1. 8th Baltic Sea Science Congress 2011 Joint research efforts for sustainable ecosystem management
Wave energy resources in Estonian territorial sea Victor Alari Marine Systems Institute at Tallinn University of Technology
MOTIVATION
Life without energy is cruel. How long will you stay cozy without electricity and fuel? Physics tells us, that energy cannot be created nor destroyed, but it can be converted from one source to another. Taking that for granted, we have used much of the fossil fuels that nature ha(d)s to offer. Fossils are ending and climate is changing due to us. What will are children think about that?
Well, as I’m in the “wave” business, I couldn’t overlook the opportunity of catching some of the waves near Estonian coast and discussing their role on tomorrows power supply. Here I go…
A glad coincidence
From the perspective of wave energy usage, high waves and ice free waters are positive. Even more positive is when they both occur in same areas. This is the case near Hiiumaa and Saaremaa islands in Estonia (Figs. 2 and 3). This implies that the assessment of wave energy potential is only meaningful for this region, considering only Estonia’s case.
Figure 2. Average significant wave height in years 1970–2001. More red means higher waves. Copied from Räämet and Soomere (2010).
DATA AND METHODS
Numerical modelling is used to assess wave energy potential.
We choose SWAN wave model and extract results for western Estonia coastal sea.
We model a two-year period covering
The years were chosen because they represent two different NAO index phases.
HIRLAM model wind fields forced SWAN. Correlation between modelled and measured wind was 0.87.
Figure 3. “Average” ice situation in years 1971–2000. White represents ice, grey open water. Copied from Granskog et al (2006).
ENGINEERING QUESTIONS
Although the probability distribution of waves is quite anisotropic (Fig. 11), it still has two peaked shape, with high waves most probably approaching from SW or NW. The best construction for wave converter is therefore linear generator buoy.
The cumulative distribution function of wave energy flux shows that 22 % of “waves” are over the mean value of 5.5 kW/m (Fig. 12). However, 40 % of fluxes are over 2 kW/m. It implies than much work is needed to find the best buoy dimensions – do we want the most out of small waves? Or do we want capturing the energy of extreme waves? etc
SWAN MODEL VALIDATION
Figure 1. What kind of energy would you prefer?
TEMPORAL VARIABILITY
The variability of energy flux during the two year period is remarkable (Fig. 8), and stems from the changing wave fields. The average was 4.7 kW/m in 2006 and 6.1 kW/m in 2007, whereas the respective maxima were 125 kW/m and 180 kW/m. However, alongside with such relatively chaotic temporal variability there exists seasonal variability (Fig. 9).
SPATIAL DISTRIBUTION
The spatial distribution of wave energy flux is not a surprise (Fig. 7). Due to wave dissipation near coast the wave energy flux also decreases and moving further to deep water (compare with Fig. 4) the flux increases. The mean averaged over is about 5.5 kW/m in the offshore areas.
Figure 4. (•) location, where wave flux data is extracted; () wave measurement and validation sites; () wind measurement and validation site.
Figure 11. Line with dots - probability of occurrence of waves depending of approach direction. Bars – light blue represents the average wave height and blue the maximum wave height coming from certain direction.
Figure 8. Temporal variability of wave energy flux at 58°31’N and 021°28’E
Figure 7. Spatial average of wave energy flux near Hiiumaa and Saaremaa Island during The colorbar unit is (kW/m).
Figure 5. Validation of the SWAN wave model.
SEASONAL AND YEARLY CHANGES
Energy flux exceeds the average value in November, December and January, and stays below the average in May, June, July and August in both years (Fig. 9). When October 2006 demonstrated fairly good productivity and was comparable with November 2006, then in 2007 the average for October was only 4.2 kW/m. The maximum for April 2007 is linked with a big wave event that can easily be seen in the time series (Fig. 8). Wave power increases during this event up to 125 kW/m
Figure 6. Validation of the SWAN wave model.
Figure 12. Cumulative frequency distribution of energy fluxes for the period
NATIONAL CONSUMPTION
Estonians are energy hungry people, like everybody. Average monthly energy consumption in 2006–2007 is clearly seasonal (Fig. 10). Energy consumption is the highest during late autumn and winter and lower during spring and early autumn. The consumption hits its low in summer months. As indicated, wave power is also characterized by seasonal variability (Fig. 9) that by and large coincides with the energy need. The results of this study show that the best months for energy production are November, December and January, i.e. the time when the energy need is also high
Figure 9. Monthly mean wave energy flux.
WHY YEARS ARE NOT BROTHERS - WAVE ENERGY RELATION WITH LARGE SCALE ATMOSPHERIC CIRCULATION
The strength of wintertime NAO index, positive or negative, determines how much westerly's Baltic Sea has and hence determines the ice and wave climate. 2005/6 winter was in NAO- phase and the Baltic Sea ice coverage was as average with low wave energy flux. On the other hand, 2006/7 NAO was in strong + phase, even the Gulf of Finland was ice free.
A strong exponential relationship (Fig. 13) holds between wintertime NAO and wave energy flux. This relation can be used as proxy for long-term hindcast of wave energy flux in the Baltic Proper.
Figure 13. Dependence of wave energy flux on wintertime NAO index.
ARE WE BETTER…?
A question arises – is the 5.5 kW/m energy flux high, medium or low. In the Baltic Sea context, this seems to be an average value. For example, Waters et al (2009) found that average wave energy flux during 8 years in the offshore Skaggerak was 5.2 kW/m. In comparison with the countries bordering an ocean, wave energy flux is one order of magnitude lower in Estonia (Table 1). However, Estonia’s wave energy potential per 1 TWh of energy consumed exceeds the wave energy potential of such big countries like Spain, UK and France as much as four times, since these countries consume considerably more energy than Estonia. In this context, these big countries could produce energy from waves, but they would not achieve a considerable increase in its share. And one cannot install an infinite number of wave energy converters into the sea in current economic situation.
EPILOGUE
I hope that someday the theory of catching waves to electricity in partially ice-covered, tideless and sometimes chaotic seas of the Baltic will come to practice in large scale.
Figure 10. Monthly mean electrical energy consumption in Estonia during
ICE - CURSE OR BLESS
Yes, we all know that around every 7th year the Baltic Proper freezes and try to catch waves then. To me this does not sound much of a problem.
We are used to fossil fuels which created an illusion, that energy flow is smooth and it comes from one certain location. It doesn’t work with renewables this way. Probably, if the Baltic Sea is fully ice covered then NAO is so low, that it will tear down wind speed also for a quite long time. This results rest of wind farms also. The point is that renewable’s are not replacement for fossil fuels, but as an alternative when nature offers us wind or waves or whatever kind of renewable. This will put the end of fossil fuels further in the future and considerably affect climate in a positive way. Meanwhile, our children can live happily and bright minds can turn the clean hydrogen energy from theory to practice.
Table 1
Nation
Consumption (TWh)
Average energy flux (kW/m)
Wave energy potential per TWh consumed energy
Iceland 8 60
7,50
Ireland 24 70
2,92
Portugal 48 50
1,04
Estonia 7
5,5
0,79
Spain
243
0,21
Engalnd
345
0,20
France
437
0,11
References
Granskog, M., Kaartokallio, H., Kuosa, H., Thomas, D.N., Vainio, J Sea ice in the Baltic Sea – A review. Estuarine, Coastal and Shelf Science, 70,
Räämet, A., Soomere, T The wave climate and its seasonal variability in the northeastern Baltic Sea. Estonian Journal of Earth Sciences, 59(1), 100–113.
Waters, R., Engström, J., Isberg, J., Leijon, M Wave climate off the Swedish west coast. Renewable Energy, 34,
Acknowledgements
I am grateful to the Estonian Meteorological and Hydrological Institute for providing wind data for validation. This study was supported through GORWIND project and Estonian Science Foundation grant nr. ETF8968.