What is a point absorber ?
The point absorber is a floating structure which absorbs the energy of the incident wave coming from every direction. These devices are small relative to the wavelength, so the direction of the wave does not affect its effectiveness. There are several different devices that use the point absorber concept, but for our study, we are going to focus on the best-known of the point converter devices : The PB150 PowerBuoy©, developed by Ocean Power Technologies (OPT).
How does it work ?The powerbuoy PB150 consist of two principal parts : • A Spar floating in a relatively stationary position thank to a mooring system and a heave plate which reduces the heave motion. • A Float moving up and down around the spar with the wave motion. The oscillations of the float at the top drive a generator contained in the Spar which will produce electrical energy. This energy is then sent to the shore by underwater cable. This type a device has several advantages, such as a low surface profile, a small horizontal footprint and is suitable for creating a wave farm. Indeed a wave propagating through an array of powerbuoys is minimally affected by each buoy, maximizing the efficiency of the buoys to convert the wave energy. Also the low profile minimizes its visibility from shore, so the aesthetic impacts of a wave farm are lessened , avoiding the negative impacts to tourism and recreation in coastal areas that an offshore wind farm would present. Another advantage of the PB150 is its ability to supply energy in a variety of weather conditions, with waves heights up to 8 meters. If there is a big storm or a hurricane which generates even larger waves, the powerbuoy automatically stops the production of energy in order to protect the mechanical system. When the wave conditions return to normal, the system continues to convert the wave energy. |
Computer simulation exemple
This video shows a simulation that OPT did in 2008 to demonstrate how the powerBuoy will work in a wave farm.
Theory of wave energy
From "Water Waves Mechanics for Engineers and Scientists," written by Robert G. Dean and Robert A. Dalrymple, we know that the total energy contained in a wave consists of two kinds: the potential energy, resulting from the displacement of the free surface and the kinetic energy, due to the fact that the water particles throughout the fluid are moving. This total energy and its transmission are of importance in determining how waves change in propagating toward shore, the power required to generate waves, and the available power for wave energy conversion devices. After derivation, the final energy equation for a linear periodic and unidirectional wave is :
With :
• E (kJ) : Energy
As mentioned before, this formula works for linear periodic and unidirectional wave. However, those types of wave are usually not encountered in the ocean, so this equation can only be considered a first-order approximation during the design phase of a buoy project. Modelling is required to determine the expected wave characteristics used in the buoy design. Finally, because of the waves encountered in the field are complex and random, prototype testing of the buoy design in the field with determine if the buoy is working as expected.
• E (kJ) : Energy
- Rho (Metric Tons / cu.m) : water density
- g (m/s-2) : gravitational acceleration
- H(m) : Average wave height
- KE : Kinetic Energy
- PE : Potential Energy
As mentioned before, this formula works for linear periodic and unidirectional wave. However, those types of wave are usually not encountered in the ocean, so this equation can only be considered a first-order approximation during the design phase of a buoy project. Modelling is required to determine the expected wave characteristics used in the buoy design. Finally, because of the waves encountered in the field are complex and random, prototype testing of the buoy design in the field with determine if the buoy is working as expected.
Characteristics
The theoretical characteristics of the power Buoy are given in the table on the left. According to the value of the power capacity (100kW), a wave farm of 30 power buoys is needed to match the output of a single wind turbine (3 MW). This comparison show that a single Power Buoy is not converting a lot of energy, however, with the small footprint of these buoys, it is possible to deploy an extended wave farm with more than 30 buoys.
|
The First Field Prototype
In 2011, the first Power Buoy was deployed off the coast of Scotland