Harnessing cold space and solar energy for renewable energy

As traditional energy methods increase in cost and harm the environment, Penn State researchers are tapping into two untapped renewable resources, the sun and space, for solutions to generate electricity and passively cool structures.

Led by Linxiao Zhu, assistant professor of mechanical engineering, the team developed and tested a dual cooling and power strategy that simultaneously harvests solar energy in a solar cell and transfers heat from the Earth through radiative cooling. They are Published Their energy solution, on March 13, is more efficient than its own components Cell Reports Physical Science.

Radiant cooling works by sending infrared light directly into space without heating the surrounding air. Zhu used a thermal camera to illustrate the concept.

Invisible, heat-resistant infrared light can only be seen with a thermal camera, which uses color to show the temperature an object emits. A human body glows orange or red, indicating high temperature, for example, a window on a cold day glows blue, indicating low temperature. Thermal infrared radiation, also known as dark matter radiation, is the energy that people and objects shed when they cool.

„In radiant cooling, infrared light is emitted from a transparent, low-iron glass,” Zhu said. „The light bounces off the glass, passes through the atmosphere without heating the surrounding air, and lands in space, which we call the cold universe.”

This process cools the surface of the radiant cooler. That cooling capacity is directed toward an object such as a building or the inside of a refrigerator.

Daylight radiant cooling was invented over a decade ago and developed as an emerging zero-carbon cooling method. As a doctoral student at Stanford University in 2014, Zhu worked on the research team that first developed diurnal radiative cooling.

„During night and day, the radiant cooler acts as a natural air conditioner 24/7,” said first author Pramit Ghosh, a mechanical engineering PhD student at Penn State. „Even on a hot day, a radiative cooler feels cool to the touch.”

Underneath the radiant cooler, the researchers positioned a solar panel so that during daylight hours, sunlight passes through the transparent radiant cooler and is absorbed by the solar cell to generate electricity.

The researchers tested their system last summer at the Penn State Sustainability Institute's Sustainability Experience Center. They found that the combined benefit of electricity generation and cooling from the dual harvesting system exceeds the electricity savings of a solar cell alone by 30%. In other words, harvesting resources together as a pair is greater than using the resources alone.

„Based on these experimental results, using the two harvesters together has the potential to significantly outperform a bare solar cell, which is a key renewable energy technology,” Zhu said.

Another advantage is unit size: since the two harvesters are stacked, they take up minimal space on the roof or floor.

„At the same time and in the same place, we can use these renewable resources together 24 hours a day,” Ghosh said.