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In this fifth article in my continuing series on renewable energy technology, I’d like to talk about ocean power. The term covers virtually any technology that extracts power from the sea, ranging from fairly well-known wave energy and tidal energy conversion systems to more experimental ideas like ocean thermal energy conversion (OTEC) and salinity gradient devices. In their own way, OTEC and salinity gradient devices are as different from wave or tidal energy converters as PV arrays are from wind turbines. Since the technologies in this group are so different, I’ll cover them in separate articles.
Today’s topic will be wave energy. Countless new technologies have emerged in the wave energy arena since Stephen Salter first threw his hat into the ring over three decades ago with the Salter Duck. They come in a bewildering array of shapes and sizes. How do these wave energy converters work?
Wave energy systems extract useful work from the most visible embodiment of the power in the ocean: waves. Waves are generated by the winds, which induce oscillations at the ocean’s surface. The stronger the wind and the longer it blows along the surface of the water, the bigger the waves. Recall from my first article on hydropower that generators work on the principle of magnetic induction: they generate electricity by spinning a loop of wire in the presence of a magnetic field. Wave energy devices work by transforming the elliptical or circular oscillation of sea water into the rotation of a generator. There’s a staggering variety in the mechanisms used to achieve this transformation. Wave energy systems are usually categorized by three traits: the way they capture wave energy, the way they convert that energy to power, and their location. Individual designs can look radically different from each other, since nearly any combination of the above traits is possible.
The options for wave energy capture are so diverse that opinions differ on how to categorize them. Point absorbers are floating devices that move up and down with the waves relative to a mooring or inertial reference. The motion can be used to pump water through a turbine, to spin a generator directly with a mechanical linkage, or to drive a linear electric generator. Point absorbers have been used successfully for years in unmanned navigation buoys. Companies like Ocean Power Technologies, Finavera Renewables and CETO Technology are developing products for utility scale power generation using arrays of point absorbers.
Attenuators, or surface following devices, are long devices with multiple segments, like floating train cars facing into the oncoming waves. As the segments bend relative to each other from variation in wave height along the length of the train, this motion can be used to turn a generator, through a mechanical linkage or by pumping a fluid through turbine. Pelamis is developing one of the best known examples of an attenuator device, and installed just over 2 MW of capacity in the Agucadoura wave farm in 2008.
Terminators are wave energy devices that extend across the wave front, perpendicular to the direction of wave travel (like the direction a surfer travels in a pipeline). Some terminators, like OceanLinx device, funnel the incoming wave energy into an oscillating water column which pushes air through a special turbine that spins in the same direction regardless of the direction of air flow. Other terminator devices use a different strategy call overtopping, where the waves slosh water up ramps into a reservoir. The sea water drains out of the reservoir through a turbine which drives a generator. Wavedragon deployed a prototype of such an overtopping device in 2003 off the coast of Denmark and is currently preparing to deploy a full scale prototype with a 7 MW capacity.
There are other types of wave energy converters a well, unique designs that don’t fit in any of the above categories. Like oscillating wave surge convertors, devices with paddle-like arms that wave back and forth around a pivoting joint as the waves pass over them. Aquamarine Power’s Oyster and BioPower Systems’ BioWave are examples of this energy capture method, though they differ in their approach to power take-off.
As if there weren’t enough variety in energy capture method, there are also several aproaches used to convert the captured wave energy into useful power. Some systems use hydraulic conversion methods, pumping sea water through a turbine using hydraulic rams or hose pumps. Hydraulic rams feature a piston in a cylinder with an inlet valve and an outlet valve. As the piston withdraws from the cylinder the inlet valve opens and low pressure sea water is drawn into the device. As the piston reverses direction and compresses, the inlet valve closes and the pressure in the cylinder increases. When the pressure exceeds a threshold the outlet valve is forced open and high pressure sea water is pumped out of the cylinder to a water turbine. Hose pumps deliver high pressure water by mimicking the human digestive tract. Rollers propagate a constriction along the length of a flexible hose, which create high pressure ahead of the pinch point and low pressure behind it. In both cases the high pressure sea water is pumped through a turbine to drive a generator.
If the turbine is located with the device, then electric power is transmitted to shore through sub-sea electric cables. Alternatively, the wave energy device can pump high pressure sea water to the shore instead, where it will drive an on-shore turbine and generator.
Some systems use air turbines to drive a generator instead of water turbines. Oscillating water column devices fall into this category.
Other systems rely on a direct mechanical coupling to a spin a generator or to drive a linear electric generator. These devices usually house the generator at sea with the wave energy device and transmit electric power to the shore via sub-sea electric cables.
To add to the spectrum of technical development in the wave energy conversion space, devices can be located in different zones as well. Shoreline systems are located at the beach. Some terminator type devices require solid mooring and are often located on the shoreline. There are near-shore devices, which are optimized for operation in shallow water. And some devices are classified as off-shore, designed for operation in deeper waters farther out to sea.The boundaries between near-shore and off-shore are a little hazy, but systems designed to work in 10-20 meters of water will not operate well in 50-70 meters of water, and vice versa.
Wave energy technology, though over thirty years old, is still not as mature as wind energy or solar PV. The biggest symptom of this is the diversity of technologies still under development. The industry hasn’t converged toward an optimal design solution or even on a handful of optimal design solutions. The main challenges that face the wave energy industry are survivability, efficiency of energy capture, and regulatory hurdles, both environmental and recreational. All of these challenges drive up the cost of deploying wave energy projects, and have (so far) prevented the field from approaching competitive footing in any but the most expensive niches in the electric utility market.
The big elephant in the room though, is survivability. The marine environment is a harsh one, and not just because of salt-water’s corrosive properties. The areas with some of the best wave resources also suffer from hurricanes and typhoons. Designing to withstand such severe storms leads to large and expensive structures like off-shore oil drilling platforms, but oil has a much higher energy density than ocean waves, and the economics just don’t support wave energy devices that expensive.
Some companies, like Sydney’s BioPower Systems, are trying to get clever about this problem with innovative design approaches. In BioPower’s case they are developing biomimetic designs which borrow from nature to improve survivability. But it’s still early days for all of the companies in this space.
Who will find the right combination of innovation and engineering to bring wave energy into the mainstream? Only time will tell. My next article will cover tidal and ocean current energy systems.
The text of this article was previously published on Associated Content.
