A team of engineers and material scientists in the Paul M. Rady Department of Mechanical Engineering at ��ƷSM����ӰƬ has developed a new technology to turn thermal radiation into electricity in a way that literally teases the basic law of thermal physics.

The breakthrough was discovered by the , led by Assistant Professor Longji Cui. Their work, in collaboration with researchers from the National Renewable Energy Laboratory (NREL) and the University of Wisconsin-Madison, was recently . 

The group says their research has the potential to revolutionize manufacturing industries by increasing power generation without the need for high temperature heat sources or expensive materials. They can store clean energy, lower carbon emissions and harvest heat from geothermal, nuclear and solar radiation plants across the globe.

In other words, Cui and his team have solved an age-old puzzle: how to do more with less.

“Heat is a renewable energy source that is often overlooked,” Cui said. “Two-thirds of all energy that we use is turned into heat. Think of energy storage and electricity generation that doesn’t involve fossil fuels. We can recover some of this wasted thermal energy and use it to make clean electricity.”

Breaking the physical limit in vacuum

High-temperature industrial processes and renewable energy harvesting techniques often utilize a thermal energy conversion method called thermophotovoltaics (TPV). This method harnesses thermal energy from high temperature heat sources to generate electricity. 

But existing TPV devices have one constraint: Planck’s thermal radiation law. 

PhD student Mohammad Habibi showcasing one of the group's TPV cells used for power generation. Habibi was the leader of both the theory and experimentation of this groundbreaking research.

“Planck’s law, one of most fundamental laws in thermal physics, puts a limit on the available thermal energy that can be harnessed from a high temperature source at any given temperature,” said Cui, also a faculty member affiliated with the Materials Science and Engineering Program and the Center for Experiments on Quantum Materials. “Researchers have tried to work closer or overcome this limit using many ideas, but current methods are overly complicated to manufacture the device, costly and unscalable.”

That’s where Cui’s group comes in. By designing a unique and compact TPV device that can fit in a human hand, the team was able to overcome the vacuum limit defined by Planck’s law and double the yielded power density previously achieved by conventional TPV designs. 

“When we were exploring this technology, we had theoretically predicted a high level of enhancement. But we weren’t sure what it would look like in a real world experiment,” said Mohammad Habibi, a PhD student in Cui’s lab and leader of both the theory and experiment of this research. “After performing the experiment and processing the data, we saw the enhancement ourselves and knew it was something great.”

The zero-vacuum gap solution using glass

The research emerged, in part, from the group’s desire to challenge the limits. But in order to succeed, they had to modify existing TPV designs and take a different approach.

“There are two major performance metrics when it comes to TPV devices: efficiency and power density,” said Cui. “Most people have focused on efficiency. However, our goal was to increase power.”

The zero-vacuum gap TPV device, designed by the Cui Research Group.

To do so, the team implemented what’s called a “zero-vacuum gap” solution into the design of their TPV device. Unlike other TPV models that feature a vacuum or gas-filled gap between the thermal source and the solar cell, their design features an insulated, high index and infrared-transparent spacer made out of just glass. 

This creates a high power density channel that allows thermal heat waves to travel through the device without losing strength, drastically improving power generation. The material is also very cheap, one of the device’s central calling cards.

“Previously, when people wanted to enhance the power density, they would have to increase temperature. Let’s say an increase from 1,500 C to 2,000 C. Sometimes even higher, which eventually becomes not tolerable and unsafe for the whole energy system,” Cui explained. “Now we can work in lower temperatures that are compatible with most industrial processes, all while still generating similar electrical power than before. Our device operates at 1,000 C and yields power equivalent to 1,400 C in existing gap-integrated TPV devices.”

The group also says their glass design is just the tip of the iceberg. Other materials could help the device produce even more power.

“This is the first demonstration of this new TPV concept,” explained Habibi. “But if we used another cheap material with the same properties, like amorphous silicon, there is a potential for an even higher, nearly 20 times more increase in power density. That’s what we are looking to explore next.”

The broader commercial impact

Assistant Professor Longji Cui (middle) and the Cui Research Group. 

Cui says their novel TPV devices would make its largest impact by enabling portable power generators and decarbonizing heavy emissions industries. Once optimized, they have the power to transform high-temperature industrial processes, such as the production of glass, steel and cement with cheaper and cleaner electricity.

“Our device uses commercial technology that already exists. It can scale up naturally to be implemented in these industries,” said Cui. “We can recover wasted heat and can provide the energy storage they need with this device at a low working temperature.

“We have a patent pending based on this technology and it is very exciting to push this renewable innovation forward within the field of power generation and heat recovery.” 

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The national awards recognize and support outstanding grad students from across the country in science, technology, engineering and mathematics (STEM) fields who are pursuing research-based master’s and doctoral degrees.

PhD students Reegan Ketzenberger, Caleb Song, and Jennifer Wu are each receiving the honor for 2024. Find out more about their research below.

Awardees receive a $37,000 annual stipend and cost of education allowance for the next three years as well as professional development opportunities.

Two mechanical engineering PhD students, Alex Hedrick and Carly Rowe, also received honorable mentions from the National Science Foundation program.

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Associate Professor Gregory Whiting and his research group are preparing for the thrill of a lifetime: two parabolic flights, each expected to provide around ten total minutes of reduced gravity to test and model how 3D printing of functional materials works in lunar gravity.

Associate Professor Gregory Whiting 

This technology is one of for testing on an aircraft that simulates spaceflight, a high-altitude balloon or a suborbital rocket. The process is meant to be iterative, providing researchers with a quick way to collect data and refine their innovations for possible inclusion into NASA missions to space and the moon. The grant will award $500,000 over the course of 18 months to Whiting and co-principal investigator Robert Street, a printed electronics expert at PARC, A Xerox Company.

So why send a 3D printer to space? One reason is so manufacturing can be completed without needing parts from Earth. If researchers can determine the best methods for printing in space, they may be able to print complex electronic devices like life support systems using multi-functional paste-like inks.

Already, Whiting’s research group, the Boulder Experimental Electronics and Manufacturing Lab has successfully printed a multi-functional composite material able to absorb and expel gasses in the lab. This material may be adapted for removal of CO₂ or other gasses in a confined space, such as the International Space Station.

Parabolic flights simulate reduced gravity environments and are useful when training astronauts or in carrying out experiments. The aircraft fly in a pattern that resembles a parabola, repeatedly climbing and descending. At the top of the parabola, reduced gravity is achieved for roughly 20-30 seconds. At the bottom of the parabola, the forces are equivalent to two times Earth’s gravity. On a typical flight, 20-30 repetitive parabolas are completed, providing time for experimentation or training.

“By far the most exciting part of this project was the first time we ran current through the printed composite,” said Jamie Thompson, a PhD student collaborating with Whiting’s group and a visiting researcher at PARC. “We could instantly feel the heat being given off by the device.”

Before they can manufacture the composite material in space, however, Whiting and his research group must first understand how gravity affects the shapes that can or cannot be printed.

“Think peanut butter,” said Whiting. “If you pull the peak up, it will stay in place for a little while, but with one-sixth the gravity, it will settle more slowly than it does on Earth. The same goes for printing these paste-like functional inks.”

To model these differences, the team will perform a series of printing challenges, including overhangs, bridges, sharp edges, curves and movements that require higher printing speeds in both Earth’s gravity and microgravity.

“We sometimes think doing things in space will be harder than on Earth, which is mostly true,” said Whiting, “but in this case, we think that the reduced gravity found on the moon could actually be quite beneficial, enabling us to print structures with these materials that would otherwise be difficult to make.”

For example, he said when printing layer upon layer on Earth, a large gap in a printed structure cannot be crossed without filling it in. With less gravity, bigger gaps could potentially be bridged.

Whiting said he is grateful for the expertise of collaborators at PARC who will deliver a test printer prepared to acquire both ground and flight data. By measuring most everything about the printing process, the researchers will have a better idea how to print desired shapes in space. Accelerometers, other sensors, image recognition software and lasers will likely be incorporated around the printer to accurately measure printed geometrical features during the flights.

Whiting, who recently joined the Multi-functional Materials Interdisciplinary Research Theme at the College of Engineering and Applied Science, said he can imagine his group working on additive manufacturing and multi-functional materials long into the future. It seems his level of excitement for this particular research is met only by the group’s excitement for the upcoming parabolic flights.

“Taking the project from materials development all the way to micro and lunar gravity testing in the near future has been an amazing process,” said Charlotte Bellerjeau, an undergraduate researcher in the BEEM Lab studying aerospace engineering. “The parabolic flights coming up are exciting, but I'm also looking forward to future applications for this technology that extend beyond spaceflight.”
 

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A five-centimeter-thick piece of cooling wood.

What if the wood used to build your house could decrease your electricity bill? In the race to save energy, researchers at ��ƷSM����ӰƬ and the University of Maryland have harnessed nature’s nanotechnology to uncover a way for buildings to dump heat.

Wood solves the problem. It is already used as a building material and is renewable and sustainable. Using tiny structures in wood known as cellulose nanofibers and the natural chambers that grow to pass water and nutrients inside a living tree, specially processed wood can radiate heat away. The results of this study were published in Science on May 9.

At the University of Maryland, Liangbing Hu along with co-first authors Tian Li and Shuaiming He and others in the Department of Materials Science have been working with wood for years. Hu’s team has invented a range of emerging wood nanotechnologies, including a transparent wood, low-cost wood batteries, super strong wood, super thermal insulating wood and a wood-based water purifier.

“This is another major advancement in wood nanotechnologies,” said Hu. “He’s group achieved cooling wood that is made of solely wood that can cool your house as a green building material.”

The team of researchers at ��ƷSM����ӰƬ led by Associate Professor Xiaobo Yin and co-first author Yao Zhai, both part of the Department of Mechanical Engineering and the Materials Science and Engineering Program, have been working on materials for radiative cooling, including thin films and paints.

“When applied to building, this game-changing structural material cools without the input of electricity or water,” said Zhai.

By removing the lignin, the part of the wood that makes it brown and strong, the researchers created a very pale wood made of cellulose nanofibers. They then compressed the wood to restore its strength. To make it water repellent, they added a super hydrophobic compound that helps protect the wood. The result was a bright white building material that could be used for roofs to push away heat from inside the building.

They took the cooling wood for testing to a farm in Arizona where the weather is always warm and sunny with low winds. There, they tested the cooling wood and found that it averaged five or six degrees Fahrenheit cooler than the ambient air temperature, even during the hottest part of the day. On average, it stayed 12 degrees cooler than natural wood, which increases in temperature when exposed to sunlight.

Total predicted cooling energy savings of midrise buildings extended for all U.S. cities based on local climate zones. 

“The processed wood uses the cold universe as heat sink and release thermal energy into it via atmospheric transparency window. It is a sustainable material for sustainable energy to combat global warming,” said Li.

The mechanical strength per weight of this wood is also stronger than steel, making it a great choice for building materials. It also damages less easily than natural wood.

To see how much energy the wood saves, the researchers calculated how much heat is used by typical apartment buildings in cities across the U.S. in all climate zones. Buildings across the U.S. that were built after 2004 would save on average 20% of cooling costs. Hot cities like Phoenix and Honolulu would save the most energy, especially if buildings had both their siding and roofs replaced with cooling wood.

"It is interesting that the same material that releases heat upon combustion can be used for cooling, offering new opportunities in green buildings," said Orlando Rojas, a professor in the Department of Bioproducts and Biosystems at Aalto University, Finland.

This research was funded by the University of Maryland. Other collaborators include Jelena Srebric’s team at the University of Maryland College Park, Professor Ronggui Yang’s team at the University of Colorado, Boulder and Ashlie Martini’s team at the University of California Merced. 

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Over the next three years, ��ƷSM����ӰƬ researchers and their partners will develop, fabricate and test a network of 3D-printed biodegradable soil sensors aimed at allowing farmers to affordably and efficiently monitor crop conditions.

Associate Professor Gregory Whiting and Research Associate Nikolaus Setiawan discuss electrical measurements. Photo credit: Cameron Douglas

According to the USDA, agriculture accounts for almost 2 percent of total energy consumption in the United States, making it an important part of our energy landscape. For this reason, researchers at ��ƷSM����ӰƬ are developing sensors that will monitor soil, environment and crop conditions so that inputs like water and fertilizer may be precisely matched to crop needs. By developing sensors to optimize inputs for greater crop yields, Mechanical Engineering Associate Professor Gregory Whiting aims to mitigate environmental losses, decrease energy use and improve farm profitability for food, feed and fuel crops.

The Department of Energy’s Advanced Research Projects Agency-Energy awarded Whiting and his team $1.69 million over three years for their project, Precision Agriculture using Networks of Degradable Analytical Sensors. PANDAS is funded as part of the Sensors for Bioenergy and Agriculture Cohort of ARPA-E’s first-ever OPEN+ program. This program seeks to develop ultra-low-energy distributed sensors to boost viability of bioenergy crops and reduce energy and water requirements for agriculture more broadly.

Aerospace engineering undergraduate student Charlotte Bellerjeau takes thermal measurements.

“It’s critical we continue to develop cutting-edge technologies that our farmers can utilize,” Sen. Cory Gardner said in a news release announcing the award. ”I’ll always advocate for and support funding for these types of programs.”

Until now, sensors have been unable to affordably measure variables like soil moisture levels and nutrient concentration continuously and at high densities. This is problematic because to maximize crop yields, hundreds or even thousands of locations per farm must be monitored. Using 3D printing and simple designs, Whiting and his team will solve this problem by creating biodegradable sensors that provide near-real-time data with a predicted cost of less than $1 per unit. These chip-less, zero-power sensors are expected to provide a 100-fold increase in information density over current precision farming solutions which will enable more effective data analytics.   

PANDAS sensors are strategically designed with the farmers’ needs in mind. They require no ongoing maintenance, can be read remotely using existing farm equipment and will accurately and continuously monitor conditions over the course of an entire season. They also do not require careful placement, allowing them to be easily distributed over large areas at high concentration.

“Perhaps most appealing is that all of the components required for sensing are printed and are able to predictably, harmlessly and completely degrade into the earth when no longer needed,” Whiting said.

Mechanical engineering PhD student Madhur Atreya prepares biodegradable electronic materials for printing.

At ��ƷSM����ӰƬ, Whiting leads the Boulder Experimental Electronics and Manufacturing Laboratory. His 15 years of experience in research and development of additively manufactured, solution processed, flexible and transient electronic devices and systems underpin the team’s current work.

For the PANDAS project, Whiting is joined by co-principal investigators Ana Claudia Arias, a professor in the Electrical and Computer Sciences Department at University of California Berkley, and Raj Khosla, professor of precision agriculture in the Department of Soil and Crop Sciences at Colorado State University.

In a news release announcing the award, Sen. Michael Bennet commended the researchers for their leadership in the field as they prepare for their next three years of research together.

“Congratulations to the researchers at ��ƷSM����ӰƬ for securing this grant and leading the way to develop innovative tools to improve farming across the country,” Bennet said.

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When a pair of tourists hiking the Alps stumbled across the frozen remains of the mummy Ötzi in 1991 they also, unknowingly, discovered the oldest known examples of tattoos in history. The 5,300-year-old body, more famously known as the Iceman, has 61 tattoos arranged in patterns of straight lines scratched across his skin. 

What amazes ��ƷSM����ӰƬ chemist Carson Bruns about those dyes, however, isn’t their age. It’s what they’re made of. 

“They’re made of the same stuff that our tattoos are made of,” said Bruns, of ��ƷSM����ӰƬ’s ATLAS Institute. “It blows my mind that we haven’t updated this technology in so long.”

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