Scientists from Kyushu University in Japan together with specialists from Hinode Holdings developed a new system for wireless energy transfer for e-vehicles using the already existing elements of urban infrastructure – the usual cast-iron hatches. This solution allows for charging e-vehicles without connecting with a socket and even during the movement turning the road pavement into a network of hidden charging stations.
The technology is based on transferring energy not through a magnetic field as in usual inductive systems, а but through an electric field created between metallic plates performing as electrodes. This is called the capacitance energy transfer. It does not need many expensive materials and cumbersome coils; hence, it may be implemented directly in road hatches without changing the pavement structure. At the same time, the screen made of earthed plates maintains the electric field between the electrodes preventing energy leakages and improving safety for both people and equipment.
To check their idea, the scientists made the system mock-up model and performed a series of trials. A cast-iron lid of 33 cm in diameter (a standard one for city utility lines) was used as a transfer electrode, and an aluminum plate fasted at the lower part of the car was used as a receiving electrode. The system operated at 13.56 MHz frequency, which is in line with the international standard for wireless industrial appliances.
In the course of the experiments, the scientists compared several materials – copper, aluminum, steel and malleable cast-iron. It turned out that cast-iron plates provide for almost the same efficiency as copper ones, even though their resistance is higher. Given only 2 mm clearance between the electrodes, the capacity reached электродами 70 pF – compatible with the best samples based on non-ferrous metals. The trials also demonstrated that rough cast surface makes the bonding stronger by increasing the real area of interaction.
When supplying 100 MW, the system reached 58%, transfer efficiency, and when the capacity was increased up to 200 MW, the system maintained about 40% efficiency. Even when the receiving electrode dislodged by 20 cm (imitating the car movement), the efficiency remained at the level of 45%. It confirms that the system is resistant to mispositioning and may be used for dynamic charging, i.e. feeding the car just when it is moving along the road.
The mechanical analysis confirmed that the cast-iron lids endure up to 25 tons load and are completely in line with the requirements for installing them in the roadway.
The researchers say that using the already existing cast-iron infrastructure elements allows for drastic savings on building the charging stations and for facilitating the technology implementation. Instead of laying the coils under roads, it will be enough to replace the standard road hatches with the modified ones connected with the power source. This opens the way to creating inexpensive, durable and almost invisible network for wireless charging of e-vehicles.
