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Wave energy potential in the Baltic Sea and theDanish part of the North Sea, with reflections on

the Skagerrak

Urban Henfridssona, Viktoria Neimanea, Kerstin Stranda,Robert Kappera, Hans Bernhoffb, Oskar Danielssonb, Mats Leijonb,Jan Sundbergb,�, Karin Thorburnb, Ellerth Ericssonc, Karl Bergmanc

aVattenfall Development AB, S-814 26 Alvkarleby, SwedenbSwedish Centre for Renewable Electric Energy Conversion, Division for Electricity and Lightning Research,

Department of Engineering Sciences, Uppsala University, Box 534, S-751 21 Uppsala, SwedencVattenfall AB, S-162 87 Stockholm, Sweden

Received 26 March 2006; accepted 18 October 2006

Available online 12 December 2006

Abstract

Wave power, along with renewable energy-generating sources like tides and streams, is

underestimated considering its advantageous physical properties and predictability. This paper

examines possible examples of wave power installations in the Baltic Sea and the Danish part of the

North Sea. Hindcasting data is used allowing estimations of wave energy generated and results show

promising areas in the North Sea, but also several parts of the Baltic Sea are of interest. The study is

based upon linear generator technique, placed on the seabed using point-absorbers arranged in

arrays of up to several thousand units. The study aims at showing the physical possibilities of wave

energy, including economical feasibility and environmental advantages of wave energy even in

moderate wave climates. With discussion from two examples in the Baltic Sea, one in the Danish

North Sea and a new pilot study site in the Swedish part of Skagerrak, this study show feasible

illustrations of wave energy takeouts. Project examples vary in size due to distance to grid, grid

voltage, and may thus be economically feasible. Examples also show considerations in societal and

nature conservation matters, including aspects such as industrial and military interests,

archaeological or marine reserves and local geology. The authors conclude that wave energy electric

see front matter r 2006 Elsevier Ltd. All rights reserved.

.renene.2006.10.006

nding author. Tel.: +4618 471 5820.

dress: jan.sundberg@angstrom.uu.se (J. Sundberg).

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conversion is an option that needs more attention and which has several advantages compared to

conventional renewable sources. Sound engineering, in combination with producer, consumer and

broad societal perspective is advised for a sustainable development of wave energy conversion.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Wave energy; Linear generator; Hindcasting; Baltic Sea; Energy potentials

1. Introduction

The world energy demand is steadily increasing along with the population growth andimproved living conditions, and the ratification of the Kyoto protocol increases the need ofCO2-free energy sources. Wave energy is a renewable energy source with high powerdensity, relatively high utilization factor, low visual impact and presumed low impact onenvironment compared to other renewable sources [1]. Still, wave power has not yet beenproperly appreciated partly due to lack of good engineering solutions.This article contains an estimation of the wave energy potential for the Baltic Sea and

the Danish parts of the North Sea. We also comment on the conditions in the Skagerrak,with emphasis on the Swedish west coast, based on earlier studies [2–5]. The Baltic Sea haspreviously been neglected in estimations of wave energy potential due to the relatively lowpower densities. However, the energy flux in the Baltic Sea is higher than that of offshorewind power in the same region and should therefore be considered. The annual average forwaves is estimated to 5 kW/m and 0.7 kW/m2 for wind. The article will also discuss limitingfactors of wave power in general and in the selected geographical areas. Consequently, thearticle consists of two parts: one general overview of the wave energy potential and itslimitations in the selected geographical area and one deeper case study of threehypothetical but site specific wave energy potentials based on user and developerperspective. The authors of the article represent both users and developers of wave energyinterest—an energy company represents the user perspective and a university department,specialized in technical development of wave energy converters, represents the technicaldeveloper and environmental perspective.The calculations and estimations in the article are based on the assumption that the

wave energy device is a small point absorber with a directly driven generator [6,7]. Thedevice has no power storage and yields a varying direct current. Multiple units areconnected to a common DC cable. The power absorption is limited during occasions withhigh wave energy density, e.g. in storms.

2. Estimation of the wave energy potential in the Baltic

A general evaluation of the wave climate in the Baltic Sea was carried out, based onhindcasting calculations of the significant wave heights Hs. Inputs to the calculations aredata of wind fields and water depth. The wind field data represents a combination of bothforecasted and observed wind, utilizing the advantages in both information sources.Forecast data covers the entire area but might include systematic errors, while observeddata can be considered as more correct, but originates from a limited number of locations.The calculations are based on four analyses per day during 1994–2004 with the sea dividedin 0.11� 0.061 (longitude, latitude) squares. The wind analyses originate from the

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European Centre for Medium-Range Weather Forecasts (ECMWF) and from the winddatabase ERA-40, while the Swedish Meteorological and Hydrological Institute (SMHI)made the wave energy calculations and the statistical processing. Local wind systems, asfor example land- and sea breeze are too small to be dissolved in the wind analysis and willbe ignored here. Hence, the precision in the wave energy calculations near shore is low. Thehindcasting data showed systematic discrepancies when compared to experimental data.Therefore, the energy density was corrected with a factor of 1.4 and the energy period by afactor of 2.1.

Initially, the analysis from Skagerrak–Kattegat was originally included from SMHI butthis data was greatly underestimated since the North Sea boundary climate conditions wasnot properly considered in the model and was therefore omitted from further analyses.Conditions in the Skagerrak will still be commented based on earlier studies [2–5].

The significant wave height, Hs, is related to the average wave energy density (Wh/m2).

E ¼rgH2

S

16½Wh=m2�, (1)

where r is the water density [kg/m3], g ¼ 9.81 [m/s2], HS the significant wave height [m].Eq. (1) is valid for linear waves. The wave energy density in the Baltic Sea, as an annual

average during the period 1994–2004, and based on hindcasting calculations, is shown inFig. 1. The wave energy density varies strongly over a year depending on the wind velocity,wind direction and the weather systems development, but with peak intensities duringautumn and winter months. The wave energy density is a measure of instantaneous waveenergy, per unit area. In order to estimate the annual gross energy yield, the wave fieldrecovery period has to be known. Based on earlier experiences and studies of the Balticarea this has approximately been assumed to 6 h and the total gross energy yield from theregion thus can be estimated (Table 1).

Fig. 1. Average annual wave energy density in the Baltic Sea (Wh/m2), calculated from hindcasting data between

1994 and 2004. Data and map obtained from SMHI.

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Table 1

Average annual gross wave energy for different areas and distance from shore in the Baltic Sea, from year 1994 to

2004

Baltic Sea (TWh)

Near shore: o5M 1.8

Off shore: 5–12M 3.2

Outside Swedish territorial water 412M, but inside the EEZ 18.1

Outside the Swedish EEZ 32.9

Total 56

Data is calculated from corrected hindcasting results.

For wave energy extraction purposes, the areas of interest are divided into sub areasdepending on distance to shore, including the 12-mile zone and the Swedish economicexclusive zone (EEZ). The areas are illustrated in Fig. 2 and the annual gross energy yieldof the Baltic Sea is summarized in Table 1.The method for estimating the energy potential out of the energy density in the Baltic

Sea is entirely based on geographical processing of the wind energy density maps withuncertainties in the classification of energy density fields.

3. Wave energy potential in the Danish part of the Nordic Sea

An investigation of the wave climate in the Danish part of the North Sea estimates theannual gross wave energy potential to be approximately 30TWh [8]. This analysis wasbased on wind data from ECMWF from the period 1979–1993 where the MIKE 21 OSW[9] hindcasting model was used.A different approach for estimating annual wave energy potential was used for the

North Sea. The wave condition was calculated in several points, including significant waveheight, zero-crossing wave period, and the direction of the waves. The annual gross waveenergy was calculated for eight wave directions, as the power flux over a line isperpendicular to the wave direction. The lines are stretching over the Danish part of theNorth Sea and the wave condition was averaged from the eight data points. The waveclimate is more energetic in the Danish part of the Nordic Sea, although the area is smaller,compared to the Baltic Sea (Table 1, Fig. 2).

4. Overview of area requirements and restrictions

Locating commercial wave array parks may become constrained by a number ofinteractions, including ongoing human activities and physical conditions, thus reducing thetheoretical gross potential. Such activities include commercial fishing, shipping channels,areas of military interest, sites of marine archaeological importance and valuable biologicalareas, including marine reserves [10–13]. Geophysical conditions further sets boundaries ofwhat are economically feasible, such as distances from land and grid, and the depths andsubstrate of the seabed. With devices placed at the seabed, the local geology and seabedconditions will affect the possibilities of placing generator units at specific sites or influenceemployed technique, while conditions at sea level are less cumbersome.

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Fig. 2. Geographical location of the three hypothetical sites, Faro and Hoburg in the Baltic Sea and the North

Sea location west off Denmark. Additionally, the pilot study site near Lysekil on the Swedish west coast is

demarked (see further in the text). Subdivision of the Baltic Sea and the Swedish west coast (Skagerrak–Kattegat)

into different areas is done depending on distance from shore and economical borders. Red dashed line denotes

near shore areas (o5miles), filled red line the 12-mile zone and blue line the EEZ-border. See also Table 1.

A point absorber could be mounted on e.g. a concrete foundation, which requiresflat seabed, commonly clay, mud or sand. On hard substrates different drillingtechniques could be employed where generators would be mounted on an axis attachedinto the rock, but hard substrates commonly are more undulating, which requires furtherconsiderations.

The Baltic Sea is the world’s largest brackish water sea (�268.000 km2), beingcomparatively shallow with an average depth of only 55m and with a maximum depthof 450m. The true Baltic includes waters south of the Aland archipelago south to thePolish coast, and includes all waters east/southeast of the Denmark (i.e. east of the121400E, see Fig. 2). Roughly 23% of the seabed is covered by harder glacial clay and 39%of softer postglacial mud/clay. The latter substrates are mostly found in deeper areas andcentrally in the sea. Postglacial sand covers 27% of the seabed and is mostly found atsouthern and eastern Baltic coasts, along with shallower banks. Only 11% consists ofharder bedrock or moraine. Considering suitable depths, below 30m, sets furtherlimitations.

The corresponding seabed cover in Kattegat and Skagerrak is �57,000 km2. Postglacialclay/sandy mud covers 48% of the area and is mainly found close to the Swedishwest coast and in the northern Skagerrak. Harder glacial clay seabed only make upto 4% and is found scattered in pockets. Postglacial sand covers 35% and is commonalong the Danish coasts in the north and east. Hard bottoms, rocky and morainebottoms cover less than 9% and is mostly found close to shore along the Swedishcoast. In the North Sea areas sandy bottoms are predominant, with pockets of moraineand muddy parts, although not exact figure was obtained. Especially along theSwedish west cost, different bottom substrates are mixed within short distances in acheckerboard manner.

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Military areas and shipping lanes should obviously be avoided and archaeological sitesnear shore are commonly known. Maps of major shipping lanes in the Baltic Sea can, forexample, be found on homepages of HELCOM, through the Maritime Accident ResponseInformation System (http://www.helcom.fi), and examples from elswhere is available in yetother publications and on the Internet. Leisure boats are unlikely to be affected as thesecommonly occur near shore only. Yet, collision risk assessment will be required. Finalconsiderations also have to include existing cables, pipelines and dredge sites.For any offshore projects, on site investigations using multi-beam sonar will be required,

both from project perspective and for obtaining necessary permits. Such surveys will givefurther seabed information including archaeological findings, WW2 wrecks, post wardumpsites, etc.Commercial fishing and areas with high biodiversity, including present and planned

marine reserves, are topics of greater concern. It is unlikely that buoy arrays would beallowed into areas with high biological values. This is still unlikely to happen with pointabsorbers, as generators will be placed in areas of depths under ca. 30m, which can beconsidered a crucial biological depth in the Baltic and other temperate seas. Thus,shallower marine banks will not come in question for point absorbers.Areas of concern for commercial fishing may become an issue and will have to be dealt

with from case to case. As in several offshore wind projects, compensatory fees may haveto be paid for loss of fishing waters. Consequently, marine life may to some extent beaffected also from wave power installations, particularly if they are areas demanding, butnot necessarily and not only negatively. Marine organisms will be attracted to equipment,an effect caused by biofouling and attraction by so-called ‘‘artificial reefs’’ and ‘‘FAD’’(fish attracting devices). Moreover, buoy array parks will hinder fishing, which in fact mayhelp protecting areas, and even populations or species. Buoy array parks could thusfunction as marine reserves [11]. Currently a program for the study of marine biologicaleffects, positive and negative, is started up in order to monitor the pilot project atIslandsberg, on the Swedish west coast (see further). Finally, it has been proposed thatpoint absorbers and other wave power techniques may in fact reduce coastal erosion [14],although this require near shore installations and windward coasts.

5. Case studies

5.1. General description of case studies

Three hypothetical sites were chosen and will be presented in order of descending parksize. These case studies were selected based on different characteristics in terms of waveclimate, geographic localization and distance to the coast; namely Faro in the north centralBaltic, Hoburgen in the southern Baltic and finally a location off the Danish west coast inthe Northern Sea. Additionally, a new pilot project site for point absorbers, at Lysekil onthe Swedish west coast, is added to the list with examples of local wave measurements andproject aims (see further).

5.1.1. Power absorption of every unit

For simplicity, point absorber size was set to 4m for all test sites. The power absorptionof a device depends on a number of factors. The absorbed power is the product of thepower available in the sea waves Pwave, the diameter of the buoy Dbuoy, and the capture

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width ratio dcap. The power in the sea waves depends on the significant wave height Hs andthe period Tp and are calculated as follows:

Pwave ¼ 0:5H2sTp. (2)

The capture width ratio of a non-phase-controlled point absorber depends on the wavecondition and the plant configuration (damping, etc.) and is found in the range of0.19–0.33 [15]. The power absorption of a plant can be tailored so that the power will besignificantly reduced for wave power larger than a certain level [16]. This is necessary inorder to not exceed the power rating Pnom of the generator and transmission system.Furthermore, there is an upper theoretical limit of the power absorption Pabs,max

determined by the volume of the buoy V, the wave height H, the period T, the density ofwater r, and the gravity constant g [17]:

Pabs;maxop4

rgVH

T. (3)

A simple model is used to determine the power output of a plant in a certain wavecondition:

Pabs ¼MINðPwaveDbuoydcap;Pnom;Pabs;maxÞ. (4)

This model assumes constant capture width ratio dcap for all power levels lower thanPnom and Pabs,max.

5.1.2. Utility factor and rated power

The utility factor, which is the ratio between the average produced power and the ratedpower, is an important economical factor [1]. The investment costs for all electricity-producing units are to a large extent set by the rated power, while incomes are governed bythe produced energy. The maximum power (rated power) of a plant can be adjusted, asdescribed above, and the utility factor can thus be chosen almost freely. A high-power limitwill give a higher average energy production but will result in a lower utility factor, and theinverse. Neither of the extremes is economically sound and trade-off studies are needed. InFig. 3, the utility factor and the mean power is plotted as a function of the maximumpower (rated power) for the different case study wave climates. By means of comparing theplant sites, the rated power of the plants was set to 10 kW. The resulting utility factor andaverage power production is listed in Table 3.

5.2. Case studies—transmission issues

The choices of farm sizes, cable dimension and voltage levels in this section should beregarded as indicative. Basic data for the three chosen sites are presented, by increasingsize, and is presented in Tables 2 and 3.

5.2.1. Hoburg, southern Gotland

There are two grid voltages available on Gotland, 33 and 75 kV. The 33 kV gridis being decommissioned, and the choice for grid connection of a wave energyfarm is therefore 75 kV. The farm size was set to 20MW installed power, i.e. 2000units of 10 kW. Transformers are required from the generation voltage (a few hundredvolts) to the grid voltage. Transformation is done in two levels, first from generation

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Fig. 3. (a–d) Utility factor and average power for different maximal power levels at site (a) the North Sea,

(b) Faro, (c) Hoburg and (d) the Baltic Sea. The power absorption of the plants are calculated with Eq. (4).

Table 2

Basic information of the three chosen sites, plus the new pilot study site on the Swedish west coast

Case study Coordinates Depth [m] Wave climate [kW/m] Distance to coast [km]

Faro N571520E191440 E140 E3a E42

Hoburg N561480E181000 E38 E4a E11

Lysekil N581120E111220 25 E3a 2

North Sea, Denmarkb N551350E061480 31 9.8 100

aThe wave climate is estimated with Eq. (1).bThe data is taken from [8].

voltage to 36 kV, and second from 36 to 75 kV. The first transformer level must then belocated offshore, whereas the second level can either be placed offshore or at the gridintegration point.

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Table 3

Utility factors, average power for the three sites and from an eastern Baltic buoy

Case study Utility factor %-hours/year Average power (kW)

Faro 0.38–3329 3.8

Hoburg 0.37–3241 3.7

Eastern Baltic Sea buoy 0.30–2628 3.0

North Sea, Denmark 0.44–3854 4.4

Here, the second transformer level is placed onshore, and one 240-mm2 cable may beused for transmission, causing cable RI2 losses of �2%. Conversion losses are estimated asfollows. Each generator has a diode rectifier with losses of �0.5%. Several units areconnected on the DC side and a common IGBT inverter introduces losses of �2%.Transformer losses are estimated to 0.7% (0.4/36 kV) and 0.5% (36/75 kV) and otherlosses (e.g. losses in breakers, connections and other equipment) can be estimated to 0.5%(see Fig. 4 for an illustration of the electrical system). The total electrical transmissionlosses in this case are in the range of 0.98� 0.995� 0.98� 0.993� 0.995� 0.995 ¼ 6.1%losses (see also Table 4).

5.2.2. Faro, northeast Gotland

The grid configuration for Faro is similar to that of the previous case, although here thesecond transformer, with 0.4% losses, was placed offshore in order to reduce losses due tothe longer transmission distance. The farm size is larger, to motivate the highertransformer installation costs, and 4000 units of 10 kW rated power makes the installedeffect 40MW. A 240-mm2 cable renders, for example, ohmic losses of 2.7%. With the sameconversion loss parameters, as in the previous case, the total transmission loss for this casetotals 0.973� 0.995� 0.98� 0.993� 0.996� 0.995 ¼ 6.6% losses (Table 4).

5.2.3. Danish North Sea

In the Danish part of the North Sea a 100MW farm could be connected to a 150 kVgrid, and which here is used for the estimation. Ten thousand 10 kW units was used in thisexample. All transformers would be placed offshore as the distance is long, 100 km, and aslosses increase rapidly with distance.

One 150 kV three-core cable with a 240mm2 cross section per phase is suggested to beused, with active power losses of 6.1%. Two transformer levels are included, 0.4/36 and36/150 kV, with losses 0.7% and 0.25%, respectively. Diode, converters and other lossesare estimated as above, and add to 3%. Total electrical transmission losses in this case are0.939� 0.993� 0.9975� 0.995� 0.98� 0.995 ¼ 9.8% losses (Table 4).

5.3. Case studies—societal and environmental aspects

5.3.1. Hoburg

Environmental and other confounding aspects vary between the three sites. Hoburgsbank has some specific aspects to consider. The most shallow parts of Hoburgs bank havebeen proposed as a marine Natura-2000 reserve and several proposals for offshorewindpower has been rejected within this area that is rich in both marine life as well aswintering seabirds, along with them several species occurring on ‘‘red lists’’. Consequently,

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Fig. 4. Schematics illustration of a wave energy conversion farm transmission system for any given size. The

output from each unit is rectified and several units are interconnected to a common DC-cable. Thereafter, output

from several DC-units are converted to AC, transformed to higher voltage and the power is transmitted to

the grid.

Table 4

Site characteristics and summary of the three sites, including efficiencies and estimates of net energy delivered

to grid

Site

Hoburg Faro Denmark

Parameter

Power flux (kW/m) 4 4 9.8

Buoy diameter (m) 4 4 4

Depth (m) 40 130 39

Distance (km), estimation 17 50 100

Farm size (MW) 20 40 100

Unit size (kW) 10 10 10

No of units 2000 4000 10000

Annual utility factor (%) 37 38 44

Annual gross power production (GWh) 65 133 385

Generator efficiency (%) 85 85 85

Transmission efficiency (%) 93.8 93.2 89.95

Annual net production (GWh) (delivered to grid) 52 105 295

Transmission/grid voltage (kV) 36/75 75/75 150/150

locating a buoy array to this area would require planning off the true shallow bank.However, good wave power areas are commonly on greater depths and would thus beplaced outside the shallower areas (+30m). A serious problem is the shipping lane,passing just east of Gotland, and which is the main route to and from St. Petersburg andthe Finnish bay. At present, the shipping lane passes over the bank but proposals foradjusting the shipping lane to the east have been presented. The suggested site, however,would not be close to the present location. The seabed substrates consist mainly of sand,

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but turning to mud and clay to the south and west. The depth is �40m and flat seabedwith little or no slope is possible to find. The area is used some for fishing although mostlyby fisherman outside of Gotland.

5.3.2. Faro

This site, east of the island Faro and northeast of Gotland, would not likely encounterspecial restrictions. Whereas the surrounding islands are built up on limestone the bottomsubstrate would most likely be softer post-glacial clay. At the proposed site the depth isbetween 100 and 150m, shallower towards Faro. The slope of the seabed is a matter ofconcern, and likely to exclude specific locations. However, there are localities within thearea that are flat and large enough, if generators are to be anchored by concretefundaments, making an array system possible.

Like elsewhere, the seabed structure would have to be investigated by multi-beam sonar,giving high resolution and which can detect anomalities in the range of a few centimetres.Sonar would also help in archaeological research and for the detection of possible wrecksin the area. The large commercial shipping lane towards St. Petersburg would likely restrictmore easterly locations. Possibly, naval interests could have additional views. Fishingoccurs around the island, both by local fishermen but also from neighbouring countries.The marine life in the area is little known but with no a priori indications of specialinterests. Other matters that would have to be investigated in projects include possiblechanges in currents. Changes in sedimentation could also be an issue. However, the arraywould be placed on the leeward side if the island, which reduces risk of effects on theisland.

5.3.3. Danish North Sea

The Danish site off the west coast of Jutland shows yet other conditions. No exact dataon seabed substrates was obtained. However, data closer to shore (�50 km east) and fromthe Horns Rev windpower site shows that most parts, along with adjacent North Sea areas,have sandy bottoms, with some pockets of sandy mud or lag deposits including rocksand gravel.

The depths of the area and distance from land makes it less likely that a projectwould intrude into known important areas for marine animals although the area islikely to harbour red listed seals (two species) and harbour porpoises, species that areclosely followed in several offshore wind projects. Shipping is intense in many parts ofthe North Sea and other offshore projects may become competitors to wave powerprojects in this area as a number of future wind projects are known from both Germanand Danish North Sea waters. The competition is less likely to be on project sites assuitable depths for wind- and wave power differ. Rather, cables, grid connectionsetc. might be factors to consider. Different projects in this area would also result incumulative aspects (added impact), which may affect other uses of the area, shippinglanes and fishing. Military use of the area is unknown as is archaeological remains.However, much must already be known within ongoing projects and site-specificsurveys would still be required. Fishing is most likely extensive periodically and requestson compensations from the fishing industry would be likely. A full view of questionsdealt with, from the EIA-process on to follow-up studies after completions of theproject can be found on the homepages of Horns Rev offshore wind farm (http://www.hornsrev.dk).

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6. The Lysekil wave power test site

Outside the Swedish West coast a wave power test site is during construction (Fig. 5).The project is intended for technical tests and verifications, as well as marine biologicalresearch connected to the point absorber technique. The latter as large-scale projects willaffect the environment and early environmental considerations is likely to enhance theproject and even lead to alterations in technique and formation of buoy arrays. Moreover,it is important to obtain early information on the extent of biofouling, which may affectthe technique chosen, wear and maintenance.The project will be built up during 5 years and used as research facility during additional

5 years. Necessary permits was obtained, mainly from the Local County Administration(Lansstyrelsen Vastra Gotaland), during 2004–2006. A full-scale EIA-process was notneeded, as the project size was limited (installed power), but was still carried out similarfashion in order to set standards for future larger projects. The site was selected due to itswell-known wave climate and yet being close to research facilities. The depth in the area,�25m, enables diving.

Fig. 5. Marine chart of the project area of the wave power project located southwest of Lysekil on the Swedish

west coast. Since June 2004 we have permission from the Local County Administration to launch and perform

research on wave power plants in the denoted area. The buoy area, fully built up, is denoted by dots in a

hexagonal pattern with a Waverider buoy in the west and two sailing marks in the north and south, respectively.

The dotted line leading onshore is the route accepted for sea cable and connection to the local grid.

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Establishing the wave power potential in the west coast of Sweden is one of the aims.Therefore, sine April 2004, the site has been equipped with a wave-measuring buoy(Waverider, [18]) that continuously measures wave height and length. The site is notoptimal for wave energy but was chosen to make research and technical work possible. Thedata presented in Fig. 6 demonstrates a time series of significant wave data important forboth the design of the converters but also for the prognoses of the potential electric energythat can be converter into the Swedish west coast.

It is yet too early to give good wave energy forecast from the measured wave data, aslonger time series are needed. However, the near shore wave climate of the Swedish westcoast facing Skagerrak is as energetic as the mid Baltic Sea [5]. Information of theexperimental site can be found at www.el.angstrom.uu.se, see further under the heading‘‘Islandsberg—wave power test site’’ in the margin.

The project site at Lysekil is situated at the southern border of a marine Natura-2000reserve, Gullmarsfjorden. Several wrecks are known in the vicinity but are not affected bythe study site and activities related to the project. Leisure boat traffic occurs near the sitebut commercial traffic is regulated in the nearby shipping lane. Fishing also occur in thearea, including the popular catching of lobsters, but a common agreement with localfishermen, in which final agreement of final adjustment of location was agreed upon,helped make the project possible. The bathymetrical data show depths of �25m. Thebottom substrate also varies extensively in the vicinity, with combinations of sand andespecially mud at most localities. The mud and clay is either harder and of pre-glacial age,or postglacial and softer and with unknown buoyancy that may or may not allowgenerators with large concrete fundaments that primarily will be used in the project. Anarea of flat sandy bottom was finally chosen was finally found and chosen as projectarea, where surrounding similar areas will be used control areas. A BACI-approach

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30Day

Sig

nif

ican

t w

ave

hei

gh

t [c

m]

Fig. 6. Measurements of significant wave height at Lysekil in November 2004. Data is obtained from the

Waverider buoy at the site and which is connected online to Uppsala University.

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(Before-After-Control-Impact assessment, [19]) will be used in several of the marineecological studies that already have started.Marine environmental questions to be covered include effects on smaller bottom

dwelling organisms but will, when the whole buoy park is in place, allow studies on fish,marine mammals and birds. A special focus is set on the effects of biofouling, i.e. thegrowth of mussels, barnacles and algae, which may cause technical wear, increasedmaintenance cost but which also is interesting from a theoretic ecological point of view[11,20]. Results from biofouling studies may have effect on e.g. counter measures,maintenance, buoy shape and dimensioning as the calculated extra weight from foulingmay exceed 300 kg.

7. Discussion

The results show that annual gross wave energy may be extensive, also in a shallow andrelatively sheltered sea as the Baltic Sea. However, the amount of energy that may becomeutilized in an economic way is dependent on a number of factors. Apart from techniquechosen, the total energy will vary locally mainly depending on fetch and thus makingleeward coasts less interesting. Naturally, as the predominant wind direction is westerly thehighest energy density is found in the eastern part of the Baltic Sea, with relatively highdensities also in the south-central part. This has also been indicated in earlier studies[3–5,21].In this study the annual wave energy is calculated to �56TWh for the true Baltic Sea.

A recent study [21], however, indicate a smaller potential (�24TWh, o50%) than thisreport. The study by Bernhoff et al. [21] should be recognized as a technical potential andpresents a possible scheme for a large-scale rollout, whereas the result from the presentwork is more to be considered as gross wave energy potential for the whole Baltic Sea.Several factors clearly affect initial investment costs and thus the potential for

development at particular sites. Distances from land have to be considered: the further thedistances the larger costs during construction, as will wave array park size. Distance willaffects cable costs; normally an extensive part of the total cost for offshore projects, andthis in turn will affect optimal park size. In this case we have more thoroughly consideredsystem losses, yet another factor to consider.The electric transmission system losses are estimated and should not be regarded as

exact values. Further, the cluster cables, i.e. the cables connecting each generator to a hub,and the cables between hubs, have not been considered in this study as the length of suchcables are limited compared with the transmission. In the overall system, from wave towire, several loss parameters has been identified, some of which are related to the electrictransmission. It should be noted that the transmission efficiency of �90–95% iscomparable with the generator efficiency.The best option would be to install a fairly large farm close to the grid. The cases shown

here illustrate three different sites and sites were chosen mainly based on the power flux inthe waves. Thorough economical analysis is required prior to any real installation plans, asseveral parameters are unknown today (e.g. installation costs, local power company lossevaluation). The choice of placing transformers offshore also requires considerations, asthe optimization is a multivariable problem. Transmission distance, cable installation cost,cost of copper, site depth and electricity price are some parameters that must be taken intoaccount.

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Several societal questions will to some extent reduce areas possible for exploitation,foremost shipping lanes, areas important for fishing and marine protected areas. Otherhuman activities or interests, such as areas of military concern or archaeological sites couldlimit access to areas but probably not to such an extent that it would reduce actualpotential. Many of these factors are also more costal, which reduces risk of conflict.Several areas would likely become restricted for future offshore projects, including wavepower. However, wave array parks, due to progressive absorption of energy, have to bedispersed and in smaller groups. Suggested inter-distances between buoy-groups in theBaltic are up to 60 km [21]. If areas of societal concern would be considered as areas offetch the true potential of energy absorption could still be close to the theoreticalmaximum. Similar argument could be used for unsuitable seabed substrates and for thefew parts of the Baltic Sea being too deep to explore. Therefore, a theoretical maximumutilization may not differ significantly from what could be practically achieved.

In the North Sea and on the Swedish west coast, the possibilities for wave power aresomewhat different. The wave climate is more energetic, and thus more energy can beharvested. Also, predominant wind directions allow establishment closer to land, whichwill reduce transmission costs, and may allow larger wave parks. Off the Danish coastmost of the seabed is relatively flat and consists mainly of sand. The seabed along theSwedish west coast is more diversified, with hard bottom substrates mixed with muddybottoms, both pre- and post glacial. Else, most other considerations are equal.

Inside the hypothetical Danish North Sea location there already exist an offshore windpark, Horns Rev, and many more wind installations are proposed in the North Sea. Willthis affect future wave parks? The answer is likely no. There is no competition for suitabledepths, at least not in a foreseeable future. Wave installations do better in deeper watersand wind power depends on shallower marine banks that unfortunately also are consideredmore ecologically important. Other factors, such as grid, transmission and additivenegative (cumulative) effects on e.g. the environment may be set final limits.

In conclusion, from the examples presented we show that the potential for future wavepower generation in moderately wave energy rich seas is substantial. Societal concerns andactivities may reduce areas to exploit but, however, more important is economicalconsiderations along with final technical considerations and solutions. Moreover, futureenergy prizes will set a framework of which projects, and under what condition, will befeasible.

Acknowledgments

The present work is the result of a co-operation between Vattenfall Development AB,Uppsala University (UU) and Vattenfall AB. UU acknowledges the Swedish Centrefor Renewable Electric Energy Conversion, financed by the Swedish Energy Agency(STEM) and the Swedish Governmental Agency for Innovation Systems (VINNOVA). Wealso wish to thank Ingemar Cato, Geological Survey of Sweden, for help in providingseabed data.

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