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      A healthy wind

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      Nearly 10 percent of today’s electricity in the United States comes from wind power. The renewable energy source benefits climate, air quality, and public health by displacing emissions of greenhouse gases and air pollutants that would otherwise be produced by fossil-fuel-based power plants.

      A new MIT study finds that the health benefits associated with wind power could more than quadruple if operators prioritized turning down output from the most polluting fossil-fuel-based power plants when energy from wind is available.

      In the study, published today in Science Advances, researchers analyzed the hourly activity of wind turbines, as well as the reported emissions from every fossil-fuel-based power plant in the country, between the years 2011 and 2017. They traced emissions across the country and mapped the pollutants to affected demographic populations. They then calculated the regional air quality and associated health costs to each community.

      The researchers found that in 2014, wind power that was associated with state-level policies improved air quality overall, resulting in $2 billion in health benefits across the country. However, only roughly 30 percent of these health benefits reached disadvantaged communities.

      The team further found that if the electricity industry were to reduce the output of the most polluting fossil-fuel-based power plants, rather than the most cost-saving plants, in times of wind-generated power, the overall health benefits could quadruple to $8.4 billion nationwide. However, the results would have a similar demographic breakdown.

      “We found that prioritizing health is a great way to maximize benefits in a widespread way across the U.S., which is a very positive thing. But it suggests it’s not going to address disparities,” says study co-author Noelle Selin, a professor in the Institute for Data, Systems, and Society and the Department of Earth, Atmospheric and Planetary Sciences at MIT. “In order to address air pollution disparities, you can’t just focus on the electricity sector or renewables and count on the overall air pollution benefits addressing these real and persistent racial and ethnic disparities. You’ll need to look at other air pollution sources, as well as the underlying systemic factors that determine where plants are sited and where people live.”

      Selin’s co-authors are lead author and former MIT graduate student Minghao Qiu PhD ’21, now at Stanford University, and Corwin Zigler at the University of Texas at Austin.

      Turn-down service

      In their new study, the team looked for patterns between periods of wind power generation and the activity of fossil-fuel-based power plants, to see how regional electricity markets adjusted the output of power plants in response to influxes of renewable energy.

      “One of the technical challenges, and the contribution of this work, is trying to identify which are the power plants that respond to this increasing wind power,” Qiu notes.

      To do so, the researchers compared two historical datasets from the period between 2011 and 2017: an hour-by-hour record of energy output of wind turbines across the country, and a detailed record of emissions measurements from every fossil-fuel-based power plant in the U.S. The datasets covered each of seven major regional electricity markets, each market providing energy to one or multiple states.

      “California and New York are each their own market, whereas the New England market covers around seven states, and the Midwest covers more,” Qiu explains. “We also cover about 95 percent of all the wind power in the U.S.”

      In general, they observed that, in times when wind power was available, markets adjusted by essentially scaling back the power output of natural gas and sub-bituminous coal-fired power plants. They noted that the plants that were turned down were likely chosen for cost-saving reasons, as certain plants were less costly to turn down than others.

      The team then used a sophisticated atmospheric chemistry model to simulate the wind patterns and chemical transport of emissions across the country, and determined where and at what concentrations the emissions generated fine particulates and ozone — two pollutants that are known to damage air quality and human health. Finally, the researchers mapped the general demographic populations across the country, based on U.S. census data, and applied a standard epidemiological approach to calculate a population’s health cost as a result of their pollution exposure.

      This analysis revealed that, in the year 2014, a general cost-saving approach to displacing fossil-fuel-based energy in times of wind energy resulted in $2 billion in health benefits, or savings, across the country. A smaller share of these benefits went to disadvantaged populations, such as communities of color and low-income communities, though this disparity varied by state.

      “It’s a more complex story than we initially thought,” Qiu says. “Certain population groups are exposed to a higher level of air pollution, and those would be low-income people and racial minority groups. What we see is, developing wind power could reduce this gap in certain states but further increase it in other states, depending on which fossil-fuel plants are displaced.”

      Tweaking power

      The researchers then examined how the pattern of emissions and the associated health benefits would change if they prioritized turning down different fossil-fuel-based plants in times of wind-generated power. They tweaked the emissions data to reflect several alternative scenarios: one in which the most health-damaging, polluting power plants are turned down first; and two other scenarios in which plants producing the most sulfur dioxide and carbon dioxide respectively, are first to reduce their output.

      They found that while each scenario increased health benefits overall, and the first scenario in particular could quadruple health benefits, the original disparity persisted: Communities of color and low-income communities still experienced smaller health benefits than more well-off communities.

      “We got to the end of the road and said, there’s no way we can address this disparity by being smarter in deciding which plants to displace,” Selin says.

      Nevertheless, the study can help identify ways to improve the health of the general population, says Julian Marshall, a professor of environmental engineering at the University of Washington.

      “The detailed information provided by the scenarios in this paper can offer a roadmap to electricity-grid operators and to state air-quality regulators regarding which power plants are highly damaging to human health and also are likely to noticeably reduce emissions if wind-generated electricity increases,” says Marshall, who was not involved in the study.

      “One of the things that makes me optimistic about this area is, there’s a lot more attention to environmental justice and equity issues,” Selin concludes. “Our role is to figure out the strategies that are most impactful in addressing those challenges.”

      This work was supported, in part, by the U.S. Environmental Protection Agency, and by the National Institutes of Health. More

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      Mining for the clean energy transition

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      In a world powered increasingly by clean energy, drilling for oil and gas will gradually give way to digging for metals and minerals. Today, the “critical minerals” used to make electric cars, solar panels, wind turbines, and grid-scale battery storage are facing soaring demand — and some acute bottlenecks as miners race to catch up.

      According to a report from the International Energy Agency, by 2040, the worldwide demand for copper is expected to roughly double; demand for nickel and cobalt will grow at least sixfold; and the world’s hunger for lithium could reach 40 times what we use today.

      “Society is looking to the clean energy transition as a way to solve the environmental and social harms of climate change,” says Scott Odell, a visiting scientist at the MIT Environmental Solutions Initiative (ESI), where he helps run the ESI Mining, Environment, and Society Program, who is also a visiting assistant professor at George Washington University. “Yet mining the materials needed for that transition would also cause social and environmental impacts. So we need to look for ways to reduce our demand for minerals, while also improving current mining practices to minimize social and environmental impacts.”

      ESI recently hosted the inaugural MIT Conference on Mining, Environment, and Society to discuss how the clean energy transition may affect mining and the people and environments in mining areas. The conference convened representatives of mining companies, environmental and human rights groups, policymakers, and social and natural scientists to identify key concerns and possible collaborative solutions.

      “We can’t replace an abusive fossil fuel industry with an abusive mining industry that expands as we move through the energy transition,” said Jim Wormington, a senior researcher at Human Rights Watch, in a panel on the first day of the conference. “There’s a recognition from governments, civil society, and companies that this transition potentially has a really significant human rights and social cost, both in terms of emissions […] but also for communities and workers who are on the front lines of mining.”

      That focus on communities and workers was consistent throughout the three-day conference, as participants outlined the economic and social dimensions of standing up large numbers of new mines. Corporate mines can bring large influxes of government revenue and local investment, but the income is volatile and can leave policymakers and communities stranded when production declines or mineral prices fall. On the other hand, “artisanal” mining operations are an important source of critical minerals, but are hard to regulate and subject to abuses from brokers. And large reserves of minerals are found in conservation areas, regions with fragile ecosystems and experiencing water shortages that can be exacerbated by mining, in particular on Indigenous-controlled lands and other places where mine openings are deeply fraught.

      “One of the real triggers of conflict is a dissatisfaction with the current model of resource extraction,” said Jocelyn Fraser of the University of British Columbia in a panel discussion. “One that’s failed to support the long-term sustainable development of regions that host mining operations, and yet imposes significant local social and environmental impacts.”

      All these challenges point toward solutions in policy and in mining companies’ relationships with the communities where they work. Participants highlighted newer models of mining governance that can create better incentives for the ways mines operate — from full community ownership of mines to recognizing community rights to the benefits of mining to end-of-life planning for mines at the time they open.

      Many of the conference speakers also shared technological innovations that may help reduce mining challenges. Some operations are investing in desalination as alternative water sources in water-scarce regions; low-carbon alternatives are emerging to many of the fossil fuel-powered heavy machines that are mainstays of the industry; and work is being done to reclaim valuable minerals from mine tailings, helping to minimize both waste and the need to open new extraction sites.

      Increasingly, the mining industry itself is recognizing that reforms will allow it to thrive in a rapid clean-energy transition. “Decarbonization is really a profitability imperative,” said Kareemah Mohammed, managing director for sustainability services at the technology consultancy Accenture, on the conference’s second day. “It’s about securing a low-cost and steady supply of either minerals or metals, but it’s also doing so in an optimal way.”

      The three-day conference attracted over 350 attendees, from large mining companies, industry groups, consultancies, multilateral institutions, universities, nongovernmental organizations (NGOs), government, and more. It was held entirely virtually, a choice that helped make the conference not only truly international — participants joined from over 27 countries on six continents — but also accessible to members of nonprofits and professionals in the developing world.

      “Many people are concerned about the environmental and social challenges of supplying the clean energy revolution, and we’d heard repeatedly that there wasn’t a forum for government, industry, academia, NGOs, and communities to all sit at the same table and explore collaborative solutions,” says Christopher Noble, ESI’s director of corporate engagement. “Convening, and researching best practices, are roles that universities can play. The conversations at this conference have generated valuable ideas and consensus to pursue three parallel programs: best-in-class models for community engagement, improving ESG metrics and their use, and civil-society contributions to government/industry relations. We are developing these programs to keep the momentum going.”

      The MIT Conference on Mining, Environment, and Society was funded, in part, by Accenture, as part of the MIT/Accenture Convergence Initiative. Additional funding was provided by the Inter-American Development Bank. More

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      A breakthrough on “loss and damage,” but also disappointment, at UN climate conference

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      As the 2022 United Nations climate change conference, known as COP27, stretched into its final hours on Saturday, Nov. 19, it was uncertain what kind of agreement might emerge from two weeks of intensive international negotiations.

      In the end, COP27 produced mixed results: on the one hand, a historic agreement for wealthy countries to compensate low-income countries for “loss and damage,” but on the other, limited progress on new plans for reducing the greenhouse gas emissions that are warming the planet.

      “We need to drastically reduce emissions now — and this is an issue this COP did not address,” said U.N. Secretary-General António Guterres in a statement at the conclusion of COP27. “A fund for loss and damage is essential — but it’s not an answer if the climate crisis washes a small island state off the map — or turns an entire African country to desert.”

      Throughout the two weeks of the conference, a delegation of MIT students, faculty, and staff was at the Sharm El-Sheikh International Convention Center to observe the negotiations, conduct and share research, participate in panel discussions, and forge new connections with researchers, policymakers, and advocates from around the world.

      Loss and damage

      A key issue coming in to COP27 (COP stands for “conference of the parties” to the U.N. Framework Convention on Climate Change, held for the 27th time) was loss and damage: a term used by the U.N. to refer to harms caused by climate change — either through acute catastrophes like extreme weather events or slower-moving impacts like sea level rise — to which communities and countries are unable to adapt. 

      Ultimately, a deal on loss and damage proved to be COP27’s most prominent accomplishment. Negotiators reached an eleventh-hour agreement to “establish new funding arrangements for assisting developing countries that are particularly vulnerable to the adverse effects of climate change.” 

      “Providing financial assistance to developing countries so they can better respond to climate-related loss and damage is not only a moral issue, but also a pragmatic one,” said Michael Mehling, deputy director of the MIT Center for Energy and Environmental Policy Research, who attended COP27 and participated in side events. “Future emissions growth will be squarely centered in the developing world, and offering support through different channels is key to building the trust needed for more robust global cooperation on mitigation.”

      Youssef Shaker, a graduate student in the MIT Technology and Policy Program and a research assistant with the MIT Energy Initiative, attended the second week of the conference, where he followed the negotiations over loss and damage closely. 

      “While the creation of a fund is certainly an achievement,” Shaker said, “significant questions remain to be answered, such as the size of the funding available as well as which countries receive access to it.” A loss-and-damage fund that is not adequately funded, Shaker noted, “would not be an impactful outcome.” 

      The agreement on loss and damage created a new committee, made up of 24 country representatives, to “operationalize” the new funding arrangements, including identifying funding sources. The committee is tasked with delivering a set of recommendations at COP28, which will take place next year in Dubai.

      Advising the U.N. on net zero

      Though the decisions reached at COP27 did not include major new commitments on reducing emissions from the combustion of fossil fuels, the transition to a clean global energy system was nevertheless a key topic of conversation throughout the conference.

      The Council of Engineers for the Energy Transition (CEET), an independent, international body of engineers and energy systems experts formed to provide advice to the U.N. on achieving net-zero emissions globally by 2050, convened for the first time at COP27. Jessika Trancik, a professor in the MIT Institute for Data, Systems, and Society and a member of CEET, spoke on a U.N.-sponsored panel on solutions for the transition to clean energy.

      Trancik noted that the energy transition will look different in different regions of the world. “As engineers, we need to understand those local contexts and design solutions around those local contexts — that’s absolutely essential to support a rapid and equitable energy transition.”

      At the same time, Trancik noted that there is now a set of “low-cost, ready-to-scale tools” available to every region — tools that resulted from a globally competitive process of innovation, stimulated by public policies in different countries, that dramatically drove down the costs of technologies like solar energy and lithium-ion batteries. The key, Trancik said, is for regional transition strategies to “tap into global processes of innovation.”

      Reinventing climate adaptation

      Elfatih Eltahir, the H. M. King Bhumibol Professor of Hydrology and Climate, traveled to COP27 to present plans for the Jameel Observatory Climate Resilience Early Warning System (CREWSnet), one of the five projects selected in April 2022 as a flagship in MIT’s Climate Grand Challenges initiative. CREWSnet focuses on climate adaptation, the term for adapting to climate impacts that are unavoidable.

      The aim of CREWSnet, Eltahir told the audience during a panel discussion, is “nothing short of reinventing the process of climate change adaptation,” so that it is proactive rather than reactive; community-led; data-driven and evidence-based; and so that it integrates different climate risks, from heat waves to sea level rise, rather than treating them individually.

      “However, it’s easy to talk about these changes,” said Eltahir. “The real challenge, which we are now just launching and engaging in, is to demonstrate that on the ground.” Eltahir said that early demonstrations will happen in a couple of key locations, including southwest Bangladesh, where multiple climate risks — rising sea levels, increasing soil salinity, and intensifying heat waves and cyclones — are combining to threaten the area’s agricultural production.

      Building on COP26

      Some members of MIT’s delegation attended COP27 to advance efforts that had been formally announced at last year’s U.N. climate conference, COP26, in Glasgow, Scotland.

      At an official U.N. side event co-organized by MIT on Nov. 11, Greg Sixt, the director of the Food and Climate Systems Transformation (FACT) Alliance led by the Abdul Latif Jameel Water and Food Systems Lab, provided an update on the alliance’s work since its launch at COP26.

      Food systems are a major source of greenhouse gas emissions — and are increasingly vulnerable to climate impacts. The FACT Alliance works to better connect researchers to farmers, food businesses, policymakers, and other food systems stakeholders to make food systems (which include food production, consumption, and waste) more sustainable and resilient. 

      Sixt told the audience that the FACT Alliance now counts over 20 research and stakeholder institutions around the world among its members, but also collaborates with other institutions in an “open network model” to advance work in key areas — such as a new research project exploring how climate scenarios could affect global food supply chains.

      Marcela Angel, research program director for the Environmental Solutions Initiative (ESI), helped convene a meeting at COP27 of the Afro-InterAmerican Forum on Climate Change, which also launched at COP26. The forum works with Afro-descendant leaders across the Americas to address significant environmental issues, including climate risks and biodiversity loss. 

      At the event — convened with the Colombian government and the nonprofit Conservation International — ESI brought together leaders from six countries in the Americas and presented recent work that estimates that there are over 178 million individuals who identify as Afro-descendant living in the Americas, in lands of global environmental importance. 

      “There is a significant overlap between biodiversity hot spots, protected areas, and areas of high Afro-descendant presence,” said Angel. “But the role and climate contributions of these communities is understudied, and often made invisible.”    

      Limiting methane emissions

      Methane is a short-lived but potent greenhouse gas: When released into the atmosphere, it immediately traps about 120 times more heat than carbon dioxide does. More than 150 countries have now signed the Global Methane Pledge, launched at COP26, which aims to reduce methane emissions by at least 30 percent by 2030 compared to 2020 levels.

      Sergey Paltsev, the deputy director of the Joint Program on the Science and Policy of Global Change and a senior research scientist at the MIT Energy Initiative, gave the keynote address at a Nov. 17 event on methane, where he noted the importance of methane reductions from the oil and gas sector to meeting the 2030 goal.

      “The oil and gas sector is where methane emissions reductions could be achieved the fastest,” said Paltsev. “We also need to employ an integrated approach to address methane emissions in all sectors and all regions of the world because methane emissions reductions provide a near-term pathway to avoiding dangerous tipping points in the global climate system.”

      “Keep fighting relentlessly”

      Arina Khotimsky, a senior majoring in materials science and engineering and a co-president of the MIT Energy and Climate Club, attended the first week of COP27. She reflected on the experience in a social media post after returning home. 

      “COP will always have its haters. Is there greenwashing? Of course! Is everyone who should have a say in this process in the room? Not even close,” wrote Khotimsky. “So what does it take for COP to matter? It takes everyone who attended to not only put ‘climate’ on front-page news for two weeks, but to return home and keep fighting relentlessly against climate change. I know that I will.” More

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      Reversing the charge

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      Owners of electric vehicles (EVs) are accustomed to plugging into charging stations at home and at work and filling up their batteries with electricity from the power grid. But someday soon, when these drivers plug in, their cars will also have the capacity to reverse the flow and send electrons back to the grid. As the number of EVs climbs, the fleet’s batteries could serve as a cost-effective, large-scale energy source, with potentially dramatic impacts on the energy transition, according to a new paper published by an MIT team in the journal Energy Advances.

      “At scale, vehicle-to-grid (V2G) can boost renewable energy growth, displacing the need for stationary energy storage and decreasing reliance on firm [always-on] generators, such as natural gas, that are traditionally used to balance wind and solar intermittency,” says Jim Owens, lead author and a doctoral student in the MIT Department of Chemical Engineering. Additional authors include Emre Gençer, a principal research scientist at the MIT Energy Initiative (MITEI), and Ian Miller, a research specialist for MITEI at the time of the study.

      The group’s work is the first comprehensive, systems-based analysis of future power systems, drawing on a novel mix of computational models integrating such factors as carbon emission goals, variable renewable energy (VRE) generation, and costs of building energy storage, production, and transmission infrastructure.

      “We explored not just how EVs could provide service back to the grid — thinking of these vehicles almost like energy storage on wheels — but also the value of V2G applications to the entire energy system and if EVs could reduce the cost of decarbonizing the power system,” says Gençer. “The results were surprising; I personally didn’t believe we’d have so much potential here.”

      Displacing new infrastructure

      As the United States and other nations pursue stringent goals to limit carbon emissions, electrification of transportation has taken off, with the rate of EV adoption rapidly accelerating. (Some projections show EVs supplanting internal combustion vehicles over the next 30 years.) With the rise of emission-free driving, though, there will be increased demand for energy. “The challenge is ensuring both that there’s enough electricity to charge the vehicles and that this electricity is coming from renewable sources,” says Gençer.

      But solar and wind energy is intermittent. Without adequate backup for these sources, such as stationary energy storage facilities using lithium-ion batteries, for instance, or large-scale, natural gas- or hydrogen-fueled power plants, achieving clean energy goals will prove elusive. More vexing, costs for building the necessary new energy infrastructure runs to the hundreds of billions.

      This is precisely where V2G can play a critical, and welcome, role, the researchers reported. In their case study of a theoretical New England power system meeting strict carbon constraints, for instance, the team found that participation from just 13.9 percent of the region’s 8 million light-duty (passenger) EVs displaced 14.7 gigawatts of stationary energy storage. This added up to $700 million in savings — the anticipated costs of building new storage capacity.

      Their paper also described the role EV batteries could play at times of peak demand, such as hot summer days. “V2G technology has the ability to inject electricity back into the system to cover these episodes, so we don’t need to install or invest in additional natural gas turbines,” says Owens. “The way that EVs and V2G can influence the future of our power systems is one of the most exciting and novel aspects of our study.”

      Modeling power

      To investigate the impacts of V2G on their hypothetical New England power system, the researchers integrated their EV travel and V2G service models with two of MITEI’s existing modeling tools: the Sustainable Energy System Analysis Modeling Environment (SESAME) to project vehicle fleet and electricity demand growth, and GenX, which models the investment and operation costs of electricity generation, storage, and transmission systems. They incorporated such inputs as different EV participation rates, costs of generation for conventional and renewable power suppliers, charging infrastructure upgrades, travel demand for vehicles, changes in electricity demand, and EV battery costs.

      Their analysis found benefits from V2G applications in power systems (in terms of displacing energy storage and firm generation) at all levels of carbon emission restrictions, including one with no emissions caps at all. However, their models suggest that V2G delivers the greatest value to the power system when carbon constraints are most aggressive — at 10 grams of carbon dioxide per kilowatt hour load. Total system savings from V2G ranged from $183 million to $1,326 million, reflecting EV participation rates between 5 percent and 80 percent.

      “Our study has begun to uncover the inherent value V2G has for a future power system, demonstrating that there is a lot of money we can save that would otherwise be spent on storage and firm generation,” says Owens.

      Harnessing V2G

      For scientists seeking ways to decarbonize the economy, the vision of millions of EVs parked in garages or in office spaces and plugged into the grid for 90 percent of their operating lives proves an irresistible provocation. “There is all this storage sitting right there, a huge available capacity that will only grow, and it is wasted unless we take full advantage of it,” says Gençer.

      This is not a distant prospect. Startup companies are currently testing software that would allow two-way communication between EVs and grid operators or other entities. With the right algorithms, EVs would charge from and dispatch energy to the grid according to profiles tailored to each car owner’s needs, never depleting the battery and endangering a commute.

      “We don’t assume all vehicles will be available to send energy back to the grid at the same time, at 6 p.m. for instance, when most commuters return home in the early evening,” says Gençer. He believes that the vastly varied schedules of EV drivers will make enough battery power available to cover spikes in electricity use over an average 24-hour period. And there are other potential sources of battery power down the road, such as electric school buses that are employed only for short stints during the day and then sit idle.

      The MIT team acknowledges the challenges of V2G consumer buy-in. While EV owners relish a clean, green drive, they may not be as enthusiastic handing over access to their car’s battery to a utility or an aggregator working with power system operators. Policies and incentives would help.

      “Since you’re providing a service to the grid, much as solar panel users do, you could be paid for your participation, and paid at a premium when electricity prices are very high,” says Gençer.

      “People may not be willing to participate ’round the clock, but if we have blackout scenarios like in Texas last year, or hot-day congestion on transmission lines, maybe we can turn on these vehicles for 24 to 48 hours, sending energy back to the system,” adds Owens. “If there’s a power outage and people wave a bunch of money at you, you might be willing to talk.”

      “Basically, I think this comes back to all of us being in this together, right?” says Gençer. “As you contribute to society by giving this service to the grid, you will get the full benefit of reducing system costs, and also help to decarbonize the system faster and to a greater extent.”

      Actionable insights

      Owens, who is building his dissertation on V2G research, is now investigating the potential impact of heavy-duty electric vehicles in decarbonizing the power system. “The last-mile delivery trucks of companies like Amazon and FedEx are likely to be the earliest adopters of EVs,” Owen says. “They are appealing because they have regularly scheduled routes during the day and go back to the depot at night, which makes them very useful for providing electricity and balancing services in the power system.”

      Owens is committed to “providing insights that are actionable by system planners, operators, and to a certain extent, investors,” he says. His work might come into play in determining what kind of charging infrastructure should be built, and where.

      “Our analysis is really timely because the EV market has not yet been developed,” says Gençer. “This means we can share our insights with vehicle manufacturers and system operators — potentially influencing them to invest in V2G technologies, avoiding the costs of building utility-scale storage, and enabling the transition to a cleaner future. It’s a huge win, within our grasp.”

      The research for this study was funded by MITEI’s Future Energy Systems Center. More

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      Engineers solve a mystery on the path to smaller, lighter batteries

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      A discovery by MIT researchers could finally unlock the door to the design of a new kind of rechargeable lithium battery that is more lightweight, compact, and safe than current versions, and that has been pursued by labs around the world for years.

      The key to this potential leap in battery technology is replacing the liquid electrolyte that sits between the positive and negative electrodes with a much thinner, lighter layer of solid ceramic material, and replacing one of the electrodes with solid lithium metal. This would greatly reduce the overall size and weight of the battery and remove the safety risk associated with liquid electrolytes, which are flammable. But that quest has been beset with one big problem: dendrites.

      Dendrites, whose name comes from the Latin for branches, are projections of metal that can build up on the lithium surface and penetrate into the solid electrolyte, eventually crossing from one electrode to the other and shorting out the battery cell. Researchers haven’t been able to agree on what gives rise to these metal filaments, nor has there been much progress on how to prevent them and thus make lightweight solid-state batteries a practical option.

      The new research, being published today in the journal Joule in a paper by MIT Professor Yet-Ming Chiang, graduate student Cole Fincher, and five others at MIT and Brown University, seems to resolve the question of what causes dendrite formation. It also shows how dendrites can be prevented from crossing through the electrolyte.

      Chiang says in the group’s earlier work, they made a “surprising and unexpected” finding, which was that the hard, solid electrolyte material used for a solid-state battery can be penetrated by lithium, which is a very soft metal, during the process of charging and discharging the battery, as ions of lithium move between the two sides.

      This shuttling back and forth of ions causes the volume of the electrodes to change. That inevitably causes stresses in the solid electrolyte, which has to remain fully in contact with both of the electrodes that it is sandwiched between. “To deposit this metal, there has to be an expansion of the volume because you’re adding new mass,” Chiang says. “So, there’s an increase in volume on the side of the cell where the lithium is being deposited. And if there are even microscopic flaws present, this will generate a pressure on those flaws that can cause cracking.”

      Those stresses, the team has now shown, cause the cracks that allow dendrites to form. The solution to the problem turns out to be more stress, applied in just the right direction and with the right amount of force.

      While previously, some researchers thought that dendrites formed by a purely electrochemical process, rather than a mechanical one, the team’s experiments demonstrate that it is mechanical stresses that cause the problem.

      The process of dendrite formation normally takes place deep within the opaque materials of the battery cell and cannot be observed directly, so Fincher developed a way of making thin cells using a transparent electrolyte, allowing the whole process to be directly seen and recorded. “You can see what happens when you put a compression on the system, and you can see whether or not the dendrites behave in a way that’s commensurate with a corrosion process or a fracture process,” he says.

      The team demonstrated that they could directly manipulate the growth of dendrites simply by applying and releasing pressure, causing the dendrites to zig and zag in perfect alignment with the direction of the force.

      Applying mechanical stresses to the solid electrolyte doesn’t eliminate the formation of dendrites, but it does control the direction of their growth. This means they can be directed to remain parallel to the two electrodes and prevented from ever crossing to the other side, and thus rendered harmless.

      In their tests, the researchers used pressure induced by bending the material, which was formed into a beam with a weight at one end. But they say that in practice, there could be many different ways of producing the needed stress. For example, the electrolyte could be made with two layers of material that have different amounts of thermal expansion, so that there is an inherent bending of the material, as is done in some thermostats.

      Another approach would be to “dope” the material with atoms that would become embedded in it, distorting it and leaving it in a permanently stressed state. This is the same method used to produce the super-hard glass used in the screens of smart phones and tablets, Chiang explains. And the amount of pressure needed is not extreme: The experiments showed that pressures of 150 to 200 megapascals were sufficient to stop the dendrites from crossing the electrolyte.

      The required pressure is “commensurate with stresses that are commonly induced in commercial film growth processes and many other manufacturing processes,” so should not be difficult to implement in practice, Fincher adds.

      In fact, a different kind of stress, called stack pressure, is often applied to battery cells, by essentially squishing the material in the direction perpendicular to the battery’s plates — somewhat like compressing a sandwich by putting a weight on top of it. It was thought that this might help prevent the layers from separating. But the experiments have now demonstrated that pressure in that direction actually exacerbates dendrite formation. “We showed that this type of stack pressure actually accelerates dendrite-induced failure,” Fincher says.

      What is needed instead is pressure along the plane of the plates, as if the sandwich were being squeezed from the sides. “What we have shown in this work is that when you apply a compressive force you can force the dendrites to travel in the direction of the compression,” Fincher says, and if that direction is along the plane of the plates, the dendrites “will never get to the other side.”

      That could finally make it practical to produce batteries using solid electrolyte and metallic lithium electrodes. Not only would these pack more energy into a given volume and weight, but they would eliminate the need for liquid electrolytes, which are flammable materials.

      Having demonstrated the basic principles involved, the team’s next step will be to try to apply these to the creation of a functional prototype battery, Chiang says, and then to figure out exactly what manufacturing processes would be needed to produce such batteries in quantity. Though they have filed for a patent, the researchers don’t plan to commercialize the system themselves, he says, as there are already companies working on the development of solid-state batteries. “I would say this is an understanding of failure modes in solid-state batteries that we believe the industry needs to be aware of and try to use in designing better products,” he says.

      The research team included Christos Athanasiou and Brian Sheldon at Brown University, and Colin Gilgenbach, Michael Wang, and W. Craig Carter at MIT. The work was supported by the U.S. National Science Foundation, the U.S. Department of Defense, the U.S. Defense Advanced Research Projects Agency, and the U.S. Department of Energy. More

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      On batteries, teaching, and world peace

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      Over his long career as an electrochemist and professor, Donald Sadoway has earned an impressive variety of honors, from being named one of Time magazine’s 100 most influential people in 2012 to appearing on “The Colbert Report,” where he talked about “renewable energy and world peace,” according to Comedy Central.

      What does he personally consider to be his top achievements?

      “That’s easy,” he says immediately. “For teaching, it’s 3.091,” the MIT course on solid-state chemistry he led for some 18 years. An MIT core requirement, 3.091 is also one of the largest classes at the Institute. In 2003 it was the largest, with 630 students. Sadoway, who retires this year after 45 years in the Department of Materials Science and Engineering, estimates that over the years he’s taught the course to some 10,000 undergraduates.

      A passion for teaching

      Along the way he turned the class into an MIT favorite, complete with music, art, and literature. “I brought in all that enrichment because I knew that 95 percent of the students in that room weren’t going to major in anything chemical and this might be the last class they’d take in the subject. But it’s a requirement. So they’re 18 years old, they’re very smart, and many of them are very bored. You have to find a hook [to reach them]. And I did.”

      In 1995, Sadoway was named a Margaret MacVicar Faculty Fellow, an honor that recognizes outstanding classroom teaching at the Institute. Among the communications in support of his nomination:

      “His contributions are enormous and the class is in rapt attention from beginning to end. His lectures are highly articulate yet animated and he has uncommon grace and style. I was awed by his ability to introduce playful and creative elements into a core lecture…”

      Bill Gates would agree. In the early 2000s Sadoway’s lectures were shared with the world through OpenCourseWare, the web-based publication of MIT course materials. Gates was so inspired by the lectures that he asked to meet with Sadoway to learn more about his research. (Sadoway initially ignored Gates’ email because he thought his account had been hacked by MIT pranksters.)

      Research breakthroughs

      Teaching is not Sadoway’s only passion. He’s also proud of his accomplishments in electrochemistry. The discipline that involves electron transfer reactions is key to everything from batteries to the primary extraction of metals like aluminum and magnesium. “It’s quite wide-ranging,” says the John F. Elliott Professor Emeritus of Materials Chemistry.

      Sadoway’s contributions include two battery breakthroughs. First came the liquid metal battery, which could enable the large-scale storage of renewable energy. “That represents a huge step forward in the transition to green energy,” said António Campinos, president of the European Patent Office, earlier this year when Sadoway won the 2022 European Inventor Award for the invention in the category for Non-European Patent Office Countries.

      On “The Colbert Report,” Sadoway alluded to that work when he told Stephen Colbert that electrochemistry is the key to world peace. Why? Because it could lead to a battery capable of storing energy from the sun when the sun doesn’t shine and otherwise make renewables an important part of the clean energy mix. And that in turn could “plummet the price of petroleum and depose dictators all over the world without one shot being fired,” he recently recalled.

      The liquid metal battery is the focus of Ambri, one of six companies based on Sadoway’s inventions. Bill Gates was the first funder of the company, which formed in 2010 and aims to install its first battery soon. That battery will store energy from a reported 500 megawatts of on-site renewable generation, the same output as a natural gas power plant.

      Then, in August of this year, Sadoway and colleagues published a paper in Nature about “one of the first new battery chemistries in 30 years,” Sadoway says. “I wanted to invent something that was better, much better,” than the expensive lithium-ion batteries used in, for example, today’s electric cars.

      That battery is the focus of Avanti, one of three Sadoway companies formed just last year. The other two are Pure Lithium, to commercialize his inventions related to that element, and Sadoway Labs. The latter, a nonprofit, is essentially “a space to try radical innovations. We’re gonna start working on wild ideas.”

      Another focus of Sadoway’s research: green steel. Steelmaking produces huge amounts of greenhouse gases. Enter Boston Metal, another Sadoway company. This one is developing a new approach to producing steel based on research begun some 25 years ago. Unlike the current technology for producing steel, the Boston Metal approach — molten oxide electrolysis — does not use the element at the root of steel’s problems: carbon. The principal byproduct of the new system? Oxygen.

      In 2012, Sadoway gave a TED talk to 2,000 people on the liquid metal battery. He believes that that talk, which has now been seen by almost 2.5 million people, led to the wider publicity of his work — and science overall — on “The Colbert Report” and elsewhere. “The moral here is that if you step out of your comfort zone, you might be surprised at what can happen,” he concludes.

      Colleagues’ reflections

      “I met Don in 2006 when I was working for the iron and steel industry in Europe on ways to reduce greenhouse gas emissions from the production of those materials,” says Antoine Allanore, professor of metallurgy, Department of Materials Science and Engineering. “He was the same Don Sadoway that you see in recordings of his lectures: very elegant, very charismatic, and passionate about the technical solutions and underlying science of the process we were all investigating; electrolysis. A few years later, when I decided to pursue an academic career, I contacted Don and became a postdoctoral associate in his lab. That ultimately led to my becoming an MIT professor. People don’t believe me, but before I came to MIT the only thing I knew about the Institute was that Noam Chomsky was there … and Don Sadoway. And I felt, that’s a great place to be. And I stayed because I saw the exceptional things that can be accomplished at MIT and Don is the perfect example of that.”

      “I had the joy of meeting Don when I first arrived on the MIT campus in 1994,” recalls Felice Frankel, research scientist in the MIT departments of Chemical Engineering and Mechanical Engineering. “I didn’t have to talk him into the idea that researchers needed to take their images and graphics more seriously.  He got it — that it wasn’t just about pretty pictures. He was an important part of our five-year National Science Foundation project — Picturing to Learn — to bring that concept into the classroom. How lucky that was for me!”

      “Don has been a friend and mentor since we met in 1995 when I was an MIT senior,” says Luis Ortiz, co-founder and chief executive officer, Avanti Battery Co. “One story that is emblematic of Don’s insistence on excellence is from when he and I met with Bill Gates about the challenges in addressing climate change and how batteries could be the linchpin in solving them. I suggested that we create our presentation in PowerPoint [Microsoft software]. Don balked. He insisted that we present using Keynote on his MacBook Air, because ‘it looks so much better.’ I was incredulous that he wanted to walk into that venue exclusively using Apple products. Of course, he won the argument, but not without my admonition that there had better not be even a blip of an issue. In the meeting room, Microsoft’s former chief technology officer asked Don if he needed anything to hook up to the screen, ‘we have all those dongles.’ Don declined, but gave me that knowing look and whispered, ‘You see, they know, too.’ I ate my crow and we had a great long conversation without any issues.”

      “I remember when I first started working with Don on the liquid metal battery project at MIT, after I had chosen it as the topic for my master’s of engineering thesis,” adds David Bradwell, co-founder and chief technology officer, Ambri. “I was a wide-eyed graduate student, sitting in his office, amongst his art deco decorations, unique furniture, and historical and stylistic infographics, and from our first meeting, I could see Don’s passion for coming up with new and creative, yet practical scientific ideas, and for working on hard problems, in service of society. Don’s approaches always appear to be unconventional — wanting to stand out in a crowd, take the path less trodden, both based on his ideas, and his sense of style. It’s been an amazing journey working with him over the past decade-and-a-half, and I remain excited to see what other new, unconventional ideas, he can bring to this world.” More

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      3 Questions: Robert Stoner unpacks US climate and infrastructure laws

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      This month, the 2022 United Nations Climate Change Conference (COP27) takes place in Sharm El Sheikh, Egypt, bringing together governments, experts, journalists, industry, and civil society to discuss climate action to enable countries to collectively sharply limit anthropogenic climate change. As MIT Energy Initiative Deputy Director for Science and Technology Robert Stoner attends the conference, he takes a moment to speak about the climate and infrastructure laws enacted in the last year in the United States, and about the impact these laws can have in the global energy transition.

      Q: COP27 is now underway. Can you set the scene?

      A: There’s a lot of interest among vulnerable countries about compensation for the impacts climate change has had on them, or “loss and damage,” a topic that the United States refused to address last year at COP26, for fear of opening up a floodgate and leaving U.S. taxpayers exposed to unlimited liability for our past (and future) emissions. This is a crucial issue of fairness for developed countries — and, well, of acknowledging our common humanity. But in a sense, it’s also a sideshow, and addressing it won’t prevent a climate catastrophe — we really need to focus on mitigation. With the passage of the bipartisan Infrastructure Investment and Jobs Act and the Inflation Reduction Act (IRA), the United States is now in a strong position to twist some arms. These laws are largely about subsidizing the deployment of low-carbon technologies — pretty much all of them. We’re going to do a lot in the United States in the next decade that will lead to dramatic cost reductions for these technologies and enable other countries with fewer resources to adopt them as well. It’s exactly the leadership role the United States has needed to assume. Now we have the opportunity to rally the rest of the world and get other countries to commit to more ambitious decarbonization goals, and to build practical programs that take advantage of the investable pathways we’re going to create for public and private actors.

      But that alone won’t get us there — money is still a huge problem, especially in emerging markets and developing countries. And I don’t think the institutions we rely on to help these countries fund infrastructure — energy and everything else — are adequately funded. Nor do these institutions have the right structures, incentives, and staffing to fund low-carbon development in these countries rapidly enough or on the necessary scale. I’m talking about the World Bank, for instance, but the other multilateral organizations have similar issues. I frankly don’t think the multilaterals can be reformed or sufficiently redirected on a short enough time frame. We definitely need new leadership for these organizations, and I think we probably need to quickly establish new multilaterals with new people, more money, and a clarity of purpose that is likely beyond what can be achieved incrementally. I don’t know if this is going to be an active public discussion at COP27, but I hope it takes place somewhere soon. Given the strong role our government plays in financing and selecting the leadership of these institutions, perhaps this is another opportunity for the United States to demonstrate courage and leadership.

      Q: What “investable pathways” are you talking about?

      A: Well, the pathways we’re implicitly trying to pursue with the Infrastructure Act and IRA are pretty clear, and I’ll come back to them. But first let me describe the landscape: There are three main sources of demand for energy in the economy — industry (meaning chemical production, fuel for electricity generation, cement production, materials and manufacturing, and so on), transportation (cars, trucks, ships, planes, and trains), and buildings (for heating and cooling, mostly). That’s about it, and these three sectors account for 75 percent of our total greenhouse gas emissions. So the pathways are all about how to decarbonize these three end-use sectors. There are a lot of technologies — some that exist, some that don’t — that will have to be brought to bear. And so it can be a little overwhelming to try to imagine how it will all transpire, but it’s pretty clear at a high level what our options are:

      First, generate a lot of low-carbon electricity and electrify as many industrial processes, vehicles, and building heating systems as we can.
      Second, develop and deploy at massive scale technologies that can capture carbon dioxide from smokestacks, or the air, and put it somewhere that it can never escape from — in other words, carbon capture and sequestration, or CCS.
      Third, for end uses like aviation that really need to use fuels because of their extraordinary energy density, develop low-carbon alternatives to fossil fuels.
      And fourth is energy efficiency across the board — but I don’t really count that as a separate pathway per se.
      So, by “investable pathways” I mean specific ways to pursue these options that will attract investors. What the Infrastructure Act and the IRA do is deploy carrots (in the form of subsidies) in a variety of ways to close the gap between what it costs to deploy technologies like CCS that aren’t yet at a commercial stage because they’re immature, and what energy markets will tolerate. A similar situation occurs for low-carbon production of hydrogen, one of the leading low-carbon fuel candidates. We can make it by splitting water with electricity (electrolysis), but that costs too much with present-day technology; or we can make it more cheaply by separating it from methane (which is what natural gas mainly is), but that creates CO2 that has to be transported and sequestered somewhere. And then we have to store the hydrogen until we’re ready to use it, and transport it by pipeline to the industrial facilities where it will be used. That requires infrastructure that doesn’t exist — pipelines, compression stations, big tanks! Come to think of it, the demand for all that hydrogen doesn’t exist either — at least not if industry has to pay what it actually costs.

      So, one very important thing these new acts do is subsidize production of hydrogen in various ways — and subsidize the creation of a CCS industry. The other thing they do is subsidize the deployment at enormous scale of low-carbon energy technologies. Some of them are already pretty cheap, like solar and wind, but they need to be supported by a lot of storage on the grid (which we don’t yet have) and by other sorts of grid infrastructure that, again, don’t exist. So, they now get subsidized, too, along with other carbon-free and low-carbon generation technologies — basically all of them. The idea is that by stimulating at-scale deployment of all these established and emerging technologies, and funding demonstrations of novel infrastructure — effectively lowering the cost of supply of low-carbon energy in the form of electricity and fuels — we will draw out the private sector to build out much more of the connective infrastructure and invest in new industrial processes, new home heating systems, and low-carbon transportation. This subsidized build-out will take place over a decade and then phase out as costs fall — hopefully, leaving the foundation for a thriving low-carbon energy economy in its wake, along with crucial technologies and knowledge that will benefit the whole world.

      Q: Is all of the federal investment in energy infrastructure in the United States relevant to the energy crisis in Europe right now?

      A: Not in a direct way — Europe is a near-term catastrophe with a long-term challenge that is in many ways more difficult than ours because Europe doesn’t have the level of primary energy resources like oil and gas that we have in abundance. Energy costs more in Europe, especially absent Russian pipelines. In a way, the narrowing of Europe’s options creates an impetus to invest in low-carbon technologies sooner than otherwise. The result either way will be expensive energy and quite a lot of economic suffering for years. The near-term challenge is to protect people from high energy prices. The big spikes in electricity prices we see now are driven by the natural gas market disruption, which will eventually dissipate as new sources of electricity come online (Sweden, for example, just announced a plan to develop new nuclear, and we’re seeing other countries like Germany soften their stance on nuclear) — and gas markets will sort themselves out. Meanwhile governments are trying to shield their people with electricity price caps and other subsidies, but that’s enormously burdensome.

      The EU recently announced gas price caps for imported gas to try to eliminate price-gouging by importers and reduce the subsidy burden. That may help to lower downstream prices, or it may make matters worse by reducing the flow of gas into the EU and fueling scarcity pricing, and ultimately adding to the subsidy burden. A lot people are quite reasonably suggesting that if electricity prices are subject to crazy behavior in gas markets, then why not disconnect from the grid and self-generate? Wouldn’t that also help reduce demand for gas overall and also reduce CO2 emissions? It would. But it’s expensive to put solar panels on your roof and batteries in your basement — so for those rich enough to do this, it would lead to higher average electricity costs that would live on far into the future, even when grid prices eventually come down.

      So, an interesting idea is taking hold, with considerable encouragement from national governments — the idea of “energy communities,” basically, towns or cities that encourage local firms and homeowners to install solar and batteries, and make some sort of business arrangement with the local utility to allow the community to disconnect from the national grid at times of high prices and self-supply — in other words, use the utility’s wires to sell locally generated power locally. It’s interesting to think about — it takes less battery storage to handle the intermittency of solar when you have a lot of generators and consumers, so forming a community helps lower costs, and with a good deal from the utility for using their wires, it might not be that much more expensive. And of course, when the national grid is working well and prices are normal, the community would reconnect and buy power cheaply, while selling back its self-generated power to the grid. There are also potentially important social benefits that might accrue in these energy communities, too. It’s not a dumb idea, and we’ll see some interesting experimentation in this area in the coming years — as usual, the Germans are enthusiastic! More

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      With new heat treatment, 3D-printed metals can withstand extreme conditions

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      A new MIT-developed heat treatment transforms the microscopic structure of 3D-printed metals, making the materials stronger and more resilient in extreme thermal environments. The technique could make it possible to 3D print high-performance blades and vanes for power-generating gas turbines and jet engines, which would enable new designs with improved fuel consumption and energy efficiency.

      Today’s gas turbine blades are manufactured through conventional casting processes in which molten metal is poured into complex molds and directionally solidified. These components are made from some of the most heat-resistant metal alloys on Earth, as they are designed to rotate at high speeds in extremely hot gas, extracting work to generate electricity in power plants and thrust in jet engines.

      There is growing interest in manufacturing turbine blades through 3D-printing, which, in addition to its environmental and cost benefits, could allow manufacturers to quickly produce more intricate, energy-efficient blade geometries. But efforts to 3D-print turbine blades have yet to clear a big hurdle: creep.

      In metallurgy, creep refers to a metal’s tendency to permanently deform in the face of persistent mechanical stress and high temperatures. While researchers have explored printing turbine blades, they have found that the printing process produces fine grains on the order of tens to hundreds of microns in size — a microstructure that is especially vulnerable to creep.

      “In practice, this would mean a gas turbine would have a shorter life or less fuel efficiency,” says Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT. “These are costly, undesirable outcomes.”

      Cordero and his colleagues found a way to improve the structure of 3D-printed alloys by adding an additional heat-treating step, which transforms the as-printed material’s fine grains into much larger “columnar” grains — a sturdier microstructure that should minimize the material’s creep potential, since the “columns” are aligned with the axis of greatest stress. The researchers say the method, outlined today in Additive Manufacturing, clears the way for industrial 3D-printing of gas turbine blades.

      “In the near future, we envision gas turbine manufacturers will print their blades and vanes at large-scale additive manufacturing plants, then post-process them using our heat treatment,” Cordero says. “3D-printing will enable new cooling architectures that can improve the thermal efficiency of a turbine, so that it produces the same amount of power while burning less fuel and ultimately emits less carbon dioxide.”

      Cordero’s co-authors on the study are lead author Dominic Peachey, Christopher Carter, and Andres Garcia-Jimenez at MIT, Anugrahaprada Mukundan and Marie-Agathe Charpagne of the University of Illinois at Urbana-Champaign, and Donovan Leonard of Oak Ridge National Laboratory.

      Triggering a transformation

      The team’s new method is a form of directional recrystallization — a heat treatment that passes a material through a hot zone at a precisely controlled speed to meld a material’s many microscopic grains into larger, sturdier, and more uniform crystals.

      Directional recrystallization was invented more than 80 years ago and has been applied to wrought materials. In their new study, the MIT team adapted directional recrystallization for 3D-printed superalloys.

      The team tested the method on 3D-printed nickel-based superalloys — metals that are typically cast and used in gas turbines. In a series of experiments, the researchers placed 3D-printed samples of rod-shaped superalloys in a room-temperature water bath placed just below an induction coil. They slowly drew each rod out of the water and through the coil at various speeds, dramatically heating the rods to temperatures varying between 1,200 and 1,245 degrees Celsius.

      They found that drawing the rods at a particular speed (2.5 millimeters per hour) and through a specific temperature (1,235 degrees Celsius) created a steep thermal gradient that triggered a transformation in the material’s printed, fine-grained microstructure.

      “The material starts as small grains with defects called dislocations, that are like a mangled spaghetti,” Cordero explains. “When you heat this material up, those defects can annihilate and reconfigure, and the grains are able to grow. We’re continuously elongating the grains by consuming the defective material and smaller grains — a process termed recrystallization.”

      Creep away

      After cooling the heat-treated rods, the researchers examined their microstructure using optical and electron microscopy, and found that the material’s printed microscopic grains were replaced with “columnar” grains, or long crystal-like regions that were significantly larger than the original grains.

      “We’ve completely transformed the structure,” says lead author Dominic Peachey. “We show we can increase the grain size by orders of magnitude, to massive columnar grains, which theoretically should lead to dramatic improvements in creep properties.”

      The team also showed they could manipulate the draw speed and temperature of the rod samples to tailor the material’s growing grains, creating regions of specific grain size and orientation. This level of control, Cordero says, can enable manufacturers to print turbine blades with site-specific microstructures that are resilient to specific operating conditions.

      Cordero plans to test the heat treatment on 3D-printed geometries that more closely resemble turbine blades. The team is also exploring ways to speed up the draw rate, as well as test a heat-treated structure’s resistance to creep. Then, they envision that the heat treatment could enable the practical application of 3D-printing to produce industrial-grade turbine blades, with more complex shapes and patterns.

      “New blade and vane geometries will enable more energy-efficient land-based gas turbines, as well as, eventually, aeroengines,” Cordero notes. “This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices.”

      This research was supported, in part, by the U.S. Office of Naval Research. More