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M3 Wave

Waves and tides offer some of the most predictable, consistent, and just generally big energy resources available. Rollouts of actual wave and tidal power installations, however, have been slow and generally limited to pilot projects so far. Part of the reason for this—along with straightforward but difficult problems like transmission—is that there is no consensus at all on what represents the best device designs to actually harness waves and tides. A couple of interesting ideas—one wave, one tidal—were on display this week at the ARPA-E Innovation Summit in Washington, D.C., that offer some clear advantages over many of the other attempts at drawing energy from the oceans.

The wave power idea is closer than the tidal energy one to rollout, with a planned open-water test for this summer. M3 Wave dispenses with all the problems that come with buoys or other above-and-below-the-surface designs by mooring a simple device to the ocean floor. The device, pictured above, involves two air chambers: as a wave passes over the top of the first chamber, the pressure inside increases, forcing air through a passageway to the second chamber. Inside the passageway is a turbine, so the passing air is actually what generates the electricity. As the wave continues on, it raises the pressure inside the second chamber, pushing the air back through the turbine—importantly, it is a bidirectional turbine—and back into the first chamber. Another wave, another cycle. Repeat.

The primary selling point here is its simple and small footprint. There is no impact on ocean view, on shipping or fishing traffic, and rough seas above won't endanger the system in any way. M3 is selling it as "expeditionary" wave power, meaning it might be brought along on a ship and deployed for things like disaster relief; the company suggests such a deployment could produce 150 to 500 kilowatts. The system will undergo open-water testing at a U.S. National Guard facility, Camp Rilea in Oregon, in August.

On the other side of the country, a group at Brown University has developed what they call an oscillating hydrofoil, intended to minimize some of the impacts of tidal power devices and increase efficiency. The hydrofoil is mounted on to the sea floor—it resembles a car's spoiler attached to a pole, essentially. As the water flows past that spoiler it oscillates, generating electricity. It is designed so that the pole can actually fold down and out of the way if necessary, allowing for ships or even wildlife (detected with sensors on the device) to pass by without incident. The team received US $750 000 in funding from ARPA-E in 2012, and will soon move to a phase II involving a medium-scale, 10-kw prototype. They have calculated that the device can achieve much better energy conversion efficiencies in tides flowing very slowly than any of the devices that are on or close to market.

The National Renewable Energy Laboratory has estimated that in the United States alone there are wave power resources totaling 252 terawatt-hours/year, with tidal power adding another 17 Twh/year. Those are big numbers, and they come without the intermittency complaints that plague wind and solar power. Any new way to catch the ocean's energy is worth a look.

Image: Goddard Space Flight Center/NASA

This is how the Earth might appear if your eyes were sensitive to infrared light.

The Earth continuously emits 100 million gigawatts of infrared heat into outer space. That’s enough to power all of humanity many thousands of times over. Capturing even a fraction of that would mean an end to our energy woes. Harvard University researchers are now proposing a way to harvest this untapped source of renewable energy.

They have come up with two designs for a device they call an “emissive energy harvester” that would convert IR radiation into usable power. Today's technology isn't sufficient for an efficient, affordable harvester, the researchers say. But they've laid out a few different paths towards such devices in the journal  Proceedings of the National Academy of Sciences .

It seems counterintuitive, but the devices generate power by emitting infrared radiation. "The [device] is emitting much more radiation than it receives," says Steven Byrnes, a postdoctoral researcher working in applied physicist Frederico Capasso’s laboratory at Harvard. "This is the imbalance that we can take advantage of to create DC power."

The first design, which they admit is not the most promising, is a heat engine running between the Earth’s surface and a cold plate. The heat flowing from the ambient surface air to the cold plate, which radiates it out into the atmosphere, would be used to do mechanical work. The concept is simple, but cooling the plate efficiently to a low enough temperature is tough, Byrnes says.

As a case study the researchers looked at how much power such a device would generate in Lamont, Oklahoma, where a facility has been measuring IR radiation intensity. They found that they would get an average of 2.7 Watts from the IR radiation emitted by a square meter of Oklahoma over 24 hours, which is pretty low for large-scale power generation.

So the researchers turn to rectifying antennas, or rectennas, devices that absorb electromagnetic radiation and convert it into direct current electricity. A rectenna is an antenna coupled with a diode. Radiation induces an AC voltage across the antenna, which the diode rectifies to DC.

The researchers argue that rectennas can be run in reverse, generating DC power while emitting radiation, rather than absorbing it. In their design, a nanoscale antenna very efficiently emits Earth's infrared radiation into the sky, cooling the electrons only in that part of the circuit. Because the diode is at a higher temperature than the antenna, current only flows from the diode to the antenna. And because the antenna acts as a resistor, this results in a voltage.

Rectennas are traditionally used to generate power from microwaves, but can be used for higher frequency radiation, all the way up to visible light. Infrared frequency rectennas are a developing technology and the proof-of-principle devices demonstrated so far would generate very little power. But technological advances could improve their efficiency, Byrnes says.

Applying solar-cooking techniques such as reflectors to heat up the rectennas could also increase efficiency. For example, in the Lamont, Oklahoma case study, raising the temperature of a rectenna-based harvester from 20° C to 100° C using solar-cooking techniques would increase the power density of a rectenna from 1.2 W/m² to 20 W/m². “Solar panels for heating and cooking are already used in much of the world,” he says. “You could easily couple that to the (infrared) harvester.”

The researchers say that IR antennas should be easy to make on large areas at a reasonable cost. The critical challenge will be making diodes that would work well at the low voltages that would be expected in the harvester. The researchers suggest a few options to get around this problem. One is to use specially designed low-voltage diodes such as tunnel diodes and ballistic diodes.

Needless to say, this vision of IR energy harvesters for renewable power rests on engineers overcoming several technical challenges. But Byrnes says that this is a new energy frontier to tackle. He imagines one day a sheet printed with thousands of tiny infrared-harvesting rectennas that could be laminated on a solar panel or integrated into a solar water heater.

Illustration: Siyuan Dai

Researchers at the University of California San Diego (UCSD) have discovered that light can cause a ripple effect on the two-dimensional (2-D) material, hexagonal boron nitride, that can be maintained long enough for the waves to be usable for practical applications. This discovery could lead to the transmission of information in computer chips, better management of heat flow in nanoscale devices, or the creation higher resolution images than is possible with light, the researchers say.

The waves in this rippling effect that the researchers observed are called phonon polaritons. Polaritons are the quasiparticles produced when any type of photon strikes an object. Specifically, phonon polaritons occur when an infrared photon strikes a material.

The UCSD researchers discovered that phonon polaritons are far smaller than light waves and can be tuned to particular frequencies and amplitudes by varying the number of layers of the boron nitride. This is the feature that makes it conceivable to use them for producing images with a higher resolution than is possible with light, say the scientists.

The research, which was published in the journal Science (“Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride”), involved focusing a laser on to the tip of an atomic force microscope as it scanned across the boron nitride. The AFM was taking measurements as infrared light from the laser struck the material.  An interference patterns were created as the phonon polariton waves traveled to the edge of the material and then reflected back.

"A wave on the surface of water is the closest analogy," said Dimitri Basov, professor of physics at the University of California, San Diego, who led the project, in a press release. "You throw a stone and you launch concentric waves that move outward. This is similar. Atoms are moving. The triggering event is illumination with light."

Basov added: "You can bounce these waves off edges. You can bounce them off defects. You can play all sorts of cool tricks with them. And of course, you can design the wavelength and amplitude of these oscillations in a way that suits your purpose."

The discovery was somewhat of a surprise. It stemmed from continuing research Basov and his colleagues have been conducting with 2-D materials, such as 2012 work in which they were experimenting with focusing infrared light on the surface of graphene to control the ripples of electrons that this caused across the surface of the material.

In this case, they were working with the insulator hexagonal boron nitride, which when combined with the conductor graphene, could make complex circuits.

In their similar work with graphene, the researchers were able to generate these phonon polaritons as well, but the waves would dissipate quickly. When using the boron nitride, the waves could be maintained for a long period of time.

"Because these materials are insulators, there is no electronic dissipation. So these waves travel further," Basov said. "We didn't expect them to be long-lived, but we are pleased that they are. It's becoming kind of practical."

Image: TU Vienna

Researchers at the University of Washington, Massachusetts Institute of Technology (MIT), and Vienna University of Technology (TU) in Austria  have all shown interesting optoelectronics results with a new two-dimensional (2-D) material called tungsten diselenide (WSe₂).

The main page of the journal Nature Nanotechnology this week looks almost as though some mistake had been made with three of the top four stories highlighting “2D crystals.” But it was no mistake and foretells of a near future in which new 2-D materials are developed that will show either improved or complementary properties to that of graphene, the granddad of 2-D materials.

Tungsten diselenide belongs to a larger group of transition metal dichalcogenides that also includes molybdenum disulfide (MoS₂). This family consists of materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium. With only a few of these transition metals having been experimented upon, it’s likely we should see more coming down the pike.

The three research groups focused on optoelectronics applications of tungsten diselenide, but each with a slightly different emphasis.

The University of Washington scientists highlighted applications of the material for a light emitting diode (LED). The Vienna University of Technology group focused on the material’s photovoltaic applications. And, finally, the MIT group looked at all of the optoelectronic applications for the material that would result from the way it can be switched from being a p-type to a n-type semiconductor.

The MIT research, which produced diodes, bears some explaining. Diodes are typically made by doping two adjacent parts of a semiconductor, one so it has excess electrons (n-type) the other so it has excess positive charge (p-type). The doping determines which way current flows through the device. Once the semiconductor is doped, that’s it; its direction is set for life. However, what is intriguing about WSe₂ is that if the researchers brought the material into close proximity with a metal electrode and tuned the voltage on that electrode from positive to negative, then the WSe₂ could be switched between p-type and n-type.

When the 2-D material is made into LEDs, as the University of Washington researchers have done, it becomes the thinnest-known LED. Standard LEDs used in electronics today are at least 10 to 20 times thicker than the device developed by the Washington researchers.

The Vienna researchers showed that a WSe₂ photovoltaic cell  is so thin that it lets 95 percent of incident light to pass through it, but it will capture a tenth of the remaining 5 percent and convert it to electricity, resulting in a high, internal conversion efficiency. This could translate to a material that would be put in the windows of buildings, allowing light through but also capable of collecting energy.

We are likely to see more potential applications for this material outside of optoelectronics. But what may be even more intriguing is what will be the next transition metal that looks to have promising properties.

Image: Seyed Mohammad Mirvakili

Ordinary fishing line and sewing thread have joined forces in the lab to create incredibly strong artificial muscles. The new artificial muscles could someday lend superhuman strength to robots and wearable exoskeletons for humans.

The twisted, coiled combinations of polymer fishing line and sewing thread can lift 100 times more weight and have 100 times greater mechanical power than the same length and weight of human muscle. U.S. researchers at the University of Texas at Dallas worked with colleagues from Australia, Canada, China, South Korea and Turkey on the breakthrough detailed in the 21 February 2014 issue of the journal Science .

"The application opportunities for these polymer muscles are vast," said Ray Baughman, the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas and director of the NanoTech Institute, in a news release. "Today's most advanced humanoid robots, prosthetic limbs, and wearable exoskeletons are limited by motors and hydraulic systems, whose size and weight restrict dexterity, force generation, and work capability."

A bundle of fishing lines with a total diameter just 10 times wider than a human hair can create an artificial muscle capable of lifting over 7 kilograms, Baughman said. If combined in parallel like biological muscles, 100 of the artificial muscles could lift about 0.7 tonnes. The muscles can also generate about 7.1 horsepower per kilogram, or the equivalent mechanical power of a jet engine.

The twisted polymer fibers have enough torsional power to spin a heavy rotor more than 10 000 revolutions per minute when heated. Additional "extreme" twisting creates coiled artificial muscles that can either contract or expand along their length when heated, depending on the direction of the twist. These coils can contract by about 50 percent of their length—much more than biological muscles, which can contract by about 20 percent. 

Such artificial muscles are usually electrically powered by resistive heating, said Carter Haines, a doctoral student in materials science and engineering at UT Dallas and lead author on the new study. The resistive heating can come from the metal coating of commercial sewing thread or from metal wires twisted together within the coiled muscles. But the muscles can also draw power from environmental temperature changes.

The new muscles could benefit technologies beyond enhancing the strength of future robots or robotic exoskeletons. Smaller bundles of the polymer muscles with a diameter thinner than a human hair could give life to more nuanced facial expressions in humanoid robots, or lend a precise touch to robotic microsurgery on the tiniest levels.

Researchers also envision the new muscles replacing typical motors in smart buildings with windows that automatically open and close in response to temperature changes, or powering tiny lab-on-a-chip devices.

The UT Dallas team led by Baughman previously experimented with artificial muscles based on carbon nanotubes twisted into bundles and infused with paraffin wax. But their latest work with the fishing line and sewing thread shows that even relatively mundane materials may have a lot to offer.

Image: Seyed Mohammad Mirvakili; Photo: Eli Paster