In maglev innovation, Chinese researchers transfer power wirelessly to moving train
- A test run in eastern China found the elevated prototype could draw powerful electricity from transmitting coils on the rail with up to 92.4 per cent efficiency
- Although a maglev train is driven forward by magnetic force, much of the equipment on board requires electricity, and existing methods have their drawbacks
The energy efficiency of a typical smartphone wireless charger is around 50 per cent. When charging a moving electric car with roadside transmitters, even more of the energy – nearly 80 per cent – is wasted, according to recent experiments.
But a maglev train in Qingdao, in China’s eastern Shandong province, designed to travel at a top speed of 600km/h (372mph) while being elevated by magnetic force, could draw powerful electricity from transmitting coils on the rail with up to 92.4 per cent efficiency.
The experimental results suggested that the wireless power transmission technology, based on extensive research and experiments by scientists and engineers around the world over the past few decades, was now “feasible for rail transport”, said lead project scientist Wu Donghua, of the CRRC Qingdao Sifang company, this month in a paper published in the Journal of Southwest Jiaotong University, a Chinese language peer-reviewed publication.
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Power supply has been a major technological challenge in high-speed maglev technology.
Although the train is driven forward by magnetic force, much of the equipment on board requires electricity.
A battery is deemed too big and heavy as a power source and wires are unable to withstand the heat and wear generated by contact with power lines at extreme speeds.
A linear motor, a traditional wireless supply method used in Japanese and German maglev trains, can produce electricity at high speed by harvesting energy directly from the magnetic driving force.
But electric turbulence occurring in the narrow gap between the train and rail could reduce the efficiency of power transmission more than 50 per cent, according to some estimates. When the train stops at a station, the linear motor cannot generate any electricity.
The wireless power transmission system developed by Wu’s team can be compared to an induction stove.
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Their system is designed to convert direct electrical current into a magnetic field with copper coils attached to the side of a rail. When the train passes over the charged coils, an antenna at its bottom cuts through the magnetic field and produces electricity in the physical process of induction.
But getting the idea to work in real life was not easy because the additional magnetic field could interfere with the magnetic force driving the train forward.
A significant proportion of the magnetic energy could also leak into the environment, cutting its overall efficiency.
To overcome these obstacles, Wu and his colleagues said they focused on some of the smaller details that were often neglected, such as the wiring method of the transmitting coil and the shape of the receiving antenna, and came up with designs never attempted before.
Their new technology could supply more than 170 kilowatts of power to the train at all speeds, exceeding the requirements of the electronic hardware on the single-car prototype.
“This efficiency is incredible,” said a Chongqing University researcher studying the application of wireless power transmission in transport.
“It can be achieved in a wireless charging experiment for electric cars, but that’s under the most favourable conditions in a laboratory, and when the car is not moving at all,” said the researcher who asked not to be named because he was not authorised to speak to the media.
The difficulty in charging a train travelling at 600km/h lies in powering up the coil, which would take at least a second.
One coil unit developed by Wu’s team was more than 20 metres (65 feet) long, and the train would pass it in less than the blink of an eye.
The Chongqing-based researcher said the proposed technology would require an extremely powerful and precise control system to switch the coils on just before the train arrived and then off after the train passed or overall efficiency would be significantly reduced.
And because of a gap between the coils, the train would also need an extremely powerful energy management system to ensure a continuous, stable supply, he added.
The idea of wireless power transfer has been around for more than a century.
China is a latecomer to the field but in recent years the government has invested a significant amount in developing the technology.
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Some surveillance devices in ultra-high voltage power grids are already powered wirelessly, according to the government.
The Chinese military has developed drones that could be powered by energy beams from the ground to remain in the air for an extended period, according to researchers involved in these projects.
