While the U.S. federal government has clean energy targets, they are not binding. Most economically developed countries have mandatory policies designed to bolster renewable electricity production. Because the U.S. lacks an enforceable federal mandate for renewable electricity, individual states are left to develop their own regulations. 

Marilyn Brown, Regents’ and Brook Byers Professor of Sustainable Systems in Georgia Tech’s School of Public Policy; Shan Zhou, an assistant professor at Purdue University and Georgia Tech Ph.D. alumna; and Barry Solomon, a professor emeritus of environmental policy at Michigan Technological University, investigated how clean electricity policies affect not only the states that adopt them, but neighboring states as well. Using data-driven comparisons, the researchers found that the impact of these subnational clean energy policies is far greater — and more nuanced — than previously known. 

Their research was recently published in the journal Proceedings of the National Academy of Sciences. 

“Analysts are asking if the U.S. should have a federal renewable mandate to put the whole country on the same page, or if individual state policies are sufficient,” Brown said. “To answer that question, it is useful to know if states with renewable energy policies are influencing those without them.”

Brown, Solomon, and Zhou examined a common clean energy policy tool: the Renewable Portfolio Standard (RPS). Adopted by more than half of U.S. states, RPSs are regulations requiring a state’s utility providers to generate a certain percentage of their electricity from renewable resources, such as wind or solar. Many of these standards are mandatory, with utility companies facing fines if they fail to reach targets within a given time.

To investigate the influence of these policies across state lines, the researchers first created a dataset that included 31 years (1991-2021) of annual renewable electricity generation data for 48 U.S. states and the District of Columbia. They then used the dataset to generate pairs of states linking each state to its geographic neighbors or electricity trading partners, allowing them to examine the influence of the RPS policy adopted by one of the pair on the renewable energy generation of the other — a total of 1,519 paired comparisons. 

“By only looking at the pairs, we can see if an RPS in one state directly affects renewable electricity generation in another state, and, if that’s the case, whether it is because they are geographic neighbors or if it’s because they are participating in the same wholesale electricity market,” Zhou said. 

Looking into the electricity market is important, because states often purchase electricity from other states through wholesale markets rather than exclusively producing their own power, and the purchased power can be generated from renewables. Utilities in some states may be allowed to meet their own RPS requirements by purchasing renewable energy credits based on the renewable electricity generated in other states. 

In their analyses, the team also considered the concept of “policy stringency.” A stringency measure evaluates a state’s renewable electricity targets relative to the amount currently produced in the state. For example, if a state requires electric utilities to generate 30% of their electricity from renewable sources by 2030 and the state already has 25%, it isn’t a very stringent policy. On the other hand, if a state has a 30% target and only uses 10% renewables currently, it has a more ambitious and stringent RPS.

Though policy experts have used the metric in related work for over a decade, the research team improved the design. 

“Our stringency variable includes interim targets as well as the existing share of renewable energy generation,” Solomon said.

The team found that the amount of renewable electricity generation in a state is not only influenced by whether that state has its own RPS, but also by the RPS policies of neighboring states. 

“We also learned that the stronger a neighboring state’s RPS policy is, the more likely a given state is to generate more renewable electricity,” Brown said. “It’s all a very interactive web with many co-benefits.”

The authors were surprised to find that a given state’s electricity trading partners did not hold the most influence over renewable generation, but rather the geographical proximity to RPS states. They suggest that past RPS policy research focusing on within-state impacts likely underestimated an RPS’s full impact. While the researchers have not yet identified all factors that can cause spillover effects, they plan to investigate this further. 

“The spillover effect is very significant and should not be overlooked by future research, especially for states without RPSs,” Zhou said. “For states without policies, their renewable electricity generation is very heavily influenced by their neighbors.”

Citation: Shan Zhou, Barry D. Solomon, and Marilyn A. Brown, “The spillover effect of mandatory renewable portfolio standards.” PNAS (June 2024). 
DOI: https://doi.org/10.1073/pnas.2313193121
 

 

- Written by Benjamin Wright -

Yuanzhi Tang knows firsthand how much of an impact BBISS can make through its programs. The associate professor in the School of Earth and Atmospheric Sciences answered a BBISS call for faculty fellowships, and later seed funding for a project related to sustainable resources. That project grew into a collaboration with Georgia Tech’s Strategic Energy Institute; the Center for Critical Mineral Solutions (CCMS), supported by the College of Sciences and co-sponsored by BBISS; SEI; the Institute for Electronics and Nanotechnology (IEN); and the Institute for Materials (IMat and IEN are now combined into the Institute for Matter and Systems). The goal of the center is to develop sustainable solutions for the grand challenges associated with critical metals and materials essential for the clean energy transition.

During her time as a faculty fellow within BBISS, Yuanzhi became familiar with the people in the organization and had the opportunity to evaluate student and faculty fellow applications. When the opportunity arose to take on the role of associate co-director of interdisciplinary research for BBISS, she was happy to accept so she could help others access resources that had shaped her growth as a researcher at Georgia Tech.

“Being part of a community of people who value interdisciplinary research on sustainability-related topics, I benefited from the interactions and engagement with BBISS and I hope to carry that forward, particularly for young faculty. They are often eager to connect but might not know where to begin. BBISS can be a starting point for them.”

With a background in geochemistry and degrees from Peking University, Stony Brook University, and a postdoc at Harvard, Yuanzhi has gained a breadth of experience that has earned her a variety of awards and recognition. As she joins BBISS in a formal role, she has some advice for early-career colleagues.

“Go to seminars, events, and organized activities, as the best ideas often come through communicating and networking with others, and that’s how you discover that your expertise is needed in other fields. Be confident in who you are as a scholar, but also go out and find ways to collaborate. Georgia Tech places value on interdisciplinary research, and this is a unique strength that you should leverage.”

Away from the office, classroom, and lab, Yuanzhi is a wife and mother of two young children. She enjoys cuddle time with the kids and navigating parenthood in an academically driven household. Her husband is also a Georgia Tech professor and together they juggle the challenges of their careers with spending quality time with the children. “We try to keep work minimal on weekends and get out of the house and enjoy what Atlanta has to offer. We love nature and appreciate that we can be close to campus, close to the city, and still have so many green places to be outside.”

As she embarks on her new role with BBISS, Yuanzhi sees parallels between being a parent, professor, and now an administrator.

“The world is changing rapidly with the explosion of information and technology. It’s a struggle to know what to teach my kids and my students. How do we prepare them for five, 10, or even 20 years from now? This feeling of responsibility connects my work and personal life. It’s challenging, but also very exciting to see how we can help them embrace changes.”

From keeping warm in the winter to doing laundry, heat is crucial to daily life. But as the world grapples with climate change, buildings’ increasing energy consumption is a critical problem. Currently, heat is produced by burning fossil fuels like coal, oil, and gas, but that will need to change as the world shifts to clean energy. 

Georgia Tech researchers in the George W. Woodruff School of Mechanical Engineering (ME) are developing more efficient heating systems that don’t rely on fossil fuels. They demonstrated that combining two commonly found salts could help store clean energy as heat; this can be used for heating buildings or integrated with a heat pump for cooling buildings.

The researchers presented their research in “Thermochemical Energy Storage Using Salt Mixtures With Improved Hydration Kinetics and Cycling Stability,” in the Journal of Energy Storage.

Reaction Redux 

The fundamental mechanics of heat storage are simple and can be achieved through many methods. A basic reversible chemical reaction is the foundation for their approach: A forward reaction absorbs heat and then stores it, while a reverse reaction releases the heat, enabling a building to use it.

ME Assistant Professor Akanksha Menon has been interested in thermal energy storage since she began working on her Ph.D.  When she arrived at Georgia Tech and started the Water-Energy Research Lab (WERL), she became involved in not only developing storage technology and materials but also figuring out how to integrate them within a building. She thought understanding the fundamental material challenges could translate into creating better storage.

“I realized there are so many things that we don't understand, at a scientific level, about how these thermo-chemical materials work between the forward and reverse reactions,” she said.

The Superior Salt

The reactions Menon works with use salt. Each salt molecule can hold a certain number of water molecules within its structure. To instigate the chemical reaction, the researchers dehydrate the salt with heat, so it expels water vapor as a gas. To reverse the reaction, they hydrate the salt with water, forcing the salt structure’s expansion to accommodate those water molecules. 

It sounds like a simple process, but as this expansion/contraction process happens, the salt gets more stressed and will eventually mechanically fail, the same way lithium-ion batteries only have so many charge-discharge cycles. 

“You can start with something that's a nice spherical particle, but after it goes through a few of these dehydration-hydration cycles, it just breaks apart into tiny particles and completely pulverizes or it overhydrates and agglomerates into a block,” Menon explained. 

These changes aren’t necessarily catastrophic, but they do make the salt ineffective for long-term heat storage as the storage capacity decreases over time. 

Menon and her student, Erik Barbosa, a Ph.D. student in ME, began combining salts that react with water in different ways. After testing six salts over two years, they found two that complemented each other well. Magnesium chloride often fails because it absorbs too much water, whereas strontium chloride is very slow to hydrate. Together, their respective limitations can mutually benefit each other and lead to improved heat storage.

“We didn't plan to mix salts; it was just one of the experiments we tried,” Menon said. “Then we saw this interactive behavior and spent a whole year trying to understand why this was happening and if it was something we could generalize to use for thermal energy storage.”

The Energy Storage of the Future

Menon is just beginning with this research, which was supported by a National Science Foundation (NSF) CAREER Award. Her next step is developing the structures capable of containing these salts for heat storage, which is the focus of an Energy Earthshots project funded by the U.S. Department of Energy’s (DOE) Office of Basic Energy Sciences.

A system-level demonstration is also planned, where one solution is filling a drum with salts in a packed bed reactor. Then hot air would flow across the salts, dehydrating them and effectively charging the drum like a battery. To release that stored energy, humid air would be blown over the salts to rehydrate the crystals. The subsequently released heat can be used in a building instead of fossil fuels. While initiating the reaction needs electricity, this could come from off-peak (excess renewable electricity) and the stored thermal energy could be deployed at peak times. This is the focus of another ongoing project in the lab that is funded by the DOE’s  Building Technologies Office.

Ultimately, this technology could lead to climate-friendly energy solutions. Plus, unlike many alternatives like lithium batteries, salt is a widely available and cost-effective material, meaning its implementation could be swift. Salt-based thermal energy storage can help reduce carbon emissions, a vital strategy in the fight against climate change.

“Our research spans the range from fundamental science to applied engineering thanks to funding from the NSF and DOE,” Menon said. “This positions Georgia Tech to make a significant impact toward decarbonizing heat and enabling a renewable future.”

Nine early-career professors will pursue cutting-edge climate mitigation research during the upcoming year as part of the Seed Grant Challenge for Climate Solutions created by the Strategic Energy Institute (SEI) and the Brook Byers Institute for Sustainable Systems (BBISS). 

Launched in April during the Frontiers in Science: Climate Action Conference and Symposium, the Challenge “provides seed funding for climate mitigation and adaptation research led by ambitious early-career faculty eager to work across disciplines,” explains Beril Toktay, Regents’ Professor and interim executive director of BBISS. 

One goal of the Challenge is to facilitate research collaboration across the Institute. “Transitioning to a sustainable, clean energy system requires concerted collaboration across diverse disciplines,” says Tim Lieuwen, Regents’ Professor, David S. Lewis, Jr. Chair, and executive director of SEI. “Initiatives like this are instrumental in paving the way for such groundbreaking interdisciplinary work.” 

The four selected proposals include researchers from five different schools and two centers, and will investigate biodiversity, coral reef resilience, lithium-ion battery recycling, and coastal resilience. “I am pleased with the range of proposals submitted by our assistant professors,” adds Susan Lozier, dean of the College of Sciences and Betsy Middleton and John Clark Sutherland Chair and professor in the School of Earth and Atmospheric Sciences. “Each proposal represents an opportunity to combine expertise from across the Institute to deepen our understanding of climate challenges and uncover possible solutions.”

Each of the following projects will receive a $15,000 seed grant to be used during the 2025 fiscal year:

Climate Solutions in the Most Biodiverse Regions on Earth: Testing Whether Warming Temperatures have set in Motion an “Escalator to Survival” in Tropical Rainforests

• Benjamin Freeman, assistant professor in the School of Biological Sciences
• James Stroud, assistant professor in the School of Biological Sciences
• Saad Bhamla, assistant professor in the School of Chemical and Biomolecular Engineering
• Amirali Aghazadeh, assistant professor in the School of Electrical and Computer Engineering

The research team seeks to test the “escalator to survival” concept, which theorizes that lowland tropical species will only be able to persist in the face of rising temperatures if they are able to shift their ranges to nearby foothills and mountains, where temperatures remain cooler. 

Macro- and Microscale Drivers of Coral Reef Resilience in a Changing Climate

• Isaiah W. Bolden, assistant professor in the School of Earth and Atmospheric Sciences
• Lauren Speare, assistant professor in the School of Biological Sciences and the Center for Microbial Dynamics and Infection

The research team will develop transformative tools to evaluate reef health and resilience; detect impending compositional changes; determine the capacity for reef regeneration; and elevate mitigation strategies that preserve reef diversity and ecosystem services.

A Workforce and Community-Engaged Team Building Approach for Lithium-Ion Battery Recycling in the U.S. Southeast: Addressing Social and Ecological Implications 

• Joe F. Bozeman III, assistant professor in the School of Civil and Environmental Engineering and the School of Public Policy
• Jennifer Hirsch, senior director of the Center for Sustainable Communities Research and Education

This project will build a transdisciplinary team to determine how to effectively unite community stakeholders, industry, social scientists, and engineers when applying for external grants to establish a U.S. southeastern hub for EV-battery lithium recycling.

Building Coastal Resilience: Science-based Adaptive Solutions to Mitigate Hurricane-Induced Compound Flooding in the Southeast U.S.

• Ali Sarhadi, assistant professor in the School of Earth and Atmospheric Sciences

This project will quantify the risks associated with hurricane-induced compound flooding in a warming climate by developing physics-based hydrodynamic and AI models. The project aims to investigate factors related to geography in climate resilience and develop science-based, cost-effective adaptation strategies through active community engagement in Savannah, Georgia and Jacksonville, Florida.