Harnessing 'Rashba spin-Seebeck effect' phenomenon will enable commercial devices to turn waste heat into electricity

April 10, 2018 by Dennis Meredith, University of California - Riverside
Credit: University of California - Riverside

Mechanical engineers at the University of California, Riverside, have reported success in using inexpensive materials to produce thermoelectric devices that transform low-level waste heat into electricity.

Their advance could enable a wide variety of commercial applications. For example, integrating thermoelectric generating devices into computer chips could enable the they produce to provide a power source. Waste heat from automobile engines could run a car's electronics and provide cooling. Photovoltaic solar cells could be made more efficient by harnessing the heat from sunlight striking them to generate more electricity.

Also, using the same basic technology, economical thermoelectric refrigerators could be produced that would be more energy efficient and with fewer moving parts than refrigerators that use compressors and coolant. Current thermoelectric refrigerators are expensive and relatively inefficient. In essence, they operate in reverse of , with an electric current applied to generate a temperature gradient that could be used in cooling.

While thermoelectric generators have been highly dependable—for example powering space probes such as the Voyager spacecraft for decades—their use has been limited by the expense and complexity of the materials they required.

However, researchers led by Assistant Professor of Mechanical Engineering Sandeep Kumar, have reported achieving significant thermoelectric energy conversion using a combination of a nickel-iron alloy and silicon. The scientific paper on their results recently appeared online in the journal Physica Status Solidi- Rapid Research Letters. Co-authors of the paper were graduate students Ravindra Bhardwaj and Paul Lou.

In their experiments, the researchers constructed a super-thin two-layer sandwich of nickel-iron Permalloy and a form of silicon called p-type silicon. The Permalloy layer was 25 nanometers thick, and the researchers produced devices in which the silicon layer had thicknesses of 100 nanometers, 25 nanometers and 5 nanometers. By comparison, a human hair is about 200 microns thick.

When the researchers applied heat to the Permalloy layer, they were able to produce an electrical voltage in the sandwich, due to a recently discovered physical phenomenon known as the spin-Seebeck effect. In this effect, a temperature gradient generates a spin current from the ferromagnetic substance, producing an electrical voltage in the silicon. The enhanced voltage produced in the was due to a mechanism called the "Rashba spin-orbit coupling."

The researchers achieved one of the largest such voltages yet measured in the device with the 5 nanometer-thick silicon layer.

"Such devices will be able to generate electricity in any situation in which there are small gradients in temperature," said Kumar. "For, example, a computer chip typically heats up to about sixty degrees Centigrade, while the surrounding temperature may be twenty-five degrees. So, such spin-thermoelectric generators integrated into computer chips could generate significant amounts of electricity from heat that would otherwise be wasted."

"Also, photovoltaic solar panels become quite warm, so integrating thermoelectric generators into such panels could generate additional electricity from that ," he said. "Thermoelectric generators could also be integrated into automobiles, where engine temperatures can reach a hundred degrees centigrade, compared to an ambient of twenty-five degrees. In such an application the generated electricity could power the car's electronics and even provide cooling."

The latest experiments constitute only the beginning of development of the new thermoelectric technology, said Kumar, who added that a provisional patent for the discovery has been submitted.

"We have only just scratched the surface of the possibilities," he said. "We believe we can generate even more energy from the same by using multiple layers, just as putting batteries together increases energy output. And, we are exploring other earth-abundant, inexpensive materials that could prove effective in spin-."

The title of the paper is "Giant enhancement in Rashba spin-Seebeck effect in NiFe/p-Si thin films."

Explore further: Exploring the thermoelectric properties of tin selenide nanostructures

More information: Ravindra G. Bhardwaj et al. Giant Enhancement in Rashba Spin-Seebeck Effect in NiFe/p-Si Thin Films, physica status solidi (RRL) - Rapid Research Letters (2018). DOI: 10.1002/pssr.201800064

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antialias_physorg
4.2 / 5 (5) Apr 10, 2018
For example, integrating thermoelectric generating devices into computer chips could enable the heat they produce to provide a power source

That seems like a weird idea. Cooling is the process of getting heat away from the source *fast*. When you start to put a generator in the way you're always slowing down the flow of heat which sorta defeats the purpose.

As for using this stuff to get energy from car engines. That might be a bit too late. By the time these nanogenerators are ready for prime time there won't be any internal combustion engines left (at least not new ones where this could be integrated into)

But the idea of backing solar cells with these might work. Depends on efficiency, though. I played around with regular Seebeck modules a few years back to build a tiny fridge (about the size of a single can of soda). The power efficiency of these things is atrocious. A regular fridge is WAY more efficient.
betterexists
not rated yet Apr 10, 2018
But, Hot Springs & Volcanoes have awful lot of heat !
rrwillsj
3 / 5 (2) Apr 10, 2018
I think these ideas have a modest utility. Numerous small, moderately productive devices. Tapping a variety of local heat sources. A conservative utilization of otherwise wasted energy.

No great leaps into the future. However a penny saved here and a penny saved there, eventually you have dollars. The key for each of us is patience and moderation in consumption.

Preventing waste is between having the maturity to temper our expectations. And continuing steady, deliberate development of efficiency in our machines.
S_Kumar
5 / 5 (3) Apr 10, 2018
Cooling is the process of getting heat away from the source *fast*. When you start to put a generator in the way you're always slowing down the flow of heat which sorta defeats the purpose
You're right but in large datacenters the processors could be cooled by liquid or heatpipes and thermal gradient is already redundant there, so it could be utilized for energy production. It's estimated that one Google query generates on average 0.2g of CO2...


That is what, we have in mind. The data centers use over 10% of the world's electricity, which is rising. And integrating the SSE generator with the chips can lead to waste heat recovery. Conventional thermoelectrics are not good for small temperature gradients at room temperature. Whereas our study shows that SSE is better in such cases.
winthrom
not rated yet Apr 10, 2018
In some situations heat is abundant and reducing weight is an important factor. I.e., aircraft. The extreme thinness of this type device argues for very low weight. In aircraft, power for everything is taken from engine power by mechanical generators (which can fail). Stripping heat lost to the environment from the engines non-power producing heat (like the exhaust pipes of an internal combustion motor or the insulation for the body of jet engines) can reduce generator sizes and fuel burn needed to run the generators. Could even eliminate generators. Depending on the aircraft, these devices could replace the mechanical generator once the engine warms up. Temperature differences between the engines and the environment are more extreme as an aircraft increases altitude, so the delta T will increase also. Battery operation before engine warm-up will cover start-up and a few minutes before transition occurs.
Da Schneib
not rated yet Apr 10, 2018
@Dr. Kumar, thanks for following this and posting. I think your ideas are very interesting. It's particularly good that the materials used are cheap, common, and non-toxic. Do you have some answers for @antialias' observations above? Will such a generator be able to keep the microprocessors and other chips from getting too hot and being damaged? This is of particular concern with graphics processors and their memory, which are noted for generating a lot of heat very quickly.

On a more general subject, has anyone considered recovering waste heat from steam-driven generation plants and generating electricity directly from it?
S_Kumar
5 / 5 (1) Apr 10, 2018
@Dr. Kumar, thanks for following this and posting. I think your ideas are very interesting. It's particularly good that the materials used are cheap, common, and non-toxic. Do you have some answers for @antialias' observations above? Will such a generator be able to keep the microprocessors and other chips from getting too hot and being damaged? This is of particular concern with graphics processors and their memory, which are noted for generating a lot of heat very quickly.

On a more general subject, has anyone considered recovering waste heat from steam-driven generation plants and generating electricity directly from it?


The first problem that we are working on is related with on-chip power generation. The temperature gradient at CPU/GPU is enough to generate electricity. But how much electricity can we generate and will that be sufficient for cooling I don't know right now.
There is lot of work to be done.
Da Schneib
not rated yet Apr 10, 2018
Thanks for your quick and thoughtful response, @Dr. Kumar. I expect it would be fruitless to talk about the internal temperature of chips without more experimentation to determine how much heat can be removed how quickly after a few more design iterations of the generator as well as possible materials changes now that the basic mechanism is proven.
antialias_physorg
1 / 5 (1) Apr 11, 2018
. But how much electricity can we generate and will that be sufficient for cooling I don't know right now.

Well, if you use the power generated for cooling of chips and this setup is more effective than cooling chips in the first place then you've just invented a perpetuum mobile (I'm not joking. There's just no way this setup can work from a physics perspective).

That said there are certainly any number of uses. Heck, you could probably cover entire house walls/roofs with this stuff if it's cheap enough to manufacture (and the energy output is worth it).

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