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      in Environment

      Concrete’s role in reducing building and pavement emissions

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      Encountering concrete is a common, even routine, occurrence. And that’s exactly what makes concrete exceptional.

      As the most consumed material after water, concrete is indispensable to the many essential systems — from roads to buildings — in which it is used.

      But due to its extensive use, concrete production also contributes to around 1 percent of emissions in the United States and remains one of several carbon-intensive industries globally. Tackling climate change, then, will mean reducing the environmental impacts of concrete, even as its use continues to increase.

      In a new paper in the Proceedings of the National Academy of Sciences, a team of current and former researchers at the MIT Concrete Sustainability Hub (CSHub) outlines how this can be achieved.

      They present an extensive life-cycle assessment of the building and pavements sectors that estimates how greenhouse gas (GHG) reduction strategies — including those for concrete and cement — could minimize the cumulative emissions of each sector and how those reductions would compare to national GHG reduction targets. 

      The team found that, if reduction strategies were implemented, the emissions for pavements and buildings between 2016 and 2050 could fall by up to 65 percent and 57 percent, respectively, even if concrete use accelerated greatly over that period. These are close to U.S. reduction targets set as part of the Paris Climate Accords. The solutions considered would also enable concrete production for both sectors to attain carbon neutrality by 2050.

      Despite continued grid decarbonization and increases in fuel efficiency, they found that the vast majority of the GHG emissions from new buildings and pavements during this period would derive from operational energy consumption rather than so-called embodied emissions — emissions from materials production and construction.

      Sources and solutions

      The consumption of concrete, due to its versatility, durability, constructability, and role in economic development, has been projected to increase around the world.

      While it is essential to consider the embodied impacts of ongoing concrete production, it is equally essential to place these initial impacts in the context of the material’s life cycle.

      Due to concrete’s unique attributes, it can influence the long-term sustainability performance of the systems in which it is used. Concrete pavements, for instance, can reduce vehicle fuel consumption, while concrete structures can endure hazards without needing energy- and materials-intensive repairs.

      Concrete’s impacts, then, are as complex as the material itself — a carefully proportioned mixture of cement powder, water, sand, and aggregates. Untangling concrete’s contribution to the operational and embodied impacts of buildings and pavements is essential for planning GHG reductions in both sectors.

      Set of scenarios

      In their paper, CSHub researchers forecast the potential greenhouse gas emissions from the building and pavements sectors as numerous emissions reduction strategies were introduced between 2016 and 2050.

      Since both of these sectors are immense and rapidly evolving, modeling them required an intricate framework.

      “We don’t have details on every building and pavement in the United States,” explains Randolph Kirchain, a research scientist at the Materials Research Laboratory and co-director of CSHub.

      “As such, we began by developing reference designs, which are intended to be representative of current and future buildings and pavements. These were adapted to be appropriate for 14 different climate zones in the United States and then distributed across the U.S. based on data from the U.S. Census and the Federal Highway Administration”

      To reflect the complexity of these systems, their models had to have the highest resolutions possible.

      “In the pavements sector, we collected the current stock of the U.S. network based on high-precision 10-mile segments, along with the surface conditions, traffic, thickness, lane width, and number of lanes for each segment,” says Hessam AzariJafari, a postdoc at CSHub and a co-author on the paper.

      “To model future paving actions over the analysis period, we assumed four climate conditions; four road types; asphalt, concrete, and composite pavement structures; as well as major, minor, and reconstruction paving actions specified for each climate condition.”

      Using this framework, they analyzed a “projected” and an “ambitious” scenario of reduction strategies and system attributes for buildings and pavements over the 34-year analysis period. The scenarios were defined by the timing and intensity of GHG reduction strategies.

      As its name might suggest, the projected scenario reflected current trends. For the building sector, solutions encompassed expected grid decarbonization and improvements to building codes and energy efficiency that are currently being implemented across the country. For pavements, the sole projected solution was improvements to vehicle fuel economy. That’s because as vehicle efficiency continues to increase, excess vehicle emissions due to poor road quality will also decrease.

      Both the projected scenarios for buildings and pavements featured the gradual introduction of low-carbon concrete strategies, such as recycled content, carbon capture in cement production, and the use of captured carbon to produce aggregates and cure concrete.

      “In the ambitious scenario,” explains Kirchain, “we went beyond projected trends and explored reasonable changes that exceed current policies and [industry] commitments.”

      Here, the building sector strategies were the same, but implemented more aggressively. The pavements sector also abided by more aggressive targets and incorporated several novel strategies, including investing more to yield smoother roads, selectively applying concrete overlays to produce stiffer pavements, and introducing more reflective pavements — which can change the Earth’s energy balance by sending more energy out of the atmosphere.

      Results

      As the grid becomes greener and new homes and buildings become more efficient, many experts have predicted the operational impacts of new construction projects to shrink in comparison to their embodied emissions.

      “What our life-cycle assessment found,” says Jeremy Gregory, the executive director of the MIT Climate Consortium and the lead author on the paper, “is that [this prediction] isn’t necessarily the case.”

      “Instead, we found that more than 80 percent of the total emissions from new buildings and pavements between 2016 and 2050 would derive from their operation.”

      In fact, the study found that operations will create the majority of emissions through 2050 unless all energy sources — electrical and thermal — are carbon-neutral by 2040. This suggests that ambitious interventions to the electricity grid and other sources of operational emissions can have the greatest impact.

      Their predictions for emissions reductions generated additional insights.  

      For the building sector, they found that the projected scenario would lead to a reduction of 49 percent compared to 2016 levels, and that the ambitious scenario provided a 57 percent reduction.

      As most buildings during the analysis period were existing rather than new, energy consumption dominated emissions in both scenarios. Consequently, decarbonizing the electricity grid and improving the efficiency of appliances and lighting led to the greatest improvements for buildings, they found.

      In contrast to the building sector, the pavements scenarios had a sizeable gulf between outcomes: the projected scenario led to only a 14 percent reduction while the ambitious scenario had a 65 percent reduction — enough to meet U.S. Paris Accord targets for that sector. This gulf derives from the lack of GHG reduction strategies being pursued under current projections.

      “The gap between the pavement scenarios shows that we need to be more proactive in managing the GHG impacts from pavements,” explains Kirchain. “There is tremendous potential, but seeing those gains requires action now.”

      These gains from both ambitious scenarios could occur even as concrete use tripled over the analysis period in comparison to the projected scenarios — a reflection of not only concrete’s growing demand but its potential role in decarbonizing both sectors.

      Though only one of their reduction scenarios (the ambitious pavement scenario) met the Paris Accord targets, that doesn’t preclude the achievement of those targets: many other opportunities exist.

      “In this study, we focused on mainly embodied reductions for concrete,” explains Gregory. “But other construction materials could receive similar treatment.

      “Further reductions could also come from retrofitting existing buildings and by designing structures with durability, hazard resilience, and adaptability in mind in order to minimize the need for reconstruction.”

      This study answers a paradox in the field of sustainability. For the world to become more equitable, more development is necessary. And yet, that very same development may portend greater emissions.

      The MIT team found that isn’t necessarily the case. Even as America continues to use more concrete, the benefits of the material itself and the interventions made to it can make climate targets more achievable.

      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

    • 113 Shares149 Views

      in Energy

      Making the case for hydrogen in a zero-carbon economy

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      As the United States races to achieve its goal of zero-carbon electricity generation by 2035, energy providers are swiftly ramping up renewable resources such as solar and wind. But because these technologies churn out electrons only when the sun shines and the wind blows, they need backup from other energy sources, especially during seasons of high electric demand. Currently, plants burning fossil fuels, primarily natural gas, fill in the gaps.

      “As we move to more and more renewable penetration, this intermittency will make a greater impact on the electric power system,” says Emre Gençer, a research scientist at the MIT Energy Initiative (MITEI). That’s because grid operators will increasingly resort to fossil-fuel-based “peaker” plants that compensate for the intermittency of the variable renewable energy (VRE) sources of sun and wind. “If we’re to achieve zero-carbon electricity, we must replace all greenhouse gas-emitting sources,” Gençer says.

      Low- and zero-carbon alternatives to greenhouse-gas emitting peaker plants are in development, such as arrays of lithium-ion batteries and hydrogen power generation. But each of these evolving technologies comes with its own set of advantages and constraints, and it has proven difficult to frame the debate about these options in a way that’s useful for policymakers, investors, and utilities engaged in the clean energy transition.

      Now, Gençer and Drake D. Hernandez SM ’21 have come up with a model that makes it possible to pin down the pros and cons of these peaker-plant alternatives with greater precision. Their hybrid technological and economic analysis, based on a detailed inventory of California’s power system, was published online last month in Applied Energy. While their work focuses on the most cost-effective solutions for replacing peaker power plants, it also contains insights intended to contribute to the larger conversation about transforming energy systems.

      “Our study’s essential takeaway is that hydrogen-fired power generation can be the more economical option when compared to lithium-ion batteries — even today, when the costs of hydrogen production, transmission, and storage are very high,” says Hernandez, who worked on the study while a graduate research assistant for MITEI. Adds Gençer, “If there is a place for hydrogen in the cases we analyzed, that suggests there is a promising role for hydrogen to play in the energy transition.”

      Adding up the costs

      California serves as a stellar paradigm for a swiftly shifting power system. The state draws more than 20 percent of its electricity from solar and approximately 7 percent from wind, with more VRE coming online rapidly. This means its peaker plants already play a pivotal role, coming online each evening when the sun goes down or when events such as heat waves drive up electricity use for days at a time.

      “We looked at all the peaker plants in California,” recounts Gençer. “We wanted to know the cost of electricity if we replaced them with hydrogen-fired turbines or with lithium-ion batteries.” The researchers used a core metric called the levelized cost of electricity (LCOE) as a way of comparing the costs of different technologies to each other. LCOE measures the average total cost of building and operating a particular energy-generating asset per unit of total electricity generated over the hypothetical lifetime of that asset.

      Selecting 2019 as their base study year, the team looked at the costs of running natural gas-fired peaker plants, which they defined as plants operating 15 percent of the year in response to gaps in intermittent renewable electricity. In addition, they determined the amount of carbon dioxide released by these plants and the expense of abating these emissions. Much of this information was publicly available.

      Coming up with prices for replacing peaker plants with massive arrays of lithium-ion batteries was also relatively straightforward: “There are no technical limitations to lithium-ion, so you can build as many as you want; but they are super expensive in terms of their footprint for energy storage and the mining required to manufacture them,” says Gençer.

      But then came the hard part: nailing down the costs of hydrogen-fired electricity generation. “The most difficult thing is finding cost assumptions for new technologies,” says Hernandez. “You can’t do this through a literature review, so we had many conversations with equipment manufacturers and plant operators.”

      The team considered two different forms of hydrogen fuel to replace natural gas, one produced through electrolyzer facilities that convert water and electricity into hydrogen, and another that reforms natural gas, yielding hydrogen and carbon waste that can be captured to reduce emissions. They also ran the numbers on retrofitting natural gas plants to burn hydrogen as opposed to building entirely new facilities. Their model includes identification of likely locations throughout the state and expenses involved in constructing these facilities.

      The researchers spent months compiling a giant dataset before setting out on the task of analysis. The results from their modeling were clear: “Hydrogen can be a more cost-effective alternative to lithium-ion batteries for peaking operations on a power grid,” says Hernandez. In addition, notes Gençer, “While certain technologies worked better in particular locations, we found that on average, reforming hydrogen rather than electrolytic hydrogen turned out to be the cheapest option for replacing peaker plants.”

      A tool for energy investors

      When he began this project, Gençer admits he “wasn’t hopeful” about hydrogen replacing natural gas in peaker plants. “It was kind of shocking to see in our different scenarios that there was a place for hydrogen.” That’s because the overall price tag for converting a fossil-fuel based plant to one based on hydrogen is very high, and such conversions likely won’t take place until more sectors of the economy embrace hydrogen, whether as a fuel for transportation or for varied manufacturing and industrial purposes.

      A nascent hydrogen production infrastructure does exist, mainly in the production of ammonia for fertilizer. But enormous investments will be necessary to expand this framework to meet grid-scale needs, driven by purposeful incentives. “With any of the climate solutions proposed today, we will need a carbon tax or carbon pricing; otherwise nobody will switch to new technologies,” says Gençer.

      The researchers believe studies like theirs could help key energy stakeholders make better-informed decisions. To that end, they have integrated their analysis into SESAME, a life cycle and techno-economic assessment tool for a range of energy systems that was developed by MIT researchers. Users can leverage this sophisticated modeling environment to compare costs of energy storage and emissions from different technologies, for instance, or to determine whether it is cost-efficient to replace a natural gas-powered plant with one powered by hydrogen.

      “As utilities, industry, and investors look to decarbonize and achieve zero-emissions targets, they have to weigh the costs of investing in low-carbon technologies today against the potential impacts of climate change moving forward,” says Hernandez, who is currently a senior associate in the energy practice at Charles River Associates. Hydrogen, he believes, will become increasingly cost-competitive as its production costs decline and markets expand.

      A study group member of MITEI’s soon-to-be published Future of Storage study, Gençer knows that hydrogen alone will not usher in a zero-carbon future. But, he says, “Our research shows we need to seriously consider hydrogen in the energy transition, start thinking about key areas where hydrogen should be used, and start making the massive investments necessary.”

      Funding for this research was provided by MITEI’s Low-Carbon Energy Centers and Future of Storage study. More

    • 75 Shares169 Views

      in Environment

      Countering climate change with cool pavements

      by

      Pavements are an abundant urban surface, covering around 40 percent of American cities. But in addition to carrying traffic, they can also emit heat.

      Due to what’s called the urban heat island effect, densely built, impermeable surfaces like pavements can absorb solar radiation and warm up their surroundings by re-emitting that radiation as heat. This phenomenon poses a serious threat to cities. It increases air temperatures by up as much as 7 degrees Fahrenheit and contributes to health and environmental risks — risks that climate change will magnify.

      In response, researchers at the MIT Concrete Sustainability Hub (MIT CSHub) are studying how a surface that ordinarily heightens urban heat islands can instead lessen their intensity. Their research focuses on “cool pavements,” which reflect more solar radiation and emit less heat than conventional paving surfaces.

      A recent study by a team of current and former MIT CSHub researchers in the journal of Environmental Science and Technology outlines cool pavements and their implementation. The study found that they could lower air temperatures in Boston and Phoenix by up to 1.7 degrees Celsius (3 F) and 2.1 C (3.7 F), respectively. They would also reduce greenhouse gas emissions, cutting total emissions by up to 3 percent in Boston and 6 percent in Phoenix. Achieving these savings, however, requires that cool pavement strategies be selected according to the climate, traffic, and building configurations of each neighborhood.

      Cities like Los Angeles and Phoenix have already conducted sizeable experiments with cool pavements, but the technology is still not widely implemented. The CSHub team hopes their research can guide future cool paving projects to help cities cope with a changing climate.

      Scratching the surface

      It’s well known that darker surfaces get hotter in sunlight than lighter ones. Climate scientists use a metric called “albedo” to help describe this phenomenon.

      “Albedo is a measure of surface reflectivity,” explains Hessam AzariJafari, the paper’s lead author and a postdoc at the MIT CSHub. “Surfaces with low albedo absorb more light and tend to be darker, while high-albedo surfaces are brighter and reflect more light.”

      Albedo is central to cool pavements. Typical paving surfaces, like conventional asphalt, possess a low albedo and absorb more radiation and emit more heat. Cool pavements, however, have brighter materials that reflect more than three times as much radiation and, consequently, re-emit far less heat.

      “We can build cool pavements in many different ways,” says Randolph Kirchain, a researcher in the Materials Science Laboratory and co-director of the Concrete Sustainability Hub. “Brighter materials like concrete and lighter-colored aggregates offer higher albedo, while existing asphalt pavements can be made ‘cool’ through reflective coatings.”

      CSHub researchers considered these several options in a study of Boston and Phoenix. Their analysis considered different outcomes when concrete, reflective asphalt, and reflective concrete replaced conventional asphalt pavements — which make up more than 95 percent of pavements worldwide.

      Situational awareness

      For a comprehensive understanding of the environmental benefits of cool pavements in Boston and Phoenix, researchers had to look beyond just paving materials. That’s because in addition to lowering air temperatures, cool pavements exert direct and indirect impacts on climate change.  

      “The one direct impact is radiative forcing,” notes AzariJafari. “By reflecting radiation back into the atmosphere, cool pavements exert a radiative forcing, meaning that they change the Earth’s energy balance by sending more energy out of the atmosphere — similar to the polar ice caps.”

      Cool pavements also exert complex, indirect climate change impacts by altering energy use in adjacent buildings.

      “On the one hand, by lowering temperatures, cool pavements can reduce some need for AC [air conditioning] in the summer while increasing heating demand in the winter,” says AzariJafari. “Conversely, by reflecting light — called incident radiation — onto nearby buildings, cool pavements can warm structures up, which can increase AC usage in the summer and lower heating demand in the winter.”

      What’s more, albedo effects are only a portion of the overall life cycle impacts of a cool pavement. In fact, impacts from construction and materials extraction (referred to together as embodied impacts) and the use of the pavement both dominate the life cycle. The primary use phase impact of a pavement — apart from albedo effects  — is excess fuel consumption: Pavements with smooth surfaces and stiff structures cause less excess fuel consumption in the vehicles that drive on them.

      Assessing the climate-change impacts of cool pavements, then, is an intricate process — one involving many trade-offs. In their study, the researchers sought to analyze and measure them.

      A full reflection

      To determine the ideal implementation of cool pavements in Boston and Phoenix, researchers investigated the life cycle impacts of shifting from conventional asphalt pavements to three cool pavement options: reflective asphalt, concrete, and reflective concrete.

      To do this, they used coupled physical simulations to model buildings in thousands of hypothetical neighborhoods. Using this data, they then trained a neural network model to predict impacts based on building and neighborhood characteristics. With this tool in place, it was possible to estimate the impact of cool pavements for each of the thousands of roads and hundreds of thousands of buildings in Boston and Phoenix.

      In addition to albedo effects, they also looked at the embodied impacts for all pavement types and the effect of pavement type on vehicle excess fuel consumption due to surface qualities, stiffness, and deterioration rate.

      After assessing the life cycle impacts of each cool pavement type, the researchers calculated which material — conventional asphalt, reflective asphalt, concrete, and reflective concrete — benefited each neighborhood most. They found that while cool pavements were advantageous in Boston and Phoenix overall, the ideal materials varied greatly within and between both cities.

      “One benefit that was universal across neighborhood type and paving material, was the impact of radiative forcing,” notes AzariJafari. “This was particularly the case in areas with shorter, less-dense buildings, where the effect was most pronounced.”

      Unlike radiative forcing, however, changes to building energy demand differed by location. In Boston, cool pavements reduced energy demand as often as they increased it across all neighborhoods. In Phoenix, cool pavements had a negative impact on energy demand in most census tracts due to incident radiation. When factoring in radiative forcing, though, cool pavements ultimately had a net benefit.

      Only after considering embodied emissions and impacts on fuel consumption did the ideal pavement type manifest for each neighborhood. Once factoring in uncertainty over the life cycle, researchers found that reflective concrete pavements had the best results, proving optimal in 53 percent and 73 percent of the neighborhoods in Boston and Phoenix, respectively.

      Once again, uncertainties and variations were identified. In Boston, replacing conventional asphalt pavements with a cool option was always preferred, while in Phoenix concrete pavements — reflective or not — had better outcomes due to rigidity at high temperatures that minimized vehicle fuel consumption. And despite the dominance of concrete in Phoenix, in 17 percent of its neighborhoods all reflective paving options proved more or less as effective, while in 1 percent of cases, conventional pavements were actually superior.

      “Though the climate change impacts we studied have proven numerous and often at odds with each other, our conclusions are unambiguous: Cool pavements could offer immense climate change mitigation benefits for both cities,” says Kirchain.

      The improvements to air temperatures would be noticeable: the team found that cool pavements would lower peak summer air temperatures in Boston by 1.7 C (3 F) and in Phoenix by 2.1 C (3.7 F). The carbon dioxide emissions reductions would likewise be impressive. Boston would decrease its carbon dioxide emissions by as much as 3 percent over 50 years while reductions in Phoenix would reach 6 percent over the same period.

      This analysis is one of the most comprehensive studies of cool pavements to date — but there’s more to investigate. Just as with pavements, it’s also possible to adjust building albedo, which may result in changes to building energy demand. Intensive grid decarbonization and the introduction of low-carbon concrete mixtures may also alter the emissions generated by cool pavements.

      There’s still lots of ground to cover for the CSHub team. But by studying cool pavements, they’ve elevated a brilliant climate change solution and opened avenues for further research and future mitigation.

      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