New Generators for All-Day Clean Energy

Thermoelectric generators get a boost—now they can supply enough renewable energy to power small devices both day and night.

By Rachil Koumproglou

Renewable sources of energy, such as solar, wind, marine, and geothermal, contribute to cutting greenhouse gas emissions. Yet we are unable to extend their use because they are incapable of supplying energy continuously.

Now, a research team from Jimei University and other collaborators from scientific centers in China have invented an autonomous electric generator that can supply energy to small outdoor devices on a 24-hour basis and under different weather conditions. The generator is self-powered because it creates electricity as a result of a temperature difference that occurs between its two ends and is caused by two natural processes: one end is heated by sunlight and the other side is cooled by emitting heat into the atmosphere.

The devices that produce electricity out of temperature differences are known as thermoelectric generators (TEGs). They use a natural thermoelectric effect that makes electrons flow from the hot to the cold side of a system.

Recently, researchers found a way to improve the heating efficiency of the TEG’s hot end by using materials that absorb sunlight from the entire solar spectrum; these are known as Solar Absorbers. In this way, the output voltage is higher and remains stable even in the night hours. However, the process to fabricate these Solar Absorbers is quite complex, which makes them an expensive solution for the hot end of TEGs.

New materials improve the use of thermoelectric generators

In the present study, the research team developed a simple and low-cost protocol for building the Solar Absorber of their thermoelectric generator. They discovered that their new material absorbs the sunlight with high efficiency by creating big temperature differences and enabling uninterrupted generation of electric current during the entire day. 

In addition, the cold end of the TEG is made of a material, known as Cooling Emitter, that enables efficient cooling by using two natural processes: it reflects the sunlight at almost 90 percent and emits heat passively in the atmosphere with extremely high efficiency (>97.94 percent). These properties are the result of radiative cooling, a natural phenomenon by which a body loses heat into the surrounding area by radiation.  

The Solar Absorber and the Cooling Emitter consist of multiple layers of fine films, making them flexible and adaptable on different surfaces. As a result, the generator is suitable for supplying energy to a great variety of devices, such as remote sensors or wearable apparatuses, and may be especially useful in remote or underdeveloped areas.

The researchers tested the new TEG under normal sunlight conditions and showed that it can generate a voltage of 166.2 mV, which is enough to charge a small device. But even when sunlight is absent (at nighttime) or is low (at daytime cloudy conditions), the TEG conserves some of the temperature differences and can generate an output voltage of 14.7 mV and 95 mV, respectively.

In short, the current work demonstrates that, owing to its innovative Solar Absorber and Cooling Emitter, the new TEG can create a temperature difference by solar heating and radiative cooling. It therefore can supply energy to small devices on a 24-hour basis while operating without any CO2 emissions.

The next goal of this project is to lower the cost of fabrication and permit the massive building of a thermoelectric generator that will have higher performance efficiency, more durability, and more suitability for further applications.  

This study was published in the journal Optics Express.

Reference

J. Liu, J., Li, D., Ma, W., Chen, Y., Dou, C., Meng, D., He, Q., Li, X., Deng, X., & Cai, H. (2023). All-day uninterrupted thermoelectric generator by simultaneous harvesting of solar heating and radiative cooling. Optics Express, 31(19), 14495–14508. https://doi.org/10.1364/OE.483531