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Water droplets are ubiquitous in nature and harvesting droplet kinetic energy has attracted a great deal of attention to meet the increasing worldwide energy demands. In this work, we report a water droplet motion energy harvester with the wafer-level fabrication method, which is based on the water–solid surface contact electrical double layer effect. A theoretical model is established to illuminate the water droplet motion energy harvesting mechanism. Guided by the model, an energy harvester is developed based on Micro Electro-Mechanical Systems compatible process technology, which achieve a wafer-level fabrication. The output performance of the energy harvester has been optimized by decreasing the electrode width and its interval. Experimental results also show that the output voltage can be improved by increasing the droplet volume and decreasing the ionic concentration. The optimized energy harvester exhibits an open circuit output peak voltage of 57.5 mV. This research provides an in-depth theoretical study and a practical guidance on water droplet kinetic energy harvesting.

Table tennis competition is voted as one of the most popular competitive sports. The referee umpires the competition mainly based on visual observation and experience, which may make misjudgments on competition results due to the referee’s subjective uncertainty or imprecision. In this work, a novel intelligent umpiring system based on arrayed self-powered acceleration sensor nodes was presented to enhance the competition accuracy. A sensor node array model was established to detect ball collision point on the table tennis table. This model clearly illuminated the working mechanism of the proposed umpiring system. And an improved particle swarm optimization (level-based competitive swarm optimization) was applied to optimize the arrayed sensor nodes distribution by redefining the representations and update rules of position and velocity. The optimized results showed that the number of sensors decreased from 58 to 51. Also, the reliability of the optimized nodes distribution of the table tennis umpiring system has been verified theoretically. The results revealed that our system achieved a precise detection of the ball collision point with uniform error distances below 3.5 mm. Besides, this research offered an in-depth study on intelligent umpiring system based on arrayed self-powered sensor nodes, which will improve the accuracy of the umpiring of table tennis competition.