Theoretical maximum efficiency and higher power output in triboelectric nanogenerators

The maximum output work and the corresponding output efficiency in energy conversion devices are key criterions, which can provide the direction of performance optimization and the range of performance improvement. In this paper, one cycle model for maximum output work is established to investigate the maximum output efficiency, and another cycle model for combined load is proposed to obtain higher output power for triboelectric nanogenerators (TENGs). By calculating the minimum input work and the maximum output work, the maximum output efficiencies of four basic modes in TENGs are derived. With the assumption of ideal parallel-plate capacitor in TENG, the maximum output efficiencies of both contact-separation (CS) mode and lateral sliding mode (LS mode), which is only determined by the maximum separation distance and shape parameter of dielectric layer (equivalent thickness or length), are similar to Carnot efficiency in heat engines. A relationship between output efficiency and output power is analyzed, which shows that the output power presents a parabolic-like curve with varying output efficiency. There is an ideal output efficiency point, at which the output power obtains the maximum. To improve the output performance, the averaged output powers of the cycles for different single loads and combined loads are calculated and compared in four basic modes of TENGs. In the combined load model, large resistance can effectively enhance the voltage during moving forward and moving backward processes, and small resistance can improve the amount of transferred charge during static process. The result shows that the averaged output power of the combined load is higher than the averaged power at both a single load and optimal matching load. Therefore, the combined load effectively improves the energy utilization of TENGs.