Generating 'Green' Electricity: Waste Heat Converted to Electricity Using New Alloy

Generating 'Green' Electricity: Waste Heat Converted to Electricity Using New Alloy

  11:01:00 PM    Science Lover

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University of Minnesota engineering researchers in the College of Science and Engineering have recently discovered a new alloy material that converts heat directly into electricity. This revolutionary energy conversion method is in the early stages of development, but it could have wide-sweeping impact on creating environmentally friendly electricity from waste heat sources.

During a small-scale demonstration in the lab, 
University of Minnesota researchers showed how 
their new material can spontaneously produce 
electricity when the temperature is raised a small 
amount. Pictured (from left) are aerospace engineering 
and mechanics professor Richard James, Ph.D. student 
Yintao Song and post-doctoral researchers Kanwal 
Bhatti and Vijay Srivastava. (Credit: Image 
courtesy of University of Minnesota)

Researchers say the material could potentially be used to capture waste heat from a car's exhaust that would heat the material and produce electricity for charging the battery in a hybrid car. Other possible future uses include capturing rejected heat from industrial and power plants or temperature differences in the ocean to create electricity. The research team is looking into possible commercialization of the technology.

"This research is very promising because it presents an entirely new method for energy conversion that's never been done before," said University of Minnesota aerospace engineering and mechanics professor Richard James, who led the research team."It's also the ultimate 'green' way to create electricity because it uses waste heat to create electricity with no carbon dioxide."

To create the material, the research team combined elements at the atomic level to create a new multiferroic alloy, Ni45Co5Mn40Sn10. Multiferroic materials combine unusual elastic, magnetic and electric properties. The alloy Ni45Co5Mn40Sn10 achieves multiferroism by undergoing a highly reversible phase transformation where one solid turns into another solid. During this phase transformation the alloy undergoes changes in its magnetic properties that are exploited in the energy conversion device.



During a small-scale demonstration in a University of Minnesota lab, the new material created by the researchers begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil. Some of this heat energy is lost in a process called hysteresis. A critical discovery of the team is a systematic way to minimize hysteresis in phase transformations. The team's research was recently published in the first issue of the new scientific journal Advanced Energy Materials.

Watch a short research video of the new material suddenly become magnetic when heated: http://z.umn.edu/conversionvideo.

In addition to Professor James, other members of the research team include University of Minnesota aerospace engineering and mechanics post-doctoral researchers Vijay Srivastava and Kanwal Bhatti, and Ph.D. student Yintao Song. The team is also working with University of Minnesota chemical engineering and materials science professor Christopher Leighton to create a thin film of the material that could be used, for example, to convert some of the waste heat from computers into electricity.

"This research crosses all boundaries of science and engineering," James said. "It includes engineering, physics, materials, chemistry, mathematics and more. It has required all of us within the university's College of Science and Engineering to work together to think in new ways."

Funding for early research on the alloy came from a Multidisciplinary University Research Initiative (MURI) grant from the U.S. Office of Naval Research (involving other universities including the California Institute of Technology, Rutgers University, University of Washington and University of Maryland), and research grants from the U.S. Air Force and the National Science Foundation. The research is also tentatively funded by a small seed grant from the University of Minnesota's Initiative for Renewable Energy and the Environment.

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A New Way to Use the Sun's Energy

A New Way to Use the Sun's Energy

  1:47:00 AM    Science Lover

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Researchers have demonstrated a new mechanism for converting both sunlight and heat into electricity.

A new type of device that uses both heat and light from the sun should be more efficient than conventional solar cells, which convert only the light into electricity.

Bright heat: Nicholas Melosh has
developed a device for simultaneously
converting the sun’s light and heat into
electricity. Melosh makes and tests the
device in this vacuum chamber in his
lab at Stanford University.
Credit: Technology Review

The device relies on a physical principle discovered and demonstrated by researchers at Stanford University. In their prototype, the energy in sunlight excites electrons in an electrode, and heat from the sun coaxes the excited electrons to jump across a vacuum into another electrode, generating an electrical current. The device could be designed to send waste heat to a steam engine and convert 50 percent of the energy in sunlight into electricity--a huge improvement over conventional solar cells.

The most common silicon solar cells convert about 15 percent of the energy in sunlight into electricity. More than half of the incoming solar energy is lost as heat. That's because the active materials in solar cells can interact with only a particular band of the solar spectrum; photons below a certain energy level simply heat up the cell.

One way to overcome this is to stack active materials on top of one another in a multijunction cell that can use a broader spectrum of light, turning more of it into electrical current instead of heat, for efficiencies up to about 40 percent. But such cells are complex and expensive to make.

Looking for a better way to take advantage of the sun's heat, Stanford's Nicholas Melosh was inspired by highly efficient cogeneration systems that use the expansion of burning gas to drive a turbine and the heat from the combustion to power a steam engine. But thermal energy converters don't pair well with conventional solar devices. The hotter it is, the more efficient thermal energy conversion becomes. Solar cells, by contrast, get less efficient as they heat up. At about 100 °C, a silicon cell won't work well; above 200 °C, it won't work at all.

The breakthrough came when the Stanford researchers realized that the light in solar radiation could enhance energy conversion in a different type of device, called a thermionic energy converter, that's conventionally driven solely by heat. Thermionic converters consist of two electrodes separated by a small space. When the positive electrode, or cathode, is heated, electrons in the cathode get excited and jump across to the negative electrode, or anode, driving a current through an external circuit. These devices have been used to power Russian satellites but haven't found any applications on the ground because they must get very hot, about 1,500 °C, to operate efficiently. The cathode in these devices is typically made of metals such as cesium.

Melosh's group replaced the cesium cathode with a wafer of semiconducting material that can make use of not only heat but also light. When light strikes the cathode, it transmits its energy to electrons in the material in a way that's similar to what happens in a solar cell. This type of energy transfer doesn't happen in the metals used to make these cathodes in the past, but it's typical of semiconductor materials. It doesn't take quite as much heat for these "preëxcited" electrons to jump to the anode, so this new device can operate at lower temperatures than conventional thermionic converters, but at higher temperatures than a solar cell.

The Stanford researchers call this new mechanism PETE, for photon-enhanced thermionic emission. "The light helps lift the energy level of the electrons so that they will flow," says Gang Chen, professor of power engineering at MIT. "It's a long way to a practical device, but this work shows that it's possible," he says.

The Stanford group's prototype, described this month in the journal Nature Materials, uses gallium nitride as the semiconductor. It converts just about 25 percent of the energy in light into electricity at 200 °C, and the efficiency rises with the temperature. Stuart Licht, professor of chemistry at George Washington University, says the process would have an "advantage over solar cells" because it makes use of heat in addition to light. But he cautions: "Additional work will be needed to translate this into a practical, more efficient device."

The Stanford group is now working to do just that. The researchers are testing devices made from materials that are better suited to solar energy conversion, including silicon and gallium arsenide. They're also developing ways of treating these materials so that the device will work more efficiently in a temperature range of 400 °C to 600 °C; solar concentrators would be used to generate such high temperatures from sunlight.

Even at high temperatures, the photon-enhanced thermionic converter will generate more heat than it can use; Melosh says this heat could be coupled to a steam engine for a solar-energy-to-electricity conversion efficiency exceeding 50 percent. These systems are likely to be too complex and expensive for small-scale rooftop installations. But they could be economical for large solar-farm installations, says Melosh, a professor of materials science and engineering. He hopes to have a device ready for commercial development in three years.

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