#Getting more electricity out of solar cells When sunlight shines on today solar cells, much of the incoming energy is given off as waste heat rather than electrical current.
In a few materials, however, extra energy produces extra electrons behavior that could significantly increase solar-cell efficiency.
An MIT team has identified now the mechanism by which that phenomenon happens, yielding new design guidelines for using those special materials to make high-efficiency solar cells.
The results are reported in the journal Nature Chemistry by MIT alumni Shane R. Yost and Jiye Lee,
and a dozen other co-authors, all led by MIT Troy Van Voorhis, professor of chemistry, and Marc Baldo, professor of electrical engineering.
In most photovoltaic (PV) materials, a photon (a packet of sunlight) delivers energy that excites a molecule,
causing it to release one electron. But when high-energy photons provide more than enough energy,
the molecule still releases just one electron plus waste heat. A few organic molecules don follow that rule.
Instead, they generate more than one electron per high-energy photon. That phenomenon known as singlet exciton fission was identified first in the 1960s.
However, achieving it in a functioning solar cell has proved difficult and the exact mechanism involved has become the subject of intense controversy in the field.
For the past four years, Van Voorhis and Baldo have been pooling their theoretical and experimental expertise to investigate this problem.
In 2013, they reported making the first solar cell that gives off extra electrons from high-energy visible light,
which makes up almost half the sun electromagnetic radiation at the Earth surface. According to their estimates, applying their technology as an inexpensive coating on silicon solar cells could increase efficiency by as much as 25 percent.
While that encouraging, understanding the mechanism at work would enable them and others to do even better.
Exciton fission has now been observed in a variety of materials all discovered like the original ones by chance. e can rationally design materials
Van Voorhis says. To support his theoretical study of electron behavior within PVS, Van Voorhis used experimental data gathered in samples specially synthesized by Baldo and Timothy Swager, MIT John D. Macarthur Professor of Chemistry.
The samples were made of four types of exciton fission molecules decorated with various sorts of pinachbulky side groups of atoms that change the molecular spacing without altering the physics or chemistry.
To detect fission rates which are measured in femtoseconds (10-15 seconds) the MIT team turned to experts including Moungi Bawendi, the Lester Wolfe Professor of Chemistry,
Van Voorhisnew first-principles formula successfully predicts the fission rate in materials with vastly different structures.
When excess energy is available in these materials, an electron in an excited molecule swaps places with an electron in an unexcited molecule nearby.
The excited electron brings some energy along and leaves some behind, so that both molecules give off electrons.
Van Voorhis says. he controversial, or xotic, mechanisms proposed more recently aren required to explain what being observed here.
The results also provide practical guidelines for designing solar cells with these materials. They show that molecular packing is important in defining the rate of fission but only to a point.
Indeed, a far more important factor is choosing a material that has the right inherent energy levels.
The researchers are pleased with the agreement between their experimental and theoretical data especially given the systems being modeled.
Van Voorhis says. hat the reason that 50 years ago they couldn compute these things
David Reichman, a professor of chemistry at Columbia University who was involved not in this research, considers the new findings very important contribution to the singlet fission literature.
and toward a focus on the electronic energy levels of selected materials. This work was performed in the Center for Excitonics,
an Energy Frontier Research center funded by the U s. Department of energy. Experimental measurements were supported by The british Engineering and Physical sciences Research Council,
and work at the Center for Functional Nanomaterials at Brookhaven National Laboratory was supported by the U s. Department of energy t
#Terahertz imaging on the cheap Terahertz imaging which is already familiar from airport security checkpoints has a number of other promising applications from explosives detection to collision avoidance in cars.
Like sonar or radar terahertz imaging produces an image by comparing measurements across an array of sensors.
Those arrays have to be very dense since the distance between sensors is proportional to wavelength.
In the latest issue of IEEE Transactions on Antennas and Propagation researchers in MIT s Research Laboratory for Electronics describe a new technique that could reduce the number of sensors required for terahertz
or millimeter-wave imaging by a factor of 10 or even 100 making them more practical.
so that light reflected by a small patch of the visual scene strikes a correspondingly small patch of the sensor array.
In lower-frequency imaging systems by contrast an incoming wave whether electromagnetic or in the case of sonar acoustic strikes all of the sensors in the array.
and intensity of the wave by comparing its phase the alignment of its troughs and crests when it arrives at each of the sensors.
As long as the distance between sensors is no more than half the wavelength of the incoming wave that calculation is fairly straightforward a matter of inverting the sensors measurements.
But if the sensors are spaced farther than half a wavelength apart the inversion will yield more than one possible solution.
Those solutions will be spaced at regular angles around the sensor array a phenomenon known as spatial aliasing.
however any given circumference around the detector is populated usually sparsely. That s the phenomenon that the new system exploits.
Think about a range around you like five feet says Gregory Wornell the Sumitomo Electric Industries Professor in Engineering in MIT s Department of Electrical engineering
and Computer science and a co-author on the new paper. There s actually not that much at five feet around you.
Keeping every tenth sensor won t work: It s the regularity of the distances between sensors that leads to aliasing.
Arbitrarily varying the distances between sensors would solve that problem but it would also make inverting the sensors measurements calculating the wave s source and intensity prohibitively complicated.
Regular irregularityso Wornell and his co-authors James Krieger a former student of Wornell s who is now at MIT s Lincoln Laboratory
and Yuval Kochman a former postdoc who is now an assistant professor at the Hebrew University of Jerusalem
instead prescribe a detector along which the sensors are distributed in pairs. The regular spacing between pairs of sensors ensures that the scene reconstruction can be calculated efficiently but the distance from each sensor to the next remains irregular.
The researchers also developed an algorithm that determines the optimal pattern for the sensors distribution.
In essence the algorithm maximizes the number of different distances between arbitrary pairs of sensors. With his new colleagues at Lincoln Lab Krieger has performed experiments at radar frequencies using a one-dimensional array of sensors deployed in a parking lot
which verified the predictions of the theory. Moreover Wornell s description of the sparsity assumptions of the theory 10 percent occupation at a given distance means one-tenth the sensors applies to one-dimensional arrays.
Many applications such as submarines sonar systems instead use two-dimensional arrays and in that case the savings compound:
One-tenth the sensors in each of two dimensions translates to one-hundredth the sensors in the complete array.
James Preisig a researcher at the Woods hole oceanographic institution and principal at JP Analytics says that he s most interested in the new technique s ability to reduce the computational burden of high-resolution sonar imaging.
This technique helps significantly with the computational complexity of using signals from very large arrays Preisig says.
I can imagine it being deployed in situations where you are using very very large arrays to get good spatial resolution
but you re processing signals on a vehicle or something where you did not have significant computational power.
In those contexts Preisig says the new technique s sparsity assumptions make perfect sense. In this context the field of view is divided into sectors Preisig says.
The field has to be sector-sparse only a subset of sectors have objects in them.
#Computer system automatically solves word problems Researchers in MIT Computer science and Artificial intelligence Laboratory, working with colleagues at the University of Washington, have developed a new computer system that can automatically solve the type of word problems common in introductory algebra classes.
In the near term, the work could lead to educational tools that identify errors in studentsreasoning
or evaluate the difficulty of word problems. But it may also point toward systems that can solve more complicated problems in geometry, physics,
and finance problems whose solutions don appear in the back of the teacher edition of a textbook.
an MIT graduate student in electrical engineering and computer science and lead author on the new paper, the new work is in the field of emantic parsing,
or translating natural language into a formal language such as arithmetic or formal logic. Most previous work on semantic parsing including his own has focused on individual sentences,
Kushman says. n these algebra problems, you have to build these things up from many different sentences,
a professor of computer science and engineering and one of his two thesis advisors, and by the University of Washington Yoav Artzi and Luke Zettlemoyer.
The researchers will present their work at the annual meeting of the Association for Computational linguistics in June.
Finding your place The researcherssystem exploits two existing computational tools. One is the computer algebra system Macsyma,
whose initial development at MIT in the 1960s was a milestone in artificial-intelligence research. For Kushman and his colleaguespurposes, Macsyma provided a way to distill algebraic equations with the same general structure into a common template.
and to produce the equation templates, the researchers used machine learning. Kushman found a website on which algebra students posted word problems they were having difficulty with,
and where their peers could then offer solutions. From an initial group of roughly 2
For the training, however, they used two different approaches or, in the parlance of machine learning, two different types of supervision.
In the first approach, they fed the system both word problems and their translations into algebraic equations 400 examples of each.
In the first case, the system, after training, was able to solve roughly 70 percent of its test problems;
the system examined hundreds of thousands of eaturesof the training examples. Some of those features related specific words to problem types:
a professor of computer science of the University of Southern California. he approach of building a generative story of how people get from text to answers is a great idea.
but it can benefit from a bunch of extra data that you haven labeled in detail. a
an MIT associate professor of biological engineering. here a general recognition that in order to understand the brain processes in comprehensive detail,
we need ways to monitor neural function deep in the brain with spatial, temporal, and functional precision, he says.
using magnetic resonance imaging (MRI) along with a specialized molecular sensor. This is the first time anyone has been able to map neural signals with high precision over large brain regions in living animals,
offering a new window on brain function, says Jasanoff, who is also an associate member of MIT Mcgovern Institute for Brain Research.
the researchers used an MRI sensor they had designed previously, consisting of an iron-containing protein that acts as a weak magnet.
When the sensor binds to dopamine, its magnetic interactions with the surrounding tissue weaken, which dims the tissue MRI signal.
The researchers also developed an algorithm that lets them calculate the precise amount of dopamine present in each fraction of a cubic millimeter of the ventral striatum.
After delivering the MRI sensor to the ventral striatum of rats, Jasanoff team electrically stimulated the mesolimbic pathway
An area known as the nucleus accumbens core, known to be one of the main targets of dopamine from the VTA,
He and his colleagues plan to build on this work by expanding their studies to other parts of the brain,
including the areas most affected by Parkinson disease, which is caused by the death of dopamine-generating cells.
Jasanoff lab is also working on sensors to track other neurotransmitters, allowing them to study interactions between neurotransmitters during different tasks.
#MIT team wins Clean energy Prize for solving solar s shade problem An MIT team whose integrated chip restores lost power to partially shaded solar panels achieving double the energy capture improvement of similar technologies won big on Monday night at the seventh annual MIT Clean energy Prize (CEP) competition.
Equipped with a promising business plan and a snappy catchphrase hade happensunified Solar took home both CEP grand prizes:
the DOE Energy efficiency and Renewable energy Clean energy Prize, worth $100, 000, and the NSTAR MIT Clean energy Prize, worth $125, 000.
Solar panels on residential rooftops that are shaded partially by clouds or trees sacrifice as much as 30 percent of their energy potential over a year.
Unified Solar technology, for the first time, integrates an entire power balance circuit onto a low-cost chip that can be integrated into a solar panel to regain that lost energy. n the real world,
shade happens, said Bessma Aljarbou, a graduate student at the MIT Sloan School of management, during Unified Solar winning pitch last night to a capacity crowd. hade brings energy loss, reliability concerns,
and a constrained market. And we have a solution. ompared to our competitors at the panel level,
we can recover twice as much energy under partial shading conditions, at a fraction of existing costs, added Arthur Chang,
an MIT Phd student in electrical engineering and computer science (EECS) who invented the technology. With the prize money, the team including students from MIT, the California Institute of technology,
and Stanford university aims to further develop the technology and launch the company. By 2015 the team aims to complete in-lab testing
and pilot their technology in outdoor solar systems. Unified Solar now becomes the finalist in the energy category in the MIT $100k Entrepreneurship Competition,
whose winner will be announced May 14. The CEP, the nation leading student-run energy business-plan competition, awarded a total of $320,
000 last night to six teams that have developed clean energy startups and innovations. More than 60 teams entered this year contest;
19 semifinalists made it to Monday grand finale. One finalist team was selected in each of three categories renewable energy (Thermovolt), energy efficiency (MF Fire),
and infrastructure and resources (ulink) with each receiving $25, 000. Four winners of the Audience Choice Awards including one from MIT earned $10, 000 each.
Since its 2008 founding at MIT, the CEP cosponsored by Massachusetts utility NSTAR and the U s. Department of energy has awarded more than $2 million to help launch clean energy startups.
Past participants have gone on to raise more than $250 million in capital said Thomas Baade-Mathiesen, a graduate student at MIT Sloan and co-managing director of the CEP. nd that even counting conservatively,
he said. Open to students at any U s. university and now partnered with Cleantech Open, the competition aims to promote clean energy innovation
and entrepreneurship across the nation and the globe, said CEP cofounder Bill Aulet, managing director of the Martin Trust Center for MIT Entrepreneurship. ustainability is not just a problem for Cambridge,
Aulet said. t a problem for the world. Existing solutions for partially shaded solar panels optimize power at the panel level.
But these bulky oxesrely on costly energy storage components such as capacitors and inductors. Failing to account for the strength or weakness of each individual PV cell,
these also only restore roughly half of lost power. But Unified Solar innovates t the cell level,
Chang said. The idea is that providing power balance for individual PV cells instead of for an entire panel allows for finer tuning of power optimization. hen youe at the cell level,
the improvement in energy capture under partial shading is basically two times better compared to panel level solutions,
Chang said. By using intrinsic parasitic components, specifically iffusion capacitancesunwanted electrical charges between two components Chang was able to eliminate the need for external energy storage
you learn a lot about properties of solar cells that people often ignored as parasitic components. People wanted to get rid of them,
but we found a way to leverage them. o one has thought ever of using the solar cell as an energy storage itself,
and that what wee effectively doing, added team member Albert Chan, a graduate student in MIT Leaders for Global Operations program.
and earned a best technical presentation award at the 2014 Applied Power Electronics Conference and Exposition, cosponsored by IEEE and the Power Sources Manufacturers Association.
and routing device that plugs into solar panels to power electronic devices, enabling a pay-as-you-go electricity system for people off the grid.
But the true novelty is in its connectivity: The ulink units can be linked to one another,
an EECS Phd student studying power electronics, said during the team pitch. t empowers users to build their own grid, from the ground up,
The top prize for energy efficiency (and an Audience Choice Award) went to a University of Maryland team, MF Fire,
and its Mulciber Stove a woodstove equipped with sensors and a control system that automates burning.
which turns solar cells into cogeneration systems. Sun exposure to solar panels produces about 0. 5 percent of wasted heat per Degree celsius increase.
Thermovolt modified solar cell captures that wasted energy and uses it to heat water for homes.
The two other audience choice awards went to REECYCLE, which reclaims rare earth elements from recycled electronics to create other resources,
and MIT team Belleds Technologies, which is developing smart LED LIGHTS that can wirelessly connect to the Internet and change colors to match people moods p
#Brewing up sustainability Each day, breweries and wineries produce thousands of gallons of wastewater, which is poured either down the drain
or shipped to treatment facilities at a high cost to the companies. Now MIT spinout Cambrian Innovation is commercializing a novel treatment system, called covolt,
The company which aims to leverage biotechnology as a way to solve environmental issues is also modifying their system to generate value from wastewater in agricultural and military fields,
raising the cost of waste disposal and causing environmental harm. But a more cyclical approach where waste is used as an energy source could provide higher profit yields
and liminate the tension between the environment and economics, says CEO Matt Silver SM 5, Phd 0, who co-founded Cambrian with Justin Buck Phd 2,
now the company chief technology officer. he treatment of wastewater, for example, consumes over 3 percent of the electricity in the United states,
yet organics in the wastewater have energy that can be extracted and used locally, Silver says. nd that the case for a lot of waste products in general.
Cambrian automated and modular Ecovolt system delivered on a flatbed and installed over a few weeks consists of a headworks unit,
which houses computers for automation and control, and expandable 20,000-gallon treatment units. In these units, microbes called xoelectrogensexecute a unique process, electromethanogenesis which is being used for the first time ever in treating wastewater.
Exoelectrogens, coated on anodes, consume the wastewater remaining organic pollutants and, in the process, generate electricity.
This electricity travels through a circuit and onto cathodes coated with separate microbes that consume that electricity
along with carbon dioxide to produce biogas at a rate of up to 100 cubic feet per minute.
This process enhances naturally occurring anaerobic digestion in wastewater Silver says, as it stabilizes treatment, improves biogas quality,
and enables a higher degree of automation. The biogas enters a connected cogeneration system for power conversion.
Depending on several site factors, this produces anywhere from 30 to 400 kilowatts of electricity. Treated wastewater exits the reactor with 80 to 90 percent of pollutants removed,
so it can be used for irrigation, equipment washing, and other things. The system can treat 10,000 to 1 million gallons of wastewater daily.
At that rate, a winery would shave about 2 pounds of CO2 per case off of its carbon footprint
through carbon-free energy generation and avoiding municipal wastewater treatment ffectively planting over 4, 400 acres of trees in a year,
Silver says. Since January, the Bear Republic and Lagunitas breweries, both in drought-stricken California, have purchased Ecovolt systems.
At current usage rates, Cambrian estimates the system will generate enough electricity to meet 25 to 50 percent of these breweriesneeds
and allow reuse of about 25 percent of their water. It could also eliminate 10 trucks of wastewater shipping per day for Lagunitas.
All in the design Increasingly, the beverage industry has employed anaerobic and aerobic wastewater treatment processes but these are expensive and difficult to adopt,
Silver says. Aerobic processes dissolving air into wastewater, where aerobic microorganisms degrade pollutants consume a lot of energy
and generate biosolids (organic materials) that are managed at cost. Processes where anaerobic microbes eat pollutants,
while producing small amounts of burnable methane, have gained popularity in the last few decades but such systems only function under specific circumstances,
are prone to failure, and are difficult to operate. Ecovolt, on the other hand, is applicable to a range of sites,
and has demonstrated a more robust treatment process, meaning better power generation and higher yields for customers and is therefore more economical,
It an architecture influenced by Silver graduate years in MIT Engineering Systems Division, which stresses reliability, scalability,
and provides real-time data thanks to using exoelectrogens as sensors. hese bugs are generating electricity,
to see how well the reactor is doing, explains Buck, who invented Cambrian sensor technologies.
With Ecovolt, Silver says, Cambrian aims to make treating and reusing water easier and more affordable,
Silver says. s our economy grows, water tables are dropping and wastewater pollution is rising, causing many companies to consider water risk in their overall strategy.
The need for more cost-effective solutions is increasingly acute in the developed world and also critical in the developing world.
We are leveraging biotechnology to provide the highest return on investment for managing water. To that end, Cambrian is working on other projects that leverage exoelectrogenic microbes to treat wastewater.
and generates electricity to power itself. Another project, funded by the National Science Foundation, uses exoelectrogens to sense nitrate in wastewater, cheaply and with very high specificity,
for the agricultural industry. arth as a spaceshipecovolt is valuable today as a solution to Earth water issues.
But the core technology began as a bit of aerospace ingenuity and has since found its way back to space.
Meeting at MIT in 2006 over a shared fondness for biotech, Silver, then a research scientist in MIT Space Systems Lab,
and Buck, a biological engineering graduate student, won a grant from the NASA Institute for Advanced Concepts program to create a life-support system that could treat waste
and generate electricity for astronauts. Soon, they came across exoelectrogens; a 1999 study had revealed that exoelectrogens could,
By the time they entered the 2010 Cleantech Open competition and won they had shifted focus to the food and beverage industry.
Cambrian successfully piloted its first system in 2010 and then, over 14 months, from 2011 to 2012
Silver says. he challenge of supporting astronauts in space is very similar to sustainability On earth,
is to leverage biotechnology to advance a sustainable ndustrial ecology, where the waste of industry is recycled to create energy
and value much like in natural ecosystems. n a natural ecosystem, there really is no such thing as waste,
Silver says. t just another resource. l
#Fast way to measure DNA repair Our DNA is under constant attack from many sources,
including environmental pollutants, ultraviolet light, and radiation. Fortunately, cells have several major DNA repair systems that can fix this damage,
which may lead to cancer and other diseases if not mended. The effectiveness of these repair systems varies greatly from person to person;
scientists believe that this variability may explain why some people get cancer while others exposed to similar DNA-damaging agents do not.
A team of MIT researchers has developed now a test that can rapidly assess several of these repair systems,
which could help determine individualsrisk of developing cancer and help doctors predict how a given patient will respond to chemotherapy drugs.
The new test, described in the Proceedings of the National Academy of Sciences the week of April 21, can analyze four types of DNA repair capacity simultaneously, in less than 24 hours.
The research team, led by professor Leona Samson, used this approach to measure DNA repair in a type of immortalized human blood cells called lymphoblastoid cells,
who is the Uncas and Helen Whitaker Professor, an American Cancer Society Professor, and a member of MIT departments of biological engineering and of biology, Center for Environmental Health Sciences,
and Koch Institute for Integrative Cancer Research. Measuring repair With the new test, the MIT team can measure how well cells repair the most common DNA lesions,
including single-strand breaks, double-strand breaks, mismatches, and the introduction of alkyl groups caused by pollutants such as fuel exhaust and tobacco smoke.
To achieve this, the researchers created five different circular pieces of DNA, four of which carry a specific type of DNA damage, also called DNA lesions.
Each of these CIRCULAR DNA strands, or plasmids, also carries a gene for a different colored fluorescent protein.
In some cases the DNA lesions prevent those genes from being expressed, so when the DNA is repaired successfully,
the cell begins to produce the fluorescent protein. In others, repairing the DNA lesion turns the fluorescent gene off.
By introducing these plasmids into cells and reading the fluorescent output, scientists can determine how efficiently each kind of lesion has been repaired.
In theory, more than five plasmids could go into each cell, but the researchers limited each experiment to five reporter plasmids to avoid potential overlap among colors.
which type of lesion the plasmid carries, as well as information about which patient cells are being tested.
Some of these differences have been linked with cancer vulnerability; for example, a genetic defect in a type of DNA repair called nucleotide excision repair often leads to a condition called xeroderma pigmentosum, in
which DNA damage caused by ultraviolet light goes unrepaired and leads to skin cancer. Scientists have identified also links between DNA repair and neurological, developmental,
and immunological disorders, but useful predictive DNA-repair-based tests have not been developed, largely because it has been impossible to rapidly analyze several different types of DNA repair capacity at once.
Samson lab is now working on adapting the new test so it can be used with blood samples taken from patients,
and potentially enabling prevention or earlier diagnosis of diseases linked to DNA repair. Such a test could also be used to predict patientsresponse to chemotherapy drugs, which often work by damaging cancer cellsdna,
or to determine how much radiation treatment a patient can tolerate. The researchers also believe this test could be exploited to screen for new drugs that inhibit
or enhance DNA repair. Inhibitors could be targeted to tumors to make them more susceptible to chemotherapy,
while enhancers could help protect people who have been exposed accidentally to DNA-damaging agents, such as radiation.
Another important application for this test could be studying fundamental biological processes such as how cells recruit backup repair systems to fill in
here an opportunity to use these multiplexed plasmids in biological assays where several repair pathways can be probed at the same time,
Graduate students Carrie Margulies and Isaac Chaim; technical assistants Siobhan Mcree and Patrizia Mazzucato; and research scientists Vincent Butty, Anwaar Ahmad, Ryan Abo,
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