Making waves

Dresser-Rand’s HydroAir turbine design that can be incorporated into an OWC structure.

With the sustained global push for cleaner electricity production, wave energy offers significant potential for the global energy mix. It is an abundant resource that can be captured through a variety of devices, including the oscillating water column. Katie Jernigan delves into wave energy’s global potential and how the oscillating water column assists in production.

A vast renewable, almost untapped energy resource can be found in the world’s oceans. According to the Lloyd’s Register Global Marine Trends 2030 report, if just 0.1% of the renewable energy available in the ocean were converted into electricity, it would supply more than five times the current global demand.

The primary types of ocean energy include wind, wave, tidal and ocean current. Wave appears to be the most promising as a recoverable resource because it contains higher energy potential than other ocean energies. K. Gunn and C. Stock Williams, in “Quantifying the Potential Global Market for Wave Power,” estimate the world’s theoretical wave power resource to be 2.11TW (or 2.1 million MW). This is almost 10 times the global installed capacity of wind, which was about 282,587 MW in 2012, according to the Global Wind Energy Council (GWEC).

Wave energy varies in different parts of the world, but is particularly abundant off of the European Atlantic coast (especially off Scottland), northern Canada, southern Africa, Australia, and the northeastern and southeastern coasts of the US. According to the Electric Power Research Institute (EPRI), the total wave energy resource along the US outer continental shelf is 2,640 TWh/yr. Of that figure, 1,170 TWh/yr is recoverable, which represents almost 1/3 of the 4,000 TWh/yr of electricity used in the US each year. By comparison, there was approximately 446 TWh of electric power produced from wind worldwide in 2011, according to the GWEC.

Dr. Sean Barrett, Technical, Wave Resource and Project Analyst of Oceanlinx Ltd., considers wave energy to be a more reliable renewable resource because waves can be predicted several days in advance, allowing for higher energy production without complete dependence of wind.

“Even when the wind is not blowing, there are still waves rolling into the beach,” said Barrett. Converting this massive potential resource into actual electrons will poses a daunting task, however. Gunn estimates that wave energy converters (WECs), a group of diverse devices that can capture energy from waves, can convert approximately 4.6% of potential energy into electricity.

WECs devices include point absorbers, attenuators and oscillating water columns (OWC), each of which is equipped with a power take-off (PTO) unit that consists of a turbine and generator. Along with the PTO unit, many devices have additional moving parts. Since the OWC does not have any other moving parts underwater, it has a competitive advantage when compared to other WECs. “The additional moving parts will reduce the efficiency of the devices, and also significantly increase the operation and management costs,” said George Laird, business development manager for Dresser-Rand. “In addition, it might affect their reliability.” An OWC is a partially submerged WEC that can be installed in various water depths: on the shoreline, near shore or offshore. Using subsea cables, energy is transferred from the OWC to the power grid.

Traditional turbines have been powered by gas or steam that flows in only one direction, whereas the air flow produced by an OWC is bidirectional (the rising and falling of the incident wave). This requires a specially designed turbine. The turbine on an OWC is above the surface line, making it easier to maintain.

“The HydroAir product makes the most of the impulse turbine design, which has a broad operating range,” Laird says. “It also uses the variable radius turbine (VRT) principle to increase efficiency and has minimal moving parts to ensure high reliability and reduced maintenance.”

Dresser-Rand’s Hydroair turbine integrated with an Oceanlinx unit. Image courtesy of Dresser-Rand.

HydroAir is Dresser-Rand’s patented turbine design that can be incorporated into the OWC structure. It contains two sets of static guide vanes that are on either side of the rotor, at a larger diameter than of the rotor. These vanes are connected by a shaped duct, used to direct the airflow. Entering the duct at a slow velocity, the air moves in a swirl as it passes through the inlet vanes. As the air passes through the narrowing duct, it accelerates and turns the rotor. The air then decelerates as it travels back through the expanding duct before passing over the outlet guide vanes. The OWC will then repeat the process during the next wave cycle.

Dresser-Rand says the goal is to “increase OWC’s power capture.” The company works with OWC developers to match the damping ratio (pressure to flow ratio) that is required to optimize the power capture for the specific device.

Australia-based Oceanlinx is one of the companies developing OWCs for wave energy. For the past 16 years, the company has developed three types of OWCs: the greenWAVE, blueWAVE and ogWAVE.

• The greenWAVE is a 1MW, a 3000 tonne, bottom- sitting device that is installed at a water depth of 10-15m. The device is able to sit under its own weight on the seabed, enabling it to be installed without anchors or mooring. Oceanlinx says this model can be incorporated into a breakwater system or seawall in the near-shore.
• The blueWAVE is a floating unit, designed to be installed and anchored at water depth of 40-70m offshore. In March 2010, Oceanlinx launched the Mk3PC project off Port Kembla, Australia, incorporating the blueWAVE device. The project was completed three months later.
• The ogWAVE is also a floating offshore unit that can also connect to offshore oil and gas platforms. Oceanlinx says it will be used to supply small island communities who rely on these platforms and diesel fuel for energy needs.

Each of Oceanlinx’s devices is equipped with airWAVE turbines. Barrett says the design allows the turbine to spin at a high efficiency in one direction, even though airflow oscillates in two directions.

Overall OWC’s power capability depends on the width of the device, suggesting that wider devices can potentially generate more power. Currently, individual OWC units are capable of producing several megawatts.

Oceanlinx’s blueWAVE comprises six OWCs designed for deepwater applications.

In October 2013, Oceanlinx announced that a 1MW greenWAVE device, located off southern Australia in Port MacDonnell, will be connected to the electrical grid in February.

The device was part of a two-year test project, federally funded through the Emerging Renewables Program (ERP) for approximately AU$4 million. It is projected to supply electricity to 1000 homes in the Port MacDonnell area. Oceanlinx says that once the greenWAVE proves to be a viable, cost-effective source of renewable energy, the company will begin installing arrays of devices around the world in 2015.

As the world looks to utilize more environmentally friendly methods of producing energy, wave energy is a sound option with its high production capability and minimal impact to the environment.

“We believe there will be an increase of wave energy devices in the future, although it is difficult to quantify the magnitude of the increase,” Laird says. “Next to wind energy, marine energy is believed to have the largest market potential in the renewable energy industry.

As wave energy converters, like the Port MacDonnell green- WAVE, are being commissioned, the enormous potential of wave energy likely means that surfers won’t be the only ones looking to "catch a wave." OE