The related paper in Nature Sustainability is here: https://rdcu.be/cECpM
More than 76% of total electricity and 40% of all energy in the United States are consumed to provide comfortable, well-lit and well-conditioned buildings, which results in a cost of more than $430 billion annually and severe global emission of greenhouse gas. The heating, ventilation and air conditioning (HVAC) systems of buildings, which directly serve for heating and cooling demands, contribute to about 40% of the building energy consumption. This huge energy consumption is causing severe environmental and economic problems. Therefore, creating energy-efficient buildings plays an increasingly important role in sustainability and reducing carbon footprint.
Engineering the radiative properties of building envelopes is emerging as a promising approach for building energy saving. For instance, near-infrared (NIR) reflective cooling coatings have been developed to reflect the invisible NIR light in the solar spectrum to decrease solar heating. Additionally, passive daytime radiative cooling materials create a surface that has high solar reflectance and high emittance in the mid-infrared (MIR) transparent window of the atmosphere for saving cooling energy by rejecting solar heating and radiating heat to the cold sky. These materials are suitable for unshaded building roofs to efficiently radiate heat to the outer space/sky, and the ideal installation locations should have long and warm summers to avoid the heating penalties in winters.
For building walls, unlike for roofs facing the sky, the MIR radiative heat exchange is more dominant with outdoor ambient surroundings than with the sky. Such radiative heat exchange is more pronounced in urban areas, where buildings are concentrated. However, traditional building materials usually show high thermal emissivity, leading to intense radiative heat exchange. In hot climates, the overheating caused by high radiation throughput from the hot environment and the sun results in increased cooling demand. Similarly, the excessive radiative heat loss from the building interior in cold climates leads to increased heating energy consumption. Therefore, a design that minimizes radiative heat transfer through the building wall envelope will be beneficial for both cooling and heating energy saving throughout the year.
In our work, we develop a category of coloured low-emissivity films for building wall thermal envelopes, providing a new solution to year-round building heating and cooling energy saving. We demonstrate high reflectance (~ 90%) in the infrared wavelength range and selective reflectance in the visible light wavelength range for desired colours. These films can help minimize radiative heat exchange between the indoor and outdoor environments, so saving energy for all-year cooling and heating, while satisfying the required aesthetical effect of buildings simultaneously. They can serve as a flexible choice for both new buildings and retrofit of existing building envelopes. Moreover, the films are easy to clean and change. Simulations show that these films can help reduce heat gain and loss up to 257.6 MJ per installation wall area annually. In the case of a typical midrise apartment building, the HVAC system can save up to 124.46 GJ (equal to 9.87% building HVAC energy consumption). By rough estimation, a global CO2 emission reduction of 1.14 billion metric tons annually could be achieved. We expect this work provides new insights for innovative energy-efficient building envelope materials that can help achieve global carbon neutrality and sustainability.
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