Solar plane sets distance record on US tour

The first manned aircraft that can fly day and night powered only by solar energy set a new distance record Thursday when it landed after the second leg of a cross-country US tour.

The Solar Impulse project, founded and led by two Swiss pilots, aims to showcase what can be accomplished without fossil fuels, and has set its “ultimate goal” as an around-the-world flight in 2015.

Solar Impulse landed in Dallas-Fort Worth, Texas at 1:08 am (0608 GMT) after an 18 hour and 21 minute flight from Phoenix, Arizona, a distance of 1,541 kilometers (950 miles), organizers said in a statement.

“This leg was particularly challenging because of fairly strong winds at the landing. It also was the longest flight — in terms of distance — ever flown by a solar airplane,” the plane’s pilot Andre Borschberg said.

“You have to understand that the pilot needs to stay awake for more than 20 hours without any autopilot,” added Borschberg, who holds the record for the longest solar-powered flight, at 26 hours.

The previous distance record was attained by Solar Impulse one year ago on a 1,116 kilometer (693 mile) flight from Switzerland to Spain.

The first leg of Solar Impulse’s US tour took place on May 3, when Swiss aviator Bertrand Piccard flew the aircraft from the San Francisco, California area to Phoenix.

On the first leg the plane — which has a slim body and four electric engines attached to enormous wings — flew quietly at an average speed of about 30 miles (49 kilometers) per hour.

Energy provided by 12,000 solar cells powered the plane’s propellers.

The plane can fly at night by reaching a high elevation of 27,000 feet (8,230 meters) and then gently gliding downward, using almost no power until the sun comes up to begin recharging the solar cells.

The US itinerary allows for up to 10 days at each stop in order to showcase the plane’s technology to the public. Another stop is planned in the US capital Washington before the trip concludes in New York in early July.

The stopovers will allow Piccard and Borschberg to share duties and rest between flights.

A dashboard showing the live speed, direction, battery status, solar generator and engine power, along with cockpit cameras of both Piccard and his view from the plane, are online at live.solarimpulse.com.

via Solar plane sets distance record on US tour | GlobalPost.

Congratulations to Captain’s Piccard and Borschberg!

Compressing air for renewable energy storage

 

Enough Northwest wind energy to power about 85,000 homes each month could be stored in porous rocks deep underground for later use, according to a new, comprehensive study. Researchers at the Department of Energy’s Pacific Northwest National Laboratory and Bonneville Power Administration identified two unique methods for this energy storage approach and two eastern Washington locations to put them into practice.

Compressed air energy storage plants could help save the region’s abundant wind power – which is often produced at night when winds are strong and energy demand is low – for later, when demand is high and power supplies are more strained. These plants can also switch between energy storage and power generation within minutes, providing flexibility to balance the region’s highly variable wind energy generation throughout the day.

“With Renewable Portfolio Standards requiring states to have as much as 20 or 30 percent of their electricity come from variable sources such as wind and the sun, compressed air energy storage plants can play a valuable role in helping manage and integrate renewable power onto the Northwest’s electric grid,” said Steve Knudsen, who managed the study for the BPA.

All compressed air energy storage plants work under the same basic premise. When power is abundant, it’s drawn from the electric grid and used to power a large air compressor, which pushes pressurized air into an underground geologic storage structure. Later, when power demand is high, the stored air is released back up to the surface, where it is heated and rushes through turbines to generate electricity. Compressed air energy storage plants can re-generate as much as 80 percent of the electricity they take in.

The world’s two existing compressed air energy storage plants – one in Alabama, the other in Germany – use man-made salt caverns to store excess electricity. The PNNL-BPA study examined a different approach: using natural, porous rock reservoirs that are deep underground to store renewable energy.

Interest in the technology has increased greatly in the past decade as utilities and others seek better ways to integrate renewable energy onto the power grid. About 13 percent, or nearly 8,600 megawatts, of the Northwest’s power supply comes from of wind. This prompted BPA and PNNL to investigate whether the technology could be used in the Northwest.

To find potential sites, the research team reviewed the Columbia Plateau Province, a thick layer of volcanic basalt rock that covers much of the region. The team looked for underground basalt reservoirs that were at least 1,500 feet deep, 30 feet thick and close to high-voltage transmission lines, among other criteria.

They then examined public data from wells drilled for gas exploration or research at the Hanford Site in southeastern Washington. Well data was plugged into PNNL’s STOMP computer model, which simulates the movement of fluids below ground, to determine how much air the various sites under consideration could reliably hold and return to the surface.

Analysis identified two particularly promising locations in eastern Washington. One location, dubbed the Columbia Hills Site, is just north of Boardman, Ore., on the Washington side of the Columbia River. The second, called the Yakima Minerals Site, is about 10 miles north of Selah, Wash., in an area called the Yakima Canyon.

But the research team determined the two sites are suitable for two very different kinds of compressed air energy storage facilities. The Columbia Hills Site could access a nearby natural gas pipeline, making it a good fit for a conventional compressed air energy facility. Such a conventional facility would burn a small

The Yakima Minerals Site, however, doesn’t have easy access to natural gas. So the research team devised a different kind of compressed air energy storage facility: one that uses geothermal energy. This hybrid facility would extract geothermal heat from deep underground to power a chiller that would cool the facility’s air compressors, making them more efficient. Geothermal energy would also re-heat the air as it returns to the surface.

“Combining geothermal energy with compressed air energy storage is a creative concept that was developed to tackle engineering issues at the Yakima Minerals Site,” said PNNL Laboratory Fellow and project leader Pete McGrail. “Our hybrid facility concept significantly expands geothermal energy beyond its traditional use as a renewable baseload power generation technology.”

The study indicates both facilities could provide energy storage during extended periods of time. This could especially help the Northwest during the spring, when sometimes there is more wind and hydroelectric power than the region can absorb. The combination of heavy runoff from melting snow and a large amount of wind, which often blows at night when demand for electricity is low, can spike power production in the region. …

via Not just blowing in the wind: Compressing air for renewable energy storage.

The solar-powered plane Solar Impulse prepares for flight around the world

The solar-powered plane Solar Impulse is preparing for a journey around the world scheduled to begin on May 1. It is powered by about 12,000 photovoltaic cells that cover its massive wings. They allow it to charge its batteries and enable it to fly day and night without jet fuel. Above, the Solar Impulse glides over the Golden Gate Bridge in San Francisco.

via The solar-powered plane Solar Impulse prepares for flight around the world – Telegraph.

‘Artificial leaf’ gains the ability to self-heal damage and produce energy from dirty water

Another innovative feature has been added to the world’s first practical “artificial leaf,” making the device even more suitable for providing people in developing countries and remote areas with electricity, scientists reported here today. It gives the leaf the ability to self-heal damage that occurs during production of energy.

Daniel G. Nocera, Ph.D., described the advance during the “Kavli Foundation Innovations in Chemistry Lecture” at the 245th National Meeting & Exposition of the American Chemical Society.

Nocera, leader of the research team, explained that the “leaf” mimics the ability of real leaves to produce energy from sunlight and water. The device, however, actually is a simple catalyst-coated wafer of silicon, rather than a complicated reproduction of the photosynthesis mechanism in real leaves. Dropped into a jar of water and exposed to sunlight, catalysts in the device break water down into its components, hydrogen and oxygen. Those gases bubble up and can be collected and used as fuel to produce electricity in fuel cells.

“Surprisingly, some of the catalysts we’ve developed for use in the artificial leaf device actually heal themselves,” Nocera said. “They are a kind of ‘living catalyst.’ This is an important innovation that eases one of the concerns about initial use of the leaf in developing countries and other remote areas.”

Nocera, who is the Patterson Rockwood Professor of Energy at Harvard University, explained that the artificial leaf likely would find its first uses in providing “personalized” electricity to individual homes in areas that lack traditional electric power generating stations and electric transmission lines. Less than one quart of drinking water, for instance, would be enough to provide about 100 watts of electricity 24 hours a day. Earlier versions of the leaf required pure water, because bacteria eventually formed biofilms on the leaf’s surface, shutting down production.

“Self-healing enables the artificial leaf to run on the impure, bacteria-contaminated water found in nature,” Nocera said. “We figured out a way to tweak the conditions so that part of the catalyst falls apart, denying bacteria the smooth surface needed to form a biofilm. Then the catalyst can heal and re-assemble.” …

via ‘Artificial leaf’ gains the ability to self-heal damage and produce energy from dirty water.

New solar-cell coating could boost efficiency

Throughout decades of research on solar cells, one formula has been considered an absolute limit to the efficiency of such devices in converting sunlight into electricity: Called the Shockley-Queisser efficiency limit, it posits that the ultimate conversion efficiency can never exceed 34 percent for a single optimized semiconductor junction.

Now, researchers at MIT have shown that there is a way to blow past that limit as easily as today’s jet fighters zoom through the sound barrier — which was also once seen as an ultimate limit. …

in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.

While others have previously “split” a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels.

This was accomplished using an organic compound called pentacene in an organic solar cell. While that material’s ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.

“Our whole project was directed at showing that this splitting process was effective,” says Baldo, who is also the director of the Center for Excitonics, sponsored by the U.S. Department of Energy. “We showed that we could get through that barrier.”

The theoretical basis for this work was laid long ago, says Congreve, but nobody had been able to realize it in a real, functioning system. “In this system,” he says, “everyone knew you could, they were just waiting for someone to do it.”

Since this was just a first proof of principle, the team has not yet optimized the energy-conversion efficiency of the system, which remains less than 2 percent. But ratcheting up that efficiency through further optimization should be a straightforward process, the researchers say. “There appears to be no fundamental barrier,” Thompson says.

While today’s commercial solar panels typically have an efficiency of at most 25 percent, a silicon solar cell harnessing singlet fission should make it feasible to achieve efficiency of more than 30 percent, Baldo says — a huge leap in a field typically marked by slow, incremental progress. In solar cell research, he notes, people are striving “for an increase of a tenth of a percent.”

Solar panel efficiencies can also be improved by stacking different solar cells together, but combining solar cells is expensive with conventional solar-cell materials. The new technology instead promises to work as an inexpensive coating on solar cells.

The work made use of a known material, but the team is now exploring new materials that might perform the same trick even better. “The field is working on materials that were chanced upon,” Baldo says — but now that the principles are better understood, researchers can begin exploring possible alternatives in a more systematic way. …

via New solar-cell coating could boost efficiency.

Power Your Car With Pee

… A scientist at Ohio University has developed a catalyst capable of extracting hydrogen from urine. That’s right. Urine. Now you can fill one tank while draining another.

Gerardine Botte claims the device uses significantly less energy than is needed to extract hydrogen from water and says it could power hydrogen fuel cell vehicles in the near future. Her electrolyzer uses a nickel-based electrode to extract hydrogen from urea (NH2)2CO, the main component in urine. Hydrogen is less tightly bound to the nitrogen in urea than to the oxygen in water, so the electrolyzer needs just 0.37 volts across the cell to oxidize the urea, according to Botte. That’s less than half the amount of energy in an AA battery and considerably less than the 1.23 volts needed to split water.

One of hydrogen’s biggest stumbling blocks to use as an alternative fuel is the amount of energy needed to produce it. And then there’s the matter of distributing it. Botte says her gadget eliminates such problems because it’s small enough to integrate into an automobile. Urine is also readily available — your body produces two to three liters of it each day, and it is the most abundant form of waste on the planet. We could treat waste water while fueling our cars.

“Urea is the same stuff we use to fertilize our flower beds. It’s a solid that dissolves in water and is therefore easy to move,” Botte told Wired.com. “An electrolyzer built into a car would eliminate the need for a hydrogen storage tank, and with the right partnership, I believe we could have pee-powered cars capable of 60 miles per gallon on the road within a year.”

Botte’s current electrolyzer prototype is about the size of a pair of CD jewel cases and can produce up to 500 milliwatts of power. That’s pretty small, but Ohio University has patented the technology and Botte says it could be scaled up to power hybrid and electric vehicles or anything else running on electricity.

“We are currently working on the chemistry of the electrolyzer,” she said. “The next step is the engineering, which should flow just fine. It would involve increasing the size of the electrolyzer, making it more efficient and testing its long-term stability.”

She says the cost of developing the technology for conventional cars would all depend on what’s powering the car. The electrolyzer would have to pull energy from a power source like a battery in order to produce hydrogen for a fuel cell. Botte also is examining how the electrolyzer could draw the power it needs from a solar panel. Hooking it up to a rooftop solar panel — like the one on the 2010 Toyota Prius — could increase efficiency as much as 40 percent, she said.

Botte hasn’t gotten much in the way of federal funding for the project, though she is working with the Department of Defense to develop electrolyzer technology for military use. …

via Power Your Car With Pee | Autopia | Wired.com

This story is from four years ago. Where is this technology now? I want to fill my Prius tank with something free.

… The utilization of wastewater for useful fuel has been gathering recent attention due to society’s need for alternative energy sources. The electrooxidation of urea found at high concentrations in wastewater simultaneously accomplishes fuel production and remediation of harmful nitrogen compounds that currently make their way into the atmosphere and groundwater. Pure hydrogen was collected in the cathode compartment at 1.4 V cell potential, where water electrolysis does not occur appreciably. It was determined that an inexpensive nickel catalyst is the most active and stable for the process.

Urine is the most abundant waste on Earth. The largest constituent of urine is urea, which is a significant organic source of H, C, O, and N. Despite the numerous benefits of using urea/urine for hydrogen production, there is not a single technology that directly converts urea to hydrogen.

In addition to sustaining hydrogen resources, such a process could denitrificate urea-rich water that is commonly purged into rivers, creeks, and tributaries from municipal wastewater treatment plants. Currently, nitrate concentration in these waters is regulated at 10 mg L, but available denitrification technologies are expensive and inefficient.

Converting urea to valuable products before it naturally hydrolyzes to ammonia, which generates gas-phase ammonia emissions and contributes to ammonium sulfate and nitrate formation in the atmosphere, will save billions of dollars spent each year on health costs. …

http://www.suttonfruit.com/pics/urea_electrolysis.pdf

Here’s the method the girls in Africa used to make a urine generator:

• Urine is put into an electrolytic cell, which cracks the urea into nitrogen, water, and hydrogen.
• The hydrogen goes into a water filter for purification, which then gets pushed into the gas cylinder.
• The gas cylinder pushes hydrogen into a cylinder of liquid borax, which is used to remove the moisture from the hydrogen gas.
• This purified hydrogen gas is pushed into the generator.
• 1 Liter of urine gives you 6 hours of electricity.

Where’s the web page showing me how to build one of these? I guess I’ll have to make it from all the clues I can fine. I doubt I could make one to power a car, but perhaps I could power a radio, or even a laptop. Additional info of use:

… The nickel electrode is reusable, but what happens in a lot of electrochemical processes is that the surface (thin top layer) of the electrode gets used up. While [it] is possible to keep polishing the surface and using the metal over and over for a very long time, the rate at which the surface requires treatment would be one of the parameters in determining how feasible this process is.

.. Combined with the chicken feather storage tanks which can allow enough non pressurized hydrogen for a normal sized car to be stored and costs around $250 to make we have a very viable solution.

… Potentiostat to control voltage Nickel electrode (piece of nickel connected to a copper wire Counter electrode (typically a platinum wire connected to a copper wire) Reference electrode (typically silver/silver chloride) You can buy … these parts at your … electrochemical dealer such as Princeton Applied Research, Bio-Logic, Gamry Instruments [just Google potentiostat]  – phys.org

Nano-sized nickel with primary particle size of 2–3nm has been successfully prepared for use as efficient anode catalysts in urea and urine fuel cells. XRD, SEM and TEM were used for characterisation of nano-sized nickel. Based on the previous communication, the performance of urea and urine fuel cells has been further improved when the relative humidity at the cathode was 100%. A maximum power density of 14.2mWcm⁻² was achieved when 1M urea was used as fuel, humidified air as oxidant. The performance of urine fuel cells operating above room temperature was also reported for the first time and a power density of 4.23mWcm⁻² was achieved at 60°C indicating potential application in urea-rich waste water treatment.  – link

Fuel cell electrodes made of nickel are 20 percent cheaper than those made of platinum. Since platinum counts for about one third of the cost of a fuel cell, replacing it with something 20% cheaper could lower the entire fuel cell’s price by a significant amount. – link

Microalgae produce more oil faster for energy, food or products

Scientists have described technology that accelerates microalgae’s ability to produce many different types of renewable oils for fuels, chemicals, foods and personal-care products within days using standard industrial fermentation.

The presentation was part of the 245th National Meeting & Exposition of the American Chemical Society (ACS) on April 7.

Walter Rakitsky, Ph.D, explained that microalgae are the original oil producers on earth, and that all of the oil-producing machinery present in higher plants resides within these single-cell organisms. Solazyme’s breakthrough biotechnology platform unlocks the power of microalgae, achieving over 80 percent oil within each individual cell at commercial scale while changing the triglyceride oil paradigm by their ability to tailor the oil profiles by carbon chain and saturation. The ability to produce multiple oils in a matter of days out of one plant location using standard industrial fermentation is a game-changer. Solazyme’s patented microalgae strains have become the workhorses of a growing industry focused on producing commercial quantities of microalgal oil for energy and food applications. Rakitsky is with Solazyme, Inc., of South San Francisco, Calif., one of the largest and most successful of those companies, which in 2011 supplied 100 percent microalgal-derived advanced biofuel for the first U.S. passenger jetliner flight powered by advanced biofuel.

In a keynote talk at the ACS meeting, Rakitsky described Solazyme’s technology platform that enables the company to produce multiple oils from heart-healthy high-oleic oils for food to oils that are tailored to have specific performance and functionality benefits in industry, such as safer dielectric fluids and oils that are the highest-value cuts of the barrel for advanced fuels. The benefits of these oils far surpass those of other oils that are currently available today.

“For the first time in history, we have unlocked the ability to completely design and tailor oils,” he said. “This breakthrough allows us to create oils optimized for everything from high-performance jet and diesel fuel to renewable chemicals to skin-care products and heart-healthy food oils. These oils could replace or enhance the properties of oils derived from the world’s three dominant sources: petroleum, plants and animals.”

Producing custom-tailored oils starts with optimizing the algae to produce the right kind of oil, and from there, the flexibility of the fermentation platform really comes into play. Solazyme is able to produce all of these oils in one location simply by switching out the strain of microalgae they use, Rakitsky explained. Unlike other algal oil production processes, in which algae grow in open ponds, Solazyme grows microalgae in total darkness in the same kind of fermentation vats used to produce vinegar, medicines and scores of other products. Instead of sunlight, energy for the microalgae’s growth comes from low-cost, plant-based sugars. This gives the company a completely consistent, repeatable industrial process to produce tailored oil at scale.

Sugar from traditional sources such as sugarcane and corn has advantages for growing microalgae, especially their abundance and relatively low cost, Rakitsky said. The company’s first fit-for-purpose commercial-scale production plant is under construction with their partner Bunge next to a sugarcane mill in Brazil. Initial production capacity will be 110,000 tons of microalgal oil annually, expanding up to 330,700 tons. In addition, the company has a production agreement with ADM in Clinton, Iowa, for 22,000 tons of oil, expandable to 110,000 tons. Ultimately, cellulosic sources of sugars from non-food plants or plant waste materials, like grasses or corn stover, may take over as those technologies reach the right scale and cost structures. … Read more

This is from Feb 22, 2008:

It’s not the first to turn to algae and biomass as a source of fuel, but upstart Solazyme seems to think it’s got a leg up on other biofuel makers and its apparently lining up the deals and big bucks to prove it. As Technology Review reports, that includes Chevron, which is now in a “testing agreement” with the start-up, and the National Institute of Standards and Technology, which dished out a $2 million grant to the company. The trick that’s attracted all that interest, it seems, is the company’s particular way of using algae to convert biomass into fuel, which takes the apparently unorthodox approach of growing them in the dark, which causes them to produce more oil than they do in the light. What’s more, Solazyme’s method also apparently allows them to use different strains of algae to produce different types of oil, including a mix of hydrocarbons that’s similar to light crude petroleum. Needless to say, all of this is still quite a ways away from finding its way into your car’s tank, but the company has demonstrated its algae-based fuel in a diesel car, so it’s at least moved beyond the lab. …link

1 Killed, 3 Hurt Accident at Ark. Nuclear Plant

One worker died and three others were injured Sunday morning when a heavy piece of equipment fell on them at an Arkansas nuclear plant, officials said.

The workers were moving the equipment from the turbine building at an Entergy Arkansas plant in Russellville, about 70 miles northwest of Little Rock. The three injured workers were taken to a hospital. The company stressed that there is no danger to the public.

The Arkansas Nuclear One plant provides about 30 percent of the state’s energy demand, according to Entergy.

“We are deeply saddened by what has happened today,” said Jeff Forbes, Entergy’s chief nuclear officer. “Our greatest sympathy is with the family and friends of the employee who lost his life, and with those who sustained injuries.”

Entergy’s two nuclear reactors were shut down after the accident.

The plant was placed under an “unusual event classification,” which is the lowest of four emergency classifications designated by the Nuclear Regulatory Commission.

Entergy provides electricity to 2.8 million customers in Arkansas, Louisiana, Mississippi and Texas.

via 1 Killed, 3 Hurt Accident at Ark. Nuclear Plant – ABC News.

New type of solar structure cools buildings in full sunlight

Homes and buildings chilled without air conditioners. Car interiors that don’t heat up in the summer sun. Tapping the frigid expanses of outer space to cool the planet. Science fiction, you say? Well, maybe not any more.

A team of researchers at Stanford has designed an entirely new form of cooling structure that cools even when the sun is shining. Such a structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into the chilly vacuum of space. Their paper describing the device was published March 5 in Nano Letters.

“People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place,” explained Shanhui Fan, professor of electrical engineering and the paper’s senior author. “We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the day time.”

The trick, from an engineering standpoint, is two-fold. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the purpose of cooling.

The second challenge is that the structure must efficiently radiate heat back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, Earth’s atmosphere simply reflects the light back down. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect—the cause of global climate change.

Two goals in one

The new structure accomplishes both goals. It is an effective a broadband mirror for solar light—it reflects most of the sunlight. It also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.

Radiative cooling at nighttime has been studied extensively as a mitigation strategy for climate change, yet peak demand for cooling occurs in the daytime.

“No one had yet been able to surmount the challenges of daytime radiative cooling—of cooling when the sun is shining,” said Eden Rephaeli, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “It’s a big hurdle.”

The Stanford team has succeeded where others have come up short by turning to nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.

“We’ve taken a very different approach compared to previous efforts in this field,” said Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”

Using engineered nanophotonic materials the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs, while it radiates heat very efficiently in the key frequency range necessary to escape Earth’s atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.

The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today’s standard 10-percent-efficient solar panels generate the about the same amount of power. That means Fan’s radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.

To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.

Radiative cooling has another profound advantage over all other cooling strategy such as air-conditioner. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain. You put it on the roof or the sides of buildings and it starts working immediately. …

via New type of solar structure cools buildings in full sunlight.

Cool! Cooling!

Sodium-air battery offers rechargeable advantages compared to Li-air batteries

Over the past few years, Li-air batteries (more precisely, Li-oxygen batteries) have become attractive due to their theoretical ability to store nearly as much energy per volume as gasoline. The key to this high energy density is the “air” part, since the batteries capture atmospheric oxygen to use in the cathode reaction instead of storing their own oxidizing agent. However, Li-air batteries have conventionally been single-use cells since they cannot be recharged, which significantly limits their applications. Now in a new study, scientists have found that replacing the lithium anode with a sodium anode may offer an unexpected path toward making metal-air batteries rechargeable while still offering a relatively high energy density.

The researchers, led by Professor Jürgen Janek and Dr. Philipp Adelhelm at the Institute of Physical Chemistry at Justus-Liebig-University Gießen in Gießen, Germany, have published their paper on rechargeable sodium-air batteries in a recent issue of Nature Materials.

Li-air batteries’ greatest appeal is their high theoretical energy density (about 3,458 Wh kg-1), which is several times higher than that of Li-ion batteries, the most commonly used battery inelectric vehicles today. However, whereas Li-ion batteries can be recharged many times while retaining most of their capacity, most Li-air batteries cannot be recharged at all. In 2011, several international groups discovered that this irreversibility is due to the instability of the Li-air battery’s electrolyte and other cell components in the presence of the reactive superoxide radical O2-, which forms as a first step during cell discharge. Only recently, improvements in rechargeability have been achieved by using gold electrodes, but the system still suffers from poor energy efficiency and large overpotentials in which some energy is lost as heat.

In the new study, the researchers demonstrated that a sodium-air (Na-air) cell does not suffer from the same negative effects on the electrolyte and energy efficiency as a Li-air cell does. This is because, while lithium and sodium are closely related chemically, they each react very differently with oxygen. When lithium reacts with oxygen, it forms LiO2 (lithium oxide), which is highly unstable and found only as an intermediate species in Li-air batteries, after which it turns into Li2O2. On the other hand, sodium and oxygen form NaO2 (sodium superoxide), a more stable compound. Since NaO2doesn’t decompose, the reaction can be reversed during charging.

(Left) When a metal-oxygen battery is discharged, metal A (e.g., lithium or sodium) is oxidized at the anode/electrolyte interface, and the resulting electron is transferred to the outer circuit. At the cathode, oxygen is reduced to a superoxide species that may form a metal superoxide in the presence of the oxidized metal A. (Right) The metal superoxide in a Li-oxygen cell is highly unstable and reacts further. (Center) The metal superoxide in a Na-oxygen cell is much more stable and doesn’t decompose further, allowing the reaction to be reversed. Image credit: P. Hartmann, et al. ©2012 Macmillan Publishers Limited.

The scientists demonstrated the reversibility of Na-air cells in their experiments. Using several techniques, including Raman spectroscopy and X-ray diffraction, the scientists confirmed that NaO2 is indeed produced during discharge, that Na and O2are separated during charging, and that the cycle can be repeated.

“We could demonstrate that by replacing lithium with sodium, the cell reaction proceeds in an unexpected and beneficial manner,” Adelhelm toldPhys.org. “The cell discharge and charge process is kinetically favored, which means that the formation and decomposition of NaO2 is very energy-efficient.”

Although this evidence of reversibility is a promising step, the reaction is far from ideal. While Na-air batteries have a theoretical energy density of 1,605 Wh kg-1, which is significantly higher than that of Li-ion batteries, it is still only about half that of Li-air batteries. And even though the Na-air batteries can be charged and discharged several times, the capacity decreases after each cycle, with negligible energy storage after eight cycles. The researchers are currently investigating the processes that limit battery lifetime.

Still, Na-air batteries have some attractive characteristics. One advantage of the Na-air battery demonstrated here is its very low overpotential, which is three or four times lower than for any Li-air or Na-air battery previously reported, resulting in fewer losses. In addition, sodium is the sixth most abundant element on Earth, while lithium resources are much more limited.

“Our results are also important from another perspective,” Adelhelm said. “NaO2 is chemically very difficult to synthesize. High temperatures, pressures and long reaction times are needed. In our battery, NaO2 forms instantly at room temperature and ambient pressure. Possibly, also other chemical compounds could be prepared this way.”

Overall, Adelhelm hopes that Na-air batteries may serve as one more option to turn to for future energy storage applications.

“A broad variety of electrochemical energy storagedevices with different properties is needed for future mobile and stationary applications,” he said. “In short, sodium’s abundance could be an important cost advantage over lithium; however, for otherwise identical batteries, the ‘lithium version’ will always provide the higher energy density. But any working metal/air battery will provide a higher energy density than current Li-ion batteries.”
…
Via PhysOrg