Energy Harvesting from Drops Impacting onto Charged Surfaces

Abstract

We use a combination of high-speed video imaging and electrical measurements to study the direct conversion of the impact energy of water drops falling onto an electrically precharged solid surface into electrical energy. Systematic experiments at variable impact conditions (initial height; impact location relative to electrodes) and electrical parameters (surface charge density; external circuit resistance; fluid conductivity) allow us to describe the electrical response quantitatively without any fit parameters based on the evolution of the drop-substrate interfacial area. We derive a scaling law for the energy harvested by such "nanogenerators" and find that optimum efficiency is achieved by matching the timescales of the external electrical energy harvesting circuit and the hydrodynamic spreading process.

Electrowetting‐Assisted Generation of Ultrastable High Charge Densities in Composite Silicon Oxide–Fluoropolymer Electret Samples for Electric Nanogenerators

“…When a spreading droplet touches the wire, a current is generated with a peak value that is proportional to the surface potential and inversely proportional to the load resistance (bottom‐left inset Figure 1f). [ 13,14 ] The surface potential U S can thus be assessed by multiplying the load resistance with the peak value of the generated current, U S = I peak R L , as illustrated in Figure 1f. As expected, | U S | is found to increase with increasing | U C |.…”

“…Moreover, Figure 1f also demonstrates the applicability of this composite electret substrate as a nanogenerator that converts a droplets’ impact energy into electrical energy, as described in detail in refs. [ 13,14 ] .…”

“…The maximum charge density reported for SiO 2 is − 7.5 mC m −2 , [ 26,58 ] which is close to the highest values of the interlayer charge density of − 5.1 mC m −2 inferred for some of our samples (see Figure S4 in the Supporting Information). Our model implies a clear roadmap toward further optimization of σ eff —one of the key parameters for maximum energy harvesting in nanogenerators [ 14 ] —while respecting the intrinsic limitations of the materials involved (see Figures S5 and S6 in the Supporting Information).…”

“…In search of clean and renewable energy sources, so‐called (tribo)electric nanogenerators that convert mechanical energy into electrical energy have recently attracted substantial attention. [ 1–16 ] The efficiency of triboelectric nanogenerators is, however, limited by the relatively small [ 1,6–9,11–14 ] and often unstable [ 5,7,8,15 ] and poorly controllable [ 5,15,17–19 ] surface charge density generated by triboelectrification. Achieving large and stable charge densities by controlled injection (or ejection) of charge carriers into a material by various methods [ 20 ] was studied before in the context of electrets: (quasi‐)permanently charged materials that are ubiquitous because of their use in the majority of (consumer) microphones, [ 20–22 ] as nowadays commonly used in smartphones.…”

“…[ 42 ] As a modification of the process, homogeneous EWCI (h‐EWCI) that suppresses the contact line singularity of the electric field and involves a dielectrically stable oxide layer underneath a hydrophobic fluoropolymer film, subsequently enabled charge densities of the order of −1 mC m −2 and higher that were homogeneous over cm 2 areas (h‐EWCI), stable over ≥100 days under ambient conditions, and showed no appreciable degradation under the impact of (highly saline) droplets. [ 13,14 ] (High stability could only be achieved for negative charging polarity. [ 42 ] ) Notwithstanding these substantial improvements compared to conventional triboelectric nanogenerators as well as corona‐based charging of electrets, the mechanisms behind the h‐EWCI process, the origin of the long‐term stability, as well as the parameters to optimize it even further have so far remained elusive.…”