Seabed carpet could harness wave energy

and wave interactions. He found that when wave-damping conditions are strong — as is the case with a muddy seabed — a considerable amount of energy is converted from “surface mode” waves (where long waves dampen faster) to “bottom mode” waves (where short waves dampen faster and impart more energy to the seabed). “If damping is strong, the overall energy absorption from the ocean is even stronger,” he explains.

Bring on the storms
The release notes that Alam believes that the CWEC presents a number of distinct advantages over existing wave-harvesting techniques. Chief among them is the fact that not only is the device resistant to storms, it actually performs better in them. Current approaches use moored floating devices or seabed-secured clam-like structures with vulnerable hinges. When waves become too energetic, these devices are designed to retreat into a protective idle mode, often by being pulled beneath the surface of the water.

The CWEC’s flat and fixed nature means that these issues are neatly sidestepped and it can continue harnessing power as the storm rolls by. Not only that, when non-linear elements of wave interaction — which increase during stormy seas — were introduced to the simulation, the efficiency of the device improved. The device also has a much broader bandwidth than most other wave-energy extractors, and can make use of any type of wave approaching from any direction.

The disadvantage of the CWEC is that its efficiency decreases with water depth, meaning that it is only suitable for use between the surf zone and depths of about twenty meters.

Popular with mariners
A completely submerged carpet structure would likely be more popular with mariners and environmentalists than traditional devices, which can pose the threat of collision for ships and entanglement for marine mammals. The energy-draining effect could also be put to good use protecting vulnerable shorelines, sheltering harbors or protecting near-offshore platforms.

It is an interesting idea but there are practical issues such as the cost of installation and maintenance, impact on [bottom-dwelling] marine life, and the impact of tides on the performance,” comments Dominic Reeve of Swansea University in the United Kingdom, who is part of the Wave Hub project, a large-scale testing facility for new wave-harvesting technologies that is based off the south-west coast of England. “If there is mobile sediment around, this carpet could well affect sediment transport — either to the detriment or advantage of itself or neighboring areas.”

Alam agrees that sedimentation is certainly one of the concerns that would face engineers if the CWEC were to be built and trialed in the field. He suggests that perhaps the device would be best deployed along rockier coastlines.

— Read more in Mohammad-Reza Alam, “Nonlinear analysis of an actuated seafloor-mounted carpet for a high-performance wave energy extraction,” Proceedings of the Royal Society (13 June 2012) (doi: 10.1098/rspa.2012.0193)