Meet the NatureNet Fellows: Haoran Yang

It was the combination of a cold night, a clothes dryer, and a poorly placed yellow floodlight that made me think our neighbor’s house was on fire. I was dialing 911 when my husband pointed out there was no smell of smoke. We looked more closely and realized the clouds billowing over the fence were from steam, not flames. Crisis averted. Our neighbors (and we) were in no danger from steam; after all, it was just the harmless by-product of the heat pouring out of the dryer vent on the side of the house.

Turns out, though, that kind of heat has a name. “It’s waste heat,” says NatureNet Science Fellow Haoran Yang. It’s also, apparently, indicative of a larger problem. “We [people] waste more than half of the total energy we generate, and considering how much energy the U.S. produces, that’s a lot of waste, and,” notes Yang, “it’s mainly in the form of heat.”

Fortunately, where there is waste, there is also opportunity. And Yang, a materials scientist and expert in nanotechnology, sees waste heat recovery as one very big, very exciting opportunity. Getting even 10 percent of that waste heat back as energy means, he says, “we could reduce energy consumption and green house gas emissions significantly.”

For Yang, his goal is not only to use nanotechnology to develop effective, widely applicable thermoelectric tools to salvage that heat and convert it to energy, but to do so in ways that limit cost and are safe for the environment.

Converting Waste Heat to Electricity

Currently pursuing his NatureNet Fellowship in Christopher B. Murray’s Lab at the University of Pennsylvania, Yang’s research is focused on developing nanostructures for “thermoelectric energy conversion,” which is, as he explains, essentially “the direct conversion of waste heat to electricity through a class of semiconductor materials.”

Put very simply, thermoelectric materials (also called thermoelectrics for short) essentially convert temperature differences into electrical energy. When thermoelectric materials are heated on one side electrons flow to the cooler side, and generate electricity. In a way, waste heat is to thermoelectrics as sunlight is to solar panels.

Yang has already made significant advances in the application of nanotechnology to the development of thermoelectrics. Right now, most thermoelectrics are still rigid, and that can limit their applicability.

As part of his Ph.D. work at Purdue University, Yang co-developed flexible nanocrystal-coated thermoelectric fibers that showed how it could be possible to eventually wrap thermoelectric fibers onto, for example, industrial pipes in power plants or other manufacturing sites. The flexible fibers would act as thermal insulating material, and also recover waste heat to produce electricity to improve energy efficiency.

And while nanostructured thermoelectrics hold great promise for a world looking for tools to adapt to climate change, produce abundant energy, and limit greenhouse gases, there are still some pretty big challenges to large-scale application. Yang’s current research touches on three of the most pressing: cost, efficiency and toxicity.

Nanotechnology for the Real World

Currently, many of the materials used for nanostructured thermoelectrics contain elements like tellurium and silver, which are scarce, expensive or both. And they may also contain toxic elements like lead and thallium, which are hazardous to human health. So, notes Yang, while certain nanostructured thermoelectrics might be effective, if they rely on toxic chemicals or rare elements, they are not sustainable in terms of either cost or maintaining a healthy environment.

That concern with sustainability is the main reason Yang pursued a NatureNet Science Fellowship to continue his research. “NatureNet is different from other fellowships,” he says. “It focuses not only on the technical aspect of a problem, but also asks us to think about the environmental impact. When we [scientists] try to develop new technology, we don’t think too much [beyond the technology]. As long as something works, we’re happy. But there are many other aspects we must consider to really push this nanotechnology into the real world.”

“I stress green chemistry in the synthesis of nanomaterials by avoiding the use of toxic or rare chemicals,” he notes. “And I take great effort to minimize possible negative environmental effects of our nanotechnology by collaborating with biologists and environmentalists. My hope is that my research will make an impact on improving the sustainability of our world.”