Efficiency metrics for energy storage devices need standardization

Solving the mystery of prematurely dead cell phone and laptop batteries may prove to be a vital step toward creating a sustainable energy grid according to Drexel researcher Dr. Yury Gogotsi. In a piece published in the November 18 edition of Science, Gogotsi, who is the head of the A.J. Drexel Nanotechnology Institute, calls for a new, standardized gauge of performance measurement for energy storage devices that are as small as those used in cell phones to as large as those used in the national energy grid.

Gogotsi is one of the featured experts, along with Bill Gates, tapped by Science to address problems that must be solved en route to the widespread use of renewable energy. His piece, co-authored with Dr. Patrice Simon of the Université Paul Sabatier in Toulouse, France, is entitled "True Performance Metrics in Electrochemical Energy Storage."

"A dramatic expansion of research in the area of electrochemical energy storage has occurred over the past due to an ever increasing variety of handheld electronic devices that we all use," Gogotsi said. "This has expanded use of electrical energy in transportation, and the need to store renewable energy efficiently at the grid level. This process has been accompanied by the chase for glory with the arrival of new materials and technologies that leads to unrealistic expectations for batteries and supercapacitors and may hurt the entire energy storage field."

The main type of energy storage device addressed in the article is the supercapacitor. Supercapacators, which are built from relatively inexpensive natural materials such as carbon, aluminum and polymers, are found in devices, ranging from mobile phones and laptop batteries to trams, buses and solar cells. While supercapacitors tend to store less energy compared to standard lithium-ion batteries, they have the ability to charge and discharge energy more quickly than batteries and can be recharged a near infinite number of times, and operate in a wider temperature range with a high efficiency.

Typically, the performance of both, batteries and supercapacitors, is presented using Ragone plots, graphs that show a relation between the energy density and the power density. For example, a Rangone plot for the battery used in an electric car shows both how far it can travel on a single charge -energy density- and how fast the car can travel -power density. An ideal energy storage device is expected to store plenty of energy and do it quickly.

The issue that Gogotsi and Simon bring to light is the idea that current metrics for grading energy storage devices, including the Ragone plot, may not provide a complete picture of the devices' capability. According to the researchers, other metrics, such as a device's cycle lifetime, energy efficiency, self-discharge, temperature range of operation and cost, must also be reported.

"This paper calls upon the community of scientists and engineers who work on supercapacitors to present data on material performance using metrics beyond the traditional Ragone plot," Simon said. "Although such plots are useful for comparing fully packaged commercial devices, they might predict unrealistic performance for packaged cells from extrapolation of small amounts of materials."

Gogotsi and Simon have a longtime research collaboration, investigating materials for supercapacitors. Their joint work has received global coverage and various awards and distinctions. Funding for the collaboration between Gogotsi and Simon is sponsored by the Partner University Fund (PUF) which supports innovative and sustainable partnerships between French and US institutions of research and higher education.

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Journal Reference:

Y. Gogotsi, P. Simon. True Performance Metrics in Electrochemical Energy Storage. Science, 2011; 334 (6058): 917 DOI: 10.1126/science.1213003

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New material can enhance energy, computer, lighting technologies

ScienceDaily (Nov. 16, 2011) — Arizona State University researchers have created a new compound crystal material that promises to help produce advances in a range of scientific and technological pursuits.

ASU electrical engineering professor Cun-Zheng Ning says the material, called erbium chloride silicate, can be used to develop the next generations of computers, improve the capabilities of the Internet, increase the efficiency of silicon-based photovoltaic cells to convert sunlight into electrical energy, and enhance the quality of solid-state lighting and sensor technology.

Ning's research team of team of students and post-doctoral degree assistants help synthesize the new compound in ASU's Nanophotonics Lab in the School of Electrical, Computer and Energy Engineering, one of the university's Ira A. Fulton Schools of Engineering.

The lab's erbium research is supported by the U.S. Army Research Office and U.S. Air Force Office of Scientific Research. Details about the new compound are reported in the Optical Materials Express on the website of the Optical Society of America.

The breakthrough involves the first-ever synthesis of a new erbium compound in the form of a single-crystal nanowire, which has superior properties compared to erbium compounds in other forms.

Erbium is one of the most important members of the rare earth family in the periodic table of chemical elements. It emits photons in the wavelength range of 1.5 micrometers, which are used in the optical fibers essential to high-quality performance of the Internet and telephones.

Erbium is used in doping optical fibers to amplify the signal of the Internet and telephones in telecommunications systems. Doping is the term used to describe the process of inserting low concentrations of various elements into other substances as a way to alter the electrical or optical properties of the substances to produce desired results. The elements used in such processes are referred to as dopants.

"Since we could not dope as many erbium atoms in a fiber as we wish, fibers had to be very long to be useful for amplifying an Internet signal. This makes integrating Internet communications and computing on a chip very difficult," Ning explains.

"With the new erbium compound, 1,000 times more erbium atoms are contained in the compound. This means many devices can be integrated into a chip-scale system," he says. "Thus the new compound materials containing erbium can be integrated with silicon to combine computing and communication functionalities on the same inexpensive silicon platform to increase the speed of computing and Internet operation at the same time."

Erbium materials can also be used to increase the energy-conversion efficiency of silicon solar cells.

Silicon does not absorb solar radiation with wavelengths longer than 1.1 microns, which results in waste of energy -- making solar cells less efficient.

Erbium materials can remedy the situation by converting two or more photons carrying small amounts of energy into one photon that is carrying a larger amount of energy. The single, more powerful photon can then be absorbed by silicon, thus increasing the efficiency of solar cells.

Erbium materials also help absorb ultraviolet light from the sun and convert it into photons carrying small amounts of energy, which can then be more efficiently converted into electricity by silicon cells. This color-conversion function of turning ultraviolet light into other visible colors of light is also important in generating white light for solid-state lighting devices.

While erbium's importance is well-recognized, producing erbium materials of high quality has been challenging, Ning says.

The standard approach is to introduce erbium as a dopant into various host materials, such as silicon oxide, silicon, and many other crystals and glasses.

"One big problem has been that we have not been able to introduce enough erbium atoms into crystals and glasses without degrading optical quality, because too many of these kinds of dopants would cluster, which lowers the optical quality," he says.

What is unique about the new erbium material synthesized by Ning's group is that erbium is no longer randomly introduced as a dopant. Instead, erbium is part of a uniform compound and the number of erbium atoms is a factor of 1,000 more than the maximum amount that can be introduced in other erbium-doped materials.

Increasing the number of erbium atoms provides more optical activity to produce stronger lighting. It also enhances the conversion of different colors of light into white light to produce higher-quality solid-state lighting and enables solar cells to more efficiently convert sunlight in electrical energy.

In addition, since erbium atoms are organized in a periodic array, they do not cluster in this new compound. The fact that the material has been produced in a high-quality single-crystal form makes the optical quality superior to the other doped materials, Ning says.

Like many scientific discoveries, the synthesis of this new erbium material was made somewhat by accident.

"Similar to what other researchers are doing, we were originally trying to dope erbium into silicon nanowires. But the characteristics demonstrated by the material surprised us," he says. "We got a new material. We did not know what it was, and there was no published document that described it. It took us more than a year to finally realize we got a new single-crystal material no one else had produced."

Ning and his team are now trying to use the new erbium compound for various applications, such as increasing silicon solar cell efficiency and making miniaturized optical amplifiers for chip-scale photonic systems for computers and high-speed Internet.

"Most importantly," he says, "there are many things we have yet to learn about what can be achieved with use of the material. Our preliminary studies of its characteristics show it has many amazing properties and superior optical quality. More exciting discoveries are waiting to be made."

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A light wave of innovation to advance solar energy: Researchers adapt classic antennas to harness more power from the sun

ScienceDaily (Nov. 10, 2011) — Some solar devices, like calculators, only need a small panel of solar cells to function. But supplying enough power to meet all our daily needs would require enormous solar panels. And solar-powered energy collected by panels made of silicon, a semiconductor material, is limited -- contemporary panel technology can only convert approximately seven percent of optical solar waves into electric current.

Profs. Koby Scheuer, Yael Hanin and Amir Boag of Tel Aviv University's Department of Physical Electronics and its innovative new Renewable Energy Center are now developing a solar panel composed of nano-antennas instead of semiconductors. By adapting classic metallic antennas to absorb light waves at optical frequencies, a much higher conversion rate from light into useable energy could be achieved. Such efficiency, combined with a lower material cost, would mean a cost-effective way to harvest and utilize "green" energy.

The technology was recently presented at Photonics West in San Francisco and published in the conference proceedings.

Receiving and transmitting green energy

Both radio and optical waves are electromagnetic energy, Prof. Scheuer explains. When these waves are harvested, electrons are generated that can be converted into electric current. Traditionally, detectors based on semiconducting materials like silicon are used to interface with light, while radio waves are captured by antenna.

For optimal absorption, the antenna dimensions must correspond to the light's very short wavelength -- a challenge in optical frequencies that plagued engineers in the past, but now we are able to fabricate antennas less than a micron in length. To test the efficacy of their antennas, Prof. Scheuer and his colleagues measured their ability to absorb and remit energy. "In order to function, an antenna must form a circuit, receiving and transmitting," says Prof. Scheuer, who points to the example of a cell phone, whose small, hidden antenna both receives and transmits radio waves in order to complete a call or send a message.

By illuminating the antennas, the researchers were able to measure the antennas' ability to re-emit radiation efficiently, and determine how much power is lost in the circuit -- a simple matter of measuring the wattage going in and coming back out. Initial tests indicate that 95 percent of the wattage going into the antenna comes out, meaning that only five percent is wasted.

According to Prof. Scheuer, these "old school" antennas also have greater potential for solar energy because they can collect wavelengths across a much broader spectrum of light. The solar spectrum is very broad, he explains, with UV or infrared rays ranging from ten microns to less than two hundred nanometers. No semiconductor can handle this broad a spectrum, and they absorb only a fraction of the available energy. A group of antennas, however, can be manufactured in different lengths with the same materials and process, exploiting the entire available spectrum of light.

When finished, the team's new solar panels will be large sheets of plastic which, with the use of a nano-imprinting lithography machine, will be imprinted with varying lengths and shapes of metallic antennas.

Improving solar power's bottom line

The researchers have already constructed a model of a possible solar panel. The next step, says Prof. Scheuer, is to focus on the conversion process -- how electromagnetic energy becomes electric current, and how the process can be improved.

The goal is not only to improve the efficiency of solar panels, but also to make the technology a viable option in terms of cost. Silicon is a relatively inexpensive semiconductor, but in order to obtain sufficient power from antennas, you need a very large panel -- which becomes expensive. Green energy sources need to be evaluated not only by what they can contribute environmentally, but also the return on every dollar invested, Prof. Scheuer notes. "Our antenna is based on metal -- aluminium and gold -- in very small quantities. It has the potential to be more efficient and less expensive."

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Shake, rattle and … power up? New device generates energy from small vibrations

ScienceDaily (Sep. 16, 2011) — Today's wireless-sensor networks can do everything from supervising factory machinery to tracking environmental pollution to measuring the movement of buildings and bridges. Working together, distributed sensors can monitor activity along an oil pipeline or throughout a forest, keeping track of multiple variables at a time.

While uses for wireless sensors are seemingly endless, there is one limiting factor to the technology -- power. Even though improvements have brought their energy consumption down, wireless sensors' batteries still need changing periodically. Especially for networks in remote locales, replacing batteries in thousands of sensors is a staggering task.

To get around the power constraint, researchers are harnessing electricity from low-power sources in the environment, such as vibrations from swaying bridges, humming machinery and rumbling foot traffic. Such natural energy sources could do away with the need for batteries, powering wireless sensors indefinitely.

Now researchers at MIT have designed a device the size of a U.S. quarter that harvests energy from low-frequency vibrations, such as those that might be felt along a pipeline or bridge. The tiny energy harvester -- known technically as a microelectromechanical system, or MEMS -- picks up a wider range of vibrations than current designs, and is able to generate 100 times the power of devices of similar size. The team published its results in the Aug. 23 online edition of Applied Physics Letters.

"There are wireless sensors widely available, but there is no supportive power package," says Sang-Gook Kim, a professor of mechanical engineering at MIT and co-author of the paper. "I think our vibrational-energy harvesters are a solution for that."

Putting the squeeze on

To harvest electricity from environmental vibrations, researchers have typically looked to piezoelectric materials such as quartz and other crystals. Such materials naturally accumulate electric charge in response to mechanical stress (piezo, in Greek, means to squeeze or press). In the past few years, researchers have exploited piezoelectric material, or PZT, at the microscale, engineering MEMS devices that generate small amounts of power.

Various groups have gravitated toward a common energy-harvesting design: a small microchip with layers of PZT glued to the top of a tiny cantilever beam. As the chip is exposed to vibrations, the beam moves up and down like a wobbly diving board, bending and stressing the PZT layers. The stressed material builds up an electric charge, which can be picked up by arrays of tiny electrodes.

However, the cantilever-based approach comes with a significant limitation. The beam itself has a resonant frequency -- a specific frequency at which it wobbles the most. Outside of this frequency, the beam's wobbling response drops off, along with the amount of power that can be generated.

"In the lab, you can move and shake the devices at the frequencies you want, and it works," says co-author Arman Hajati, who conducted the work as a PhD student at MIT. "But in reality, the source of vibration is not constant, and you get very little power if the frequency is not what you were expecting."

To address the problem, some researchers have taken a "power in numbers" approach, simply increasing the number of cantilever beams and PZT layers occupying a chip. However, Kim and Hajati say this tactic can be wasteful, and expensive.

"In order to deploy millions of sensors, if the energy harvesting device is $10, it may be too costly," says Kim, who is a member of MIT's Microsystems Technology Laboratories. "But if it is a single-layer MEMS device, then we can fabricate [the device for] less than $1."

Bridging the power divide

Kim and Hajati came up with a design that increases the device's frequency range, or bandwidth, while maximizing the power density, or energy generated per square centimeter of the chip. Instead of taking a cantilever-based approach, the team went a slightly different route, engineering a microchip with a small bridge-like structure that's anchored to the chip at both ends. The researchers deposited a single layer of PZT to the bridge, placing a small weight in the middle of it.

The team then put the device through a series of vibration tests, and found it was able to respond not just at one specific frequency, but also at a wide range of other low frequencies. The researchers calculated that the device was able to generate 45 microwatts of power with just a single layer of PZT -- an improvement of two orders of magnitude compared to current designs.

"If the ambient vibration is always at a single frequency and does not vary, [current designs] work fine," says Daniel Inman, professor of aerospace engineering at the University of Michigan. "But as soon as the frequency varies or shifts a little, the power decreases drastically. This design allows the bandwidth to be larger, meaning the problem is, in principle, solved." Inman adds that going forward, the MIT group will have to aim lower in the frequencies they pick up, since few vibrations in nature occur at the relatively high frequency ranges captured by the device.

Hajati says the team plans to do just that, optimizing the design to respond to lower frequencies and generate more power.

"Our target is at least 100 microwatts, and that's what all the electronics guys are asking us to get to," says Hajati, now a MEMS development engineer at FujiFilm Dimatix in Santa Clara, Calif. "For monitoring a pipeline, if you generate 100 microwatts, you can power a network of smart sensors that can talk forever with each other, using this system."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by Jennifer Chu.

Journal Reference:

Arman Hajati, Sang-Gook Kim. Ultra-wide bandwidth piezoelectric energy harvesting. Applied Physics Letters, 2011; 99 (8): 083105 DOI: 10.1063/1.3629551

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Nano bundles pack a powerful punch: Solid-state energy storage takes a leap forward

ScienceDaily (Aug. 23, 2011) — Rice University researchers have created a solid-state, nanotube-based supercapacitor that promises to combine the best qualities of high-energy batteries and fast-charging capacitors in a device suitable for extreme environments.

A paper from the Rice lab of chemist Robert Hauge, to be published in the journal Carbon, reported the creation of robust, versatile energy storage that can be deeply integrated into the manufacture of devices. Potential uses span on-chip nanocircuitry to entire power plants.

Standard capacitors that regulate flow or supply quick bursts of power can be discharged and recharged hundreds of thousands of times. Electric double-layer capacitors (EDLCs), generally known as supercapacitors, are hybrids that hold hundreds of times more energy than a standard capacitor, like a battery, while retaining their fast charge/discharge capabilities.

But traditional EDLCs rely on liquid or gel-like electrolytes that can break down in very hot or cold conditions. In Rice's supercapacitor, a solid, nanoscale coat of oxide dielectric material replaces electrolytes entirely.

The researchers also took advantage of scale. The key to high capacitance is giving electrons more surface area to inhabit, and nothing on Earth has more potential for packing a lot of surface area into a small space than carbon nanotubes.

When grown, nanotubes self-assemble into dense, aligned structures that resemble microscopic shag carpets. Even after they're turned into self-contained supercapacitors, each bundle of nanotubes is 500 times longer than it is wide. A tiny chip may contain hundreds of thousands of bundles.

For the new device, the Rice team grew an array of 15-20 nanometer bundles of single-walled carbon nanotubes up to 50 microns long. Hauge, a distinguished faculty fellow in chemistry, led the effort with former Rice graduate students Cary Pint, first author of the paper and now a researcher at Intel, and Nolan Nicholas, now a researcher at Matric.

The array was then transferred to a copper electrode with thin layers of gold and titanium to aid adhesion and electrical stability. The nanotube bundles (the primary electrodes) were doped with sulfuric acid to enhance their conductive properties; then they were covered with thin coats of aluminum oxide (the dielectric layer) and aluminum-doped zinc oxide (the counterelectrode) through a process called atomic layer deposition (ALD). A top electrode of silver paint completed the circuit.

"Essentially, you get this metal/insulator/metal structure," said Pint. "No one's ever done this with such a high-aspect-ratio material and utilizing a process like ALD."

Hauge said the new supercapacitor is stable and scaleable. "All solid-state solutions to energy storage will be intimately integrated into many future devices, including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates," he said.

Pint said the supercapacitor holds a charge under high-frequency cycling and can be naturally integrated into materials. He envisioned an electric car body that is a battery, or a microrobot with an onboard, nontoxic power supply that can be injected for therapeutic purposes into a patient's bloodstream.

Pint said it would be ideal for use under the kind of extreme conditions experienced by desert-based solar cells or in satellites, where weight is also a critical factor. "The challenge for the future of energy systems is to integrate things more efficiently. This solid-state architecture is at the cutting edge," he said.

Co-authors of the paper include graduate student Zhengzong Sun; James Tour, the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, and Howard Schmidt, adjunct assistant professor of chemical and biomolecular engineering, all of Rice; Sheng Xu, a former graduate student at Harvard; and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry at Harvard University, who developed ALD.

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Journal Reference:

Cary L. Pint, Nolan W. Nicholas, Sheng Xu, Zhengzong Sun, James M. Tour, Howard K. Schmidt, Roy G. Gordon, Robert H. Hauge. Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates. Carbon, 2011; 49 (14): 4890 DOI: 10.1016/j.carbon.2011.07.011

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Revived EcoEnergy program to advise on energy use

The federal government is reviving an energy efficiency program meant to encourage consumers and companies to cut their energy use.

Natural Resources Minister Joe Oliver is putting $78 million over two years into collecting and spreading information about buying more efficient vehicles, houses, buildings and appliances.

He says the program will assess existing technology and encourage higher energy standards, leading to an anticipated reduction in emissions of four megatonnes by 2016.

The goal is to push provinces to strengthen their building codes, and to provide consumers with detailed information that will allow them to compare energy-efficient products to more conventional products.

The money will also help beef up energy efficiency measurements for appliances, and encourage better energy labelling and training for workers.

"These initiatives will help Canadians and Canadian businesses save money while reducing greenhouse-gas emissions," Oliver told reporters.

The funding only lasts two years because the government is facing budget constraints, Oliver said.

"We have to balance the environmental objectives with the fiscal situation."

Environmentalists welcomed the announcement, but said it was only a small fraction of what is needed to get Canadians to embrace a more energy-efficient lifestyle.

Tim Weis with the Pembina Institute said the government money will fund solid and useful information and set some higher standards, but it's a far cry from a co-ordinated national strategy with concrete targets to improve energy use.

"It's a foundation to build on," he said.

The program launched Wednesday is the latest version of the EcoEnergy efficiency initiative, a multibillion-dollar program that began in 2007 but expired last March.

In the last budget, the wildly popular home-retrofit side of the EcoEnergy program was extended for one year, with $400 million. At the same time, the government set aside an additional $86 million over two years to promote energy efficiency, but did not release details on how the money would be spent.

Wednesday's announcement explains the renewal of the smaller and less-known efficiency initiatives. Officials did not immediately respond to requests to explain why the amount of money had dropped to $78 million from $86 million.

Oliver stressed that the $78 million investment will create jobs, but added that he did not have a number for new positions.

Rather, officials explained that department would spend the money on research, analysis and gathering information which would encourage industry to invest in energy-efficient options for consumers -- indirectly creating green jobs.

"This is about job creation and a clean energy future for Canadians," Oliver said.

Ottawa has committed to a 2020 target of reducing greenhouse gas emissions by 17 per cent from 2005 levels, mainly by regulating industrial production in lock-step with the United States.

Environment Canada has attributed much of the rise in emissions over the past decade to an increase in oil-and-gas production and a surge in vans, SUVs and trucks on the road.

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UCLA polarizing filter lets LCDs harvest and recycle energy

Last updated: Friday, August 12, 2011

LCD is the predominant technology used in most of the displays we include in devices today. And while the display technology is constantly being updated to allow for better efficiency and image output, LCDs still account for the majority of energy used in portable devices. Engineers at UCLA have been working to rectify that problem,

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UCLA polarizing filter lets LCDs harvest and recycle energy

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