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    How light can vaporize water without the need for heat

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    It’s the most fundamental of processes — the evaporation of water from the surfaces of oceans and lakes, the burning off of fog in the morning sun, and the drying of briny ponds that leaves solid salt behind. Evaporation is all around us, and humans have been observing it and making use of it for as long as we have existed.

    And yet, it turns out, we’ve been missing a major part of the picture all along.

    In a series of painstakingly precise experiments, a team of researchers at MIT has demonstrated that heat isn’t alone in causing water to evaporate. Light, striking the water’s surface where air and water meet, can break water molecules away and float them into the air, causing evaporation in the absence of any source of heat.

    The astonishing new discovery could have a wide range of significant implications. It could help explain mysterious measurements over the years of how sunlight affects clouds, and therefore affect calculations of the effects of climate change on cloud cover and precipitation. It could also lead to new ways of designing industrial processes such as solar-powered desalination or drying of materials.

    The findings, and the many different lines of evidence that demonstrate the reality of the phenomenon and the details of how it works, are described today in the journal PNAS, in a paper by Carl Richard Soderberg Professor of Power Engineering Gang Chen, postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang.

    The authors say their study suggests that the effect should happen widely in nature— everywhere from clouds to fogs to the surfaces of oceans, soils, and plants — and that it could also lead to new practical applications, including in energy and clean water production. “I think this has a lot of applications,” Chen says. “We’re exploring all these different directions. And of course, it also affects the basic science, like the effects of clouds on climate, because clouds are the most uncertain aspect of climate models.”

    A newfound phenomenon

    The new work builds on research reported last year, which described this new “photomolecular effect” but only under very specialized conditions: on the surface of specially prepared hydrogels soaked with water. In the new study, the researchers demonstrate that the hydrogel is not necessary for the process; it occurs at any water surface exposed to light, whether it’s a flat surface like a body of water or a curved surface like a droplet of cloud vapor.

    Because the effect was so unexpected, the team worked to prove its existence with as many different lines of evidence as possible. In this study, they report 14 different kinds of tests and measurements they carried out to establish that water was indeed evaporating — that is, molecules of water were being knocked loose from the water’s surface and wafted into the air — due to the light alone, not by heat, which was long assumed to be the only mechanism involved.

    One key indicator, which showed up consistently in four different kinds of experiments under different conditions, was that as the water began to evaporate from a test container under visible light, the air temperature measured above the water’s surface cooled down and then leveled off, showing that thermal energy was not the driving force behind the effect.

    Other key indicators that showed up included the way the evaporation effect varied depending on the angle of the light, the exact color of the light, and its polarization. None of these varying characteristics should happen because at these wavelengths, water hardly absorbs light at all — and yet the researchers observed them.

    The effect is strongest when light hits the water surface at an angle of 45 degrees. It is also strongest with a certain type of polarization, called transverse magnetic polarization. And it peaks in green light — which, oddly, is the color for which water is most transparent and thus interacts the least.

    Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study.

    They have named this the photomolecular effect, by analogy with the photoelectric effect that was discovered by Heinrich Hertz in 1887 and finally explained by Albert Einstein in 1905. That effect was one of the first demonstrations that light also has particle characteristics, which had major implications in physics and led to a wide variety of applications, including LEDs. Just as the photoelectric effect liberates electrons from atoms in a material in response to being hit by a photon of light, the photomolecular effect shows that photons can liberate entire molecules from a liquid surface, the researchers say.

    “The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction,” says Xiulin Ruan, professor of mechanical engineering at Purdue University, who was not involved in the study. “It could help us gain new understanding of how sunlight interacts with cloud, fog, oceans, and other natural water bodies to affect weather and climate. It has significant potential practical applications such as high-performance water desalination driven by solar energy. This research is among the rare group of truly revolutionary discoveries which are not widely accepted by the community right away but take time, sometimes a long time, to be confirmed.”

    Solving a cloud conundrum

    The finding may solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight have often shown that they are absorbing more sunlight than conventional physics dictates possible. The additional evaporation caused by this effect could account for the longstanding discrepancy, which has been a subject of dispute since such measurements are difficult to make.

    “Those experiments are based on satellite data and flight data,“ Chen explains. “They fly an airplane on top of and below the clouds, and there are also data based on the ocean temperature and radiation balance. And they all conclude that there is more absorption by clouds than theory could calculate. However, due to the complexity of clouds and the difficulties of making such measurements, researchers have been debating whether such discrepancies are real or not. And what we discovered suggests that hey, there’s another mechanism for cloud absorption, which was not accounted for, and this mechanism might explain the discrepancies.”

    Chen says he recently spoke about the phenomenon at an American Physical Society conference, and one physicist there who studies clouds and climate said they had never thought about this possibility, which could affect calculations of the complex effects of clouds on climate. The team conducted experiments using LEDs shining on an artificial cloud chamber, and they observed heating of the fog, which was not supposed to happen since water does not absorb in the visible spectrum. “Such heating can be explained based on the photomolecular effect more easily,” he says.

    Lv says that of the many lines of evidence, “the flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.” That temperature profile “is a signature” that demonstrates the effect clearly, he says.

    Zhang adds: “It is quite hard to explain how this kind of flat temperature profile comes about without invoking some other mechanism” beyond the accepted theories of thermal evaporation. “It ties together what a whole lot of people are reporting in their solar desalination devices,” which again show evaporation rates that cannot be explained by the thermal input.

    The effect can be substantial. Under the optimum conditions of color, angle, and polarization, Lv says, “the evaporation rate is four times the thermal limit.”

    Already, since publication of the first paper, the team has been approached by companies that hope to harness the effect, Chen says, including for evaporating syrup and drying paper in a paper mill. The likeliest first applications will come in the areas of solar desalinization systems or other industrial drying processes, he says. “Drying consumes 20 percent of all industrial energy usage,” he points out.

    Because the effect is so new and unexpected, Chen says, “This phenomenon should be very general, and our experiment is really just the beginning.” The experiments needed to demonstrate and quantify the effect are very time-consuming. “There are many variables, from understanding water itself, to extending to other materials, other liquids and even solids,” he says.

    “The observations in the manuscript points to a new physical mechanism that foundationally alters our thinking on the kinetics of evaporation,” says Shannon Yee, an associate professor of mechanical engineering at Georgia Tech, who was not associated with this work. He adds, “Who would have thought that we are still learning about something as quotidian as water evaporating?”

    “I think this work is very significant scientifically because it presents a new mechanism,” says University of Alberta Distinguished Professor Janet A.W. Elliott, who also was not associated with this work. “It may also turn out to be practically important for technology and our understanding of nature, because evaporation of water is ubiquitous and the effect appears to deliver significantly higher evaporation rates than the known thermal mechanism. …  My overall impression is this work is outstanding. It appears to be carefully done with many precise experiments lending support for one another.”

    The work was partly supported by an MIT Bose Award. More

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      In a surprising finding, light can make water evaporate without heat

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      Evaporation is happening all around us all the time, from the sweat cooling our bodies to the dew burning off in the morning sun. But science’s understanding of this ubiquitous process may have been missing a piece all this time.

      In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate.

      After carrying out a series of new experiments and simulations, and reexamining some of the results from various groups that claimed to have exceeded the thermal limit, a team of researchers at MIT has reached a startling conclusion: Under certain conditions, at the interface where water meets air, light can directly bring about evaporation without the need for heat, and it actually does so even more efficiently than heat. In these experiments, the water was held in a hydrogel material, but the researchers suggest that the phenomenon may occur under other conditions as well.

      The findings are published this week in a paper in PNAS, by MIT postdoc Yaodong Tu, professor of mechanical engineering Gang Chen, and four others.

      The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy, the researchers say. And it might play an important part in many industrial processes such as solar-powered desalination of water, perhaps enabling alternatives to the step of converting sunlight to heat first.

      The new findings come as a surprise because water itself does not absorb light to any significant degree. That’s why you can see clearly through many feet of clean water to the surface below. So, when the team initially began exploring the process of solar evaporation for desalination, they first put particles of a black, light-absorbing material in a container of water to help convert the sunlight to heat.

      Then, the team came across the work of another group that had achieved an evaporation rate double the thermal limit — which is the highest possible amount of evaporation that can take place for a given input of heat, based on basic physical principles such as the conservation of energy. It was in these experiments that the water was bound up in a hydrogel. Although they were initially skeptical, Chen and Tu starting their own experiments with hydrogels, including a piece of the material from the other group. “We tested it under our solar simulator, and it worked,” confirming the unusually high evaporation rate, Chen says. “So, we believed them now.” Chen and Tu then began making and testing their own hydrogels.

      They began to suspect that the excess evaporation was being caused by the light itself —that photons of light were actually knocking bundles of water molecules loose from the water’s surface. This effect would only take place right at the boundary layer between water and air, at the surface of the hydrogel material — and perhaps also on the sea surface or the surfaces of droplets in clouds or fog.

      In the lab, they monitored the surface of a hydrogel, a JELL-O-like matrix consisting mostly of water bound by a sponge-like lattice of thin membranes. They measured its responses to simulated sunlight with precisely controlled wavelengths.

      The researchers subjected the water surface to different colors of light in sequence and measured the evaporation rate. They did this by placing a container of water-laden hydrogel on a scale and directly measuring the amount of mass lost to evaporation, as well as monitoring the temperature above the hydrogel surface. The lights were shielded to prevent them from introducing extra heat. The researchers found that the effect varied with color and peaked at a particular wavelength of green light. Such a color dependence has no relation to heat, and so supports the idea that it is the light itself that is causing at least some of the evaporation.

      The puffs of white condensation on glass is water being evaporated from a hydrogel using green light, without heat.Image: Courtesy of the researchers

      The researchers tried to duplicate the observed evaporation rate with the same setup but using electricity to heat the material, and no light. Even though the thermal input was the same as in the other test, the amount of water that evaporated never exceeded the thermal limit. However, it did so when the simulated sunlight was on, confirming that light was the cause of the extra evaporation.

      Though water itself does not absorb much light, and neither does the hydrogel material itself, when the two combine they become strong absorbers, Chen says. That allows the material to harness the energy of the solar photons efficiently and exceed the thermal limit, without the need for any dark dyes for absorption.

      Having discovered this effect, which they have dubbed the photomolecular effect, the researchers are now working on how to apply it to real-world needs. They have a grant from the Abdul Latif Jameel Water and Food Systems Lab to study the use of this phenomenon to improve the efficiency of solar-powered desalination systems, and a Bose Grant to explore the phenomenon’s effects on climate change modeling.

      Tu explains that in standard desalination processes, “it normally has two steps: First we evaporate the water into vapor, and then we need to condense the vapor to liquify it into fresh water.” With this discovery, he says, potentially “we can achieve high efficiency on the evaporation side.” The process also could turn out to have applications in processes that require drying a material.

      Chen says that in principle, he thinks it may be possible to increase the limit of water produced by solar desalination, which is currently 1.5 kilograms per square meter, by as much as three- or fourfold using this light-based approach. “This could potentially really lead to cheap desalination,” he says.

      Tu adds that this phenomenon could potentially also be leveraged in evaporative cooling processes, using the phase change to provide a highly efficient solar cooling system.

      Meanwhile, the researchers are also working closely with other groups who are attempting to replicate the findings, hoping to overcome skepticism that has faced the unexpected findings and the hypothesis being advanced to explain them.

      The research team also included Jiawei Zhou, Shaoting Lin, Mohammed Alshrah, and Xuanhe Zhao, all in MIT’s Department of Mechanical Engineering. More

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      in Environment

      AI pilot programs look to reduce energy use and emissions on MIT campus

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      Smart thermostats have changed the way many people heat and cool their homes by using machine learning to respond to occupancy patterns and preferences, resulting in a lower energy draw. This technology — which can collect and synthesize data — generally focuses on single-dwelling use, but what if this type of artificial intelligence could dynamically manage the heating and cooling of an entire campus? That’s the idea behind a cross-departmental effort working to reduce campus energy use through AI building controls that respond in real-time to internal and external factors. 

      Understanding the challenge

      Heating and cooling can be an energy challenge for campuses like MIT, where existing building management systems (BMS) can’t respond quickly to internal factors like occupancy fluctuations or external factors such as forecast weather or the carbon intensity of the grid. This results in using more energy than needed to heat and cool spaces, often to sub-optimal levels. By engaging AI, researchers have begun to establish a framework to understand and predict optimal temperature set points (the temperature at which a thermostat has been set to maintain) at the individual room level and take into consideration a host of factors, allowing the existing systems to heat and cool more efficiently, all without manual intervention. 

      “It’s not that different from what folks are doing in houses,” explains Les Norford, a professor of architecture at MIT, whose work in energy studies, controls, and ventilation connected him with the effort. “Except we have to think about things like how long a classroom may be used in a day, weather predictions, time needed to heat and cool a room, the effect of the heat from the sun coming in the window, and how the classroom next door might impact all of this.” These factors are at the crux of the research and pilots that Norford and a team are focused on. That team includes Jeremy Gregory, executive director of the MIT Climate and Sustainability Consortium; Audun Botterud, principal research scientist for the Laboratory for Information and Decision Systems; Steve Lanou, project manager in the MIT Office of Sustainability (MITOS); Fran Selvaggio, Department of Facilities Senior Building Management Systems engineer; and Daisy Green and You Lin, both postdocs.

      The group is organized around the call to action to “explore possibilities to employ artificial intelligence to reduce on-campus energy consumption” outlined in Fast Forward: MIT’s Climate Action Plan for the Decade, but efforts extend back to 2019. “As we work to decarbonize our campus, we’re exploring all avenues,” says Vice President for Campus Services and Stewardship Joe Higgins, who originally pitched the idea to students at the 2019 MIT Energy Hack. “To me, it was a great opportunity to utilize MIT expertise and see how we can apply it to our campus and share what we learn with the building industry.” Research into the concept kicked off at the event and continued with undergraduate and graduate student researchers running differential equations and managing pilots to test the bounds of the idea. Soon, Gregory, who is also a MITOS faculty fellow, joined the project and helped identify other individuals to join the team. “My role as a faculty fellow is to find opportunities to connect the research community at MIT with challenges MIT itself is facing — so this was a perfect fit for that,” Gregory says. 

      Early pilots of the project focused on testing thermostat set points in NW23, home to the Department of Facilities and Office of Campus Planning, but Norford quickly realized that classrooms provide many more variables to test, and the pilot was expanded to Building 66, a mixed-use building that is home to classrooms, offices, and lab spaces. “We shifted our attention to study classrooms in part because of their complexity, but also the sheer scale — there are hundreds of them on campus, so [they offer] more opportunities to gather data and determine parameters of what we are testing,” says Norford. 

      Developing the technology

      The work to develop smarter building controls starts with a physics-based model using differential equations to understand how objects can heat up or cool down, store heat, and how the heat may flow across a building façade. External data like weather, carbon intensity of the power grid, and classroom schedules are also inputs, with the AI responding to these conditions to deliver an optimal thermostat set point each hour — one that provides the best trade-off between the two objectives of thermal comfort of occupants and energy use. That set point then tells the existing BMS how much to heat up or cool down a space. Real-life testing follows, surveying building occupants about their comfort. Botterud, whose research focuses on the interactions between engineering, economics, and policy in electricity markets, works to ensure that the AI algorithms can then translate this learning into energy and carbon emission savings. 

      Currently the pilots are focused on six classrooms within Building 66, with the intent to move onto lab spaces before expanding to the entire building. “The goal here is energy savings, but that’s not something we can fully assess until we complete a whole building,” explains Norford. “We have to work classroom by classroom to gather the data, but are looking at a much bigger picture.” The research team used its data-driven simulations to estimate significant energy savings while maintaining thermal comfort in the six classrooms over two days, but further work is needed to implement the controls and measure savings across an entire year. 

      With significant savings estimated across individual classrooms, the energy savings derived from an entire building could be substantial, and AI can help meet that goal, explains Botterud: “This whole concept of scalability is really at the heart of what we are doing. We’re spending a lot of time in Building 66 to figure out how it works and hoping that these algorithms can be scaled up with much less effort to other rooms and buildings so solutions we are developing can make a big impact at MIT,” he says.

      Part of that big impact involves operational staff, like Selvaggio, who are essential in connecting the research to current operations and putting them into practice across campus. “Much of the BMS team’s work is done in the pilot stage for a project like this,” he says. “We were able to get these AI systems up and running with our existing BMS within a matter of weeks, allowing the pilots to get off the ground quickly.” Selvaggio says in preparation for the completion of the pilots, the BMS team has identified an additional 50 buildings on campus where the technology can easily be installed in the future to start energy savings. The BMS team also collaborates with the building automation company, Schneider Electric, that has implemented the new control algorithms in Building 66 classrooms and is ready to expand to new pilot locations. 

      Expanding impact

      The successful completion of these programs will also open the possibility for even greater energy savings — bringing MIT closer to its decarbonization goals. “Beyond just energy savings, we can eventually turn our campus buildings into a virtual energy network, where thousands of thermostats are aggregated and coordinated to function as a unified virtual entity,” explains Higgins. These types of energy networks can accelerate power sector decarbonization by decreasing the need for carbon-intensive power plants at peak times and allowing for more efficient power grid energy use.

      As pilots continue, they fulfill another call to action in Fast Forward — for campus to be a “test bed for change.” Says Gregory: “This project is a great example of using our campus as a test bed — it brings in cutting-edge research to apply to decarbonizing our own campus. It’s a great project for its specific focus, but also for serving as a model for how to utilize the campus as a living lab.” More

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      in Environment

      MIT engineering students take on the heat of Miami

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      Think back to the last time you had to wait for a bus. How miserable were you? If you were in Boston, your experience might have included punishing wind and icy sleet — or, more recently, a punch of pollen straight to the sinuses. But in Florida’s Miami-Dade County, where the effects of climate change are both drastic and intensifying, commuters have to contend with an entirely different set of challenges: blistering temperatures and scorching humidity, making long stints waiting in the sun nearly unbearable.

      One of Miami’s most urgent transportation needs is shared by car-clogged Boston: coaxing citizens to use the municipal bus network, rather than the emissions-heavy individual vehicles currently contributing to climate change. But buses can be a tough sell in a sunny city where humidity hovers between 60 and 80 percent year-round. 

      Enter MIT’s Department of Electrical Engineering and Computer Science (EECS) and the MIT Priscilla King Gray (PKG) Public Service Center. The result of close collaboration between the two organizations, class 6.900 (Engineering For Impact) challenges EECS students to apply their engineering savvy to real-world problems beyond the MIT campus.

      This spring semester, the real-world problem was heat. 

      Miami-Dade County Department of Transportation and Public Works Chief Innovation Officer Carlos Cruz-Casas explains: “We often talk about the city we want to live in, about how the proper mix of public transportation, on-demand transit, and other mobility solutions, such as e-bikes and e-scooters, could help our community live a car-light life. However, none of this will be achievable if the riders are not comfortable when doing so.” 

      “When people think of South Florida and climate change, they often think of sea level rise,” says Juan Felipe Visser, deputy director of equity and engagement within the Office of the Mayor in Miami-Dade. “But heat really is the silent killer. So the focus of this class, on heat at bus stops, is very apt.” With little tree cover to give relief at some of the hottest stops, Miami-Dade commuters cluster in tiny patches of shade behind bus stops, sometimes giving up when the heat becomes unbearable. 

      A more conventional electrical engineering course might use temperature monitoring as an abstract example, building sample monitors in isolation and grading them as a merely academic exercise. But Professor Joel Volman, EECS faculty head of electrical engineering, and Joe Steinmeyer, senior lecturer in EECS, had something more impactful in mind.

      “Miami-Dade has a large population of people who are living in poverty, undocumented, or who are otherwise marginalized,” says Voldman. “Waiting, sometimes for a very long time, in scorching heat for the bus is just one aspect of how a city population can be underserved, but by measuring patterns in how many people are waiting for a bus, how long they wait, and in what conditions, we can begin to see where services are not keeping up with demand.”

      Only after that gap is quantified can the work of city and transportation planners begin, Cruz-Casas explains: “We needed to quantify the time riders are exposed to extreme heat and prioritize improvements, including on-time performance improvements, increasing service frequency, or looking to enhance the tree canopy near the bus stop.” 

      Quantifying that time — and the subjective experience of the wait — proved tricky, however. With over 7,500 bus stops along 101 bus routes, Miami-Dade’s transportation network presents a considerable data-collection challenge. A network of physical temperature monitors could be useful, but only if it were carefully calibrated to meet the budgetary, environmental, privacy, and implementation requirements of the city. But how do you work with city officials — not to mention all of bus-riding Miami — from over 2,000 miles away? 

      This is where the PKG Center comes in. “We are a hub and a connector and facilitator of best practices,” explains Jill Bassett, associate dean and director of the center, who worked with Voldman and Steinmeyer to find a municipal partner organization for the course. “We bring knowledge of current pedagogy around community-engaged learning, which includes: help with framing a partnership that centers community-identified concerns and is mutually beneficial; identifying and learning from a community partner; talking through ways to build in opportunities for student learners to reflect on power dynamics, reciprocity, systems thinking, long-term planning, continuity, ethics, all the types of things that come up with this kind of shared project.”

      Through a series of brainstorming conversations, Bassett helped Voldman and Steinmeyer structure a well-defined project plan, as Cruz-Casas weighed in on the county’s needed technical specifications (including affordability, privacy protection, and implementability).

      “This course brings together a lot of subject area experts,” says Voldman. “We brought in guest lecturers, including Abby Berenson from the Sloan Leadership Center, to talk about working in teams; engineers from BOSE to talk about product design, certification, and environmental resistance; the co-founder and head of engineering from MIT spinout Butlr to talk about their low-power occupancy sensor; Tony Hu from MIT IDM [Integrated Design and Management] to talk about industrial design; and Katrina LaCurts from EECS to talk about communications and networking.”

      With the support of two generous donations and a gift of software from Altium, 6.900 developed into a hands-on exercise in hardware/software product development with a tangible goal in sight: build a better bus monitor.

      The challenges involved in this undertaking became apparent as soon as the 6.900 students began designing their monitors. “The most challenging requirement to meet was that the monitor be able to count how many people were waiting — and for how long they’d been standing there — while still maintaining privacy,” says Fabian Velazquez ’23 a recent EECS graduate. The task was complicated by commuters’ natural tendency to stand where the shade goes — whether beneath a tree or awning or snaking against a nearby wall in a line — rather than directly next to the bus sign or inside the bus shelter. “Accurately measuring people count with a camera — the most straightforward choice — is already quite difficult since you have to incorporate machine learning to identify which objects in frame are people. Maintaining privacy added an extra layer of constraint … since there is no guarantee the collected data wouldn’t be vulnerable.”

      As the groups weighed various privacy-preserving options, including lidar, radar, and thermal imaging, the class realized that Wi-Fi “sniffers,” which count the number of Wi-Fi enabled signals in the immediate area, were their best option to count waiting passengers. “We were all excited and ready for this amazing, answer-to-all-our-problems radar sensor to count people,” says Velasquez. “That component was extremely complex, however, and the complexity would have ultimately made my team use a lot of time and resources to integrate with our system. We also had a short time-to-market for this system we developed. We made the trade-off of complexity for robustness.” 

      The weather also posed its own set of challenges. “Environmental conditions were big factors on the structure and design of our devices,” says Yong Yan (Crystal) Liang, a rising junior majoring in EECS. “We incorporated humidity and temperature sensors into our data to show the weather at individual stops. Additionally, we also considered how our enclosure may be affected by extreme heat or potential hurricanes.”

      The heat variable proved problematic in multiple ways. “People detection was especially difficult, for in the Miami heat, thermal cameras may not be able to distinguish human body temperature from the surrounding air temperature, and the glare of the sun off of other surfaces in the area makes most forms of imaging very buggy,” says Katherine Mohr ’23. “My team had considered using mmWave sensors to get around these constraints, but we found the processing to be too difficult, and (like the rest of the class), we decided to only move forward with Wi-Fi/BLE [Bluetooth Low Energy] sniffers.”

      The most valuable component of the new class may well have been the students’ exposure to real-world hardware/software engineering product development, where limitations on time and budget always exist, and where client requests must be carefully considered.  “Having an actual client to work with forced us to learn how to turn their wants into more specific technical specifications,” says Mohr. “We chose deliverables each week to complete by Friday, prioritizing tasks which would get us to a minimum viable product, as well as tasks that would require extra manufacturing time, like designing the printed-circuit board and enclosure.”

      Play video

      Joel Voldman, who co-designed 6.900 (Engineering For Impact) with Joe Steinmeyer and MIT’s Priscilla King Gray (PKG) Public Service Center, describes how the course allowed students help develop systems for the public good. Voldman is the winner of the 2023 Teaching with Digital Technology Award, which is co-sponsored by MIT Open Learning and the Office of the Vice Chancellor. Video: MIT Open Learning

      Crystal Liang counted her conversations with city representatives as among her most valuable 6.900 experiences. “We generated a lot of questions and were able to communicate with the community leaders of this project from Miami-Dade, who made time to answer all of them and gave us ideas from the goals they were trying to achieve,” she reports. “This project gave me a new perspective on problem-solving because it taught me to see things from the community members’ point of view.” Some of those community leaders, including Marta Viciedo, co-founder of Transit Alliance Miami, joined the class’s final session on May 16 to review the students’ proposed solutions. 

      The students’ thoughtful approach paid off when it was time to present the heat monitors to the class’s client. In a group conference call with Miami-Dade officials toward the end of the semester, the student teams shared their findings and the prototypes they’d created, along with videos of the devices at work. Juan Felipe Visser was among those in attendance. “This is a lot of work,” he told the students following their presentation. “So first of all, thank you for doing that, and for presenting to us. I love the concept. I took the bus this morning, as I do every morning, and was battered by the sun and the heat. So I personally appreciated the focus.” 

      Cruz-Casas agreed: “I am pleasantly surprised by the diverse approach the students are taking. We presented a challenge, and they have responded to it and managed to think beyond the problem at hand. I’m very optimistic about how the outcomes of this project will have a long-lasting impact for our community. At a minimum, I’m thinking that the more awareness we raise about this topic, the more opportunities we have to have the brightest minds seeking for a solution.”

      The creators of 6.900 agree, and hope that their class helps more MIT engineers to broaden their perspective on the meaning and application of their work. 

      “We are really excited about students applying their skills within a real-world, complex environment that will impact real people,” says Bassett. “We are excited that they are learning that it’s not just the design of technology that matters, but that climate; environment and built environment; and issues around socioeconomics, race, and equity, all come into play. There are layers and layers to the creation and deployment of technology in a demographically diverse multilingual community that is at the epicenter of climate change.” More

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      in Energy

      Powering the future in Mongolia

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      Nestled within the Tuul River valley and embraced by the southern Khentii Mountain Range, Ulaanbaatar (UB), Mongolia’s largest city, presents itself as an arena where nature’s forces wage an unrelenting battle against human resilience. The capital city is an icy crucible, with bone-chilling winters that plummet temperatures to an astonishing -40 degrees Fahrenheit (-40 degrees Celsius). Mongolia, often hailed with the celestial moniker of “The Land of the Eternal Blue Sky,” paradoxically succumbs to a veil of pollution and energy struggles during the winter months, obscuring the true shade of the cherished vista.

      To understand the root of these issues, MIT students from classes 22.S094 (Climate and Sustainability Systems: Decarbonizing Ulaanbaatar at Scale) and 21A.S01 (Anthro-Engineering: Decarbonization at the Million-Person Scale) visited Mongolia to conduct on-site surveys, diving into the diverse tapestry of local life as they gleaned insight from various stakeholder groups. Setting foot on Mongolian soil on a crisp day in January, they wasted no time in shaking off the weariness of their arduous 17-hour flight, promptly embarking on a waiting bus. As they traversed the vast expanse of the countryside, their eyes were captivated by snow-laden terrain.

      That is, until a disconcerting sight unfolded — thick smog, akin to ethereal pillars, permeated the cityscape ahead. These imposing plumes emanated from the colossal smokestacks of Ulaanbaatar’s coal-fired power plants, steadfastly churning electricity and heat to fuel Mongolia’s central and district energy systems. Over 93 percent of the nation’s energy comes from coal-fired power plants, where the most considerable load is caused by household consumption. Nevertheless, with nearly half of Ulaanbaatar’s population disconnected from the central heating networks, one of Mongolia’s most significant sources of pollution comes from coal-burning stoves in the residential settlements known as the ger districts. Over the past three decades, since the democratic revolution in 1990, Mongolians have grappled with escalating concerns surrounding energy provision, accessibility, and sustainability.

      Engineers who think like anthropologists

      “We find ourselves compelled to venture on-site, engaging in direct conversations with the locals, and immersing ourselves in the fabric of daily life to uncover what we don’t know,” emphasized Michael Short, professor in MIT’s Department of Nuclear Science and Engineering and faculty lead of MIT’s NEET Climate and Sustainability Systems thread, shortly before heading to Mongolia.

      The Ulaanbaatar Project sprang from a multiyear collaboration between MIT and the National University of Mongolia (NUM). Shedding light on the matter, Professor Munkhbat Byambajav of the Department of Chemical and Biological Engineering at NUM underscored the paramount importance of mitigating environmental pollution at an economic scale to alleviate the heavy burden borne by the people.

      Class 22.S094 is offered through MIT’s New Engineering Education Transformation (NEET) program, which allows students with multidisciplinary interests to collaborate across departments within four different subject areas, or threads. In this capstone project, students consider ways to decarbonize a city like Ulaanbaatar, transitioning from burning coal briquettes to a more sustainable, energy-efficient solution, given several parameters and constraints set by the local context.

      One of the ideas students have recently explored is a thermal battery made with molten salt that can store enough energy to heat a ger for up to 12 hours with added insulation for cooling curve regulation. The Mongolian ger, meaning home, is a dome-like portable dwelling covered in felt and canvas, held together by ropes traditionally crafted of animal hair or wool. Over several semesters, students have been testing a version of their proposed idea on campus, working with a prototype that weighs around 35 pounds.

      Nathan Melenbrink, the lead instructor of NEET’s Climate and Sustainability Systems (CSS) thread, believes that the complexity of the Ulaanbaatar capstone project allows students to reject the one-way solution approach and instead consider challenges with a nonprescriptive mindset. The uniqueness of the CSS thread is that students are asked to build on the previous findings from the past cohort and iterate on their designs each year. This workflow has allowed the project to mature and advance in ways that may not be feasible within a semester schedule. When asked how the recent trip impacted the ongoing research back on campus, Melenbrink states, “In light of the recent trip to Mongolia, students are beginning to see the impact of cultural immersion and social awareness leveraging the technical scope and rigor of their work.”

      Course 21A.S01, taught by Professor Manduhai Buyandelger of the MIT Anthropology Section, proved instrumental in deepening students’ understanding of the intricate dynamics at play. She asks, “The prototype works in the lab, but does it work in real life once you factor in the challenges in the larger structures of delivery, production, and implementation in Mongolia?”

      This recognition of the social dimensions of engineering permeated the early stages of the UB project, engaging all participants, including students from MIT and NUM, professionals residing in Mongolia, and local nongovernmental organizations, fostering what Buyandelger aptly describes as “a collaboration on multiple scales: trans-disciplinary and transcontinental.” Lauren Bonilla, co-lecturer for the anthropology course, was crucial in devising the first onsite trip to Mongolia. Drawing upon her extensive ethnographic research in Mongolia that spans decades, Bonilla remarks, “To me, engineering is a highly social discipline.” She further stresses how anthro-engineering elevates the social dimensions of engineering by critically questioning the framing of problems and solutions, stating, “It draws on anthropological insights and methods, like ethnography, to bring a human face to the users of a technology and adds complexity and nuance to the social constraints that limit designs.”

      Making of khorkhog

      Amidst the frigid atmosphere, a traditional Mongolian ger stands in front of the Nuclear Science Laboratory at the National University of Mongolia, emitting warm steam from its roof. The faculty and students of NUM organize a welcoming event inside the ger, inviting everyone to partake in a khorkhog cookout. Earlier that week, a remark from the Mongolian energy representative stood out during one of the presentations: “We need powerful heat. Solar is not enough, and electricity is not enough. Mongolians need fire,” he had emphasized.

      Indeed, the culinary delight known as khorkhog demands the relentless embrace of scorching flames. The process involves a large metal jug, stones, fire, and lamb. With skillful precision, the volunteer chef places the fire-heated stones and large pieces of lamb into the cooking container, triggering a cascade of steam that fills the ger, accompanied by the sounds of sizzling and hissing. Everyone waits patiently as the cook carefully inspects the dish, keenly listening for signs of readiness. And when the time comes, a feast is shared among all, complemented by steam-cooked potatoes, freshly sliced onions, and vegetables. In this moment, the presence of fire symbolizes a profound connection with the heart of Mongolian culture, evoking a deep resonance among the gathered crowd as they partake in this cherished staple meal.

      The distance between two points

      Familiar faces form a grid on the computer screen as the standing meeting between the students in Massachusetts and Ulaanbaatar begins. Sharing the morning (evening in Mongolia) for updates has been a critical effort by both sides to stay engaged and make decisions together. NEET CSS students in Cambridge proceeded to share their latest findings.

      Lucy Nester, a nuclear science and engineering major, has been diligently working on developing a high-efficiency electrical heating solution for individual consumers. Her primary focus is leveraging the discounted electricity rates available in the ger districts and utilize existing infrastructure. Recognizing the importance of maximum flexibility in heating the brick, Nester emphasizes the “no one-size-fits-all” solution. She shares the results of her test trials, which involve both inductive and resistive heating methods, outlining the advantages and disadvantages of each approach. Despite her limited experience in electrical engineering and circuit building, Nester has impressively overcome the steep learning curve. She enthusiastically describes her UB trip as “one of the most remarkable experiences I’ve had during my time at MIT.”

      Darshdeep Grewal, a dedicated materials science and engineering major with a strong passion for data science and computation, has been diligently conducting research on convection heating using COMSOL Multiphysics. In his investigation, Grewal explores the correlation between air temperature and heating, investigates the impact of convecting air arrangement on the heating process, and examines the conditions that may contribute to overheating. Leveraging his expertise in computational workflows, Grewal presents an impressive collection of heatmap simulations derived from the extensive data accumulated by his team throughout the project. Recognizing the immense value of these simulations in modeling complex scenarios, he highlights the importance of running experiments concurrently with simulations to ensure accurate calibration of results, stating, “It’s important to stay rooted in reality.”

      Arina Khotimsky, another materials science and engineering major, has actively engaged in NEET’s Climate and Sustainability Systems thread since her sophomore year. Balancing the demands of her final semester at MIT and the upcoming review of 22.S094, Khotimsky reveals how she has seamlessly integrated her project involvement into her energy studies minor. Reflecting on her journey, she remarks, “Working on the Ulaanbaatar project has taught me the significance of taking local context into account while suggesting solutions as an engineer.” Khotimsky has been tirelessly iterating and refining the insulation box prototype, which holds the thermal battery and controls the rate at which the battery releases heat. In addition, the on-site observations have unveiled another design challenge — ensuring the insulation box functions as a secure and dependable means of transportation. 

      To “engineer” means to contrive through one’s deliberate use of skills. What confronted the UB Project team on site was not the limitations of skill or technology, but the real-world constraints often amiss in the early equation: the people and their everyday lives. With over 6,195 miles of distance between the two groups, it takes more than just dedication to make a collaboration blossom. That may be the desire for a positive impact. Moreover, it may be the goal of cultivating a healthier relationship with energy that spans a million-person scale. No matter where you are, there is no one solution to the complex story of energy. This progressive realization brings the two teams together every two weeks in virtual space, bridging the distance between the two points.  More

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      Asegun Henry wins National Science Foundation’s Alan T. Waterman Award

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      The National Science Foundation (NSF) today named Asegun Henry, an associate professor in MIT’s Department of Mechanical Engineering, as a 2023 recipient of its Alan T. Waterman Award. This award is the NSF’s highest honor for early-career researchers and provides funding for research in any science or engineering field. 

      This is the second year NSF has chosen to honor three researchers with the award. Henry is the sixth faculty member from MIT to receive this honor in its 47-year history, and is only the second mechanical engineer to ever win the award. In addition to a medal, Henry and his fellow awardees, Natalie S. King of Georgia State University and William Anderegg from the University of Utah, will each receive $1 million over five years for research in their chosen field of science.

      “I am thrilled to congratulate this year’s Waterman awardees, three outstanding scientists who are courageously tackling some of the most challenging societal problems through their ingenuity and innovative mindset,” says NSF Director Sethuraman Panchanathan. “Their pioneering accomplishments are precisely what the Waterman Award was created to recognize, and I look forward to their tremendous contributions in the future.”

      NSF recognizes Henry as an international thermal science and engineering leader. Henry has made breakthrough advances in nanoscale heat transfer and high-temperature energy systems. He directs the Atomistic Simulation and Energy (ASE) Research Group at MIT, focusing on heat transfer at the atomic level. He also works on developing technologies that can help mitigate climate change, addressing many problems from the atomic to the gigawatt scale.

      Henry and colleagues have led the development of several technological breakthroughs, setting a world record for the highest-temperature pump, using an all-ceramic mechanical pump to move liquid metal above 1,400 degrees Celsius, as well as the world record for thermophotovoltaic efficiency.

      “It has been challenging to push the field towards acceptance of new ideas, and it has been a path fraught with resistance and questioning of the validity of our results,” says Henry. “Receiving this award is vindicating and will impact my career greatly as it helps validate that the advances we’ve pioneered really do register as major contributions, and I pride myself on the impact of my work.”

      The Waterman Award will be presented to Henry at a ceremony held in Washington on May 9 during the National Science Board meeting.  More

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      Solving a longstanding conundrum in heat transfer

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      It is a problem that has beguiled scientists for a century. But, buoyed by a $625,000 Distinguished Early Career Award from the U.S. Department of Energy (DoE), Matteo Bucci, an associate professor in the Department of Nuclear Science and Engineering (NSE), hopes to be close to an answer.

      Tackling the boiling crisis

      Whether you’re heating a pot of water for pasta or are designing nuclear reactors, one phenomenon — boiling — is vital for efficient execution of both processes.

      “Boiling is a very effective heat transfer mechanism; it’s the way to remove large amounts of heat from the surface, which is why it is used in many high-power density applications,” Bucci says. An example use case: nuclear reactors.

      To the layperson, boiling appears simple — bubbles form and burst, removing heat. But what if so many bubbles form and coalesce that they form a band of vapor that prevents further heat transfer? Such a problem is a known entity and is labeled the boiling crisis. It would lead to runaway heat, and a failure of fuel rods in nuclear reactors. So “understanding and determining under which conditions the boiling crisis is likely to happen is critical to designing more efficient and cost-competitive nuclear reactors,” Bucci says.

      Early work on the boiling crisis dates back nearly a century ago, to 1926. And while much work has been done, “it is clear that we haven’t found an answer,” Bucci says. The boiling crisis remains a challenge because while models abound, the measurement of related phenomena to prove or disprove these models has been difficult. “[Boiling] is a process that happens on a very, very small length scale and over very, very short times,” Bucci says. “We are not able to observe it at the level of detail necessary to understand what really happens and validate hypotheses.”

      But, over the past few years, Bucci and his team have been developing diagnostics that can measure the phenomena related to boiling and thereby provide much-needed answers to a classic problem. Diagnostics are anchored in infrared thermometry and a technique using visible light. “By combining these two techniques I think we’re going to be ready to answer standing questions related to heat transfer, we can make our way out of the rabbit hole,” Bucci says. The grant award from the U.S. DoE for Nuclear Energy Projects will aid in this and Bucci’s other research efforts.

      An idyllic Italian childhood

      Tackling difficult problems is not new territory for Bucci, who grew up in the small town of Città di Castello near Florence, Italy. Bucci’s mother was an elementary school teacher. His father used to have a machine shop, which helped develop Bucci’s scientific bent. “I liked LEGOs a lot when I was a kid. It was a passion,” he adds.

      Despite Italy going through a severe pullback from nuclear engineering during his formative years, the subject fascinated Bucci. Job opportunities in the field were uncertain but Bucci decided to dig in. “If I have to do something for the rest of my life, it might as well be something I like,” he jokes. Bucci attended the University of Pisa for undergraduate and graduate studies in nuclear engineering.

      His interest in heat transfer mechanisms took root during his doctoral studies, a research subject he pursued in Paris at the French Alternative Energies and Atomic Energy Commission (CEA). It was there that a colleague suggested work on the boiling water crisis. This time Bucci set his sights on NSE at MIT and reached out to Professor Jacopo Buongiorno to inquire about research at the institution. Bucci had to fundraise at CEA to conduct research at MIT. He arrived just a couple of days before the Boston Marathon bombing in 2013 with a round-trip ticket. But Bucci has stayed ever since, moving on to become a research scientist and then associate professor at NSE.

      Bucci admits he struggled to adapt to the environment when he first arrived at MIT, but work and friendships with colleagues — he counts NSE’s Guanyu Su and Reza Azizian as among his best friends — helped conquer early worries.

      The integration of artificial intelligence

      In addition to diagnostics for boiling, Bucci and his team are working on ways of integrating artificial intelligence and experimental research. He is convinced that “the integration of advanced diagnostics, machine learning, and advanced modeling tools will blossom in a decade.”

      Bucci’s team is developing an autonomous laboratory for boiling heat transfer experiments. Running on machine learning, the setup decides which experiments to run based on a learning objective the team assigns. “We formulate a question and the machine will answer by optimizing the kinds of experiments that are necessary to answer those questions,” Bucci says, “I honestly think this is the next frontier for boiling,” he adds.

      “It’s when you climb a tree and you reach the top, that you realize that the horizon is much more vast and also more beautiful,” Bucci says of his zeal to pursue more research in the field.

      Even as he seeks new heights, Bucci has not forgotten his origins. Commemorating Italy’s hosting of the World Cup in 1990, a series of posters showcasing a soccer field fitted into the Roman Colosseum occupies pride of place in his home and office. Created by Alberto Burri, the posters are of sentimental value: The (now deceased) Italian artist also hailed from Bucci’s hometown — Città di Castello. More

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      Using excess heat to improve electrolyzers and fuel cells

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      Reducing the use of fossil fuels will have unintended consequences for the power-generation industry and beyond. For example, many industrial chemical processes use fossil-fuel byproducts as precursors to things like asphalt, glycerine, and other important chemicals. One solution to reduce the impact of the loss of fossil fuels on industrial chemical processes is to store and use the heat that nuclear fission produces. New MIT research has dramatically improved a way to put that heat toward generating chemicals through a process called electrolysis. 

      Electrolyzers are devices that use electricity to split water (H2O) and generate molecules of hydrogen (H2) and oxygen (O2). Hydrogen is used in fuel cells to generate electricity and drive electric cars or drones or in industrial operations like the production of steel, ammonia, and polymers. Electrolyzers can also take in water and carbon dioxide (CO2) and produce oxygen and ethylene (C2H4), a chemical used in polymers and elsewhere.

      There are three main types of electrolyzers. One type works at room temperature, but has downsides; they’re inefficient and require rare metals, such as platinum. A second type is more efficient but runs at high temperatures, above 700 degrees Celsius. But metals corrode at that temperature, and the devices need expensive sealing and insulation. The third type would be a Goldilocks solution for nuclear heat if it were perfected, running at 300-600 C and requiring mostly cheap materials like stainless steel. These cells have never been operated as efficiently as theory says they should. The new work, published this month in Nature, both illuminates the problem and offers a solution.

      A sandwich mystery

      The intermediate-temperature devices use what are called protonic ceramic electrochemical cells. Each cell is a sandwich, with a dense electrolyte layered between two porous electrodes. Water vapor is pumped into the top electrode. A wire on the side connects the two electrodes, and externally generated electricity runs from the top to the bottom. The voltage pulls electrons out of the water, which splits the molecule, releasing oxygen. A hydrogen atom without an electron is just a proton. The protons get pulled through the electrolyte to rejoin with the electrons at the bottom electrode and form H2 molecules, which are then collected.

      On its own, the electrolyte in the middle, made mainly of barium, cerium, and zirconium, conducts protons very well. “But when we put the same material into this three-layer device, the proton conductivity of the full cell is pretty bad,” says Yanhao Dong, a postdoc in MIT’s Department of Nuclear Science and Engineering and a paper co-author. “Its conductivity is only about 50 percent of the bulk form’s. We wondered why there’s an inconsistency here.”

      A couple of clues pointed them in the right direction. First, if they don’t prepare the cell very carefully, the top layer, only about 20 microns (.02 millimeters) thick, doesn’t stay attached. “Sometimes if you use just Scotch tape, it will peel off,” Dong says. Second, when they looked at a cross section of a device using a scanning electron microscope, they saw that the top surface of the electrolyte layer was flat, whereas the bottom surface of the porous electrode sitting on it was bumpy, and the two came into contact in only a few places. They didn’t bond well. That precarious interface leads to both structural de-lamination and poor proton passage from the electrode to the electrolyte.

      Acidic solution

      The solution turned out to be simple: researchers roughed up the top of the electrolyte. Specifically, they applied acid for 10 minutes, which etched grooves into the surface. Ju Li, the Battelle Energy Alliance Professor in Nuclear Engineering and professor of materials science and engineering at MIT, and a paper co-author, likens it to sandblasting a surface before applying paint to increase adhesion. Their acid-treated cells produced about 200 percent more hydrogen per area at 1.5 volts at 600 C than did any previous cell of its type, and worked well down to 350 C with very little performance decay over extended operation. 

      “The authors reported a surprisingly simple yet highly effective surface treatment to dramatically improve the interface,” says Liangbing Hu, the director of the Center for Materials Innovation at the Maryland Energy Innovation Institute, who was not involved in the work. He calls the cell performance “exceptional.”

      “We are excited and surprised” by the results, Dong says. “The engineering solution seems quite simple. And that’s actually good, because it makes it very applicable to real applications.” In a practical product, many such cells would be stacked together to form a module. MIT’s partner in the project, Idaho National Laboratory, is very strong in engineering and prototyping, so Li expects to see electrolyzers based on this technology at scale before too long. “At the materials level, this is a breakthrough that shows that at a real-device scale you can work at this sweet spot of temperature of 350 to 600 degrees Celsius for nuclear fission and fusion reactors,” he says.

      “Reduced operating temperature enables cheaper materials for the large-scale assembly, including the stack,” says Idaho National Laboratory researcher and paper co-author Dong Ding. “The technology operates within the same temperature range as several important, current industrial processes, including ammonia production and CO2 reduction. Matching these temperatures will expedite the technology’s adoption within the existing industry.”

      “This is very significant for both Idaho National Lab and us,” Li adds, “because it bridges nuclear energy and renewable electricity.” He notes that the technology could also help fuel cells, which are basically electrolyzers run in reverse, using green hydrogen or hydrocarbons to generate electricity. According to Wei Wu, a materials scientist at Idaho National Laboratory and a paper co-author, “this technique is quite universal and compatible with other solid electrochemical devices.”

      Dong says it’s rare for a paper to advance both science and engineering to such a degree. “We are happy to combine those together and get both very good scientific understanding and also very good real-world performance.”

      This work, done in collaboration with Idaho National Laboratory, New Mexico State University, and the University of Nebraska–Lincoln, was funded, in part, by the U.S. Department of Energy. More

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      Q&A: Randolph Kirchain on how cool pavements can mitigate climate change

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      As cities search for climate change solutions, many have turned to one burgeoning technology: cool pavements. By reflecting a greater proportion of solar radiation, cool pavements can offer an array of climate change mitigation benefits, from direct radiative forcing to reduced building energy demand.

      Yet, scientists from the MIT Concrete Sustainability Hub (CSHub) have found that cool pavements are not just a summertime solution. Here, Randolph Kirchain, a principal research scientist at CSHub, discusses how implementing cool pavements can offer myriad greenhouse gas reductions in cities — some of which occur even in the winter.

      Q: What exactly are cool pavements? 

      A: There are two ways to make a cool pavement: changing the pavement formulation to make the pavement porous like a sponge (a so-called “pervious pavement”), or paving with reflective materials. The latter method has been applied extensively because it can be easily adopted on the current road network with different traffic volumes while sustaining — and sometimes improving — the road longevity. To the average observer, surface reflectivity usually corresponds to the color of a pavement — the lighter, the more reflective. 

      We can quantify this surface reflectivity through a measurement called albedo, which refers to the percentage of light a surface reflects. Typically, a reflective pavement has an albedo of 0.3 or higher, meaning that it reflects 30 percent of the light it receives.

      To attain this reflectivity, there are a number of techniques at our disposal. The most common approach is to simply paint a brighter coating atop existing pavements. But it’s also possible to pave with materials that possess naturally greater reflectivity, such as concrete or lighter-colored binders and aggregates.

      Q: How can cool pavements mitigate climate change?

      A: Cool pavements generate several, often unexpected, effects. The most widely known is a reduction in surface and local air temperatures. This occurs because cool pavements absorb less radiation and, consequently, emit less of that radiation as heat. In the summer, this means they can lower urban air temperatures by several degrees Fahrenheit.

      By changing air temperatures or reflecting light into adjacent structures, cool pavements can also alter the need for heating and cooling in those structures, which can change their energy demand and, therefore, mitigate the climate change impacts associated with building energy demand.

      However, depending on how dense the neighborhood is built, a proportion of the radiation cool pavements reflect doesn’t strike buildings; instead, it travels back into the atmosphere and out into space. This process, called a radiative forcing, shifts the Earth’s energy balance and effectively offsets some of the radiation trapped by greenhouse gases (GHGs).

      Perhaps the least-known impact of cool pavements is on vehicle fuel consumption. Certain cool pavements, namely concrete, possess a combination of structural properties and longevity that can minimize the excess fuel consumption of vehicles caused by road quality. Over the lifetime of a pavement, these fuel savings can add up — often offsetting the higher initial footprint of paving with more durable materials.

      Q: With these impacts in mind, how do the effects of cool pavements vary seasonally and by location?

      A: Many view cool pavements as a solution to summer heat. But research has shown that they can offer climate change benefits throughout the year.

      In high-volume traffic roads, the most prominent climate change benefit of cool pavements is not their reflectivity but their impact on vehicle fuel consumption. As such, cool pavement alternatives that minimize fuel consumption can continue to cut GHG emissions in winter, assuming traffic is constant.

      Even in winter, pavement reflectivity still contributes greatly to the climate change mitigation benefits of cool pavements. We found that roughly a third of the annual CO2-equivalent emissions reductions from the radiative forcing effects of cool pavements occurred in the fall and winter.

      It’s important to note, too, that the direction — not just the magnitude — of cool pavement impacts also vary seasonally. The most prominent seasonal variation is the changes to building energy demand. As they lower air temperatures, cool pavements can lessen the demand for cooling in buildings in the summer, while, conversely, they can cause buildings to consume more energy and generate more emissions due to heating in the winter.

      Interestingly, the radiation reflected by cool pavements can also strike adjacent buildings, heating them up. In the summer, this can increase building energy demand significantly, yet in the winter it can also warm structures and reduce their need for heating. In that sense, cool pavements can warm — as well as cool — their surroundings, depending on the building insolation [solar exposure] systems and neighborhood density.

      Q: How can cities manage these many impacts?

      A: As you can imagine, such different and often competing impacts can complicate the implementation of cool pavements. In some contexts, for instance, a cool pavement might even generate more emissions over its life than a conventional pavement — despite lowering air temperatures.

      To ensure that the lowest-emitting pavement is selected, then, cities should use a life-cycle perspective that considers all potential impacts. When they do, research has shown that they can reap sizeable benefits. The city of Phoenix, for instance, could see its projected emissions fall by as much as 6 percent, while Boston would experience a reduction of up to 3 percent.

      These benefits don’t just demonstrate the potential of cool pavements: they also reflect the outsized impact of pavements on our built environment and, moreover, our climate. As cities move to fight climate change, they should know that one of their most extensive assets also presents an opportunity for greater sustainability.

      The MIT Concrete Sustainability Hub is a team of researchers from several departments across MIT working on concrete and infrastructure science, engineering, and economics. Its research is supported by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation. More