# Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview
## 3 Wireless Power Transfer of Power Industry Optical Wireless Communications
### Dynamic Optical Wireless Power Transfer  
OWPT is one of the most appropriate ways to extend the transmission distance while keeping the relevant system components relatively simple [117]. Some companies, for example, [118], are already investigating the technology for use in consumer electronics and other areas. At the same time, some research institutions and institutions around the world are conducting far-field WPT research on OWPT, such as [119, 120]. An OWPT system consists of three parts: the transmitter of light, transmitting environment, and receiver of light. Therefore, the OWPT system efficiency is deduced from [98], that is:  
\[\lambda_{\text{{OWPT}}}=\lambda_{T}\times\lambda_{C}\times\lambda_{R} \tag{3}\]  
where \(\lambda_{T}\) stands for the light transmitter energy efficiency, which is for the ratio of output energy to consumed energy. \(\lambda_{C}\) is the optical transfer through the environment, and shows how much energy is lost when light is traveling in the considering environment such as air, pure water or sea water, etc. \(\lambda_{R}\) of the optical receiver; respectively. The correctness of the formula is verified by experimental tests and could be used to explain the measurement efficiency of OWPT system. To verify the proposed dynamic optical WPT concept, [PERSON] et al. first created a simple experiment, as shown in Figure 29. Here, solar energy is collected and stored in a device including DC-DC and DC-alternating current converters, and Li-ion batteries. Then utilizing the device to power the LED light bulbs of a small EV. This LED bulb consumes 8 W electricity as a light emitter, while a small solar panel is connected to another toy electric car as an energy receiver. The LED-solar panel  
Figure 28: The solar panels on the roof of electric vehicles (EVs) receive laser light from infrastructure along arterial roads; **(a)** sunlight illuminates directly at photovoltaic cells on car roof; **(b)** photovoltaic cells on the car roof illuminated by laser generated from solar-pumped [113]. WPT system introduced could work as far as 5 cm. The maximum WPT efficiency is 2% when the distance is almost zero, and the maximum WPT efficiency is 1.57% when the distance is 3 cm [117]. Figure 30a shows the situation where the blade is at a standstill without WPT, while Figure 30b shows that after the LED bulb moves to a certain distance, WPT does occur due to the rotation of the blade. Similar to the testing of ground vehicles, the dynamic OWPT system of this UAV is inefficient. Since its instrument is simple, it is predictable. Described above experimental OWPT efficiency is very low, it can also be seen in other systems, for instance used in underwater systems [121]. However, using high quality solar cells and light sources, higher OWPT efficiency can be expected. Theoretically, the upper limit of the energy conversion efficiency of traditional single-junction solar cells is 30%. This is because most of the solar energy that impacts the battery is not absorbed or converted into heat. Scientists are studying various solar cell  
Figure 30: Experiment setup for demonstration of dynamic OWPT between a moving light source and a moving object: (**a**) no WPT; (**b**) with WPT [117]. Figure 29: Experiment setup for dynamic optical wireless power transfer (WPT) between ground EVs using visible light [117]. designs to eliminate these limitations in conversion efficiency and reduce energy loss. The current world record for four-junction solar cells is 46% [122]. In [123; 124], the GaSb-based solar cell designed by researchers at George Washington University in 2017 has a photoelectric conversion efficiency of 44.5%. If the energy conversion efficiency of solar cells exceeds 50%, it will have a great impact on the cost of power generation [122]. This is still far behind the efficiency of resonant inductively coupled systems, but is acceptable in certain applications, for example, wireless charging for consumer electronics devices.