Wave Climate and Wave Energy Content
Before building the WEC, the acquired data from the wave measuring buoy was studied to determine e.g. a suitable stroke length. Recently a more thorough analysis of the wave climate at the test site has been made. In Fig. 7, 15,650 data points from 2007 have been plotted. The data is from all months except for August, and each point represents a half hour average. For every half hour, the corresponding values for energy period T
E and significant wave height H
S have been calculated. The left plot in Fig. 7 lists the occurrence of different sea states in 2007, whereas the figure to the right describes what sea states carry the greatest amount of energy over a year. The average energy flux during 2007, excluding August, was 3.4 kW/m.
While the most frequent sea state has a value for T
E around 4 sec and H
S less than 0.5 m, the main energy contribution comes from the more energetic sea states. Due to its relatively low energy flux, the wave climate at the Lysekil test site would not be ideal for commercial wave power production. That was, however, not the motive for choosing the site; it was chosen due to its proximity to the marine research stations described above under Test site.
WEC Output and Forces
The generated output from the WEC has been continuously recorded in the measuring station. A time window of voltage and power is given in Fig. 8. The significant wave height during the half hour from which Fig. 8 is taken was 2.3 m, and the load applied at the measuring station was 2.2 Ohm. A force transducer was connected between the generator and the buoy in the same way as can be seen in Fig. 3. The force from the weight of the translator added to the force from the springs in mid position is around 20 kN. Accelerometers were placed inside the buoy and have been used to calculate the vertical buoy speed. Figure 8 shows the measured buoy line force, the vertical speed of the buoy, the load voltage and power in the load during a time period of 24 s. A detailed description of the events illustrated in Fig. 8 is given below.
The force in the buoy line of the WEC has been measured and compared with electrical power and output voltage during a set of waves. The measured time sequence is shown in Fig. 8. At first, the buoy line force is around 40 kN. At t = 0.5 s, the translator is near its top position, which is shown by the peak in the force curve that indicates that the translator is in contact with the upper end stop. The buoy starts to move from the crest to the trough. From this point onward, the line force is reduced. The buoy and translator is accelerated downward by the springs and the translator mass and voltage is induced in the generator. This downward motion takes approximately 2 s. At approximately t = 2 s to t = 3 s, the downward motion of the buoy is larger than the downward motion of the generator translator. This results in that the line slackens and the resulting line force is almost zero. Maximum power is reached during minimum and maximum line force, assumed within generator stroke length. If the line is slack when the translator reaches the bottom, the generator stroke length is shorter than the wave height. At t = 4 s, the translator changes its direction, which is shown by that the buoy speed becomes positive and by the change in phase order in the voltage curve. The buoy is now lifted by the hydrodynamic forces acting on its bottom surface, i.e. it is lifted by the wave until the translator reaches the upper end stop a second time. This is visible as a peak in the force curve at t = 8 s. Some of the kinetic energy of the translator and buoy has now been converted to potential energy, which is stored in the end stop spring. The translator is now at stand still and energy is not produced until the wave height is lower than the top position of the buoy. At t = 9 s, the translator is descending again. The line force is then about 25 kN, decreasing towards 20 kN and below.
A higher level of damping (power extraction) results in bigger difference between the vertical motion of the wave and the speed of the translator. This will in turn result in a higher line force when the wave lifts the buoy and a lower line force when the buoy descends. This effect is clearly visible at e.g. t = 11 s when the translator and buoy moves downwards. If the damping is very large the line might slacken when the buoy moves downwards which then results in a line force close to zero.
At t = 12.5 s, the translator again changes direction and starts moving upwards (see the speed curve). This time, the translator turns almost instantaneously without any time at standstill. This is most obvious looking at the voltage curve. At t = 15.5 s, the translator again changes direction after hitting the upper end stop. The end stop spring is most likely compressed to a maximum and the line force peaks, reaching a maximum of 60 kN. The end stop spring then recoils the translator some 0.2 m, which is visible as a small voltage pulse in the voltage curve. The translator decelerates for a little less than a second before it continues to descend. The analyzed wave sequence ends with a smooth voltage pulse as the buoy is lifted by a wave crest at t = 21 s. The translator moves its full stroke length until it hits the upper end stop, again resulting in a force peak of some 55 kN at t = 23.5 s.
The performance of the WEC can be seen in Fig. 9. The figure shows the power absorbed by the buoy (excluding generator iron losses and mechanical losses) when the WEC is connected to a resistive load of 4.9 ohm per phase. Each data point is the average of one half hour of measurements. As can be seen, the data points in the region of 4–10 kW/m are sparser than in the region below 4 kW/m. This is due to the low occurrence of more energetic sea states during the test period. The spread in absorbed power is also larger in the region of more energetic sea states than in the calm seas. A study on the variation in power absorption at different resistive loads was made in (Waters et al. 2007).
As previously mentioned, a conversion of the generator power output is needed before grid connection. The first step in this conversion is to rectify the voltage from each generator to a DC voltage, interconnect them in parallel, and filter the DC voltage. In order to study how this system will be designed in the future, the existing system in the measuring station was complemented with a six pulse diode rectifier and a capacitive filter. A more detailed description of the system is given in Boström et al. (2008). The total capacitance value of the ultra capacitors in the filter is 12.2 F. The high capacitive value enables energy storage that is needed for a system including only one generator. A resistive load is connected in parallel with the capacitors and by changing the resistive value, different load characteristics were achieved in the operation of the generator. The purpose with the experiment was to study how the generator operates when it is connected to a non-linear load. The level of power smoothing can then be analyzed.
Figure 10 shows one of the three phase voltages, measured before the rectifier, and the DC-voltage, measured after the filter. The result shows that the filter can smooth out the voltage from the linear generator to a stable DC voltage. During shorter time periods as in Fig. 10, the power after the filter will also be constant. If the system is studied during longer time periods (hour-scales) there will be a variation in the produced power, see Fig. 11. The variations occur due to changes in the sea state. To overcome sea state variations with capacitors, unreasonably large energy storage devices would have to be used.
The sediment samples from 2004 contained in total 309 individuals of 68 different species. The results of infauna sampling showed that there is a significantly higher species abundance in the buoy area compared to the control area (ANOSIM Global R = 0.259; p = 0.004) (Fig. 11). Monitoring studies at the research site before constructions showed polychaete worms were the most abundant species in the sediment (Fig. 12).
Comparing total species (S), species richness (d) and Shannon Wiener biodiversity, it all was significantly higher in the buoy area compared to the control area (Fig. 13, Table 1). Anyway, we only found very small, juvenile organisms, there were no red listed species found in those areas and there is no concern about extinction of sensitive local species.