Monthly Archives: July 2011
31 July 2011
ANTARCTICA is rising like a cheese soufflé: slowly but surely. Lost ice due to climate change and left-over momentum from the end of the last big ice age mean the buoyant continent is heaven-bound.
Donald Argus of NASA’s Jet Propulsion Laboratory in Pasadena, California, and colleagues used 15 years of GPS data to show that parts of the Ellsworth mountains in west Antarctica are rising by around 5 millimetres a year (Geophysical Research Letters, DOI: 10.1029/2011gl048025). Elsewhere on the continent, the rise is slower.
A faster rise has been seen in Greenland, which is thought to be popping up by 4 centimetres a year.
Ongoing climate change could be partly to blame: Antarctica is losing about 200 gigatonnes of ice per year, and for Greenland the figure is 300 gigatonnes. Earth’s continents sit on viscous magma, so the effect of this loss is like taking a load off a dense foam mattress.
But there is another possible contributor. “The Earth has a very long memory,” says Argus. As a result, “there is also a viscous response to ice loss from around 5000 to 10,000 years ago going on”.
Despite this effect, the known ice loss at both poles suggests that embedded in the local rises is a signal of current climate change – researchers just have to tease it out.
another way to make electrical generation cheap and ubiquitous
Sun-Free Photovoltaics: Materials Engineered to Give Off Precisely Tuned Wavelengths of Light When Heated
ScienceDaily (July 31, 2011) — A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light — selected to match the wavelengths that photovoltaic cells can best convert to electricity — makes the new system much more efficient than previous versions.
The key to this fine-tuned light emission, described in the journal Physical Review A, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat — whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other source — the pitted surface radiates energy primarily at these carefully chosen wavelengths.
Based on that technology, MIT researchers have made a button-sized power generator fueled by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged instantly, just by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope that steadily produces heat from radioactive decay, could generate electricity for 30 years without refueling or servicing — an ideal source of electricity for spacecraft headed on long missions away from the sun.
According to the U.S. Energy Information Administration, 92 percent of all the energy we use involves converting heat into mechanical energy, and then often into electricity — such as using fuel to boil water to turn a turbine, which is attached to a generator. But today’s mechanical systems have relatively low efficiency, and can’t be scaled down to the small sizes needed for devices such as sensors, smartphones or medical monitors.
“Being able to convert heat from various sources into electricity without moving parts would bring huge benefits,” says Ivan Celanovic ScD ’06, research engineer in MIT’s Institute for Soldier Nanotechnologies (ISN), “especially if we could do it efficiently, relatively inexpensively and on a small scale.”
It has long been known that photovoltaic (PV) cells needn’t always run on sunlight. Half a century ago, researchers developed thermophotovoltaics (TPV), which couple a PV cell with any source of heat: A burning hydrocarbon, for example, heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, generating electricity. The thermal emitter’s radiation includes far more infrared wavelengths than occur in the solar spectrum, and “low band-gap” PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can. But much of the heat is still wasted, so efficiencies remain relatively low.
An ideal match
The solution, Celanovic says, is to design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity, while suppressing other wavelengths. “But how do we find a material that has this magical property of emitting only at the wavelengths that we want?” asks Marin Soljačić, professor of physics and ISN researcher. The answer: Make a photonic crystal by taking a sample of material and create some nanoscale features on its surface — say, a regularly repeating pattern of holes or ridges — so light propagates through the sample in a dramatically different way.
“By choosing how we design the nanostructure, we can create materials that have novel optical properties,” Soljačić says. “This gives us the ability to control and manipulate the behavior of light.”
The team — which also includes Peter Bermel, research scientist in the Research Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and Michael Ghebrebrhan, a postdoc in RLE — used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.
This powerful approach — co-developed by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others — has been widely used to improve lasers, light-emitting diodes and even optical fibers. The MIT team, supported in part by a seed grant from the MIT Energy Initiative, is now working with collaborators at MIT and elsewhere to use it to create several novel electricity-generating devices.
Mike Waits, an electronics engineer at the Army Research Laboratory in Adelphi, Md., who was not involved in this work, says this approach to producing miniature power supplies could lead to lighter portable electronics, which is “critical for the soldier to lighten his load. It not only reduces his burden, but also reduces the logistics chain” to deliver those devices to the field. “There are a lot of lives at stake,” he says, “so if you can make the power sources more efficient, it could be a great benefit.”
The button-like device that uses hydrocarbon fuels such as butane or propane as its heat source — known as a micro-TPV power generator — has at its heart a “micro-reactor” designed by Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems Technology Laboratories. While the device achieves a fuel-to-electricity conversion efficiency three times greater than that of a lithium-ion battery of the same size and weight, Celanovic is confident that with further work his team can triple the current energy density. “At that point, our TPV generator could power your smartphone for a whole week without being recharged,” he says.
Celanovic and Soljačić stress that building practical systems requires integrating many technologies and fields of expertise. “It’s a really multidisciplinary effort,” Celanovic says. “And it’s a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion.”
Note: The full version of the MITEI story is available at: http://web.mit.edu/mitei/research/spotlights/making-electricity-with-photovoltaics.html
i could power a bunch of things with transparent, flexible batteries.
Researchers fabricate electrodes that are too small to see.
| July 25, 2011
By Duncan Graham-Rowe of NaturemagazineFlexible, transparent lithium-ion batteries have been made by a team of researchers at Stanford University in California, a technological leap that could spawn see-through electronic gadgets such as translucent iPads.
Many electronic components can be fabricated to be transparent, but so far this hasn’t been possible for the power supply, says materials scientist Yi Cui, who led the work, which is published today in Proceedings of the National Academy of Sciences.
Batteries are normally made up of a pair of electrodes separated by an electrolytic solution, with something to conduct the current to an external circuit, and packaging to hold it all together. But only the electrolyte is naturally transparent, says Cui. And to make matters worse, he says, these components need to be piled on top of each other, which means all of them must be clear for the device to be transparent.
Nothing to see here
While transparent materials can easily be found for the casing and conductors, the electrodes tend to be naturally opaque. Cui’s solution was to make the components so small that they are beyond the resolution of human eyes. This is done by creating each electrode out of a very fine mesh, instead of a solid sheet or rod of the material. Because no individual strand within the mesh is wider than 35 micrometers, they are too small for the eye to detect and so when stacked they appear transparent, much like looking through two very fine sieves.
Creating such tiny features out of active electrode materials is tricky, says Cui. Lithographic fabrication techniques are capable of creating structures at this scale but these processes involve using solvents that would be harmful to the electrodes.
So Cui’s group used microfabrication techniques to first create a grid pattern mould out of silicon. Then a 100-micrometre-thick layer of polydimethylsiloxane (PDMS), a flexible, transparent polymer, was applied using a technique called electro-spinning. When the PDMS was removed from the silicon an aqueous slurry solution containing the active electrode material was applied to it, which filled the grid-like trenches to form a mesh, says Cui.
Cui and his colleagues show that they can create lithium-ion batteries that let 60% of the light that hit them pass right through, and an energy density of 10 watt-hours per liter. “That’s very transparent,” says Cui. Not as good as glass, but clear enough to read text through.
But 10 watt-hours per liter is a low energy density. The energy densities of conventional Lithium-ion batteries are typically an order of magnitude greater than this, says Hiroyuki Nishide, a chemist at Waseda University in Tokyo, whose own group is working on a type of transparent battery that uses nitroxide radicals as charge carriers.
Cui agrees, but says it should be relatively easy to push the energy density to roughly half that of conventional batteries. Making the electrode meshes deeper will increase the volume of the active material and the amount of stored energy without making the battery any more visible, he says.
Besides the aesthetic appeal of creating transparent gadgets, this sort of battery could also help reduce the size of portable devices, says Nishide. For example integrating the power supplies of smart phones within their displays should make them more compact. But the main innovation here seems to be the fabrication technique, which may not be limited to just lithium-ion batteries. This strategy seems pretty versatile and may be applicable to a range of different battery types, says Nishide.
This article is reproduced with permission from the magazine Nature. The article was first published on July 25, 2011.