Innovative offshore hydraulics improves the efficiency and economics of wave-energy converters.
As the world slowly embraces renewable energy, most efforts today focus on solar and wind. But tapping the power of the oceans could be the next forefront of “green” energy.
For one reason, the payoff could be huge. According to the Paris-based International Energy Agency, harnessing ocean waves, tides, currents and temperature gradients to generate electric power could someday exceed the world’s current annual electricity demand of about 20,000 TWh. More realistically, IEA predicts globally installed wave and tidal power arrays could have a capacity of more than 330 GW by 2050. That’s on par with today’s wind energy output, which supplies about 4% of current global electricity demand.
A recent report by the Electric Power Research Institute (EPRI) says usable wave energy along the U.S. coastline is over 1,000 TWh/year, and predicts that wave power could eventually provide 10% of total U.S. electricity demand.
And in Europe, a number of wave-energy converters have already been deployed. Larger-scale projects of up to 40 MW are expected by 2020 with wholesale market roll-out in 2035, according to the European Ocean Energy Association. Projections are to install up to 100 GW of wave and tidal power capacity in the next three decades.
Weighing the prospects
While such predictions may be optimistic, ocean energy nonetheless offers a number of advantages over other renewables. For instance, waves pack a lot of power, as anyone who has been to the beach can attest. Sea water is about 850x heavier than air, so waves have a much greater energy potential compared to wind or solar.
Along the European Atlantic coast the energy density in waves is around 2 to 3 kW/m², around 5x that of wind and more than 10x that of solar, according to researchers at Marine Power Systems, a wave-energy converter (WEC) developer in Swansea, U.K. This means energy can be harvested from rather compact devices, and sizeable amounts of power can be supplied from wave farms that occupy relatively little space on the ocean floor. Furthermore, ocean-energy devices can be located near coastal population centers, which would eliminate the need for lengthy power transmission lines.
Granted, the size and frequency of waves varies, but their energy potential is more consistent and predictable compared to that of wind and solar. Wave motion can be forecast days ahead, making it a reasonably reliable power source that’s better suited for grid balancing.
On the other hand, successful wave-energy generation faces significant obstacles. As an emerging technology, WECs and the power they generate are currently not cost competitive with even other renewables and, thus, they rely heavily on subsidies for R&D.
There are environmental concerns that the devices themselves, as well as underwater anchor cables and transmission lines, could harm sea life and delicate marine ecosystems. And more research is needed to better predict the reliability of components submerged in corrosive and turbulent seawater.
On the technology side, numerous wave-harvesting devices have evolved—with varying levels of success—but an optimal design has not emerged. Wave-energy converters harness the energy of surface waves through a number of different mechanisms. For example, powered buoys, or point absorbers, allow relative, up-and-down movement between the buoy and a stationary, submerged base. Oscillating converters are near-shore devices that capture energy from wave surges. These are bottom-hinged devices where a large arm moves back-and-forth in response to wave movement.
Such designs convert wave action into powerful reciprocating action. Many WECs use that to generate high-pressure hydraulic flow—often with hydraulic cylinders—and capture that energy in a power take-off (PTO) device to generate electricity.
Currently, the go-it-alone approach to R&D for offshore hydraulics by individual manufacturers has hindered the success of WECs. Now, however, two projects, in Europe and Australia, aim to develop a standard “plug-in” PTO that is commercially available to any WEC manufacturer. The goal is to develop an efficient and reliable system using proven components that is both economical and easy to maintain.
Ocean energy company BioPower Systems, Sydney, Australia, has developed an oscillating converter called the bioWAVE, whose design is said to be based on the swaying motion of sea plants caused by ocean waves. The 26-m-high structure mounts on the sea floor, and three buoyant arms hinged to the base act as inverted pendulums. Two actions, the rising and falling sea surface and back-and-forth wave movements, cause the arms to slowly oscillate. A pivot near the bottom lets the structure continuously self-orient with the wave direction, ensuring that the WEC efficiently captures energy from a wide range of incoming waves. And to protect the bioWAVE in extreme conditions, say hurricanes, a ballast system fills the arms with water, which drop and lie flat against the seabed. Afterward, driving out the water refloats the arms.
WECs face the difficult prospect of converting the large forces and slow motions inherent in ocean waves into a steady flow of electricity. BioPower Systems has reportedly solved this problem by developing an onboard PTO—called the O-Drive—that converts WEC motion into hydraulic power, which in turn is used to run an electric generator.
The back-and-forth movement of the arms produces reciprocating motion in two large hydraulic cylinders mounted on the structure. These cylinders pump high-pressure, variable-flow fluid to the O-Drive. Fluid routes through a manifold and is stored in a bank of accumulators. From there, the accumulators supply a steady flow to a hydraulic motor that, in turn, directly couples to a standard electric generator. The generator rectifies and smooths the output to produce stable, utility-grade electricity that is transmitted through a subsea cable to the on-shore grid. Low-pressure hydraulic fluid exits the motor and flows to a reservoir and, eventually, returns to the cylinders to complete the hydraulic circuit.
The sealed O-Drive houses the hydraulic circuit, electric generator and sophisticated processors that autonomously control the equipment, monitor systems and provide real-time feedback to shore-based operators.
The O-Drive can be detached from a moored WEC, which simplifies and cuts the cost of periodic maintenance. A retrieval rig connects to and raises the O-Drive using ballast compartments and air-powered winches. Once at the surface, the rig and O-Drive are towed to shore.
A pilot demonstration of the bioWAVE began last December off the southern coast of Australia. It uses a single 250-kW O-Drive. The plan is to operate and evaluate the system for at least one year. If successful, the company intends to produce a larger, 1-MW commercial version of bioWAVE that would use four 250-kW O-Drive modules. Ultimately, multi-unit wave-energy farms will deliver utility-scale renewable power to onshore distribution grids.
The O-Drive power conversion module is designed as a “plug-in” PTO for ocean-energy systems. BioPower Systems plans to commercialize the O-Drive and offer it to other project developers. According to company officials, it could provide a substantial boost to the global wave-energy sector by offering a standard PTO suitable for any WEC. The O-Drive can also connect to rotary hydraulic pumps, rather than cylinders, for use on tidal-energy converters.
Wave-energy firms Albatern, Roslin, Scotland and Carnegie Wave Energy UK, Redruth, England along with drive and control manufacturer Bosch Rexroth, Lohr am Main, Germany are collaborating on a project called WavePOD (Wave Power Offtake Device) with the aim of designing a standard, self-contained offshore electricity generator. The consortium also includes Irish utility ESB, which is developing the Westwave wave farm off the west coast of Ireland.
According to the companies, the project focuses on one of the biggest challenges in wave energy—how to generate electricity reliably and cost effectively. The WavePOD PTO is a hydraulic-electric generator housed in a sealed nacelle that transforms the reciprocating motion of most WECs into electrical power offshore, which can then be transmitted through cables to shore.
In a news release on the program, Tim Sawyer, project development officer at Carnegie Wave Energy said, “Working with Bosch Rexroth we intend to create an industry-enabling technology which will be available as a commercial product for a range of different ocean energy technologies. This makes a huge amount of sense for the industry—rather than every company developing its own power offtake technology, WavePOD will be a standard product that frees companies to pursue the development of their own unique machines, without having to worry about converting their technology’s motion into electricity.”
A tenth-scale, 80-kW prototype is currently on test at the Institute for Fluid Power Drives and Controls (IFAS) at RTWH Aachen University, Germany. The test rig simulates WEC motion to generate hydraulic flow to the WavePOD. According to a paper published in the International Journal of Fluid Power, a basic system has two hydraulic cylinders connected to high and low-pressure lines, and check valves control flow from the cylinders to the WavePOD. An accumulator smooths flow of high-pressure fluid to a variable-displacement hydraulic motor, which then turns a generator. Other hydraulic components include a reservoir, feed pumps, pressure relief valves and filters. Mineral oil serves as the hydraulic fluid.
But various WECs interact with waves differently, so a standard PTO must be designed to operate efficiently over a range of inputs. While the basic concept uses a single accumulator in the high-pressure line, the actual set-up uses two sets of accumulators with different pre-charge pressures to better reduce pressure fluctuations. Additional accumulators pre-charge the low-pressure lines to the cylinders to ensure adequate flow and avoid cavitation during fast movements.
Also, instead of a single hydraulic motor and electrical generator, the prototype has three same-size motors connected to generators. This set-up increases performance at low-power wave conditions by only running a single motor at higher efficiency, rather than two or all three at low efficiency. Fixed-speed, asynchronous generators simplify grid connections.
The goal of testing is to gage the performance and reliability of individual components and the complete offshore hydraulics system. Start-up was in November 2014, and results are expected to be published in the near future.
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