Technology to extract energy from ocean waves and tides is still emerging.The various wave and tidal units under development fall into one of six broad technology concepts. To date, only a few technologies have progressed to long-term testing of a full-scale prototype in natural waters.
By Andree J. Houle and Roger J. Bedard
Ocean energy is a term used to describe all forms of renewable energy derived from the sea, including: wave, tidal, and ocean current. The technology to convert wave, tidal, or current resources to electricity is here today, albeit in its infancy.
Work in this field actually began in the 1970s. At that time, the United Kingdom regarded wave power as an alternative to nuclear generation and had the most aggressive research and development program in the world. The program contributed to important research on optimal control and tuning of wave power conversion devices. However, the program stalled as oil prices dropped in the late 1970s and, as a result, government funding stopped in the early 1980s.
Over the past decade, industry, government, and academia research programs have established an important foundation for the emerging ocean energy industry. Several factors have renewed interest in ocean energy, including: increasing oil prices, climate change awareness, energy security issues, and new pressure to implement renewable energy generation pursuant to corporate goals or government mandates. Recent research regarding wave generation and advances in the offshore industry have led to new designs for this technology, some of which have been tested at sea and have produced electricity.
Today, several small companies – backed by government organizations, private industry, utilities, and venture capital firms – are leading the commercialization of technologies to generate electricity from ocean energy resources.
Waves are generated by the influence of the wind on the ocean surface, which causes ripples, then chop, then fully developed seas, and finally swells. In deep water, the energy in waves can travel for thousands of miles before being dissipated on land.
The PowerBuoy, from Ocean Power Technology, is an example of a point absorber. These devices absorb wave energy from all directions.
Wave energy extraction is complex, and many device designs have been proposed. Energy conversion technologies can be classified according to the type of displacement and the reaction system employed. Various hydraulic or pneumatic power take-off systems are used. For many wave energy conversion devices, the mechanical motion of the displacer is converted directly to electrical power (direct-drive). These devices can be bottom-mounted or floating and vary in size, orientation, and distance from shore. Below are four of the best-known offshore technology concepts for converting wave energy into electricity.
Devices that are small compared to a typical wavelength and that can absorb energy in all directions are called “point absorbers.” Waves that most typically are used for energy production using these devices vary from 40 to more than 300 meters in length. Point absorbers are bottom-mounted or floating structures that absorb energy from all directions, such as a floating buoy. The power take-off system may take a number of forms – such as a closed hydraulic system or a linear inductor – depending on the configuration of displacers or reactors. About 66 percent of the wave energy devices currently being developed are point absorbers.
A terminator is a device that reflects or absorbs all the energy in a wave. One type of terminator is an overtopping device that uses a floating reservoir structure, typically with reflecting arms to focus the wave energy. As waves arrive, they overtop the device and are restrained in the reservoir. The head of the collected water as it flows back out to sea turns conventional low-head hydro turbines, and the turbines are coupled to generators to produce electricity. Only about 7 percent of the wave energy devices currently being developed are overtopping terminators.
Oscillating water column (OWC)
An oscillating water column device harnesses the motion of ocean waves as they pressurize a pocket of trapped air. This device is a partially submerged chamber with air trapped above the water surface. As waves enter and exit the chamber, the water surface moves up and down and acts like a piston. The air trapped above the water level is compressed and decompressed by this movement to generate an alternating stream of high-velocity air through an exit blow hole. This air is channeled through a turbine-generator to produce electricity. About 20 percent of the wave energy devices currently being developed are oscillating water column devices.
Linear absorber or attenuator
A linear absorber, also called an attenuator, uses less than the full amount of energy in a wave. The linear absorber structure is oriented roughly parallel to the direction of wave propagation and is composed of multiple sections that rotate in pitch and yaw relative to each other. That motion is used to pressurize an internal hydraulic fluid. This pressurized fluid turns a turbine that is coupled to a generator to produce electricity. Only about 7 percent of the wave energy devices currently being developed are linear absorbers or attenuators.
Status of the technologies
Today’s wave energy conversion technologies are the result of many years of testing, modeling, and development. Installed capacity is about 4 MW worldwide in Portugal (2 MW), Ireland (1 MW), and the United Kingdom (1 MW). Most of the devices are engineering prototypes.
The first shore-based, grid-connected wave power unit to be installed was an oscillating water column system built into the coastline of the Island of Islay in Scotland in 2000. The 500-kW device was developed by Wavegen in cooperation with Queen’s University Belfast.
Then, in 2003, Wave Dragon installed the first subscale grid-connected wave power unit. The 57-meter-wide prototype was an exact replica (at a scale of 1:4.5) of a 260-meter-wide 4-MW machine with a capacity of 20 kW. Because of its subscale nature, this unit was deployed in a protected bay in Nissum Bredning in Denmark. Experimental data was collected from this unit before a load cell in one of the key mooring lines failed during a storm and the unit was beached.
In July 2004, Pelamis was the first company to deploy a full-scale grid-connected wave power unit, in open seas at the European Marine Energy Center (EMEC) in Orkney, Scotland. Based on successful testing of this 750-kW unit at EMEC, Pelamis announced the first commercial sale of an offshore wave power plant in May 2005 to Enersis. After testing of a 2.25-MW pilot plant, the 30-MW plant is being deployed off the coast of Portugal and will begin operating in the summer of 2008.
Horizontal-axis tidal turbines, like this unit from Verdant Power, are oriented so the axis of rotation is roughly parallel to the water stream.
In addition, a number of demonstration projects are ongoing and planned in Australia, Canada, China, Ireland, Japan, Portugal, Spain, the United Kingdom, and the U.S. If these early demonstration schemes prove successful, wave farms 50 to 100 MW in capacity could be deployed within the next five to eight years.
Tidal energy occurs as a result of a mass of water moving with speed and direction, caused by the gravitational forces of the sun and moon and centrifugal and inertial forces on the earth. Because of its proximity to the earth, the moon exerts roughly twice the tide-raising force of the sun. The gravitational forces of the sun and moon and the centrifugal/ inertial forces caused by the rotation of the earth around the center of mass of the earth-moon system create two “bulges” in the earth’s oceans: one closest to the moon and the other on the opposite side of the globe. These bulges result in two tides each day, which is the dominant tidal pattern in most of the world’s oceans.
Lucid Energy’s vertical-axis tidal turbine is oriented so the axis of rotation is roughly perpendicular to the water stream.
Energy conversion devices are placed in the flowing tidal stream to harness the kinetic power of the water and convert it to electricity. Unlike traditional hydroelectric generation, these units do not require a dam or impoundment. Tidal energy extraction is complex, and many device designs have been proposed. Tidal turbines are grouped into two types:
– Horizontal axis, in which the axis of rotation is horizontal with respect to the ground and roughly parallel to the water stream; and
– Vertical axis, in which the axis of rotation is vertical with respect to the ground and roughly perpendicular to the water stream.
About 57 percent of the tidal devices being developed are horizontal axis; 43 percent are vertical axis.
The subsystems of these type of converters include: a blade or rotor, which converts energy in the water to rotational shaft energy; a drive train, which usually consists of a gearbox and generator; a tower that supports the rotor and drive train; and other equipment, including controls, electrical cables, and interconnection equipment.
Status of the technologies
In May 2003, a 300-kW experimental SeaFlow unit supplied by Marine Current Turbines was installed 1 kilometer off the coast of North Devon in the United Kingdom. This is the world’s first marine renewable energy system to be installed in the seabed. Based on the experimental success of the SeaFlow machine, which is still used for experimental purposes, MCT has designed, built, and recently installed a 500-kW machine called SeaGen in the Strangford Lough in Northern Ireland.
In the U.S., Verdant Power installed an array of six 35-kW grid-connected turbines in the East River (a tidal estuary) in New York in early 2007.
In addition, a number of tidal demonstration projects are ongoing and planned in Canada, Italy, Korea, the United Kingdom, and the U.S. If these early demonstration schemes prove successful, arrays 1 to 10 MW in capacity could be deployed within the next five to eight years.
There are about 60 technology developers with ocean energy conversion devices that have been tested at the prototype stage, and those devices are at various stages of development. The time period for ocean wave and tidal technologies to go from conceptual level to deployment of a long-term full-scale prototype tested in natural waters is five to ten years. The technology is still emerging and is where wind technology was 15 to 20 years ago. It is too early to know which technology will turn out to be the most cost-effective, reliable, and environmentally sound.
Of the current technologies, only a few dozen have progressed to rigorous sub-scale laboratory tow or wave-tank model testing. Only two dozen have advanced to short-term (days to months) tests in natural waters. Even fewer (about half a dozen) have progressed to long-term (more than one year) testing of a full-scale prototype in natural waters. The first commercial wave energy plant is being deployed in Portugal, but the first commercial tidal plant using the type of technology discussed in this article has not yet been realized.
Ms. Houle and Mr. Bedard may be reached at Electric Power Research Institute, 3420 Hillview Avenue, Palo Alto, CA 94304; (1) 650-855-2059 (Houle) or (1) 650-855-2131 (Bedard); E-mail: firstname.lastname@example.org or rbedard@ epri.com.
Andree Houle is project engineer and Roger Bedard is ocean energy leader with the Electric Power Research Institute (EPRI).