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Cheap Solar Cells Offer Hydrogen Hope

Perovskite solar cells have made remarkable progress in the past few years, and the rising efficiency of the low-cost devices has begun to rival conventional silicon cells. Now perovskite photovoltaics have been used to split water to produce hydrogen, offering a promising route to the clean-burning fuel.

Hydrogen is very slowly gaining ground as a transportation fuel, but it has a dirty secret: most of the world’s supply of the gas is made via reactions with methane and steam at high temperatures, releasing the greenhouse gas carbon dioxide in the process. Developing a less-polluting source of hydrogen could help to bolster its green credentials and speed its adoption. Using electricity from renewable sources to split water by electrolysis, however, is far too expensive today.

That’s partly because it takes a voltage of at least 1.23 V to split water, with commercial systems running at about 1.8 V to 2.0 V. Conventional silicon or cadmium-telluride solar cells cannot deliver that voltage, because their bandgap is not wide enough. So three or four cells must be connected in series to reach the electrolysis threshold. At large scales, such systems are therefore uneconomical.

Enter the clean energy savior du jour: perovskites, which have a wide band gap, enabling each cell to produce a relatively beefy voltage of up to 1.5 V. What’s more, they rely on a cheap and easy-to-manufacture light-absorbing layer such as methyl ammonium lead iodide. (The cell’s name refers to this material’s crystal structure). And since 2009, improvements in the chemistry and design of the cells’ various  layers have pushed their efficiency from just 3.8 percent to a whopping 17.9 percent, with some labs reporting unconfirmed results up to 19 percent.

Michael Grätzel at the Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland, one of the pioneers of perovskite cells, has now shown, along with his colleagues, that using just two cells in parallel is enough to start the hydrogen fizzing out of water. Their research was published this week in Science.

His team connected the cells to cheap, efficient electrocatalysts of their own design that use the solar power to split water. One electrode delivers electrons to water, splitting the molecules into hydrogen gas and hydroxide anions. The other electrode uses those hydroxide anions to produce oxygen gas and release more electrons back to the circuit.

Previous electrocatalysts like platinum and iridium are cursed by high costs, and cheaper materials that contain cobalt or molybdenum are finicky. Electrolysis systems generally need the water to be spiced with acid or base to help the current flow, but here's the rub: Electrocatalysts that are good at generating hydrogen are generally only stable if the water is acidic, while those adept at generating oxygen prefer basic conditions.

Grätzel’s system builds on previous work with an electrode made of  iron and nickel. His team coated porous nickel “foam” with iron and nickel hydroxide, and tested the electrodes in a basic solution of sodium hydroxide. They found that the electrodes generated oxygen more effectively than a platinum-nickel electrode, and were almost as good at generating hydrogen— quite a surprise, given that this reaction usually runs far more slowly in basic solution. “The real advance is being able to use this less-than-ideal catalyst, because the voltage of the perovskite cell is so high,” says Thomas Hamann at Michigan State University in East Lansing, who wrote a commentary on the work in Science.

In the researchers’ prototype system, a pair of perovskite cells converted light into electricity with an efficiency of 15.7 percent, and generated a healthy 2.0 V. That was more than enough to make hydrogen and oxygen stream from the electrodes. Measuring the rate of gas production, Grätzel’s team calculated that the overall solar-to-hydrogen conversion efficiency was 12.3 percent. That matches the performance of a gallium-indium-phosphide/gallium-arsenide tandem cell linked to platinum catalytic electrodes—but at a much lower cost, says Hamann. Indeed, of all similar systems that use cheap, abundant materials, nothing has topped 10 percent efficiency, he adds.

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Video: EPFL

As perovskite cells continue to improve, Grätzel’s team reckons that the overall solar-to-hydrogen conversion efficiency could rise to 15 percent. That’s in line with the trajectory of the US Department of Energy's goals for solar hydrogen production: 15 percent efficiency by 2015, 20 percent efficiency by 2020, and significant cuts to the cost of the system’s components.

If those targets could be met, hydrogen might also be used to bank excess energy generated by solar power or wind turbines and be deployed when these facilities idle or when demand surges. In theory, storing energy in the form of free hydrogen could be more convenient than pumped hydropower, and a lot cheaper than batteries.

There is one major drawback: Grätzel’s perovskite cells degraded after only a few hours. “Stability of the perovskites is a known issue,” says Jingshan Luo, part of Grätzel’s team at EPFL. But he points out that other researchers have recently shown that perovskite solar cells can be made much more resistant to moisture or heat, and in some cases, made to operate for more than 1000 hours without any loss in performance.

If the stability issues can be conquered, Hamann suggests that there are other ways to improve the performance of the system. Rather than using two perovskite cells, one of them could be placed on top of a silicon cell. Since silicon has a smaller band gap, it would absorb light from the red end of the spectrum that currently streams through the semi-transparent perovskite cell. This could halve the area of solar panels needed, increase the overall efficiency of the solar energy conversion, and boost hydrogen production. “That’s exactly what I’m doing now!” says Luo excitedly.

This strategy comes with a cost, however: It would lower the overall voltage of the tandem cell, requiring a more efficient hydrogen-evolving electrode. Luo is working on this, too, and says he and his colleagues are making good progress: “Most perovskite groups are focused on the cells, and they’re not experts in solar fuel generation,” he says. “We’re doing research on both sides.”

Solar Concentrator Borrows Water Cooling from IBM Supercomputer

Flat photovoltaic panels have come to dominate the global solar market thanks to a dramatic drop in panel prices over the past five years. A Swiss start-up, using technology that IBM Research–Zurich developed for one of its supercomputers, is challenging the status quo with a dish-shaped solar concentrator that produces both electricity and hot water.

The start-up, Airlight Energy, developed the dish and has created a spin-off company—called Dsolar for “dish solar”—and plans to release the concentrator in 2017. It will be targeted at off-grid communities in areas like deserts that get a lot of direct sunlight but want for both electricity and hot water. In developed countries, the 10-meter-high concentrator can be used for on-site power generation at corporate campuses or hotels, says Ilaria Besozzi, business development manager at Airlight Energy.

The parabolic dish, which automatically reorients to track the sun throughout the day, is made up of 36 elliptical reflectors that concentrate light onto very efficient, multijunction solar cells to produce electricity. These types of cells use materials tuned for different wavelengths and in this device will be able to convert about 30 percent of sunlight into current, says IBM.

Typically, the solar cells exposed to concentrated sunlight are air cooled. Airlight Energy hopes to increase the electrical output—and produce hot water—by using liquid cooling. Behind each set of solar cells is a structure that houses IBM’s water-cooling technology, which is now used in its SuperMUC supercomputer in Germany. Water flows through a network of small tubes, or “microchannels,” etched into a layer of silicon to wick away the heat from the solar cells, says Bruno Michel, the manager of advanced thermal packaging at IBM Research–Zurich. In supercomputers, the liquid coolers are attached directly to the processors, where most of the heat is generated, says Michel.

Because the dish uses active cooling, the solar cells should be able to withstand a very high concentration of light—the equivalent of 2,000 suns—and still operate for 25 years. The higher concentration has the side effect of heating the cooling water to about 90 ºC. At that temperature, the water can be used to power desalination systems or, oddly enough, a particular type of heat-driven cooling system, Michel says. In full sunlight, the dish is designed to generate 12 kilowatts of electricity and 20 kW of heat.

Concentrated photovoltaic (CPV) systems have been around for decades, and the technology offers, at least in theory, a number of advantages. They can generate more power in a given amount of space and offer higher conversion efficiency than flat panels can. In fact, by generating both heat and electric power, Airlight Energy says it converts 80 percent of sunlight into usable energy. But the cost of CPV technology and its complexity have kept it from becoming commonplace with utilities.

To keep costs low, Airlight Energy is using two materials not normally used in solar concentrators. Rather than have glass mirrors to concentrate light, the mirrors will be made of the same thin plastic foil used to wrap Swiss chocolates, says Besozzi. The main structure of the dish will be made of a specialized concrete, which can be precisely molded, doesn’t shrink, and costs far less than metals or plastic building materials, Airlight Energy says.

Engineering a system to simultaneously generate electricity and process the thermal energy will be difficult, says Sarah Kurtz, principal scientist at the U.S. National Renewable Energy Laboratory. For example, if the heat-driven cooling fails, the dish needs to quickly steer away from the sun or transfer the heat in another way. Using thin-film mirrors lowers the cost, but first-of-a-kind systems are typically expensive. “The challenge is to reduce the cost of the other components, but at 2,000 times concentration, this is feasible,” she says. Having IBM’s technology behind the product gives it commercial credibility, notes Matthew Feinstein, an analyst at Lux Research, in Boston. Traditionally, CPV systems were targeted at utilities, but many industrial companies, such as data center operators and manufacturers, can easily make use of the solar dish’s thermal energy, he adds. Still, “commercialization and early adoption have always been a challenge for unique solar technologies,” he says.

This article was updated on 16 October 2014.

Getting Power to 12 Billion People

With 7.5 billion inhabitants and growing, Earth is speeding towards the end of its ability to sustain humans. Yet it is still a surprise to most that a new study predicts that the world’s human population could grow to as high as 12.3 billion by 2100—2 billion more than many experts' previous projections.

“We found there’s a 70 percent probability the world population will not stabilize this century,” statistician Adrian Raftery says. “Population, which had sort of fallen off the world’s agenda, remains a very important issue.”

Raftery, a professor of statistics and sociology at the University of Washington and a research affiliate at the school's Center for Studies in Demography and Ecology, worked with colleagues to develop models of population growth that looked at fertility and mortality data country by country and predicted the most likely population trends. The findings, published in Science last week, are part of the first U.N. population report to use Bayesian statistics, a comprehensive method of statistics that incorporates more variables to better predict outcomes.

“Previous models worked in a very deterministic way” and were filled with uncertainty, says Patrick Gerland, a statistician with the U.N. Population Division and collaborator on the study. He explains that the new models look at multiple forms of data stretching back at least 60 years—basically a summation of each country’s past experience—and use those numbers to run nearly 10,000 future population simulations for each country. The researchers determined, with an 80 percent probability, that the world population would increase to between 9.6 billion and 12.3 billion by 2100.

The research group noticed separate trends in the developing world. Fertility rates in Asia and South America have slowed and will eventually level off. The same, however, cannot be said of Africa, where birth rates have hardly slowed down. Gerland and his group calculated a 95 percent probability that by 2100, Africa could be home to as many as 5.7 billion people, a staggering figure.

The study raises concerns over how to meet the energy needs of growing populations. And like all discussions before, the answer is rife with uncertainty. “There’s no real consensus as to how that will happen,” says Raftery.

“There’s a complexity that’s very hard to predict in the long term,” says Gerland. “Not everyone in the world consumes the same energy with the same purpose.” Roger Pielke Jr., a professor of environmental studies at the University of Colorado Boulder, cites differences in diet, health, and transportation among the slew of variables that must be considered.

Currently, sub-Saharan Africa maintains sustainability because its populace consumes very little energy. But because an increased population would require more economic development, a much larger energy infrastructure would need to be built to get power to everyone. And more people enjoying a better standard of living would cause sub-Saharan Africa’s energy needs to jump exponentially—well before it caused a reduction in fertility rates and would help stabilize population growth. According to a 2011 report (pdf) by Morgan Bazilian of the Joint Institute for Strategic Energy Analysis, “sub-Saharan Africa would need to increase its installed electricity capacity by 33 times to reach the level of energy use enjoyed by South Africans—and 100 times to reach that of Americans.”

Pielke thinks China may actually be an illustration of how African countries and developing nations elsewhere could accommodate the energy needs of a booming population. However, China's success at providing for its residents' energy needs has come at a tremendous environmental cost, especially due to the use of air-fouling coal-fired power plants. Pielke cautions that careful discussion is needed before policy makers decide how best to meet their countries' energy demands.

That discussion will have to come soon. There will be a great need for better access to energy in the future, and “there are a lot of challenges to face,” says Gerland. 

Nuclear Shutdowns Put Belgians and Britons on Blackout Alert

A bad year for nuclear power producers has Belgians and Britons shivering more vigorously as summer heat fades into fall. Multiple reactor shutdowns in both countries have heightened concern about the security of power supplies when demand spikes this winter.

In Belgium, rolling blackouts are already part of this winter's forecast because three of the country's largest reactors—reactors that normally provide one-quarter of Belgian electricity—are shut down.

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"Someday" is Now for Solar and Wind Power, says Lazard

Large wind and solar power farms have the economics to go toe-to-toe with the cheapest fossil fuel-based power supplies in the United States according to the venerable financial advisory firm Lazard Ltd. Thanks to falling costs and rising efficiency, reports Lazard in an analysis released this week, utility-scale installations of solar panels and wind turbines now produce power at a cost that's competitive with natural gas and coal-fired generating stations—even without subsidies.  

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Wave Power's Uncertain Future

Buried in wave power device company Ocean Power Technologies' latest quarterly report and press release is a seemingly backwards move: one major project in Australia has been terminated, while another signature project in Oregon is in the process of "winding down." A company determined to build commercial-scale wave power facilities is apparently not interested, at least at the present moment, in building commercial-scale wave power facilities. Instead, OPT will focus their efforts on "next generation designs," an idea that experts say is probably the only way to go at the moment.

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Japan One Step Closer to Nuclear Restart

Japan's Nuclear Regulation Authority granted approval on Wednesday for a restart of the nuclear reactors at the Sendai plant on the southern island of Kyushu, potentially paving the way for the first such plants to come back online after the Fukushima disaster in March 2011.

The agency issued a release saying that the Sendai plant's units 1 and 2 "were deemed to meet the NRA's new regulatory requirements," which they claim are among the most stringent in the world. The approval comes after a review of 18,600 pages of documentation from the plant's owner, Kyushu Electric Power, and apparent safety measure improvements put in place after the earthquake and tsunami at Fukushima.

The country's 48 nuclear reactors have all been quiet since the crisis began, and the government has waffled on just what part nuclear power will play in the electricity mix in the future. Though the NRA has issued this approval for Sendai, the New York Times reports that it will likely be months before an actual restart of the reactors can take place. Local governments near the plant must issue approvals before the reactors can be switched on, and prime minister Shinzo Abe will be responsible for a final approval.

The road to this point has been rocky for Japan. In September 2012, the country essentially committed to doing away with nuclear power. Only a few months later, though, the newly formed NRA issued new safety regulations that would allow the shuttered plants to restart. Those rules went into effect by last summer, which paved the way for the Sendai decision this week.

The backdrop to all this is that the public is apparently still quite skeptical of nuclear power in a country where earthquakes, volcanic eruptions, and tsunamis aren't likely to disappear any time soon. The Sendai plant, for example, sits particularly close to the site of a current volcano warning from the Japan Meteorological Agency ("Do not approach the volcano"). Though most of Japan fits this description, the island of Kyushu is indicated as particularly high risk on the USGS seismic hazard maps.

The Times reports that public opinion polls are still sour on nuclear power; the NRA reviewed more than 17,000 comments submitted on Sendai's safety measures, far more than the agency expected to receive. Whether the public's skepticism or the government's support end up winning out for Japan's nuclear future remains to be seen.

Solar Cookers Get Hot

Of all the innovations that could markedly benefit humankind, a reliable solar cooker remains one of the most imperative. Countless people across the developing world still cook their food by burning wood or even cow dung, causing respiratory problems and severe deforestation in some regions.

The quest for a practical solar cooker has gone on for decades and produced dozens of models ranging from $70 cheapies to $400 deluxe models. But none of them have caught on significantly for use in the developing-world because they can’t store heat. Without the ability to store heat, a cooker cannot be used, for example, on cloudy days.

But now a group based at the Norwegian University of Science and Technology (NTNU) in Trondheim is reporting a breakthrough in solar cookers. They say it will lead, within a year and a half, to the production of practical solar cookers that can store enough heat during a sunny day to work for an entire additional day, even if it is cloudy. The breakthrough was recognized with a US $8,000 award from the Norwegian Ministry of Petroleum and Energy this past August.

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Report: Tesla Gigafactory a Well-Played Overreach

Tesla Motors, which this summer announced a partnership with Panasonic to build a battery “Gigafactory,” may be overreaching. A new industry analyst’s report questions whether the demand for the company's electric cars will be big enough within the next five to eight years to warrant the factory’s projected $5 billion investment. 

According to the new report, by 2020 Tesla could be facing substantial overcapacity, with too many batteries and not enough EV cars — and other applications like renewable grid energy storage — to put them in.

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How to Turn Tires Into Batteries for Electric Cars

Scrapped car tires have been used for construction material and recycled into floor mats and shoe soles. Now they might find their way back into hybrid and electric cars, but under the hood. Scientists at Oak Ridge National Laboratory have devised a method to convert tire rubber into a carbon material for lithium-ion battery anodes.

Conventional lithium anodes are made of the graphite, a natural carbon material that is mined. The Oak Ridge researchers, led by materials chemist Parans Paranthaman, have found an alternative waste source for the carbon anode.

In a paper published online in the journal RSC Advances, they outlined a straightforward process to convert waste tire rubber into nanoporous carbon. They first pulverize waste tire rubber into tiny micrometer-sized pieces, which they break down in a hot sulfuric acid bath. The resulting rubber slurry is filtered, washed and turned into a solid cake. Heating the cake in the presence of nitrogen gives a highly porous carbon black material with pores that were less than 2nm in diameter.

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