Estimation of the wave energy in the Italian offshore

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1 Journal of Coastal Research SI pg - pg 617 ICS2011 (Proceedings) Poland ISSN Estimation of the wave energy in the Italian offshore D. Vicinanza, L. Cappietti, V. Ferrante and P. Contestabile Dept. of Civil Engineering Dept. of Civil and Environmental Engineering Second University of Naples, Aversa University of Firenze, Firenze 81031, Italy 50139, Italy [email protected] [email protected] ABSTRACT Vicinanza, D., Cappietti, L., Ferrante,V. and Contestabile, P., Estimation of the wave energy in the Italian offshore. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium),. Szczecin, Poland, ISSN The offshore wave energy potentials of the Italian seas has been studied by analyzing the wave measurements carried out by the Italian Wave Buoys Network. The annual and monthly average offshore wave power, varies between 1,6 kw/m and 9.05 kw/m. The Adriatic sea shows an average value around 2 kw/m, the smallest value around Italian coasts as expected. The Ionian, North and Middle Tyrrhenian seas are a bit more energetic reaching a value of about 3 kw/m whereas the South Tyrrhenian is characterized by a value of 4 kw/m. A completely different behavior is highlighted for the Alghero buoy (north-west Sardinia island) where the estimated power reaches the value up to 9 kw/m. ADDITIONAL INDEX WORDS: Wave energy, Wave power, Renewable energy, Italy INTRODUCTION The present interest of world energy market towards renewable energy is clear. Among all other possibilities the wave energy is emerging albeit with the inevitable difficulties concerning its conversion but its high specific density contributes to increase the investment opportunities for this relatively new source. The world-wide contribution of wave energy has the potential to be of the same order of magnitude of the world electrical energy consumption (Angelis-Dimakis et al., 2010). Wave power is conventionally rated in terms of the energy flux crossing an imaginary deep water contour and expressed in kw/m. Europe has a long coastline overlooking the Atlantic, the Mediterranean and the North Seas. Obviously, the most significant resource is on the oceanic wavefront. The wave energy flux generally increase rising the coastline from South to North in the latitude range. The Portuguese coast can be considered as the lower boundary of the range where the highest energy ocean waves are concentrated. It is characterised by an annual technically available wave source of about 5 GW with a average flux of 30 kw/m (Mollison and Pontes, 1992). Spain, as showed by a recent study (Iglesias et al., 2009) reveals an offshore average wave power to exceed 30 kw/m, rising 50 kw/m in north-western corner of the Iberian Peninsula. In the southwest of France, the annual power levels up to 40 kw/m (Clément et al., 2002). The United Kingdom is characterised by a offshore power level of kw/m, for an annual wave source of 120 GW (Thorpe, 2000). In Ireland, offshore wave energy source has been estimated (Lewis, 1999) in TWh, with a winter wave power regularly exceeds 150kW/m. An high density of power characterize the North Sea coast, but its relatively limited length and the shallow coastal water in several areas reduce the source potentially exploitable. Norway has an exceptional wave energy resources of the order of 400 TWh/year, which in terms of power corresponds to a range of kw/m. The total incident wave energy of Denmark is around 30 TWh, corresponding to a power between 7 and 24 kw/m coming from a westerly direction. Smaller values are found at 13 locations spread out over varying depth and distance from land and well representative of the west coast of Sweden, with a maximum of 5.2 KW/m (Waters et al., 2009), which should be correspond to 5-10 TWh/year. On the Mediterranean side, the power levels decrease drastically. Spanish and French coasts are characterized by a average wave energy less than 5 kw/m. In the Aegean and Ionian Seas the wave climate range between 4 and 11 kw/m in some Greek hot spots (Athanassoulis and Skarsoulis, 1992; Pontes et al., 1996). Albeit there are a lot of countries involved in the development of Wave Energy Converters (hereafter WECs), the efforts have been largely uncoordinated thus that a wide variety of new WECs constantly appears on the scene (Falnes and Lovseth, 1991). Most WEC technologies remain in the development/research stage but a significant number of these have reached the demonstration phase. Nowadays the problem is that the Wave Energy Sector is still in a very immature phase for which the production of electricity based on these technologies is not profitable. The largest problem in harvesting wave energy is obtaining reliability of the technology and bringing the cost down (Margheritini et al., 2009; Vicinanza and Frigaard, 2008). A detailed design/optimization must take into account the actual wave climate existing on the proposed location for the WECs installation. In other words, the evaluation of the wave energy potentials and how it is temporally distributed is the primary information needed for a feasibility study on WECs. At present an extensive and accurate estimation of wave energy for the Italian seas is not available. The only available wave measurements are carried out by the Italian Wave Buoys Network (IWN), active since These measurements give an accurate description of the wave climate at the sites of buoy location but their coverage is still poor in spatial and time extension. A first attempt to assess the offshore European wave energy resource using an high accurate data was made through the European Wave Energy Atlas - WERATLAS (Pontes et al., 1996). The methodology adopted has settled the basis for further 613

2 Estimation of the wave energy in the Italian offshore development of some atlas at a regional scale. Estimations of the nearshore wave energy potentials have been provided by the ONDATLAS project, that gives data relative to 20-metre depth of the Portughese coast (Pontes et al., 2005), and by the Atlas of UK Marine Renewable Energy Resources that provides information over the extent of the UK Continental Shelf (UK Dept. of Trade & Industry, 2004). Worth to be mentioned is the EUROWAVES project, developed with EU support, in which all possible sources of wave measurements were considered to form the data set whose accuracy had been carefully evaluated (Athanassoulis et al., 1998). The most recent atlas on the Mediterranean area is the MEDATLAS (Gaillard et al., 2004; Cavaleri, 2005). The data set include model data, originally from the archive of the European Centre for Medium-Range Weather Forecasts, satellite observation and part of Italian Wave Network measurements. However for the Italian seas the available atlas do not provide information at the required level. Considering that the Italian energy market is expected to undergo substantial changes in the next years, the access to reliable information at these early stages of discussion is crucial. The wave energy could have the potentiality to contribute in the Italian electricity production at a comparable level of the renewable energies presently in the market (i.e. solar and wind energies). A first assessment of wave energy around Italy was developed under a founded project by the Second University of Naples titled PRIST 2007 (Progetti di Ricerca di Rilevante Interesse Scientifico e Tecnologico). According to the aim of this project, addressed to upgrade wave energy into the Italian discussion on renewable energy, a website has been also realized ( Within the website, the novel Italian Wave Energy Atlas has been published. Through an interactive geographical map of Italy it is possible to download the wave climate plots based on the monthly and yearly data for each buoy of the IWN. Future aims of the project will be to investigate in detail the most promising areas for wave power exploitation and promote the developing of WEC prototypes in selected sites in Italy. ANALYSIS OF THE DATA The analysis of WECs performance is considerably more complex than the computation of the wave energy flux as response to parametric wave height and period. When considering the performance of a particular model of WEC, it may not be easy to decouple its power capture behavior from the specific characteristics of the wave climate of a location. Albeit the range of variability of wave period associated with a certain wave height is relatively narrow, the energy distribution in the frequency domain, the grouping factor, the spreading and the temporal distribution of calms could play an important role. Further it should be qualified any performance description in order to consider the location-effect. In this sense, it is obvious that the water depth represents the primary location-specific variable. In general, a particular attention should be operated to the refraction and diffraction processes that affect waves propagation. In water depths greater than about 100m these aspects can be not considered so that the power generation of a device can generally be adequately characterized entirely in terms of available wave energy on site. Hence a rough estimates of the available mean power in kw/m in selected areas representative of the whole Italian coast, represent the first phase for a feasibility study of wave energy utilization. The present work has been based on the wave data records supplied by the Italian Buoys Network, (IWN), that is actives since July 1989 ( Figure 1. Location map of the IWN wave buoys. The IWN wave measurements are available offshore the sites highlighted in Fig. 1 and Table 1. From 1989 to about 2002, each wave buoy collected 30 minutes of wave measurements every three hours but in presence of wave heights greater than 1.5m the measurements were continuous. From 2002 the wave measurements have been always continuous and the wave characteristics parameters refer to 30 minutes time intervals. In any case the data set comprises the spectrum zero-moment wave height (H m0 ), the mean period (T m ), and wave direction (θ m ). Table 1: IWN wave buoys information N Buoy Missed Data (%) Number of Years 1 Alghero Ancona Cagliari Capo Comino Capo Gallo Capo Linaro Catania Cetraro Crotone La Spezia Mazara del Vallo Monopoli Ortona Ponza Punta della Maestra In the previous preliminary estimation of the wave energy potentials, in Vicinanza et al. 2009, only the sub-set of the threehourly wave data were used while in the present work all available wave data (i.e. three-hourly or half-hourly when available) have been used in order to increases the accuracy of this estimation. METODOLOGY To increase the accuracy of the estimation of wave energy based on the IWC buoys, a gross stochastic errors detection phase has 614

3 Vicinanza et al. been applied. It was performed considering that wave data (H, T) must be compatible with the fetch related physical limit of waveform for fully developed sea condition. The values of wave height and period exceeding the obtained thresholds have been considered as errors and removed. Subsequently, the data processing has regarded the missing data problem. Missing values reduce the representativeness of the sample and it can severely disturb the conclusions drawn from the data. In order to get a conservative estimation in case of lack in the time series, missed data have been considered as calm condition (i.e. H m0 =0). However, to test the sensitivity of the results of this approach, also H m0 =1m, 2m and 3m has been used in presence of lacks (the corresponding T m0,-1 has been computed as 4H 0.5 m0 ). This analysis has shown that the estimated wave powers do not differs substantially (i.e. less than 10%) if wave heights of 0m, 1m, 2m and 3m are used to fill the missed data. Then a statistical analysis was carried out on a monthly, seasonal and annual mean wave parameters. Regard the wave energy calculation it is worth to remark that all wave buoys used in the present analysis are located in deep waters. For regular waves, the sum of kinetic and potential energy density per unit area, can be computed according to the known relationship: 2 E t = ρg H 8 (1) where ρ is the sea water density and g is the gravity acceleration. The energy flux across a vertical section of unit width perpendicular to the wave propagation direction is equal to: P = E t C g (2) in which C g is the group velocity. In case of deep water, C g can be expressed as: g C g = (3) 2ω where the wave frequency, ω, is 2π T. For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. Wave height computation is based on zero-order moment of the spectral function and readily estimated as: H m 0 = 4 m 0 (4) Moreover the wave period is derived as follow: m 1 T e = (5) m0 where m n represents the spectral moment of order n. To physically comprehend the meaning of the energy period it should be considered as the period of an energy equivalent regular wave or, in other words, as the period corresponding to the weighted average of the wave energy. Hence, after appropriate simplification and substituting H m0 and T e into equations similar to those describing the power of regular waves, the wave power, under a wave crest 1 meter wide is given by: 2 ρ H 2 T m0 e P = g (6) 64π Seawater density depends on salinity and temperature, which vary spatially and temporally. For this work an average value of ρ = 1025 Kg/m 3 was adopted. From each three-hours and halfhours wave data pair (H m0, T e ), measured at each buoy, it was computed the related power series in kw/m. Moreover it was also computed the average of the power series to get the monthly and yearly mean wave power. RESULTS In Table 2 has been summarised the assessment of the monthly and yearly mean wave power at each buoy. On the basis of the number of the missed data, as reported in Table 1, it is worth to note that the wave power estimation carried out for the buoy number 3, 4, 5, 8 and 15 has been based on short time series (around 2 years). Thus these results are poorly significant and need more analysis using longer wave time series as those coming out from the hindcast models. To enable an immediate visual identification of the wave power climate, the results are graphically represented for each buoy of the IWN with polar diagrams assembled in Fig. 2. More detailed information and the wave climate plots based on the monthly data are published on the website As expected, the Adriatic sea shows an average value around 2 kw/m, the smallest value around Italian coasts. The Ionian, North and Middle Tyrrhenian seas are a bit more energetic reaching a value of about 3 kw/m whereas the South Tyrrhenian is characterized by a value of 4 kw/m. The best resourced area is the North-West area of Sardinia Island, represented by the Alghero buoy, whose highlights a completely different behavior. The site is subject to direct approach of swells from distant storms of one of the most perturbed region of the Mediterranean Sea. In fact, it is influenced by rapid change in currents caused by winds from the east and north Europe. Here the estimated power reaches the value up to 9 kw/m in agreement with Filianoti (2000). The high energy potential for West Italian offshore (Sardinia) is also showed from wave climate registered around the west coast of Sicily Island. Sicily has mid-mediterranean location, positioning at the end of the passage formed from Sardinia and Tunisian coast. This configuration creates a localized weather system that encourage the coming of the wave energy resources from the far field. However, the data set of the buoy operating in Mazara del Vallo provide point wave power assessment of 4.75 kw/m, which is only half of the power reaching Alghero. It is important to note that Mazara del Vallo wave buoy is located offshore of the southwestern coast, in a naturally sheltered site, so that its primary and secondary fetch sectors result quite reduced. The south facing shores, in fact, experience more gentle swell conditions associated with weaker seas generated by damped winds passing through the Tunisia and Sicily island. Hence, greater energetic conditions are expected for the Sicilian western coast. Table 2. Average monthly and yearly wave power at IWN buoys. Buoy Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual kw/m na na na

4 Estimation of the wave energy in the Italian offshore Figure 2. Wave power climate for the IWN wave buoys. 616

5 Vicinanza et al. CONCLUSIONS Wave energy is a renewable and pollution-free energy source that could have the potentiality to contribute in the Italian electricity market, benefiting to a better definition of the national strategies on energy supplies. In order to prove if this is a realistic opportunity, an estimation of the wave energy in the Italian offshore has been carried out. Overall results confirm that the wave energy perspectives of the Italian seas is relatively low if compared to the other European countries faced to the ocean. By means of the analysis of available wave buoy data, it has been highlighted that the west coast of Sardinia and Sicily Islands are the more energetic sites thus that the exploitation of wave power could be considered at these sites before than elsewhere around Italian coasts. In fact, the higher values have been obtained for the Alghero and Mazara del Vallo buoys where respectively 9.05 kw/m and 4.75 kw/m have been computed. The Mazara del Vallo buoy location has a southwestern exposition which modify the wind and wave flow and create a more localized weather patterns reducing the direct approach of highest sea swell conditions. This situation reduces the representativeness of this buoy for the west Sicilian coast, where greater energetic conditions are expected. Hence it seems to be reasonable to make a detailed estimation of a Wave Energy Converter (WEC) installation at least at these locations. In principle there is not "the best" commercial leading technology that can be used everywhere but it is expected that, depending on the selected site characteristics, some technologies could be more efficiently used than others. Due to the stochastic nature of wave energy, the performance of a WEC is compromises between the optimum values at various sea conditions. The tuning parameters may be established at each sea state. Hence, in general, WECs with a flexible optimization are more suitable. Nevertheless, in that phase it will be critically important to ensure that the harvesting of the wave power may represent really a sustainable energy source and do not become environmentally costly. ACKNOWLEDGEMENTS The work discussed here is part of the Second University of Naples founded project PRIST 2007 (Progetti di Ricerca di Rilevante Interesse Scientifico e Tecnologico) titled Convertitori di Energia Ondosa per la produzione di Energia Elettrica (Wave Energy Converters for Electrical Production). The first Author gratefully acknowledges Second University of Naples for supporting this innovative research and encouraging mobility of researchers. LITERATURE CITED Angelis-Dimakis A, et al., Methods and tools to evaluate the availability of renewable energy sources. Renew Sustain Energy Rev, doi: /j.rser Athanassoulis, G.A., Gerosthatism, Th.P., Stefanakos, Ch.N. and Holevas, H., Coastline and bathymetry of the European nearshore area. 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