Milky Way May Have Formed 'Inside-Out:' Gaia Provides New Insight Into Galactic Evolution

Jan. 20, 2014 — A breakthrough using data from the Gaia-ESO project has provided evidence backing up theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way's disc -- the vast collection of giant gas clouds and billions of stars that give our Galaxy its 'flying saucer' shape.

    

By tracking the fast-produced elements, specifically magnesium in this study, astronomers can determine how rapidly different parts of the Milky Way were formed. The research suggests that stars in the inner regions of the Galactic disc were the first to form, supporting ideas that our Galaxy grew from the inside-out.
Using data from the 8-m VLT in Chile, one of the world's largest telescopes, an international team of astronomers took detailed observations of stars with a wide range of ages and locations in the Galactic disc to accurately determine their 'metallicity': the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.
Immediately after the Big Bang, the Universe consisted almost entirely of hydrogen and helium, with levels of "contaminant metals" growing over time. Consequently, older stars have fewer elements in their make-up -- so have lower metallicity.
"The different chemical elements of which stars -- and we -- are made are created at different rates -- some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes," said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.
Massive stars, which have short lives and die as 'core-collapse supernovae', produce huge amounts of magnesium during their explosive death throes. This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.
The team have shown that older, 'metal-poor' stars inside the Solar Circle -- the orbit of our Sun around the centre of the Milky Way, which takes roughly 250 million years to complete -- are far more likely to have high levels of magnesium. The higher level of the element inside the Solar Circle suggests this area contained more stars that "lived fast and die young" in the past.
The stars that lie in the outer regions of the Galactic disc -- outside the Solar Circle -- are predominantly younger, both 'metal-rich' and 'metal-poor', and have surprisingly low magnesium levels compared to their metallicity.
This discovery signifies important differences in stellar evolution across the Milky Way disc, with very efficient and short star formation timescales occurring inside the Solar Circle; whereas, outside the Sun's orbit, star formation took much longer.
"We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form," said Maria Bergemann from Cambridge's Institute of Astronomy, who led the study.
"This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out."
The findings offer new insights into the assembly history of our Galaxy, and are the part of the first wave of new observations from the Gaia-ESO survey, the ground-based extension to the Gaia space mission -- launched by the European Space Agency at the end of last year -- and the first large-scale survey conducted on one the world's largest telescopes: the 8-m VLT in Paranal, Chile.
The study is published online today through the astronomical database Astro-ph, and has been submitted to the journal Astronomy and Astrophysics.
The new research also sheds further light on another much debated "double structure" in the Milky Way's disc -- the so-called 'thin' and 'thick' discs.
"The thin disc hosts spiral arms, young stars, giant molecular clouds -- all objects which are young, at least in the context of the Galaxy," explains Aldo Serenelli from the Institute of Space Sciences (Barcelona), a co-author of the study. "But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity."
During the latest research, the team found that:

• Stars in the young, 'thin' disc aged between 0 -- 8 billion years all have a similar degree of metallicity, regardless of age in that range, with many of them considered 'metal-rich'.
• There is a "steep decline" in metallicity for stars aged over 9 billion years, typical of the 'thick' disc, with no detectable 'metal-rich' stars found at all over this age.
• But stars of different ages and metallicity can be found in both discs.

"From what we now know, the Galaxy is not an 'either-or' system. You can find stars of different ages and metal content everywhere!" said Bergemann. "There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs -- that's how we know these two discs probably exist -- but they could have very different origins."
Added Gilmore: "This study provides exciting new evidence that the inner parts of the Milky Way's thick disc formed much more rapidly than did the thin disc stars, which dominate near our Solar neighbourhood."
In theory, say astronomers, the thick disc -- first proposed by Gilmore 30 years ago -- could have emerged in a variety of ways, from massive gravitational instabilities to consuming satellite galaxies in its formative years. "The Milky Way has cannibalised many small galaxies during its formation. Now, with the Gaia-ESO Survey, we can study the detailed tracers of these events, essentially dissecting the belly of the beast," said Greg Ruchti, a researcher at Lund Observatory in Sweden, who co-leads the project.

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Get Used to Heat Waves: Extreme El Nino Events to Double

Jan. 19, 2014 — Extreme weather events fueled by unusually strong El Ninos, such as the 1983 heatwave that led to the Ash Wednesday bushfires in Australia, are likely to double in number as our planet warms.

 

An international team of scientists from organizations including the ARC Centre of Excellence for Climate System Science (CoECSS), the US National Oceanic and Atmospheric Administration and CSIRO, published their findings in the journal Nature Climate Change.

"We currently experience an unusually strong El Niño event every 20 years. Our research shows this will double to one event every 10 years," said co-author, Dr Agus Santoso of CoECSS.
"El Nino events are a multi-dimensional problem, and only now are we starting to understand better how they respond to global warming," said Dr Santoso. Extreme El Niño events develop differently from standard El Ninos, which first appear in the western Pacific. Extreme El Nino's occur when sea surface temperatures exceeding 28°C develop in the normally cold and dry eastern equatorial Pacific Ocean. This different location for the origin of the temperature
increase causes massive changes in global rainfall patterns.
"The question of how global warming will change the frequency of extreme El Niño events has challenged scientists for more than 20 years," said co-author Dr Mike McPhaden of US National Oceanic and Atmospheric Administration.
"This research is the first comprehensive examination of the issue to produce robust and convincing results," said Dr McPhaden.
The impacts of extreme El Niño events extend to every continent across the globe.
The 1997-98 event alone caused $35-45 US billion in damage and claimed an estimated 23,000 human lives worldwide.
"During an extreme El Niño event countries in the western Pacific, such as Australia and Indonesia, experienced devastating droughts and wild fires, while catastrophic floods occurred in the eastern equatorial region of Ecuador and northern Peru," said lead author, CSIRO's Dr Wenju Cai
In Australia, the drought and dry conditions induced by the 1982-83 extreme El Niño preconditioned the Ash Wednesday Bushfire in southeast Australia, leading to 75 fatalities.
To achieve their results, the team examined 20 climate models that consistently simulate major rainfall reorganization during extreme El Niño events. They found a substantial increase in events from the present-day through the next 100 years as the eastern Pacific Ocean warmed in response to global warming.
"This latest research based on rainfall patterns, suggests that extreme El Niño events are likely to double in frequency as the world warms leading to direct impacts on extreme weather events worldwide."
"For Australia, this could mean summer heat waves, like that recently experienced in the south-east of the country, could get an additional boost if they coincide with extreme El Ninos," said co-author, Professor Matthew England from CoECSS.

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Egypt: Sarcophagus Leads to the Tomb of a Previously Unknown Pharaoh, from 3,600 Years Ago

Jan. 16, 2014 — Archaeologists working at the southern Egyptian site of Abydos have discovered the tomb of a previously unknown pharaoh: Woseribre Senebkay -- and the first material proof of a forgotten Abydos Dynasty, ca. 1650-1600 BC. Working in cooperation with Egypt's Supreme Council of Antiquities, a team from the Penn Museum, University of Pennsylvania, discovered king Senebkay's tomb close to a larger royal tomb, recently identified as belonging to a king Sobekhotep (probably Sobekhotep I, ca. 1780 BC) of the 13th Dynasty.

  

The discovery of pharaoh Senebkay's tomb is the culmination of work that began during the summer of 2013 when the Penn Museum team, led by Dr. Josef Wegner, Egyptian Section Associate Curator of the Penn Museum, discovered a huge 60-ton royal sarcophagus chamber at South Abydos. The sarcophagus chamber, of red quartzite quarried and transported to Abydos from Gebel Ahmar (near modern Cairo), could be dated to the late Middle Kingdom, but its owner remained unidentified. Mysteriously, the sarcophagus had been extracted from its original tomb and reused in a later tomb -- but the original royal owner remained unknown when the summer season ended.
In the last few weeks of excavations, fascinating details of a series of kings' tombs and a lost dynasty at Abydos have emerged. Archaeologists now know that the giant quartzite sarcophagus chamber derives from a royal tomb built originally for a pharaoh Sobekhotep -- probably Sobekhotep I, the first king of Egypt's 13th Dynasty. Fragments of that king's funerary stela were found just recently in front of his huge, badly robbed tomb. A group of later pharaohs (reigning about a century and a half later during Egypt's Second Intermediate Period) were reusing elements from Sobekhotep's tomb for building and equipping their own tombs. One of these kings (whose name is still unknown) had extracted and reused the quartzite sarcophagus chamber. Another king's tomb found just last week is that of the previously unknown pharaoh: Woseribre-Senebkay.
A Lost Pharaoh and a Forgotten Dynasty
The newly discovered tomb of pharaoh Senebkay dates to ca. 1650 BC during Egypt's Second Intermediate Period. The identification was made by Dr. Wegner and Kevin Cahail, Ph.D. student, Department of Near Eastern Languages and Civilizations, University of Pennsylvania. The tomb of Senebkay consists of four chambers with a decorated limestone burial chamber. The burial chamber is painted with images of the goddesses Nut, Nephthys, Selket, and Isis flanking the king's canopic shrine. Other texts name the sons of Horus and record the king's titulary and identify him as the "king of Upper and Lower Egypt, Woseribre, the son of Re, Senebkay."
Senebkay's tomb was badly plundered by ancient tomb robbers who had ripped apart the king's mummy as well as stripped the pharaoh's tomb equipment of its gilded surfaces. Nevertheless, the Penn Museum archaeologists recovered the remains of king Senebkay amidst debris of his fragmentary coffin, funerary mask, and canopic chest. Preliminary work on the king's skeleton of Senebkay by Penn graduate students Paul Verhelst and Matthew Olson (of the Department of Near Eastern Languages and Civilizations) indicates he was a man of moderate height, ca. 1.75 m (5'10), and died in his mid to late 40s.
The discovery provides significant new evidence on the political and social history of Egypt's Second Intermediate Period. The existence of an independent "Abydos Dynasty," contemporary with the 15th (Hyksos) and 16th (Theban) Dynasties, was first hypothesized by Egyptologist K. Ryholt in 1997. The discovery of pharaoh Senebkay now proves the existence of this Abydos dynasty and identifies the location of their royal necropolis at South Abydos in an area anciently called Anubis-Mountain. The kings of the Abydos Dynasty placed their burial ground adjacent to the tombs of earlier Middle Kingdom pharaohs including Senwosret III (Dynasty 12, ca. 1880-1840 BC), and Sobekhotep I (ca. 1780 BC). There is evidence for about 16 royal tombs spanning the period ca. 1650-1600 BC. Senebkay appears to be one of the earliest kings of the "Abydos Dynasty." His name may have appeared in a broken section of the famous Turin King List (a papyrus document dating to the reign of Ramses II, ca. 1200 BC) where two kings with the throne name "Woser...re" are recorded at the head of a group of more than a dozen kings, most of whose names are entirely lost.
The tomb of pharaoh Senebkay is modest in scale. An important discovery was the badly decayed remains of Senebkay's canopic chest. This chest was made of cedar wood that had been reused from the nearby tomb of Sobekhotep I and still bore the name of that earlier king, covered over by gilding. Such reuse of objects from the nearby Sobekhotep tomb by Senebkay, like the reused sarcophagus chamber found during the summer, provides evidence that suggests the limited resources and isolated economic situation of the Abydos Kingdom which lay in the southern part of Middle Egypt between the larger kingdoms of Thebes (Dynasties 16-17) and the Hyksos (Dynasty 15) in northern Egypt. Unlike these numbered dynasties, the pharaohs of the Abydos Dynasty were forgotten to history and their royal necropolis unknown until this discovery of Senebkay's tomb.
"It's exciting to find not just the tomb of one previously unknown pharaoh, but the necropolis of an entire forgotten dynasty," noted Dr. Wegner. "Continued work in the royal tombs of the Abydos Dynasty promises to shed new light on the political history and society of an important but poorly understood era of Ancient Egypt."

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Massive Exoplanets May Be More Earth-Like Than Thought: 'Super-Earths' Likely to Have Oceans, Continents

Jan. 8, 2014 — Massive terrestrial planets, called "super-Earths," are known to be common in our galaxy, the Milky Way. Now a Northwestern University astrophysicist and a University of Chicago geophysicist report the odds of these planets having an Earth-like climate are much greater than previously thought.

Nicolas B. Cowan and Dorian Abbot's new model challenges the conventional wisdom which says super-Earths actually would be very unlike Earth -- each would be a waterworld, with its surface completely covered in water. They conclude that most tectonically active super-Earths -- regardless of mass -- store most of their water in the mantle and will have both oceans and exposed continents, enabling a stable climate such as Earth's.

Cowan is a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), and Abbot is an assistant professor in geophysical sciences at UChicago.

"Are the surfaces of super-Earths totally dry or covered in water?" Cowan said. "We tackled this question by applying known geophysics to astronomy.

"Super-Earths are expected to have deep oceans that will overflow their basins and inundate the entire surface, but we show this logic to be flawed," he said. "Terrestrial planets have significant amounts of water in their interior. Super-Earths are likely to have shallow oceans to go along with their shallow ocean basins."

In their model, Cowan and Abbot treated the intriguing exoplanets like Earth, which has quite a bit of water in its mantle, the rocky part that makes up most of the volume and mass of the planet. The rock of the mantle contains tiny amounts of water, which quickly adds up because the mantle is so large. And a deep water cycle moves water between oceans and the mantle. (An exoplanet, or extrasolar planet, is a planet outside our solar system.)

Cowan presented the findings at a press conference, "Windows on Other Worlds," held Jan. 7 at the 223rd meeting of the American Astronomical Society (AAS) annual meeting in Washington, D.C.

He also will discuss the research at a scientific session to be held from 2 to 3:30 p.m. EST Wednesday, Jan. 8, at the AAS meeting (Potomac Ballroom D, Gaylord National Resort and Convention Center). The study will be published Jan. 20 in theAstrophysical Journal.

Water is constantly traded back and forth between the ocean and the rocky mantle because of plate tectonics, Cowan and Abbot said. The division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity.

Accounting for the effects of seafloor pressure and high gravity are two novel factors in their model. As the size of the super-Earths increase, gravity and seafloor pressure also go up.

"We can put 80 times more water on a super-Earth and still have its surface look like Earth," Cowan said. "These massive planets have enormous seafloor pressure, and this force pushes water into the mantle."

It doesn't take that much water to tip a planet into being a waterworld. "If Earth was 1 percent water by mass, we'd all drown, regardless of the deep water cycle," Cowan said. "The surface would be covered in water. Whether or not you have a deep water cycle really matters for planets that are one one-thousandth or one ten-thousandth water."

The ability of super-Earths to maintain exposed continents is important for planetary climate. On planets with exposed continents, like Earth, the deep carbon cycle is mediated by surface temperatures, which produces a stabilizing feedback (a thermostat on geological timescales).

"Such a feedback probably can't exist in a waterworld, which means they should have a much smaller habitable zone," Abbot said. "By making super-Earths 80 times more likely to have exposed continents, we've dramatically improved their odds of having an Earth-like climate."

Cowan and Abbot accede that there are two major uncertainties in their model: that super-Earths have plate tectonics and the amount of water Earth stores in its mantle.

"These are the two things we would like to know better to improve our model," Cowan said. "Our model is a shot from the hip, but it's an important step in advancing how we think about super-Earths.
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Universe Measured to One-Percent Accuracy: Most Precise Calibration Yet of Cosmic 'Standard Ruler'

Jan. 8, 2014 — Today the Baryon Oscillation Spectroscopic Survey (BOSS) Collaboration announced that BOSS has measured the scale of the universe to an accuracy of one percent. This and future measures at this precision are the key to determining the nature of dark energy.

"One-percent accuracy in the scale of the universe is the most precise such measurement ever made," says BOSS's principal investigator, David Schlegel, a member of the Physics Division of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab). "Twenty years ago astronomers were arguing about estimates that differed by up to fifty percent. Five years ago, we'd refined that uncertainty to five percent; a year ago it was two percent. One-percent accuracy will be the standard for a long time to come."

BOSS is the largest program in the third Sloan Digital Sky Survey (SDSS-III). Since 2009, BOSS has used the Sloan Foundation Telescope at the Apache Point Observatory in New Mexico to record high-precision spectra of well over a million galaxies with redshifts from 0.2 to 0.7, looking back over six billion years into the universe's past. Schlegel says, "We believe the BOSS database includes more redshifts of galaxies than collected by all the other telescopes in the world."

BOSS will continue gathering data until June, 2014. However, says Martin White, a member of Berkeley Lab, a professor of physics and astronomy at the University of California at Berkeley, and chair of the BOSS science survey team, "We've done the analysis now because we have 90 percent of BOSS's final data and we're tremendously excited by the results."

Baryon acoustic oscillations (BAO) are the regular clustering of galaxies, whose scale provides a "standard ruler" to measure the evolution of the universe's structure. Accurate measurement dramatically sharpens our knowledge of fundamental cosmological properties, including how dark energy accelerates the expansion of the universe.

Combined with recent measures of the cosmic microwave background radiation (CMB) and supernova measures of accelerating expansion, the BOSS results suggest that dark energy is a cosmological constant whose strength does not vary in space or time. Although unlikely to be a flaw in Einstein's General Theory of Relativity, the authors of the BOSS analysis note that "understanding the physical cause of the accelerated expansion remains one of the most interesting problems in modern physics."

Among other cosmic parameters, says White, the BOSS analysis "also provides one of the best-ever determinations of the curvature of space. The answer is, it's not curved much."

Calling a three-dimensional universe "flat" means its shape is well described by the Euclidean geometry familiar from high school: straight lines are parallel and triangles add up to 180 degrees. Extraordinary flatness means the universe experienced relatively prolonged inflation, up to a decillionth of a second or more, immediately after the big bang.

"One of the reasons we care is that a flat universe has implications for whether the universe is infinite," says Schlegel. "That means -- while we can't say with certainty that it will never come to an end -- it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe."

The BOSS analysis is based on SDSS-III's Data Releases 10 and 11 (DR 10 and DR 11) and has been submitted for publication in the Monthly Notices of the Royal Astronomical Society.

Ripples in a sea of galaxies

The BOSS analysis incorporates spectra of 1,277,503 galaxies and covers 8,509 square degrees of the sky visible from the northern hemisphere. This is the largest sample of the universe ever surveyed at this density. When complete, BOSS will have collected high-quality spectra of 1.3 million galaxies, plus 160,000 quasars and thousands of other astronomical objects, covering 10,000 square degrees.

Periodic ripples of density in visible matter ("baryons," for short) pervade the universe like raindrops on the surface of a pond. Regular galaxy clustering is the direct descendant of pressure waves that moved through the hot plasma of the early universe, which was so hot and dense that particles of light (photons) and particles of matter, including protons and electrons, were tightly coupled together. Invisible dark matter was also part of the mix.

By 380,000 years after the big bang, however, the temperature of the expanding mixture had cooled enough for light to escape, suffusing the newly transparent universe with intense radiation, which in the 13.4 billion years since has continued to cool to today's faint but pervasive cosmic microwave background.

Minute variations in the temperature of the CMB record periodicity in the original density ripples, of which the European Space Agency's Planck satellite has made the most recent and most accurate measures. The same periodicity is preserved in the clustering of the BOSS galaxies, a BAO signal which also mirrors the distribution of underlying dark matter.

Regular clustering at different eras, starting with the CMB, establishes the expansion history of the universe. BOSS collaborator Beth Reid of Berkeley Lab translates the two-dimensional sky coordinates of galaxies, plus their redshifts, into 3-D maps of the density of galaxies in space.

"It's from fluctuations in the density of galaxies in the volume we're looking at that we extract the BAO standard ruler," she says. "To compare different regions of the sky on an equal footing, first we have to undo variations from atmospheric effects or other patterns caused by how we observe the sky with our telescope." The results depend crucially on accurate measures of redshifts, which disclose the galaxies' positions in space and time. But galaxies don't move in lock step.

"When galaxies are close together their mutual gravitational attraction pushes them around and interferes with attempts to measure large-scale structure," Schlegel says. "Their peculiar motion makes it hard to write a formula for overall gravitational growth."

However, says Reid, "We have a very good model for what these distortions look like. The galaxy density field shows you where there are concentrations of matter, and the peculiar velocity field points in the direction of the net effect of all the local over- and under-densities."

"The BOSS data are awe-inspiring," says Martin White, "but many other pieces had to be put into place before we could get what we're after out of the data." Complex computer algorithms were essential for reconciling the inherent uncertainties. "We made thousands of model universes in a computer, and then observed them as BOSS would do and ran our analysis on them to answer the questions of 'What if?'"

By gauging how well their algorithms could analyze these model universes, known as "mocks" and based on catalogues of realistic but artificial galaxies, the experienced BOSS team was able to assess and fine-tune the algorithms when they were applied to the real BOSS data.

The National Energy Research Scientific Computing Center (NERSC), based at Berkeley Lab, was critical to the analysis and the creation of the mocks. Says White, "NERSC set aside resources for us to push analyses through quickly when we were up against deadlines. They provide a virtual meeting place where members of the collaboration from all around the world can come together on a shared platform, with both the data and the computational resources they need to perform their research."

BOSS has now provided the most accurate calibration ever of BAO's standard ruler. The universe's expansion history has been measured with unprecedented accuracy during the very stretch of ancient time, over six billion years in the past, when expansion had stopped slowing and acceleration began. But accurate as they are, the new BOSS results are just the beginning. Greater coverage and better resolution in scale are essential to understanding dark energy itself.

The proposed Dark Energy Spectroscopic Instrument (DESI), based on an international partnership of nearly 50 institutions led by Berkeley Lab, would enable the Mayall Telescope on Kitt Peak in Arizona to map over 20 million galaxies, plus over three million quasars, in 14,000 square degrees of the northern sky. By filling in the missing eons that BOSS can't reach, DESI could sharpen and extend coverage of the expansion history of the universe from the first appearance of the cosmic background radiation to the present day.

In the meantime, BOSS, ahead of schedule for completion in June, 2014, continues to be the premier instrument for mapping the universe
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Organic Mega Flow Battery Promises Breakthrough for Renewable Energy

Jan. 8, 2014 — A team of Harvard scientists and engineers has demonstrated a new type of battery that could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources such as wind and solar far more economical and reliable.

The novel battery technology is reported in a paper published in Nature on January 9. Under the OPEN 2012 program, the Harvard team received funding from the U.S. Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E) to develop the innovative grid-scale battery and plans to work with ARPA-E to catalyze further technological and market breakthroughs over the next several years.

The paper reports a metal-free flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.

The mismatch between the availability of intermittent wind or sunshine and the variability of demand is the biggest obstacle to getting a large fraction of our electricity from renewable sources. A cost-effective means of storing large amounts of electrical energy could solve this problem.

The battery was designed, built, and tested in the laboratory of Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard School of Engineering and Applied Sciences (SEAS). Roy G. Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, led the work on the synthesis and chemical screening of molecules. Alán Aspuru-Guzik, Professor of Chemistry and Chemical Biology, used his pioneering high-throughput molecular screening methods to calculate the properties of more than 10,000 quinone molecules in search of the best candidates for the battery.

Flow batteries store energy in chemical fluids contained in external tanks -- as with fuel cells -- instead of within the battery container itself. The two main components -- the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity), and the chemical storage tanks (which set the energy capacity) -- may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.

By contrast, in solid-electrode batteries, such as those commonly found in cars and mobile devices, the power conversion hardware and energy capacity are packaged together in one unit and cannot be decoupled. Consequently they can maintain peak discharge power for less than an hour before being drained, and are therefore ill suited to store intermittent renewables.

"Our studies indicate that one to two days' worth of storage is required for making solar and wind dispatchable through the electrical grid," said Aziz.

To store 50 hours of energy from a 1-megawatt power capacity wind turbine (50 megawatt-hours), for example, a possible solution would be to buy traditional batteries with 50 megawatt-hours of energy storage, but they'd come with 50 megawatts of power capacity. Paying for 50 megawatts of power capacity when only 1 megawatt is necessary makes little economic sense.

For this reason, a growing number of engineers have focused their attention on flow battery technology. But until now, flow batteries have relied on chemicals that are expensive or difficult to maintain, driving up the energy storage costs.

The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow battery technology now in development, but its cost sets a rather high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells.

The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.

"The whole world of electricity storage has been using metal ions in various charge states but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy," Gordon said. "With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good."

Aspuru-Guzik noted that the project is very well aligned with the White House Materials Genome Initiative. "This project illustrates what the synergy of high-throughput quantum chemistry and experimental insight can do," he said. "In a very quick time period, our team honed in to the right molecule. Computational screening, together with experimentation, can lead to discovery of new materials in many application domains."

Quinones are abundant in crude oil as well as in green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.

To back up a commercial wind turbine, a large storage tank would be needed, possibly located in a below-grade basement, said co-lead author Michael Marshak, a postdoctoral fellow at SEAS and in the Department of Chemistry and Chemical Biology. Or if you had a whole field of turbines or large solar farm, you could imagine a few very large storage tanks.

The same technology could also have applications at the consumer level, Marshak said. "Imagine a device the size of a home heating oil tank sitting in your basement. It would store a day's worth of sunshine from the solar panels on the roof of your house, potentially providing enough to power your household from late afternoon, through the night, into the next morning, without burning any fossil fuels."

"The Harvard team's results published in Nature demonstrate an early, yet important technical achievement that could be critical in furthering the development of grid-scale batteries," said ARPA-E Program Director John Lemmon. "The project team's result is an excellent example of how a small amount of catalytic funding from ARPA-E can help build the foundation to hopefully turn scientific discoveries into low-cost, early-stage energy technologies."

Team leader Aziz said the next steps in the project will be to further test and optimize the system that has been demonstrated on the bench top and bring it toward a commercial scale. "So far, we've seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles," he said. He also expects to achieve significant improvements in the underlying chemistry of the battery system. "I think the chemistry we have right now might be the best that's out there for stationary storage and quite possibly cheap enough to make it in the marketplace," he said. "But we have ideas that could lead to huge improvements."

By the end of the three-year development period, Connecticut-based Sustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when there's a need. Sustainable Innovations anticipates playing a key role in the product's commercialization by leveraging its ultra-low cost electrochemical cell design and system architecture already under development for energy storage applications.

"You could theoretically put this on any node on the grid," Aziz said. "If the market price fluctuates enough, you could put a storage device there and buy electricity to store it when the price is low and then sell it back when the price is high. In addition, you might be able to avoid the permitting and gas supply problems of having to build a gas-fired power plant just to meet the occasional needs of a growing peak demand."

This technology could also provide very useful backup for off-grid rooftop solar panels -- an important advantage considering some 20 percent of the world's population does not have access to a power distribution network.

William Hogan, Raymond Plank Professor of Global Energy Policy at Harvard Kennedy School, and one of the world's foremost experts on electricity markets, is helping the team explore the economic drivers for the technology.

Trent M. Molter, President and CEO of Sustainable Innovations, LLC, provides expertise on implementing the Harvard team's technology into commercial electrochemical systems.

"The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind," Aziz said. "A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I'm excited that we have a good shot at it."

In addition to Aziz, Marshak, Aspuru-Guzik, and Gordon, the co-lead author of the Nature paper was Brian Huskinson, a graduate student with Aziz; coauthors included research associate Changwon Suh and postdoctoral researcher Süleyman Er in Aspuru-Guzik's group; Michael Gerhardt, a graduate student with Aziz; Cooper Galvin, a Pomona College undergraduate; and Xudong Chen, a postdoctoral fellow in Gordon's group.

This work was supported in part by the U.S. Department of Energy's Advanced Research Project Agency-Energy (ARPA-E), the Harvard School of Engineering and Applied Sciences, the National Science Foundation (NSF) Extreme Science and Engineering Discovery Environment (OCI-1053575), an NSF Graduate Research Fellowship, and the Fellowships for Young Energy Scientists program of the Foundation for Fundamental Research on Matter, which is part of the Netherlands Organization for Scientific Research (NWO)
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