L7: Wave Energy Converter Technology Flashcards
Popular Power Take Off Systems
Hydraulic piston system Accumulation Air turbine Hydraulic turbine Direct mechanical drive system Direct electrical drive system Dielectric elastomer Inertia PTO
Hydraulic cylinder system
Consists of 3 main parts:
Chamber filled with hydraulic oil
A piston rod
A pump
Piston will contract when the pump feeds oil into the upper part and will extend when the pump feeds hydraulic oil into the lower part of the chamber
Check valves
Part of hydraulic piston system.
Allow fluid to flow in only 1 direction.
Including check valves, the hydraulic oil can only flow from the high pressure accumulator side to the low pressure accumulator side, regardless of whether the piston is moving up or down.
Accumulation
Regulates the amount of hydraulic oil flow into the turbine in order to prevent rapid change of flow rate into the turbine which then provides a relatively stable or constant electricity output to the grid.
Almost all power take off systems have some sort of accumulation.
Air turbine
Most oscillating water column devices use air turbines as the power take off.
When the airflow passes the turbine, the turbine will rotate due to the lift force generated by the aero foil. Due to the reciprocated character of the air flow movement inside the chamber, the air turbine needs to be able to rotate in the same direction irrespective of the flow direction. Common solutions to that is to use the well known wells turbine which uses symmetrical aero foil.
Hydraulic turbines
Similar to air turbines but for liquid
Direct Mechanical Drive System
Uses a gear system to convert motions into a rotation form. Dyno torch is a good example of this technology.
When you press crank, the energy provided by you is then transferred to the gear system and fed to the generator.
Dielectric elastomer
A flexible material that can produce electric charges when deformed. Relatively new application in the wave energy industry and is normally used on some deformable wave energy converters like balloons.
Inertia PTO
A mass connected to the generator. When the wave energy converter moves, mainly a rotational motion, the generator will continue to rotate due to the inertia of the mass.
How does the Power Take Off system affect the wave energy converter?
According to Newton’s 3rd law of motion, when the wave energy converter applies a force to the power take off system the power take off system will also apply a force to the wave energy converter. Therefore, to account for the effect of the power take off system, we need to add the power take off system force into the wave energy dynamic equation.
Mathematical representation of PTO.
PTO force can be represented as a damping force which is normally proportional to the velocity. This however depends on the specific PTO system. In order to accurately assess the power performance of a wave energy design, it is necessary to model the PTO system correctly.
Wave Energy Converter Classifications
Point absorber Attenuator Oscillating wave surge converter Terminator Oscillating water column device Overtopping devices
The point absorber
Has a relatively small size and is not sensitive to wave direction, meaning it can theoretically achieve the same energy extraction performance with omni direction waves. A distinguishable feature of this device is the antenna or point absorber effect. That is that the point absorber can extract energy not only from the wave coming in front of it but also from waves that pass by the side.
Attenuator
Normally laid parallel to the main direction of wave propagation. When in operation, the device looks like it’s riding the wave. As the device needs to ride the wave, it is a directional type of device. The orientation of the device has to be facing the incoming wave to maximise power extraction.
Oscillating wave surge converter
This device mainly used the surge component of water particle motion, which provides moment to the flap of the device and the device will pitch about a pivot.
