Temperature Adaptive Radiative Coating

Cool roof

For decades, remarkable efforts have been put into decreasing energy consumption to fight the catastrophic energy crisis and climate change. Energy saving on temperature regulation in buildings is one of the most important topics. For instance, according to U.S. Energy Information Administration (EIA), 51% of energy in residential buildings is invested in space heating and air conditioning. To save this part of the energy, one mainstream solution is the cool roof. The cool roof materials are designed to have a strong solar reflection to minimize the heat intake from the sun, while having a high thermal emissivity in the atmospheric window (8-14um) to effectively release heat to outer space. Currently, the global cool roof market has great growth and can be reached $38.20 billion in 2028 with a compound annual growth rate (CAGR) of 7.8% during the forecast period (2021-2028) [1]. Also, according to the Cool Roof Rating Council, there are more than 3000 cool roof products on the market [2].

However, cool roofs constantly emit heat from the roof during cold nights or winters. This heat loss will cause more energy waste in space heating. In some climate areas that have strong temperature variations, the extra energy consumption in space heating can even outweigh the saved cooling energy. Thus, from an annual perspective, the energy saving of cool roofs is limited by the overcooling issue.

Temperature Adaptive Radiative Coating (TARC)

Is there a solution to the overcooling issue? The answer is yes. Temperature Adaptive Radiative Coating (TARC) is a material that can switch its emissivity (in the atmosphere window) between low and high at zero external energy input. When the ambient temperature is high, like in summer daytime, TARC has a high emissivity so it works similarly to traditional cool roofs. When the temperature drops below a comfortable temperature set point, TARC will automatically switch to a low emissivity state to keep the heat under itself. As a smart, energy-free "air conditioner", TARC can outperform traditional cool roofs in most climate areas, especially those with large day-to-night, summer-to-winter temperature variations [3].

Fig. 1 Comparison between cool roofs and TARC

Tuning the thermal emissivity is not an easy job, as all matter with a temperature greater than absolute zero emits thermal radiation, and most non-metal material has high emissivity [4]. Vanadium dioxide (VO2) is a phase-transition material that has a metal-insulator-transition (MIT) at ~67 C. The phase transition temperature can be further tuned by tungsten (W) doping in a broad range [5]. Thus TARC can work with different switching temperatures including the comfortable indoor temperature (~22 C).

The efficient switching of TARC is made by a VO2 photonic meta-structure, an infrared transparent matrix, and a back mirror. When the temperature is lower than the phase transition temperature of W-doped VO2, the VO2 is in the insulator state and almost transparent to thermal infrared light. Thus, the whole system works like a metal mirror that has very low emissivity. In the other case, when VO2 performs like metal at high temperatures, the thermal emission will be drastically boosted by the photonic resonance in the structure. The following figure shows the color change of TARC in infrared photos taken by a FLIR camera.

Fig. 2 Thermal infrared image of TARC and two reference materials with low and high emissivity

To evaluate the energy-saving potential of TARC, extensive simulation with real climate data and TARC properties was carried out. The minimum energy saving from replacing the best-performing commercial roof material with TARC was calculated. The result for cities representing 15 climate areas in the U.S. shows that TARC can save more energy than all commercial roof materials in 12 out of 15 cities. For an average flat house in the U.S., the largest red circle on the map represents an extra energy saving that equals 10% of annual electricity consumption.

I and my colleagues realized the world's first TARC. And In December 2021, the research paper was published in Science [3].

Fig. 3 Energy-saving map of TARC for U.S. cities. The energy-saving value was calculated by replacing the locally best-performing commercial roof material with TARC. Red means positive energy saving while blue means negative.

Scalable TARC (s-TARC)

The scalable TARC (s-TARC) is aimed at scaling up TARC at a low cost. A new WVO2/polymer-based structure is used to improve the flexibility and lifetime of this material. The color of s-TARC can also be tuned during production to reach an optimized solar absorption for climates. The s-TARC has passed a series of in-lab/outdoor tests and the publication and patents are in progress.

The photos below show an s-TARC panel and colored s-TARC. More results will be added to this page after the public disclosure of the research paper & patents.

The future of TARC

As we have successfully developed TARC and s-TARC, we are aiming for more applications to make our inventions better serve the society and environment:

We are currently exploring TARC in more forms such as paint and fabric.
We are looking for expanding the application cases of our current TARC film material, such as tents, vehicles, and consumer electronics.

References

Cool Roof Market Research Report: Information by Roof Type, Product Type, Application, and Region - Forecast till 2030, market research future.
CRRC roof directory, Cool Roof Rating Council.
Tang, Kechao, Kaichen Dong, Jiachen Li, Madeleine P. Gordon, Finnegan G. Reichertz, Hyungjin Kim, Yoonsoo Rho et al. "Temperature-adaptive radiative coating for all-season household thermal regulation." Science 374, no. 6574 (2021): 1504-1509.
What Is Emissivity? (EE Tip #133), Circuit Cellar.
Lee, Sangwook, Kedar Hippalgaonkar, Fan Yang, Jiawang Hong, Changhyun Ko, Joonki Suh, Kai Liu et al. "Anomalously low electronic thermal conductivity in metallic vanadium dioxide." Science 355, no. 6323 (2017): 371-374.

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