Climate in crisis

Record-setting heat waves, droughts and wildfires across the Western United States and Canada, deadly flooding in Germany, torrential rains in India, and other extreme events have ravaged communities around the globe. Greenhouse gases from human activities, like burning fossil fuels to power cars and factories, are raising temperatures and disrupting the world’s climate in an unprecedented way.

But although we’ve pushed the planet to the brink, it is not too late to pull it back. By prioritizing the cutting of carbon dioxide drastically and as soon as possible, we may yet reverse the course of climate change.

“We need to do a lot of work in a lot of different sectors, from the water sector, to iron and steel, to cement, to chemicals manufacturing, to everything else,” said Eric Larson, senior research engineer at the Andlinger Center for Energy and the Environment, which houses many researchers focused on securing our energy and environmental future. “Pick your favorite topic, and you’ll be able to make a contribution.”

Larson is a lead author on a report first issued in December 2020 that maps out what it would take to reach a “Net-Zero America,” where the amount of greenhouse gases the nation emits is offset by an equal amount removed. The report, the final version of which appeared in October 2021, lays out five pathways to decarbonizing the U.S. economy and reaching net-zero emissions by 2050.

These pathways can be achieved with today’s technologies — electric vehicles, heat pumps, wind and solar power, nuclear energy, and fossil fuels and biomass with carbon capture and storage. To transform our energy landscape in ways that enable people to live to their full potential across the planet, however, we’ll need new approaches and technologies, such as those being developed in Princeton’s School of Engineering and Applied Sciences.

Advances in reclaiming carbon from wastewater, lithium-ion-battery recycling, innovative building materials and new approaches to urban infrastructures are active areas of research at Princeton.

Wasted carbon no more

Think of carbon dioxide, and you might envision smokestacks and car tailpipes. But carbon is in solid waste and wastewater as well.

“Carbon dioxide, wastewater, all the food and garbage we throw away — these all contain wasted carbon, just in different forms,” said Zhiyong Jason Ren, professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment.

And these are not small quantities of carbon. A study by the U.S. Department of Energy found that the amount of chemical energy stored in the liquid waste stream is about equal to 11 billion gallons of gasoline per year, which is about 8% to 10% of the U.S. gasoline demand. In other words, if we could somehow capture the carbon and reuse it, we could dramatically reduce the amount of new carbon that we dig out of the ground.

Ren works on water infrastructure, looking at ways to reduce the carbon emissions associated with water purification and wastewater treatment, while recovering carbon to make new fuels and chemicals. He cites New York City as an example where remedying wastewater treatment could yield significant benefits. The city’s infrastructure and populace rely heavily on fossil fuel-based energy and emit large amounts of greenhouse gases, and the city also spends over $1 billion per year maintaining the water and wastewater infrastructure.

By installing systems that capture carbon from wastewater and convert it to fuels, chemicals and plastics, the city could turn a money-losing waste treatment center into a revenue center, Ren said. According to Ren’s calculations, this approach could generate millions of dollars in income for the city.

“When you clean up wastewater, at the same time you can generate renewable fuels, chemicals and bioplastics, and this is just one of the ways in which infrastructure and decarbonization are connected,” Ren said. “All these sectors are linked.”

To put wastewater to work, Ren and his team rely on microbes that digest carbon and generate free electrons. Under normal conditions, the microbes breathe and grow by consuming oxygen from the surrounding environment, then transferring electrons to the oxygen molecules. With Ren’s microbial electrochemistry platform, researchers sidetrack the flow of electrons, collect them and put them to work.

“We basically steal electrons from bacteria,” Ren said. “The bacteria transfer their electrons not to oxygen but to our electrode.”

The electrons can be transformed into electrical current, used in the production of fuels or even used to split water to release hydrogen for fuel cells.

This source of electrons could also drive salt removal — turning seawater into drinking water — or facilitate ocean mining for critical minerals like lithium. Typically, desalinization plants filter water via reverse osmosis, in which massive amounts of water are pushed through a filter to recover a relatively small amount of drinking water.

By taking advantage of the electrical potential generated between electrodes, these charged ions, salts and minerals will be extracted and recovered, resulting in separate outputs of clean water and concentrated mineral solutions.

Ren recently received a prestigious award from the Water Research Foundation to map greenhouse gas emissions in the water sector and help the industry decarbonize. “Water, energy and climate are intrinsically connected,” Ren said, “so it is imperative to understand how they are related and to develop cross-disciplinary solutions to tackle these challenges.”

Battery recycling

With the increasing popularity of electric vehicles and the role they play in getting to a net-zero economy, lithium-ion-battery recycling is a growing area of research.

A group of researchers at Princeton have developed a new process for recycling lithium-ion batteries using clouds of charged particles called plasmas to recover valuable metals such as cobalt and other battery materials, while avoiding the harsh chemical washes and high temperatures that are used in existing recycling methods.

The four-person team — which includes Bruce Koel, professor of chemical and biological engineering; Yiguang Ju, the Robert Porter Patterson Professor of Mechanical and Aerospace Engineering; Chao Yan, a postdoctoral researcher; and Xiaofang Yang, formerly a postdoctoral researcher at Princeton — founded a startup called Princeton NuEnergy to develop the technology.

Existing recycling strategies can recover materials like lithium, nickel and cobalt from spent batteries, but the processes are inefficient, environmentally harmful and costly. First, batteries are shredded to expose their various internal components, which are then separated. The electrode materials are then either smelted or dissolved in acid.

Next, there are multiple rounds of chemical separation to obtain the target metals, which finally must be remade into a functional form that can be used again as battery electrode materials. “Going through all these steps generates a tremendous amount of liquid waste, and consumes a lot of resources in the process,” Koel said.

Instead, the team uses several innovations for the component separation steps, then applies an electrically charged gas or plasma to remove contaminants and other impurities from the electrodes. The charged ions of the plasma react chemically with the impurities, cleaning them away without destroying the structure and composition of the still-usable material underneath.

The process can reclaim roughly 95% of a battery’s original lithium and other metals — which is comparable to the performance of existing recycling methods but cuts both energy use and CO2 emissions by more than half, while consuming far less water.

The researchers are now tailoring the process for each of the most commonly used varieties of lithium-ion battery, while also laying the groundwork to deploy the method at a large scale. “Transferring from the lab scale to the industrial scale is always challenging,” Yan said. “But as we become increasingly reliant on lithium-ion batteries, the prospect of being able to efficiently regenerate them is worth the effort.”

Better blocks

The net-zero future will also require changes to how we build things. When it comes to construction, concrete is essential. This mixture of sand, rocks, water and cement powder is one of the most-used materials on the planet, second only to water in its service to global society. But the critical ingredient, cement powder, is responsible for vast amounts of CO2 emissions.

Claire White, an associate professor in the Department of Civil and Environmental Engineering and the Andlinger Center, is an expert on concrete. White is developing low-emissions alternatives to the traditional Portland cement powder, an industry staple since it was invented in the 1800s.

“Around 7% to 8% of all CO2 emissions come from cement manufacturing,” White said. “Per ton of Portland cement powder you produce, you get about 900 kilograms of CO2 being emitted.”

Many of those emissions occur during the manufacturing process, when crushed limestone and clay are baked at 2600 degrees Fahrenheit. As the limestone heats up, it releases CO2.

One tactic to shrink the environmental impact is to use less Portland cement powder in the concrete mixture — replacing some of it with other materials, such as byproducts like fly ash from the coal industry. But White and her group are going one step further: They’re aiming for concrete mixtures that don’t need any Portland cement powder.

White’s current focus is on materials called alkali-activated calcined clays. These are mineral-rich clays that have been exposed briefly to intense heat, then mixed with an alkaline solution such as sodium silicate. The alkali causes the calcined clays to dissolve and form the binding cement “gel” that holds concrete together.

To determine the strength and durability of the resulting alternative concretes, White uses computer modeling and techniques like X-ray scattering to understand how the atoms interact. “We do a lot of this fundamental characterization to understand what’s going on at the nanoscale,” White said.

Some of the mixtures have the potential to outperform traditional concrete under harsh conditions. With support from the National Science Foundation and the Department of Energy, White’s group is developing fire-resistant concretes that can help minimize damage from wildfires, as well as concretes that stand up to the strongly acidic environment found in sewage systems.

But there are hurdles to overcome before these novel cements can be deployed on a large scale. “All of our construction codes are based on Portland-cement concrete,” White said.

Another impediment is the cost. Cement alternatives are expensive, and alternative concretes can cost twice as much as traditional concrete. These materials are currently manufactured for industries that require higher levels of purity than what is needed for making cement — clays are used in making paper, for example, while certain alkali activators are used in food production. To make these materials affordable for use in construction, manufacturing methods will have to evolve.

Addressing these challenges will take time, but White believes that more sustainable cements will eventually transform the concrete industry.

Urban transformation

Lowering emissions to reach net-zero targets must be done in ways that meet the needs of society and individuals. Environmental engineer Anu Ramaswami thinks a lot about this challenge. “Cities are a really important action arena,” Ramaswami said. “They contribute to more than 70% of global greenhouse gas emissions.”

Ramaswami is director of the Sustainable Healthy Cities Network, a National Science Foundation-funded network based at Princeton in which researchers from major universities work with industry and policy partners to make cities simultaneously more livable and better for the planet. She sees urban infrastructure as a set of seven interconnected systems — water, energy, food, shelter, transportation, waste management and public spaces.

“I study the city as a sort of the ultimate engineered system,” said Ramaswami, who is a chemical engineer by training and holds joint appointments in the Department of Civil and Environmental Engineering, Princeton’s High Meadows Environmental Institute, and the Princeton Institute of International and Regional Studies. “I ask how we can think of redesigning and innovating cities in ways that advance human and planetary outcomes.”

The seven systems are highly interdependent and work across geographic scales that extend beyond city limits, Ramaswami and her team showed in an influential paper published in 2016 in Science.

For example, greenhouse gas emissions associated with cities can come from far outside the city’s boundaries, from power plants and trucking routes that supply the city. “I pioneered methodologies for cities to account for their emissions all along the supply chain,” she said.

For Ramaswami, there are four factors that tell whether a city’s infrastructure is doing its job: environment, ealth, wellbeing and equity. This last one is especially important.

Infrastructure is at the heart of urban inequality. For example, transportation is the gateway for access to jobs, health care and many other human needs. Access to energy — such as electricity to power your laptop — is essential for finding a job and connecting with services and with one another.

“These physical infrastructure systems are really foundational,” Ramaswami said. “Without them, all other things become that much more difficult.”

But measuring outcomes such as equity can quickly become subjective. Ramaswami and her team wanted to find a reliable way to measure social equity across cities. They partnered with electric utilities to look at energy usage patterns in two cities, Tallahassee, Florida, and St. Paul, Minnesota, looking for patterns of inequality. They found that the amount and intensity of energy usage varies by income but also by race independent of income, yielding important metrics of inequality. The study was published in the Proceedings of the National Academy of Sciences in June 2021.

“Now we have a methodology to actually measure infrastructure inequality, unpacking income and race effects,” she said.

Ramaswami is working on a scenarioplanning tool that enables cities to bring all the sectors together to identify how changes to various sectors will return benefits to the environment, as well as health, well- being and equity. The tool will allow users to make policy choices — such as land-use planning and incentives for electric vehicles — with the goal of informing pathways toward a net-zero future.

She is also developing partnerships with entities and governments in India. Using machine learning and other technologies, her team has developed an energy-usage database for all 600-plus districts in India.

“That’s sort of a new frontier,” said Ramaswami, who is Princeton’s Sanjay Swani ’87 Professor of India Studies and director of the M.S. Chadha Center for Global India, which is focused on interactions between India and the world on issues such as water, climate, health and the arts. “How do you go from working with a few cities, to providing data and models to all cities? That is the challenge, as well as a huge opportunity to inform beneficial infrastructure transformations across large numbers of cities.”

Shaping the future

As countries around the world consider ways to decarbonize the global economy, designing and deploying new infrastructure provides an opportunity to make deliberate choices about the shape of our societies.

Traditionally, planning new or expanded infrastructure has been based around a single goal: cost- effectiveness. “Federal and state agendas are informed by computational energy system models that optimize for costs,” said Erin Mayfield, a former postdoctoral scholar at Princeton’s High Meadows Environmental Institute, Andlinger Center and Carbon Mitigation Initiative, who recently became Dartmouth’s Hodgson Family Assistant Professor of Engineering. “Basically, they’re trying to select least-cost technology options to reach some type of emissions target.”

But the real world is far more multidimensional. A change in infrastructure — like closing down a natural-gas facility or building a new wind farm — could mean an increase or decrease in levels of employment, property values and local air quality. It’s a delicate balance that links environmental policy and planning to questions of social equity.

Mayfield builds computational models that optimize multiple objectives rather than just prioritizing low cost, giving policymakers tools to incorporate the goals of different stakeholders when making decisions. These goals could be minimizing air pollution-related mortality in vulnerable communities or job creation across regions. And these policy changes, Mayfield said, “might not translate to much more cost, or might not mean more cost at all.”

Toward 2050

One of the conclusions in the Net-Zero America report is that the annual cost of implementing the pathway to carbon neutrality by 2050 is about the same as what the country already spends on energy, or about 4% to 6% of gross domestic product (GDP).

“Net-zero pathways for the U.S. require spending a similar fraction of GDP that we spend on energy today, but we have to immediately shift investments toward new clean infrastructure instead of existing systems,” said Jesse Jenkins, an assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment. Jenkins was a lead author of the Net-Zero America report with Larson and Chris Greig, the Theodora D. ’78 & William H. Walton III ’74 Senior Research Scientist at the Andlinger Center.

If the U.S. can realize such a shift, there is hope that other countries might be able to as well.

By 2050, will the planet still be experiencing unprecedented flash floods, town-consuming fires and crippling droughts? That’s an unanswerable question today, but through the research being conducted at Princeton and other institutions, combined with policy decisions and new innovations, our planet may stand a chance.

Reaching a Net-Zero America
The Net-Zero America project aims to inform and ground political, business, and societal conversations regarding what it would take for the U.S. to achieve an economy-wide target of net-zero emissions of greenhouse gases by 2050. The study provides granular guidance on what getting to net-zero really requires and on the actions needed to translate pledges into tangible progress.

The two-year research effort was funded primarily by the High Meadows Environmental Institute’s Carbon Mitigation Initiative, a research program focused on studying and addressing climate change that is funded by BP, and ExxonMobil through the Andlinger Center’s Princeton E-ffiliates Partnership.