Graphene device puts fuel-efficient cars in pole position

Graphene device puts fuel-efficient cars in pole position
Graphene ballistic rectifier. Credit: University of Manchester

A graphene-based electrical nano-device has been created which could substantially increase the energy efficiency of fossil fuel-powered cars.

The nano-device, known as a 'ballistic rectifier', is able to convert heat which would otherwise be wasted from the car exhaust and engine body into a useable electrical current.

Parts of car exhausts can reach temperatures of 600 degrees Celsius. The recovered can then be used to power additional automotive features such as air conditioning and power steering, or be stored in the car battery.

The nano-rectifier was built by a team at The University of Manchester led by Professor Aimin Song and Dr. Ernie Hill, with a team at Shandong University. The device utilises graphene's phenomenally high electron mobility, a property which determines how fast an electron can travel in a material and how fast electronic devices can operate.

The resulting device is the most sensitive room-temperature rectifier ever made. Conventional devices with similar conversion efficiencies require cryogenically low temperatures.

Even today's most efficient can only convert about 70% of energy burned from fossil fuels into the energy required to power a car. The rest of the energy created is often wasted through exhaust heat or cooling systems.

Greg Auton, who performed most of the experiment said: "Graphene has exceptional properties; it possesses the longest carrier mean free path of any electronic material at room temperature.

"Despite this, even the most promising applications to commercialise graphene in the electronics industry do not take advantage of this property. Instead they often try to tackle the the problem that graphene has no band gap."

Professor Song who invented the concept of the ballistic rectifier said: "The working principle of the ballistic rectifier means that it does not require any band gap. Meanwhile, it has a single-layered planar device structure which is perfect to take the advantage of the high to achieve an extremely high operating speed.

"Unlike conventional rectifiers or diodes, the ballistic rectifier does not have any threshold voltage either, making it perfect for energy harvest as well as microwave and infrared detection".

Graphene was the world's first two-dimensional material, isolated in 2004 at The University of Manchester, since then a whole family of other 2D materials have been discovered.

The advantage of a graphene-based nano-rectifier is its high conversion efficiency from an alternating current to a direct current at room temperature, enabled by the extremely high electron mobility achieved in this work.

The Manchester-based group is now looking to scale up the research by using large wafer-sized graphene and perform high-frequency experiments. The resulting technology can also be applied to harvesting wasted heat energy in power plants.


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Graphene drives potential for the next-generation of fuel-efficient cars

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Jun 02, 2016
70% efficient??? I thought the best efficiency of a gasoline car was only around 35%

Jun 02, 2016
70% efficient??? I thought the best efficiency of a gasoline car was only around 35%


Think hybrid electric....

Jun 02, 2016
The 70% efficiency is reversed, and is more like 30% efficiency max.(modern engines are far better heaters than thrusters)

Wiki says, "Modern gasoline engines have a maximum thermal efficiency of about 25% to 30% when used to power a car. In other words, even when the engine is operating at its point of maximum thermal efficiency, of the total heat energy released by the gasoline consumed, about 70-75% is rejected as heat without being turned into useful work, i.e. turning the crankshaft"

Jun 02, 2016
Checked it out, found a good explanation of how a ballistic rectifier works: http://personalpa...fier.htm but the thing in this article is more sophisticated: look at the shape of it. Think about a bunch of ping pong balls that are being pushed out the two wings on the right and left and think about the only direction they can go; it's the same idea but a different geometry.

So, the basic idea is to make a Maxwell's Demon that always shuffles electrons, say, down, using nanotech features to direct the electrons.

It's the *shape* of the graphene, not its doping, that directs the electrons. For the EEs, this means it has no bandgap. The efficiency is vastly improved by this.

Jun 03, 2016
We can already see a winner in automotive propulsion - electric. All we are waiting for is the correct battery technology and the ICE is dead.

Jun 03, 2016
70% efficient??? I thought the best efficiency of a gasoline car was only around 35%


70% is the theoretical Carnot efficiency of a gasoline engine when the average combustion temperature stays below the melting point of aluminium.

The most efficient internal combustion engines in general however do come above 50% and with a bag of tricks like compounding, can approach 70%.

All we are waiting for is the correct battery technology and the ICE is dead.


And you can wait forever for that, because batteries are fundamentally inconvenient and cumbersome to carry and recharge. Developments in SOFC technology will produce a more flexible and ultimately cheaper option for electric cars, because they bypass the whole issue where you can't carry electrons in a bucket, and where batteries require too many too rare and precious materials.


Jun 03, 2016
Sorry, but what a waste of time.

Any knowledge that increases efficiency is good. Especially since combustion engines are still the defacto standard. If an easy addon could lower the amount of fossil fuels burned then that helps, too. That EVs are the way to go (which I fully agree with) is not an argument against this.

Also you can't just go to a scientist and say: "Don't research X, research this Y". Scientists are highly specialized. If someone is able to do research in X then it's by no means a given (read: rather unlikely) that he can do research in Y.

Jun 03, 2016
If you click on the link you get this.

"The average car currently loses around 70% of energy generated through fuel consumption to heat. Utilising that lost energy requires a thermoelectric material which can generate an electrical current from the application of heat."

Thus the author was wrong about the 70% efficiency. BTW I was taught that the Carnot cycle predicted a maximum efficiency of 50%

The link also stated that the devices efficiency was 3-5 % but did not mention the temperature differential required for operation. All in all I can see this device as being useful in cars if it can be made small enough and for the right price. It might be even more useful in an industry that generates a lot of waste heat.

Jun 03, 2016
Sorry, but what a waste of time.

I'll add to what @antialias said and point out that rectifiers with no bandgap would be *extremely* useful in electric vehicles and hybrids.

It looks like you only read the title and didn't delve into the details, @RM07.

Jun 03, 2016
Developments in SOFC technology will produce a more flexible and ultimately cheaper option for electric cars, because they bypass the whole issue where you can't carry electrons in a bucket, and where batteries require too many too rare and precious materials.
Yeah, but they have that temperature problem.

And battery tech has been burgeoning; there are a lot of organizations out there spending a lot of money on it. We'll have some environmentally friendly batteries with high performance using non-rare materials fairly soon.

Personally I'm an "all of the above" kinda guy.

Jun 03, 2016
Did some more research, and found out ordinary graphene nanoribbons have resistivity about 3x that of pure copper (see here: https://arxiv.org...24.pdf). This is *far* better than silicon, the material of choice for semiconductor rectifiers, and we haven't even included the bandgap in the efficiency yet.

A layered graphene ballistic rectifier looks from this like it can outperform a silicon rectifier by a very great deal. What one would do to construct such a device is iterate on the deposition of graphene layers and fabricate a connection to all the layers preferably afterward in a single step. Power handling would be superior due to the lack of bandgap, as well as the better bulk resistivity; combining these, such devices would probably far outperform any semiconductor rectifier on the market today, even many hooked in parallel, and if the fabrication can be performed with high yield they could beat semiconductor rectifiers on price as well.

[contd]

Jun 03, 2016
[contd]
All things considered the applications of such devices as that discussed in this article should go far beyond simply improving fuel efficiency of carnot engines.

Using a device like this in the switch of a switched-mode power converter should yield unprecedented efficiency; this leads to all sorts of applications in renewables, not to mention increased efficiency and decreased power dissipation in most manufactured electronic products from stereo consoles to computers to cordless electric shavers. Furthermore, it improves the efficiency not only of produced power from PV and wind systems, but of batteries to store their power for off-hours service.

Finally, if they can use these things to generate power from the exhaust of a carnot engine, then they can use them to generate power from just about any heat differential.

So this is a much deeper subject than it at first appears to be.

Jun 04, 2016
"Using a device like this in the switch of a switched-mode power converter should yield unprecedented efficiency; this leads to all sorts of applications in renewables, not to mention increased efficiency and decreased power dissipation in most manufactured electronic products from stereo consoles to computers to cordless electric shavers. Furthermore, it improves the efficiency not only of produced power from PV and wind systems, but of batteries to store their power for off-hours service."

Yes, that is all well and good but 3 to 5 % conversion efficiency is the financial equivalent of a brick wall unless this product can be produced at a very low price.

Jun 04, 2016
Finally, if they can use these things to generate power from the exhaust of a carnot engine, then they can use them to generate power from just about any heat differential.

Something simple - sun heated ground. Or a rock. Or ice...
and so on.
Much application here...
Even a brick wall, MR...;-)

Jul 03, 2016
Could similar efficiency gains be had in the solar world?
graphene101.com - sharing anything and everything about graphene with our readers.

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