Japan's Tohoku Tsunami Created Icebergs in Antarctica

Before (left) and after (right) photos of the Sulzberger Ice Shelf illustrate the calving event associated with the Japan earthquake and resulting tsunami that occurred on March 11, 2011. The icebergs have just begun to separate in the left image. Credit: European Space Agency/Envisat

Science Daily — A NASA scientist and her colleagues were able to observe for the first time the power of an earthquake and tsunami to break off large icebergs a hemisphere away.

The birth of an iceberg can come about in any number of ways. Often, scientists will see the towering, frozen monoliths break into the polar seas and work backwards to figure out the cause.

Kelly Brunt, a cryosphere specialist at Goddard Space Flight Center, Greenbelt, Md., and colleagues were able to link the calving of icebergs from the Sulzberger Ice Shelf in Antarctica following the Tohoku Tsunami, which originated with an earthquake off the coast of Japan in March 2011. The finding, detailed in a paper published online in theJournal of Glaciology, marks the first direct observation of such a connection between tsunamis and icebergs.

So when the Tohoku Tsunami was triggered in the Pacific Ocean on March 11 this spring, Brunt and colleagues immediately looked south. All the way south. Using multiple satellite images, Brunt, Emile Okal at Northwestern University and Douglas MacAyeal at University of Chicago were able to observe new icebergs floating off to sea shortly after the sea swell of the tsunami reached Antarctica.

To put the dynamics of this event in perspective: An earthquake off the coast of Japan caused massive waves to explode out from its epicenter. Swells of water swarmed toward an ice shelf in Antarctica, 8,000 miles (13,600 km) away, and about 18 hours after the earthquake occurred, those waves broke off several chunks of ice that together equaled about two times the surface area of Manhattan. According to historical records, this particular piece of ice hadn't budged in at least 46 years before the tsunami came along.

And as all that was happening, scientists were able to watch the Antarctic ice shelves in as close to real-time as satellite imagery allows, and catch a glimpse of a new iceberg floating off into the Ross Sea.

"In the past we've had calving events where we've looked for the source. It's a reverse scenario -- we see a calving and we go looking for a source," Brunt said. "We knew right away this was one of the biggest events in recent history -- we knew there would be enough swell. And this time we had a source."

Scientists first speculated in the 1970s that repeated flexing of an ice shelf -- a floating extension of a glacier or ice sheet that sits on land -- by waves could cause icebergs to break off. Scientific papers in more recent years have used models and tide gauge measurements in an attempt to quantify the impact of sea swell on ice shelf fronts.

The swell was likely only about a foot high (30 cm) when it reached the Sulzberger shelf. But the consistency of the waves created enough stress to cause the calving. This particular stretch of floating ice shelf is about 260 feet (80 meters) thick, from its exposed surface to its submerged base.

When the earthquake happened, Okal immediately honed in on the vulnerable faces of the Antarctic continent. Using knowledge of iceberg calving and what a NOAA model showed of the tsunami's projected path across the unobstructed Pacific and Southern oceans, Okal, Brunt and MacAyeal began looking at what is called the Sulzberger Ice Shelf. The Sulzberger shelf faces Sulzberger Bay and New Zealand.

Through a fortuitous break in heavy cloud cover, Brunt spotted what appeared to be a new iceberg in MODerate Imaging Spectroradiometer (MODIS) data.

"I didn't have strong expectations either way whether we'd be able to see something," Brunt said. "The fastest imagery I could get to was from MODIS Rapid Response, but it was pretty cloudy. So I was more pessimistic that it would be too cloudy and we couldn't see anything. Then, there was literally one image where the clouds cleared, and you could see a calving event."

A closer look with synthetic aperture radar data from the European Space Agency satellite, Envisat, which can penetrate clouds, found images of two moderate-sized icebergs -- with more, smaller bergs in their wake. The largest iceberg was about four by six miles in surface area -- itself about equal to the surface area of one Manhattan. All the ice surface together about equaled two Manhattans. After looking at historical satellite imagery, the group determined the small outcropping of ice had been there since at least 1965, when it was captured by USGS aerial photography.

The proof that seismic activity can cause Antarctic iceberg calving might shed some light on our knowledge of past events, Okal said.

"In September 1868, Chilean naval officers reported an unseasonal presence of large icebergs in the southernmost Pacific Ocean, and it was later speculated that they may have calved during the great Arica earthquake and tsunami a month earlier," Okal said. "We know now that this is a most probable scenario."

MacAyeal said the event is more proof of the interconnectedness of Earth systems.

"This is an example not only of the way in which events are connected across great ranges of oceanic distance, but also how events in one kind of Earth system, i.e., the plate tectonic system, can connect with another kind of seemingly unrelated event: the calving of icebergs from Antarctica's ice sheet," MacAyeal said.

In what could be one of the more lasting observations from this whole event, the bay in front of the Sulzberger shelf was largely lacking sea ice at the time of the tsunami. Sea ice is thought to help dampen swells that might cause this kind of calving. At the time of the Sumatra tsunami in 2004, the potentially vulnerable Antarctic fronts were buffered by a lot of sea ice, Brunt said, and scientists observed no calving events that they could tie to that tsunami.

"There are theories that sea ice can protect from calving. There was no sea ice in this case," Brunt said. "It's a big chunk of ice that calved because of an earthquake 13,000 kilometers away. I think it's pretty cool."

Study of Abalone Yields New Insights Into Sexual Reproduction

Abalone live in ocean crevices, shown here at San Diego's Point Loma. (Credit: Eric Hanauer)

Science Daily  — In new research that could have implications for improving fertilization in humans and other mammals, life scientists studied interactions between individual sperm and eggs in red abalone, an ocean-dwelling snail, and made precise chemical measurements and physical models of these interactions. They are the first scientists to do so.

"If we can understand the basic physics, chemistry and biology of reproduction, then moving from one species to the next is like dotting I's and crossing T's," said the study's lead author, Richard Zimmer, a UCLA distinguished professor of ecology and evolutionary biology.By simulating the natural habitat of the abalone in the laboratory, the scientists were able to determine the conditions under which sperm-egg encounters and fertilization were most likely to occur.Red abalone live in ocean crevices and spawn year-round, with females releasing several million eggs and males releasing up to 10 billion sperm directly into the ocean, Zimmer said.

In 2002, Zimmer's research team identified a molecule called tryptophan that is released by female abalone eggs to attract sperm. Now Zimmer and Jeffrey Riffell, an assistant professor of biology at the University of Washington, report that the released tryptophan creates a plume around the egg, greatly enlarging the target area for sperm, in much the same way using a larger tennis racket increases the chances of hitting the ball. The plume increases the egg size by a factor of five, the researchers said.

In addition, the egg has to release very little tryptophan to increase its target area, Zimmer and Riffell report in the journalProceedings of the National Academy of Sciences. The research is currently online and will be in published an upcoming print edition of the journal.

"We established that less than 1 percent of an egg's tryptophan reserves are used by eggs to communicate with sperm," Zimmer said. "The egg does not want to give up tryptophan. The egg sequesters tryptophan and releases just a whiff -- just enough to attract a sperm. It's an effective, evolved trick to enhance the likelihood of an encounter between sperm and egg. There is essentially no cost to the egg for this very effective communication system."

An abalone egg also uses tryptophan for constructing an embryonic nervous system and building neurotransmitters, as well as for other purposes, Zimmer said.

But the success of the egg's plume in attracting sperm, as well as sperm motility and other elements of the fertilization process, are greatly influenced by ocean flow conditions. Therefore, the biologists analyzed the physics of fluid motion in mediating sperm-egg interactions.

"The effect is huge, and was previously unsuspected," said Zimmer, who is a member of UCLA's neuroscience program. "The physics of fluid motion has a profound consequence on the ability of sperm to navigate and find an egg, and therefore on fertilization. We have identified the principal mechanisms by which eggs and sperm communicate and interact within an environment that has fluid motion. The method by which sperm in humans search for and find an egg seems to be the same process as in abalone. Similar fluid dynamics operate whether in the turbulent ocean environment or within a mammalian reproductive tract.

"It appears the forces imposed by fluid motion have acted as selective forces in the evolution of the communication system between sperm and egg within abalone. I expect that we will be able to describe the specific environmental conditions within the human reproductive system that will maximize the likelihood of contact between sperm and egg. The physical and chemical environments are actually quite similar."

Zimmer and Riffell, who was formerly a graduate student in Zimmer's lab, conducted fieldwork at San Diego's Point Loma. They employed tiny sensors in a novel way, using Doppler acoustic technology to measure fluctuations of fluid motion in the environment where abalone naturally reproduce.

The biologists also devised an experimental technique by which they could generate the type of water flow and physical forces found in those habitats in a specialized tank. They used a computer and laser-based imaging system to study and quantify individual interactions between single sperm cells and single eggs, at long distances, away from any walls or boundaries. They are the first scientists to conduct such research.

"We now know properties of fluid motion that relate to the forces imposed at a microscopic scale on individual sperm and individual eggs as they interact," Zimmer said.

As part of the study, Zimmer and Riffell also developed mathematical models that describe the plumes of tryptophan that come off of eggs under a variety of physical conditions.

"We have theoretical models of what the plumes should look like, and images of eggs with sperm swimming around them," Zimmer said. "We mapped the empirical data on top of the theoretically predicted plumes. The question was whether sperm's attraction to eggs could be predicted from our theoretical models of the physics of what the plume looks like. The two mapped onto each other beautifully. The interaction could be predicted."

Because aspects of fluid flow in coastal ocean environments are remarkably similar to those in the human reproductive tract, the research could lead to methods to identify which human donors' sperm are the "most vigorous," with the highest probability of fertilizing an egg, Zimmer said. It may also suggest how to add fluid motion to maximize the probability of sperm fusing with an egg.

The study was funded by the National Science Foundation, the National Institutes of Health, the National Oceanic and Atmospheric Administration, and UCLA's Council on Research.

Scientists have been trying to describe interactions between microorganisms in oceans and chemical plumes for years. Zimmer and Riffell are the first to achieve the chemical measurements and develop the physical models.

Zimmer has received a new three-year federal award from the National Science Foundation for further studies in humans, abalone and sea urchins, working with Riffell and Roman Stocker, an associate professor of in civil and environmental engineering at the Massachusetts Institute of Technology. Stocker is building microscopic devices that will allow the researchers to study a wide variety of chemical and physical environmental conditions.

Zimmer, Riffell and former UCLA postdoctoral scholar Patrick Krug isolated tryptophan and determined its function.

"Sexual reproduction and fertilization are controlled to a significant degree by chemical communication," Zimmer said. "We are learning how chemical communication occurs.

Scientist Develops Virus That Targets HIV: Using a Virus to Kill a Virus

USC chemical engineering professor, Pin Wang. (Credit: Image courtesy of University of Southern California)

Science Daily — In what represents an important step toward curing HIV, a USC scientist has created a virus that hunts down HIV-infected cells.

"If you deplete all of the HIV-infected cells, you can at least partially solve the problem," said Wang, chemical engineering professor with the USC Viterbi School of Engineering.

Dr. Pin Wang's lentiviral vector latches onto HIV-infected cells, flagging them with what is called "suicide gene therapy" -- allowing drugs to later target and destroy them.

The process is analogous to the military practice of "buddy lasing" -- that is, having a soldier on the ground illuminate a target with a laser to guide a precision bombing strike from an aircraft.

Like a precision bombing raid, the lentiviral vector approach to targeting HIV has the advantage of avoiding collateral damage, keeping cells that are not infected by HIV out of harm's way. Such accuracy has not been achieved by using drugs alone, Wang said.

So far, the lentiviral vector has only been tested in culture dishes and has resulted in the destruction of about 35 percent of existing HIV cells. While that may not sound like a large percentage, if this treatment were to be used in humans, it would likely be repeated several times to maximize effectiveness.

Among the next steps will be to test the procedure in mice. While this is an important breakthrough, it is not yet a cure, Wang said.

"This is an early stage of research, but certainly it is one of the options in that direction," he said.

Wang's research, which was funded by the National Institutes of Health, appears in the July 23 issue of Virus Research

Hybrid Solar System Makes Rooftop Hydrogen

This is the hybrid system schematic. (Credit: Nico Hotz)

Science Daily  — While roofs across the world sport photovoltaic solar panels to convert sunlight into electricity, a Duke University engineer believes a novel hybrid system can wring even more useful energy out of the sun's rays.

For his analysis, Hotz compared the hybrid system to three different technologies in terms of their exergetic performance. Exergy is a way of describing how much of a given quantity of energy can theoretically be converted to useful work.Instead of systems based on standard solar panels, Duke engineer Nico Hotz proposes a hybrid option in which sunlight heats a combination of water and methanol in a maze of glass tubes on a rooftop. After two catalytic reactions, the system produces hydrogen much more efficiently than current technology without significant impurities. The resulting hydrogen can be stored and used on demand in fuel cells.

"The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter, compared to 5 to 15 percent for the conventional systems in the summer, and 2.5 to 5 percent in the winter," said Hotz, assistant professor of mechanical engineering and materials science at Duke's Pratt School of Engineering.

The paper describing the results of Hotz's analysis was named the top paper during the ASME Energy Sustainability Fuel Cell 2011 conference in Washington, D.C. Hotz recently joined the Duke faculty after completing post-graduate work at the University of California-Berkeley, where he analyzed a model of the new system. He is currently constructing one of the systems at Duke to test whether or not the theoretical efficiencies are born out experimentally.

Hotz's comparisons took place during the months of July and February in order to measure each system's performance during summer and winter months.

Like other solar-based systems, the hybrid system begins with the collection of sunlight. Then things get different. While the hybrid device might look like a traditional solar collector from the distance, it is actually a series of copper tubes coated with a thin layer of aluminum and aluminum oxide and partly filled with catalytic nanoparticles. A combination of water and methanol flows through the tubes, which are sealed in a vacuum.

"This set-up allows up to 95 percent of the sunlight to be absorbed with very little being lost as heat to the surroundings," Hotz said. "This is crucial because it permits us to achieve temperatures of well over 200 degrees Celsius within the tubes. By comparison, a standard solar collector can only heat water between 60 and 70 degrees Celsius."

Once the evaporated liquid achieves these higher temperatures, tiny amounts of a catalyst are added, which produces hydrogen. This combination of high temperature and added catalysts produces hydrogen very efficiently, Hotz said. The resulting hydrogen can then be immediately directed to a fuel cell to provide electricity to a building during the day, or compressed and stored in a tank to provide power later.

The three systems examined in the analysis were the standard photovoltaic cell which converts sunlight directly into electricity to then split water electrolytically into hydrogen and oxygen; a photocatalytic system producing hydrogen similar to Hotz's system, but simpler and not mature yet; and a system in which photovoltaic cells turn sunlight into electricity which is then stored in different types of batteries (with lithium ion being the most efficient).

"We performed a cost analysis and found that the hybrid solar-methanol is the least expensive solution, considering the total installation costs of $7,900 if designed to fulfill the requirements in summer, although this is still much more expensive than a conventional fossil fuel-fed generator," Hotz said.

Costs and efficiencies of systems can vary widely depending on location -- since the roof-mounted collectors that could provide all the building's needs in summer might not be enough for winter. A rooftop system large enough to supply all of a winter's electrical needs would produce more energy than needed in summer, so the owner could decide to shut down portions of the rooftop structure or, if possible, sell excess energy back to the grid.

"The installation costs per year including the fuel costs, and the price per amount of electricity produced, however showed that the (hybrid) solar scenarios can compete with the fossil fuel-based system to some degree," Hotz said. 'In summer, the first and third scenarios, as well as the hybrid system, are cheaper than a propane- or diesel-combusting generator."

This could be an important consideration, especially if a structure is to be located in a remote area where traditional forms of energy would be too difficult or expensive to obtain.

Hotz's research was supported by the Swiss National Science Fund. Joining him in the study were UC-Berkeley's Heng Pan and Costas Grigoropoulos, as well as Seung H. Ko of the Korea Advanced Institute of Science and Technology, Daejon.

Five Ways Apple Should Spend Its $76 Billion

Credit: Technology Review

COMPUTING

If you had more money than the U.S. government, what would you spend it on?

• BY PAUL BOUTIN

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Last week, Apple reported that it now has $76 billion in cash reserves. As many wags pointed out, that's more than the cash-strapped U.S. government has left. On Tuesday, Apple also briefly surpassed Exxon Mobil to become the world's most highly valued company, at more than $340 billion in stock-market valuation.

With tens of billions of dollars to throw around and super-high investor confidence, shouldn't Apple reinvest in some cutting-edge R&D that could make it even more successful?

Apple has already shown the value of introducing unique new technologies for its products: the iPhone's advanced touchscreen and the MacBook's one-piece aluminum case, for example. Apple has also begun bringing CPU chip design and production closer to home, giving it another technological advantage.

So it wouldn't be a stretch for Apple to spend some of its cash on bringing new technologies into existence that competitors couldn't touch. Never mind buying Hulu or some other company. Here are five ways could Apple actually invent the future, and thwart other makers of phones, tablets, and computers.

5. Color screens that work in the sunshine

As much as I love printed books, I'd much rather tote a skinny little iPad for my on-the-go reading. But here in sunny Los Angeles, I can't see the color screen when I try to read outdoors. There's no way to read a book on a tablet at the beach, or in the park.

Of course mobile displays for reading in direct sunlight are already available, such asthose on Amazon's Kindle, but they're only black-and-white, and they refresh at a painfully slow, page-turning speed. Now that a large chunk of my media consumption is in color, these displays don't cut it. Surely a daytime color display isn't impossible. With Apple's spare cash, could a breakthrough be around the corner?

4. Wireless network quality

Before Steve Jobs unveiled the original iPhone in 2007, he and his company pulled off a feat most pundits would have considered impossible: They got AT&T to change the way its voicemail system worked. Instead of forcing users to listen to all messages in order—a throwback to cassette-tape answering machines, and also good for getting customers to run up their minutes—the iPhone let its users view all messages onscreen at once and play only those they tapped.

But iPhone owners still complain bitterly about the quality of wireless service. AT&T drops calls, and Verizon won't let you make a voice call and use an Internet app at the same time. The audio quality of voice calls on any phone, through any carrier, seems to have gotten worse rather than better. If Apple could fix these issues, iPhone calls could become a premium feature rather than a joke. It might require a multi-billion-dollar investment in wireless network infrastructure, but we know who's got the money to spend.

3. Hands-free interfaces

Yes, I've figured out how to swipe at the latest version of Mac OS X. But you know what would be even better? Being able to wave at my iPhone instead of having to fumble with the keypad. Once you've tried Oblong Technology's hands-free interface, even a touchscreen seems dated.

The only problem is that Oblong's system is still too expensive for the mass market. Apple is legendary for turning Xerox's high-end mouse-and-menu workstations into affordable Macintoshes in the early 1980s. Couldn't they do the same with a hands-free interface?

2. Education

Another form of R&D: Give your products to a whole bunch of kids. In 2002, Apple began helping the state of Maine leapfrog its students ahead of wealthier states by giving Maine's schools a special deal on notebooks. Every seventh- and eighth-grader in Maine gets an Apple laptop that they can take home after school. Classrooms have wireless networks. Not only are the kids learning to use the tools they'll someday encounter in real-world jobs, but they're also being trained to prefer Apple over Windows. Apple has long focused on the educational market for both ideological and marketing reasons. Now would be a good time for a big national giveaway on MacBooks or iPads for future geniuses—and future customers.

1. Reinvent the battery

What's the biggest problem with your phone, laptop, or music player? It runs out of juice when you're nowhere near a power supply to recharge it. Even with a less thirsty CPU, energy-saving software, and premium batteries packed into as much internal space as possible, Apple's products can't hold enough power for a full day of heavy use for most customers.

Battery technology has advanced much more slowly than chips and displays. Apple's approach to product design—don't just think outside the box, replace the box entirely—could change the way mobile gadgets are powered. Is there a battery technology waiting to be discovered that blows past lithium-ion tech?

Could a new kind of battery be recharged without a special power adapter, or even without a wall socket? If my phone is about to conk out, could I get it to last a few minutes longer by shaking it? I'm fond of my Android phone, but its less-than-all-day battery life has caused me plenty of problems, and before day's end I often run down both the battery in the phone and the spare battery I carry with me. If Apple offered an iPhone that I could use in the real world for a week without a recharge, I'd switch on the spot.

Of course, what has made Apple so special for decades isn't fulfilling my wishes, but going beyond them. Dear Steve Jobs: Please bring me yet another gadget I would never have even thought of. Now more than ever, you can afford to do that.

Researchers Hack Mobile Data Communications

Affected devices: Researchers have broken the encryption used to protect GPRS, which sends data among many smart electronic devices, including the vehicle tracking system shown here.

Credit: Coban Group

COMMUNICATIONS

The encryption protecting mobile-device data transmission is permeable.

• BY ERICA NAONE

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Researchers plan to show today how to break the encryption that protects information sent over the General Packet Radio Service (GPRS), a standard commonly used to send data to and from mobile devices and from other devices such as smart meters. This breach makes it possible to listen in on data communications such as e-mail, instant messages, and Web browsing on smartphones, as well as updates from automated industrial systems.

The researchers, who will make their announcement at the Chaos Communication Camp, a hacker event taking place near Berlin, Germany, previously cracked the Global System for Mobile Communications (GSM), which is used to carry calls among 80 percent of the world's mobile phones. GPRS is an older technology that often supplements GSM, for example, when faster 3G connections are unavailable. Smartphones, including the iPhone, use GPRS when operating on Edge networks (when the network connection says "E" rather than "3G"). Phones that don't support 3G use GPRS all the time. Both GSM and GPRS are used worldwide, though in the United States, some major carriers, including Verizon and Sprint, use a competing standard.

Phones might be the most familiar devices affected by the research, says Karsten Nohl, founder of Security Research Labs, a Berlin-based research consultancy that conducted the work. But the standard is also used in some cars, automated industrial systems, and electronic toll booths. "It carries a lot of sensitive data," Nohl says.

Security researchers haven't looked at the GPRS standard much in the past, Nohl says, but since more and more devices are using GPRS, he believes the risk posed by poor security is growing.

Nohl's group found a number of problems with GPRS. First, he says, lax authentication rules could allow an attacker to set up a fake cellular base station and eavesdrop on information transmitted by users passing by. In some countries, they found that GPRS communications weren't encrypted at all. When they were encrypted, Nohl adds, the ciphers were often weak and could be either broken or decoded with relatively short keys that were easy to guess.

The group generated an optimized set of codes that an attacker could quickly use to find the key to protecting a given communication.  The attack the researchers designed against GPRS costs about 10 euros for radio equipment, Nohl says.

GPRS has not suffered very many security problems in the past, says Jukka Nurminen, a professor of data communications at Aalto University in Finland who spent 25 years at the Nokia Research Center. If the researchers have truly achieved what they claim, Nurminen says, many mobile communications could be much less secure. Depending on the mobile operator and subscription plan, some devices maintain a GPRS connection at all times, particularly those whose users access e-mail and instant message applications from their phones.

However, Nurminen adds, it might be possible to mitigate the risk by encrypting communications when they are sent, using common e-mail and Web-browsing tools. He notes that GPRS security is also affected by regulations in different countries and that some laws undermine security by requiring governments to be able to break into communications if necessary.  

The GSM Association, a London-based organization representing mobile operators, handset makers, and other industry interests, regulates GPRS as well as GSM. The organization says it is reviewing Nohl's research but has not yet learned enough to comment.

Nohl says companies will be negligent if they ignore the risks. He suggests that mobile applications take steps now to use encryption such as SSL, which already protects much of the sensitive information sent over the Internet. Nohl hopes that cellular network companies will require better authentication among devices and base stations communicating over GPRS. He also believes the ciphers used by the standard should be upgraded.

Energy Storage for Solar Power

Stored sunlight: A rendering shows BrightSource’s new thermal storage design. The two large tanks will store molten salt, which can be used to generate steam to drive a turbine.
Credit: BrightSource

ENERGY

Startup BrightSource announces a new system that could allow future solar plants to run at night.

• BY KEVIN BULLIS

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BrightSource Energy has become the latest solar thermal power company to develop a system for generating power when the sun isn't shining. The company says the technology can lower the cost of solar power and make it more reliable, helping it compete with conventional sources of electricity.

The company, based in Oakland, California, is building one of the world's largest solar thermal power plants. The 392-megawatt solar plant in Ivanpah, California, however, will not include the storage technology. Instead, BrightSource is working with utilities to determine which future projects could best benefit from storage.

Solar thermal systems use mirrors to focus sunlight, generating temperatures high enough to produce steam to drive a turbine. One of the advantages of the solar thermal approach, versus conventional photovoltaics that convert sunlight directly into electricity, is that heat can be stored cheaply and used when needed to generate electricity. In all solar thermal plants, some heat is stored in the fluids circulating through the system. This evens out any short fluctuations in sunlight and lets the plant generate electricity for some time after the sun goes down. But adding storage systems would let the plant ride out longer periods of cloud cover and generate power well into, or even throughout, the night. Such long-term storage could be needed if solar is to provide a large share of the total power supply.

BrightSource is using a variation on an approach to storage that's a decade old: heating up a molten salt—typically, a combination of sodium and potassium nitride—and then storing it in a tank. To generate electricity, the molten salt is pumped through a heat exchanger to generate steam. BrightSource CEO John Woolard says one big factor in making this technology economically attractive is the use of power towers—in which mirrors focus sunlight on a central tower—that generate higher temperatures than other solar thermal designs. That higher temperature makes it possible to store more energy using a smaller amount of molten salt. "It's a much more efficient system and much more cost effective, overall," he says.  

Storage allows a thermal power plant to run more hours in the day, so they can more quickly recover the cost of expensive steam turbines and generators. Woolard says that while a solar thermal plant without storage can generate electricity about 2,700 hours a year, BrightSource's storage system increases that to 4,300 hours. The increased output more than offsets the added cost of storage. A study from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, estimates that storage in a power tower system could cut costs per kilowatt hour by 25 to 30 percent.

At least two other companies are pairing power tower technologies with molten salt storage. Torresol Energy has built such a system at a 19.9-megawatt solar thermal power plant near Seville, Spain, and demonstrated that it can run the power plant through the night using stored heat. In the United States, Solar Reserve plans to build a power tower with molten salt storage in Riverside County, California. 

In addition to lowering costs, the storage system also improves the economics of solar thermal power by increasing the price that utilities are willing to pay for the electricity. Storage decreases the need for utilities to invest in backup power for smoothing out variations in power. Utilities will also pay a higher price for power they can count on at any given moment to make up for increases in demand. And the storage system lets the plant sell power into the evening, when power prices are higher in some locations.

Storage technology may be essential if solar thermal technology is to compete with photovoltaic solar panels, which have been coming down in price, says Mark Mehos, an NREL researcher. "BrightSource plants that don't have energy storage probably generate electricity at about the same price as a plant that uses photovoltaics," he says. "So all things being equal, they would like to be able to deliver that at a higher value."