects of interactions of marine animals and

habitats with all parts of MRE devices and systems.

In most nations, permitting processes for MRE development are driven by environmental legislation

and regulations that require a significant level of evidence to support conclusions about whether

potential impacts are acceptable. Most of the time, collecting sufficient data to satisfy that level of

certainty presents challenges for the emerging MRE sector.

This perception of high risk by the regulators results from the lack of understanding of the features

and operation of MRE systems, the relative newness of the technologies, as well as external pressure

from other stakeholders who fear competition and degradation of the ocean space.

It is important that areas consented by governmental authorities for the purpose of marine current

energy farms or offshore wind farms do not overlap with protected habitats, shipping lanes etc. In the

UK, Crown Estate is the governmental authority who has to give permission for the potential

implementation of this project, as it owns most of the UK’ seabed.

6.5 Comparison with other alternatives

6.5.1 Photovoltaic devices

Photovoltaic devices are devices , which have the ability to convert the solar power into electrical

power. They require large areas, therefore their installation either on the substation roof or on the

wind turbine nacelle roof does not result to the production of adequate power due to limited space

(Berggren, 2013). Tidal kites can reach the load demand without the need of space on the WTs or

substation. Another disadvantage of the photovoltaics is that they cannot generate electricity during

night hours, when the navigation lights are essential for the offshore wind farms. During wintertime,

Northern Europe experiences short days and long nights, thus photovoltaics would not be a viable

solution for these areas.

28

6.5.2 Wave power farms

Wave power is one more emerging marine renewable energy source. However, the technology is not

commercial yet and the installation and construction cost is quite high. Moreover, around the

Aberdeen wind farm, the wave height is not quite significant, compared to the wind speed. This is not

good for wave energy exploitation, but it is positive for the installation of the tidal devices in the area.

7. Energy storage solutions

In the case study, the WTs are equipped with UPS-system, which has a lead-acid battery series and

can provide power for up to 12 hours. This is enough considering the fact that tides can provide energy

more often within a time frame of 12 hours. However, the tidal kites will also require the use of a

storage system, which will allow the storage of tidal power during the peak power rates when the is no

disconnection from the grid and thus no emergency needs for them to cover. Two types of batteries

are examined as energy storage solutions with focus on the primary operation and survivability of the

tidal energy devices, as well as in cases of energy excess: lead acid batteries and lithium ion batteries.

It is necessary that the tidal device’s loads are identified as most of them will require constant power

supply, therefore, the energy requirements for the onboard energy storage system will be more clear.

7.1 Rechargeable battery types

a. Lead acid batteries

There are two types of lead acid batteries, the Valve Regulated Lead Acid (VRLA) battery and the

Absorbed Glass Mat (AGM) battery (Murray et al., 2017). The first one consists of cell containers,

which are sealed so they can prevent possible leakage, and a pressure relief non-return valve (Murray

et al., 2017). The valve controls the safe release of the oxygen and the hydrogen formed during

charging. The Absorbed Glass Mat (AGM) battery is a variation of the sealed VRLA battery. In this

sealed battery, the acid is absorbed between the positive and negative plates and bound by a fiberglass

mat (Murray et al., 2017).

The Lead acid batteries are an established technology, which requires minimum parts and charging-

discharging through a range of standard drives. The batteries have demonstrated a high capability of

providing large power in highly humid environments, with large vibration and movement range, in

different temperatures, over many years with many charge and discharge cycles (Murray et al., 2017).

The disadvantages of these batteries are their large weight, their power density, which is less than

lithium ion batteries and the safety issues due to hydrogen gas, which requires clearances and

ventilation. Through generally produced hydrogen will remain below flammable concentration and

will rise and dissipate through walls and fittings.

b. Lithium ion batteries

The lithium-ion family is divided into three major battery types:lithium-ion-cobalt or lithium-cobalt

(LiCoO2), lithium-ion-manganese or lithium-manganese (LiMn2O4) and lithium-ion-phosphate or

lithium-phosphate (LiFePO4) (Buchmann, 2015). During installation, commissioning and operation

of lithium ion batteries, the following must be taken into consideration (Murray et al., 2017):

● Operation above 80oC or operation of defective batteries can result in the bursting of the Li-

ion batteries and discharge of liquid electrolytes and flammable gases. There is risk of the

electrolytes and electrodes reacting with water and humidity. Serious health and

environmental hazard can result from the direct contact with the components of the battery.

For all the above reasons, adequate physical protection and ventilation must be provided

(Murray et al., 2017).

● Damaged batteries are dangerous and they should be surrounded with dry sand, chalk powder

(CaCO3) or Vermiculite and if possible stored outdoors.

29

● Smoke alarms are required at the installation location.

● It is recommended to store the batteries in a cool and dry place, protected from sunlight

exposure, radiation and heat sources.

● The battery system should not be covered or concealed so it can release waste heat optimally.

● Only approved battery inverters and chargers with safety and compatibility issues within a

range of hundred volts dc are suitable.

● The Battery Management System (BMS) often with a Controller Area Network (CAN) data

interface is responsible for the control of the voltage and temperature values for the individual

battery cells (Murray et al., 2017).

● Two persons are suggested for the installation and disassembly with associated PPE.

● Completely discharging the battery may permanently damage it.

These batteries are more energy dense and lighter than lead acid batteries. They present good cycle

life parameters, which are ideal for solar photovoltaic charging profiles. The established technology of

these batteries has lead to their increased popularity in all applications and their cost reduction.

However, there are a few disadvantages, such as the fact that there are many recently reported heavy

publicized failures, where explosions and fires affected the development of prototypes (Murray et al.,

2017). There are Increased health and environment concerns compared to lead acid batteries,

especially in a prototype MRE system which is at risk of sinking or internal compartments flooding.

Furthermore, these batteries are expensive, since they require special equipment and inverters.

Table 11: Characteristics of lead-acid and Li-ion batteries (Buchmann, 2015).

Battery characteristics

Parameters Lead-acid Li-ion

Cell voltage (V) 2 3.3-3.8

Specific energy (Wh/kg) 30-50 90-190

Fast-charge time 8-16 h 4 hours and less

Maintenance requirement 3-6 months N/A

8. Environmental impacts

The examination of the environmental impacts of the projects is important for the optimal scaling and

siting of the tidal stream turbines. Nowadays, there is not adequate and accurate data regarding the

potential environmental impacts from tidal energy devices and systems as the technology is relatively

new. However, a lot of information, that has been gathered from the hydropower and offshore wind

industry, as well as from ship propellers and offshore oil industry, can be used as guidance to

determine these impacts. Furthermore, a few assessments, which have been performed on testing sites

mainly, have identified a number of potential environmental impacts as a result of the tidal energy

development. The most noticeable impacts are presented here (Copping et al., 2016; OES, 2017):

1. The tidal devices and the foundation structure are the main sources of disturbance in the

water column during the installation phase and the removal phase after the lifetime of the

devices has expired. This disturbance may include sediment transport and water quality

change due to flow changes.

2. During the operation of the devices, there is also some disturbance caused in the water

column due to the possible device wake or blade strike. Although there is high concern about

30

the changes in biodiversity in the surrounding area, the installed stationary structures can

attract fouling organisms, which function as fish aggregating devices or artificial reefs and

they become food source for many marine animals.

3. During both installation and operation, there is some noise caused. This noise might be

unbearable to some marine species, which may seek other locations instead. The duration

length of the installation might take from several weeks to several months, therefore it is

important that mitigation measurements are taken. The noise from the tidal devices during

their operation is significantly of lower amplitude than other industrial uses.

4. One of the biggest concerns of the stakeholders is the potential chemical contamination that

might occur from the use of paints and coatings for the tidal devices.

5. The electromagnetic field (EMF) emissions from the generator, export cables or other

electronics could alter the navigation and the hunting ability of the marine species, who are

electro- or magneto-sensitive. However, the exposure of marine animals to EMFs due to the

tidal system can be estimated and compared with the exposure of the marine animals to EMFs

due to other subsea cables used for power and telecommunication, bridges and tunnels. These

cables have been present for years and their emissions are measurable.

6. The possibility of overlapping and competition with current marine uses, such as fishing,

shipping and conservation.

7. Potential collision of the tidal blades with the marine animals. Although there is such risk, the

tidal turbine blades rotate in slower speeds than ship propellers or hydropower turbines.

Therefore, they provide the marine animals with the necessary time frame they need to avoid

any collision and in circumstances where a collision occurs, the damage is quite small and the

animals can survive (Copping et al, 2015).

Tidal kites are characterised by small dimensions compared to other tidal devices and are not visible

from the surface. Therefore, they cannot draw negative attention aesthetically. However, like the rest

of the tidal devices, there is potential for noise and vibrations that might disturb the surrounding

environment, the potential for water flow disturbance caused by the tether and collision risk with

marine animals due to the flight path of the tidal kite (O’Driscoll, 2012). Acknowledging the danger

associated with the offshore environment, the developer should take actions to minimise the risks to

humans, marine animals and the equipment from the design phase already.

The regulators, stakeholders and developers should all collaborate for the mitigation of the negative

impacts that tidal devices may cause. Although it seems that the negative impacts are not as

significant, the uncertainty and lack of definitive data forces the regulators to be cautious with the

tidal renewable energy interactions and perceive them to be of high risk (Copping et al, 2016; OES,

2017).

9. Monitoring

One of the most important parts of the project is the monitoring, which aims to ensure that the project

is implemented as planned. The monitoring part includes the documentation of the tasks required for

the project and their performance or potential consequences. Not only the project remains within

scope without extra costs or delays, but it can be also used as an example for future implementations.

9.1 Software and tools for monitoring

The right planning and array design of potential ocean marine energy sites is quite important for their

optimum exploitation. The wrong placement of the tidal energy devices might result in undesired total

energy yield. Estimating the energy yield for tidal energy arrays accurately enables the stakeholders to

have confidence in the return on investment.

31

Technology achievements nowadays can offer the most advanced and safe planning and monitoring

solutions for the tidal parks. Some of these commercial products are the following:

1. TidalFarmer is the first commercially available tidal farm 3D array modelling and planning

tool, created by the Energy Technologies Institute (ETI) as part of the PerAWaT (Performance

Assessment of Wave and Tidal Array Systems) R&D project and it is launched by DNV GL.

The tool can estimate the energy yield and perform resource assessments for given site

specific conditions with high accuracy and minimum cost risks and project uncertainty. The

tool can used to test the tidal array virtually before construction.

2. SMARTtide (Simulated Marine Array Resource Testing) is a 2D hydrodynamic model for

tidal energy modelling projects of the UK’s continental shelf. The output is GIS compatible

file. With the use of SMARTtide, it is easier to assess the interactions between tidal energy

systems around the UK and around other marine structures and identify the possible impacts

of the different tidal farm layouts and locations on the tidal resource. The model ensures the

limitation of environmental changes and monitors the sediment mobility and coastal erosion.

This way, the exploitation of tidal energy around the UK is monitored effectively.

10. Uncertainties

Regarding the tidal devices, maintainability is a major issue since failures are very likely to happen.

The components of the tidal system should be protected by sea-water ingress, temperature variations

and corrosion risks. Another concern is the variability within the tidal current, which results in load

variability upon the support structure, the tidal turbine and the gearbox. As a result, the performance

of the tidal devices decreases and they might be led to failure.

Some key factors that can affect the energy capture of the tidal devices (Fraenkel, P. 2012) are the

choice of location, their position in the water column (75% of energy is in top half of water column),

the rotor area and the device efficiency. The energy capture is proportional to both rotor area and

efficiency. The selection of a suitable location for the placement of the tidal stream generators can be

realised primarily with the use of navigational data and to some extent numerical modelling.

However, high quality survey data would be required for full scale deployment. In order to maximize

the total power output of the energy system in the selected area, the investigation of different

combinations of generators should be performed.

There is considerable uncertainty attached to the quality of current observations, which is directly

influenced by the fact that to deploy and maintain a current meter is 4-10 times as expensive as to do

similar activities to measure water levels. Current speed and direction can vary greatly over short

distances. Moreover, currents are strongly influenced by local conditions and can change in unknown

ways when local circumstances change (e.g. natural processes move sand bars or reshape the bottom).

One of the major uncertainties regarding the permissions and regulations, which will apply in this

project is the fact that UK has announced its separation from the European Union. This decision will

most probably affect and change the current trade policies, the funding policies for renewable energy

projects by EU and the regulatory framework that is being currently applied.

During the construction and installation of the tidal devices, the weather conditions provide the

highest degree of uncertainty, since they cannot be predicted during the project planning. In the case

of unpredictable weather conditions, the project might be delayed for short or long term and cost a lot

more than it was initially planned. Therefore, a gap for bad weather conditions should be added on the

project timeline. This gap is considered in this project.

32

11. Results and Discussion

From the resource assessment performed and the device alternatives provided in this study, the

Aberdeen wind farm was selected for further investigation with the use of tidal kites as suitable

devices for the project. The tidal power plant was assessed technically and financially whether it can

provide affordable back-up power to the wind farm during grid disconnections.

From table 12 and figure 19, it is evident that the tidal devices are a much more expensive solution

than diesel generators. The cost for four emergency generators 200 kW or two standby generators 1

MW is almost one-third of the total cost for the tidal kites. However, as it is mentioned in section 5.1,

the cost of the tidal kites nowadays is estimated to be much lower, almost half of the values presented

here. Due to confidentiality reasons and lack of access of the proper documentation, the values from

2015 are being used. Nevertheless, if the cost of tidal kites is soon officially confirmed that it is half of

what is stated in this report, then the costs for the tidal kites and diesel generators would be

comparably close to each other.

The biggest part of the total cost of tidal kites is a result of their high capital cost, which has

considerably decreased in the last years and will further decrease after their availability in the market.

On the other hand, the biggest part of the total cost of the diesel generators is created due to their

demanding maintenance, especially when they are located on offshore substations.They require

increased human refuelling intervention, on weekly, monthly or yearly basis. Eventually, the

operational cost for the use of diesel generators in long term can reach the capital cost of tidal devices,

whose operational costs are quite low. Although diesel generators are a proven and robust solution,

they cause air and noise pollution and negative environmental impacts due to GHG emissions.

The tidal devices can produce energy without the need of fuel, which in case of excess can provide to

the grid directly. Moreover, the benefits for the environment are more significant with the use of tidal

devices. The costs for the production and installation is expected to drop more in the near future, with

the increase of their commercialisation. Furthermore, the tidal kites will make use of the existing

infrastructure in Aberdeen, while producing back-up power, but also extra power every day.

Table 12: Estimation of the total costs for the tidal devices and the diesel generator.

7 Tidal kites 4 Generators 200kW

2 Generators 1MW

Total cost (including foundation and installation costs) in mil SEK for 20 years

301.6 89.6 - 92 116.6 - 122.6

The total cost for 7 tidal kites is approximately 301.6 MSEK, but they also provide profit of 36.8 MSEK

in 20 years, as it was calculated in section 5.1. This means that their “real” cost reduces to 264.8

MSEK.

From figure 19, it is evident that the cost for 7 tidal kites is quite high, but the cost for four emergency

generators 200 kW or two standby generators 1 MW is almost one-third of their price. However, for

the standby generators, the continuous operation for long length of time might not be feasible due to

their size and will only result to production of 25% of its total rated power. That is, because, these

generators have greater cooling and temperature controlling needs when the power production is

required for long time (more than 12-24 hours). In that case, two or more generators would be needed

to cover the average load need of 616 kW for the wind farm. Therefore, the three solutions are

financially comparable to each other in the long term and if the grid failures occur for more than 12

hours.

33

Figure 19: Chart with the total costs of the potential backup system solutions: 7 tidal kites, the 4 emergency

generators 200 kW and the 2 standby generators 1 MW.

The total cost for the Aberdeen Bay wind farm is approximately 3964,3 MSEK. In order to determine

whether the tidal kites would be a good investment and should or should not be installed, their

estimated total installation and operation cost is being compared as part of the total cost of the

Aberdeen wind farm project. As it is shown in figure 21, the cost for the tidal kites project is equal to

the 7.6% of the total cost for the Aberdeen wind farm. This cost is quite high if the tidal kites would

operate only as back-up energy system. However, as mentioned previously, the tidal kites will be able

to produce energy on daily basis, whether it is needed for emergency disconnections or not. This

energy is extra energy, which can be distributed to the grid and thus function as hybrid system with

the offshore wind farm. The tidal kites would use the already existing infrastructure, which does not

add more expenses in the already estimated capital costs and their maintenance would be part of the

entire project’s expenses.

Figure 20: Total costs of the potential backup system solutions as part of the total cost for the Aberdeen farm project.

34

12. Conclusion

Tidal stream energy has the potential to be a competitive low carbon energy source for the future of

the UK energy system, as well as worldwide. Smaller devices that can extract energy from lower

velocities and higher depths are necessary with tidal kites currently paving the way.

This study shows that tidal kites can be an efficient backup solution for the auxiliary services of

offshore wind farms like the Aberdeen Bay wind farm. Not only can the tidal power provide all the

necessary power during emergency disconnections with only a few devices, but also generate excess of

energy, which can be stored or distributed to the grid. The cost of this solution might seem quite high

compared to the use of traditional diesel generators, but tidal kites have less operational costs and

produce more energy in the long term. The capital costs of these devices are expected to decrease due

to the technology advancement and the steadily increasing competition in the marine renewable

energy market.

In the Aberdeen wind farm case study, seven tidal kites with average power 700 kW and rated power

3.5 MW are able to provide adequate power to the offshore wind farm for three months without grid

connection, whereas they can provide excess of energy on daily basis when grid disconnection does

not occur. The total cost for the project would be 301.6 MSEK.

The cost for the project is quite high compared to other solutions according to the current market

situation in offshore wind and in general, the renewable energy industry. Therefore, at the moment, a

project like this might not be feasible without high investment risks for companies like Vattenfall. In

the near future, it is expected that the cost of the tidal devices will decrease and this project can be

reevaluated and possibly implemented as it is suggested here.

13. Recommendations for future studies and further work

In future studies, more locations should be assessed, from which the most suitable should be selected,

by taking into account both wind and tidal resource potential. As the marine energy technology

advances, the tidal devices will be able to extract energy from lower tidal stream velocities equally in

small and big depths and most importantly, in lower prices. Thus the combination of wind and tidal

farms in a hybrid system will be less demanding financially. Unfortunately, due to limited time and

confidentiality reasons from the manufacturers’ side, a detailed cost estimation, cost of energy (COE)

and profitability analysis (e.g. net present value NPV, internal rate of return IRR) was not possible for

this study. However, this is something that should be definitely performed in the future before any

project implementation.

Apart from the regular offshore wind farms, floating offshore wind farms should be considered as they

might be a good solution for bigger sea depths. Floating offshore wind farms are still in an early stage

of development, but the first floating wind farm, Hywind Scotland, shows that its successful

performance until now might encourage the building of more floating wind farms in the future. Tidal

testing centres with this specific role could be established in the future, where offshore wind is

combined with tidal device testing. Moreover, thorough Life Cycle Analysis (LCA) should be

performed for investigating the environmental impacts of new technologies before any installation.

A detailed bathymetry and water column velocity profile should be designed in future tidal resource

assessments for potential tidal power plant construction and installation. The current study is using a

rather idealised environment, which differs significantly from real site performance characteristics.

35

Technology advancement will favor energy storage options as well, providing higher storage capacity

in lower costs. Therefore the investigation of further storage options in the future is recommended.

36

References

Bahaj, A.S., Myers, L.E. 2003. Fundamentals applicable to the utilisation of marine current turbines

for energy production. Renew Energy 28:2205–2211.

Baker, J.R. 1983. Features to aid or enable self-starting of fixed pitch low solidity vertical axis wind

turbines. 15(1–3):369–390.

Berggren, J., 2013. Study of auxiliary power systems for offshore wind turbines: an extended analysis

of a diesel gen-set solution. Uppsala Universitet.

Bernas, M., Kang, D.H., Koo, J., Nerurkar, A. 2015. Tidal and Wind Power Plant Proposal, EE 526 –

Renewable Energy in Power Systems. University of Southern California. Viterbi School of

Engineering. Ming Hsieh Department of Electrical Engineering.

Bitowt, M., Johansson, M. 2013. Design of a tidal power park and a wave power park with a techno-

economical approach. A study of subsea components, optimisation of the collection grids and an

investigation of the transmission with the opportunity to share the cable with a wind park. Chalmers

University of Technology.

Black and Veatch, 2004. Phase I. UK tidal stream energy resource assessment.The Carbon Trust.

Black and Veatch, 2005. Phase II. UK tidal stream energy resource assessment. The Carbon Trust. 10-

31.

Blunden, L. S., Bahaj, A. S. 2006. Initial evaluation of tidal stream energy resources at Portland Bill.

UK. Renew Energy. 31:121–132.

Charlier, R. H. 2003. A ‘‘Sleeper’’ awakes: tidal current power. Renewable and Sustainable Energy

Reviews. 7: 515–29.

Coles, D. S., Blunden, L. S., Bahaj, A.S. 2017. Assessment of the energy extraction potential at tidal

sites around the Channel Islands. Energy. 124: 171-186.

Copping, A., Grear, M., Jepsen, R., Chartrand, C., Gorton, A. 2015. Understanding the potential risk to

marine mammals from collision with tidal turbines. Proceeding of the 3rd Marine Energy Technology

Symposium, METS2015, Washington, D.C.

Copping, A., Sather, N., Hanna, L., Whiting, J., Zydlewski, G., Staines, G., Gill, A., Hutchison, I.,

O'Hagan, A., Simas, T., Bald, J., Sparling, C., Wood, J., Masden, E. 2016. Annex IV 2016 State of the

Science Report: Environmental Effects of Marine Renewable Energy Development Around the World.

de Alegria, I., Martin, J. L., Kortabarria, I., Andreu, J., Ereno, P.I. 2009. Transmission alternatives for

offshore electrical power. Renewable and Sustainable Energy Reviews 13:1027–38.

deCastro, M., Costoya, X., Salvador, S., Carvalho, D., Gómez-Gesteira, M., Sanz-Larruga, F. J.,

Gimeno, L. 2018. An overview of offshore wind energy resources in Europe under present and future

climate. Annals of the New York Academy of Sciences.

DTI, 2007. Economic viability of a simple tidal stream energy capture device.

Eamon, R. 2015. North seas countries offshore grid initiative. Outcomes from the 2015 Ostend

Conference and pathways for the dutch presidency. E3G.

37

Garrett, C., Cummins, P. 2007. The efficiency of a turbine in a tidal channel. J. Fluid Mech. 588:243–

251

Ghefiri, K.; Bouallègue, S.; Garrido, I.; Garrido, A.J.; Haggège, J. 2017. Complementary Power Control

for Doubly Fed Induction Generator-Based Tidal Stream Turbine Generation Plants. Energies, 10:

862.

Greaves, D., Iglesias, G. 2018. Wave and Tidal Energy. Hoboken, NJ: Wiley p.124

Hardisty, J. 2007. Assessment of Tidal Current Resources: Case Studies of Estuarine and Coastal

Sites, Energy & Environment, Vol. 18, pp. 233-249.

Holmes, R., Bulat, J., Henni, P., Holt, J., James, C., Kenyon, N., Leslie, A., Long, D., Morri, C.,

Musson, R., Pearson, S., Stewart, H. 2004. DTI Strategic Environmental Assessment Area 5 (SEA5):

Seabed and superficial geology and processes. British Geological Survey

Johnstone, C.M., Pratt, D., Clarke, J. A., Grant, A. D. 2013. A Techno-Economic Analysis of Tidal

Energy Technology, Renewable Energy. 49: 101-106.

Jonkman, J., Butterfield, S., Musial, W., Scott, G.. 2009. Definition of a 5-MW Reference Wind

Turbine for Offshore System Development. Technical Report NREL/TP-500-38060.

JRC. 2014. Ocean Energy Status Report

Koca, K., Kortenhaus, A., Holtz, H., Angelelli, E., Zanuttigh, B., Kirca, Ö., Bas, B., Elginöz, N., Bagci,

T., Sarmiento, J., Armesto, J.A., Guanche, R., Losada, I. 2013. Energy converters. MERMAID project

deliverable: D 3.3. Seventh framework programme.

Magagna, D., Monfardini, R., Uihlein, A. 2016. JRC Ocean Energy Status Report: 2016 Edition. EUR

28407 EN. Luxembourg Publications Office of the European Union.

Maulud, K. N. A., Karim, O.A., Sopian, K., Aziz, S.N.F.A. 2009. Determination of Tidal Energy

Resource Location in East Coast of Peninsular Malaysia Using Geographical Information System

McAdam, R., Holsby, G., Oldfield, M., McColloch, M. 2010. Experimental testing of the transverse

horizontal axis water turbine. IET Renew Power Gener 4(6):510–518

McBreen, F., Askew, N., Cameron, A., Connor, D., Ellwood, H., Carter, A. 2011, UK SeaMap 2010

Predictive mapping of seabed habitats in UK waters. JNCC Report No. 446, Peterborough

Murray, D. B., Gallagher, P., Duffy, B., McCormack, V. 2017. Energy storage solutions for offshore

wave and tidal energy prototypes. Twelfth International Conference on Ecological Vehicles and

Renewable Energies (EVER). Institute of Electrical and Electronics Engineers (IEEE).

Ocean Energy Forum, 2016. Ocean Energy Forum: Ocean Energy Strategic Roadmap 2016, building

ocean energy for Europe.

O'Doherty, T., O'Doherty, D. M., Mason‐Jones, A. 2018. Tidal Energy Technology. Wave and Tidal

Energy. 105-150.

38

O’Driscoll, B. 2012. Ecological hazard identification of the Minesto ‘Deep Green Commercial’ tidal

stream turbine. Master of science thesis. Department of Energy and Environment. Division of

Environmental Systems Analysis. Chalmers University of Technology. Goteborg, Sweden

OES, 2017. Annual report: An overview of ocean energy activities in 2017. The Executive Committee of

Ocean Energy Systems.

Pugh, D., Woodworth, P. 2014. Sea-level science. Understanding tides, surges, tsunamis & mean sea-

level changes. 1:36.

REpower. Wind turbine REpower 6M - General requirements for an offshore emergency power

supply. Technical report V-6.1-GP.EL.03-A-A (EN).

REpower systems SE. 6M - The further development of the successful offshore turbine repower 5m.

Technical report.

Roberts, A., Thomas, B., Sewell, P., Khan, Z., Balmain, S., Gillman, J. 2016. Current tidal power

technologies and their suitability for applications in coastal and marine areas. J. Ocean Eng. Mar.

Energy 2: 227-245.

Rourke, F.O., Boyle, B., Reynolds, A. 2010. Marine current energy devices: Current status and possible

future applications in Ireland.Renewable and Sustainable Energy Reviews 14: 1026-1036.

Salvador, S., Gimeno, L., Sanz Larruga, F.J. 2018. Streamlining the consent process for the

implementation of offshore wind farms in Spain, considering existing regulations in leading European

countries. Ocean & Coastal Management.

Samo, K.A., Rigit, A.R.H., Baharun, A. 2016. Mapping of Tidal Energy Potential based on High and

Low Tides for Sabah and Sarawak. ENCON 2016. MATEC Web of Conferences 87, 02007.

Thake, J. 2005. Development, installation and testing of a large-scale tidal current turbine. Technical

report T/06/00210/00/REP, Department of Trade and Industry, UK.

TPOcean, 2016. Strategic Research Agenda for Ocean Energy.

UNFCCC, 2015. Conference of the Parties. Adoption of the Paris Agreement.

Vanhellemont, Q., Ruddick, K. 2014. Turbid wakes associated with offshore wind turbines observed

with Landsat 8.Royal Belgian Institute for Natural Sciences (RBINS), Operational Directorate Natural

Environment, Brussels, Belgium

Yang, B., Shu, X.W. 2012. Hydrofoil optimization and experimental validation in helical vertical axis

turbine for power generation from marine current. Ocean Eng 42:35–46

WindEurope, 2018. Wind in power 2017. Annual combined onshore and offshore wind energy

statistics.

Other references

4C Offshore, 2018. Available at:

https://www.4coffshore.com/windfarms/windfarms.aspx?windfarmId=UK36

39

ABP Marine Environmental Research, 2018. UK Renewables Atlas. Available at:

https://www.renewables-atlas.info/explore-the-atlas/

American Generators, 2015. How much does a commercial generator typically cost?. American

Generators, Sales & Service, LLC. Available at:

https://www.americangeneratorsmi.com/about/blog/How-Much-Does-a-Commercial-Generator-

Typically-Cost_AE12.html

AREG, 2018. Delivering a renewable future. Available at:

http://www.aberdeenrenewables.com/about-renewables/marine/

Axelsson, U. 2018. Senior Engineer at Vattenfall R&D. Personal communication September 2018

Buchmann, I. 2015. BU-107: Comparison table of secondary batteries. Available at:

https://wecanfigurethisout.org/ENERGY/Lecture_notes/Electrification_of_Tranportation_Supporti

ng_Materials%20/Secondary%20(Rechargeable)%20Batteries%20%E2%80%93%20Battery%20Univ

ersity.pdf

EMEC, 2018a. Tidal devices. EMEC: European Marine Energy Centre. Available at:

http://www.emec.org.uk/marine-energy/tidal-devices/

EMEC, 2018b. Tidal clients. EMEC: European Marine Energy Centre. Available at:

http://www.emec.org.uk/about-us/our-tidal-clients/

European Commission, 2016. European Commission: SET Plan-Declaration of Intent on Strategic

Targets in the context of an initiative for Global Leadership in Ocean Energy. Available at:

https://setis.ec.europa.eu/system/files/integrated_set-plan/declaration_of_intent_ocean_0.pdf

Fraenkel, P. 2012. Tidal Stream Turbines: a Technical Update. Aberdeen. Available at:

http://www.all-energy.co.uk/__novadocuments/14983

Global CCS Institute, 2018. Tidal device principles (device classification by EMEC). Available at:

https://hub.globalccsinstitute.com/publications/marine-energy-uk-state-industry-report-2012/51-

tidal-device-principles-device-classification-emec

Gov.uk, 2013. Wave and tidal energy: part of the UK’s energy mix. An explanation of wave and tidal

stream energy in the UK. Available at: https://www.gov.uk/guidance/wave-and-tidal-energy-part-of-

the-uks-energy-mix

HM Government, 2008. Climate change act. Available at:

http://www.legislation.gov.uk/ukpga/2008/27/contents

Jeroense, Marc: MJ Marcable Consulting AB. Personal communication September 2018

Kay, M. 2017. The Four Different Types of Tides. Sciencing. Available at: https://sciencing.com/list-

7653299-four-different-types-tides.html

Marine Current Turbines. SeaGen-S 2 MW brochure. Available at:

https://www.atlantisresourcesltd.com/wp/wp-content/uploads/2016/08/SeaGen-Brochure.pdf

Minesto, 2017. Minesto’s technology transforms ocean energy into one of the most cost effective

energy resources, new cost of energy analysis states. Available at:

40

https://minesto.com/news-media/minesto%E2%80%99s-technology-transforms-ocean-energy-one-

most-cost-effective-energy-resources-new

Munich RE, 2014. Maintaining Emergency and Standby Engine-Generator Sets. Available at:

https://www.munichre.com/site/hsb/get/documents_E316703384/hsb/assets.hsb.group/Document

s/Knowledge-Center/447-Recommended-Practice-for-Maintaining-Emergency-and-Standby-Engine-

Generator-Sets.pdf

OpenHydro, 2018. Open Centre Turbine. Available at: http://www.openhydro.com/technology/Open-

Centre-Turbine

Slot, I. Senior Engineer at Vattenfall Vindkraft A/S. BA Wind. Personal communication September

2018.

41

42

Appendix I

Estimated costs for 66kV sea cables with Cu conductors (Axelsson, 2018).

Estimated costs for 33kV sea cables with Cu conductors (Axelsson, 2018).

43

www.kth.se