Recently, Professor Zhang Zhihua and Academician Guo Wanlin's team at Nanjing University of Aeronautics and Astronautics combined high-speed photography and multi-physics simulation to reveal the generation mechanism of ultra-high voltage of a drip power device with two electrodes, establish the analytical relationship between the output electrical signal and the device geometry and the parameters related to water drop movement, and increase the output voltage to 1030V through the optimal design of the device. Energy conversion efficiency increased to 3.3%. The research results were published in Advanced Functional Materials under the title "Dynamical Mechanism for Reaching Ultrahigh Voltages from a Falling Droplet."
The author first observed the dynamic behavior of the spreading, contact with the upper electrode and contraction of water droplets when they hit the surface of the power generation device at different positions by using a high-speed camera, and recorded the electrical signal output by the device simultaneously (FIG. 1a). By comparing the contact length (l) between the water drop and the upper electrode and the contact area (S) of the surface with the recorded voltage (U) and its corresponding charge transfer (q), the authors found that the voltage U is positively correlated with the rate of change of l, that is, dl/dt (Figures 1b, c). Using the distribution of charge on the two electrodes obtained from the results of multi-physics simulation (FIG. 1d), the author established a model to express the change of charge on the upper electrode over time as a function of l, S, the thickness of the polymer in the device, the relative dielectric constant, the surface charge density, the concentration of water drop salt and other parameters. The authors used models to predict electrical signals at different impact locations, and the results were consistent with experimental measurements (Figure 2). Parametric scanning results show that surface charge density, polymer thickness, external resistance, drop salt concentration and dl/dt (related to the distance px from the impact point to the upper electrode and the device tilt Angle α) have significant effects on the electrical signal, and are verified by experiments (FIG. 3). By optimizing the above parameters, the authors significantly improved the output voltage and energy conversion efficiency (Figure 4).
The research reveals the mechanism of the ultra-high voltage generated by the drip power generation device, and also provides a new idea for understanding, exploring and realizing other water-volt devices that use the mechanical energy in water efficiently.
Figure 1: When a water drop strikes the surface of a drip-generating device with two electrodes and comes into contact with the upper electrode, an electrical signal is generated between the two electrodes. (a) Schematic diagram of the experimental device. The illustration in the upper left corner shows the xz section near the contact line, illustrating the mechanism of drip power generation. (b, c) Evolution of the change rate (dl/dt) of the measured electrical signal (U) and contact line length (l) with time when the impact distance (px) is 5.5mm and 0.8mm. The illustration shows the evolution of charge transfer (q) and l with time in the corresponding case. (d) The simulated distribution of the dimensionless surface charge density (σ/σ0) across the two electrodes, with positive charges in red and negative charges in blue.
Figure 2: Comparison of U and q predicted using the model and experimentally measured. The experimental device is placed horizontally (α=0°) in (a-d) with an Angle of 45° (α=45°) from the horizontal plane in (e, f). The distance (px) from the impact point to the upper electrode is 5.5 mm in (a, b) and 0.8 mm in (c, d).
Figure 3: Optimizing output voltage peak (Umax) and energy (E) by adjusting key parameters. (a-e) Effect of electrode diameter (dt), surface charge density (σ0), polymer thickness (lp), external resistance (R) and salt concentration (c) on Umax. (f) The effect of R on E. The blue and red curves are the original results and the optimized results respectively.
Figure 4: High voltage output using optimized parameters. (a, b) The influence of external resistance (R) on the peak output voltage (Umax) and energy (E) when the optimization parameters are used. (c) Comparison of model predictions with experimentally measured electrical signals when using optimization parameters.
Dr. Hongbo Zhang is the first author of the paper, Academician Wanlin Guo, Professor Zhuhua Zhang and Dr. Minmin Xue are co-corresponding authors. The work was supported by the National Key Research and Development Program, the National Natural Science Foundation and the Natural Science Foundation of Jiangsu Province, and part of the calculation was completed in the High Performance Computing Center of Nanjing University of Aeronautics and Astronautics.
Link: https://doi.org/10.1002/adfm.202315912