Logo: to the web site of Uppsala University

This paper presents and discusses the wave climate off the Swedish west coast It is based on 8 years (1997-2004) of wave data from 13 sites, nearshore and offshore, in the Skagerrak and Kattegat. The data is a product of the WAM and SWAN wave models calibrated at one site by a wave measurement buoy. It is found that the average energy flux is approximately 5.2 kW/m in the offshore Skagerrak, 2.8 kW/m in the nearshore Skagerrak, and 2.4 kW/m in the Kattegat. One of the studied sites, i.e. site 9, is the location of a wave energy research site run by the Centre for Renewable Electric Energy Conversion at Uppsala University. This site has had a wave power plant installed since the spring of 2006, and another seven are planned to be installed during 2008. Wave energy as a renewable energy source was the driving interest that led to this study and the results are briefly discussed from this perspective.

Little is known about the effects of offshore energy installations on the marine environment, and further research could assist in minimizing environmental risks as well as in enhancing potential positive effects on the marine environment. While biofouling on marine energy conversion devices on one hand has the potential to be an engineering concern, these structures can also affect biodiversity by functioning as artificial reefs. The Lysekil Project is a test park for wave power located at the Swedish west coast. Here, buoys acting as point absorbers on the surface are connected to generators anchored on concrete foundations on the seabed. In this study we investigated the colonisation of foundations by invertebrates and fish, as well as fouling assemblages on buoys. We examined the influence of surface orientation of the wave power foundations on epibenthic colonisation, and made observations of habitat use by fish and crustaceans during three years of submergence. We also examined fouling assemblages on buoys and calculated the effects of biofouling on the energy absorption of the wave power buoys. On foundations we demonstrated a succession in colonisation over time with a higher degree of coverage on vertical surfaces. Buoys were dominated by the blue mussel Mytilus edulis. Calculations indicated that biofouling have no significant effect in the energy absorption on a buoy working as a point absorber. This study is the first structured investigation on marine organisms associated with wave power devices

The ocean are largely an untapped source of energy. However, compared to other energies, power fluctuations for ocean waves are small over longer periods of time. This paper present a grid-oriented approach to electricity production from ocean waves, utilizing a minimal amount of mechanical components.

The full-scale direct-driven wave energy converter developed at Uppsala University has been in offshore operation at the Swedish west coast since 2006. Earlier simulations have now been validated by full-scale experiment with good agreement. Based on that, a theoretical model for a passive system having optimum amplitude response at frequencies coinciding with Swedish west coast conditions has been developed. The amplitude response is increased by adding supplementary inertia by use of the additional mass from a submerged body. A sphere with neutral buoyancy is chosen as the submerged body and modeled as being below the motion of the waves. The model is based on potential linear wave theory and the power capture ratio is studied for real ocean wave data collected at the research test site. It is found that the power capture ratio for the two body system can be increased from 30% to 60% compared to a single body system. Increased velocity in the system also decreases the value for optimal load damping from the generator, opening up the possibility to design smaller units.

We calculate the distribution of energy flux as a function of the distance below the surface for propagating polychromatic gravity fluid surface waves. Linear theory has been used to derive closed-form expressions for the energy flux as a function of depth. In this context we discuss the power distribution for real ocean waves measured off the west coast of Sweden and compare this to the energy flux distribution for waves with Pierson-Moskowitz and Bretschneider spectral distributions. This is done in order to get an improved understanding of how to improve the power absorption in wave energy converters, and this is also discussed in this paper.

For a complete understanding of a wave energy conversion device, it is important to know how the proposed device moves in the water, how this motion can be measured, and to what extent the motion can be predicted or simulated. The magnitude and character of the motion has impacts on engineering issues and optimization of control parameters, as well as the theoretical understanding of the system. This paper presents real sea measurements of buoy motion and translator motion fora wave energy system using a linear generator. Buoy motion has been measured using two different systems: a land-based optical system and a buoy-based accelerometer system. The data have been compared to simulations from a Simulink model for the entire system. The two real sea measurements of buoy motion have been found to correlate well in the vertical direction, where the measured range of motion and the standard deviation of the position distributions differed with 3 and 4 cm, respectively. The difference in the horizontal direction ismore substantial. The main reason for this is that the buoy rotation about its axis of symmetry was not measured. However, used together the two systems give a good understanding of buoy motion. In a first comparison, the simulations show good agreement with the measured motion for both translator and buoy.

The total instantaneous energy transport can be found for polychromatic waves when using the deep water approximation. Expanding this theory to waves in waters of finite depth, we derive an expression for the total instantaneous energy transport for polychromatic fluid gravity waves based on potential theory with linearized free surface boundary conditions. We present the results for time series of wave elevation measured at the Uppsala University wave energy research test site. We show that a significant proportion of the total instantaneous energy transport is not accounted for when using the deep water theory. This is important since many wave energy conversion devices under development will operate in waters that do not fulfil the deep water criteria.

Based on an earlier conceptual model of a two body system point absorbing wave energy converter tuned to resonance in Swedish west coast sea states, an extended coupled hydrodynamic, mechanic, and electromagnetic model has been developed. The hydrodynamic characteristics of the two body system are studied in the frequency and time domain, while its response to real Swedish west coast sea states are studied in the time domain, by using a wave energy converter model with two independently moving bodies connected to a direct driven linear generator with non-linear damping. The two body system wave energy converter gives nearly 80% power capture ratio in irregular waves. The resonant behaviour is shown to be sensitive to the shape of the spectrum, and the distance between the two bodies is shown to have a large effect on the power absorption.