Dual-mode heating and cooling depending on the weather

“We show that state-of-the-art heating/cooling performance can be accomplished on the same footprint of building with proper materials science and thermal engineering”Po-Chun Hsu

Researchers at Duke University in the US have developed a device that with a reversible thermal contact that allows switching between two modes for heating or cooling a building, a breakthrough that could reduce heating, ventilation and air conditioning (HVAC) energy use by up to 20% if widely deployed. The nanomaterial-based device is based on a combination of mechanics and materials science that harnesses or expels certain wavelengths of light while being transportable and retaining thermal contact with the building.

As reported in Nature Communications [Li et al. Nat. Commun. (2020) DOI: 10.1038/s41467-020-19790-x], the sheets of the nanomaterial could be used to either bounce heat away or absorb it. Depending on conditions, rollers in the device move a sheet back and forwards to expose either heat-trapping materials on one half or cooling materials on the other. One of the materials acts to absorb the sun's energy and trap existing heat, while the other reflects light and allows heat to escape through the atmosphere and out into space.

The sheets have a polymer composite as the base that can expand or contract by electricity so that the device stays in contact with the building for transmitting energy and can still be disengaged for the rollers to switch between modes. The cooling part of the sheet involves an ultra-thin silver film covered by a thinner layer of clear silicone, combining to reflect the sun's rays. When heating is required, the electrical charge releases and the rollers pull the sheet along a track, changing the cooling, reflective section of the sheet for the heat-absorbing section.

To heat the “building” underneath, they used an ultra-thin layer of copper topped by a layer of zinc-copper nanoparticles. By ensuring the nanoparticles are a certain size and spaced at a specific distance, they interact with the copper such that light is trapped on their surface so that the material can absorb over 93% of the heat from the sun.

Although most previous attempts to reduce greenhouse gas emissions focus on either heating or cooling, in temperate climates both are necessary throughout the year. It is hoped the device could also operate in tandem with existing HVAC systems rather than replace them. As team leader Po-Chun Hsu told Materials Today, “We show that state-of-the-art heating/cooling performance can be accomplished on the same footprint of building with proper materials science and thermal engineering”.

The team are now exploring the best design approach to make the device scalable for manufacturing, and to overcome issues such as the long-term wear and tear of the moving parts and costs of the materials, as well as to improve its functionality.