Even if you do your best to eat local, chances are most of the fruits and vegetables you consume come from far away -- especially if you live in a big city. Water and land for growing crops are hard to come by in urban areas. Finding more sustainable methods for growing produce in urban areas would have enormous benefits. A pilot project by Georgia Tech’s Yongsheng Chen, a professor in the School of Civil and Environmental Engineering, aims to use wastewater from the campus to do just that. 

“The overarching goal is trying to figure out a way to use wastewater nutrients to grow produce in urban areas so we can decentralize vegetable production,” Chen said. A grant provides $5 million over five years from the U.S. Department of Agriculture’s National Institute of Food and Agriculture (NIFA) to create and operate a hydroponic growing system using domestic wastewater extracted from the Georgia Tech campus sewer system. It is the largest USDA award Georgia Tech has received. 

“Currently we treat wastewater by taking all the nutrients from it,” said Chen. “Then we have to use an energy-intensive process to synthesize and add fertilizer to the food production process.” 

The proposed anaerobic membrane biological treatment process will transfer organic contaminants into biogas and remove pathogens such as E. coli to ensure food safety, but the nutrients (nitrogen, phosphorus and potassium, for example) will remain. By using a smart membrane or nanomaterials to extract trace contaminants like endocrine disruptors, heavy metals and pharmaceuticals, the nutrients that are left can be pumped through a vertical hydroponic system to grow produce without adding fertilizer. The project will monitor water and produce quality and measure contamination from chemicals and microbes continuously.

The overall goal, said Chen, is to show that using the nutrients and water resources from domestic wastewater (DWW) in an urban controlled environment agriculture system (CEAs) is socially, environmentally and financially sustainable and can easily be replicated in other cities. The project will closely track nutrient requirements, energy needed to produce, handle and transport the fruits and vegetables, and water needs to determine what resources are needed to support this kind of CEA system (DWW-CEAs).

Ecological network analysis for DWW-CEA coupling will track material and energy flows across components that produce, consume and recycle food. Using a geodesign approach, Chen’s team will then compare data from traditional agriculture and DWW-CEAs to see how the system performs and how it could be designed to perform better in terms of water, energy and nutrient needs.

“Our model will have options to calculate energy consumption for the system, water consumption, water balance and nutrient balance,” said Chen. “We’ll conduct a life-cycle analysis and techno-economic analysis to evaluate whether this type of system will be commercially feasible or profitable in different locations, not just Atlanta.”

Chen will use machine learning in the controlled growing environment to seek a “recipe” for each plant type: the ideal amount of nutrients, growing temperature and humidity needed for lettuce, for example, so that each head of greens will taste the same. The project also provides an opportunity to test a number of other technologies, such as using solar power for cooling or biogas extracted from the wastewater and discarded food to power a micro chiller. 

Of course, showing that such a DWW-CEA system is feasible and profitable is one hurdle – another is getting consumers on board with the way the produce is produced. “If we are going to decentralize this system, what are the implications for policy related issues?” Chen asked. “Will people buy products produced by wastewater?” The project will involve working with a number of collaborators at Georgia Tech and in Atlanta, including Kaye Husbands Fealing, professor and chair of the School of Public Policy at Georgia Tech, and the Mayor’s Office of Sustainability.

“We want to change the current wastewater treatment practice, step back a little bit and think outside the box,” Chen said. “This could have a big impact locally, regionally or even nationally and internationally.”

This research is supported by the U.S. Department of Agriculture’s (USDA) National Institute of Food and Agriculture (NIFA), Agriculture and Food Research Initiative (AFRI) Water for Food Production Systems (Grant 2018-68011-28371). 

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Solar power accounts for less than two percent of U.S. electricity, but could make up more than that if the cost of electricity generation and energy storage for use on cloudy days and at nighttime were cheaper.

A Purdue University-led team that included researchers from Georgia Tech have developed a new material and manufacturing process that would make one way to use solar power – as heat energy – more efficient in generating electricity.

The innovation is an important step for putting solar heat-to-electricity generation in direct cost competition with fossil fuels, which generate more than 60 percent of electricity in the U.S.

“Storing solar energy as heat can already be cheaper than storing energy via batteries, so the next step is reducing the cost of generating electricity from the sun's heat with the added benefit of zero greenhouse gas emissions,” said Kenneth Sandhage, Purdue’s Reilly Professor of Materials Engineering.

The research, which was done at Purdue in collaboration with the Georgia Institute of Technology, the University of Wisconsin-Madison and Oak Ridge National Laboratory, published in the journal Nature on October 18. 

Solar power doesn't only generate electricity via panels in farms or on rooftops. Another option is concentrated power plants that run on heat energy. 

Concentrated solar power plants convert solar energy into electricity by using mirrors or lenses to concentrate a lot of light onto a small area, which generates heat that is transferred to a molten salt. Heat from the molten salt is then transferred to a "working" fluid, supercritical carbon dioxide, that expands and works to spin a turbine for generating electricity.

To make solar-powered electricity cheaper, the turbine engine would need to generate even more electricity for the same amount of heat, which means the engine needs to run hotter. 

The problem is that heat exchangers, which transfer heat from the hot molten salt to the working fluid, are currently made of stainless steel or nickel-based alloys that get too soft at the desired higher temperatures and at the elevated pressure of supercritical carbon dioxide.

Inspired by the materials his group had previously combined to make composite materials that can handle high heat and pressure for applications like solid-fuel rocket nozzles, Sandhage worked with Asegun Henry – formerly at Georgia Tech, but now at the Massachusetts Institute of Technology – to conceive of a similar composite for more robust heat exchangers.

Two materials showed promise together as a composite: The ceramic zirconium carbide, and the metal tungsten.

Purdue researchers created plates of the ceramic-metal composite. The plates host customizable channels for tailoring the exchange of heat, based on simulations of the channels conducted at Georgia Tech by Devesh Ranjan's team.

“We simulated the printed circuit heat exchanger, which contains channels that are straight and parallel with semi-circular cross sections two millimeters in diameter,” said Ranjan, associate professor in the George W. Woodruff School of Mechanical Engineering. “The thickness of each plate in the printed circuit heat exchanger stack and the spacing between the channels were then determined from the maximum allowed stresses for each type of material, with a factor of safety added.”

Mechanical tests by Edgar Lara-Curzio’s team at Oak Ridge National Laboratory and corrosion tests by Mark Anderson’s team at Wisconsin-Madison helped show that this new composite material could be tailored to successfully withstand the higher temperature, high-pressure supercritical carbon dioxide needed for generating electricity more efficiently than today’s heat exchangers.

An economic analysis by Georgia Tech and Purdue researchers also showed that the scaled-up manufacturing of these heat exchangers could be conducted at comparable or lower cost than for stainless steel or nickel alloy-based ones.

“Ultimately, with continued development, this technology would allow for large-scale penetration of renewable solar energy into the electricity grid,” Sandhage said. “This would mean dramatic reductions in man-made carbon dioxide emissions from electricity production.”

A patent application has been filed for this advancement. The work is supported by the U.S. Department of Energy, which has also recently awarded additional funding for further development and scaling up the technology.

This story was provided by Purdue University.

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Writer: Kayla Wiles, Purdue University

A team of semiconductor researchers based in France has used a boron nitride separation layer to grow indium gallium nitride (InGaN) solar cells that were then lifted off their original sapphire substrate and placed onto a glass substrate. 

By combining the InGaN cells with photovoltaic (PV) cells made from materials such as silicon or gallium arsenide, the new lift-off technique could facilitate fabrication of higher efficiency hybrid PV devices able to capture a broader spectrum of light. Such hybrid structures could theoretically boost solar cell efficiency as high as 30 percent for an InGaN/Si tandem device.

The technique is the third major application for the hexagonal boron nitride lift-off technique, which was developed by a team of researchers from the Georgia Institute of Technology, the French National Center for Scientific Research (CNRS), and Institut Lafayette in Metz, France. Earlier applications targeted sensors and light-emitting diodes (LEDs).

“By putting these structures together with photovoltaic cells made of silicon or a III-V material, we can cover the visible spectrum with the silicon and utilize the blue and UV light with indium gallium nitride to gather light more efficiently,” said Abdallah Ougazzaden, director of Georgia Tech Lorraine in Metz, France and a professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE). “The boron nitride layer doesn’t impact the quality of the indium gallium nitride grown on it, and we were able to lift off the InGaN solar cells without cracking them.”

The research was published August 15 in the journal ACS Photonics. It was supported by the French National Research Agency under the GANEX Laboratory of Excellence project and the French PIA project “Lorraine Université d’Excellence.”

The technique could lead to production of solar cells with improved efficiency and lower cost for a broad range of terrestrial and space applications. “This demonstration of transferred InGaN-based solar cells on foreign substrates while increasing performance represents a major advance toward lightweight, low cost, and high efficiency photovoltaic applications,” the researchers wrote in their paper.

“Using this technique, we can process InGaN solar cells and put a dielectric layer on the bottom that will collect only the short wavelengths,” Ougazzaden explained. “The longer wavelengths can pass through it into the bottom cell. By using this approach we can optimize each surface separately.”

The researchers began the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1,300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick, and produces crystalline structures that have strong planar surface connections, but weak vertical connections. 

The InGaN attaches to the boron nitride with weak van der Waals forces, allowing the solar cells to be grown across the wafer and removed without damage. So far, the cells have been removed from the sapphire manually, but Ougazzaden believes the transfer process could be automated to drive down the cost of the hybrid cells. “We can certainly do this on a large scale,” he said.

The InGaN structures are then placed onto the glass substrate with a backside reflector and enhanced performance is obtained. Beyond demonstrating placement atop an existing PV structure, the researchers hope to increase the amount of indium in their lift-off devices to boost light absorption and increase the number of quantum wells from five to 40 or 50.

“We have now demonstrated all the building blocks, but now we need to grow a real structure with more quantum wells,” Ougazzaden said. “We are just at the beginning of this new technology application, but it is very exciting.”

In addition to Ougazzaden, the research team includes Georgia Tech Ph.D. students Taha Ayari, Matthew Jordan, Xin Li and Saiful Alam; Chris Bishop and Simon Gautier from Institut Lafayette; Suresh Sundaram, a researcher at Georgia Tech Lorraine; Walid El Huni and Yacine Halfaya from CNRS; Paul Voss, an associate professor in the Georgia Tech School of ECE; and Jean Paul Salvestrini, a professor at Georgia Tech Lorraine and adjunct professor in the Georgia Tech School of ECE.

CITATION: Taha Ayari, et al., “Heterogeneous Integration of Thin-Film InGaN-Based Solar Cells on Foreign Substrates with Enhanced Performance,” (ACS Photonics 2018) https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00663

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Writer: John Toon

It’s been eight years since Zhong Lin Wang launched pioneering research into triboelectric nanogenerators, effectively creating an entirely new field of study into materials that produce  an electrical charge when in motion.

This week, Wang, the Hightower Chair and Regents’ Professor in the Georgia Tech School of Materials Science and Engineering, was named the winner of the Eni Award for Energy Frontiers.

The award is one of three awarded each year by the Italian-based oil and gas company Eni, which established the prize a decade ago with a goal of being similar to the Nobel prize for energy. The award  recognizes researchers who have made significant contributions to the industry.

Wang’s research uncovered a new pathway to harvesting energy from a variety of sources such as wind, ocean currents or sound vibrations.

“This is a great honor for me and recognition of the tremendous potential we have to capture the random mechanical energy that surrounds us every day,” Wang said. “Triboelectric nanogenerators have broad applications for harvesting energy from human activities such as rotating tires, mechanical vibration and more, with great applications in self-powered systems for personal electronics, environmental monitoring, and medical.”

The triboelectric  nanogenerators, which are fabricated  from layers of plastic and metal, use a combination of the triboelectric effect and electrostatic induction to generate small amount of electrical power from mechanical motion such as rotation, sliding or vibration.

Wang’s team in recent years has demonstrated the use of the triboelectric nanogenerators in applications such as a fabric that creates energy when in motion and a self-powered  computer keyboard.

Wang, who joined Georgia Tech in 1995, has long focused his research into small things that make a big impact. After researching carbon nanotubes, he shifted to zinc oxide nanowires and nanobelts. The latter formed the foundation of another discovery, the piezoelectric nanogenerator, which also captures mechanical energy generated from bending the zinc oxide material.

“These self-powered nanosystems have applications not just in powering small Internet of Things devices such as wearable electronics, but also have the potential to make a significant impact in addressing societal challenges on a large scale – such as using triboelectric nanogenerators to harvest energy from ocean waves, which, unlike solar energy, could be more reliable and less dependent on whether it’s day or night, or whether it’s sunny or cloudy.”

The award will be presented on October 22 at the Quirinal Palace in Rome.

With an inexpensive setup based on a wheelchair and a tablet computer, BBISS Fellow and School of Civil and Environmental Engineering Professor Randall Guensler has helped Atlanta catalog 1,200 miles of sidewalks.

As residents will tell you, sometimes those paths can be a bumpy, cracked mess. But it’s difficult for cities to keep track. That’s why Guensler and his students have been working for several years on a simple system to help communities assess the condition of their sidewalks.

Their latest project includes cataloging 200 miles in an Atlanta suburb in Cobb County.

Along with graduate student Daniel Walls, Guensler demonstrated the system to WABE’s Stephannie Stokes:

As it rolls along, the tablet records video of the sidewalk and any rumbling the wheelchair experiences. The computer also documents the location.

Guensler’s students then review the data back at the lab to create an inventory of sidewalks — and any problems, like cracks or obstructions.

Guensler said cities can use the inventories to make sure they’re meeting federal requirements to accommodate people with disabilities.

Otherwise, local governments can face lawsuits, like Atlanta has.

The city, for its part, has said it is working to comply with federal rules. And Guensler said cities around the country — not just Atlanta — have neglected their sidewalks.

Sidewalks tend only to have a lifespan of about 40 years.

“They’re really not difficult to maintain. It’s just that we don’t consider them to be streets,” Guensler said.

In other words, cities don’t consider the sidewalks to be part of their overall transportation system.

Walls, the graduate student, said this research has made him pay more attention.

“It’s almost impossible for me to not recognize bad sidewalks now,” Walls said.

Story courtesy of GT School of Civil and Environmental Engineering.
Listen to the full story on WABE NPR Radio.