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Fall 2017

Electricity-Free Cooling

Daytime cooling demonstrated using abundant materials in a simple device

Zachary Lebens-Higgins

The diagram illustrates the processes that influence the performance of the electricity-free cooling device. The device has a simple three-layer structure: a polymer coating, fused silica mirror and thin silver back reflector. Even in sunlight, the device remains cool due to highly transparent top layers and a silver back reflector. These researchers identified that thermal emission across a wider wavelength range in the infrared can improve cooling. One of the main barriers to higher device cooling power is identified as atmospheric convection/conduction. Image courtesy of Zachary Lebens-Higgins, NECCES EFRC

On a hot, sunny day, surfaces such as asphalt can become blisteringly hot. Yet some materials stay cooler than the surrounding air even in direct sunlight. Scientists at the Light‑Material Interactions in Energy Conversion (LMI) Energy Frontier Research Center demonstrated over 8 °C cooling below the air’s temperature using a simple device made from abundant, inexpensive materials. These devices open the door for passive cooling, cutting energy consumption by air conditioners and fans.

To keep cool, the device needs to emit power to the surroundings while minimizing the power absorbed. Power is absorbed through thermal radiation from the sun and atmosphere and also through conduction/convection from contact between the air and device. Power is emitted from the device through thermal radiation. While all objects emit thermal radiation (including humans), some objects are better than others, depending on their emissivity, which is a measure of how well an object can emit thermal radiation.

The device engineered by the researchers at LMI has a simple three-layer structure: a polymer coating, fused silica mirror and thin silver back reflector. Each of these layers plays an important role in maximizing cooling power. The layers were first selected to minimize heating in direct sunlight. The polymer coating and fused silica mirror absorb only a tiny fraction of sunlight. The silver back layer is highly reflective and prevents sunlight from passing through the device.

High emissivity of the device in the infrared enables cooling below air temperature. However, good thermal emitters are also good absorbers, and the device will absorb radiation in the same wavelength region of high emissivity. Thermal radiation from the air is then absorbed by the device in wavelength regions of overlap between their respective emissivity/absorption windows. Previously, complex nanostructures were designed to only have high emissivity/absorption in a very selective infrared window. This avoided absorption of thermal radiation from the air as this radiation was thought to significantly limit the achievable device’s cooling power. In contrast, the polymer coating and silica mirror layers emit in a wide infrared window that overlaps with thermal radiation from the air. 

Surprisingly, the device engineered by the LMI researchers could achieve better performance than these complex nanostructures, despite infrared regions of emissivity/absorption overlapping with air. By modeling the cooling power, the researchers at LMI identified that a wider emissivity window can be preferable when the device is only a few degrees Celsius below the surrounding air. In this case, the parasitic conduction/convection from the air is responsible for limiting the cooling ability of the structure.

By moving away from the complex nanostructured devices to a simple three-layer device, these researchers took an important step in the development of electricity-free cooling devices. While more work is needed in material selection and design to optimize the emissivity/absorptivity spectrum, these researchers’ use of abundant materials in a simple device structure increases the feasibility of these devices for a range of cooling applications.

Acknowledgments

This work is part of the Light-Material Interactions in Energy Conversion Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Kou JL, Z Jurado, Z Chen, S Fan, and AJ Minnich. 2017. “Daytime Radiative Cooling Using Near-Black Infrared Emitters.” ACS Photonics 4(3):626-630. DOI: 10.1021/acsphotonics.6b00991

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Cool Down without Electricity

Early research shows simple three-layer device cools air by 8 °C

Even in sunlight, the device remains cool due to highly transparent top layers and a silver back reflector. Image courtesy of Nathan Johnson, Pacific Northwest National Laboratory

This summer, people in Washington state faced a tough choice: Roast in a closed-up house or turn on old air conditioners that bring in wildfire smoke. Researchers are looking at the science behind materials that can provide innovative cooling. At Light-Material Interactions in Energy Conversion (LMI), a team created a device that drops the surrounding temperature by 8 °C without using electricity. The device has three layers: a silver reflector, a special mirror and a thin coating. The device, made with readily available materials, stays cool. Why? It doesn’t absorb much sunlight and emits light across a wide range of the spectrum. Previous approaches emitted light in a narrower range. The science behind these devices opens the door for electricity-free cooling, cutting the energy consumption of air conditioners and fans. The California Institute of Technology leads LMI.

More Information

Kou JL, Z Jurado, Z Chen, S Fan, and AJ Minnich. 2017. “Daytime Radiative Cooling Using Near-Black Infrared Emitters.” ACS Photonics 4(3):626-630. DOI: 10.1021/acsphotonics.6b00991

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.