Laser-based wireless power transmission (LWPT) technology is considered a relatively new technology to be used in long-range wireless power transfer applications such as unmanned aerial vehicles and orbiting satellites.1,2 The LWPT system has two major components: a laser diode (LD) and a photovoltaic (PV) array, as shown in the below figure.3 In any application, the end-to-end efficiency of a system must be considered. This system efficiency includes the efficiencies of the LD and PV array, as these are responsible to limit the efficiency of an implemented system. Most of the latest research on LWPT technology mainly focuses on device-level techniques and hardware implementations. However, there are certain studies that focus on enhancing the efficiency characteristics of the LD and PV array so that the system-efficiency characteristic of the entire system is still ambiguous.
In order to scrutinize the system-efficiency characteristic, the LWPT system in this article is modeled as an optically coupled DC/DC converter, as shown in the below figure. As it is clear from the figure that the current has a direct effect on the performance of the LD, the efficiency of the system due to input current is analyzed theoretically and measured experimentally. It is concluded that the system efficiency can be influenced by the duty cycle of LD input current, which makes some contribution to the LWPT technology field. With the help of the relationship between transmission power and the system efficiency under different input-current duty-cycle conditions, the guideline for the system control method can be provided. Keeping in view the above circumstances, the system can be optimized by efficiently utilizing the LD and PV cell.
Theoretical analysis of the system-efficiency characteristic
Efficiency characteristics of the LD
In wireless power transmission applications, the LD can be powered by continuous current (CC mode) or pulse current (pulse mode).4 The below figure (top) shows the key waveform of the LD under CC and pulse mode. First, we investigate the relationship of LD efficiency and its input current, as the performance of the LD is influenced by its input current. It is evident from the below figure (bottom) that for the same output optical power, the smaller the duty cycle of the LD input current, the higher the efficiency of the LD.
Efficiency characteristics of PV arrays
Driving the LD in pulse mode offers the advantage of high efficiency. So it is quite natural to investigate the mechanism of how the efficiency of a PV array changes with continuous pulse-incident optical power. The voltage and current of a PV array at maximum power point are dependent on irradiance level and cell temperature.
In standard environmental conditions, the irradiance level is 1,000 W/m2 and the cell temperature is 25˚C.
Increasing temperature degrades the efficiency of the PV cell. In order to overcome this problem, a cooling system is used in the LWPT system to maintain the temperature of the cells at the lowest level as possible. This cooling system helps in achieving the maximum performance of the PV cell. Hence, it is concluded that temperature can be ignored.
The figure below shows the graph of efficiency versus the incident optical power under different duty cycles of the laser pulse. It can be easily seen in the graph that the smaller the duty cycle, the higher the efficiency will be. Hence, it is concluded that, for efficient performance of PV cells at high laser intensities, the PV array should be illuminated by the pulse laser.
Efficiency characteristics of the system
For an LWPT system, the losses in LD and PV array are the major contributing factors to the system rather than other components. So the efficiency of the system is determined by the efficiency of the LD and PV. As mentioned earlier, the duty cycle has the same effect on the efficiencies of both the LD and PV array in the case of specialized PV cells. Therefore, system efficiency will increase as duty cycle decreases.
In this article, we have discussed and investigated the LWPT system. There have been few studies regarding this system and those studies have been taken into consideration in this article. In a nutshell, system efficiency is directly related to the duty cycle of LD input current; the smaller the duty cycle, the higher the efficiency of the system. Hence, controlling the duty cycle of the pulse laser is the key factor in optimizing the system efficiency.
1Efficiency Evaluation of Laser Based Wireless Power Transmission System. Weiyang Zhou, Department of Electrical and Computer Engineering, University of Michigan-Dearborn, USA, firstname.lastname@example.org; Ke Jin, College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China,
2R. Mason, “Feasibility of Laser Power Transmission to a High-Altitude Unmanned Aerial Vehicle,” Rand Corporation, 2011.
3D. E. Raible. “High intensity laser power beaming for wireless power transmission,” Master’s Thesis, Department of Electrical and Computer Engineering, Cleveland State University, Cleveland, OH, May 2008.
4Weiyang Zhou, and Ke Jin, “Efficiency Evaluation of Laser Diode in Different Driving Modes for Wireless Power Transmission,” IEEE Trans. Power Electron., Vol. 30, No. 11, pp. 6,237–6,244, 2015.
5K. J. Sauer, T. Roesslor, and C. W. Hansen, “Modeling the irradiance and temperature dependence of photovoltaic modules in PVsyst,” IEEE Journal. of Photovoltaics., Vol. 5, No. 1, pp. 152–158, 2015.
6O. Hohn, A. W. Walker, A. W. Bett and H. Helmers, “Optimal laser wavelength for efficient laser power converter operation over temperature,” Appl. Phys. Lett., Vol. 108, No. 24, pp. 1–9, 2016.