Hybrid Solutions for Energy Extraction in Coastal Environment

Abstract

The higher requests concerning the large scale implementation of the renewable energy imposed by the EU directives implies a substantial enhancement of the renewable energy extraction all over Europe. Wind turbines entered in the last decades gradually in the common landscape and the success of the wind power industry renewed the interest in discovering what might work in the sea. On the other hand, it becomes quite obvious nowadays that wave power will play an important role in the global energy portfolio. Combined offshore wind-wave projects, also known as hybrids, hold great potential down the line when wave technologies will become more established. At that point, wave production might compensate for the intermittent offshore wind, while economies of scale developed from offshore wind could accelerate cost reduction for wave components. Despite a certain degree of uncertainty related to the variability in the wave-wind climate, improvements in the accuracy of evaluating the environmental data in coastal areas would also enhance the accuracy of the predictions that future energy converters yield. Another important problem that arises together with the implementation of the energy farms in the coastal environment is related to the correct assessment of their impact on the littoral dynamics. From this perspective, the proposed work presents the main challenges related to the wave energy extraction in the nearshore. This includes also the identification of the best European locations from the point of view of the synergy between the wind and the wave power.

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Energy Procedia 00 (2016) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2016 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of ICACER 2017
2017 2nd International Conference on Advances on Clean Energy Research, ICACER 2017,
7-9 April 2017, Berlin, Germany
Hybrid solutions for energy extraction in coastal environment
Eugen RUSU*, Florin ONEA
Department of Mechanical Engineering, ‘‘Dunarea de Jos’’ University of Galati, Domneasca” Street, 47, Galati 800008, Romania
Abstract
The higher requests concerning the large scale implementation of the renewable energy imposed by the EU directives implies a
substantial enhancement of the renewable energy extraction all over Europe. Wind turbines entered in the last decades gradually
in the common landscape and the success of the wind power industry renewed the interest in discovering what might work in the
sea. On the other hand, it becomes quite obvious nowadays that wave power will play an important role in the global energy
portfolio. Combined offshore wind-wave projects, also known as hybrids, hold great potential down the line when wave
technologies will become more established. At that point, wave production might compensate for the intermittent offshore wind,
while economies of scale developed from offshore wind could accelerate cost reduction for wave components. Despite a certain
degree of uncertainty related to the variability in the wave-wind climate, improvements in the accuracy of evaluating the
environmental data in coastal areas would also enhance the accuracy of the predictions that future energy convertors yield.
Another important problem that arises together with the implementation of the energy farms in the coastal environment is related
to the correct assessment of their impact on the littoral dynamics. From this perspective, the proposed work presents the main
challenges related to the wave energy extraction in the nearshore. This includes also the identification of the best European
locations from the point of view of the synergy between the wind and the wave power.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under the responsibility of the organizing committee of ICACER 2017.
Keywords: European nearshore; wind power; wave energy; synergy wind-waves.
* Corresponding author. Tel.: +40-740-205-534; fax: +40-236-461-353.
E-mail address: erusu@ugal.ro

Eugen RUSU and Florin ONEA/ Energy Procedia 00 (2016) 000–000
1. Introduction
Nowadays, a significant percentage of the global energy market is sustained by the processes related to fossil fuel
consumption. At the beginning of the Industrial Revolution (in 1760), the hydrocarbures resources were abundant
and at a lower price, but with the population growth the demand for energy drastically increased to the point where
we can ask how long they will be available. As an alternative to this issue, a possible solution may result from the
conversion of the natural resources, especially from those specific to the marine areas [1, 2]. Since almost 71% of
the Earth's surface is covered by water, the energy potential is tremendous, especially in the vicinity of the shoreline
where the wind and wave resources can be efficiently captured. In terms of the European market can be mentioned
that the wind energy is on an ascendant trend, being reported at the end of 2015 a cumulated capacity of 12.9 GW
provided by 84 offshore farms located in 11 European countries. In general, the average water depth and the distance
from the shoreline are estimated to be around 27 m and 43.3 km, respectively, while the largest grid-connected
project can reach a maximum capacity of 600 MW [3].
The intermittence of the natural resources is considered to be a major issue for the energy production, since the
conditions of the marine environments can significantly vary between different geographical regions or between
various time intervals [4-6]. In general, the coastal sectors located close to the ocean are considered to be the first
option for a marine renewable project taking into account that the energy density is more consistent. Nevertheless,
since the extreme conditions of these areas can significantly influence the performances of a wind/wave generator
during the recent time it was taken also into account the viability of similar projects in enclosed basins, such as
Mediterranean, Black or Caspian seas [7, 8].
In order to tackle the fluctuation of the energy output, a solution will be to couple the wind and wave systems into
a so called hybrid farm, which will extract two sources of energy from a single site by using the same infrastructure.
At this point, it is important to mention that most of the wave energy converters (WECs) are still in the research and
development stage, and by using the know-how from the offshore wind industry it is possible to accelerate the
development of this emerging sector.
Taking into account all the above mentioned aspects, the objective of the present work is to provide some insights
regarding the wind-wave synergy and to draw some lines regarding the applications of these projects.
2. Methods and materials
In the present work three main directions are considered. The first is related to the assessment of the synergy of
wind and wave conditions in the vicinity of several European offshore wind farms. The marine conditions are
highlighted by analyzing the datasets coming from the European Centre for Medium-Range Weather Forecasts
(denoted as ECMWF) [9] which covers the 10-year time interval: January 2005- December 2014. From the analysis
of these results, it was possible to further identify, which site presents the best conditions for a mixed wind-wave
energy project. Several parameters were taken into account [10, 5], among which the wind power (Pwind - in Kw/m2)
and the wave power (Pwave in Kw/m). This can be defined as:
2
3
10
U
Pa
wind
ρ
=;
π
ρ
64
2
2
sesw
wave
HTg
P= (1)
where: a
ρ
- represents the air density (≈1.225 kg/m3); 10
U- the wind speed reported at 10 m height above the sea
level; w
ρ
- the density of the sea water (1025 kg/m3); g - the acceleration of gravity (9.81 m/s2).
Another direction is focused on the performance assessment of several WECs, which could operate in various
geographical environments. By combining the power matrix of several wave generators (Wave Bob, Pelamis,
Oceantec and Ceto) with the local sea state, it is possible to estimate their performances in terms of the power
output. In the final part of the paper, the viability of a generic wave farm to provide coastal protection against the
wave conditions reported in the vicinity of the Portuguese nearshore is discussed. For the proposed case study the
ISSM interface (Interface for SWAN and Surf Models) was implemented. This connects the models SWAN

Author name / Energy Procedia 00 (2016) 000–000
(Simulating Waves Nearshore) for waves with NSSM (Navy Standard Surf Model) for the longshore currents [11].
3. Results
As in any project, which is focused on the natural resources, the first step in the development of a renewable
project is to assess the consistency and the seasonal patterns of the conditions of the marine environment. Figure 1
focuses on this aspect, where the wind conditions (U10) reported in the vicinity of some European offshore wind
farms are indicated by means of the wind roses distribution. In this case, were considered the wind dataset simulated
at the European Center for Medium-Range Weather Forecasts (denoted also as ECMWF). The analysis focuses on
the total (TT), summer (ST) and winter time interval (WT), respectively, where the last one was considered the time
interval from October to March, while ST represents the rest. As expected, these sites are defined by a good
potential, reporting frequently U10 values which exceed the 9 m/s limit, these being in fact, one of the main factors
which were taken into account when was decided to start a commercial wind project.
Fig. 1. Wind roses (at 10 m above the sea level) corresponding to some reference sites, where in the center the distribution for the total time (TT)
is indicated. The results are based on the ECMWF data, being reported for the 10-year time interval 2005-2014.
Since most of the offshore wind parks are located in the marine environment, they are frequently under the
action of the waves, which could be converted into electricity. In Table 1, besides the U10 parameter it was also
presented the significant wave height (Hs in meters), in order to estimate which site will be more promising (bolded
values) in terms of a wind-wave project. Following these results, it can be mentioned that the sites Gode Wind,
Sceirde and Wave Hub present important resources in terms of wind and wave energy, compared to Horns Rev 3
and Hywind which are defined only by relevant wind energy.
Table 1. U10 and Hs parameters - mean values reported for several European offshore projects. The results are based on the ECMWF data, being
processed for the 10-year time interval 2005-2014, considering only the TT interval [data processed from 2].
Site* → O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14
Parameter ↓
U10 (m/s) 8.26 7.92 7.9 7.67 6.94 6.82 6.72 6.54 6.45 6.12 6.11 5.95 5.89 3.19
Hs (m) 1.68 2.43 2.08 1.23 1.52 0.96 1.43 0.46 1.66 2.17 1.61 0.81 1.8 1.57
*O1 - Gode Wind (DE); O2 - Scierde (IE); O3 - Wave Hub (UK); O4 - Horns Rev 3 (DK); O5 - Hywind (NO); O6 - Greater Gabard (UK); O7 -
Aberdeen (UK); O8 - InFLOW (FR); O9 - PLOCAN (ES); O10 - WindFloat (PT); O11 - Oleron (FR); O12 - Teesside (UK); O13 - Floatgen
(FR); O14 - BALEA (ES)

Eugen RUSU and Florin ONEA/ Energy Procedia 00 (2016) 000–000
On an opposite side, we notice that the projects WindFloat and Floatgen, which are mainly known for the wind
projects, seem to present also relevant wave conditions. Further on, the main wind and wave patterns reported by the
sites presented in Table 1 will be discussed in more details. In Figure 2 the distribution of the wave roses for five
reference sites is illustrated. From these, it can be highlighted the site Sceirde, which is defined by a narrow
directional distribution aligned in the west direction, for which the wave heights of 4 m (or higher) represent a
common occurrence. A wider distribution of the waves is noticed in the vicinity of the sites Hywind and Gode Wind
for which the waves are predominantly spread from the southwest to the northeast sector, in some cases being
noticed also waves coming from the coastline (the eastern sector).

Fig. 2. Wave roses, where in the center the distribution for the total time (TT) is indicated. For each plot of the figure, the upper subplot is related
to the summertime (ST) while the lower subplot is associated with the winter time (WT).
An inter-annual variation of the wind and wave conditions (mean values) is presented in Figure 3 for the 10-
year time interval 2005-2014. In general, the two marine parameters present relatively smaller variations, with the
mention that the year 2010 is characterized by smaller values. Regarding the U10 parameter, it can be observed that
the site Gode Wind is characterized by more energetic values, compared to Hywind, where a minimum value of 6.3
m/s may be encountered. For the wave heights, the site Sceirde presents a maximum of 2.66 m (in 2011), while
much lower values indicate this time by Floatgen and Gode Wind, respectively.
Fig. 3. Inter-annual variations of the wave and wind parameters considered for the 10-year time interval 2005-2014, where: (a) U10-mean values;
(b) Hs-mean values.

Author name / Energy Procedia 00 (2016) 000–000
Since Gode Wind and Sceirde are among the sites with a good wind and wave potential, Figure 4 illustrates the
distribution of the energy level throughout a histogram plot. In terms of the wind energy potential it can be observed
that in the interval 0-0.6 Kw/m2 most of the values are grouped, with the mention that the site Gode Wind presents a
peak in the range 0.2-0.4 Kw/m2 compared to Sceirde, where the values from the interval 0-0.2 Kw/m2 may account
for a maximum of 28%. For the wave power, the Gode Wind site is defined by a dominant distribution of the waves
in the range 0-25 Kw/m, while near the Sceirde project the distribution is more flattened, most of the values being
grouped in the interval 0-50 Kw/m.
Fig. 4. The power density of the wave and wind parameters reported for the sites Sceirde and Gode Wind. The results are represented in terms of:
(a) and (b) Pwind; (c) and (d) Pwave.
Comparing with the wind industry, where the three blade turbine gain popularity over the last decades, in the case
of the WEC systems, there is not clear what will work better in the future. This is because the wave energy is
continuously changing, reporting different density power for various water depths (offshore, nearshore and onshore),
each wave generator being designed to work better for a particular combination of wave heights and periods, which
are defined in the power matrix of each manufacturer. In Figure 5 some devices which are in the final stage of
development are presented. These systems are deployed in the offshore areas where the harshest marine conditions
already tested their survival ability skills. The theoretical performances of these systems will be further discussed in
this paper, in order to assess their efficiency for various coastal environments.
Fig. 5. Some state-of-the-art WEC systems (Wave Bob, Ceto and Pelamis) defined by the rated capacity located below 1000 kW.

Eugen RUSU and Florin ONEA/ Energy Procedia 00 (2016) 000–000
The performance of a WEC and the geographical positions of a site are closely related factors, being also
influenced by the capacity of the generator and the distance to the coastline. It is well known that the best results can
be obtained from the sites located between 30o and 60o latitude (north and south), especially close to the coastal
environments which are aligned on the western side. In Figure 6, there were evaluated the performances of some
WECs rated below 1000 kW, from which it can be noticed that the Ceto system presents much lower performance,
which are directly related to his capacity. The best results are noticed near the site S3, which is located in the coastal
environment of the South America continent (southern extremity).
Fig. 6. Mean values of the power output (in MW) reported by some WECs rated below 1000 kW. The results are based on the 15-year interval of
ECMWF data (2000 - 2014) [data processed from 12].
Besides the energy production, a hybrid marine farm may be considered a viable alternative for the coastal
protection [13-15]. Figure 7 presents a case study where a generic wave farm was defined in the central part of the
Portuguese coastal environment. The waves are coming from the western sector, and in this case it can be mentioned
that the best protection is obtained throughout the farm configuration denoted as WF1, which is the closest one to
the shore (1 km).
Fig. 7. Variation of the Hs values in the presence of a generic wave farm located in the sector Pinheiro da Cruz, Portugal. The results are reported
for a winter time scenario, considering a high absorption farm and four study cases: (a) no farm; (b) WF1 - 1 km from the coast; (c) WF2 - 4 km
from the coast; (d) WF3 - 7 km from the coast [data processed from 16].
Regarding the wave farm configurations WF2 and WF3, in the presence of the WEC systems the incoming waves

Author name / Energy Procedia 00 (2016) 000–000
are significantly reduced, but as they pass this line, the wave fields tend to rapidly regenerate. The drawback of this
effect is that, an additional line of WECs needs to be added in order to provide a suitable coastal protection.
In the vicinity of the coastal environment, on a local scale, the breaking waves represent the main cause for the
occurrences of the longshore currents. From the results presented in Table 2, there can be noticed different patterns
of these currents in the presence of a wave farm, the maximum values being severely influenced by the shielding
effect. In this case study, it can be mentioned that points P2 and P3 reveal an enhance of the current speed (reported
in m/s), comparing to the group P4-P7 where a minimum of 0.06 m/s is observed for the scenario WF2 where the
farm was defined at a 4 km from the shore.
Table 2. Variation of the longshore currents (maximum values in m/s) in the presence of a wave farm located close to the Portuguese coast.
Results reported only for the winter season [data processed from 16].
Reference point → P1 P2 P3 P4 P5 P6 P7
Case studies ↓
NO FARM 0.65 0.41 0.35 0.43 0.63 0.81 0.76
WF1 0.65 0.42 0.25 0.25 0.41 0.8 0.76
WF2 0.67 0.71 0.6 0.13 0.06 0.55 0.67
WF3 0.63 0.72 0.63 0.25 0.15 0.4 0.53
4. Conclusions
The renewable industry is a dynamical environment defined by multiple opportunities, especially in the case of
the waves and offshore wind. Therefore, it is important to have access to a solid database in order to evaluate in
detail the energy profile of a specific site, and also to predict the performances of a generator. Offshore wind
industry is more developed, and this may represent an advantage for the development of the WEC systems which
are still in the early stage of development. This can be considered a paradox if we take into account that, the first
wave generator was registered in 1800 compared to 1890 which is related to the first windmill (on land), and also by
the fact that there are multiple WEC projects in progress, most of them being developed in Europe. Nevertheless, by
extracting the wind and wave energy from a single site, it is possible to establish a consistent source of energy, and
at the same time to protect the coastal areas against the erosion processes will are directly related to the intensity and
direction of the waves.
In terms of the coastal protections, there is a growing interest to understand better how a wave farm could
provide protection against the abrasive action of the breaking waves, with the mention that in most of the cases the
variations of the local currents is ignored. It is important to evaluate the sediment dynamics from a holistic
approach, since by protecting a coastal sector from the wave action it is possible to trigger a domino effect where the
nearby beach areas could be seriously affected by an accentuated current flow. At this moment, the concept of
combining the wave generators with an existing offshore wind farm it is still in an infancy stage, this approach being
mainly tested on small scale projects. Since this is an emerging concept and the marine areas represent an interesting
environment for the renewable energy production, this opens the way to various research directions that may be also
related to the development of new WEC solutions or the implementation of such projects in enclosed seas defined
by moderate marine resources. From this perspective, the present work gives a more comprehensive picture of the
synergy between the wind and the wave energy resources in various coastal environments.
Acknowledgement
This work was supported by COSMOMAR project, leaded by Grigore Antipa Research Institute in Constanta in
the framework of the STAR program.

Eugen RUSU and Florin ONEA/ Energy Procedia 00 (2016) 000–000
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