A synthetic “seabed carpet” that mimics the wave-damping effect of a muddy seafloor could be used to extract energy from waves passing over it, according to an engineer in the US. As well as offering a new way to produce clean and cheap electricity, the carpet – which has not yet been built – could be used to protect coastal areas against strong waves and provide areas of safe haven for boats in stormy seas.

The ability of muddy seafloors to dampen ocean waves is well documented at various locations around the world. In the Gulf of Mexico, fishermen have learned to steer their boats into a local muddy spot known as the “mud hole” when a storm is brewing. Here, the wave–mud interaction is so strong that the storm waves are damped within a distance of a couple of wavelengths (100–200 m) and the boats there are completely safe.

Springs and generators

“If mud can seriously take so much energy out of ocean waves, then why don’t we use this idea to design a wave-energy convertor that’s very efficient?” asks the carpet’s inventor, Mohammad-Reza Alam of the University of California, Berkeley. He came up with the idea using a viscoelastic “carpet of wave-energy conversion” (CWEC) placed over a network of vertically oriented springs and generators on the coastal seafloor. The flexible carpet responds just like mud: as waves pass overhead, they induce dynamic ripples and undulations in its sprung surface, and these perturbations can be used to generate electricity.

Modelling the interaction of ocean waves with the proposed carpet, Alam was able to show that the system can easily absorb 50% of incident wave energy over short distances of about 10 m. For typical North Sea waves, the simulation suggested energy absorption rates of up to 6.5 kW m^–2, which is more than double the maximum possible with wind turbines and at least 20 times greater than currently achieved by solar-power convertors.

Short waves are better

All oceanic motion is a combination of long and short waves. It has long been known that the short waves associated with choppy seas are dampened faster than far-reaching long waves (swells) for various reasons, but it was only recently that surprising field observations revealed that short waves are actually much better than long waves at imparting their energy to a muddy seafloor in shallow waters.

Alam developed a computer simulation that took into account hundreds of different waves 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

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 20 m.

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 harbours 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 UK, 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 neighbouring areas.”

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

The work is published in the Proceedings of the Royal Society A.