When most people think of charging a battery, they probably picture a wall outlet connected to a battery via a charging cable. In such conventional charging, excess electrons are forced into the anode of the battery through a medium (such as a cable) from a power source (such as a wall outlet).
Electrons flowing from a power source to a battery through a cable. Image courtesy of Let’s Talk Science
In recent years, another type of charging has become popular: wireless charging. In wireless charging, a battery is charged over the air when placed near a wireless power transmitter. One popular application of wireless charging has been smartphone charging. By placing a smartphone close to a wireless charger, consumers can charge their phones without a conventional cable.
Recently, researchers from Sejong University in South Korea claimed a feat in this space, achieving a wireless power transfer of nearly 100 feet using a laser-based device.
The researcher’s prototype includes a transmitter that has an erbium-doped fiber amplifier optical power source. The receiver uses a ball lens retroreflector to increase performance. Image courtesy of Jinyong Ha/Sejong University
Far-field Wireless Charging Leverages Electromagnetism
To better understand this recent study, it may be helpful to review the primary category of wireless charging the researchers investigated: far-field wireless charging.
Far-field (also called radiative) wireless power transfer (WPT) uses microwave or laser beaming. In this method, an electromagnetic beam is directed at an object to be charged via a transmitter. A receiver on the object converts the incoming electromagnetic beam to electrical current to charge a battery. Although basic far-field WPT may seem like a recent invention, it has been around since the 1960s when Raytheon demonstrated a microwave-powered helicopter.
High-level block diagram of long-range wireless power transfer. Image courtesy of Imperial College London
In a typical far-field WPT system, an antenna/rectifier system (sometimes referred to as a rectenna) is directed at a receiver, which converts the incoming RF signal to DC voltage. To achieve this RF-to-DC conversion, a non-linear device such as a diode is used in conjunction with a low-pass filter and load.
Basic rectenna circuit used for RF-to-DC conversion. Image courtesy of Imperial College London
Even though this basic circuit may seem simple, there are three critical challenges that have been a key focus of recent research into far-field WPT: range, efficiency, and non-line-of-sight (NLoS) applications. NLoS refers to situations in which the energy receiver may not be in the direct path of the energy transmitter due to an obstruction.
Sejong University Tackle Far-field Wireless Power Transfer With Laser
One of the main shortcomings of RF transmission is that it can’t send as much power over long distances as a laser. In a large common area such as an airport terminal or library, wireless power transfer to charge smartphones, for example, must occur with sufficient power over long distances.
Far-field WPT aims to operate efficiently with sufficient range. Image courtesy of Sejong University
Recently, researchers at Sejong University in South Korea unveiled a wireless charging system that uses infrared light to transmit 400 mW of power over 98 feet. Infrared light is electromagnetic radiation with a wavelength longer than visible light, typically defined as 700 nm to 1 mm.
The Sejong University researchers introduced a novel erbium-doped fiber optical source and a receiver. Using an erbium-doped source, the team said, allows for greater power transfer over a longer range than traditional laser amplifiers.
Basic structure of the erbium-doped fiber amplifier. Image courtesy of RP Photonics Encyclopedia
The key innovation behind the new system is an optimized form of distributed laser charging. Distributed laser charging separates the optical components of a traditional laser into a transmitter and receiver. When the transmitter and receiver are within line of sight, power is transmitted. When an obstruction occurs, the system automatically switches to safe mode.
In pursuit of large-scale wireless charging, what are the greatest technical challenges engineers must overcome? Share your thoughts in the comments below.
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