A team working at the new NGI National Graphene Institute within Manchester University has developed a graphene-based electrical nanodevice, which could increase the energy efficiency of fossil fuel-powered cars.
The 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.
Car exhausts can reach temperatures of 600 degrees Celsius. The recovered energy is used to power additional automotive features like air conditioning and power steering, or it can be stored in the car battery.
Professor Aimin Song and Dr. Ernie Hill led the team responsible in Manchester, who worked with other scientists in Shandong University. The device utilises graphene’s phenomenally high electron mobility – the property, which determines how fast an electron can travel in a material and how fast electronic devices are able to operate. The result is the most sensitive room-temperature rectifier ever made. Conventional equipment with similar conversion efficiencies requires cryogenically low temperatures.
Even today’s most efficient internal combustion engines 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.
Graphene has exceptional properties; it possesses the longest carrier mean free path of any electronic material at room temperature. It has no band gap, which has been viewed as a problem for some other potential commercial applications.
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 singlelayered planar device structure, which is perfect to take advantage of the high electronmobility 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”.
The advantage of a graphene-based nanorectifier is its outstanding 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 next stage for the Manchester based team will be to up-scale their work. To do this they will use large wafer-sized graphene to perform high-frequency experiments. The result could be technology that may gather wasted heat energy within power plants.