Wave Energy Systems
While there is a wide range of wave energy designs being tested, the method for connecting these facilities to the electrical grid are largely the same.
Wave Energy System Types
Ocean wave energy technologies are still in the early development and demonstration phase. There are four predominant wave energy technologies: point absorbers, attenuators, terminators, and overtopping devices. These four technologies are classified according to their size and orientation.
Point absorbers have a small horizontal dimension compared with the vertical dimension. Attenuators are elongated floating structures whose length, which is about one wavelength or longer, is aligned in the direction of the wave propagation. Terminator devices are aligned perpendicular to the direction of wave propagation. Overtopping devices are reservoirs that are filled by incoming waves to levels above the average surrounding ocean.
A point absorber device consists of a buoy that is connected to components that move relative to each other due to the rising and falling of waves. This mechanical energy drives an electrical generator. The electrical energy is fed down a single umbilical cable to a junction on the seabed. Several devices can be connected together and linked to shore through a single underwater cable. The device can either float or be anchored to the sea floor. An individual point absorber device may produce up to 11 MW of electricity, although most current designs would produce much less.
Attenuators are long, multi-segment floating structures anchored so they are aligned perpendicular to the direction of wave travel. The segments flex at hinged joints as a wave passes along the device. The mechanical motion of the flexing is converted to electrical energy using hydraulic motors and generators. The electrical energy is fed down a single umbilical cable to a junction on the seabed. Several devices can be connected together and linked to shore through a single underwater transmission cable. The energy generating capacity of a single attenuator device can be up to 1 MW.
Terminators are oriented perpendicular to the direction of wave travel and are often located onshore or near shore. One example, shown to the right, is the oscillating water column. This system consists of a chamber, which is a fixed structure with its bottom open to the sea. The wave motion inside the chamber alternately compresses and decompresses the air that exists above the water level inside the chamber. As a result, an alternating stream of high-velocity air is generated. This airflow is driven through a duct to a turbine generator that is used to generate electricity.
For an offshore device, the electrical energy is fed down a single umbilical cable to a junction on the seabed. Several devices can be connected together and linked to shore through a single underwater cable. The energy generating capacity of a single terminator device can be up to 1.5 MW.
Overtopping devices generally are anchored in open water and consist of reservoirs that are filled by wave action to levels above the surrounding sea level. The collected water is released from the reservoir to drive a turbine and generator to produce electrical energy in the same way a hydroelectric plant does. Overtopping devices have been designed and tested for both onshore and floating offshore. The Wave Dragon (shown on the right) is an example of an offshore overtopping device.
The electrical energy is fed down a single umbilical cable to a junction on the seabed and is then linked to shore through a single underwater transmission cable. The energy generating capacity of a single overtopping device can be up to 11 MW.
Wave Energy Facility Size
An individual wave energy device is capable of producing up to 11 MW, depending upon the design of the device. A utility-scale installation of most types of devices would include many wave energy devices, depending upon the intended use. Utility-scale wave energy facilities would generate a large amount of electricity that would be transmitted from a near-shore wave energy farm to many users through a transmission system, similar to that of any other commercial power plant.
A wave farm at Agucadoura, Portugal, consists of three attenuators manufactured by Pelamis Wave Power. It produces 2.25 MW, enough to power 2,000 homes. The wave farm will ultimately contain 30 such devices, which will occupy approximately 250 acres.
An individual overtopping device could occupy from 0.4 to 9 acres. Because there are no large systems in place, the ultimate size of these facilities is unknown. Estimates used in the OCS Alternative Energy and Alternate Use PEIS indicate that commercial-scale wave energy facilities likely would occupy less than 2 km2 (1.9 mi2).
Primary Wave Energy Facility Components
Wave Energy Devices
The major component of a wave farm is the device that captures the mechanical energy of the ocean waves. Each device is attached to the sea floor via various anchoring systems. Turbines or hydraulic motors and generators produce the electricity. Power cables from each device are connected to a seabed cable. Grid synchronization occurs via a variable speed drive and step-up transformer to a suitable voltage level.
Generated electricity is brought to shore via a standard submarine electrical cable, which is usually installed in a trench in the seafloor and under the beach at the shore. The cable runs to an interconnection substation that might be as small as 225 ft2 (25 m2). From that point, the electricity is transferred to consumers through the land-based transmission grid.
The submarine electrical cable would be connected to a local power distribution grid or a long-distance power transmission grid. The control and monitoring of devices and transformers would be carried out remotely using fiber-optic cables or other communication devices.