Objective In Laser Wireless Power Transmission (LWPT) system, Maximum Power Point Tracking (MPPT) technology is a key factor to improve the power transfer efficiency by adjusting the circuit parameters at the receiver in real time, so that the output power of the laser photovoltaic cell reaches the maximum value. Due to the error of the aiming system and the obstruction of the object, the photovoltaic cell array will be subjected to uneven laser irradiation, and the photovoltaic cell array will generate different photogenerated currents, which leads to the current mismatch between the photovoltaic cells in the array, and the output power-volt characteristic curves is showing multi-peak, and the output power will be significantly reduced. The Distributed Maximum Power Point Tracking (DMPPT) technique can effectively reduce the current mismatch between the photovoltaic cells in the string of the array. Aiming at the problem of current mismatch in LWPT system, DMPPT technology is applied to LWPT system according to the characteristics of high laser irradiation power density but low power of Gallium Arsenide (GaAs) cell. It is proposed to replace the traditional Boost circuit with a Parallel-Type Boost circuit (PT-Boost) to reduce the input current ripple, so as to effectively improve the tracking accuracy of each MPPT submodule of DMPPT system and the overall circuit efficiency of DMPPT.
Methods The working state of PT-Boost circuit is theoretically analyzed (Fig.5). A theoretical model of the DMPPT system is established and its output characteristice is analyzed; The above theoretical model is used to build a set of DMPPT simulation system, simulate and analyze the input current ripples of the PT-Boost circuit relative to the Boost circuit in the MPPT sub-module of the DMPPT system, and set up different LWPT scenarios to verify the feasibility of the DMPPT system. Finally, the DMPPT experimental platform (Fig.8) is built to test the specific performance of the DMPPT system.
Results and Discussions The simulation results show that under the same GaAs cell model parameters, the current ripple is reduced and the MPPT accuracy is higher when utilizing the PT-Boost circuit as a DC/DC converter compared to the traditional Boost circuit (Fig.6). And under different shading conditions, the difference between the maximum power point of the DMPPT output and the total power of the GaAs battery array is within 2%(Fig.7). which shows that it is feasible to utilize the DMPPT system to reduce the current mismatch of each cell among the strings of the GaAs battery array. A DMPPT test system was established, and the experimental results showed that the tracking efficiency of the DMPPT system based on the PT-Boost circuit was improved by 3.6% to 93.5% compared with the traditional Boost circuit (Fig.9). Based on the above study, laser wirelessenergy transmission scenarios with 0%, 25% and 50% shading were set up, and the overall circuit efficiency of DMPPT reached 93%, 92.6% and 90.3%, respectively (Fig.10).
Conclusions In the study of LWPT system, the non-uniform distribution of laser irradiation leads to the occurrence of multi-peak conditions in the output of photovoltaic cells array, and the output power decreases. In this paper, The DMPPT system is used to reduce the current mismatch between cells in the photovoltaic cell array string, thus reducing the occurrence of multi-peak conditions and improving the output power. In addition, the PT-Boost circuit is utilized to replace the traditional Boost circuit and the phase relationship between the branch currents is changed by shunting in parallel, which reduces the current ripple of the input current of circuit and effectively improves the tracking accuracy of the DMPPT. The simulation verifies the feasibility of the DMPPT system to reduce the output current mismatch of each cell between battery array strings under different laser wireless energy transfer scenarios. The DMPPT test system was built, which simulates different shading scenarios by blocking part of with the sunshade. The test results show that the overall circuit efficiency of the DMPPT reaches 93%, 92.6% and 90.3%, respectively. Compared with using a traditional Boost circuit as the DC/DC converter of the DMPPT system, the DMPPT system has an improvement of about 2% in the laser irradiation non-uniformity scenario by using a PT-Boost circuit as the DC/DC converter of the DMPPT system. The results are instructive for the maximum power point tracking of laser wireless power transmission under uneven laser irradiation scenarios.
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