By Kelley Freund
Photography by Craig Mahaffey ’98
Like other research universities, Clemson grapples with how to help provide the world sustainable energy that is clean, efficient, scalable and portable. Reaching that goal will require expertise and innovation from across a number of disciplines, solving problems from their respective perspectives and a collective gaze. It also depends on preparing the next generation of sustainable engineers. Fortunately, Clemson has a collection of veteran faculty who lead their fields — and bright graduate students ready to carry the torch further.
Clemson is not alone. With an increasing global population, climate change and the environmental impact of fossil fuel, the world is looking more and more toward sustainable power sources — and solving the challenges that come with using them. We know we can use the wind or solar power, but we need energy when the sun doesn’t shine or the wind doesn’t blow. So where do we store it? What other sources of power can we usea? How do we make this energy more efficient? From simple generators to batteries to waste heat, Clemson researchers are looking for the answers and working to create new cost-effective technology that will power our future.
IN THE BEEHIVE
The more we rely on renewable energy, the more important that simplicity and cost effectiveness become. What if you could harness waste mechanical energy that comes from routine activities such as walking or typing to power more than 300 LEDs with a device that costs 60 cents to produce in a matter of five minutes?
That’s what researchers at the Clemson Nanomaterials Institute (CNI) did when they developed an ultra-simple triboelectric nanogenerator (U-TENG), which transforms mechanical motion into electricity to power lights or electronic devices. Unlike existing technologies, Clemson’s U-TENG is made up of materials that are commercially available and easily mass produced, like the plastic that makes up water bottles and high-temperature tape.
“We focus our attention on new materials development at the nanoscale,” says CNI director Apparao Rao. “If we have a good material, then we try to make that a better material. If you can tune its properties, then you can get the material to do more things and better things for humanity. That is the driving force of what we do at CNI.”
The team made the U-TENG electrically conductive by adding a layer of indium tin oxide so that current could flow through. By applying a repetitive motion, like tapping your foot, to bring the two materials into contact, it transforms mechanical energy into electricity. When you tap your foot, the plastic comes into contact with the tape. When you lift your foot, the force is released and the electrons in the plastic and the tape redistribute between the two materials, but not evenly. The plastic becomes more positive than it was and the tape more negative, and this generates voltage.
With no battery, the U-TENG can’t run out of power. Put it on your shoes, and the LEDs embedded in the shoes light up if you’re running or walking in the dark, increasing safety. Put it under a walkway and harvest energy as people walk by. You can even monitor traffic flow as a car drives over a U-TENG.
According to Sriparna Bhattacharya, a research assistant professor in astronomy and physics, the broad spectrum of research performed at CNI is based not only on renewable energy conversion materials, but also on batteries and supercapacitors to store the energy. Researchers at CNI are interested in bringing new types of carbon to battery research to make a product that can charge quickly, last more cycles and won’t catch fire.
The team at CNI is trying to develop techniques or devices that will eventually be used by the general public. With that in mind, Rao explains that one has to be very conscious about the cost, and in that sense, Clemson is unique.
“Let’s go back to the battery example,” Rao says. “We make these new nanostructured forms of carbon, but can we come up with a scalable manufacturing process for such nanocarbons? For many people, scalable production is very challenging. Often, academic labs make just enough material to do the research development for a thesis. But we developed these blueprints. One can scale these up if we want to take that to the consumer market. You don’t see that happening in most places. It’s a win-win situation for us. We do great fundamental science, plus we have a product that’s very appealing to industry collaborators.”
BUILDING A BETTER BATTERY
Batteries and their storage ability have major implications for renewable energy, which is why Rao and his team at CNI aren’t the only ones at Clemson researching them.
When we pull up to a gas station, we fill our tank up in less than five minutes and are able to drive for more than five hours. Someday Rajendra Bordia, chair of the materials science and engineering department, hopes to replicate the same results with a lithium ion battery.
Lithium ion batteries are used in a variety of devices from cell phones to computers to electric cars. They are very light, which makes them a popular choice, but their energy density (the amount of energy you can store per unit of weight) is still on the low side. The other problem is they take a long time to charge.
Bordia’s group is working on addressing these two problems, creating strategies to increase the energy density of the batteries and design tweaks to make them faster charging, all the while maintaining the safety of the batteries.
“It’s a crowded field,” Bordia says. “There are a lot of people, all the way from fundamental researchers to automobile companies working on this particular topic. The thing we bring to the table that’s unique is that we work on microstructure design.”
While most scientists are developing new materials to improve battery performances, Bordia and his team focus on the design of the microstructure of the material: the electrodes of the battery. Chemistry Professor Stephen Creager and Bordia have teamed up to work on battery electrodes that have an exotic architecture. The electrodes are made from a slurry that gets frozen, which is known as a freeze casting technique. The slurry has water as a solvent, and it makes ice crystals that the scientists remove by freeze drying. This leaves behind big holes that fill up with electrolytes and allows the battery to be charged faster.
“We think that this is a unique niche for us because if somebody else comes up with the next best material, then we can still use our microstructure design concepts,” Bordia says.
IS IT HOT IN HERE?
But even if scientists can come up with innovative methods and materials for speeding up the process to charge a battery, you still have to have energy available to charge it. For example, let’s say you’re camping and your cellphone battery dies. You can use the waste heat from your campfire and a thermoelectric charger to power your phone.
Any heat source such as geothermal heat, ocean heat, solar heat and even body heat can be used for creating electricity in thermoelectric chargers or generators — it’s known as thermoelectric energy conversion. These chargers and generators are mostly used for remote areas or unmanned sites and are reliable sources of power in these situations because they can work day and night, perform under all weather conditions and can work without battery backup.
Terry Tritt, a Clemson alumnus and current chair of the department of physics and astronomy, has been conducting research on thermoelectric energy conversion for more than 20 years. The idea behind the concept is to recycle waste heat and utilize it for electrical energy. NASA has been using this concept for decades on missions like Voyager and Cassini, and will use it again with the next Mars rover in 2020. Since these probes are too far away to use solar energy, and they’re out in space too long to use any sort of chemical energy, Tritt says NASA clads a plutonium core with thermoelectric materials and that serves as the power source.
But a broader application of thermoelectrics requires developing higher-performance, eco-friendly materials. Tritt and his Clemson colleague Jian He recently published an invited review on the state of thermoelectric energy in the journal Science, discussing what makes a good thermoelectric material and the “tuning knobs” (like charge or composition of electrons) that one can vary to manipulate a material’s properties and maximize its performance. The measure of this performance is called a ZT. An inexpensive bulk material with a ZT between 2.5 and 3 is what Tritt calls the “holy grail of thermoelectrics.” ZTs above 2.5 have been found in many nanomaterials, but Tritt says these can’t carry much heat load, nor are they available in bulk.
But several years ago, Tritt and his colleagues teamed up with a group in China to use nanocomposites (they put nanomaterials within bulk materials) and saw improved performance — the compound not only exhibited good thermoelectric properties, but it also comprises non-toxic and abundantly available elements with high chemical and thermal stability. These kinds of developments could eventually turn out more efficient and cost effective thermoelectric energy devices to provide clean energy technology. Tritt says his students, both graduates and undergraduates, are instrumental to this type of research coming out of his lab.
“We are needing more and more energy every day,” Tritt says. “Because of the applications and the importance, students are excited to be working on this. What’s more exciting than giving them a project that they feel is going to make a difference?”