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Harvesting energy from droplets

Scientists of the University of Twente and South China Normal University designed an electrical generator that can harvest energy from impacting droplets and other sources of mechanical energy. Their paper recently appeared in Advanced Materials.

Global warming is an increasing threat, demanding the utilization for renewable energy sources. Running water in the form of (rain) droplets is a ubiquitous source of mechanical energy that can be converted into electrical energy. Earlier attempts to harvest this energy using various techniques were either limited in efficiency or in stability, or required an external voltage.

The electrical generator can be explained as being a permanently charged capacitor, also known as an electret. The researchers injected charges into an insulating layer of this capacitor by employing a novel charging method based on electrowetting – the modification of the ability of liquids to maintain contact with a solid surface with an electric field. An electrical current is generated upon impact of a droplet when the induced counter-charges on the capacitor are redistributed.

The magnitude of this current is governed by the number of injected charges. By using composite samples with a high strength they were able to increase the number of injected charges significantly. Smart design of the electrodes allowed the charge redistribution in their generator to occur most efficiently. The researchers managed to convert 11.8% of the mechanical energy of an impacting droplet into electrical energy, which is a significant improvement compared to the efficiency of similar devices. Furthermore, they demonstrated that the energy harvesting efficiency does not degrade after 100 days, requiring only a single 15 minute charging cycle before long-term application.

Their approach is not limited to droplet-based energy harvesting. Niels Mendel, one of the authors says: “Our method can also be considered for other applications where mechanical energy needs to be converted into electrical energy, for example in wearables, from tidal waves, or for sensing.” More research is needed to design a generator that can harvest energy from rain efficiently enough.

The paper is titled “Charge Trapping-Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets”, and has been published in Advanced Materials on the 6th of July. The paper can be read online here. A second paper by the same team that focuses on the physical principles of the energy harvesting process is titled “Energy harvesting from drops impacting onto charged surfaces” and is about to appear in Physical Review Letters.

The research has been performed by Dr. Hao Wu (South China Normal University, University of Twente), Niels Mendel, Stijn van der Ham, Prof. Frieder Mugele (all Physics of Complex Fluids Group (PCF); University of Twente), Prof. Lingling Shui (South China Normal University) and Prof. Guofu Zhou (South China Normal University, Academy of Shenzhen Guohua Optoelectronics).

The research project is supported by the National Key R&D Program of China, National Natural Science Foundation of China, Program for Chang Jiang Scholars and Innovative Research Teams in Universities, Program for Guangdong Innovative and Entrepreneurial Teams, Science and Technology Project of Guangdong Province, Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, Science and Technology Program of Guangzhou, MOE International Laboratory for Optical Information Technologies, and the 111 Project.”

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