Power distribution networks (PDNs), have become a key technology focus as the task of efficiently providing power to products becomes more difficult. Because PDNs are routed to all devices that need power, any noise or transients on power rails propagate throughout the system. This makes analyzing your system’s power integrity becomes even more important. See tips for power supply measurements and selecting the best tools for making these measurements in this application note.

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A research team from the Georgia Institute of Technology has developed a power management system that could allow fitness bands and smart watches to harvest energy from human motions like running, walking, or tapping your fingers.

The research, published last month in the journal Nature Communications, could significantly increase the efficiency of triboelectric generators—tiny electronic devices that convert body movements and mechanical vibrations into electricity. The team reports that their new power management system can increase the efficiency of these devices from about one percent up to 60%.

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Zhong Ling Wang, a professor in the Georgia Tech School of Materials Science and Engineering and member of the research team, led the development of the team's first triboelectric nanogenerators in 2012. Since then, Wang and his colleagues have invented several generators that take advantage of the triboelectric effect, or the property of certain materials to produce an electric current when rubbed against each other. Earlier this month, the research team developed a flag that could generate electricity by flapping in the wind. And last year, the researchers created a fabric that could harvest energy from human motion.

The new power management system has two stages. First, it captures alternating current from a triboelectric generator in a small capacitor. When the capacitor is full, a circuit funnels the electricity into a larger capacitor or battery. This second storage device then supplies direct current at the low voltages required for portable electronics and wearable devices.

The research could represent a major step toward solving the tricky problem of harvesting energy from irregular human motion. Ordinarily, alternating current can be converted into direct current using a transformer. But normal transformers require a constant number of cycles per second. By contrast, walking or finger tapping produce mechanical energy with fluctuating amplitude and variable frequencies.

The two-stage design of the new system can account for this problem, says Simiao Niu, a Georgia Tech graduate research assistant. “The first stage of our system is matched to the triboelectric nanogenerator,” he says, “and the second stage is matched to the application it will be powering.”

The power management system is capable of amplifying the very small electric current produced by triboelectric generators—up to 330 times more power output than the generator alone. When tested with finger tapping, the system generated a continuous direct current of 1.044 mW. The power management system needs a small amount of power to work, but the milliwatt-level power is more than enough to compensate, says Niu.

"It doesn't matter what kind of mechanical motion or what frequency of mechanical motion you have, as long as the energy input is high," says Niu. The system also helps to convert the high-voltage and low-current power from triboelectric generators for low-voltage, high-current portable electronics.

Wang says that the next step for the power management system, which has only been confirmed in a proof-of-concept experiment, will be to shrink the circuitry to the point where it can be used in an actual device. He adds that the system is also compatible with piezoelectric and pyroelectric generators, which also produce alternating current.

The international accord drafted by 195 countries at the Paris climate talks last year is focused on keeping the average global temperature increase below 1.5 degrees Celsius. The agreement will require not only using more renewable energy and reducing carbon emissions, but also a reevaluation of how a renewable power grid will work.

Now, a research team led by the Lawrence Livermore National Laboratory (LLNL) in California has proposed a new method for satisfying these requirements. Writing in the journal Mechanical Engineering, the researchers outlined a system that uses carbon emissions from power plants to store renewable energy for the power grid.

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The system, which the research team described as a vast underground battery, would store thermal energy in underground carbon dioxide reservoirs when demand on the power grid was low. The system would convert that energy into electricity and dispatch it to the power grid when demand is high.

The research comes as wind turbines, solar farms, and other renewable sources are increasingly being used in a power grid dominated by “baseload” power plants, which generate a constant flow of electricity. In contrast, the power output of renewable sources varies widely based on the time of day, seasons, location, and weather.

According to Thomas Buscheck, the paper’s author and director of LLNL’s Geochemical, Hydrological, and Environmental Science division, if “you want to store the large quantities of renewable energy necessary to balance seasonal supply-demand mismatches and store it efficiently, we believe the best way to do that is underground.”

The underground storage system involves pumping supercritical CO₂, a highly concentrated liquid version of carbon dioxide, into tunnels carved in sedimentary rock. Once underground, the highly pressurized CO₂ pushes brine trapped in the rock up production wells. At the surface, the brine can be heated by energy from concentrating solar power plants, wind turbines, and nuclear and other baseload power plants.

Once heated, the brine is sent into the reservoirs to store thermal energy. The geothermal heat and huge amount of pressure underground prevent significant heat loss. When renewable energy cannot fulfill power grid demands, the pressurized CO₂ and brine can both be released and their thermal energy converted to power. The brine could be used in a steam-powered generator, while the heated CO₂ could drive turbines by itself. Both fluids could be reheated and sent back down into the reservoirs.

Another benefit of the system is that it locks away CO₂ that would otherwise be released into the atmosphere, fueling climate change. Buscheck says at least four million tons of CO₂ could be stored underground each year over 30 years. That is equivalent to the CO₂ impact of a 600-megawatt coal plant. To relieve the enormous pressure of the stored CO₂, which will make it difficult to keep underground permanently, some of the brine could be removed to create water through desalination.

Burning fossil fuels to generate electricity accounts for about 31% of the carbon dioxide emissions in the United States—more than any other source, including transportation. This translates into more than two billion metric tons of carbon emissions per year, according a 2013 study by the Environmental Protection Agency.

Buscheck says the United States is a very suitable candidate for the new system, but his reasons are less about the country’s carbon footprint and more about its geology. The sedimentary rock formations required for this system cover about half of the United States, he says.

The project falls within the U.S. Energy Department's Grid Modernization program, which aims to integrate renewable energy sources into the grid, while making it more resilient and secure against cyberattacks. On Thursday, Energy Secretary Ernest Moniz announced that the laboratory would start 14 new power grid research projects this year. Buscheck and his LLNL colleagues worked on the project with researchers from Ohio State University, the University of Minnesota, and Terracoh Inc., a company that designs underground CO² storage systems.

The underground battery concept has been in development for seven years and has been validated in computer models, but Buscheck notes that more research is necessary. Fundamentally, however, “the concept is based on proven technology,” he says. “There are no showstoppers.”

Even though they have around three times the energy density of lead acid and NiMH, lithium-ion batteries can be very unstable. The Li-ion batteries in electric vehicles have been known to catch fire following accidents and front-end collisions. The Federal Aviation Administration has even considered a moratorium on shipping batteries for fear that they could ignite during flight.

Over the last month, as the press have raised the questions of their overall safety, and engineers have guaranteed that failures are extremely rare, two research projects have invented Li-ion batteries with internal fail-safes to prevent the catastrophic failures that cause them to catch fire.

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When severely damaged, overcharged due to a malfunction, or contaminated by metallic dust in the production process, Li-ion batteries can short-circuit and overheat. Too much heat triggers a chain reaction known as a thermal runaway, in which the flammable electrolyte inside the battery catches fire. Low-quality separators between the battery’s electrodes can also cause thermal runaways.

The most recent research to address these problems comes out of Stanford University. Researchers have developed a temperature sensor that can shut down the battery before it overheats, preventing a thermal runaway. In addition, the sensor is capable of restarting the battery automatically once it cools down.

Writing in the journal Nature Energy, the researchers said that they had based their design on wearable sensors for measuring body temperature. The sensor is made out of a thin film of elastic polyethelene, and embedded in the film are tiny nickel particles coated in graphene.

“We attached the polyethylene film to one of the battery electrodes so that an electric current could flow through it,” says Zheng Chen, lead author of the paper. “To conduct electricity, the spiky particles have to physically touch one another. But during thermal expansion, polyethylene stretches. That causes the particles to spread apart, making the film nonconductive so that electricity can no longer flow through the battery.”

The polyethylene film expands when the battery’s temperature is above 160°F, causing the spike to separate and the battery to shut down. When the temperature falls back under that threshold, the polyethylene contracts and the battery generates electricity again. The temperature threshold can be adjusted with different materials, according to the research team.

Another study from Penn State University takes a similar approach, using sensors to shut down the Li-ion battery at high temperatures. The Penn State team worked closely with large-format batteries—the kind used in electric vehicles and other gadgets—that require more power than cell phones and computers.

The team worked with a grant from the U.S. Department of Energy’s Computer Aided Engineering for Electric Drive Vehicle batteries (CAEBAT) project, underlining the need for Li-ion batteries that can survive accidents, as well as adapt to wide changes in weather and temperature. Its research was published in the journal Scientific Reports earlier this month.

The transition to electric vehicles will serve as one of the main tests of Li-ion battery safety. Automobile and battery companies are working to increase their energy density, at the same time making them smaller and lighter, in an attempt to extend the range of electric vehicles. At these higher densities, battery failures have the potential to cause more violent thermal runaways.

Chao-Yang Wang, a professor of mechanical, chemical, and materials engineering at Penn State, notes “you are compressing more and more energy into a smaller space, and you have to careful when you do that.”

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