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Photonic Crystal Uses Coldness of the Universe to Chill Solar Panels on Earth

Last December, researchers at Stanford University developed a passive radiator that uses outer space as a universe-size heat sink. It absorbs ambient heat and then emits it at a very specific infrared band (between 8 and 13 micrometers), for which the Earth’s atmosphere is completely transparent. So the radiator can transfer the heat entirely off-world.

Stanford's radiator is cheap to produce (or so they say), but it would be fighting for rooftop space with all the solar panels that we (should) have up there. In work published today in PNAS, the Stanford researchers describe the performance of a prototype photonic crystal cooling system that can sit on top of a solar cell and cool it by up to 13 degrees Celsius—boosting the amount of electricity that it generates.

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Wearable Uses Your Local EM Field to Track Your Electronics Use

A prototype of a wearable device can sense what appliance you’re using. Engineers at the University of Washington developed MagnifiSense, a wrist-worn magnetic sensing system that tracks your interaction with specific devices, such as a microwave or hair dryer. Based on which device is detected, the system infers what activity you’re performing: Turning on a stove implies that you’re cooking, for example.

Tracking an individual’s daily activity could help monitor and perhaps reduce a user’s energy footprint, or it could feed data to smart home applications and provide safety alerts for the elderly. The Washington team described MagnifiSense and its potential uses in research [pdf] presented at the 2015 ACM International Joint Conference on Pervasive and Ubiquitous Computing in Japan last week.

MagnifiSense works because each appliance generates a distinct electromagnetic radiation pattern. MagnifiSense uses off-the-shelf magneto-inductive sensors to capture a wide spectrum of frequencies near the user. This allows the wearable device to identify the radiation of the particular components—motors, rectifiers, and various modulators—that make up the pattern. Using signal processing and machine learning techniques, the system can use the combination of components to distinguish one device from another.

 Edward Wang, lead researcher and a PhD student in electrical engineering at the University of Washington gave a hairdryer as an example:

The frequency component of a hairdryer is that there’s a motor that spins, so there’s going to be some changing frequency that has to do with the motor’s speed… There’s also the power that it draws, which is 60 Hz in America. The 60 Hz component can be seen in our signal. So, if it doesn’t have a 60 Hz component signal, then it’s not plugged into the wall.

These type of characteristics, also known as domain knowledge, are gathered into a feature set, which is similar to a template, says Wang. After hundreds of different hairdryer templates are fed into the system, it learns to identify the behavior of a hairdryer. Then, when the template of an unknown device is fed into the system, it compares it against existing templates to determine a match.  

The team studied MagnifiSense’s performance in 16 homes and on 12 commonly used appliances in the kitchen, living room, and bathroom. It also studied the user’s interaction with various devices. In a 24-hour period, MagnifiSense successfully identified 25 of the 29 interactions.  

Although this technology seems promising, researchers still need to work out a few kinks. People tend to interact with multiple electronics simultaneously. But, the current prototype can’t detect when multiple electronics are used concurrently.

“Due to the nature of the signal, they add linearly,” Wang says. “The sensor sees A plus B plus C.” This means that if you turn on the stove and also use the blender, the system detects the appliance closest to you- not both. He also says they’re trying to miniaturize the wearable device. 

Japanese Paper Cutting Trick for Moving Solar Cells

To maximize the amount of electricity that solar cells generate, solar panels can be tilted to track the position of the sun over the course of a day. Conventional solar trackers can increase yearly energy generation by 20 to 40 percent, but they can be costly, heavy and bulky, limiting their widespread implementation.

Now materials scientist Max Shtein and his colleagues at the University of Michigan at Ann Arbor have developed novel solar cells that integrate tracking into their design. The design involves a variation of origami known as kirigami, which uses both folding and cutting to create unique structures. They detailed their findings in the 8 September online edition of the journal Nature Communications.

The scientists cut kirigami designs into a 3-micron-thick flexible crystalline gallium arsenide solar cells mounted on plastic sheets. A solar cell array of this type can tilt in three dimensions in a highly controllable manner when its edges are tugged. So a quick pull can make it flex so that it is at the best angle for catching rays.

The researchers found that their new devices could generate roughly as much power as solar cells mounted on conventional trackers. Moreover, the kirigami trackers proved to be electrically and mechanically robust, with no appreciable decrease in performance after more than 300 cycles of activity.

Shtein and his colleagues suggest that kirigami solar panels could be simple, inexpensive and lightweight, and have widespread rooftop, mobile, and spaceborne applications. They added that kirigami systems might also be useful for phased array radar and optical beam steering.

The scientists are now exploring whether mounting solar cells onto more durable materials such as spring steel could make kirigami systems even more robust. 

Artificial Leaf Is 10 Times Better at Generating Hydrogen from Sunlight

A "hydrogen economy" sounds just about as green and eco-friendly as it gets. Fuel cells that combine hydrogen with ambient oxygen in the air can generate electricity with naught but pure water as a byproduct—which is great if you hate pollution and are thirsty. The problem we face now is the source of our hydrogen: the vast majority of it comes from fossil fuels, specifically natural gas. And while transforming methane into hydrogen is 80 percent efficient, that other 20 percent is carbon dioxide.

The vast majority of the accessible clean hydrogen on Earth is locked up with oxygen in water, but breaking apart H2O into an O and a useful H or two isn't a particularly environmentally-friendly or efficient process to get involved in. The fantasy is an "artificial leaf," a passive, inexpensive thing that you can stick in water and expose to sunlight, then watch as it bubbles off all the hydrogen and oxygen you need. People have been working on these, but Caltech has just made an enormous amount of progress with an artificial leaf that, according to the researchers, "shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more."

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A city skyline with imaginary wind turbines just off shore.

Renewable Energy is Good for Your Health

Renewable energy projects and energy efficiency measures—particularly those that replace coal-fired power plants—will not only decrease carbon emissions but may also have major health implications worth millions of dollars, according to researchers at Harvard University.

Public health experts evaluated the impact of four different renewable energy or energy efficiency installations in six locations in the mid-Atlantic and Great Lakes regions and came up with a model to simulate and compare the climate and health benefits of each of the 24 scenarios. Depending on the site and installation, they found that benefits ranged from US $5.7 million to $210 million per year.

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Concentrator Photovoltaics: The Next Step Towards Better Solar Power

Today’s concentrator photovoltaic (CPV) technologies have shown promising potential for more efficient solar power. The latest systems are said to be capable of handling the power of a hundred suns. Yet prototypes have failed to compete with cheaper flat panel solar systems that dominate the market. The U.S. Department of Energy’s Advanced Research Projects Agency (ARPA-E) is determined to push CPV to the next level. On 24 August, at the Clean Energy Summit, U.S. President Barack Obama and Energy Secretary Ernest Moniz announced a program called MOSAIC that will invest $24 million into CPV solar technology development.

Why can’t today’s CPV systems compete? The concentrators can only convert direct sunlight into energy, missing out on the large fraction of sunlight diffracted by clouds and the atmosphere. Manufacturing costs of concentrator apparatuses have also prevented CPV from reaching mass production.

That’s where the MOSAIC initiative comes in. The 11 new CPV programs under MOSAIC’s umbrella are investigating an array of system designs to address cost-efficiency and performance challenges. The list of projects include economical micro-PV cell construction, waveguiding solar concentrators, and single-junction cells that will maximize concentration under indirect and diffuse sunlight.

“ARPA-E is supporting new technology that can help the industry progress even more, but even where it is today is quite exciting,” says Sarah Kurtz, a research fellow working on CPV technology (separately from the MOSAIC effort) at the U.S. National Renewable Energy Laboratory (NREL) in Colorado.

For a large-scale commercial flat plate solar panel system, efficiency is approximately 16 to 20 percent, while a typical CPV system is 25 to 30 percent. In engineering labs, efficiency test results show that the gap between CPV technology and flat planel photovoltaics is even greater. Research groups have created CPV cells that convert more than 40 percent of the light that strikes them to electric current—the highest marks received in testing environments. Three of these groups’ systems have even passed the 46 percent mark.

“This means that these results are very repeatable,” says Keith Emery, a principal scientist who measures solar cell efficiency at the National Center for Photovoltaics at NREL. “I wouldn’t be surprised that by the next two or three years, an individual research group will reach 50 percent efficiency. Fifty percent is a realistic goal that people have on the drawing board.”

CPV maximizes efficiency by using multiple optical elements such as mirrors and lenses to reflect light into a super concentrated beam that is aimed at a solar cell. Machinery adjusts the panels throughout the day so that the cells are exposed to a maximum amount of direct sunlight. The technique is similar to the way you would move a magnifying glass while burning your name into a piece of wood, explains Kurtz.

The optical elements make it possible to use smaller, higher performance solar cells. The miniaturized cells make it easier to modulate their movement to prevent overheating and degradation. Some labs have even constructed cells as small as 1 square millimeter; they can generate more power in less space than flat panels can.

The next push is to make CPV materials that are even greener and cheaper than flat plate photovoltaics. Engineers are testing mirrors constructed from recyclable plastic, with aluminum-based reflective coatings, says Kurtz.

As the energy industry slowly transitions old fossil fuel plants into photovoltaic and other renewable power plants, solar energy can become a larger part of the electric grid.

“There is a rumor that solar is the technology of the future,” Kurtz says. “It has grown a lot, but it is still a minuscule part of the electricity we use.” While photovoltaics provide only about 1 percent of the electricity generated in the U.S., the rate of solar installation is increasing every year. “If we maintain industry growth rates long enough, solar could be at something like 25 percent of the world’s electricity production,” says Kurtz.

Why You Probably Don't Care About a Fuel Cell iPhone That Can Run for a Week

A smartphone powered by a fuel cell that can run for an entire week without recharging sounds absolutely amazing. The Telegraph is reporting that a British fuel cell company called Intelligent Energy has managed to stuff a fuel cell inside of an iPhone 6, allowing the phone to run for an entire week on a single charge.

Sort of.

As with anything that sounds absolutely amazing, it's not that simple, and the truth is likely not something that's worth getting excited about at all.

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Ultrathin Solar Cells for Lightweight and Flexible Applications

Photovoltaic cells are finding a host of new applications, even powering airplanes. An example is the Solar Impulse 2 plane, which is blanketed by photocells that can keep it airborne indefinitely.  Although their conversion efficiency, at 22 percent, is comparatively good, covering 200 square meters with the 130-micrometer-thick cells adds significant weight to the plane.

A team of scientists from the Johannes Kepler University Linz in Austria reported in the 25 August edition of Nature Materials Online that they have created prototype solar cells that are a mere 3 micrometers thick. 

“We are interested in developing a solar cell technology that is the most lightweight and has the highest  power per weight that is possible in a research lab,” says physicist Martin Kaltenbrunner, the spokesperson for  the team.  Although the ultrathin cells’ conversion efficiency is 12 percent, they weigh about 100 times less than the lightest solar cells currently available. The researchers reached an interesting milestone: A square meter of solar cell weighing 5.2 grams produced 120 watts. “It is an absolute record in power per weight,” says Kaltenbrunner.

For their light harvesting material, the Austrian researchers used organolead halide perovskite, a hybrid organic material that is now viewed as a promising alternative to silicon. Says Kaltenbrunner:

The neatest thing about the perovskites is that they are direct band gap materials and can be made very thin, but can still absorb a lot of light. So a couple of hundred nanometers are actually enough to collect all the photons in the absorption range that it has and convert them to electron-hole pairs.

For the creation of these thin perovskite layers, the scientists used solution processing to deposit the thin layer on the transparent electrode. “We used that technique but we had come up with a new method to get a nice pinhole film on a very rough substrate,” says Kaltenbrunner. “Our foils are very thin but also very rough,  and it is challenging to process nanometer thick layers on them without having too many defects.” The other, non-transparent electrode, which could be either gold, aluminum  or chrome, also serves to increase the efficiency of the cell by reflecting back the photons that still passed through. The total thickness of the device, including the two 100-nanometer-thick electrodes, the substrate, and protective layers is 3 micrometers.

The researchers’ processing tools allowed the creation of films only as large as about 10 centimeters, but in the future, roll to roll processing should be possible, says Kaltenbrunner.

At this stage of the research, the device is vulnerable to oxygen and water, but functions at full capacity with artificial sunlight for several days. The team says it made demonstrators that operate in air, and that a half year later, the gadgets are still functional. “We had this little zeppelin and a few days ago we tried it, and [the solar panel] still propels it."

Because these layers are so thin, the film is very flexible and elastic, which results in other interesting advantages besides low weight. By creating the foil on a stretched substrate and then releasing the tension, the deposited foil wrinkles up. “You get this microtexture of tiny valleys that collect more light and, per unit area, we get a higher efficiency,” says Kaltenbrunner.  The flexibility provides yet another advantage: “You fabricate on a large scale in a flat geometry a very thin electronically functional foil, and then you laminate it to whatever object you want,” Kaltenbrunner points out.  “The car industry is very interested in this.”

Photovoltaics that are almost weightless may end up in everything that flies, such as weather balloons, small unmanned planes, and even drones, says Kaltenbrunner.

What about applications in space? It is still too early to answer this question because it is not known whether the film could withstand the intense particle radiation. “It would be really exciting to have partners on board that can test this, like ESA or NASA,” says  Kaltenbrunner. For example, one could have solar sails that might also be solar cells. “Interestingly, this was the driving force to thin down solar cells and make them lightweight.” Kaltenbrunner explains. “This started in the 1980s; there were papers on silicon solar cells on plastic substrates just for this purpose.” 

Supercritical Carbon Dioxide Can Make Electric Turbines Greener

The U.S. Department of Energy (DOE) is on the hunt for technologies that can support a smarter electric grid. It is currently devoting millions of dollars, via its SunShot Initiative, to create more efficient photovoltaic systems. But in addition to the solar power that is SunShot's focus, the DOE it is looking to improve conventional electric power generation. Sandia National Laboratories in Albuquerque, New Mexico, along with its new partners, has received $8 million to make advances in supercritical carbon dioxide gas turbines.

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Synchronization Controls Could Help Smooth Microgrids

Circadian rhythms and microgrids might not seem to have much in common, but in the world of theoretical mathematics, they do.

New research published Friday in Science Advances suggests that the same mathematical models that help scientists better understand and explain biological phenomena could also be applied to making small, islanded power grids run more efficiently.

Microgrids come in various flavors, but usually they are localized grids that can disconnect from the larger power grid and operate independently. More recently, microgrids have combined clean energy generation and more traditional generation (such as diesel or natural gas turbines) to deliver both heat and electric power. They manage current with energy storage and controls.

Because the grids are small, they’re prone to more severe fluctuations in voltage and frequency than are larger grids, which can more easily smooth fluctuations across their wider systems.

This is where a well known mathematical model for synchronization, the Kuramoto model, can help, says Per Sebastian Skardal, assistant professor of mathematics at Trinity College and lead author of the paper.

The Kuramoto model is a phase oscillator model that defines each oscillator as just a phase angle. The behavior of each phase depends on its interaction with the other phases, explains Skardal.

The model has helped to explain the synchronization of various processes, including the rhythmic flashing of fireflies and neurons firing in the brain. “With the power grid, on the other hand, we are going one step further and using what we know about networks and synchronization to make deliberate choices to improve the functionality of a given system,” says Skardal.

Skardal and his collaborators found that in islanded microgrids that are disconnected from the large power grid, there are essentially a few problematic oscillators. They tend to prefer a frequency that is either much higher or lower than the other oscillators in the network, or they’re loosely coupled to the network. These problematic oscillators could be a set of solar panels that have widely variable output, or a large power draw that turns on and off suddenly.

Skardal says the model should apply regardless of the microgrid configuration. Although the work is firmly theoretical at this point, Skardal and his collaborator Alex Arenas, professor of physics at Rovira i Virgili University in Spain, are interested in applied mathematics, which would take further studies and engineering work.

The model would ultimately inform control systems for microgrids, giving the ability to identify the problem oscillators and adjust them in real time. Preventing or minimizing the grid fluctuations within a microgrid could potentially reduce costs because fewer inverters would be needed or a more simplified and standardized control scheme could be implemented.

Whether this research would apply to a microgrid when it is connected to the main power grid, or help to limit fluctuations in larger power grids whose stability is affected by the variability of inputs from renewable energy sources, is yet to be seen.

Skardal says he believes both may be future applications of the current research, but it is too early to say for sure. In the case of a microgrid that is not islanded, “I believe that the intuition behind the idea will hold,” says Skardal. But the Kuramoto network model would have to be adjusted to take some external forcing, which would be the influence of the larger grid, into account. 

For large power grids with high levels of renewable energy generation, the complexity of the system would make the model more complicated, but the same math could potentially apply.

But power grids aren’t the only things that could benefit from this line of research, the authors argue. “We hypothesize that our findings here may potentially shed some light on the control of synchronization in other contexts,” they conclude in the paper, “such as cardiac physiology and neuroscience.”


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