Energy, all at sea

Floating wind turbines could capture the energy of higher wind speeds further out to sea and address some of the noise and unsightliness complained about by those with turbines closer to home.

Wind turbines represent one of the most reliable renewable energy solutions, along with solar power and tidal and hydroelectric power. As wind turbine designs increase their size they also get noisier and become more of an eyesore. The solution is either to site them remotely on dry land or to build them at sea with the tower embedded in the seabed of shallow waters, but this restricts them to near-shore waters with depths no greater than 50 metres, which means they cannot utilise the strong winds further out to sea.

Now, naval architect Dominique Roddier of Berkeley, California-based Marine Innovation & Technology has, together with his team, published a feasibility study of a novel platform design – WindFloat – that, as the name suggests uses floating wind turbines. The study is published in the Journal of Renewable and Sustainable Energy this month.

Floating wind turbines could use stronger offshore winds
Floating wind turbines could use stronger offshore winds (Credit: Roddier et al/JRSE/American Institute of Physics)

Roddier and colleagues, Christian Cermelli, Alexia Aubault, and Alla Weinstein, have tested a 1:65 scale model in a wave tank, which shows that a three-legged floating platform, based on existing gas and oil offshore platform designs. The team explains the main issue: “A floater supporting a large payload (wind turbine and nacelle) with large aerodynamic loads high above the water surface challenges basic naval architecture principles due to the raised centre of gravity and large overturning moment,” they say. In other words at first glance such a rig would capsize very easily. However, after several years work, their results show that the current design is stable enough to support a 5-megawatt wind turbine, the largest turbine that currently exists. These mammoth turbines are 70 metres tall and have rotors the size of a football field. Just one, Roddier says, produces enough energy “to support a small town.”

The next step is to continue construction of a prototype with electricity operator Energias de Portugal that will help the developers understand the life-cycle cost of such projects and to refine the economic model. The prototype will be tested in open water by the end of summer 2012, Roddier says. “The WindFloat [design] is envisioned to be located 15-20 km offshore so as to minimize risks/nuisance to the general public, and to mitigate the view impact from the coastline,” the team adds.

Links

J Renewable Sustainable Energy, 2010, 2, 3, 033104
Marine Innovation & Technology

Flat-packed particles

Graphene, a Manchester University discovery, is a material comprising sheets of carbon just one atom thick; graphene is like a single layer of graphite. However, it was the discovery that it has some peculiar electronic properties because of the existence of massless quasiparticles that has led to an explosion of interest in this material. Some researchers suggest that ultimately it will become the material that gives us a post-silicon world in computing.

Now, US scientists have made the first observation of the energy bands of complex particles within graphene known as plasmarons. This small step is an important one in understanding graphene and using it to develop devices for that future of ultrafast chemical computers.

At Berkeley Lab’s Advanced Light Source, an international team led by Aaron Bostwick and Eli Rotenberg have shown that these composite plasmaron particles are vital in generating graphene’s unique properties. “Graphene’s true electronic structure can’t be understood without understanding the many complex interactions of electrons with other particles.”

The electric charge carriers in graphene are negative electrons and positive holes, which in turn are affected by plasmons, oscillations in the density of the material that travel like sound waves through a sea of electrons. A plasmaron is “simply” a charge carrier coupled to a plasmon. “Although plasmarons were proposed theoretically in the late 1960s, and indirect evidence for them has been found, our work is the first observation of their distinct energy bands in graphene, or indeed in any material,” Rotenberg says. The team reported details of their findings in the journal Science in May.

Top: graphene structure. Bottom: a theoretical model of plasmaron interactions in graphene, sheets of carbon one atom thick.

The relationships between charge carriers, plasmons, and plasmarons will be important in the development of plasmonics, the architecture analogous to electronics in conventional silicon semiconductor circuitry. An important aspect of studying these relationships is to produce flat graphene sheets; graphene is usually rumpled like unmade bed linen. “One of the best ways to grow a flat sheet of graphene is by heating a crystal of silicon carbide,” Rotenberg explains, “and it happens that our German colleagues Thomas Seyller from the University of Erlangen and Karsten Horn from the Fritz Haber Institute in Berlin are experts at working with silicon carbide. As the silicon recedes from the surface it leaves a single carbon layer.”

With flat graphene sheets in hand, the team used a beam of low-energy, or soft, X-rays to analyse the materials. The resulting data provided them with an image of the electronic bands created by the electrons themselves. Even from the initial experiments, the team suspected graphene’s behaviour was more complicated than simple theory would suggest and seemed to hint at the existence of bare electrons. Since bare electrons cannot exist, the researchers postulated the fuzziness in their image was due to charge carriers emitting plasmons. Additional experiments with graphene sheets isolated from their support material revealed that electrons detached by the X-rays can leave behind either an ordinary hole or a hole bound to a plasmon – a plasmaron, explains Rotenberg.

“By their nature, plasmons couple strongly to photons, which promises new ways for manipulating light in nanostructures, giving rise to the field of plasmonics,” Rotenberg says. “Now we know that plasmons couple strongly to the charge carriers in graphene, which suggests that graphene may have an important role to play in the merging fields of electronics, photonics, and plasmonics on the nanoscale.”

Links

Science, 2010, 328, 999-1002
Eli Rotenberg homepage

Scrubbing up knowledge of submarine volcanoes

A study of the shape of pumice from three adjacent submarine lava dome volcanoes in the western Pacific reveal that explosive volatility driven by the movement of molten magma is lower in deeper water. The shape of pumice stones, which are formed by expansion of magmatic volatiles as the magma rises to the sea surface, is different depending on the water depth and so can be a useful indicator of the evolution and eruption of underwater volcanoes.

Sharon Allen of the ARC Centre of Excellence in Ore Deposits and the School of Earth Sciences, at the University of Tasmania, Hobart, Australia and colleagues Richard Fiske of the Smithsonian Institution, Washington, DC, and Yoshihiko Tamura of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in Yokosuka Japan, used sampling and observations collected by a remotely operated vehicle of the three adjacent submarine lava dome volcanoes of the Sumisu, Izu-Bonin arc in the Western Pacific.

Domes of the volcanic complex have summits at ocean depths of 1100, 600, 245, and 95 metres and are mantled with pumice that is chemically identical but size, distribution, and surface texture varies enormously across the volcanic range.

Sharon Allen
Sharon Allen

According to a report in the May issue of the journal Geology, pumice generated from lava domes at water depths of more than 500 metres formed as a thick carapace on dense rock whereas at water depths less than 500 m pumice is blasted out. At shallower than 500 metre depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts (rock fragments) enclosed by quenched margins and pockmarked by coarse [centimetre-sized] vesicles, a rock fragment within which is trapped a bubble of gas, the team explains.

The study shows that an increase in hydrostatic pressures over a range of 12 megapascals [120 times atmospheric pressure] reduces volatile-driven explosivity of the dome-forming eruptions the team says, it does not affect the formation of rocky “bubbles, the vesicles. “We conclude that metre-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 metres, and its morphology can be used to distinguish between explosive and effusive origins,” they conclude.

Links

Geology, 2010, 38(5), 391-394.
Sharon Allen homepage