In many schools and universities marine energy is taught in the offshore energy department. As for much of marine energy theory it is the same as theory on ship motions. The big difference is that at ships technology is used to keep the energy of waves as much as possible out of the vessels, while in wave energy the task is to capture this energy. Waves are build up bij wind striking the water surface. There is a relationship between this. See next graph.
Waves are not only a surface phenom, but go deep under the surface and make a large quantity of water move. Actually it is this motion that travels over the ocean like domino stones with a speed of up to 80km/h.
After traveling a longway, even if there is no wind at the surface, these wave arrive at the coasts. Here they transform into shallow water waves and break on the beaches. A famous spot is Nazaree in Portugal where waves that have traveled over a long distance come at a place where the seabed is shaped so, that the waves merge and grow to huge sizes.
A very special wave at Nazaree in Portugal.
Luckily for us most waves are much smaller. The statistic height versus occurrence of waves is actually one of the largest challenges in the wave-power technology. The average wave is importent for the total energie production, and as such the income. The highest waves makes the devices expensive, as they have survive these heigh waves.
The graph below is a so called scatter diagram. It gives for a certain site the historical vallues of the occurrence of a certain waves
The scatter diagram above shows the occurrence of a certain sea-state.
The wave height and period given are however a number that represents a spectrum of waves. The graph below shows how these waves vary. the black line represents the waves.
The graph show also how the symphony device is behaving in these waves. It even shows the amount of power is taken from the generator. This is done by applying an electromagnetic force that counteracts the motion of the turbine and therefor slows down the the motion of the device. It also shows a relation ship between the wave height and the generator force.
The symphony is an underwater WEC (wave energy converter). In the middle position of the top part of moving cap it is ca. 6 m submerged. On the bottom there is a small winch.
This winch controls the draft and makes the device move with the tides. It also puls the device more down when heavy storms are coming. As in the scatter diagram is shown , most of the waves are below Hs 4.5 m. This value is a statistic wave height. The average wave in this seastate is much lower while the highest wave can be theoretical 1,8 – 2 times the significant wave. Again here a statistic weight is geven see graph below. It shows the amount of individual wave heights in a seastate.
In the operation of the device this means that for the heighest waves (above 5m) the generator can not dampen the motion alone and the device will run in its end buffers. As the buffers are springs (see technology of the symphony) this doesn’t matter. This is a huge benefit of the structural membrane.
How does the symphony operate?
Inside the Symphony is double acting structural roll membrane. Inside this membrane is a liquid. This liquid is pressurized by the gas on top of the liquid. As the gap from the upper membrane is larger than the lower the liquid moves in and out of the pressure chamber as the hull moves up and down. As the air volume decreases the pressure increases. This increase gives a higher force on the membrane and forces the hull upwards once the pressure in the wave pushing on top of the device, decreases, the hull starts to move up. The air volume together with the membrane acts like a spring. If the mass and spring are tuned well. This is done by the size of the air volume, the device resonates at the dominant wave frequency. If you look at the time series in the graph shown before you can see that device even on low waves moves at its full stroke. During this motion the forse of the wave multiplied bij the stroke represents the energy captured. In this way even from low waves power can be taken.
[The black line represents the wave, the blue line represents the motion.]
Of course there is a limit to this all. As the device moves it radiates a wave as well. This is called “hydrodynamic damping” if the waves are very small. These radiated wave absorb all the energy. In those waves the device starts to limits its motion. In the test models this is done with wave that are smaller than 0,2 m.
The turbine in the middle slows down the motion. As one of the rotor has “fingers” and one only pockets the rotor is pushed around by the pressurised water. The generator creates a counter force.
As the hull moves down the water comes from the outside to the middle like shown in the picture above. the next half cycle the turbine reverses and water flows out.
The generator is directly connected to the turbine wheel. As the magnets are on a large diameter and the velocity of the rotor goes up to almost 400 rpm, much less magnets and coils are needed and a large cost reduction is created compared to another option; the linear generator.
Comparing the Symphony (principle) to other devices.
Because the symphony is a resonant system, it is capable of extracting more energy than most other systems. The simple reason for this is that many other systems, especially buoyant systems have a too stiff spring. The spring is based on its displacement of water as a function of displacement. As the mass is fixed. ( the displacement of water in its middle position. Morst buoyant systems have natural period of ca. 5 seconds. This makes them move with the waves instead of a motion that is slightly delayed. See both figures below.
the left figure represent a “normal” buoy , while the right finger a resonant system is. The blue line is the wave, the thin not dashed line the position and the dashed line the velocity. The pressure of the wave times velocity gives the power out. We see at the left figure lower power out and during part of the motion even a negative power. As the device down moves as the wave is stil positive. Buoyant systems have found ways to make this problem smaller.
A very clever solution is by Corr power. They introduced a hydro-pneumatic spring that creates a force that increases a more if it is further from its equilibrium. This is called a negative spring. At another principle the sparr-buoy they increase the mass by adding volume with the same density as the water. Both systems are capable of resonating which results in a much higher efficiency (up to 300% more) .
We believe that being underwater for survivability and with the features of symphony and the same high efficiency Symphony is one of the best options to survive and be high efficient. It will al depend on the reliability of the membrane and the novel turbine.
These technologies will be extensively tested before the system goes in the water.